Discovery of nuclear fission

Nuclear fission of the uranium atom: special stamp of the German Federal Post Office from 1979

Former Kaiser Wilhelm Institute for Chemistry, today Hahn-Meitner-Bau of the Free University of Berlin Coordinates: 52 ° 26 ′ 53.2 ″  N , 13 ° 17 ′ 3.1 ″  E

The discovery of nuclear fission on December 17, 1938 at the Kaiser Wilhelm Institute for Chemistry in Berlin is one of the most important and momentous events in the history of the natural sciences . When uranium was irradiated with neutrons , uranium fission products were formed , among other things. a. the first to be detected barium . This decisive result of a nuclear physics and radiochemical experiment by Otto Hahn was supplemented by chemical analyzes by his assistant Fritz Straßmann . In an interdisciplinary collaboration, this unexpected result was interpreted theoretically and physically for the first time in January 1939 by Lise Meitner and Otto Frisch .

The starting point was the experiments of Enrico Fermi , who irradiated uranium with neutrons in 1934. Over the years Hahn, Meitner and Straßmann tried to elucidate the processes observed. Independently of this, from 1937 onwards, a working group around Irène Joliot-Curie at the Radium Institute in Paris also devoted itself to the same topic. Initially, all working groups pursued the hypothesis that the irradiation produced elements heavier than uranium (so-called transuranium elements ). In December 1938 there was an unexpected result: Hahn and Straßmann used special chemical separation and analysis processes to prove that the reaction products observed were radioactive barium isotopes that did not occur in nature . The experiments evidently resulted in a "bursting" of the atomic nucleus - as Hahn put it - which the chemists could not theoretically explain, especially since this "bursting" contradicted the previous physical models of an "indivisible" atom .

Lise Meitner, who had to leave National Socialist Germany in the summer of 1938 due to her Jewish descent and had emigrated to Sweden via Holland with Hahn's help, was informed by letter from Otto Hahn about the progress of the Berlin experiments. In January 1939 she and her nephew Otto Frisch were able to work out a first nuclear physics interpretation of Hahn-Straßmann's results and publish it together with Frisch in February 1939 in the English journal Nature . Their model described the uranium nucleus as an electrically charged drop of liquid that was set in motion by the capture of the neutron in such a way that it split into two fragments of approximately the same size, releasing a high level of energy. Fresh this previously unknown were nuclear reaction type the name "nuclear fission" ( fission ), who prevailed rapidly internationally. The Hahn-Straßmann and Frisch-Meitner publications in January and February 1939 triggered an extraordinary response among natural scientists because nuclear fission opened up a new source of energy of a previously unknown magnitude, nuclear energy .

Preliminary remarks on nomenclature and spelling

In the context of this article, the terms common in nuclear chemistry and physics in the 1930s are used, even if they are no longer in use today or have been replaced by new terms, e.g. B. atomic weight by atomic mass . For radiochemical preparations, the terms such as "activity", "body" or "β-emitter" used in the protocols and publications of that time are retained, often in combinations such as "13-min activity" or "3.5 hour body" , whereby the half-life (HWZ) is meant with the respective time . The time units used are s ( seconds ), m ( minutes ), h ( hours ), d ( days ) and a ( years ).

The names of individual radioactive nuclides that were customary at the time , such as B. Mesothorium 1 (MsTh1) or Thorium X (ThX), both of which are not thorium but radium isotopes, are retained, but supplemented by the current notation.

The abbreviation KWI is used for the Kaiser Wilhelm Institute for Chemistry in Berlin.

The state of research in the early 1930s

The periodic table of the elements

The working groups that took part in the search for the transuranium elements after Fermi's discovery were faced with the problem of identifying a new radioactive substance formed in only imponderable quantities by the neutron irradiation. The periodic system of the chemical elements plays an important role here. However, since it differs in its form at that time from the arrangement of the elements that is usual today, it is shown and explained here (simplified).

H  1

He   2

Li  3

Be  4

B  5

C  6

N  7

O  8

F  9

Ne  10

Well 11

Mg 12

Al 13

Si 14

P 15

P. 16

Cl 17

Ar  18

K 19

About 20

Sc 21

Ti 22

V 23

Cr 24

Mn 25

Fe 26

Co 27

Ni 28

Cu 29

Zn 30

Ga 31

Ge 32

As 33

Se 34

Br 35

Kr  36

Rb 37

Sr 38

Y 39

Zr 40

Nb 41

Mon 42

Ma 43

Ru 44

Rh 45

Pd 46

Ag 47

CD 48

In 49

Sn 50

Sb 51

Te 52

J 53

X  54

Cs 55

Ba 56

La 57

Hf 72

Ta 73

W 74

Re 75

Os 76

Ir 77

Pt 78

Au 79

Hg 80

Tl 81

Pb 82

Bi 83

Po 84

? 85

Rn  86

? 87

Ra 88

Ac 89

Th 90

Pa 91

U 92

? 93

? 94

? 95

? 96

Lanthanide noide:

La 57

Ce 58

Pr 59

Nd 60

? 61

Sm 62

Eu 63

Gd 64

P 65

Dy 66

Ho 67

He 68

Tm 69

Yb 70

Cp 71

Periodic Table of the Elements (1938)

The chemical elements are arranged in the order of their atomic number (atomic number) from left to right in the form of a table, whereby a new row of the table is started at periodic intervals, some of which are the same, but also increase rapidly. The elements then standing one below the other belong to a group , they show a similar chemical behavior, which is why they are also referred to as chemically homologous elements .

The elements 61 ( Promethium ), 85 ( Astat ) and 87 ( Francium ) were still unknown at that time, the elements 43 (Masurium) and 71 (Cassiopeium) are now called Technetium and Lutetium, respectively . At that time the symbol J (today I) was common for the element iodine , for xenon the symbol X (today Xe).

The transuranic elements that might arise or were expected during the neutron irradiation of uranium were classified according to the ideas of the time in the still free fields of the periodic system to the right of the uranium (the figure shows these places provided for the elements 93 to 96 at that time). Accordingly, element 93 was considered to be homologous to manganese , masurium and rhenium , element 94 would then be a homologous element to iron , ruthenium and osmium , etc. This is where the names Eka-Rhenium , Eka- and used in the cited publications originate. Osmium , Eka-Iridium and Eka-Platinum for these as yet unknown elements 93–96.

It was only years later that it turned out that this arrangement does not apply to actinium and the elements that follow it (including the transuranic elements); they belong to the group of actinides . The chemical similarity between element 93 and rhenium expected in 1938 was therefore a fallacy, as was that between element 94 and iron.

In the meantime, the chemical elements with the ordinal numbers 93 to 96 have been given the following names:

Neptunium (Np),  Plutonium (Pu),  Americium (Am) and Curium (Cm).

Several (radioactive) isotopes are known of all of them.

The discovery of artificial radioactivity

Up until 1933, radioactive chemical elements were only understood to be the naturally occurring radioactive elements. These are the chemical elements with high atomic numbers 84 to 92 and the radioactive daughter substances formed when they decay. There are also some lighter chemical elements in nature such as B. Potassium and Rubidium , in which individual isotopes are radioactive; their natural radioactivity was also known at that time.

In 1934, the couple Irène Joliot-Curie and Frédéric Joliot succeeded in converting atomic nuclei of non-radioactive elements into a neighboring chemical element and making them "radioactive" by irradiating them with alpha particles from such natural radiation sources - that is to say by means of nuclear reactions in today's language . In contrast to the naturally radioactive isotopes, such isotopes were called “artificially radioactive”. The conversion of the elements was limited to the lighter chemical elements, because the heavier elements had a high positive nuclear charge which prevented the also positively charged α-particle from penetrating the atomic nucleus.

The Joliot-Curie couple received the Nobel Prize in Chemistry in 1935 for their discovery of this artificial radioactivity.

The neutron irradiation of chemical elements by Fermi (1934)

In Rome, Enrico Fermi continued Joliot-Curie's experiments in 1934/35 to produce artificially radioactive elements. For irradiation, however, he used the electrically uncharged neutron discovered by James Chadwick in 1932 , which, in contrast to the α-particles used by Joliot-Curie, can also penetrate atomic nuclei with a high atomic number unhindered. Irradiation was carried out on 68 of the 89 chemical elements known at the time, and a transformation was observed for 47 elements, which always proceeded according to the following scheme for heavy elements: The neutron hitting the atomic nucleus is absorbed into the nucleus, an isotope of the irradiated element is created with a mass number one unit higher. This atomic nucleus is unstable (radioactive) and disintegrates into a stable isotope of the next higher element with emission of a β ^- particle ( electron ); for example:

${\displaystyle {}_{\ 47}^{109}\mathrm {Ag} +{}_{0}^{1}\mathrm {n} \ \rightarrow \ {}_{\ 47}^{110}\mathrm {Ag} \ {\xrightarrow[{\text{24 s}}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 48}^{110}\mathrm {Cd} }$

(n: neutron; Ag: silver ; Cd: cadmium ; β ^- : beta radiation ; 24 s: half-life ( half-life ) of the radioactive silver isotope ¹¹⁰ Ag)

In these experiments, Fermi found that the radiation from the samples is particularly intense when there is a thick layer of hydrogen-containing substance (e.g. water or paraffin ) between the neutron source and the sample , a moderator in today's language . Frequent collisions of the neutrons with the hydrogen nuclei ( protons ) slow down their speed considerably. The probability that such slow (“thermal”) neutrons will be captured by an atomic nucleus of the sample is much greater than with unrestrained neutrons.

In the literature cited, this method is referred to as "enhanced" radiation.

The undetected nuclear fission of uranium in the search for transuranium elements

In the period that followed, Fermi in Rome tried to produce transuranium elements by irradiating uranium (the heaviest known element in the periodic system with atomic number 92 at the time) with neutrons. In Berlin and Paris, the working groups headed by Hahn / Straßmann / Meitner and Joliot-Curie / Savitch are attempting to detect transuranic elements using the assumptions made at the time about the chemical relationships of these as yet unknown elements that do not occur in nature.

According to the scheme given above for the silver, this reaction would have to proceed

${\displaystyle {}_{\ 92}^{238}\mathrm {U} +{}_{0}^{1}\mathrm {n} \ \rightarrow \ {}_{\ 92}^{239}\mathrm {U} \ {\xrightarrow[{\text{ }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 93}^{239}\mathrm {Element} }$

and thus lead to a new element with the ordinal number 93.

Today we know that ²³⁸ U reacts with neutrons according to this scheme, but this reaction is completely superimposed by the fission of ²³⁵ U, which was then ignored . Its numerous fission products were mistaken for transuranium elements by researchers in the 1930s and described as such, as can be seen from the following articles. Because uranium reacts with neutrons differently than assumed at the time and the suspected family relationships did not apply at all, the way to clarify the true facts was long and arduous.

Scheme of the splitting of ²³⁵ U into barium and krypton

What really happened:

${\displaystyle {}_{\ 92}^{235}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{236}\mathrm {U} \ \rightarrow \ {}_{\ 56}^{139}\mathrm {Ba} \ +\ {}_{36}^{95}\mathrm {Kr} \ +\ 2\ {}_{0}^{1}\mathrm {n} }$

This is only one example of more than 100 cleavages of ²³⁵ U that occur , namely the one with the barium chemically detected by Hahn and Straßmann at the end of 1938. The fact that there was nuclear fission was confirmed by Frisch and Meitner in the days between Christmas 1938 and New Year 1939; that this split only affects the uranium isotope 235 was only recognized by Niels Bohr in 1939 .

The neutron irradiation of uranium by Fermi in 1934

When irradiating uranium, instead of a new substance, Fermi found five radioactive products with 10 s, 40 s, 13 min, 40 min and a half-life of around 24 h. Fermi tried to chemically identify one of these bodies, the one with a half-life of 13 minutes. He could rule out belonging to uranium or one of its lighter neighboring elements and therefore expressed the assumption that the atomic number of the element could be greater than 92. The fact that the body behaved similarly to the elements manganese and rhenium, which were held to be homologous, spoke in favor of element 93 , but Fermi did not want to rule out the possibility of an atomic number 94 or 95, since he assumed that the chemical properties of these elements were those of Element 93 might be quite similar.

Fermi did not use the term transuranium for these new elements; it only became naturalized later. The physicist Fermi did not undertake any further attempts to elucidate the course of the reaction or to chemically identify the conversion products. In 1938 he was awarded the Nobel Prize in Physics for his discoveries. After the award ceremony in Stockholm, he did not return to fascist Italy, but emigrated with his wife to the USA.

The neutron irradiation of uranium by the KWI 1934–1938

The only institute in Germany that dealt with radiochemical and at the same time nuclear physics issues was the Kaiser Wilhelm Institute for Chemistry in Berlin (KWI). The then director of the institute, to which the chemical-radioactive department was also subordinate, was the radiochemist Otto Hahn , the head of the physical-radioactive department was the physicist Lise Meitner . Both had known each other since 1907, both were habilitated at the University of Berlin , both had been appointed professors (1910 and 1926). In 1933, however, Lise Meitner's license to teach was revoked because of her Jewish descent, but as an Austrian she was initially able to continue her work at the (non-state) KWI.

The chemist Fritz Straßmann continued to work in Hahn's department. In 1929, after completing his doctorate, he was hired as a scholarship holder and in 1935 received an assistant position at the institute.

Fermi's observations during the neutron irradiation of uranium were taken up by this team at the KWI in Berlin towards the end of 1934. They prompted the search for transurans, because the physicist Fermi was unable to chemically identify the five reaction products; This was only possible in the radiochemical laboratories in Berlin and Paris.

Thus began a search for transurans in Berlin, which lasted a full four years and led to the discovery of nuclear fission.

Natural uranium and its derivatives

Uranium, which is mainly found in nature in the form of pitchblende (U ₃ O ₈ ), consists of the isotopes ²³⁸ U (99.28%), ²³⁵ U (0.715%) and ²³⁴ U (0.005%). All three isotopes are α-emitters and decay in long decay series of α- and β-emitters, until finally stable final states are reached for lead ( ²⁰⁶ Pb and ²⁰⁷ Pb, respectively ) (see decay series as well as uranium-radium series and uranium-actinium Row ). Chemically pure uranium only contains the initial elements, namely the three isotopes of uranium and the rapidly newly forming secondary products of ²³⁸ U, the thorium isotope UX1 and the protactinium isotope UX2:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} \ {\xrightarrow[{{4,5}\ \cdot \ {10^{9}}{\ a}}]{{\text{ }}\mathrm {\alpha } }}\ {}_{\ 90}^{234}\mathrm {Th\ (UX1)} \ {\xrightarrow[{\text{ 24,1 d }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 91}^{234}\mathrm {Pa\ (UX2)} \ {\xrightarrow[{\text{ 1,17 m }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 92}^{234}\mathrm {U} \ {\xrightarrow[{{2,5}\ \cdot \ {10^{5}}{\ a}}]{{\text{ }}\mathrm {\alpha } }}\ {}_{\ 90}^{230}\mathrm {Th} }$

If a commercially available uranium compound is irradiated with neutrons, the activity of the resulting reaction product with the weak neutron sources of that time is much weaker than the activity of the two simulated natural secondary products ²³⁴ Th and ²³⁴ Pa. Therefore, immediately before the neutron irradiation, these two disruptive secondary products have to be separated off (the uranium “clean”).

Pre-purified uranium

The pre-cleaning is done by the process described below, a co-precipitation of the secondary products on iron hydroxide. The uranium which remains in solution is then precipitated with ammonia as ammonium diuranate (NH ₄ ) ₂ U ₂ O ₇ (ammonium pyrouranate), filtered off and dried. Immediately after this separation of the Th and Pa isotopes, the uranium is a pure alpha emitter , but the two isotopes are reproduced immediately. However, they only reached 50% of their equilibrium activity after 24.1 days, so you can work with the purified uranium preparation for a few days (irradiate, measure, analyze) without having to fear any significant influence from the two β-radiating daughter substances.

The duration of exposure to uranium

In order to achieve sufficiently measurable activities despite the purification of the uranium, larger amounts (10 to 100 grams of ammonium diuranate) have to be irradiated. In general, the amount and thus the activity of the reaction product increases with the duration of the neutron irradiation; however, it approaches a final value when the exposure time z. B. corresponds to ten times the half-life of the new radioactive body. If one irradiates only for the duration of one half-life, the activity has reached 50% of this end value, with two half-lives 75%, with three half-lives 87.5% etc. An irradiation duration of more than 3 half-lives hardly brings an increase in activity and is therefore makes little sense.

However, if the reaction product has a half-life of days or weeks, one must be content with an irradiation time that is shorter than z. B. is a half-life and accept the lower activity of the new body.

The irradiation and measuring equipment

The chemical and physical equipment used at that time is now in the Deutsches Museum in Munich. The equipment and devices that had to be housed in different rooms in the KWI are arranged on a table in the museum ; as far as radioactive substances are concerned, they are replaced by dummies. Compared with the measuring devices available today, the facilities at that time were extremely simple.

The neutron sources for irradiating the uranium are powdered mixtures of beryllium or beryllium sulfate and about 100 mg of radium sulfate each, melted airtight in glass tubes.

If the six sources available at the time were not needed for irradiating several samples, they could be used together to irradiate only one sample and thus increase the activity of the reaction product. If the uranium was to be irradiated with slow (thermal) neutrons, the uranium and sources were located in a paraffin block closed on all sides, which is provided with holes to accommodate the neutron sources and the uranium.

After the irradiation, the uranium sample was dissolved in water or acid and the radioactive substance formed was separated from the uranium. This was done in one of the radiochemical laboratories of the KWI with the help of special chemical separation methods that had to be fast enough for short-lived bodies. Of the devices required for this chemical is on the work table only one feeding bottle having an attached suction to see on the deposited radioactive sample and was then dried and a round smooth filter of 3 cm in diameter.

In a third room, the activity of this sample was measured with a Geiger-Müller counter tube . The counter tubes were made in the institute's workshop from aluminum tubes approx. 2 cm in diameter and 6 cm in length. The wall thickness was previously reduced to approx. 0.1 mm by turning on the lathe so that the β-radiation to be measured is not already absorbed in the counter tube wall. A steel wire inserted in the middle was insulated from the tube with hard rubber stoppers attached on both sides, and a glass tube also embedded was used to pump out and fill the counter tube with a gaseous argon - alcohol mixture.

During the measurement, the counter tube was embedded horizontally in a dismountable lead block in order to shield against external influences, in which there is an opening underneath the counter tube, into which a lead “boat” can be inserted, in whose recess the filter with the sample was previously glued . In this way, the radioactive sample was largely correct in shape on the outer wall of the counter tube. Several anode batteries connected in series were used to supply the counter tube with power (1000–1500 volts DC) . The counting wire is connected to the positive terminal and the housing to the negative terminal of the battery via a high resistance.

If a β ^- particle (electron) emitted by the sample to be measured gets through the aluminum wall of the counter tube into the inside of the tube, it is accelerated in the electric field towards the counting wire. In this way, the argon atoms it hits are ionized, the electrons formed in the process also fly to the counting wire and form further electrons through ionization along the way. This electron avalanche flows through the counting wire and resistor to the earthed positive pole. This current pulse is amplified by a tube amplifier and registered by a mechanical counter. If the counter tube is filled with alcohol, the discharge automatically extinguishes after a short time, so that the counter tube is then ready to detect another electron.

Working with unpredictable amounts of radioactive substances

When searching for the transuranium elements, the researchers at the KWI were faced with the problem of a relatively large amount of a water- or acid-soluble uranium compound (approx. 10 to 100 g), a new, radioactive substance formed in tiny, imponderable quantities by the neutron irradiation - essential less than 1 picogram ( ^10-12  grams) - chemically identifiable.

Even for the most sensitive analytical methods known at the time, spectral analysis and mass spectrometry , this amount is much too small. However, the detection of the ionizing radiation of this new substance is far more sensitive than the two analytical methods just mentioned. For working with imponderable radioactive substances, special "radiochemical" working and analysis techniques had therefore been developed since the discovery of radioactivity (1898), in which this detection strength is combined with the selectivity of chemical separations. The two following chemical separation methods are used again and again.

The separation by coprecipitation

It has been suggested that one or more of the five activities observed by Fermi are the unknown element 93. A look at the periodic table of the chemical elements in its former form shows element 93 below rhenium, so it should be homologous to it and the elements manganese and masurium (today technetium ) above it and its chemical compounds should be similar to that of rhenium , Masurium and Manganese behave. Masurium could not be used for comparison, however, as no compounds of this element were known. For a similarity analysis, only rhenium and manganese remained.

A chemical reagent was now sought with which the rhenium could be precipitated from an aqueous solution of rhenium and uranium, but the uranium remained in solution. One such reagent is hydrogen sulfide , it forms sulfides with the elements just mentioned , but with the following different properties.

Uranium sulfide :	soluble in dilute acids
Manganese sulfide :	soluble in dilute acids
Rhenium sulfide :	insoluble even in strong (undiluted) hydrochloric acid

From the different solubility of manganese and rhenium sulfide, one can conclude that a sulfide of element 93 is at least as insoluble in hydrochloric acid as rhenium sulfide.

So the irradiated uranium is dissolved in strong hydrochloric acid, a small amount (a few milligrams) of a soluble rhenium salt is added and gaseous hydrogen sulfide is introduced. Insoluble black rhenium sulfide Re ₂ S _{7 forms} , and the sulfide of the newly formed element should be carried along if element 93 is involved.

This process is known as co-precipitation. The extremely small amounts of substance of a chemically related element are also incorporated into the crystal lattice of the rhenium sulfide in the form of their sulfide and are also precipitated. The precipitate is filtered off and in this way the isotopes of the expected element 93 produced by the irradiation have been separated from the uranium.

If it is expected that the uranium reaction products also include elements 94 to 96, then coprecipitation as sulfide is also recommended, but this time in medium-strength hydrochloric acid and with a z. B. platinum salt as a carrier substance; elements 93 to 96 would also be dropped here.

The separation of homologous substance mixtures through fractional crystallization

If the active substance is to be separated from the inactive carrier, the property of chemical similarity of the compounds, which is desired in the co-precipitation, turns out to be a great obstacle in the separation. The process of fractional crystallization (or fractional precipitation), which was developed by Carl Auer von Welsbach around 1885 in order to separate the chemically very similar elements of the lanthanides (rare earths; elements 57 to 71), helps here .

This method was tested for radiochemistry by Marie Curie in the years 1899–1902 when the radium was purified from the processing residues of the pitchblende.

She was faced with the task of separating the radium-containing barium chloride (approx. 8 kg BaCl ₂ per ton of processing residues) already isolated from the residues in weighable quantities from the barium in order to be able to examine it by spectral analysis and determine its atomic mass .

To do this, she dissolved the barium chloride (in quantities of around 1 kg) in hot, distilled water and boiled the solution until the first crystals appeared. On cooling, part of the barium chloride then crystallized out, and beautiful, firmly adhering crystals (fraction A; top fraction) formed at the bottom of the dish, from which the supernatant mother liquor could easily be poured off after cooling. The mother liquor was then again evaporated to saturation in a second (smaller) dish; after cooling and “ decanting ” (pouring off the mother liquor), it received crystal fraction B (tail fraction). When comparing the activity of both crystal fractions, M. Curie found that fraction A was about five times more radioactive than fraction B. The reason for this is the lower water solubility of radium chloride compared to barium chloride Solution was present) "enriched" in the first crystal fraction of the barium chloride during coprecipitation.

Marie Curie had to repeat this process (dissolving, evaporating, crystallizing out, decanting) countless times and again and again with new amounts of barium chloride containing radium in order to finally obtain a few milligrams of barium-free radium. A more detailed description of this very laborious and extremely tedious work can be found in the article on the discovery of radioactivity . In connection with nuclear fission, however, the following information from M. Curie is of interest:

If, instead of water, dilute or even strong hydrochloric acid is used to dissolve the barium-radium-chloride, the solubility of both chlorides is reduced and the separating effect between the two components is also considerably increased; the accumulation of radium in the top fraction is therefore considerably greater than in an aqueous solution. The accumulation of radium in the top fraction is even greater if the isolation of the radium-containing barium from the pitchblende residues is not carried out with barium and radium chloride, but in the form of their bromides (i.e. with barium bromide + radium bromide ).

Measuring the activity with the counter tube

The precipitated inactive carrier substance, in whose crystal lattice the almost weightless amount of the radioactive reaction product is enclosed, was filtered off and dried, the filter glued into a lead boat and inserted into the lead block of the counter tube. The β ^- radiation of the preparation was then measured at certain time intervals. The duration of a counter tube measurement could be chosen freely; naturally, the weaker the activity of the sample, the longer it had to be. The number of counting pulses per minute was then calculated from the measured value.

Before or after each measurement of a radioactive preparation, the background effect of the counter tube had to be measured and its value subtracted from the activity of the preparation. The background is the number of counting pulses per minute that are registered by the counter tube when there is no radioactive sample in the lead block. The cause of this blank value of around 10-20 pulses / minute is essentially cosmic radiation .

If the activity of the sample determined in each case is plotted graphically as a function of time, the half-life of the emitter can be determined from the course of the curve. If the precipitation contains several radioactive isotopes, the half-life can also be determined here for each isotope if these values ​​are not too close to one another.

A transuran among the reaction products

Until the summer of 1938, work at the KWI in Berlin was unaffected by the political events in Germany. The nuclear physicist Meitner, the radiochemist Hahn and the analyst Straßmann (the man for the separation experiments) complemented each other perfectly. The team believed they had found out that the successfully separated substances were three beta-emitting uranium isotopes and seven secondary products that had formed during or after the neutron irradiation of uranium.

uranium		Element 93		Element 94		Element 95		Element 96		Element 97
10 s	→	2.2 min	→	59 min	→	66 h	→	2.5 h	→	?
40 s	→	16 min	→	5.7 h	→	60 d	→	?
23 min	→	?

All of these radioactive bodies can be created by both rapid and, with much better yield, thermal neutrons.

However, no secondary products could be found for the substance listed in third place (with a half-life of 23 min), so it was assumed that its first secondary product, i.e. element 93, is a very long-lived isotope that did not occur during the observation period noticeably decays with radiation emission. As was later demonstrated by McMillan, in this one case it was actually not a fission product of ²³⁵ U, but a reaction of the neutron with ²³⁸ U according to the originally assumed reaction:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} +{}_{0}^{1}\mathrm {n} \ \rightarrow \ {}_{\ 92}^{239}\mathrm {U} \ {\xrightarrow[{\text{ 23 m }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 93}^{239}\mathrm {EL93} }$

A Transuran was really created.

To separate the uranium isotopes, elements 93 to 96 were first removed by precipitation on platinum sulfide and then the uranium used for irradiation was precipitated as sodium-magnesium-uranyl acetate in the filtrate. If these two precipitations took place very quickly, the 40s body could still be measured in the uranium precipitate.

The knowledge gained about the chemical and nuclear physical properties of the transuranium elements was summarized in two publications from 1937. The chemical similarity with the homologous elements rhenium, osmium, iridium and platinum and thus also the classification in the periodic system as transuranic elements seemed certain.

The nuclear physics findings support this interpretation, even if it is difficult to understand how three different product lines can be created from ²³⁸ U. However, the Berlin team did not think of naming the new chemical elements.

In July 1938 Hahn, Meitner and Straßmann published their last joint publication.

As an Austrian, Lise Meitner was initially unaffected by Hitler's race laws, which had been enacted since 1933 . However, with the annexation of Austria to Germany in March 1938, this situation changed, so she emigrated to Stockholm to the Nobel Institute for Physics in mid-July 1938. However, she was in lively correspondence with Otto Hahn; she was informed about all further work at the KWI.

The neutron irradiation of uranium in Paris 1937–1938: The 3.5 hour activity

Irène Joliot-Curie and Paul Savitch in Paris found a β ^- emitter with a half-life of 3.5 hours while irradiating uranium with neutrons in 1937 , but its chemical identification proved extremely difficult.

Since the "3.5 h-body" could be precipitated together with lanthanum , an element from the 3rd group of the periodic system , Joliot-Curie and Savitch initially assumed that it was with due to the assumptions made at the time about chemical relationships the substance in question must be an isotope of the next higher element of this group, i.e. actinium . Only a little later, however, they realized that the “3.5 h body” has very similar properties to lanthanum, but that actinium and lanthanum can be separated from actinium and lanthanum through fractional precipitation, so it cannot be actinium or lanthanum.

When their results were published in July 1938, Joliot-Curie and Savitch finally interpreted their results in such a way that the "3.5 h body" must be a transuranic element. Regardless of its very close resemblance to lanthanum, they could imagine "that this body has the atomic number 93 and that the transuranic elements found by Hahn, Meitner and Straßmann so far are the elements 94 to 97."

The chemical proof of a nuclear fission of uranium

Berlin, autumn 1938: The “radium” isotopes

The publication of Joliot-Curie and Savitch in Berlin became known in mid-October 1938. These were new points of view and at the same time a challenge for Hahn and Straßmann. The Berlin team had always started the separation of the reaction products with a joint precipitation on platinum sulfide. The "3.5 h body", as Joliot-Curie and Savitch reported, is not included. Their separation process begins with a precipitation of poorly soluble potassium lanthanum sulfate.

Hahn and Straßmann start from the following considerations when examining these results. Actinium, which was postulated by Paris at least for a certain period of time, could arise from uranium in the following steps:

The neutron hitting the uranium nucleus knocks an α-particle out of the nucleus in an (n, α) process and a thorium isotope is created:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} +{}_{0}^{1}\mathrm {n} \rightarrow {}_{\ 90}^{235}\mathrm {Th} +{}_{2}^{4}\mathrm {He} }$

Radium is formed from the ²³⁵ Th through α-decay

${\displaystyle {}_{\ 90}^{235}\mathrm {Th} \rightarrow {}_{2}^{4}\mathrm {He} +{}_{\ 88}^{231}\mathrm {Ra} }$   or in abbreviated form   ${\displaystyle {}_{\ 90}^{235}\mathrm {Th} \ {\xrightarrow {\alpha }}\ {}_{\ 88}^{231}\mathrm {Ra} }$

and from this actinium by β ^- decay:

${\displaystyle {}_{\ 88}^{231}\mathrm {Ra} \ {\xrightarrow[{\text{ }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 89}^{231}\mathrm {Ac} }$

If the actinium thus formed is also precipitated in potassium lanthanum sulfate, the radium-231 formed as an intermediate can then be carried away as poorly soluble radium sulfate and thus accompany the actinium into the precipitate. In the subsequent fractionation, radium would get into the top fraction and thus falsify the result.

Hahn and Straßmann test this hypothesis by precipitating barium chloride from the irradiated sample. This precipitation is actually radioactive, so radium must have also been precipitated. Because of the different half-lives measured, the precipitation even contains at least three beta-emitting radium isotopes, which in turn simulate three beta-emitting actinium isotopes and these in turn three thorium isotopes which are not further characterized.

In a communication published in early November 1938, they put it this way:

${\displaystyle \mathrm {Ra} _{1}^{}\ {\xrightarrow[{\mathrm {\sim } \ {\text{25 Min.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac} _{1}^{}\ {\xrightarrow[{\mathrm {\sim } \ {\text{40 Min.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

${\displaystyle \mathrm {Ra} _{2}^{}\ {\xrightarrow[{\mathrm {\sim } \ {\text{110 Min.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac} _{2}^{}\ {\xrightarrow[{\mathrm {\sim } \ {\text{4 Stdn.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

${\displaystyle \mathrm {Ra} _{3}^{}\ {\xrightarrow[{\text{mehrere Tage}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac} _{3}^{}\ {\xrightarrow[{\mathrm {\sim } \ {\text{60 Std.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

However, this finding is problematic in several respects. Three radium-actinium pairs are found which differ in their half-lives. They must all have the same mass number 231 because the atomic nucleus ²³⁸ U hit by a neutron can only have been converted into radium according to the following scheme, which has already been described in its individual steps:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} \ (n,\alpha )\ {}_{\ 90}^{235}\mathrm {Th} \ {\xrightarrow {\alpha }}\ {}_{\ 88}^{231}\mathrm {Ra} }$

Of this ²³¹ Ra there are obviously the three "versions" Ra ₁ , Ra ₂ and Ra ₃ with different half-lives!

Cases were well known in which there are two versions of an atomic nucleus with different half-lives; this phenomenon is called nuclear isomerism and is attributed to different energy states of the same nucleus. A threefold nuclear isomerism, which is also transferred to the secondary products actinium-231 during radioactive decay, was, however, completely new and difficult to explain.

But the postulated (n, α) -process also causes problems because it would have to take place with slow neutrons, but energy-rich (fast) neutrons would actually be required to knock an α-particle out of the uranium nucleus that has been hit. On the other hand, the same phenomenon had already been observed with the irradiation of thorium, where three radium isotopes with different half-lives (<1 minute, 15 minutes, 4 hours) were assigned to the ²²⁹ Ra:

${\displaystyle {}_{\ 90}^{232}\mathrm {Th} +{}_{0}^{1}\mathrm {n} \rightarrow {}_{\ 88}^{229}\mathrm {Ra} \ +{}_{2}^{4}\mathrm {He} }$

Otto Hahn reported on this state of knowledge in mid-November 1938 in Copenhagen at the Bohr Institute. There he also meets Lise Meitner and Otto Frisch . The nuclear physicists Meitner, Bohr and Frisch are skeptical about the nuclear isomerism, but have no satisfactory explanation for the findings. Further experiments are therefore necessary; it is about the question: "Are the radium isotopes denoted by Ra ₁ , Ra ₂ and Ra ₃ really radium?"

In order to decide this, one has to separate or at least enrich the radioactive Ra isotopes from the barium, which are only present in imponderable quantities in the barium-radium fraction . This is made possible by the fractional crystallization of barium chloride or barium bromide already described.

Before it comes to that, however, a fourth, very short-lived radium isotope ²³¹ Ra is found, so Hahn and Straßmann change the sample names of the previous three decay series. The half-lives are now also more precisely determined, so the following scheme results:

${\displaystyle \mathrm {Ra\ I?} _{}^{}\ {\xrightarrow[{\mathrm {<} \ {\text{1 Min.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac\ I\ } {\xrightarrow[{\mathrm {<} \ {\text{30 Min.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

${\displaystyle \mathrm {Ra\ II} _{}^{}\ {\xrightarrow[{{\text{14}}\ \mathrm {\pm } \ {2Min.}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac\ II\ } {\xrightarrow[{\mathrm {\sim } \ {\text{2,5 Std.}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

${\displaystyle \mathrm {Ra\ III} _{}^{}\ {\xrightarrow[{{\text{86}}\ \mathrm {\pm } \ {6Min.}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac\ III\ } {\xrightarrow[{\mathrm {\sim } \ {\text{mehrere Tage?}}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

${\displaystyle \mathrm {Ra\ IV} _{}^{}\ {\xrightarrow[{\text{250-300 Std.}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Ac\ IV\ } {\xrightarrow[{\text{40 Std.}}]{{\text{ }}\mathrm {\beta } }}\mathrm {\ Th?} }$

Separation attempts

It is only thanks to the fact that most of Fritz Straßmann's laboratory journals have survived that details about the conduct of the experiments and the measurement results of the KWI in the last weeks of 1938 are still generally known and accessible today.

The originals are in the estate of Fritz Straßmann in the main state archive of Koblenz , the protocol booklet “Chem. II ”(No. 192, November 1938 to February 1939), however, in the Deutsches Museum. Facsimiles of individual protocol pages can be found in the book by F. Krafft mentioned in the section “Other literature”, Volume 95 of the Landesarchivverwaltung Rheinland-Pfalz and the two essays by G. Herrmann.

The details given in the following experiments (sample quantities, irradiation time, measurement date, measurement results, etc.) are taken from these laboratory journals.

Fractionation with barium chloride

The first radium-barium fractionation is carried out on November 25, 1938. It is described in more detail as an example:

11 g of ammonium diuranate had been irradiated with decelerated (thermal) neutrons for 16 hours; the irradiation was ended at 11:36 a.m. The sample is then dissolved in hydrochloric acid, and after the addition of 1.5 g of barium chloride (as a carrier substance) the barium is precipitated with strong hydrochloric acid; the radium isotopes formed by the irradiation are also precipitated. The precipitate is filtered off and separated from the uranium solution. The mixture of barium and radium chloride is then dissolved in hot water and strong hydrochloric acid is added dropwise until the solution becomes cloudy. When the solution is cooled, some of the barium chloride crystallizes out and is filtered off; by cooling the filtrate further, two further crystal fractions are obtained. After the first crystal fraction has been separated off, the counter tube measurements can then be started. In the time from 12:05 p.m. to 1:22 p.m., the counting rate in each case of equal amounts of the three fractions (350 mg BaCl ₂ ) is measured alternately with the same counter tube. The activity of the samples decreases continuously during this period (for the first fraction from 387 to 132 pulses / minute), this is due to the radioactive decay of RaII and RaIII. A sudden decrease in activity between the first and second or the second and third crystal fractions cannot be seen, however. An accumulation of the radium isotopes in the top fraction has not taken place.

The fractionations with barium bromide

The next attempt is made just three days later. This time the uranium sample has been irradiated for 45 hours, the co-precipitation of the reaction products takes place again on barium chloride, but the subsequent fractionation with barium bromide. For this purpose, the mixture of barium and radium chloride is precipitated in the form of their carbonates and the precipitate is dissolved in hydrobromic acid HBr. The solution is fractionally crystallized by adding further HBr drop by drop. This time, four crystal fractions of barium bromide are the result.

You wait about an hour before starting the measurements, then only RaIII is available. Three of these four fractions are measured alternately. Here, too, the activity is the same for all samples at the same point in time; the half-life of the RaIII decreases accordingly.

In a third attempt, on December 8th, with long irradiated uranium, fractionation was carried out with the long-lived RaIV, again with bromide. Here, too, the same result, all parliamentary groups have the same activity, there is also no enrichment of the RaIV.

Control attempts

In the search for a plausible explanation for this abnormal behavior of radium, Hahn and Straßmann consider it possible that the radium isotopes are not accumulated in the top fraction of the barium salts because their concentration in the carrier substances is too low; in any case much less than in the classical work on the isolation of radium.

As a countercheck, they therefore add different amounts of naturally radioactive radium isotopes to barium chloride ( ²²⁴ Ra or ²²⁸ Ra; see indicator tests ) and fractionated. As expected, these (real) radium isotopes accumulated in the top fraction of the barium, even if they were only added to the barium in very low concentrations.

Indicator trials

The experiments described so far have consistently shown that in the fractional crystallization of the radium isotopes RaII, RaIII and RaIV with barium chloride or barium bromide, the enrichment to be expected for radium in the top fraction does not occur. The natural radium isotopes ²²⁴ Ra and ²²⁸ Ra, on the other hand, behaved as usual in separate test series.

Now it made sense to combine the two series of experiments: to mix the “artificial” radium produced from uranium through neutron irradiation with natural radium and to fractionate it together. The natural radium is then the indicator for the chemical behavior of this element under the selected conditions; an artificial radium must behave in exactly the same way.

Hahn and Straßmann regard such an attempt as an experimentum crucis . They use the radium isotope Mesothorium 1 (MsTh1; ²²⁸ Ra), a β ^- emitter, which was discovered by Hahn in 1908 , as an indicator . It is the daughter substance of natural thorium, the initial member of the thorium decay series :

${\displaystyle {}_{\ 90}^{232}\mathrm {Th} \ {\xrightarrow[{{1,4}\ \cdot \ {10^{1}}{^{0}}{\ a}}]{{\text{ }}\mathrm {\alpha } }}{}_{\ 88}^{228}\mathrm {Ra\ (MsTh1)} \ {\xrightarrow[{6,7\ a}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 89}^{228}\mathrm {Ac\ (MsTh2)} \ {\xrightarrow[{6,13\ h}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 90}^{228}\mathrm {Th} \ {\xrightarrow[{1,9\ a}]{{\text{ }}\mathrm {\alpha } }}\ }$ ……

The actinium isotope Mesothorium2 (MsTh2; ²²⁸ Ac) emerges from ²²⁸ Ra , also an early discovery by Hahn. It serves as an indicator for a similar experiment with the artificial actinium (see section AcIV - MsTh2 ).

RaIII - MsTh1

The attempt to test RaIII is carried out on December 17, 1938. After a uranium sample has been irradiated overnight, it stands for 2.5 hours to allow the short-lived RaI and RaII to decay. The secondary products AcI and AcII are then removed and purified MsTh1 is added as an indicator. 3 grams of barium bromide are added to co-precipitate the RaIII and the MsTh1.

The fractionation results in three fractions each with 500 mg BaBr ₂ . In order to decide how the two radium isotopes behave, the activity of the fractions must be measured over several days, since there are three contributions to this:

RaIII,	which disintegrates with a half-life of 86 minutes,
MsTh1	( 228 Ra), the activity of which remains constant during the duration of the experiment (HWZ: 6.7 a),
MsTh2	( 228 Ac), which remains in the mother liquor during fractionation, but is reproduced from MsTh1 during the measurement period (HWT: 6.13 h).

The result can only be determined on the basis of a graphic evaluation. The curves show a steep drop in activity in the first few hours, which is caused by the decay of RaIII. This decrease is slowed down and finally converted into an increase in activity by the MsTh2, which is reproduced from MsTh1 and which has reached its final value after approx. 60 hours and is then in radioactive equilibrium with its long-lived parent substance MsTh1. This end value is 67.6 / 25/11 pulses / minute for the three fractions. It states that the MsTh1 is present in the barium bromide fractions 1 to 3 in concentrations which behave as 6.1 : 2.3 : 1; the natural isotope of radium has thus, as expected, been heavily enriched in the top fraction.

The proportion of activity of RaIII in each of the three fractions is obtained if the proportions assigned to MsTh1 and the simulated MsTh2 are subtracted from the activity measured in each case. If you enter these adjusted measured values ​​as a function of time in a diagram with a semi-logarithmic scale, the measuring points for each fraction lie on a straight line, the inclination of which corresponds to the half-life of RaIII; If this straight line is extrapolated to the point in time zero, activities of 81, 72 and 81 pulses / minute result for the three fractions. The concentration of RaIII in the three barium bromide fractions is therefore constant within the error limits of the measurements (± 10% in view of the low counting rates), and there was no accumulation. In this experiment, RaIII does not behave like the added radium indicator MsTh1, but like natural barium.

AcIV - MsTh2

If the radium isotopes are not radium but barium, then the above-mentioned series of decays with the supposed radium isotopes (cf. Berlin, autumn 1938: The "radium" isotopes )

Ra → Ac → Th →?

the new range

Ba → La → Ce →?

So the Ac isotope is not actinium, but should be lanthanum. Hahn and Straßmann also check this conclusion in a further indicator experiment two days later. In a uranium sample that has been irradiated for several days, they subject the AcIV (half-life: 40 hours), which is reproduced from RaIV, to fractional crystallization using lanthanum oxalate. The oxalates of lanthanum and actinium are sparingly soluble in dilute acids, but actinium oxalate is not as soluble as lanthanum oxalate; in a fractional crystallization actinium oxalate would have to accumulate in the last fraction, the tail fraction.

The naturally radioactive Ac isotope MsTh2 (half-life: 6.1 hours) is added as an indicator. As expected, MsTh2 accumulates in the tail fraction, but the AcIV is evenly distributed over all four fractions; so it is not actinium, it behaves like lanthanum.

Berlin, December 1938: The chemical proof of barium

Experimental set-up with which Otto Hahn and Fritz Straßmann discovered nuclear fission on December 17, 1938 in Berlin ( Deutsches Museum , Munich )

All these separation, control and indicator experiments show that the bodies RaII, RaIII and RaIV, which were previously thought to be radium isotopes, cannot be isotopes of the chemical element radium. Chemically, they behave in exactly the same way as the inactive carrier element barium. Hahn and Straßmann have to draw the only possible conclusion from this: The supposed radium isotopes are not radium, they are radioactive isotopes of the chemical element barium.

Hahn and Straßmann summarized the results of these investigations in a report that was submitted to the editorial staff of the journal Die Naturwissenschaften on December 22, 1938 and was published on January 6, 1939. The authors are “reluctant to announce their strange results”. As chemists, you do not doubt that the supposed radium isotopes are radioactive barium isotopes. As nuclear chemists who are “in a certain way close to physics”, they are reluctant to give a physical interpretation of the process, as it contradicts all previous experience in nuclear physics. In a letter to Lise Meitner of December 19, 1938, Hahn becomes clearer when he writes: “ Perhaps you can suggest some fantastic explanation. We know ourselves that it (note: the uranium nucleus) cannot actually burst in a battery. "

Indicator experiment with radium isotope ThX

In a further paper that was submitted at the end of January 1939, Hahn and Straßmann then publish the measurement curves of their indicator experiment with RaIII-MsTh1 and barium bromide, and they report on the results of further indicator experiments with RaIII and RaIV (instead of RaIII and RaIV they now use BaIII and write BaIV). The indicator is the isotope ²²⁴ Ra (Thorium X), an α-emitter, which is detected via a β ^- -radiating secondary product. The fractionation of the radium-barium mixture takes place by precipitation of the poorly soluble chromates , which are particularly suitable for this purpose. The evaluation of the measurement curves again brings the same result: the “real” radium isotope (ThX) is very strongly enriched in the top fraction of BaCrO ₄ , the supposed radium (BaIII or BaIV) is evenly distributed over the individual BaCrO ₄ fractions.

Cycle test

In a final indicator experiment, Hahn and Straßmann examine whether inactive barium and the supposed radium activities are inseparable even when the following “cycle” is carried out.

The 12d body Ra (IV) is separated from a uranium which has been irradiated for a long time after 3 g of BaCl _{2 have been added} . About 0.5 g of this now radioactive barium chloride are retained, the remaining amount is successively converted into the following chemical compounds:

Barium succinate (amber acid barium)

Barium nitrate

Barium carbonate

Barium chloride

Barium ferric mannitol (organic barium iron compound)

Barium chloride

wherein the respective compound is precipitated or crystallized out and, after being separated from the aqueous solution, is converted into the next compound.

The same quantities of both the retained barium chloride and the repeatedly converted barium chloride are then measured alternately under the same counter tube. Over a period of 14 days, both preparations always have the same activity at the same time. The mixing ratio between barium and RaIV has not changed during the cycle, Ra (IV) also behaves like barium in this experiment.

The chemical proof of further fission products

Radioactive noble gases

If the uranium nucleus with its 92 nuclear charges splits off a barium nucleus with the nuclear charge 56, the element with the nuclear charge 36 remains, i.e. an isotope of the noble gas krypton . Since a direct measurement of radioactive noble gases appears difficult, Hahn and Straßmann use indirect evidence based on the decay products of krypton, namely the radioactive elements 37 and 38 ( rubidium and strontium ). Strontium is the easiest to isolate. Like barium, it belongs to the group of alkaline earth metals , but can be easily separated from barium, since strontium chromate , unlike barium chromate, is soluble in dilute acids. A corresponding experiment confirms the assumption: Radioactive strontium is found and also its secondary product, also a β ^- emitter.

In a second experiment, they prove in a completely different way that a noble gas is present and that it breaks down into an alkali element. To do this, they irradiate an aqueous solution of uranyl nitrate and let a stream of air bubble through it during the irradiation in order to drive out noble gas fission products (krypton or xenon). The air flow then passes through a second vessel with water, which absorbs the water-soluble decay products (rubidium or cesium) that arise from the noble gas. The heavy alkali elements together with inactive cesium carriers are then precipitated from this solution.

In the publication it remains to be seen whether it is rubidium (from krypton) or cesium (from xenon). A little later it turns out that both element pairs are present.

The chemical proof of a nuclear fission of the thorium

The report not only provides “definitive evidence for the formation of barium from uranium” , but also provides evidence that barium isotopes are also formed when thorium is irradiated with neutrons.

In their previous publication Hahn and Straßmann had mentioned irradiation experiments on thorium, which they had carried out together with Lise Meitner in 1938 and in which "radium" isotopes had also been found, which from the current perspective could also be radioactive barium isotopes. The renewed test that has now taken place (with thorium X as an indicator for radium and a fractionation in the form of chromates) confirms this assumption.

The physical interpretation of nuclear fission

Even before the first manuscript was finished, on December 19, 1938, Otto Hahn had informed Lise Meitner in a letter of these results and asked her whether she had any nuclear physics explanation for the observed behavior of uranium ( “bursting in barium?” ). Without waiting for her answer, he wrote to her two days later: "We cannot keep silent about our results, even if they are physically absurd." She replied to his first letter on December 21, 1938:

"It seems to me that the assumption of such an extensive bursting seems very difficult, but we have experienced so many surprises in nuclear physics that one cannot simply say: it is impossible."

Theoretical considerations

Lise Meitner spends Christmas with Swedish friends in the small town of Kungälv near Gothenburg and has asked her nephew Otto Frisch to visit her there. Frisch is a physicist with Niels Bohr at the Institute for Theoretical Physics in Copenhagen. In his own words, it will be the most meaningful visit of his life, because within a few days they will find the solution to the problem together.

Until now, fragments larger than protons or helium nuclei (alpha particles) have never been split off from an atomic nucleus; according to well-founded theoretical concepts in nuclear physics, the emission of even larger particles was extremely unlikely. However, the latest analysis results from Berlin contradict this view.

Meitner and Frisch therefore fall back on a model in which atomic nuclei are compared with electrically charged water droplets. In this concept, the large uranium nucleus is a rather unstable structure, which - according to you - can vibrate so violently when a neutron is caught that it splits into two smaller nuclei of roughly the same size. These two atomic nuclei repel each other strongly because of their positive nuclear charges. They estimate the energy released as a result to be around 200 million electron volts (= 200 MeV); that is far more than any other process of any kind could provide.

Meitner and Frisch refer to this previously unknown process as the “fission process” (based on the biological process of cell division), in German it becomes the terms “splitting process” and “nuclear fission”. Frisch vividly described these considerations and conclusions, which he and his aunt came to while walking in the snow, in his book “What I remember” (see section “Other literature”).

Meitner and Frisch report on their findings in a short note that was submitted to the journal "Nature" on January 16, 1939 and published on February 11, 1939. Otto Hahn received a copy of the manuscript on January 24, 1939.

Frisch is going back to Copenhagen at the beginning of January. When he informed Niels Bohr about the latest results from Berlin and Kungälv, he slapped his forehead with his hand and shouted: “Oh, what idiots we were all! We could have foreseen that. "

Physical confirmation of the fission

The theoretical considerations of Meitner and Frisch are followed by a publication by Otto Frisch, in which he experimentally confirms the concept of a split into two high-energy fragments developed in Ref.

Frisch is looking for the postulated fragments of high speed with an atomic number of about 40 to 50 and an atomic weight of about 100 to 150.Despite their very high energy of up to 100 MeV, these atomic nuclei should only have a range of a few millimeters because they high charge ionize the surrounding air very strongly and thus be slowed down quickly. Such processes can be measured with an ionization chamber .

Frisch uses a very small ionization chamber, only 3 cm in diameter, into which he inserts a uranium preparation on a copper sheet. Since the range of the fragments depends on the gas filling in the chamber (the lower the atomic weight of the filling gas, the greater it is), he fills them with hydrogen.

The Ra-Be neutron source (with 300 mg radium) is placed under the ionization chamber in such a way that a hollow paraffin cylinder can be pushed over the neutron source (to slow down the neutrons). By means of a suitable electrical circuit with a thyratron , Frisch ensures that the connected counter only registers strongly ionizing particles (with an energy greater than 20 MeV); however, all weaker current impulses, especially the uranium alpha particles (4.2 MeV), are suppressed.

When irradiated with unbraked neutrons, the measuring apparatus registers about 15 fission products per minute; if the neutrons are slowed down by the paraffin layer, twice as many of these high-energy fragments are measured. The results are similar when irradiating thorium, but slow neutrons do not intensify the effect. With this experiment (carried out on January 13 and 14, 1939), Frisch succeeded in physically demonstrating the fission of uranium and thorium and confirming the postulated, unusually high release of energy.

Supplementary observations on nuclear fission

The publication of the reports in January and February 1939 triggered a veritable flood of investigations worldwide. At the end of February more than 40 papers were published in which the nuclear fission of uranium by neutrons is also demonstrated.

The chain reaction in the uranium isotope 235

Niels Bohr developed the concept proposed by Meitner and Frisch further and came to the conclusion that the uranium is not split at the main isotope ²³⁸ U, but at the rare uranium isotope ²³⁵ U, which is only 0.7% present in natural uranium. A working group led by Frédéric Joliot finds that two to three neutrons are released per fission during uranium fission. This creates the possibility of a chain reaction in which these neutrons split further uranium nuclei.

In a detailed paper, Siegfried Flügge examines whether and how the energy content of the atomic nuclei in such a "uranium machine" can be made technically usable.

Fission product pairs

In their report Hahn and Straßmann also made some assumptions about the mass number of the barium isotopes; thereafter BaIII should have the atomic weight 139 and BaIV the atomic weight 140. Both are isotopes that are not found in naturally occurring barium.

If one now assumes that during the fission of uranium, together with each of the two barium isotopes mentioned above, both two and three free neutrons can occur, the following fission processes result for these two fragments alone:

${\displaystyle {}_{\ 92}^{235}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{236}\mathrm {U} \ \rightarrow \ {}_{\ 56}^{139}\mathrm {Ba} \ +\ {}_{36}^{95}\mathrm {Kr} \ +\ 2\ {}_{0}^{1}\mathrm {n} }$

${\displaystyle {}_{\ 92}^{235}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{236}\mathrm {U} \ \rightarrow \ {}_{\ 56}^{139}\mathrm {Ba} \ +\ {}_{36}^{94}\mathrm {Kr} \ +\ 3\ {}_{0}^{1}\mathrm {n} }$

${\displaystyle {}_{\ 92}^{235}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{236}\mathrm {U} \ \rightarrow \ {}_{\ 56}^{140}\mathrm {Ba} \ +\ {}_{36}^{94}\mathrm {Kr} \ +\ 2\ {}_{0}^{1}\mathrm {n} }$

${\displaystyle {}_{\ 92}^{235}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{236}\mathrm {U} \ \rightarrow \ {}_{\ 56}^{140}\mathrm {Ba} \ +\ {}_{36}^{93}\mathrm {Kr} \ +\ 3\ {}_{0}^{1}\mathrm {n} }$

Hahn and Straßmann already indicated in their report that the fission is not limited to the combination of barium + krypton; a few weeks later they can then also detect the element combination Xe + Sr as another fission product pair, and finally also Cs + Rb.

If one looks at the above-mentioned and further gap pairs in the ordered sequence

${\displaystyle {}_{53}^{}\mathrm {I} \ \ \ +\ {}_{39}^{}\mathrm {Y} }$

${\displaystyle {}_{54}^{}\mathrm {Xe} \ +\ {}_{38}^{}\mathrm {Sr} }$

${\displaystyle {}_{55}^{}\mathrm {Cs} \ +\ {}_{37}^{}\mathrm {Rb} }$

${\displaystyle {}_{56}^{}\mathrm {Ba} \ +\ {}_{36}^{}\mathrm {Kr} }$

${\displaystyle {}_{57}^{}\mathrm {La} \ +\ {}_{35}^{}\mathrm {Br} }$

so you can see that both the first-named partners (I …… La) and the second (Y …… Br) are each successive elements in the periodic system, with both groups clearly differing in their number of masses. The uranium splitting takes place asymmetrically; As it turns out in the following years, the heavy partners are grouped around the mass number 138, the light ones around the mass 95. If the rarer products are included, around forty elements are represented in the fission product mixture, from copper to the heavy lanthanides several isotopes of each of these elements.

Fission product decay chains

The primary fission products, which arise immediately after the neutron capture of a uranium nucleus as a result of its fission, are transformed by β ^- decay and form chains of decay until a stable nucleus is reached. From a primary cleavage product lanthanum z. B. creates the chain of decay

La → Ce → Pr → Nd (stable)

another chain from a primary barium

Ba → La → Ce → Pr (stable)

Reaction products of the irradiation thus arise both primarily through cleavage and secondarily through decay.

In the experiments in Rome, Paris and Berlin, all of these fission products were present in the neutron-irradiated uranium, but those involved did not specifically look for them!

The 3.5-hour body of Joliot-Curie and Savitch is identified in retrospect as an isotope mixture that mainly consists of the fission product ⁹² Y (half-life: 3.5 hours), an isotope of the chemical element yttrium that is present in the same Group of the periodic table stands like lanthanum.

The equivalence of mass and energy

Already in 1905 Albert Einstein concluded from his special theory of relativity that mass and energy can be converted into one another ( c = speed of light):

${\displaystyle E=\Delta m\cdot c^{2}}$

This equation applies to every chemical or physical process in which energy E (mostly in the form of heat) occurs or is consumed; However, the mass changes Δm occurring here are so small in normal energy processes (combustion) that they cannot be measured experimentally.

Energy and mass change in nuclear fission

Atomic masses of the reaction partners involved in a uranium fission according to the "Table of the Nuclides", Rutherford Laboratory:

isotope	Mass of the isotope ( amu )
neutron	1.008664
235 U	235.043922
139 Ba	138.908835
140 Ba	139.910599
93 kr	92.931265
94 kr	93.934362
95 kr	94.939840

The reason for the tremendous release of energy during nuclear fission is a conversion of mass into energy, which occurs due to the properties of the atomic nucleus: The mass of an atomic nucleus is always less than the sum of the masses of its components (protons and neutrons = nucleons ). This mass defect is the “binding energy” of the atomic nucleus; it would be released if it were assembled from its protons and neutrons. The binding energy increases with the number of nucleons in the nucleus, but not evenly. Instead, it is calculated per nucleon for medium-weight atomic nuclei such as e.g. B. the fission products significantly larger than for the atomic nuclei of uranium. This excess of binding energy is released during the fission process when two fission product nuclei are formed from one uranium.

From the exact values ​​known today for the atomic masses of the reactants, the change in mass that occurs during nuclear fission can easily be determined and the energy released can then be calculated from this.

From these values ​​results for the nuclear reaction

²³⁵ U + n → ¹³⁹ Ba + ⁹⁴ Kr + 3 n

a mass change of 0.183 amu; the fission products on the right-hand side of the equation are lighter than the uranium isotope 235 including the neutron. At the similar nuclear reaction

²³⁵ U + n → ¹⁴⁰ Ba + ⁹⁴ Kr + 2 n

the mass defect is even larger at 0.190  amu . From the original mass of about 236 amu, 0.183 amu and 0.19 amu have disappeared; this mass fraction of about 0.08 percent has been released as energy, the size of which can be calculated using the equation given by Einstein. Since the mass of 1 amu corresponds to an energy of 931.494 MeV, 0.19 amu mass defects result in an energy gain of around 177 MeV.

Meitner and Frisch did not have the exact data for the atomic mass, so they calculated the kinetic energy of the two fragments of the uranium nucleus (which repel each other because of their positive nuclear charges) from Coulomb's law , using them for the distance between the two spherical fission products had to insert the sum of the core radii, the size of which they could only estimate. However, with the 200 MeV they calculated, they were remarkably good.

Comparison of nuclear energy and conventional energy

Energy conversion tables show that a mass loss of 1 amu  corresponds to an energy of joules. For 0.19 amu this is about  joules. Compared to the amount of heat generated in a chemical reaction, such as the burning of carbon, ${\displaystyle 1{,}492\cdot 10^{-10}}$ ${\displaystyle 0{,}3\cdot 10^{-10}}$

C + O ₂ → CO ₂ + 393,000 joules,

seems to be released very little energy in the nuclear fission. However, in the case of a nuclear reaction, the energy specification relates to a single atom, whereas in a chemical reaction, it relates to the amount of substance 1 mol, i.e.  atoms (see Avogadro's constant ). The mass loss of the nuclear fission of 1 mole of ²³⁵ U, i.e. 235 grams, corresponds to an energy of  Joule. ${\displaystyle 6{,}022\cdot 10^{23}}$ ${\displaystyle 1{,}7\cdot 10^{13}}$

Converted to the heating of water, one can bring about 1 liter of water to the boil by burning 12 grams of carbon; in the case of nuclear fission of 235 grams of ²³⁵ U, it is  liters of water (= 50,000 cubic meters). ${\displaystyle 5\cdot 10^{7}}$

The actual transuranic elements

With the discovery of nuclear fission, the interpretation of all previous experiments in Berlin had become questionable. The supposed transuranic elements now turned out to be different fission products of uranium. The search for the transurans began again.

Element 93: Neptunium

The starting point is the 23-minute body, which the KWI team found in 1937 and correctly identified as uranium isotope 239 (see section A Transuran under the reaction products ).

As a β ^- radiator, this isotope must decay in element 93; all searches for it in the co-precipitations of the sparingly soluble sulphides remained unsuccessful in Berlin.

The discovery of element 93 and the identification of precisely this isotope 239 of element 93 was achieved in 1940 in Berkeley (USA) by E. McMillan and PH Abelson. As a neutron source , they use the existing cyclotron , which was then the world's largest . With the help of a Be-D reaction (bombardment of a beryllium preparation with deuterons )

${\displaystyle {}_{4}^{9}\mathrm {Be} +{}_{1}^{2}\mathrm {D} \ \rightarrow \ {}_{\ 5}^{10}\mathrm {B} \ +{}_{0}^{1}\mathrm {n} }$

it generates a neutron stream that corresponds to that of a radium-beryllium neutron source with around 1200 kg of radium.

After intensive bombardment of a thin uranium layer, in addition to the 23-minute activity of ²³⁹ U, a further β ^- radiation with a half-life of 2.3 days can be measured on the preparation without prior chemical separation could:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} +{}_{0}^{1}\mathrm {n} \ \rightarrow \ {}_{\ 92}^{239}\mathrm {U} \ {\xrightarrow[{\text{ 23 min}}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 93}^{239}\mathrm {EL93} \ {\xrightarrow[{\text{ 2,3 d}}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 94}^{239}\mathrm {EL94} }$

This connection is clearly demonstrated chemically in the course of the investigations. Contrary to what was expected, however, element 93 does not turn out to be a homologous element to rhenium. It is not precipitated together with platinum sulfide, but behaves quite differently, namely very similar to uranium. This also makes it clear why the search for element 93 in Berlin and elsewhere failed: due to incorrect assumptions about the chemical properties of this element.

McMillan and Abelson published their discovery in mid-June 1940; it became known in Berlin at the end of August. The name Neptunium is later chosen based on the sun-distant planets Uranus, Neptune and Pluto.

Hahn and Straßmann are now trying again to find the product of the 23-minute body. However, since the β ^- radiation of ²³⁹ Np is very low in energy (around 0.4 MeV), you can only do this after building a particularly thin-walled counter tube. They are developing a chemical process to isolate this neptunium isotope from large quantities of neutron-irradiated uranium. They can confirm and significantly supplement the chemical properties described by McMillan and Abelson. They also come to the conclusion that element 93 cannot be a homologous element to rhenium, so the periodic table of the elements in the arrangement of 1938 needs to be corrected.

Element 94: plutonium

Since neptunium-239 is a β ^- emitter, it has to decay into an isotope 239 of element 94, later called plutonium . McMillan and Abelson are therefore looking for this new element in very strong preparations of neptunium-239, but without success and with the conclusion that it must be a very long-lived and therefore rarely radiating isotope.

With increasing knowledge of the fission process it becomes clear that this plutonium isotope should be fissile as easily as ²³⁵ U. The search at the Berkeley cyclotron is therefore intensified, now as a secret research.

You will find it in a detour. During the irradiation of uranium with deuterons, GT Seaborg , E. McMillan , JW Kennedy and AC Wahl observed the following nuclear reaction:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} +{}_{1}^{2}\mathrm {H} \ \rightarrow \ {}_{\ 93}^{238}\mathrm {Np} +2\ {}_{0}^{1}\mathrm {n} }$

The Np isotope 238 decays according to

${\displaystyle {}_{\ 93}^{238}\mathrm {Np} \ {\xrightarrow[{\text{ 2,1 d }}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 94}^{238}\mathrm {Pu} \ {\xrightarrow[{\text{ 88 a }}]{{\text{ }}\mathrm {\alpha } }}\ {}_{\ 92}^{234}\mathrm {U} }$

in ²³⁸ Pu, a long-lived α-emitter, which in turn simulates a uranium isotope. The chemical properties of plutonium are studied at ²³⁸ Pu. It turns out to be closely related to neptunium and uranium and is very difficult to separate from these elements. With this knowledge, however, it is then possible to find the plutonium isotope 239, an alpha emitter with a half-life of 24,000 years:

${\displaystyle {}_{\ 92}^{238}\mathrm {U} \ +\ {}_{0}^{1}\mathrm {n} \rightarrow \ {}_{\ 92}^{239}\mathrm {U} \ {\xrightarrow[{\text{23 min}}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 93}^{239}\mathrm {Np} \ {\xrightarrow[{\text{2,33 d}}]{{\text{ }}\mathrm {\beta } \mathrm {^{-}} }}\ {}_{\ 94}^{239}\mathrm {Pu} \ {\xrightarrow[{\text{24000 a}}]{{\text{ }}\mathrm {\alpha } }}\ {}_{\ 92}^{235}\mathrm {U} }$

²³⁹ Pu is, as expected, very easily fissionable by neutrons; neutrons are also emitted, so that a chain reaction is also possible with this element.

McMillan and Seaborg were jointly awarded the Nobel Prize in Chemistry in 1951 for their discoveries.

In Germany there is no search for the secondary product of ²³⁹ Np, i.e. ²³⁹ Pu. Hahn and Straßmann, who worked intensively with element 93, had to regard this as hopeless in view of their own weak neutron sources after the unsuccessful attempts by McMillan and Abelson.

Other transuranic elements

The elements 95 ( americium ) and 96 ( curium ), which were also discovered in the USA in 1944/45, do not continue the uranium-like properties, but are based strongly on actinium. Seaborg concludes from this that in the periodic table of actinium a series homologous to the lanthanoids, which begins with actinides and should extend to element 103 (lawrencium), a prediction that was confirmed in the following decades.

The series of transactinoids then begins with element 104 . At present (2006) their chemical properties up to and including element 108 have been researched. In the periodic system , the element 107 (Bohrium) is now under the rhenium, with chemical properties that were assumed for the element 93 in the 1930s.

Conclusion

Reviews of the people directly involved in the discovery of nuclear fission unanimously express:

"The discovery of nuclear fission arose, without anyone having looked for it, from years of consistent pursuit of sometimes unexpected radiochemical test results."

In nuclear physics, in the production of artificially radioactive isotopes, as well as in the decay of naturally radioactive elements, only nuclear transformations were observed until the end of 1938, in which the newly created elements were very close to the original element in the periodic system. In the race to synthesize the first transuranic elements from uranium, it was assumed in Rome, Berlin and Paris that the radioactive substances observed could be nothing more than isotopes of uranium or its closest neighbors, to the right or, if necessary, to the left of it in the periodic system . It was also assumed that the transuranic elements are chemically similar to rhenium and the platinum metals.

An assumption made by Ida Noddack as early as 1934 was therefore ignored in Rome as well as in Berlin and Paris:

"It is conceivable that when heavy nuclei are bombarded with neutrons, these nuclei disintegrate into several larger fragments that are isotopes of known elements, but not neighbors of the irradiated elements."

She had no influence on the further development.

Irène Joliot-Curie and Paul Savitch came closer to the solution in the summer of 1938, however, by postulating that they had found a transuran with completely different chemical properties during the neutron irradiation of uranium. This prompts Otto Hahn and Fritz Straßmann to search for radium isotopes and they find several. To corroborate this finding, they examine their chemical behavior in detail, with a completely unexpected result: the supposed radium isotopes are radioactive barium isotopes; the uranium nucleus splits into lighter elements after capturing a neutron.

Hahn and Straßmann published this result “reluctantly” in early 1939, but they were able to present further chemical evidence within a few weeks. At the same time, the nuclear physicists Lise Meitner and Otto Frisch, who were asked for help with a plausible explanation, found a qualitative model for the novel process, and Frisch succeeded in first confirming it through a physical experiment.

The response has been extraordinary. By December 1939, more than 100 publications dealing with the problem of nuclear fission had appeared.

Looking back, Lise Meitner wrote:

“The discovery of the fission of uranium by Otto Hahn and Fritz Straßmann opened a new era in human history. The scientific achievement on which this discovery is based seems so admirable to me because it was achieved purely chemically without any theoretical guidance. "

And in an interview with ARD on March 8, 1959, she added:

“It succeeded with an unusually good chemistry from Hahn and Straßmann, with a fantastic chemistry that really nobody else could at that time. The Americans learned it later. But at that time Hahn and Straßmann were really the only ones who could do that because they were such good chemists. You have really demonstrated a physical process with chemistry, so to speak. "

Fritz Straßmann specified in the same interview:

“Professor Meitner stated earlier that the success is due to chemistry. I have to correct them a little. Because chemistry has only managed to isolate the individual substances, but not an exact identification. To do this, Professor Hahn's method was necessary. So that's his merit. "

Nuclear fission, however, has consequences that go far beyond the sciences because with each fission process, in addition to the two fragments, some neutrons are also released, which trigger further fission processes in other uranium nuclei. This chain reaction makes it possible to use the extremely high fission energy technically. It only takes three years to implement this idea. The first functional nuclear reactor , built by Enrico Fermi at the University of Chicago, became "critical" in December 1942, that is, the chain reaction ran independently, the reactor produced energy. In July 1945 the first atomic bomb was detonated (in the desert of Alamogordo / New Mexico); the Janus head of nuclear fission thus comes to light.

Honors

1945	Otto Hahn was awarded the Nobel Prize in Chemistry in 1944 for the discovery and radiochemical proof of nuclear fission . (Presentation in December 1946).
1966	For the joint work that led to the discovery of nuclear fission, Lise Meitner , Otto Hahn and Fritz Straßmann received the Enrico Fermi Prize from the US Atomic Energy Commission (USAEC).

Individual evidence

1. ^ E. Fermi: Possible Production of Elements of Atomic Number Higher than 92 . Nature 133 (1934) pp. 898-899.
2. ^ S. Curie: Investigations on the radioactive substances . Dissertation from Marie Curie; translated by W. Kaufmann. Die Wissenschaft, first volume, pp. 23–33. Braunschweig: Vieweg (1904).
3. ↑ L. Meitner, O. Hahn, F. Straßmann: About the conversion series of uranium that are generated by neutron radiation . Zeitschrift für Physik 106 (1937) pp. 249-270.
4. ↑ O. Hahn, L. Meitner, F. Straßmann: About the transuranic elements and their chemical behavior . Reports of the German Chemical Society 70 (1937) pp. 1374-1392.
5. ↑ O. Hahn, L. Meitner, F. Straßmann: A new long-lived conversion product in the trans-uranium series . Die Naturwissenschaften 26 (1938) pp. 475–476.
6. ^ ^{A b} I. Curie, P. Savitch: Sur les radioéléments formés dans l'uranium irradié par les neutrons II . Le Journal de Physique et le Radium 9 (1938) pp. 355-359.
7. ↑ O. Hahn, F. Straßmann: About the formation of radium isotopes from uranium by irradiation with fast and slowed neutrons . Die Naturwissenschaften 26 (1938) pp. 755-756.
8. ↑ ^{a b} L. Meitner, F. Straßmann, O. Hahn: Artificial transformation processes when the thorium is irradiated with neutrons; Appearance of isomeric series by splitting off α rays. Zeitschrift für Physik 109 (1938) pp. 538-552.
9. ↑ ^{a b c d} O. Hahn, F. Straßmann: About the detection and behavior of the alkaline earth metals formed when the uranium is irradiated with neutrons . Die Naturwissenschaften 27 (1939) pp. 11–15.
10. ↑ ^{a b c d e} O. Hahn, F. Straßmann: Evidence of the formation of active barium isotopes from uranium and thorium through neutron irradiation; Evidence of further active fragments in the uranium fission . Die Naturwissenschaften 27 (1939) pp. 89-95.
11. ↑ ^{a b c d} L. Meitner, OR Frisch: Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction . Nature 143 (1939) pp. 239-240.
12. ↑ ^{a b} O. R. Frisch: Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment. Nature 143 (1939) pp. 276-277.
13. ↑ N. Bohr: Resonance in Uranium and Thorium Disintegrations and the Phenomenon of Nuclear Fission . Physical. Review 55 (1939) pp. 418-419.
14. ↑ H. von Halban, F. Joliot, L. Kowarski: Liberation of Neutrons in the Nuclear Explosion of Uranium . Nature 143: 470-471 (1939).
15. ↑ H. von Halban, F. Joliot, L. Kowarski: Number of Neutrons Liberated in the Nuclear Fission of Uranium . Nature 143: 680 (1939).
16. ↑ S. Flügge: Can the energy content of atomic nuclei be made technically usable? Die Naturwissenschaften 27 (1939) pp. 402-410.
17. ^ O. Hahn, F. Straßmann: About the fragments when the uranium bursts . Die Naturwissenschaften 27 (1939) p. 163.
18. ^ O. Hahn, F. Straßmann: Further fission products from irradiating uranium with neutrons . Die Naturwissenschaften 27 (1939) pp. 529-534.
19. ^ ^{A b} E. McMillan and PH Abelson: Radioactive Element 93 . Physical Review 57 (1940) pp. 1185-1186; doi : 10.1103 / PhysRev.57.1185.2 .
20. ↑ F. Straßmann, O. Hahn: About the insulation and some properties of the element 93 . Die Naturwissenschaften 30 (1942) pp. 256–260.
21. ^ GT Seaborg, E. McMillan, JW Kennedy, AC Wahl: Radioactive Element 94 from Deuterons on Uranium . Physical Review 69 (1946) pp. 366-367; doi : 10.1103 / PhysRev.69.367 .
22. ^ JW Kennedy, GT Seaborg, E. Segrè, AC Wahl: Properties of Element 94 . Physical Review 70 (1946) pp. 555-556; doi : 10.1103 / PhysRev.70.555 .
23. ↑ M. skull: chemistry of super heavy elements . Angewandte Chemie 118 (2006) pp. 378-414.
24. ^ O. Hahn: The discovery of the fission of uranium . FIAT Review of German Science 1939-1946, Nuclear Physics and Cosmic Rays, Part 1. Dieterich, Wiesbaden (1948) pp. 171-178.
25. ↑ L. Meitner: Paths and wrong ways to nuclear energy . Naturwissenschaftliche Rundschau 16 (1963) pp. 167-169.
26. ^ F. Straßmann: Nuclear Fission - Berlin, December 1938, private print, Mainz 1978.
27. ^ I. Noddack: About the element 93. Angewandte Chemie 47 (1934) pp. 653-655.
28. ^ LA Turner: Nuclear Fission . Reviews of Modern Physics 12 (1940) pp. 1-29.
29. ^ Lise Meitner: Memories of Otto Hahn . S. Hirzel Verlag, Stuttgart 2005. P. 74.
30. ^ Lise Meitner: Memories of Otto Hahn . S. Hirzel Verlag, Stuttgart 2005. p. 50.

Other literature

Books

• Hahn, Otto: From radiothor to uranium fission . Friedr. Vieweg & Sohn, Braunschweig 1962. New edition (Ed .: Dietrich Hahn) with a foreword by Prof. Dr. Kurt Starke. Vieweg, Braunschweig-Wiesbaden 1989; ISBN 3-528-08413-8 .
• Hahn, Otto: My life . F. Bruckmann, Munich 1968. Paperback edition (Ed: Dietrich Hahn) from Piper, Munich-Zurich 1986; ISBN 3-492-00838-0 .
• Hahn, Otto: Experiences and knowledge (Hg: Dietrich Hahn). With an introduction by Prof. Dr. Karl Erik Zimen, Econ Verlag, Düsseldorf-Vienna 1975; ISBN 3-430-13732-2 .
• Wohlfarth, Horst: 40 Years of Nuclear Fission - An Introduction to the Original Literature . Scientific Book Society, Darmstadt 1979; ISBN 3-534-08206-0 .
• Hahn, Dietrich : Otto Hahn - founder of the atomic age. A biography in pictures and documents . Preface by Reimar Lüst , foreword by Paul Matussek , introduction by Walther Gerlach . List Verlag, Munich 1979; ISBN 3-471-77841-1 .
• Frisch, Otto Robert: What I remember - physics and physicists of my time . Scientific Publishing Company, Stuttgart 1981; ISBN 3-8047-0614-2 .
• Krafft, Fritz : In the shadow of sensation - life and work of Fritz Straßmann . Verlag Chemie, Weinheim 1981; ISBN 3-527-25818-3 .
• Gerlach, Walther and Hahn, Dietrich: Otto Hahn - A researcher's life of our time . Scientific Publishing Company, Stuttgart 1984; ISBN 3-8047-0757-2 .
• Hahn, Dietrich: Otto Hahn - life and work in texts and pictures . Foreword by Carl Friedrich von Weizsäcker . Insel Taschenbuch Verlag, Frankfurt / M, 1988; ISBN 3-458-32789-4 .
• Brommer, Peter and Herrmann, Günter: Fritz Straßmann (1902–1980) - co-discoverer of nuclear fission . Inventory of the estate and comments on the attempts at nuclear fission. Publications of the Landesarchivverwaltung Rheinland-Pfalz Volume 95. Verlag der Landesarchivverwaltung Rheinland-Pfalz, Koblenz 2001; ISBN 3-931014-57-6 .
• Meitner, Lise : Memories of Otto Hahn (Ed .: Dietrich Hahn). S. Hirzel Verlag, Stuttgart 2005; ISBN 3-7776-1380-0 .

Magazines

• H. Menke, G. Herrmann: What were the transuranic elements of the thirties in reality? Radiochimica Acta 16 (1971) pp 119-123.
• W. Seelmann-Eggebert: About the discovery of nuclear fission. A historical review . Chimia 33 (1979) pp. 275-282.
• F. Krafft: An early example of interdisciplinary team work. On the discovery of nuclear fission by Hahn, Meitner and Straßmann . Physikalische Blätter 36 (1980) pp. 85-89, 113-118.
• Dietrich Hahn : Otto Hahn and Fritz Straßmann . Physikalische Blätter 37 (1981) pp. 44-46.
• P. Brix : The momentous discovery of uranium fission - and how it came about . Physikalische Blätter 45 (1989) pp. 2-10.
• G. Herrmann: Five decades ago: From the "transurans" to nuclear fission . Angew. Chemie 102 (1990) pp. 469-496.
• G. Herrmann: The Discovery of Nuclear Fission - Good Solid Chemistry Got Things on the Right Track . Radiochimica Acta 70/71 (1995) pp. 51-67.

Web links

• Otto Hahn and Fritz Straßmann's desk in the Deutsches Museum in Munich
• Chronicle of the discovery with extracts from the measurement logs