Helium-3

from Wikipedia, the free encyclopedia
Structural formula
3 He
General
Surname Helium-3
Molecular formula 3 He
Brief description

colorless and odorless gas

External identifiers / databases
CAS number 14762-55-1
EC number 238-822-9
ECHA InfoCard 100.035.278
PubChem 6857639
Wikidata Q533498
properties
Molar mass 3.0160293191 (26) g mol −1
Physical state

gaseous

boiling point

3.197 K

safety instructions
GHS labeling of hazardous substances
04 - gas bottle

Caution

H and P phrases H: 280
P: 410 + 403
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Helium-3 ( 3 He) is one of the two stable isotopes of helium , along with helium-4 . Its nucleus contains two protons and one neutron .

The main area of ​​application of helium-3 is low-temperature research: in mixture cryostats , temperatures of only a few thousandths of a Kelvin above absolute zero are achieved by using helium-3 and helium-4 . Helium-3 also plays a role in neutron detectors (see counter tube ).

Helium-3 is very rare on earth . The earth's atmosphere consists of only 5.2  ppm of helium. Of this helium, in turn, only a small proportion (0.000138% or 1.38 ppm) is 3 He. In total, this corresponds to a proportion of the total atmosphere of 7.2 · 10 −12 or 3000 to 4000 t. In natural helium sources, the ratio of 3 He / 4 He can be slightly higher or lower than in the earth's atmosphere. The reason for this is that the cosmic helium introduced during the formation of the earth originally contained 0.01% (100 ppm) helium-3, but later outgassed and was more or less diluted by the helium-4 produced during radioactive alpha decay .

The main source of helium-3 on earth is currently the decay of tritium . Tritium can be produced artificially in nuclear reactors . Helium-3 collects as a decay product in boosted nuclear weapons containing tritium and must be removed from them regularly.

History and discovery

The helion , the nucleus of the helium-3 atom, consists of two protons and, in contrast to normal helium with two neutrons, only one neutron. Helium-3 and tritium were first observed in 1934 by the Australian nuclear physicist Mark Oliphant at the University of Cambridge in the Cavendish Laboratory when he irradiated deuterium with accelerated deuterons . Be running nuclear fusion reactions from where helium-3 and tritium produced.

Later in 1939 Luis Walter Alvarez deepened his understanding of both substances through experiments on the cyclotron at the Lawrence Berkeley National Laboratory .

Based on theoretical considerations, helium-3 was expected to be a radionuclide until Alvarez could detect traces of it in samples of natural helium. Since these samples were of geological origin and millions of years old, it had to be tritium, which turns into helium-3 with a half-life of a few years, and not the other way around, as originally assumed. Alvarez was also able to determine the half-life of tritium. Protium and helium-3 are the only stable nuclides that contain more protons than neutrons.

Occurrence

Helium-3 is an original nuclide that escapes from the earth's crust into the atmosphere and from there into space over millions of years . It is believed that helium-3 is a natural cosmogenic nuclide , as it can be formed when lithium is bombarded with neutrons. The latter are released during spontaneous fission and nuclear reactions with cosmic rays . Tritium is also continuously formed in the earth's atmosphere through reactions between nitrogen and cosmic rays, which over time breaks down to helium-3. The absolute amounts are small, however; the total amount of natural tritium in the biosphere is estimated at 3.5 kg.

Helium-3 is more common in the earth's mantle (typical ratio of helium-3 to helium-4 of 1:10 4 ) than in the earth's crust and atmosphere (typical ratio of 1:10 6 ). The reason is that the earth's crust outgasses towards the atmosphere, and helium reproduced from radioactive decay is always helium-4. In areas with high volcanic activity, where mantle plumes rise from the earth's mantle, there is often a higher helium-3 concentration.

Frequency in the solar system

A concentration of helium-3 that is about a thousand times higher than terrestrial sources, but still very low, is suspected on the moon, where it was deposited by the solar wind in the upper layer of the regolith over billions of years and then neither released nor released due to a lack of volcanic and biogenic activity has been diluted.

On the gas planets , helium-3 is found in the original cosmic, correspondingly higher ratio to helium-4. It is believed that this relationship is similar to that in the solar nebula, from which the sun and planets later formed. Compared to Earth, the proportion of helium in the gas planets is very high, since their atmosphere - unlike the Earth's atmosphere - can permanently bind this gas. The mass spectrometer of the Galileo space probe made it possible to measure the ratio of helium-3 to helium-4 in Jupiter's atmosphere . The ratio is about 1:10 4 , i.e. 100 ppm. It is roughly in the range of the ratio in the regolith of the moon. However, the ratio in the earth's crust is a factor of 10 2 lower (i.e. between 2 and 20 ppm), which is mainly due to the outgassing of the original helium with the simultaneous entry of new helium-4 through alpha decay of uranium , thorium and their daughter nuclides.

Artificial production, commercial extraction

Some of the helium-3 and tritium in the earth's atmosphere are of artificial origin. In particular, tritium is produced as a by-product of nuclear fission : Sometimes during nuclear fission, a third, light nucleus is emitted in addition to the two medium-weight fission products; in 7% of these ternary decays or in 0.1% of the total decays, tritium is one of the fission products. In addition, fission neutrons activate part of the deuterium always contained in the cooling water to form tritium. If even heavy water (deuterium oxide) is used as the coolant, which enables the reactor to be operated with non-enriched natural uranium (e.g. CANDU reactor), in addition to tritium as a fission product, about 1 kg of tritium is also formed in the cooling water for every 5 gigawatt years of thermal produced Power. Some of this tritium is removed from the cooling water in a commercial plant in order to market it (around 2.5 kg per year), for example for use in fluorescent paints .

Tritium decays to helium-3 with a half-life of 12.3 years. When spent fuel assemblies are dismantled in a reprocessing plant after a decay of one to two decades , a large part of the tritium has already decayed to helium-3. This is released into the environment as a harmless gas. But also the tritium cannot be completely retained in a WAA. In addition, tritium is released in accidents with nuclear reactors and nuclear weapons tests . The tritium that got into the biosphere also decays further to helium-3 there.

In addition, considerable amounts of pure tritium are deliberately produced in national nuclear reactors for nuclear weapons through the irradiation of lithium-6 . The tritium is used together with deuterium as a fusion booster to improve the ignition behavior of nuclear weapons and to increase their energy release. Since the tritium breaks down to helium-3, it must be replaced regularly. At the same time, the helium-3 formed is removed. But helium-3 is also formed in the central tritium store of the US Department of Energy . The helium-3 obtained in this way is sold, primarily through Linde Gas . The company operates a plant to filter out the last remains of tritium from the helium gas.

Due to the declining number of active nuclear weapons, the reduction and, in some cases, the complete suspension of tritium production by the US Department of Energy and, at the same time, the increasing number of applications, there is now a shortage of helium-3. The current annual consumption of helium-3 is around 60,000 liters of gas (approx. 8 kg). The price has increased from $ 100 to $ 2,150 per liter of helium-3 gas. Possible options for the future include building up tritium production for civil purposes or increased tritium extraction from the cooling water in existing reactors, as well as the distillation of helium-3 from fresh helium that has already been liquefied for cooling purposes. The latter still has to be cooled further from the already reached temperature of 4 K to around 1 K with considerable effort, but in the countercurrent process, after distilling off, the running off helium-4 can precool the fresh helium that runs after it. It would be very expensive to selectively extract only helium-3 from natural gas if the helium-4 is not used. A helium-3 breakdown on the moon has been considered.

Physical Properties

Boiling point and critical point

Due to the relatively large mass difference of almost 25%, helium-3 and helium-4 show more clear differences in their properties than the isotopes of heavy elements. The boiling point of helium-4 is 4.23  K , that of helium-3 is only 3.19 K, corresponding to a temperature ratio of 4: 3, which corresponds almost exactly to the core mass ratio of 4: 3. Since the temperature is linear to the total energy of the respective atom, the energies per nucleon are almost the same at the respective boiling point of helium-3 and helium-4. For comparison: Hydrogen boils at 21.15 K, but the doubly heavy deuterium only has a boiling point of 23.57 K that is 11% higher. The critical temperature , beyond which it is no longer possible to differentiate between liquid and gas, is for helium -3 at 3.35 K, for helium-4 at 5.3 K.

The considerable difference in the boiling temperature can be used to distill helium-3 from a helium-3 / helium-4 mixture: at 1.25 K the vapor pressure of helium-3 is, for example, 3.17 kPa, that of helium-3 4 only 0.115 kPa. At temperatures below 0.86 K, helium-3 and helium-4 even begin to spontaneously separate. In order to achieve helium-3 concentrations of less than 10% or greater than 90% in this way, very low temperatures below 0.3 K are required.

Zero point energy

Furthermore, with its high symmetry (two protons, two neutrons, two electrons) and total spin  0 , helium-4 belongs to the bosons . Helium-3, on the other hand, has spin  ½   and is therefore a fermion . Due to its fermionic properties, helium-3 has a significantly higher zero-point energy than helium-4: Due to the Pauli principle , all helium-3 atoms must be in different states, while - at sufficiently low temperatures - there are any number of helium-4 atoms can be in the basic state at the same time. The zero point energy of the ground state is also higher due to the lower mass. As a result, helium-3 atoms vibrate more strongly, so that they are less densely packed in the liquid state than with helium-4: Liquid helium-3 has a density of 59 g / at the boiling point (3.19 K, 1 bar pressure) L, liquid helium-4, despite the higher temperature (4.23 K) more than double with 125 g / L. The enthalpy required for evaporation is 0.026 kJ / mol, less than a third of that of helium-4 with 0.0829 kJ / mol.

Superfluidity

While helium-4 becomes superfluid at 2.17 K , this does not occur with helium-3 until 2.491 mK. The common theories of the superfluidity of helium-3 state that two helium-3 atoms come together to form a Cooper pair and thus form a boson. There is a similar effect with electrical superconductors , where according to the BCS theory , two fermionic electrons each come together to form a Cooper pair. With helium-4, this intermediate step is not necessary, it becomes superfluid directly.

Because of the very low temperatures at which superfluidity occurs with helium-3, it was discovered relatively late. In the 1970s, David Morris Lee , Douglas Dean Osheroff and Robert Coleman Richardson even observed two phase transitions along the melting curve, which were soon interpreted as two superfluid phases of helium-3. The transition to a superfluid occurs at 2.491 mK on the melting curve. For this discovery, they were awarded the Nobel Prize in Physics in 1996.

In 2003 Anthony James Leggett also won the Nobel Prize in Physics for a better understanding of the superfluid phases of helium-3. In a magnetic field-free space there are two independent superfluid phases of helium-3, namely the A phase and the B phase. The B phase is the low temperature and low pressure phase, which has an isotropic energy gap. The A phase is the high pressure and high temperature phase, which can also be stabilized by a magnetic field and has two nodes in its energy gap.

The presence of two phases is a clear indication that 3 He is an unusual superfluid (or superconductor), since another symmetry, besides the calibration symmetry, is required for the two phases, which is broken spontaneously. In fact, it is a p- wave superfluid, with spin one ( ) and angular momentum one ( ). The basic state then corresponds to a vectorial added total angular momentum . Excited states have a total angular momentum , which corresponds to excited collective pair modes. Because of the extreme purity of the superfluid 3 He, these collective modes could be investigated with higher accuracy than in any other unusual pairing system. The high purity is achieved because all materials except 4 He have long since frozen at the low temperatures and sunk to the bottom, and all 4 He segregates and is present in a separate phase. The latter still contains 6.5% of 3 He, which would not segregate even at absolute zero, but which does not interfere here, since the remaining 3 He dissolved in 4 He does not become superfluid.

polarization

Because of the spin  ½   the 3 He atom carries a magnetic moment. In the magnetic field, more of these moments are parallel to the magnetic field than antiparallel to it, this effect is called spin polarization . 3 He gas becomes slightly magnetic itself under the influence of an external magnetic field. At room temperature, however, the numerical difference between the parallel and anti-parallel aligned magnets is small, since the average energy per atom at this temperature is much higher than the energy split in the magnetic field, despite the high gyromagnetic ratio of helium. With the technique of hyperpolarization , however, it is possible to achieve degrees of polarization of up to 70%. Due to the low level of interaction between the nuclear spins and the environment, once generated hyperpolarized helium-3 can be stored in pressure tanks for up to 100 hours.

use

Cryogenics

The most important area of ​​application for helium-3 is cryogenics . A helium-3- absorption refrigerating machine works with pure 3 He and reaches temperatures as low as 0.2 to 0.3 K. The 3 He- 4 He mixture cooling utilizes the spontaneous demixing of 3 He and 4 He to cool down to a few thousandths of a Kelvin: In the heavier 4 He-rich phase, however, some 3 He still dissolves . If 3 He is distilled off from the mixed phase, the 3 He content in the 4 He phase is reduced , and 3 He flows in from the pure 3 He phase into the 4 He-rich phase. When the 3 He is dissolved in the 4 He phase, heat energy is consumed and the temperatures drop.

In addition to its use as a refrigerant, 3 He is itself an intensive research subject in low-temperature physics.

Neutron detection

Helium-3 is used for neutron detection in counter tubes because it has a large cross-section for thermal neutrons for the nuclear reaction

n + 3 He → 3 H + 1 H + 0.764 MeV,

which generates the charged recoil nuclei tritium (T, 3 H) and protium (p, 1 H).

Polarizer for neutrons

Since the absorption of neutrons by helium-3 is highly spin- dependent, the aforementioned hyperpolarized helium-3 can be used to generate spin-polarized thermal neutron radiation. The neutrons with the appropriate spin for the absorption are intercepted by helium-3, while those with the unsuitable spin are not.

medicine

Hyperpolarized helium-3 is very suitable for MRI exams . This can be used, for example, to observe the inflow and outflow of gas into the lungs. Normally, the gas - which is a thousand times thinner compared to body tissue - cannot be seen in MRI images, but the hyperpolarization amplifies the signal accordingly. This allows the airways to be visualized in the MRI, non-ventilated parts of the lungs to be found or the oxygen partial pressure to be measured. This method can be used for diagnosis and treatment control in chronic diseases such as chronic obstructive pulmonary disease (COPD).

Theoretical use in fusion reactors

It has been proposed to use 3 He as a fuel in a hypothetical second or third generation of fusion reactors . Such fusion reactors would have great advantages in terms of reducing radioactivity . Another possible advantage would be that the emitted protons , which carry the energy gain of the helium-3 fusion reaction, could be captured by electrical and magnetic fields and their energy could be converted directly into electricity.

Helium-3 offers energy-supplying nuclear reactions with deuterium or - although technically even more difficult to realize - with itself (see Deuterium / Helium-3 and Helium-3 / Helium-3 ). Both reactions are well known from accelerator experiments. The feasibility as an energy source lies at least many decades in the future.

The amounts of helium-3 that would be needed to replace fossil fuels are more than four orders of magnitude higher than current world production. The total energy released during the 2 H- 3 He fusion is 18.4 MeV. This corresponds to 493 MWh per mole (corresponds to 3 g) of 3 He. If this energy could be converted completely into electricity, 145 t 3 He per year would be needed for the current global energy demand for electricity alone . This contrasts with a production of 8 kg per year.

literature

Web links

Individual evidence

  1. ↑ a b c d data sheet Helium- 3 He, 99.9999 atom%, 99.995% (CP) from Sigma-Aldrich , accessed on March 22, 2014 ( PDF ).
  2. ^ G. Audi, AH Wapstra, C. Thibault: The AME 2003 atomic mass evaluation. (II). Tables, graphs and references. In: Nuclear Physics A. Volume 729, 2003, pp. 337-676, doi: 10.1016 / j.nuclphysa.2003.11.002 .
  3. ↑ Yonghua Huang, GB Chen, V. Arp, Ray Radebaugh: Equation of state and thermophysical properties of helium-3. In: Proceedings of the ICEC. Volume CR06-379, 2007, pp. 1-6 ( PDF; 5 MB ).
  4. ↑ nuclide data from atom.kaeri.re.kr ( Memento from July 7, 2013 in the web archive archive.today )
  5. ↑ MLE Oliphant, Harteck, P .; Rutherford, E .: Transmutation Effects Observed with Heavy Hydrogen . In: Proceedings of the Royal Society A . 144, No. 853, 1934, pp. 692-703. bibcode : 1934RSPSA.144..692O . doi : 10.1098 / rspa.1934.0077 .
  6. ^ Lawrence and His Laboratory: Episode: A Productive Error . News magazine Publication. 1981. Retrieved September 1, 2009.
  7. ↑ EN Slyuta: The Estimation of Helium-3 Probable Reserves in Lunar Regolith . In: 38th Lunar and Planetary Science Conference., P. 2175.
  8. ↑ FH Cocks: 3 He in permanently shadowed lunar polar surfaces . In: Icarus . 206, No. 2, 2010, pp. 778-779. bibcode : 2010Icar..206..778C . doi : 10.1016 / j.icarus.2009.12.032 .
  9. ↑ Fa WenZhe & Jin YaQiu: Global inventory of Helium-3 in lunar regoliths estimated by a multi-channel microwave radiometer on the Chang-E 1 lunar satellite , December 2010
  10. ↑ HB Niemann, SK Atreya, GR Carignan, TM Donahue, JA Haberman, DN Harpold, RE Hartle, DM Hunten, WT Kasprzak, PR Mahaffy, TC Owen, NW Spencer, SH Way: The Galileo probe mass spectrometer: composition of Jupiter's atmosphere . In: Science. Volume 272, number 5263, May 1996, ISSN  0036-8075 , pp. 846-849, doi: 10.1126 / science.272.5263.846 , PMID 8629016 .
  11. ↑ Marcus Wöstheinrich: Emission of ternary particles from the reactions 229 Th (n th , f), 233 U (n th , f) and 239 Pu (n th , f) . Tübingen 1999, DNB  963242830 , p. 9 , urn : nbn: de: bsz: 21-opus-349 (dissertation, University of Tübingen).
  12. ^ O. Serot, C. Wagemans, J. Heyse: New Results on Helium and Tritium Gas Production From Ternary Fission. In: International conference on nuclear data for science and technology. AIP Conference Proceedings. 2005, 769, pp. 857-860, doi: 10.1063 / 1.1945141 .
  13. ^ A. Fiege: Tritium . Report KfK-5055, Karlsruhe Nuclear Research Center, 1992. ISSN  0303-4003 .
  14. ↑ Whitlock, Jeremy: Section D: Safety and Liability - How does Ontario Power Generation manage tritium production in its CANDU moderators? Canadian Nuclear FAQ, accessed September 19, 2010 .
  15. ^ Tritium and the environment , accessed October 10, 2014
  16. ↑ Hisham Zerriffi: Tritium: The environmental, health, budgetary, and strategic effects of the Department of Energy's decision to produce tritium . Ed .: Institute for Energy and Environmental Research. January 1996 ( PDF ).
  17. ^ A b Dana A. Shea, Daniel Morgan: The Helium 3 Shortage: Supply, Demand, and Options for Congress . Congressional Research Service, December 22, 2010, accessed March 11, 2017 (PDF).
  18. ↑ Physics Projects Deflate for Lack of Helium-3 . Spectrum.ieee.org. Retrieved November 8, 2011
  19. ↑ Technology Review : Energy from the Moon, August 31, 2007, accessed July 15, 2015
  20. ↑ 'Kohlrausch Practical Physics, tables and diagrams for Chapter 3, pages 342 and 343 ( Memento of the original from October 24, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.ptb.de
  21. ↑ teragon's Summary of cryogen Properties teragon Research., 2005
  22. ↑ DD Osheroff, Richardson, RC; Lee, DM: Evidence for a New Phase of Solid He 3 . In: Physical Review Letters . 28, No. 14, 1972, pp. 885-888. bibcode : 1972PhRvL..28..885O . doi : 10.1103 / PhysRevLett.28.885 .
  23. ↑ DD Osheroff, Gully, WJ; Richardson, RC; Lee, DM: New Magnetic Phenomena in Liquid He 3 below 3 mK . In: Physical Review Letters . 29, No. 14, 1972, pp. 920-923. bibcode : 1972PhRvL..29..920O . doi : 10.1103 / PhysRevLett.29.920 .
  24. ↑ AJ Leggett: Interpretation of Recent Results on He 3 below 3 mK: A New Liquid Phase? . In: Physical Review Letters . 29, No. 18, 1972, pp. 1227-1230. bibcode : 1972PhRvL..29.1227L . doi : 10.1103 / PhysRevLett.29.1227 .
  25. ^ Dilution Refrigeration ( Memento of February 8, 2010 in the Internet Archive ). cern.ch
  26. ↑ NCNR Neutron Spin Filters . Ncnr.nist.gov (April 28, 2004). Retrieved November 8, 2011
  27. ↑ ILL 3 He spin filters . Ill.eu (October 22, 2010). Retrieved November 8, 2011
  28. ^ SANS polarization analysis with nuclear spin-polarized 3He . In: J. Appl. Cryst. . 33, 2000, pp. 771-774. doi : 10.1107 / S0021889800099817 .
  29. ↑ Neutron Spin Filters: Polarized 3He . NIST.gov
  30. ^ TA Altes, M. Salerno: Hyperpolarized gas MR imaging of the lung . In: Journal of Thoracic Imaging . tape 19 , no. 4 , 2004, p. 250-258 , PMID 15502612 .
  31. ↑ John Santarius: Lunar 3 He and Fusion Power (PDF) September 28, 2004. Retrieved June 9, 2014.
  32. ↑ Mark Williams: Mining the Moon: Lab experiments suggest that future fusion reactors could use helium-3 gathered from the moon . In: MIT Technology Review . August 23, 2007. Retrieved June 9, 2014.