Scintillation counter

from Wikipedia, the free encyclopedia
Scintillation dose rate meter with uranium sample

A scintillation counter - more rarely, but more precisely also as a scintillation detector - is a measuring device based on scintillation to determine the energy and intensity of ionizing radiation .

history

Fig. 1: Simple scintillation counter

The scintillation measurement is one of the oldest measurement methods for the detection of ionizing or X-ray radiation . Originally, a zinc sulfide screen was held in the beam path and the scintillation events were either counted as flashes or, in the case of X-ray diagnostics, viewed as an image (Fig. 1). Rutherford used a scintillation counter called a spinariscope to investigate the scattering of α-particles on atomic nuclei.

Layout and function

Fig. 2: Structure of a scintillation counter

In the head of the measuring device there is a scintillator protected against external light (and moisture, e.g. when using the very hygroscopic sodium iodide) , in which several flashes of light are triggered (indirectly) by the ionizing radiation, the number of which depends on the energy of the incident radiation depends. These very weak flashes of light release electrons from the photocathode of the photomultiplier behind it ( photo effect ). These electrons are multiplied like an avalanche by collisions with the electrodes in the photomultiplier. An easily measurable current pulse can then be picked up at the anode , the amplitude of which depends on the energy of the incident radiation. In the case of particularly compact scintillation counters, a sensitive photodiode is used instead of the photomultiplier .

Depending on the scintillator, a scintillation counter is suitable for measuring alpha , beta , gamma or neutron radiation .

Both inorganic salts and organic plastics or liquids can be used for the transparent scintillation material (see scintillator ). Inorganic substances have the advantage that they can be used to achieve a higher density, which improves the absorption capacity and thus the sensitivity of the meter for gamma radiation . A frequently used substance is sodium iodide (NaI), which is doped with small amounts of thallium (Tl, approx. 0.1%) for this purpose . Other materials are, for example, lanthanum chloride (LaCl 3 ) or cesium iodide (CsI) and the sensitive for higher energy gamma radiation bismuth germanate (BGO) (Bi 4 Ge 3 O 12 ), and with Ce 3+ doped Lutetiumyttriumoxyorthosilicat , Luyo [SiO 4 ], or Lutetium oxyorthosilicate , Lu 2 O [SiO 4 ].

With a scintillation counter, β and γ spectra can be recorded (see gamma spectroscopy ), which is not possible with a Geiger-Müller counter tube , for example .

The energy resolution of scintillation counters is i. A. better than proportional counters , but again not as good as those of semiconductor detectors , e.g. B. silicon detectors for particle radiation or cooled germanium detectors for gamma radiation.

commitment

Scintillation counters have been in practical use in various areas for a long time. Thus, for example, in nuclear medicine of the method positron emission tomography scintillation counter as detectors for the annihilation - photon arranged annularly to three-dimensional cross-sectional images of organs to be generated.

Another main area of ​​application of scintillators is the detection of gamma quanta in calorimeters in particle physics. They are also often used to trigger other detectors that provide more detailed information and in hodoscopes .

By combining two different types of scintillators in one detector, with the help of the so-called Δ E-E measurement, conclusions can be drawn not only about the energy E of the detected particles, but also about their mass. A thin, fast scintillator, which only slows down the particles to be detected a little (loss of energy ), and a thicker, slower scintillator, which then completely catches the particles, are used. The light from the two scintillators can then be captured with a single light detector. Because of the different reaction times of the scintillators, the signals can be electronically separated and put into relation.

Use as a primary detector in scanning electron microscopes (so-called Everhart-Thornley detector ) is also important.

Liquid scintillation counter

An important area of ​​application for scintillation counters is the measurement of the concentration of radioactively labeled substances, e.g. B. in biochemistry . In most cases, small amounts ( specific activities ) of radionuclides such as tritium ( 3 H), carbon -14 ( 14 C) or sulfur -35 ( 35 S) have to be determined, and it is precisely these nuclides that only give less beta radiation (β radiation) Energy that is strongly absorbed in matter. A liquid scintillator is best suited here, in which the substance sample to be measured is dissolved, so that almost all beta electrons emitted are recorded by the scintillator. As with other scintillation counters, the flashes of light are converted into electrical pulses by a photomultiplier and fed to a counting arrangement.

(Liquid scintillators, which are used in sealed glass or metal-glass vessels like solid scintillators and are used to measure fast neutrons, must be distinguished from this technology. )

The β-emitting sample to be measured should be distributed as homogeneously as possible in the solution in order to achieve the best counting yield (efficiency). The liquid therefore consists of the following components:

solvent

The first task of the solvent is to dissolve the actual scintillator and the sample to be measured. Furthermore, it has to absorb the energy of the radiation as excitation energy and transfer it to the dissolved scintillator. Aromatic solvents are particularly suitable for this dual task. Commonly used are toluene , xylene and cumene or pseudocumene .

Scintillators

The actual scintillator has the task of taking the excitation energy from the solvent and converting it into light quanta. It must therefore be readily soluble in the solvent and emit a fluorescence spectrum suitable for the photocathodes of the photomultiplier. The chemical names of the scintillators are a bit unwieldy in practice. It has therefore become common practice to only use abbreviations.

Here are some examples of scintillators:

  • PBD = 2-phenyl-5- (4-biphenyl) -1,3,4-oxadiazole
  • PPO = 2,5-diphenyloxazole
  • BBOT = 2,5-bis [5'-tert-butyl-benzoxazolyl (2 ')] thiophene
  • POPOP = p-bis-5-phenyl-oxazolyl (2) -benzene

additions

If the substance to be measured (the sample system) is not soluble in the previously mentioned components solvent and scintillator, additives are used, which mostly have the task of solubilizers. For example, the addition of alcohol to toluene achieves a limited absorption capacity for aqueous samples. However, this reduces the effectiveness of the scintillator.

Scintillation process

If a β-particle is emitted in the liquid scintillator, the solvent molecules are excited along the path of the particle. The excited solvent molecules transfer their energy to the scintillator, which emits the absorbed excitation energy as fluorescent light. The number of excited molecules depends on the path length of the emitted electron in the scintillator. The number of photons emitted per decay is therefore dependent on the energy of the primary radiation particle.

Structure of the measuring device

Fig. 3: Photomultiplier tube
Fig. 4: Schematic structure of a liquid scintillation counter
Fig. 5: Spectrum of a β emitter

In the measuring device, the light flashes generated in the scintillator per decay act are converted into electrical impulses in a photomultiplier (see Fig. 3). Since the gain in the photomultiplier is proportional, that is, all pulses are increased by the same factor, the height of the electrical pulse at the output is proportional to the energy of the observed particle.

At normal ambient temperature, electrical impulses (noise, dark current ) are generated in the photomultiplier , which are counted during the sample measurement but were not triggered by the sample. To suppress this, two photomultipliers and an electrical circuit are used that only allow counting impulses that were emitted simultaneously by both photomultipliers ( coincidence measurement ) (see Fig. 4).

The measured spectrum does not contain information from the sample in all areas, but in some cases also from other sources, e.g. B. the cosmic radiation . To reduce this measuring subsurface to use discriminators . A discriminator unit allows certain areas of the spectrum to be selected; other areas are excluded from the count (discriminated against). For the measurement of a β-emitter, the device is set so that the pulse rate maximum lies in the window area between the lower and upper discriminator threshold. By using several discriminator units, measurements can be made in several energy ranges at the same time.

Modern devices have multi-channel analyzers . This makes it possible to assign each measured β-particle to a small energy window. If all energy windows (channels) are shown next to each other in a coordinate system, an energy spectrum is obtained (see Fig. 5).

Possible errors due to the "quench" and the efficiency control

Fig. 6: Effect of the quench on the energy spectrum

If not all of the fluorescent light reaches the photomultipliers due to chemical substances or a coloration of the sample with the same radiation energy of the particles, the spectrum is shifted to lower energies. Some of the pulses become too small to be counted. The yield drops. This effect is called quench.

There are basically two types of quench:

Chemical quench

When the energy is transported from the excited solvent molecule to the photon-generating scintillator, part of the energy is transferred to non-fluorescent molecules. Instead of the desired photons, heat is generated.

Optical quench

Photons already emitted by the scintillator are absorbed in the solution itself. This process is called "optical quench".

Both types of quench lead to the same effect, the pulse spectrum is shifted towards lower energies (see Fig. 6). As a result, the pulse spectrum moves out of the optimally set window, and the efficiency changes. Therefore, when measuring with liquid scintillators, it is always necessary to check the efficiency of the measurement and to make corrections when comparing samples that were measured with different levels of efficiency. There are essentially two methods of controlling the efficiency:

Determination of efficiency with "internal standard"

When determining the efficiency with the "internal standard", the sample to be measured is placed in a sample container with a scintillator and measured. Then an exactly known activity (the internal standard) is added to the sample and measured again. The difference in the count rates between the second and the first measurement gives the count rate of the internal standard. This counting rate is divided by the known activity used, giving the efficiency for measuring the internal standard. This efficiency is then transferred to the measurement of the sample. A prerequisite for this method is that the addition of the internal standard does not result in any further quenching, i.e. the efficiency does not change.

Determination of efficiency according to a "key figure method"

The key figure method uses the effect that the quench shifts the pulse spectrum towards lower energies. A key figure is obtained from this shift, to which the efficiency is assigned via a calibration measurement. The graphical representation of the efficiency as a function of the associated key figure is called the quench curve . The measured values ​​can only be compared directly for samples with the same characteristic number. When comparing samples with different characteristic numbers , a quench correction must first be carried out. The associated efficiency is determined from the key figure with the help of the recorded quench curve and the count rate found is converted to the absolute activity. The oldest measure method is the channel ratio method . With the channel ratio method, in addition to the optimally set measuring channel, a second channel is used as a comparison channel, the window width of which is reduced by increasing the lower threshold or decreasing the upper threshold compared to the measuring channel. In this way, two counting rates are obtained for each measurement of a sample, the counting rate in the shortened comparison channel always being lower. If the counting rate of the comparison channel is divided by the counting rate of the measuring channel, a quotient is obtained which is independent of the size of the counting rates and only depends on the position of the spectrum in the two channels. If the position of the pulse spectrum in the measurement windows is shifted due to different quenching, this shift has different effects in the measurement channel and comparison channel, and the quotient of the counting rates changes. In this way you first have a parameter for the efficiency. For samples with the same quotient, the pulse spectrum lies in the same window area; they are measured with the same efficiency. Samples with different quotients are measured with different degrees of efficiency. Using a calibration series with precisely known activities but different quenching, a quotient is obtained as a key figure for each degree of efficiency in the measuring channel.

Two methods are used to obtain the channel ratio quotients as a key figure. According to the first method, the pulse rates of the sample are used to obtain the two count rates in the different channels. The key figure obtained from the sample spectrum is therefore called the sample channel ratio . Since long measurement times are often required for the shortened channel, especially for the shortened channel, because of the often low counting rates of the samples to statistically secure the results, a gamma emitter is usually brought from the outside to the sample vial and a β via the Compton electrons in the scintillator solution -Sample generated comparable spectrum. This pulse spectrum is subject to a quenching effect similar to that of the sample spectrum. Since high counting rates can be generated in this way, short measuring times are sufficient to determine the quotient. If the quotient is obtained from the Compton spectrum of an external gamma source, this method is called the external standard channel ratio . The actual measurement of the sample is separated in time from the determination of the quotient and takes place either before or after the determination of the key figure.

Sample preparation for liquid scintillation measurement

Directly measurable samples in a homogeneous phase

In the past, scintillators had to be manufactured in-house for many applications. Today there is a wide range that covers almost all possible uses. It is important to be able to characterize your samples well in order to get the right cocktail after consulting a manufacturing company. Many samples therefore no longer need to be processed for the measurement, unless, for example, they are very cloudy or contain highly quenching substances. Scintillators, which can absorb a great deal of water (over 50%), have become very important in environmental measurement technology. When the measurement sample is placed in the scintillator, it is important to shake the sample well. Then it must be checked whether a homogeneous phase is present. If the cocktail's absorption capacity is used, it must be checked whether a phase separation takes place. Samples that are cloudy after shaking often clear up after some time. Cooling the samples has proven to be effective in minimizing the appearance of luminescence.

Directly measurable samples in heterogeneous phase

Finely distributed solids or larger volumes of liquids can be measured in the heterogeneous measuring system. It is important to distribute the samples very finely in the scintillator system in order to establish good contact with the scintillator. However, due to the self-absorption of the radiation in the sample particles, an exact determination of the efficiency becomes difficult. The dispersion of the sample in the scintillator must be stabilized by suitable emulsifiers or gel formers. Finely divided silica gel (Cab-O-Sil) is used as a gel former . Depending on the sample phase, a distinction is made between measurements in emulsions and suspensions. The group of measurements in the heterogeneous phase also includes the introduction of filter strips or similar material with radioactive substance firmly adhering to it and insoluble in the scintillator.

Measurable samples after sample conversion

Absorption of gaseous samples

It is often necessary to measure gases containing 14 C, 35 S or 3 H. In the case of 3 H, it can be assumed that the activity to be determined is bound as water. In this case, the water is frozen out of the air with a cold trap. If the contamination is present as dust particles, these can be deposited on filters. These filters are placed directly in the scintillator. Acid gases are absorbed in an organic base such as benzethonium hydroxide (Hyamin 10-X), ethanolamine or phenylethylamine ) and dissolved in the scintillator system.

Solubilization

During the solubilization, the high molecular structure of the biological sample material is degraded to such an extent that a homogeneous solution is made possible with the help of solubilizers. Quaternary ammonium bases have proven particularly useful as degradation reagents and solubilizers . In addition, enzymatic hydrolysis or digestion with formic acid is also used . In the case of samples that are embedded in a polyacrylamide gel , N , N '-diallyl tartariamide can be used for the gel instead of N , N ' -methylene bisacrylamide , and the gel can be dissolved by glycol cleavage after the separation run with sodium periodate solution .

Sample combustion

The most radical form of sample conversion in biological samples is sample combustion. Either in solution (wet oxidation) or by combustion in an oxygen atmosphere (dry oxidation), the substance is burned to CO 2 and water. The resulting CO 2 is absorbed and taken to the measurement. With dry oxidation it is possible to separate CO 2 and the water produced and in this way to measure 3 H and 14 C separately from a sample.

literature

Web links

Wikibooks: Physical basics of nuclear medicine / scintillation counter  - learning and teaching materials

Individual evidence

  1. ↑ Kevin Klipsch: Neutron activation analytical investigations for the determination of radio-ecological parameters from the long-term entry of 129 I , diploma thesis Uni Hannover (2002).
  2. ↑ Cocktails for measurements in the scintillation counter (PDF; 271 kB).
  3. ↑ A quick method for measuring α − emitters with an extracting scintillator and pulse decay analysis (PDF; 170 kB).