Scintillation counter

As a scintillation counter - less common but more accurately as a scintillation detector - refers to a system based on the scintillation meter to determine the energy and the intensity of ionizing radiation.

• 4.7.1 Directly measurable samples in the homogeneous phase
• 4.7.2 Directly measurable samples in heterogeneous phase
• 4.7.3 After the sample conversion measurable samples 4.7.3.1 Absorption of gaseous samples
• 4.7.3.2 solubilization
• 4.7.3.3 sample combustion

History

The scintillation is one of the oldest methods of measurement for the detection of ionizing radiation or X-ray. Originally a zinc sulphide screen was held in the beam path and the scintillation counted as either flashes or in the case of X-ray diagnostics considered as an image ( Fig. 1). A designated as Spinthariscope scintillation counter was used to atomic nuclei of Rutherford to study the scattering of α - particles.

Design and function

In the head of the instrument is located against the outer light ( and moisture, eg when using the very hygroscopic sodium iodide ) protected scintillator, several flashes are triggered in which the ionizing radiation ( indirect), the number of the energy of the incident radiation depends. This very faint flashes of light set from the photocathode of the photomultiplier mounted behind it releases electrons ( photoelectric effect ). These electrons are multiplied by impact avalanche at the electrodes in the photomultiplier. At the anode, then a well- measurable current pulse can be decreased, the amplitude of which is dependent on the energy of the incident radiation. In particularly compact scintillation a sensitive photodiode is used instead of the photomultiplier.

Depending on the scintillator is to use a scintillation counter for the measurement of alpha, beta, gamma or neutron radiation.

For the transparent scintillation both inorganic salts and organic plastics or liquids in question (see scintillator ). Inorganic substances have the advantage, that one can achieve a higher density with them, which improves the absorption capacity, and thus the sensitivity of the counter for gamma radiation. A frequently used substance is sodium iodide ( NaI ), which for this purpose with small amounts of thallium (Tl, about 0.1 %) is doped. Other materials include lanthanum chloride ( LaCl3 ) or cesium iodide (CsI ), as well as the sensitive for higher -energy gamma rays bismuth germanate (BGO ) ( Bi4Ge3O12 ) and with Ce3 doped Lutetiumyttriumoxyorthosilicat, Luyo [ SiO4 ] or Lutetiumoxyorthosilicat, Lu2O [ SiO4 ].

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

The energy resolution of scintillation counters is A., better than that of proportional counters, but again not so good as that of semiconductor detectors, such as silicon or particle detectors cooled germanium detectors for gamma radiation.

Use

Scintillation counters are used in various long practical use. Be arranged in a ring in the method of positron emission tomography as scintillation detectors for the annihilation photons to provide three-dimensional cross-sectional images of organs, for example, in nuclear medicine.

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 used in Hodoskopen.

By combining two different kinds of scintillators in a detector, not only draw conclusions about the energy E of the detected particles, but also to the mass may be made with the aid of so-called e- AE measurement. Here are a thinner, faster scintillator only slightly decelerating (energy loss ) and a thicker, slower scintillator used particles to be detected, then the fully absorbs the particles. The light from the two scintillators can then be collected with a single light detector. The signals can be electronically separated due to the different response times of the scintillators and taken into consideration.

Also important is the use as a primary detector in scanning electron microscopes (so-called Everhart -Thornley detector ).

Liquid scintillation

An important application for scintillation counter is to measure the concentration of radioactively labeled substances, for example, in biochemistry. It usually small quantities (activities) of radionuclides such as tritium (3H ), carbon -14 ( 14C) or sulfur -35 ( 35S ) must be determined, and it is this nuclides give also only beta radiation ( β radiation) low energy which is strongly absorbed in matter. This is the best, a liquid scintillator in which the substance to be measured the sample is dissolved, so that almost all emitted electrons are collected by beta scintillator. The light flashes are as other scintillation converted by a photomultiplier into electrical pulses, and supplied to a counter device.

( To be distinguished from this technique are liquid scintillators, which are used in sealed glass or metal and glass vessels as solid scintillators and fast neutrons are used for measurement. )

The measured β -radiative sample is to be distributed homogeneously in the solution in order to obtain the best counting efficiency (efficiency ). The liquid is a matter of following components:

Solvent

The solvent 's first task is to solve the actual scintillator and the sample to be measured. Furthermore, it must be the energy of the radiation record as excitation energy and transferred to the scintillator dissolved. For this double task is particularly suitable aromatic solvents. Commonly used are toluene, xylene and cumene or pseudocumene.

Scintillators

The actual scintillator has the task to take on the excitation energy from the solvent and convert into light quanta. So it has to be readily soluble in the solvent and emit a suitable for the photocathode of the photomultiplier fluorescence spectrum. The chemical names of the scintillators are a bit unwieldy for practical use. It has come to be a matter only to 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

Additives

If ( sample system ) is not soluble substance to be measured in the previously mentioned components solvent and scintillator, additives are used, which have the task of solubilizers most. For example, a conditional capacity for aqueous samples is achieved by the addition of alcohol to toluene. However, the effectiveness of the scintillator decreases.

Szintillationsvorgang

Emitted β - particles in a liquid scintillator, as along the path of the particle excited solvent molecules. The excited solvent molecules transfer their energy to the scintillator, which emits the excitation energy taken over 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 thus dependent on the energy of the primary Strahlungsteilchens.

Structure of the instrument

In measuring the resulting per decay event in the scintillator light flashes are converted into electrical pulses in a photomultiplier tube ( see Figure 3). Since the gain in the photomultiplier is proportional, that is, all of the pulses get larger by the same factor, the height of the electrical pulse at the output proportional to the energy of the observed particle.

At normal ambient temperatures in the photomultiplier caused electrical impulses (noise, dark current ), which are counted in the sample measurement, but were not triggered by the sample. To suppress this, one uses two photomultiplier and an electrical circuit that allows only those pulses for counting that were emitted by two photomultipliers simultaneously ( coincidence measurement ) ( see Figure 4).

The measured spectrum does not contain information in all areas of the sample, but partly also from other sources, such as cosmic radiation. To reduce this measurement subsoil to use discriminators. A discriminator unit allows the selection of specific regions of the spectrum; other areas are excluded from the count ( discrimination ). For the measurement of a β - emitter, the device is adjusted so that the maximum count rate is in the panel, between the upper and lower discriminator. By using several Diskriminatoreinheiten can be measured in several energy ranges simultaneously.

Modern devices have multi-channel analyzers. Thereby, it is possible to assign each measured energy β - particles a small window. If in a coordinate system, all power windows (channels) shown side by side, we obtain an energy spectrum ( see Figure 5).

Possibility of error by the "quench ", and the efficiency of control

If at the same radiation energy of the particles does not arrive by chemical substances or coloring the sample all fluorescent light to the photomultipliers, the spectrum is shifted to lower energies. A portion of the pulse is too small to be counted. The yield decreases. This effect is called quenching.

Basically two types of quenching:

Chemical quench

When energy transfer from the excited solvent molecule to the photon generating scintillator part of the energy is transferred to not capable of fluorescence molecules. Instead of the desired photon generates heat.

Optical quenching

Already emitted by the scintillator photons are absorbed in the solution itself. This process is referred to as " optical quench".

Both Quencharten lead to the same effect, the pulse spectrum is shifted to lower energies down ( see Figure 6). Thus, the pulse spectrum from the window once optimally adjusted wanders out, the efficiency changes. Therefore, it is always necessary in the measurement with liquid scintillators, to control the efficiency of the measurement and the comparison of samples with different efficiencies were measured to make corrections. To check the efficiency, there are essentially two methods:

Efficiency measurement with " internal standard "

In determining the efficiency with " internal standard " the sample to be measured is given and measured with scintillator in a sample cup. Then a well- known activity ( the internal standard ) is added to the same sample vessel and measured again. The difference in count rate between the second and the first measurement gives the count of the internal standard. This count is divided by the known type employed, the result is the efficiency for the measurement of the internal standard. This efficiency is then transferred to the measurement of the sample. It is assumed in this method that the addition of the internal standard, no further quench occurs, the efficiency is not so changed.

Efficiency test for a " code " method

The ratio method uses the effect that carried through the quench was a shift of the pulse spectrum to lower energies. This shift a figure is obtained which is assigned via a calibration measurement of efficiency. The graphical representation of the efficiency as a function of the associated code is referred to as quench curve. Only in samples with the same code, the measured values ​​can be directly compared. In the comparison of samples with different code a first quench must be performed. From the figure, the corresponding efficiency is calculated using the recorded quench determined and the count rate found converted to the absolute activity. The oldest measure method is the channel ratio method. The channel ratio method in addition to the optimal set measuring channel, a second channel is used as a reference channel whose window width is reduced by increasing the lower threshold or decreasing the upper threshold compared to the measuring channel. In this way is obtained for each measurement of a sample, two count rates, the count rate in the reference channel is always less shortened. Dividing the pulse rate of the reference channel count rate by the measurement of the channel, we obtain a quotient, which is independent of the size of the count rates and depends only on the position of the spectrum in the two channels. If by different quench the position of the pulse spectrum shifted in the measurement windows, so this shift affects the measuring and reference channels from different, the ratio of the count rates change. In this way, one has firstly a parameter for the efficiency. For samples with the same quotient, the pulse spectrum is in the same pane, they are measured with the same efficiency. Samples with different ratios are measured with different efficiencies. About a calibration series with exactly known activities but different quenching efficiency is obtained for each of the channel is thus a quotient as a measure.

For the extraction of the channel ratio quotient as a measure, two methods are used. After the first process is used for obtaining the count rates in the two different channels, the pulse rate of the sample. The code obtained from the sample spectrum is therefore called sample channel ratio. Since, due to the often low counting rates of the sample to statistical assurance of the results, especially for the shorter channel long measurement times are often required, a gamma emitter is usually placed on the outside of the vial for the quotient and the Compton electrons in Szintillatorlösung one of the β sample produced similar spectrum. This pulse spectrum is under a similar quenching effect as the sample spectrum. Since high count rates can be produced in this manner, sufficient for the determination of the quotient of short measurement times. If the quotient obtained through the Compton spectrum of an external gamma source, referred to this process as external standard channel ratio. The actual measurement of the sample is thus separated in time from the determination of the quotient and takes place either before or after the code provision.

Sample preparation for liquid scintillation

Directly measurable samples in the homogeneous phase

Previously had to be produced in-house for many applications scintillators. Today there is a wide range that takes into account almost all applications. It is important to characterize their samples well to get then in agreement with a manufacturer the right cocktail. Therefore, many samples need to measure not to be processed, unless they are for example very cloudy or have strong quenching substances. In the environmental monitoring scintillators have acquired great significance which can absorb a lot of water ( about 50%). When the measurement sample is added to the scintillator, it is important to shake the sample well. Then you have to check whether a homogeneous phase. When utilizing the capacity of the cocktail has to be checked whether a phase separation takes place. Samples that are cloudy after shaking, clarify often after some time. The cooling of the samples has been proven to minimize luminescence phenomena.

Directly measurable samples in heterogeneous phase

Finely divided solids or larger volumes of liquids can be measured in the heterogeneous measurement system. It is important to distribute the sample very finely in the scintillator system to make good contact with the scintillator. However, an accurate determination of efficiency is difficult to self-absorption of radiation in the sample particles. The dispersion of the sample in the scintillator must be stabilized by suitable emulsifiers or gelling agent. Gel formers among other finely divided silica (Cab -O -Sil ) is used. Depending on the sample stage differentiates measurements in emulsions or suspensions. The group of measurements in heterogeneous phase includes the introduction of filter strips or similar material with it, adherent and non-soluble in the scintillator radioactive substance.

After sample conversion measurable samples

Absorption of gaseous samples

It is often necessary to measure 14C, 35S or 3H- containing gases. 3H at can be assumed to be determined that the activity is bound as water. In this case, the water freezes in a cold trap from the air. If the contamination and dust particles before, they can be deposited on filters. These filters are placed directly into the scintillator. Acid gases are absorbed in an organic base such Benzethoniumhydroxid ( Hyamine 10 -X), ethanolamine, or phenylethylamine ) and dissolved in the scintillator system.

Solubilization

In solubilizing the high molecular weight structure of the biological sample material is reduced so far, that a homogeneous solution with the aid of solubilizing agents is possible. As mining reagents and solubilizers particularly quaternary ammonium bases have proved successful. In addition, enzymatic hydrolysis or digestion with formic acid is commonly used. For samples which are embedded in a polyacrylamide gel, can be for the gel in place of N, N'-methylenebisacrylamide N, N'- diallyltartardiamide use, and dissolve the gel after the separating operation with sodium periodate by glycol cleavage.

Sample combustion

The most radical form of samples conversion of biological samples is the sample combustion. In this case either in solution (wet oxidation) or by combustion in an oxygen atmosphere (dry oxidation) is burned the substance to CO2 and water. The resulting CO2 is absorbed and taken for measurement. In dry oxidation, it is possible to separate CO2 and water formed and measured separately in this way, 3H and 14C from a sample.