The interaction between radiation emitted by radioactive isotopes and substances will directly or indirectly produce ionization and excitation effects, which can be used to detect the existence of radioactivity and the nature and intensity of radioactive isotopes. The instruments used to record the quantity of various rays, measure the intensity of rays and analyze the energy of rays are collectively called probes. There are various instruments and methods to measure rays, as MacKay said in 1953: "Whenever a physicist observes a new effect caused by atomic particles, he tries to make a detector by using this new effect." Generally speaking, detectors are divided into two categories. One is the "track" detector, such as photographic emulsion, cloud chamber, bubble chamber, spark chamber, dielectric particle detector and photochromic detector, which is mainly used in the field of high-energy particle physics. 2. Signal detectors, including ionization counter, proportional counter, Geiger counter, scintillation counter, semiconductor counter and Cherenkov counter, are more and more widely used in low-energy nuclear physics, radiation chemistry, biology, biochemistry, molecular biology and geology, especially scintillation counter is one of the necessary instruments in biochemical and molecular biology research.

First, the scintillation detector

1. detection principle

Scintillation detector consists of scintillator, photomultiplier tube, power supply and amplifier-analyzer-scaler system. Modern scintillation detectors are usually equipped with a computer system to process the measurement results. When the ray passes through the scintillator, the scintillator is excited by the ionization of the ray and emits light with a certain wavelength. These photons hit the photocathode of photomultiplier tube, producing photoelectric effect and releasing electrons. Electrons flow through the multistage cathode circuit of the photomultiplier tube, are amplified step by step or are electric pulses, input into the electronic circuit part, and then recorded by the scaler. The number of electrons produced by photocathode is proportional to the number of photons irradiated on it, that is, the more radioactive isotopes, the more flicker caused on scintillator and the more pulses recorded by instrument. The measurement results can be expressed by counting rate, that is, the number of ray counts per minute (cpm). Modern counting devices can usually give data such as decay rate, counting times per minute (abbreviated as dpm), counting efficiency (E), measurement error, etc. Scintillation detector is a nuclear detector with rapid development and wide application in recent years, and one of its core structures is scintillator. Scintillator largely determines the quality of the counter.

2. Fluorescent materials

Scintillator is a material that can absorb energy and re-emit part of the absorbed energy in the form of light in about one microsecond or less. Scintillators are divided into two categories: inorganic scintillator and organic scintillator. The essential property of scintillators is that they should be highly transparent to the photons emitted by themselves. The ratio at which a scintillator absorbs some photons emitted by itself varies with the scintillation material. Inorganic scintillators [such as NaL (TL) and ZnS (Ag)] are almost 100% transparent, while organic scintillators (such as anthracene, plastic scintillators and liquid scintillators) are generally less transparent. Several scintillators commonly used now are: (1) inorganic crystals, mainly alkali metal iodide with or without impurities; (2) All organic crystals are unsubstituted or substituted aromatic hydrocarbons; (3) liquid organic solution, namely liquid scintillator; (4) The organic solution in the plastic solution is the solid solution scintillator.

3. Photomultiplier tube

It is one of the important parts of scintillation detector. Its components are photocathode and multiplier electrode. The photocathode is used to convert the light signal of the scintillator into an electrical signal, and the multiplier electrode is used as an amplifier with a magnification greater than 106. Electrons generated on the photocathode fly to the multiplier electrode through acceleration, and the multiplication of electrons occurs at each multiplier electrode. The multiplication coefficient of the multiplier electrode is proportional to the applied voltage, so the power supply of the photomultiplier tube must be very stable to ensure the minimum change of the multiplication coefficient and no incidence. When using photomultiplier tube to detect low-energy nuclear radiation, dark current must be reduced. Maintaining a low room temperature in the measurement space environment is an effective method to reduce the dark current of photomultiplier tubes.

Second, the crystal scintillation count (crystal scintillation count)

1. detection principle

Unlike alpha and beta particles, gamma rays are similar to light and other electromagnetic radiation. When they interact with matter, they are not directly ionized, but absorbed according to one of the following three mechanisms: photoelectric effect, Compton effect and the generation of electron pairs. In the photoelectric effect, each photon will retain all its energy until it interacts with the orbital electrons of the atoms in the absorbing material. In this process, the photon gives all the energy to the electron, and the electron is emitted at high speed, so the photon no longer exists. The emitted electrons are called photoelectrons, which ionize their energy like beta particles, while other atoms are consumed. In Compton effect, the incident γ photon with hv energy interacts with orbital electrons of atoms in absorbing materials. In this process, photons give their energy to orbital electrons to make them emit, and then photons with smaller energy hv' are "scattered" in the form of conservation of energy and momentum. The emitted electrons are called recoil electrons, also known as Compton electrons. Compton electrons dissipate energy in the same way as β particles, and scattered photons are further absorbed by photoelectric or Compton processes. When an electron pair is generated, the energy of some incident photons is converted into mass according to Einstein's equation: e = mc2, where e is the energy expressed by er (Lego), m is the mass expressed by G, and c is the speed of light, in cm/s, the incident gamma photons are annihilated in an unknown way in the nuclear field of an atom of the absorbing substance, and then there is only one particle, a negative electron and a positron. It is neutralized by an electron in the absorbing material, and this annihilation process leads to the generation of a pair of γ photons, each with an energy of 0.5 1MeV, which is finally absorbed by the photoelectric effect Compton effect. Gamma rays are easily absorbed by substances with high electron density, such as lead, because they have no mass and have strong penetrating power. Atoms with high atomic number z are directly related to high electron density. As far as detectors are concerned, some inorganic salts can effectively absorb gamma photons and emit photons whose intensity is proportional to the absorbed gamma ray energy. For example, thallium-activated sodium iodide has high atomic number Z of iodine atom, high density (specific gravity 3.67), high photon yield per absorbed unit energy and good light transmittance of crystal, so it is very effective for detecting gamma rays.

2. Detection device

A solid crystal device for detecting gamma photons includes a "sealed" thallium activated sodium iodide crystal placed on the surface of a photomultiplier tube. The "sealed" crystal is a solid cylindrical thallium-activated sodium iodide, and its top and periphery are covered with aluminum layers to avoid light and moisture, because sodium iodide crystals easily absorb moisture. In order to improve the reflectivity, the sodium iodide crystal is sealed with a glass sheet and directly contacted with the surface of the photomultiplier tube, and some silicone oil is added to realize optical matching. The whole device is opaque. Gamma rays easily penetrate the aluminum layer on the surface of the crystal, and then are absorbed by the high-efficiency crystal. The crystal emits visible light, and its energy is proportional to the energy of the incident gamma rays. Then, the photomultiplier tube converts visible light energy into electric pulses. The proportional nature of various energy conversion processes (from the emission of gamma photons to the generation of electric pulses) and the absorption nature of gamma photons ensure that gamma radioisotopes can be counted and quantified by crystal scintillation. Crystal γ counter is usually designed to effectively detect photoelectric effect and Compton effect. But the detection effect decreases with the increase of photon energy. For the size of sodium iodide crystal used in most commercial gamma counters, photoelectric effect is dominant at low photon energy, for example, below 400keV, and Compton effect is dominant near 1MeV. Between these two energies, the two effects occur at almost the same frequency, and it is difficult to detect the generation of electron pairs because of the small crystal size used. In addition, adding scintillator (such as POPOP or TP) into plastic solvent (such as polyethylene toluene) to form thin slices can be used to detect high-energy β rays, such as 1.7 1MeV high-energy β rays emitted by 32P. The earliest used zinc sulfide crystal is very thin and contains a trace of silver as an activator, which can be used to detect alpha rays.

3. Qualitative and quantitative analysis of crystal scintillation counting.

Radioisotope chromium decays mainly through electron capture, with a half-life of 27.8 days. Due to electron capture, the atomic number of the atom decreases by 1, thus becoming an isotope of vanadium. The frequency of vanadium decay to the ground state by electron capture is 9 1%, which leads to the subsequent emission of weak X-rays of -5 kev, which is generally difficult to detect, because X-rays from the sample can penetrate into the sodium iodide crystal before it. 5 1Cr has a 9% probability of decaying to the excited nuclear state of vanadium by electron capture, and decays to the stable ground state immediately by emitting-320keV gamma rays, which are easily detected. Using crystal scintillation counter to observe 5 1Cr, a sharp peak is observed at 320keV, which is the result of the loss of γ photon energy by photoelectric effect, but this process does not lose all energy, so at low energy, a series of wide and inconspicuous peaks appear due to Compton effect loss, which is called Compton edge from the bottom of photoelectric peak to the opposite side of valley. The energy is lower than the diffusion peak of Compton region, which is caused by the backward scattering of γ -rays to the absorption material, and the scattered photon energy is low. All kinds of γ -ray radioisotopes have their characteristic photoelectric peaks, which can be used for identification. Gamma-ray counting measurement of various samples is to compare the counting rate with the counting rate of total radioactivity or standard source, so as to calculate the percentage of sample radioactivity in total radioactivity or standard source, and thus obtain the sample radioactivity.

4. Instrument performance evaluation

Crystal scintillation counters are basically square or cylindrical, and sodium iodide (thallium) is used as scintillator to detect gamma rays, so the crystal scintillation counter for detecting gamma rays is also called gamma counter (gamma-count-r). Generally speaking, the performance of γ counter is compared according to its 662keV photoelectric peak resolution of 137Cs. The resolution of the detection system is a measure of the broadening degree of the photoelectric peak, which is defined as the peak width at half the maximum peak height (keV) divided by the maximum pulse height (keV) of the photoelectric peak and then multiplied by 100. If the photomultiplier tube works in the best condition, the resolution can reach 7%. However, the general well-shaped crystal counter has poor resolution due to poor optical characteristics, and the resolution value is about 12%. The higher the γ -ray energy, the better the resolution of the photoelectric peak.

Three. Liquid scintillation counting

The scintillator used in liquid scintillation counting is liquid, that is, the scintillator is dissolved in an appropriate solution to make scintillation liquid, and the radioactive substance to be measured is placed in the scintillation liquid for measurement. The application of liquid scintillation counter can achieve the superior geometric measurement condition of 4π solid angle, and the self-absorption of light source can also be ignored. It has high detection efficiency for α rays and low-energy β rays (such as 3H and 14C) with low energy, short range and easy absorption by air and other substances. Liquid scintillation counter is the first choice for measuring α -rays and low-energy β -rays

1. detection mechanism

The process of photon generation by scintillation liquid is that the radiant energy emitted by radioactive source is first absorbed and excited by solvent molecules. When the excitation energy propagates in the solvent, it is transferred to the scintillator (solute), which causes the excitation of scintillator molecules. When the scintillator molecules return to the ground state, they emit photons, which pass through the transparent scintillation liquid and the wall of the sample bottle, are received by the photocathode of the photomultiplier tube, and then produce photoelectrons, which are amplified by the potential amplifier of the photomultiplier tube and then received by the anode to form electric pulses, thus completing the conversion from radiant energy to light energy to electric energy.

2. Flashing liquid

Scintillation liquid used in liquid scintillation counting system refers to other components except radioactive samples in scintillation bottles, mainly organic solvents and solutes (scintillators), and sometimes other additives are added for sample preparation or improving counting efficiency. ⑴ Solvent: In this series of energy transfer links from the beta source emitting beta rays to the photon emitted by Xiao cathode, the energy transfer efficiency is very low, and only a small part of the emitted energy is used to emit photons, among which the energy transfer efficiency between the radiation source and the solvent is about 5% ~ 10%. The choice of solvent mainly depends on its solubility in scintillator and the efficiency of transferring radioactive energy to scintillator. If the pulse height generated by a scintillator with a certain concentration in toluene solution is 100%, then all those that can generate more than 80% of the pulse height are defined as solvents, those that can gradually reduce the pulse height with the increase of its concentration are called diluents, and those that can significantly reduce the pulse height when the concentration is very low are called quenchers. In the liquid scintillation counting system, a good solvent should meet the following conditions: ① high solubility to scintillator; ② The transfer efficiency of radioactive sources is high; (3) high transparency to scintillation photons; ④ Radioactive samples can be dissolved with or without the help of solvent; ⑤ Freeze at the working temperature of the counter; ⑥ A uniform measuring solution can be formed. Generally speaking, alkylbenzene is the best solvent, such as toluene and xylene. In addition, anisole is also a good solvent. In addition, for samples with more water content, 1, 4- dioxygen is not used as the solvent, because this organic compound has polarity, which can well dissolve scintillator and samples with more water content and improve counting efficiency. However, it has the disadvantages of high price and high freezing point, and peroxide with strong quenching effect will be produced after long-term release. It must be purified before use and 0.00 1 should be added.

So as to inhibit deterioration of the purified dioxane. The solvent in scintillation liquid accounts for about 99%, so its purity is an important factor affecting the quality of scintillation liquid. The contents of non-luminous impurities, oxygen and water in the solvent are related to the quenching degree. In principle, the solvent should have scintillation purity, that is, it contains no or little quenching components that affect scintillation counting. It has been proved that "analytically pure" reagents can be used directly without purification.

⑵ Scintillator: In the liquid scintillation counting system, scintillator, also called phosphor, is the solute of scintillation liquid. According to their fluorescence characteristics and functions, they can be divided into two categories, namely, the first scintillator and the second scintillator.

① First scintillator: (primary scintillator): Common first scintillator: triphenyl (TP): Chemical structure It is one of the earliest scintillators. Its counting rate is high and its price is relatively cheap, but its dielectric performance is not high at low temperature or in aqueous solution. 2,5-Benzoxazole (PPO): Chemical Structure It is a widely used scintillator at present. It can be well dissolved in common solvents, even in the case of water. The solubility in toluene is above 200 g/L, the chemical properties are stable and the price is relatively cheap. But its biggest disadvantage is obvious concentration quenching (self-quenching), that is, with the increase of PPO concentration in the solvent, the counting efficiency decreases. 2- phenyl -5-(4- diphenyl)-1, 3,4- oxazole (PBD): Its chemical structure makes it one of the most effective scintillators known. Compared with PPO, it can tolerate concentration quenching, but its solubility is low, especially at low temperature and in the presence of water-containing samples, and its dosage is twice that of PPO, which is expensive. 2-(4- tert-butylphenyl) -5-(4- diphenyl)-1, 3,4, oxadiazole (butyl -PBD): Its chemical structure is higher in solubility than PBD, and its biggest advantage is that it is insensitive to chemical quenching and color quenching, so it can obtain higher counting efficiency. ② Second scintillator (secondary scintillator): The main function of the second scintillator is to absorb the photons emitted by the first scintillator, and then re-emit the fluorescence with a longer wavelength band, which can increase the photon yield. At a high concentration, the second scintillator plays a part of the same function as the first scintillator (that is, it receives the quenching energy of the excited solvent molecules and emits fluorescence), and in addition, it can compete with the quenching factor, thereby reducing the degree to which the first scintillator is quenched. In one or more of the following cases, the second scintillator must be added to the scintillation liquid: a. The sample contains a compound that directly quenches the first scintillator; B, the concentration of the first scintillator is too high, which causes strong self-quenching, and the emitted spectral range does not match the photomultiplier tube; C. The photocathode of the photomultiplier tube of the counter has better spectral response to longer wavelength; D. The tested sample has obvious absorption in the near ultraviolet region.

The commonly used second scintillator is 1, 4,2-bis (5- phenyloxazole) benzene (POPOP). Its solubility is low, which is 65438 0.2g/L in toluene system and 65438 0.5g/L in dioxane. Because of its slow dissolution rate, heating is usually needed to promote its dissolution. It is the second scintillator widely used at present. 1, 4 bis 2(4- methyl -5- phenyloxazolyl)-benzene (DMPOPOP): Its solubility is higher than that of POPOP, which is 2.3g/L in toluene series and 0.8g/L in dioxane, and its dissolution speed is also fast, but it is not as efficient as that of POPOP and needs higher concentration. In addition, there are p-bis (0- methylphenylethyl) benzene, (bis-MSB) and 2-(4'- biphenyl) -6- phenylbenzoxazole (PBBO). The fluorescence wavelength of several commonly used primary scintillators is between 3460 and 3800 angstroms, while the maximum spectral response wavelength of Cs-sb photocathode is 4000. Therefore, for the photocathode of Cs-Sb material, only the primary scintillator can not transfer energy well, and the counting efficiency is very low. After adding the secondary scintillator, the emission spectrum wavelength increases to 4 180-4300 angstrom, which improves the spectral response with Cs-Sb photocathode, better energy transfer and higher counting efficiency. Cs-K-Sb is a double-base photomultiplier tube, and its maximum spectral response wavelength is shorter than that of Cs-Sb. Therefore, better counting efficiency can be achieved without the secondary scintillator. However, considering the functions of secondary scintillator, secondary scintillator is usually used in practical work.

In addition to the solvent and scintillator, some other components are sometimes added to the scintillation liquid. In order to increase the solubility of scintillation liquid to aqueous samples, cosolvent should be added; In order to improve the counting efficiency, an anti-quenching agent is added. Organic solvents such as toluene and xylene are very polar and have poor solubility in water. When the sample contains more water, it is difficult to dissolve xylene in toluene into a transparent homogeneous mechanism even if the sample volume is small. Sometimes, although the water content of the sample is not large, its radioactivity level is very low. In order to obtain the count that meets the requirements of statistical error in a short measuring time, it is often necessary to increase the volume of the sample, which is equivalent to increasing the moisture content, and such samples are not well miscible with toluene or xylene. Therefore, a certain amount of polar organic solvents, such as methanol, ethanol, glycol ether, etc. , should be added. These solvents act as a bridge between nonpolar solvents and water molecules, and are miscible with toluene and xylene. \par cosolvent has a great quenching effect, so its dosage should be limited, so the water content it can contain is also limited. Among them, ethylene glycol ether is a common cosolvent because of its high polarity and small chemical quenching effect. Quenching agents are usually used for the measurement of samples with high water content, or when dioxane is used as a solvent. Because naphthalene has a strong quenching effect, it is very important to add an anti-quenching agent naphthalene to improve the counting efficiency. Naphthalene is also a fluorescent substance, which can counteract some quenching effects, but naphthalene can't be used with terphenyls, especially in toluene and xylene solvents, otherwise the counting efficiency is very low. In liquid scintillation counter, the optimal volume of scintillation liquid can be changed within a certain range. In order to obtain high counting efficiency, a smaller volume should be used. Especially for the 3H sample, the smaller volume of scintillation liquid can also reduce the background count (about 0.5 CPM/ml scintillation liquid) and reduce the self-absorption of the sample. When the sample contains quencher, if the volume of scintillation liquid increases, quenching can be reduced by dilution.

3. Detection device

It is very important to introduce a very sensitive photomultiplier tube into liquid scintillation counting for detecting α rays with low penetration and β rays with low energy (such as 3H, 14C, etc.). ). Single photomultiplier tube liquid scintillation counter using photomultiplier tube will produce higher background count and lower detection efficiency due to the thermal noise of photomultiplier tube and the phosphorescence emitted by the sample after being irradiated by light. The double-tube coincidence liquid scintillation counter is composed of two photomultiplier tubes with roughly the same performance index and a coincidence circuit. The coincidence circuit can only pass the signals generated by two photomultiplier tubes at the same time, so only the signals observed by the two photomultiplier tubes at the same time within the resolution time of the coincidence circuit are recorded, and the random pulses generated by thermal noise or phosphorescence are deducted, which effectively reduces the instrument background and improves the detection efficiency. The detection efficiency of the system can reach more than 50%. In the liquid scintillation counting system, the pulse voltage formed by the anode of photomultiplier tube has a linear relationship with the number of electrons collected by the anode at one time. When the magnification of the photomultiplier tube is constant (depending on the stability of high voltage), the more photoelectrons produced by the photocathode, the more electrons finally reach the anode, and the number of photoelectrons depends on the number of photons. Under normal circumstances, the number of photons released by scintillator molecules is directly proportional to the β -ray energy produced by radioactive isotope decay. Because radiant energy will be consumed more or less in the process of transmission and energy conversion, there is an approximate linear relationship between radiant energy and the number of photons emitted. This shows that the liquid scintillation meter can do energy spectrum research and analyze radioactive isotopes with different energies to achieve qualitative purposes. For example, 3H, 14C

Dual-channel liquid scintillation counter can simultaneously determine double-labeled samples. The number of pulse voltage generated by anode in unit time is linear with the number of radioisotopes in scintillation bottle and isotope decay rate, and proportional to the radioactive intensity in the sample, which is the quantitative basis for liquid scintillation measurement. For example, on the premise of knowing the detection efficiency of liquid scintillation counter, by measuring a radioactive sample, we can know how many micro curies or how many Bekkerel the radioactive intensity in the sample is.

4. Application of Double Labeled Isotope Measurement

One of the characteristics of liquid scintillation counter is that it can do double isotope analysis, and it is equipped with two or more independent pulse height analyzer multichannel devices, pulse addition and linear gate devices. Under the best counting condition of each isotope, isotopes emitting different energies can be distinguished. Assuming that a sample contains 3H and 14C, we adjust the channel 1 in the multichannel device of the pulse amplitude analyzer in the instrument to the equilibrium point of 3H (the best working condition), and the 3H and 14C standard samples are dissolved in the same solvent, using the same scintillator as the experimental samples. Firstly, the blank samples are measured, and then the experimental samples and standard samples are counted.

In order to make the double-label measurement successful, the β spectra of the two radioisotopes must be different enough to meet the separation requirements of pulse height analysis. When the energy spectra of two isotopes are too close, such as 14C and 35S, they must be chemically separated and counted separately. In the double standard measurement, the commonly used paired isotopes are 3H and 14C, 3H and 35S, 3H and 32P, 14C and 32P. In short, in the measurement of double isotope labeling, the following two conditions should be met: first, isotopes with higher energy can be counted as much as possible without interference from isotopes with lower energy; Secondly, an optimal condition is selected to calculate the energy of low-energy isotopes in double-labeled samples.

5. Preparation of liquid scintillation counting samples

The preparation of liquid scintillation measurement is a very important operation, and the success of the operation directly affects the counting efficiency. The following four factors should be considered in the selection of sample preparation method: (1) the physical and chemical characteristics of the sample to be tested, the type of scintillation liquid used and whether it is necessary to convert the sample into a more suitable form for measurement; ⑵ Attention should be paid to the types of isotopes contained in the samples and the samples containing 3H; (3) When the radioactive intensity of the sample is low, the expected radioactive level requires strict preparation methods; ⑷ The economy and convenience of the preparation process, especially when the number of samples is large. The general principle is that the radioactivity of prepared samples must reach appropriate statistical accuracy within a short measurement time, and the most important thing is to reduce the "quenching" factor as much as possible in the process of sample preparation.

Preparation of (1) homogeneous sample

Fat-soluble samples can be directly added to the scintillation liquid of toluene and xylene system. For samples with water content less than 3%, the scintillation liquid of toluene and xylene system is still used, but polar solvents such as ethanol, methanol or glycol ether need to be added to help dissolve, and the ratio of cosolvent to toluene is usually 3:7. If necessary, partial quenching effect is cancelled to improve counting efficiency. When the water content is high, it is best to use 100 ml ethylene glycol ether. 20ml of ethylene glycol, 8 g of POPOP 500mg of popop, naphthalene 150g, and finally added to 1l with dioxane. This formula contains a lot of water and is quite efficient. However, it should be noted that dioxane is easy to form peroxide, which will lead to chemiluminescence, so it should be stored away from light, or zinc particles or other antioxidants should be added during the storage process to remove peroxide.

⑵ Preparation of heterogeneous samples

① emulsion count: Triton X- 100, surfactant, which is a widely used emulsifier. Its chemical structural formula: its hydrophilic end attracts polar molecules such as water, and its hydrophobic end attracts nonpolar molecules such as toluene. The physical properties of emulsion change with the increase of water content. When the formula of toluene scintillation liquid and Triton X- 100 is 2: 1 (v/v), the emulsion with water content below 15% is transparent. With the increase of water content, two different phases will appear. The separated emulsion is unstable and cannot be used for measurement. When the moisture continues to increase, a stable emulsion is formed, and the liquid is transparent or opaque at this time. The phase separation of emulsion is related to temperature. When the temperature drops from 17℃, the counting efficiency increases linearly by about 10%, reaching the maximum value between 4-0℃. When the temperature decreases, the counting efficiency no longer increases. Usually, the emulsion is heated to 40℃ first, then cooled without oscillation and kept at 4℃ for 2-4 hours. The different distribution of solute between organic phase and water phase is the key to determine the efficiency of emulsion measurement and counting. The efficiency of emulsion measurement is sometimes higher than that of homogeneous measurement, because quenching substances mainly remain in the water phase and do not affect the energy transfer process in the organic phase. In homogeneous solution, all the components in the system are in close contact with each other, so any quenching effect can be shown.

② Suspension measurement: For samples such as inorganic salts with extremely low solubility in toluene-based scintillation liquid, gel technology can be used to form suspension measurement liquid. After pretreatment, the sample was made into particles of the same size, and then it was made into suspension in a system containing gel. For suspension measurement, the following requirements are necessary: ① solid materials should be well crushed, and white or colorless uniform powder particles are required to avoid light absorption; (2) It is required that the sample is really insoluble in scintillation liquid, otherwise the counting efficiency of dissolved and insoluble parts is different, which leads to unstable counting and difficult to repeat the results. The advantage of suspension measurement is that the sample is insoluble in solvent, so the sample quenching is minimal. In suspension measurement, aluminum stearate and castor oil derivative (thixin) are used as gelling agents.

And silica particles (Cab-o-sil). For the suspension containing 3.5 ~ 4.0% Cab-o-sil, in order to obtain higher counting efficiency, Cab-o-sil can also reduce the adsorption of radioactivity on the wall of counting bottle. Generally, when preparing samples, cab-o-sil is often added first, and then radioactive samples are added, so that radioactivity can be adsorbed on suspended particles more and counting efficiency can be improved. Suspension measurement can be used not only for the determination of solid inorganic salts, but also for aqueous solutions and tissue homogenates, and also for the radioactive measurement of thin layer chromatography. When in use, only the chromatographic substances need to be crushed and simply mixed with the gel. If the analyte can be partially eluted from the chromatographic support and dissolved in the scintillation liquid, this method cannot be used.

③ Bracket measurement: Similar to suspension measurement, any sample insoluble in scintillation liquid can be placed on the bracket and then immersed in scintillation liquid for counting. There are many kinds of supports, such as paper, filter paper, glass fiber filter paper, acetate fiber film and so on. The position of the bracket in the counting bottle has a direct influence on the counting. Usually, the bottom of the bottle is flat when measuring, and the diaphragm does not exceed the flash liquid level. Keeping the bracket and measuring cup dry can obtain high counting efficiency and measurement repeatability. In addition to the small quenching effect, the measurement of stent has an outstanding advantage, that is, more samples can be measured at one time. Because in the same measuring bottle, with the increase of the number of overlapping diaphragms (within 10), the counting rate increases linearly and the counting efficiency remains unchanged, which is very suitable for the measurement of water-containing samples with low radioactivity level. \ Among the supports above \par, cellulose acetate film and glass fiber filter paper are better than ordinary filter paper, because ordinary filter paper is almost opaque to photon propagation, so the counting efficiency is very low.