GB/T 11743-1989 Gamma spectrum analysis method for radionuclides in soil
Some standard content:
National Standard of the People's Republic of China
Gamma speciromery method of analysing Tadianuchides in soil
1 Subject and scope of application
GB11743-89
This standard specifies the conventional method for analyzing the specific activity of natural or artificial radionuclides in soil using high-resolution semiconductor or Na1(11) spectrometers.
This standard is applicable to the analysis of the specific activity of radionuclides in soil using a laboratory spectrometer. The counting rate of the sample to be tested is less than 10°cpm, and the activity should be higher than the detection limit of the spectrometer. 2 Terminology
2.1 Half width fullwidth at half maximum The distance between two points at half the peak height of a distribution curve consisting of a single peak on the spectrometer in units of energy or channel number. 2.2 Energy resolution encrgyresolutian For a given energy composition, the minimum relative difference between two digital energies that a radiation spectrometer can distinguish. In general applications, energy resolution is expressed by dividing the half-width of the peak in the energy distribution curve measured by the spectrometer for monoenergetic particles by the energy corresponding to the position.
2.3 Background count rate backgrounal countrate In the energy spectrum, the count rate caused by other factors except the radioactivity of the sample. 2.4 Detection limit lower limit of detection The lowest activity that a spectrometer can detect under a given confidence level 2.5 Detector efficiency detection efficiency The ratio of the number of particles measured by the detector to the number of particles of this type incident on the detector in the same time interval. 2.6 Detection efficiency detection efficiency Under certain detection conditions, the ratio of the number of particles measured to the total number of particles of this type emitted by the radiation source in the same time interval. 3 Instruments and Devices
3-Y Spectrometer
3.1.1 Detectors
3.1.1.1 Sodium Iodide Detector Na1(T1)1: A cylindrical Na1(TI) detector with a size of not less than 7.5cm×7.5cm can be used to measure soil samples. It is best to use a low-potassium Na1(TI) sample and a low-potassium photomultiplier tube. The entire sample is sealed in a sealed container with a light-transmitting window. The sample and the photomultiplier tube form an optical coupling medium. The resolution of the detector for 661.6keV light drop of 13s should be less than 9%. 3.1.1.2 Semiconductor detector: Semiconductor detectors of different materials and types can be used according to the energy range of the rays. The best sample for the measurement is a single-ended coaxial lead lithium or high-purity germanium detector: its energy resolution for °Co1332.5keV rays should be less than 3kcV, and the relative detection efficiency should be not less than 15%.
Approved by the Ministry of Health of the People's Republic of China on October 6, 1989 and implemented on July 1, 1990
3.1.2 Shielding
GB1174389
The detector device should be placed in a metal shielding room with an equivalent lead equivalent of not less than 10cm. The distance between the inner wall of the shielding room and the surface of the sample is greater than 13cm. On the inner surface of the lead room, there should be a multi-layer inner shielding material with a gradually decreasing atomic yield, such as: 0.4mm copper, 1.6mm pick and 2-3mm thick organic glass. The shielding room should have a door or hole for taking and placing samples. 3.1.3 High-voltage power supply
There should be a high-voltage power supply to ensure the stable operation of the detector, and its ripple voltage should not exceed 0.01%. For semiconductor detectors, the high voltage should be continuously adjustable from 0 to 5kV without any discontinuity.
3.1.4 Spectrum amplifier
There should be an amplifier with waveform adjustment that matches the preamplifier and pulse height analyzer. 3.1.5 Pulse height analyzer
The number of channels of the NaI (T1) spectrometer should be no less than 256, and the number of channels for high-resolution semiconductor spectrometers should be no less than 4096. 3.1.6 Readout device
There are a variety of readout devices to choose from. For example: recorder, XY plotter printer, image digital terminal, etc. 3.1.7 Analysis and calculation device
The spectrum system can be connected to a computer such as a special machine or microcomputer to process the spectrum data through the computer. 3.2 Measurement container
Select sample boxes of different sizes and shapes according to the amount of samples and the shape and size of the detector, such as: a cylindrical sample box with a narrow bottom equal to or smaller than the detector diameter or a ring sample box matching the detector size. The container should be made of plastic with low natural radionuclides content, such as A or polyolefin.
4 Spectrometer Calibration
4.1 Energy Calibration
Use a calibration source containing known nuclides to calibrate the energy response of the energy spectrum system. The energy calibration range should be from 50 to 3000 kcV. The monoenergetic or polyenergetic nuclides suitable for energy calibration are: 21Am (59.5 keV), 13Ba (81.0 keV), aCd (88.0 keV), Co (122.1 keV), ulCe (145. 4 keV), 51Cr (320. 1 kcV), 137Cs (661. 6 keV). 1Mn (834. 8 ke:V), 60co (i 173. 2 keV, 1 332. 5kcV). 20FTI (2614.7kcV) and Eu that emits multiple rays, etc. The energy scale includes at least 10 scale points with energy uniformly distributed in the required scale energy range, and records the characteristic ray energy and the corresponding total energy peak position of the calibration source. You can draw a graph on a straight line or fit the data with a minimum straight line or parabola. If the nonlinearity exceeds 0.5%, the product analysis should not be performed. The stability of the instrument is good, and the possibility of change is small.
4.2 Efficiency scale
The total energy peak efficiency is the ratio of the total energy peak area (count rate) of the observed specified energy formula to the number of photons emitted by the photon. For soil samples, uranium, radium, chrysene, and potassium standard sources are required for efficiency calibration. The standard source used for efficiency calibration should have a geometric shape that is alternate with the sample being tested, and the matrix density and effective atomic number should be as close as possible to the sample being tested. The detection efficiency of the detector for the rays incident on it is a function of the ray energy. After finding the full energy peak detection efficiency of monoenergetic gamma rays of different energies, the relationship between detection efficiency and ray energy can be drawn in the coordinate dimension (efficiency curve) or the weighted least squares curve fitting of the experimental points can be used by computer to obtain the efficiency curve. In the range of 503000keV, the use of logarithmic polynomial fitting can provide good results. The expression is as follows: In the formula (1): where: - experimental efficiency value:
E, corresponding ray energy;
a. - fitting constant.
The uncertainty of the efficiency calibration should be less than 2%. 5. Preparation of bulk standard source
GB11743-89
? Bulk standard source used for energy spectrum efficiency calibration must meet the requirements of good uniformity, accurate nuclide content, stability, and sealing. Bulk standard source is made by mixing a simulated matrix with a standard solution or standard mineral powder of a certain specific nuclide, and the inhomogeneity is less than 2%. 5.! Select materials with low radioactive background, easy to mix evenly, and similar density to the sample to be tested as the simulation matrix. For the bulk standard source of soil sample crystals with a filling density of 0.8~1.6 μm/cm, a certain proportion of zirconium oxide and silicon dioxide is the most suitable as the simulation matrix. 5.2 The activity of the bulk standard source should be moderate, generally 10 to 30 times that of the sample to be tested. The specific multiple depends on the amount and strength of the sample. 5.3 The prepared bulk standard source should be placed in a sample box and sealed for 3 to 4 weeks to allow uranium, radium and their short-lived daughters to reach equilibrium before measuring the mother.
5.4 The total uncertainty of the bulk standard source should be controlled within +5%. 6. Sample preparation
The soil sample, after removing foreign matters such as debris and gravel, is dried at [00℃ to constant weight, crushed and sieved (40-~60! months), weighed, and then placed in a sample box of the same shape and volume as the calibration source, sealed, and left for 3 to 4 weeks before measurement. 7. Measurement
The simulated matrix background spectrum and the empty sample box background spectrum should be measured. When calculating the net area of the full energy peak of the standard source, the full energy peak count of the standard source should be subtracted from the corresponding simulated matrix background count, and the full energy peak count of the soil sample should be subtracted from the corresponding empty sample box background count. When measuring the calibration source, its position relative to the detector should be at the same time as the measurement of the soil sample: the measurement time is determined according to the strength of the measured standard source or sample. The statistical error of the measurement count of the standard source should be less than ± 2%. The statistical error of the counting of radionuclides in the soil sample requires that uranium is less than ± 20%, radium, proton, potassium is less than + 10%, and 137C is less than ± 1!%. The confidence level is 95%.
8Spectrum analysis method
8.1. Determine the specific activity of ten radionuclides in the sample by relative comparison method: Use various computer spectrum analysis methods such as total peak product method, function fitting method and least square fitting method to calculate the total energy peak area of each characteristic light peak in the standard source and sample spectrum. Then calculate the calibration coefficient of each standard source according to formula (2):
The activity (Bq) of the th nuclide in the standard source is C, which is the total energy peak area of the th characteristic peak of the th nuclide in the standard source (counts/kg). Then the specific activity (Bq/kg) of the th nuclide in the measured sample is: C(AA)
Where: Agh—the total energy peak area of the th characteristic peak of the th nuclide in the measured sample, counts/s; Agh—the background count rate of the light peak corresponding to Agh, counts/s; W—the net weight of the measured sample, ks;
D,—the relative correction coefficient of the th nuclide corrected to the sampling time. This method is applicable to the case where a standard source of the nuclide to be measured is available. (2)
(3)
8.2 Determine the specific activity of the nuclide in the sample from the efficiency curve. According to the efficiency curve after efficiency calibration or the fitting function of the efficiency curve, the efficiency value corresponding to a certain energy gamma ray is obtained, and then the specific activity of the nuclide is calculated using formula (4), (Bq/kg) GB 11743 89
A,— A
, = P,-nWD,
Where: the efficiency value corresponding to the characteristic peak of the first ray; P,—the probability of the jth nuclide emitting the i-th ray. The meanings of An.Am, W and D, are the same as those described in 8.I. This method is applicable to the case where there is no standard source of the nuclide to be measured but the efficiency curve is available. 64
8.3 Determine the nuclide content using the inverse matrix method. The specific activity of natural radionuclides 23*U, 232Th, 2Ra, \K and fission product 137Cs in soil can also be solved by the inverse short array method and the U content can be calculated more accurately. Using this method, the response matrix must be determined first. All standard source nuclides that determine the response short array must include all the nuclides to be determined in the sample to be determined. The characteristic channel areas selected for different nuclides must not overlap. The original measurement for the characteristic channel area selection is:
. For nuclides that emit multiple energy rays, the characteristic channel area should select the ray full energy peak area with the largest branching ratio; b. If the emission probability of the rays of several energies is similar, then the gamma-ray peak area with less energy contribution from other nuclides should be selected;
c. If the ray peaks with the highest emission probability of two nuclides overlap, then one of the nuclides can only take its secondary gamma-ray as the characteristic peak:
d. The selection of the characteristic channel width is to minimize the drift effect of the multi-channel analyzer and the overlap of adjacent peaks. The correct selection of the characteristic channel area is the basis for the inverse matrix method to analyze the energy spectrum. The inverse matrix is used to solve the specific activity of radionuclides in the soil. Generally, the characteristic channel areas selected for each nuclide are 92.6keV (28U), 32keV or 609.4keV (22Ra), 238.6keV or 583.1keV (22Th), 1460.8keV (\K) and 661.6keV (1\Cy). After obtaining the net counting rate of a certain characteristic area in the spectrum of a mixed sample of multiple nuclides, the specific activity of the th nuclide in the sample (Bg/kg) can be calculated as shown in formula (5):
x,=WD,
Q:=WD,
-- 1.2,--.7
Formula: a,=WD,
Q:=WD,
-- 1.2,--.7
Formula: a,=WD,
Q:=WD,
-- 1.2,--.7
Formula: a,=WD,
Q:=WD,
-- 1.2,--.7
Formula: a,=WD,
Q:=WD,
-- 1.2,--.7
Formula: c,=WD,
Q:=WD,
-- 1.2,--.7
Formula: a,=WD,
Q ... -- 1.2,--.7
Formula: a,=WD,
Q:=WD,
-- 1.2,--.7
-- 1.2,--.7
-- 1.2,--.7
-- 1.2, 8.4 When analyzing the spectrum using the method in 8.1 or 8.2, 352.0, 609.4, 1120.4, 1764.7 keV can be selected for 226Ra, and 238.6, 583.1, 911.1, 2614.7 kcV can be selected for 1232Th. Their results should be consistent within the counting error range. The specific activity of 226Ra and 1232Th can be given by the average of the results of the selected characteristic light peaks. 8.5 Interference and influencing factors
When determining a nuclide in a mixture containing complex radiators, the degree of interference of other nuclides is determined by the individual nuclides. If multiple nuclides can be considered to exist in approximately equal proportions from the perspective of radiometric measurement, then interference will occur when the light peaks cannot be completely resolved. If multiple nuclides are considered to exist in unequal proportions in the mixture from the perspective of radiometric measurement, and the nuclear system with higher energy is dominant, then there will be serious interference in interpreting the small peaks of lower energy in the spectrum. For example: the main radiation of the system is 92.6keV of 23\Th, but the system has a 93.4keV X-ray. When the nuclide content of the sample is ionized, the 93.4keV X-ray peak will seriously interfere with the 92.6keV peak of the system. The background of the spectrometer system is another important interference factor. In addition, the density of the sample also affects the analysis results. Measures such as repeated peak analysis, shielding, background reduction, and density correction can be taken to reduce the influence of various factors on the results. Report
GB 11743-89
9.1 When reporting the analysis results of soil samples, the specific activity and corresponding counting standard deviation of all nuclides in the sample whose activity exceeds the detection limit should be reported, and the confidence level (95% confidence level) adopted should be indicated. Others such as spectrum analysis error and calibration error also need to be noted in the report. The sample standard deviation (%) can be calculated using formula (6): 9.2
wherein: N, the number of peak or channel area counts; N, the corresponding background counts;
t, sample counting time:
—background counting time.
-+++++++-( 6)
GB 1174389
Appendix A
? Inverse matrix method in energy spectrum analysis
【Reference）
In the energy spectrum of a mixed sample of multiple nuclides, the counting rate of a certain energy peak characteristic channel area, in addition to the contribution of the nuclide corresponding to the peak, is also superimposed with the Compton contribution of the radiation of the nuclide emitting higher energy rays, and the photoelectric peak contribution of other isotope rays with similar energy bases. Therefore, after deducting the empty sample box negative from the spectrum of the mixed radiator, the counting rate of a certain energy peak channel area should be the sum of the contributions of each nuclide in the channel area, that is:
Where: 5
-the serial number of the nuclide in the mixed sample;
the serial number of the characteristic channel area:
the total number of nuclides contained in the mixed sample i= J,2,.-.m
C, the count rate of the mixed sample spectrum in the :th characteristic channel area: X, the unknown intensity of the th nuclear disorder in the sample; a.
the response coefficient of the th characteristic channel area to the 5th nuclide can be calculated according to formula (A2)1,
formula (A1) can be:
the count rate of the th nuclide standard spectrum in the :th characteristic channel area soil decay number (activity) of the th isotope standard source x, =
j=1,2,.**,m
（A2）
The response matrix (, can be determined by experiment, and the inverse matrix a, can be obtained. Therefore, the counting rate of the characteristic channel area of each response of the sample can be measured to calculate the activity of various nuclides. When the soil contains natural radionuclides and Cs, the inverse matrix program of 5 characteristic channels can simultaneously calculate the activities of 28U, 22Ih, 3Ra, \K and 13\Cs in the soil. Appendix B
Detection limit of Y spectrum analysis
(reference)
Since the specific activity of radionuclides in soil is generally very low, the spectrum analysis of radionuclides in soil is carried out under the existence of strong random interference. Weak radioactivity measurement. The lower detection limit (Lower limit Dclecion, or LLD) of an energy spectrum is the lowest activity that the system can confidently detect under a given confidence level. The lower detection limit can be approximately expressed as:
LLD(K.K)S
(B1)
where K. -
GB11743—89
The upper percentile value of the standard normal variable corresponding to the pre-selected risk probability of misjudging the presence of radioactivity (a); the value corresponding to the pre-selected confidence level (1-B) of the detected radioactivity; the standard deviation of the net sample radioactivity.
If the α and β values are at the same level, then K. -K,, LLD ^ 2 KS.
If the total sample radioactivity is close to the background, it can be further optimized: LLD22V2K
Where:
Background spectrum measurement time:
-background counts corresponding to a certain full-energy peak in the background spectrum. For different α values, K values are shown in Table BI. B
·(B2)
（B3)
In formula (B3), the detection limit is based on the counting rate. Considering the nuclide characteristics, detection efficiency, and sample. That is, converting the counting rate into the lower detection limit expressed in activity
Appendix (
? Table of natural radionuclides with ray emission rate greater than 1>% (dream)
Energy, key
Emission rate, %
Half-life
Generation method
Energy, kev
CB 11743: :89
Continued Table C1
Probability, %
Half-life
71x10'
71×10
71×10a
71x10'g
16. (12×10abzxz.net
Generation method
Energy.kev
1 499. CL
1 631. 01
GB11743—89
Continued Table CI
Child rate, %
Note: ① In the half-life column, L indicates the donor of long-lived human radionuclides ② In the energy column, D indicates double line, T indicates triple line, X indicates Indicates X-rays. Half-life
Generation method
292 Th
Additional remarks:
This standard was proposed by the Department of Health Supervision, Ministry of Health. GB
11743—89
This standard was drafted by the Industrial Hygiene Laboratory, Ministry of Health. The main drafters of this standard were Zhang Shurong and Ren Tianshan. This standard was interpreted by the Industrial Hygiene Laboratory, Ministry of Health, the technical unit entrusted by the Ministry of Health.
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