CN106290429B - A kind of PGNAA characteristic gamma rays power spectrum backoff algorithm - Google Patents

Abstract

The invention discloses a kind of PGNAA equipment characteristic gamma ray power spectrum backoff algorithms.The principle of the algorithm is the proportionality constant k by the way that measured matter is calculated_Ei(i=1~1024), by the band information carrying N, the airborne signals N0 that are mounted on the ionisation chamber (6) on PGNAA equipment (1) subsequent controlled nucleon operated belt conveyor scale, operation is compensated to characteristic gamma ray power spectrum, solves the power spectrum distortion phenomenon caused by tested material own absorption characteristic gamma ray.For PGNAA equipment characteristic gamma ray during penetrating tested material arrival gamma-ray detector, part gamma ray can have the phenomenon that distortion by tested material own absorption, finally obtained characteristic gamma ray power spectrum.

Description

A PGNAA Characteristic Gamma Ray Spectrum Compensation Algorithm

Technical field:

The invention relates to an energy spectrum compensation algorithm, in particular to a PGNAA characteristic gamma ray energy spectrum compensation algorithm.

Background technique:

Prompt gamma neutron activation analysis (PGNAA) is a rapid, non-contact multi-element analysis technique, which has been widely used in building materials, coal, thermoelectricity, metallurgy, mining and other industries. The principle of the prompt gamma neutron activation analysis technology is that the measured material reacts with thermal neutrons to generate characteristic gamma rays, which pass through the measured material and reach the gamma ray detector to form a nuclear pulse signal. The pulse signal is converted into a digital signal through a multi-channel processor, and finally forms a characteristic gamma ray energy spectrum. When the characteristic gamma ray penetrates the measured material, part of the rays will be absorbed by the measured material itself, so that the final characteristic gamma ray energy spectrum cannot represent the real characteristic gamma ray energy spectrum. When the total amount of the measured material occurs When , the attenuation of characteristic gamma rays will also change, and the finally obtained gamma ray energy spectrum will be distorted.

Invention content:

In order to solve the problem of characteristic gamma ray energy spectrum distortion, as shown in Figure 1, the present invention installs a nuclear belt scale after the PGNAA equipment. The nuclear belt scale is composed of a ¹³⁷ Cs radiation source and an ionization chamber. The signal transmission of the ionization chamber Compute the host for the PGNAA device.

Theoretically, the more the measured material is, the more characteristic gamma rays will be generated by neutron activation. However, since the material itself also has an attenuation effect on gamma rays, the characteristic gamma rays collected by the detector will also increase with the increase of materials. e exponential decay:

I _Ei ＝I0 _Ei ×exp(-μ _mEi t _m ) (1)

In the formula, IO _0i represents the intensity of the characteristic gamma ray with energy Ei, I _Ei represents the intensity of the gamma ray with energy Ei after passing through the material, and _μmEi represents the quality of the characteristic gamma ray with energy Ei in the material The attenuation coefficient is related to the atomic number of the material and the energy of the ray, and t _m is the mass thickness of the material. It can be seen from formula 1 that if μ _mEi and t _m are known, the attenuation degree of characteristic gamma rays can be quantified and described, so that the compensation operation of restoring the measured characteristic gamma ray amount to the real characteristic gamma ray amount can be performed mathematically.

The present invention installs a nuclear belt scale behind the PGNAA equipment, adopts ¹³⁷ Cs radiation source, ionization chamber transmission structure, and the nuclear belt scale follows the following mathematical relationship:

N＝N0×exp(-μ _m0 t _m ) (2)

In the formula, N represents the ionization chamber count when there is material, N0 represents the ionization chamber count when the belt is empty, μ _m0 represents the mass attenuation coefficient corresponding to the ¹³⁷ Cs radioactive source, then it can be deduced Substituting t _m into Equation 1, we can get:

In the formula When the material composition does not change much, the proportionality coefficient is approximately constant, so we only need to obtain the constant k _Ei , and the attenuation degree of the characteristic gamma ray energy spectrum can be obtained by counting N and N0 in the ionization chamber, and then the energy The spectrum is corrected.

The relationship between the mass attenuation coefficient and the linear absorption coefficient is as follows:

μ _m = μ/ρ (4)

Where μ _m is the mass attenuation coefficient, μ is the linear absorption coefficient, and ρ is the material density.

The linear absorption coefficient μ of a substance is defined as follows:

In the formula, N _i represents the atomic number density of the i-th element in the substance, is the reaction cross-section between the gamma photon of the i-th element and the element, and m indicates that the substance is composed of m elements.

The calculation formula of N _i is as follows:

In the formula, m _ρ represents the mass density of the substance, M _i represents the atomic weight of the element, and f _i represents the proportion of the i-th element in the substance (after normalization).

In this way, as long as we know the approximate element composition of the substance to be measured, according to the curve of the reaction cross section of gamma photon and various elements-gamma ray energy, we can calculate the corresponding ¹³⁷ Cs radioactive source through formula 4, formula 5 and formula 6 The mass attenuation coefficient μ _m0 and the mass attenuation coefficient μ _mEi of the characteristic gamma ray with energy Ei in the material, and then obtain the relationship curve between the proportional constant k _Ei and the gamma ray energy, according to the ionization chamber count N, N0 and formula 3 The compensation and correction of the energy spectrum can be realized.

Beneficial effect:

By adopting the present invention, the characteristic gamma ray energy spectrum of the PGNAA equipment is compensated and calculated, the problem of distortion of the characteristic gamma ray energy spectrum is solved, and the detection accuracy of the PGNAA equipment can be improved.

Description of drawings:

Fig. 1 is a schematic diagram of the present invention, Fig. 2 is a graph of the photon section of each element, Fig. 3 is a graph of proportionality constant k _E after calculation, and Fig. 4 is a graph of characteristic gamma ray energy spectra before and after compensation.

1-PGNAA equipment, 2-PGNAA computing host, 3-conveyor belt, 4-measured material, 5- ¹³⁷ Cs radiation source, 6-ionization chamber.

The specific implementation method:

The specific implementation of the compensation algorithm consists of two parts: calculation of the proportional constant k _Ei matrix, and compensation operation.

1. Calculate the proportionality constant k _Ei matrix

Here we assume that the measured material is cement raw meal, assuming that the characteristic gamma rays are generated at the center of the thickness of the measured material, and the cement raw meal is mainly composed of CaCO ₃ , SiO ₂ , Al ₂ O ₃ , Fe ₂ O ₃ , H ₂ O composition, the general composition of each substance is shown in _Table 1, and the tested material mainly contains seven elements: H, C, O, Al, Si, Ca, and Fe. See Figure 2 for the photon cross-section curves of the elements.

Table 1 Component content of cement raw meal

substance CaCO ₃ SiO ₂ Al ₂ O _{3 Fe2O3 _} H ₂ O mass ratio 75% 15% 3% 2% 5%

Table 2 Atomic ratio f _i of cement raw meal elements

element Ratio f _i Ca 0.126127 Si 0.042042 Al 0.009892 Fe 0.004204 h 0.093427 C 0.126127 o 0.598181

According to the known photon cross-section data of each element, the atomic weight of each element, the ratio of each element, and Formula 6, Formula 5, and Formula 4, the mass attenuation coefficient μ _m0 corresponding to the ¹³⁷ Cs radiation source and the characteristic gamma ray with energy Ei can be calculated The mass attenuation coefficient μ _mEi in the material, and then obtain the proportionality constant k _Ei (i=1-1024, corresponding to 10keV-10.24MeV energy) matrix, the k _E curve is shown in Figure 3.

μ _m = μ/ρ (4)

2. Compensation calculation

Given the characteristic gamma ray energy spectrum I _Ei (i=1~1024), constant k _Ei (i=1~1024), and ionization chamber signals N, N0, the compensated characteristic gamma ray can be calculated by formula 3 Energy spectrum I0 _Ei (i=1~1024).

In the application of cement raw meal detection, the energy spectrum region we are more concerned about is 2.8MeV ~ 10.24MeV, so we only compensate this part of the energy spectrum in the calculation, as shown in Figure 4, the curve I is the measured cement raw material. The energy spectrum of the material (2.8MeV～10.24MeV is enlarged by 15 times), and the curve I0 is the energy spectrum curve after compensation calculation (the compensation calculation interval is 2.8MeV～10.24MeV, and the curve in this interval is enlarged by 15 times).

Claims (1)

1. a kind of PGNAA characteristic gamma rays power spectrum backoff algorithm, which is characterized in that include the following steps：

Step 1: according to measured matter elemental composition ratio, each element photon reaction cross-section and mass attentuation coefficient and linearly Absorption coefficient relationship, substance linear absorption coefficient define and calculate separately energy in 2.8-10.24 with the atom number density of element Mass attentuation coefficient of the gamma-rays of MeV in the various elements of measured matterWith¹³⁷The γ of 0.662 MeV of Cs radioactive sources Mass attentuation coefficient of the ray in the various elements of measured matter, according to formulaCalculate proportionality constant k_Ei,Wherein i=1 ~ 1024 of Ei；

Step 2: on controlled nucleon operated belt conveyor scale¹³⁷Cs radioactive sources are in ionisation chamber（6）Formed belt on have tested material band information carrying N and It is the airborne signals N0 of sky on belt；

Step 3: PGNAA equipment measures the neutron activation Characteristic γ ray power spectrum I of tested material on belt_Ei, wherein i=1 of Ei ~1024；

Step 4: utilizing formulaCalculate the Characteristic γ ray power spectrum after compensating approachIts I=1~1024 of middle Ei.