A passive efficiency calibration method with Monte Carlo simulation in segmented gamma scanning
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摘要: 针对200 L核废物桶分段γ扫描（SGS）过程中的效率刻度问题，提出了一种效率刻度函数模型，采用MCNP程序计算不同基质密度和γ射线能量条件下的离散断层效率，经过多元非线性回归获取函数参数，从而建立效率刻度函数，实现核废物桶SGS断层效率刻度。对核废物桶样品进行实验分析，结果表明：对于桶内基质分别为密度0.310 g·cm−3的硅酸铝、密度0.595 g·cm−3的木质纤维，桶内核素分别为活度3.110×105 Bq的点源137Cs、活度1.371×105 Bq的点源60Co，在桶内仅有单个点源存在的核素分布极端不均匀情况下，桶内核素活度重建误差在−37.68%～31.52%范围内。本文的方法能够准确有效实现核废物桶SGS断层效率矩阵计算，并确定核废物桶内放射性核素活度，满足实际检测要求。Abstract: In this work, an efficiency calibration function model is presented to calculate the efficiency of segment in segmented gamma scanning (SGS) for 200 L nuclear waste drum. Discrete SGS efficiencies are simulated with MCNP for different densities and gamma ray energies. Parameters of function are determined by using multivariate nonlinear regression method with the efficiencies. The SGS efficiency calibration function is constructed to calculate the efficiency matrix. Aluminum silicate with density of 0.310 g·cm−3, wood fiber of 0.595 g·cm−3, point sources 137Cs with activity of 3.110×105 Bq and 60Co of 1.371×105 Bq are used to construct samples of drum for SGS analysis. Result shows: for the extremely heterogeneous radioisotope distribution of only a point source placed at 8 different positions in the drum, errors of reconstructed activities are −37.68%～31.52%. Overall, the reconstructed activity is in agreement with the true activity. This method effectively and accurately achieves SGS efficiency calculation and reconstruction of activity.
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Key words:
- nuclear waste drum /
- segmented gamma scanning /
- Monte Carlo /
- efficiency calibration /
- function /
- MLEM algorithm
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图 8 两种刻度函数参数下活度重建结果对比
Figure 8. Comparison of reconstructed activity with two sets of parameters (F1 and F2 represent parameters calibrated by aluminum silicate and wood fiber respectively, as shown in Table 1)
表 1 效率刻度函数参数
Table 1. Parameters of efficiency function
calibrating material spacing parameter a1 a2 a3 a4 a5 a6 R2 aluminum silicate 0 layer between detector and segment −8.214 −0.322 −0.681 −0.034 0.101 −0.601 0.998 1 layer between detector and segment −8.376 −0.303 −0.870 −0.051 0.110 −0.564 0.998 2 layers between detector and segment −10.094 −0.015 −2.014 −0.014 0.224 0.346 0.996 wood fiber 0 layer between detector and segment −8.152 −0.312 −0.676 −0.031 0.102 −0.713 0.999 1 layer between detector and segment −8.341 −0.293 −0.888 −0.047 0.111 −0.637 0.998 2 layers between detector and segment −10.052 0.011 −2.098 −0.015 0.231 0.276 0.996 
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表 2 137Cs和60Co核素的重建活度和误差
Table 2. Reconstructed activities and errors of 137Cs and 60Co
samples
No.activity in aluminum silicate/Bq(error/%) activity in wood fiber/Bq 0.662 MeV 1.173 MeV 1.332 MeV 0.662 MeV 1.173 MeV 1.332 MeV 1# 2.869×105(−7.74) 1.479×105(7.89) 1.403×105(2.33) 1.938×105(−37.68) 1.119×105(−18.35) 1.102×105(−19.54) 2# 3.025×105(−2.74) 1.591×105(16.08) 1.532×105(11.77) 2.138×105(−31.25) 1.265×105(−7.68) 1.197×105(−12.6) 3# 3.096×105(−0.46) 1.646×105(20.08) 1.512×105(10.32) 2.235×105(−28.15) 1.273×105(−7.09) 1.211×105(−11.64) 4# 3.079×105(−0.99) 1.468×105(7.07) 1.396×105(1.79) 2.129×105(−31.56) 1.239×105(−9.6) 1.199×105(−12.48) 5# 3.134×105(0.78) 1.582×105(15.39) 1.513×105(10.36) 2.199×105(−29.3) 1.265×105(−7.64) 1.215×105(−11.34) 6# 3.243×105(4.29) 1.640×105(19.61) 1.568×105(14.34) 2.474×105(−20.44) 1.347×105(−1.7) 1.302×105(−4.93) 7# 3.445×105(10.78) 1.739×105(26.81) 1.640×105(19.66) 2.972×105(−4.43) 1.567×105(14.35) 1.463×105(6.81) 8# 3.680×105(18.32) 1.803×105(31.52) 1.745×105(27.24) 3.579×105(15.07) 1.794×105(30.97) 1.708×105(24.65) 9# 3.316×105(6.63) 1.646×105(20.07) 1.572×105(14.68) 2.670×105(14.13) 1.442×105(5.28) 1.377×105(0.54)
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