强干扰下的^(196)Au和44Sc^(m)放射性活度测量装置研制

Development of Radioactivity Measurement Device of ^(196)Au and ^(44)Sc^(m) under Strong Interference

在^(197)Au(n,2n)^(196)Au和^(45)Sc(n,2n)^(44)Sc^(m)的中子谱平均截面测量中,^(196)Au和^(44)Sc^(m)活度低、半衰期短,受其(n,γ)活化产物干扰严重。为准确测量^(196)Au和^(44)Sc^(m),放射性活度测量装置基于反符合原理采取了以下技术措施:首先,使用液闪测量体系探测^(198)Au和^(46)Sc衰变发射的β-射线,利用β-射线反符合抑制康普顿本底;其次,使用环形NaI晶体构建便携式的反康普顿谱仪,利用反康普顿原理抑制康普顿本底。该活度测量装置利用数字谱仪将上述两套反符合系统融合为一,取得了良好实验效果。对^(196)Au和^(44)Sc^(m)的MDA分别降低到原来的16.7%和36%,在0.2 MBq ^(198A)u和^(46)Sc的同位素强干扰条件下,MDA分别小于0.7 Bq和0.3 Bq。

In the spectral mean cross section measurements of 197 Au(n,2n)196 Au and 45 Sc(n,2n)44 Sc^(m),the activation products 196 Au and 44 Sc^(m) have low activity and shorter half-life,but the activation products 198 Au and 46 Sc from neutron capture reactions of 197 Au(n,g)and 45 Sc(n,g)have strong activity and longer half-life,which results in poor signal-to-noise ratio of gamma-ray characteristic peaks,and increases the measuring uncertainty of 196 Au and 44 Sc^(m) activity.In order to accurately measure 196 Au and 44 Sc^(m),a radioactive activity measuring device based on anti-coincidence principle was developed.The main techniques used were as follows:Firstly,liquid flash measurement system has been used to detect the beta-ray emitted by the decay of 198 Au and 46 Sc,basing on the anti-coincidence principle,the beta-ray would be used to suppress Compton background caused by gamma-rays emitted by the two nuclides above;Secondly,a portable anti-Compton spectrometer has been constructed using a ring sodium iodide crystal,and the Compton background caused by the decay of 198 Au and 46 Sc has been further suppressed by the anti-Compton principle.Thirdly,a thin layer of lead shield has been placed between the radioactive source distributed evenly in the liquid scintillation and a well-shaped high purity germanium(HPGe)crystal to absorb the X-rays emitted by the decay of the 196 Au,as results,the coincidence between the X-rays and the characteristic gamma-rays emitted by 196 Au in the HPGe crystal reduces significantly,and the detection efficiency of this measurement device for 196 Au increases too.By using the digital spectrometer,the liquid flash measurement system and the anti-Compton spectrometer integrated into one,the anti-coincidence effect could be superimposed and good experimental results were obtained.For 198 Au and 46 Sc,the anti-coincidence inhibition ratio is 87.1%and 86.9%respectively.The detection efficiency for 44 Sc m is 31.3%.After increasing the shielding layer,the detection efficiency for 196 Au increases greatly,from 5.5%when the shielding layer is not available up to 11.9%.Under the condition of strong isotope interference(198 Au or 46 Sc of 0.2 MBq),the minimum detectable activity(MDA)of 24 h detecting to 196 Au is less than 0.7 Bq,and to 44 Sc^(m) is less than 0.3 Bq.Applying the double anti-coincidence measuring device established in this paper to the activity measurement of 196 Au and 44 Sc m could effectively reduce the uncertainty of 197 Au(n,2n)196 Au and 45 Sc(n,2n)44 Sc^(m) average cross-section measurement.