Development of Radioactivity Measurement Device of 196Au and 44Scm under Strong Interference

Abstract

In the spectral mean cross section measurements of 197Au(n,2n)196Au and 45Sc(n,2n)44Scm, the activation products 196Au and 44Scm have low activity and shorter halflife, but the activation products 198Au and 46Sc from neutron capture reactions of 197Au(n,g) and 45Sc(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 196Au and 44Scm activity. In order to accurately measure 196Au and 44Scm, 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 198Au and 46Sc, 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 198Au and 46Sc 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 196Au, as results, the coincidence between the Xrays and the characteristic gammarays emitted by 196Au in the HPGe crystal reduces significantly, and the detection efficiency of this measurement device for 196Au 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 198Au and 46Sc, the anticoincidence inhibition ratio is 871% and 869% respectively. The detection efficiency for 44Scm is 313%. After increasing the shielding layer, the detection efficiency for 196Au increases greatly, from 5.5% when the shielding layer is not available up to 11.9%. Under the condition of strong isotope interference (198Au or 46Sc of 02 MBq), the minimum detectable activity (MDA) of 24 h detecting to 196Au is less than 07 Bq, and to 44Scm is less than 03 Bq. Applying the double anticoincidence measuring device established in this paper to the activity measurement of 196Au and 44Scm could effectively reduce the uncertainty of 197Au(n,2n)196Au and 45Sc(n,2n)44Scm average crosssection measurement.