CARR堆冷中子瞬发伽玛活化分析系统及实验研究

利用中国先进研究堆（CARR）在国内首次开展了冷中子瞬发伽玛活化分析（CNPGAA）实验，采用定制加长的电制冷高纯锗（HPGe）探测器和先进的数字多道谱仪DSPEC®-502进行测量，获得了NH₄Cl样品中元素冷中子瞬发伽玛谱和本底谱等数据，同时利用伽玛放射源¹⁵²Eu、¹³⁷Cs、⁶⁰Co以及NH₄Cl产生的瞬发伽玛射线对探测器在宽能区0.1~8 MeV进行能量刻度。为降低环境辐射本底，HPGe探测器外围采用环形锗酸铋（BGO）康普顿谱仪，10 cm铅以及含⁶Li和¹⁰B材料对中子束流准直屏蔽。此外，利用金片活化法测量了CARR堆运行功率为15 MW时有无冷源情况下冷中子导管B(CNGB)末端1 m处的中子注量率，结果显示有冷源时中子注量率可提高一个量级。

Experimental Research on Cold Neutron Prompt Gamma-ray Activation Analysis System at CARR

Prompt gamma-ray activation analysis (PGAA) is a neutron beam technique in which the gamma rays are measured during sample irradiation, which is used for the analysis of light elements such as H, B, C, N, as well as for the heavy elements Cd, Sm, Gd. PGAA can be performed using either a cold or thermal beam of neutrons. Due to the advantages of minimizing gamma-ray background and maximizing neutron flux by cold neutron guide, the cold neutron prompt gamma-ray activation analysis (CNPGAA) is used primarily for certification of inorganic standard reference materials (SRMs) and measurement of trace hydrogen and boron in materials. The CNPGAA experiment was firstly carried out at China Institute of Atomic Energy (CIAE) using the China Advanced Research Reactor (CARR) with cold neutron source (CNS). In the experiment, a customized and extended electro-mechanical cooler for high purity germanium (HPGe) detector and advanced digital multichannel spectrometer DSPEC®-502 were used to measure the cold neutron prompt gamma-ray spectrums of system background and NH₄Cl sample. The energy calibration was constructed for HPGe detector in the energy range from 0.1 MeV to 8 MeV using decay gamma rays produced by ¹⁵²Eu, ¹³⁷Cs, ⁶⁰Co and prompt gamma rays produced by NH₄Cl sample with neutron capture. In order to reduce the background of ambient radiation, the HPGe detector was surrounded successively by an annual bismuth germinate compton (BGO) suppressed spectrometer, 10 cm lead and materials for neutron beam collimator including elements of ⁶Li and ¹⁰B. In addition, the neutron flux was measured with and without CNS using the gold activation method at one meter far from cold neutron beam guide B (CNGB) exit of CARR under operating power of 15 MW. The results show that the neutron flux can be increased more than one order of magnitude by adding the CNS.