In-situ calibration of the efficiency of a yttrium activation detection system by the accelerator-based DD fusion neutron source
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摘要: 在包括激光惯性约束聚变（ICF）、Z箍缩及稠密等离子聚焦（DPF）等脉冲聚变装置上，活化法作为中子通量测量与产额诊断的手段，得到了广泛的应用。利用无机闪烁体探测器，测量89Y核与中子非弹散反应产生的909 keV的单能伽马射线，可以实现DD中子通量的准确测量。采用金属钇作为活化靶，利用LaBr3:Ce闪烁体伽马探测器建立了中子活化原位探测系统。针对钇活化核半衰期仅有15.663 s的特点，对其在连续束流辐照下的累积过程进行了物理分析，建立了通过加速器DD中子源进行入射中子探测效率标定的实验方法。实验中，利用伽马探测器兼顾中子通量监测与活化伽马测量，模拟了钇靶活度随中子通量变化的过程，实现了该活化探测系统对入射中子探测效率的原位标定。Abstract: On the pulsed fusion sources such as laser ICF device, Z pinch facility and dense plasma focus device, the neutron activation method are widely applied, which can measure the neutron flux and diagnose the neutron yield from the source. Based on the inorganic scintillation detector, the 909 keV monoenergetic gammas, which are emitted from decay of the activated yttrium nuclei after the inelastic scattering on neutrons, can be measured, and the flux of the DD fusion neutrons can be diagnosed. In this work, an activation detection system using yttrium is developed, in which the LaBr3:Ce scintillator detector is chosen as the gamma sensitive material. The accumulation process of yttrium activation products under continuous irradiation has been physically analyzed, with respect to their half-life of only 15.663 s. An experimental method of calibrating the incident neutron detection efficiency by accelerator-based DD neutron source is thus established. In the experiments, the gamma detector is served as both neutron flux rate monitor and activation gamma measurements. The variation of radiation activity of the yttrium target with the neutron flux rate are simulated. Therefore, the in-situ calibration of the detection efficiency of this yttrium activation system for incident neutrons is achieved, with an accuracy of about 3.8%.
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图 1 LaBr3:Ce伽马探测器在远离束流靶头与靠近束流靶头两种情况下所测本底脉冲高度谱
Figure 1. Pulse height spectra measured when the detector is close to the target (orange) and away from the target (blue)
图 3 两种测量条件下获得的时间-脉冲高度二维事件分布
Figure 3. Time-PSD distribution of the events measured under different conditions
图 4 无钇靶情况下探测器记录事件的计数率变化
Figure 4. Count rates of the events measured when no yttrium disk is installed
图 5 两种厚度钇靶情况下，根据测量结果还原靶片上中子通量变化
Figure 5. Using the yttrium disks with two different thickness, the neutron fluxes are reconstructed from the measurements
图 6 不同初始活度下，对钇靶活度变化的模拟
Figure 6. Simulated of the radioactivation of the yttrium disk varieds with time
图 7 取不同的时间单元，对钇靶活度变化的模拟
Figure 7. Simulation of the radioactivation of the yttrium disk for different time bin width
图 8 停束后，QDC值在[
1800 ,2200 ]的事件计数率变化Figure 8. Count rates of the events of which the QDC values are between
1800 and2200 表 1 两种靶厚的中子活化探测系统标定结果
Table 1. Results of calibration with 2 different thickness of the yttrium disk
No. thickness/mm n1 n2 ϕn(t0)/s−1 η 1 10 11196 8047 6.09×105 2.29×10−4 2 20 12832 7594 7.25×105 3.20×10−4 
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