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瞬发γ射线法测量(n,xnγ)反应截面实验中,在线的实验本底对测量结果影响显著,是影响测量结果的关键因素。为了达到降低在线实验本底一个量级的目标,通过蒙特卡罗模拟程序给出了屏蔽体和准直器的改进方案,最终选取的屏蔽方案:在原有屏蔽体上加厚30 cm(C2H4)n+9 cm Pb,原屏蔽墙加厚54 cm重混凝土,再在屏蔽体和屏蔽墙上共同加厚2 cm厚的铅(Pb);准直孔开孔形状在圆柱形、圆锥形和对称双锥形这三种方案中准直效果和能量单一性方面对称双锥形准直孔最好。在屏蔽体改造完成后,利用尺寸为?5.08cm×5.08 cm型液体闪烁体探测器(BC501)测量了改造后距准直孔右方径向距离70 cm处透射出来的中子和γ射线的相对强度,在扣除无束流天然本底后,有束流的中子本底降低了7.75倍,γ本底降低了38.5倍,改造效果达到了测量要求。
Instantaneous γ-ray measurement (n, xnγ) reaction cross-section experiment, the online experimental background has a significant impact on the measurement results, is the key factor affecting the measurement results. In order to achieve the goal of reducing the magnitude of online experiments by one order of magnitude, the improved scheme of shield and collimator was given by Monte Carlo simulation program. Finally, the shielding scheme was selected: thickened 30 cm (C2H4) on the original shield, n + 9 cm Pb, the original shield wall thickening of 54 cm heavy concrete, and then together in the shield and the shield wall thickness of 2 cm thick lead (Pb); collimation hole hole shape cylindrical, conical and symmetrical Biconical collimation of the three programs in the three aspects of energy uniformity and symmetry biconical collimation holes the best. After the reconstruction of the shielding body, the neutron and γ-ray transmitted by the distance of 70 cm to the right of the collimation hole after the reconstruction was measured by using a liquid scintillator detector (BC501) measuring 5.08 cm × 5.08 cm in size Relative intensity, after subtracting the natural background without beam, the background of the beam with neutron drops by 7.75 times and the γ background by 38.5 times, the improvement effect meets the measurement requirements.