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 cmPb,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φ5.08 cm×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.
In the experiment of measuring (n,xnγ) reaction cross section with prompt γ ray method, the experiment background has a significant influence on the result and is the key factor. In order to achieve the goal of reducing a certain amount of the experiment background, the improvement scheme of shield and collimator was given through Monte Carlo method and the shielding scheme was finally selected: add 30 cm(C2H4)n+9 cm Pb on the original shield, 54 cm heavy concrete on the original shield wall and then 2 cm thick lead(Pb) on the shield and shield wall. The collimation effect and energy uniformity of symmetrical double cone collimation hole are the best among cylindrical, conical, and symmetrical double cone. After the completion of the transformation of shield, the relative intensities of the transmitted neutron and γ rays from the 70 cm in right radial of the transformed collimation hole were measured with φ5.08 cm × 5.08 cm type liquid scintillator detector(BC501). After deducting the natural background of no beam current, the neutron background of the beam current is reduced by 7.75 times and theγ background is reduced by 38.5 times, which meets the requirement.
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