基于WLS光纤及SiPM读出的反符合探测器单元研究

Anti-coincidence detector unit based on WLS fiber and SiPM readout

中高能区中完全运动学测量是研究丰中子奇异核结构与性质的常用实验方法。反符合(Veto)探测器是CSR-RIBLLII(CoolingStorageRing-RadioactiveIonBeamLineinLanzhou)外靶实验终端(ExternalTarget Facility,ETF)开展丰中子测量的关键设备之一,其功能是消除带电粒子干扰以提高丰中子的有效事例数。然而,原有的Veto探测器存在探测效率低、均匀性差等缺点,可能导致实验与理论计算结果出现偏差。为了解决原有探测器的问题,设计了一种新的Veto探测器单元构型,即采用在Veto探测器单元中嵌入波长位移转换光纤(Wave Length Shifter Fiber,WLS),并使用硅光电倍增管(Silicon Photomultiplier,SiPM)读出的方案。搭建了专门的测试平台并对新型Veto探测器单元进行细致研究,其关键性能参数反符合效率可大于99.9%,相比于原有的Veto探测器提升了22.74%。这一研究结果为CSR-RIBLLII外靶实验终端提供了有效的升级方案,为下一步丰中子奇异核的实验研究奠定了良好的基础。

[Background]Complete kinematic measurements in the medium or high-energy region is a common experimental method to study the structure and properties of exotic nuclides on the neutron-rich side.The experiment setup in the Cooling Storage Ring-Radioactive Ion Beam Line in Lanzhou(CSR-RIBLLII),a typical nuclear external target facility,comprises many detectors with different requirements.The anticoincidence(Veto)detector is an essential part of the external target facility for eliminating the interference of charged particles and measuring medium or high-energy neutrons with high reliability and performance by combining them with a neutron wall detector.The original Veto detector with photomultiplier(PMT)readouts has many disadvantages,such as low detection efficiency and poor uniformity,resulting in significant differences or contradictions between experimental and calculation results.[Purpose]This study aims to upgrade the original Veto detector using wave length shifter fiber(WLS)and silicon photomultiplier(SiPM)to improve the detection efficiency of charged particles.[Methods]Firstly,a new configuration for the anticoincidence Veto detector unit was designed and the detector thickness was increased by 5 mm compared to the previous Veto detector,resulting in a final thickness of 1 cm.The Veto detector was embedded with 15,7,and 3 WLS fibers from both ends,and read using SiPM.Furthermore,to systematically explore the performance of the detector unit,a linear relationship was calibrated between the number of photons of the SiPMs and the number of Analog-to-Digital Converter(ADC)channels.This relationship was used to accurately calculate the threshold value,laying a foundation for calculating detection efficiency.Then,based on Multi-Wire Proportional Chamber(MWPC),a detection efficiency test platform was established,and time position conversion and track selection data analysis methods were developed as test methods.Finally,a detailed test on the whole and each part of the anticoincidence Veto detector unit was carried out on the MWPC test platform.[Results]Test results show the highest anticoincidence efficiencies of SiPMs at both ends for the Veto detector embedded with 15,7,and 3 WLS fibers are 99.99%,99.94%,and 99.82%,respectively;increased by over 22.74%compared with the original Veto detector.[Conclusions]The new Veto detector based on WLS fiber and SiPM readout meets the needs of the CSR-RIBLLII external target facility.