Research On Anticoincidence Detection Technique In HIRFL-CSR External Target Facility

The frontier research in nuclear science focuses on exploring the atomic nucleus structure and properties far from the beta stability line,significantly impacting the development of experimental techniques and theoretical nuclear models.Investigating neutron-rich nuclei,especially those near magic numbers,contributes to a better understanding of nuclear structure.The CSR-RIBLL2 secondary radioactive ion beam(RIB)experimental terminal in Lanzhou,China,utilizes neutron wall detectors to study neutron-rich nuclei.However,as the research direction deviates from the beta stability line,the number of target neutron-rich nuclei events decreases,resulting in deviations in experimental results.Therefore,it is significant to suppress various possible sources of background events and increase the number of effective neutron-rich events to study and develop neutron-rich nuclei.Regarding the abovementioned issue,this study designs a veto detector to reduce or eliminate the probability of charged particles being misidentified as neutrons.The veto detector is placed before the neutron wall detectors to enable joint measurements of neutron-rich nuclei.The main research contents of this article are as follows:(1)A thin and low atomic number(Z)plastic scintillator with dimensions of 150 cm ×18cm × 1cm is chosen as the material for the veto detector.The probability of misidentifying neutrons as charged particles when passing through the veto detector is only 1.57%.This effectively reduces background interference from various sources and improves the number of effective neutron-rich events.(2)Due to edge effects,the detection efficiency of individual veto detector units is relatively low,at only 77.61%.In order to reduce the influence of the edge effect on detection efficiency,a simulation platform using Monte Carlo methods is employed to simulate the veto detector unit.The chosen approach involves parallel readout signals from 9 silicon photomultipliers(Si PMs)at both ends of the veto detector unit,thus reducing the influence of edge effects on the detection efficiency.(3)A platform for detecting weak signals down to single photons is established.The linear relationship between the Si PM single photoelectron and the number of ADC channels is calibrated.The simulated photon spectrum threshold is set at 14 photons.The detection efficiency of the veto detector unit is calculated by accumulating the number of photons above the threshold,yielding a value of 99.82%.(4)The veto detector unit is further tested using the MWDC(Multi-Wire Drift Chamber)platform.The unit is divided into five parts for testing,resulting in detection efficiencies of 99.91%,99.91%,99.93%,99.92%,and 99.90% for each part,respectively.The overall detection efficiency is measured at 99.57%,representing a maximum improvement of 22.32% compared to the original veto detector unit.(5)Ultimately,a veto detector consisting of 9 veto detector units with dimensions of150 cm × 18 cm × 1cm is chosen,and they are arranged in overlapping positions with a1 cm gap to minimize the "dead zone" area.This configuration completely covers the neutron wall detectors.The results demonstrate that the new veto detector design meets the requirements of the CSR-RIBLL2 secondary RIB experimental terminal.Therefore,the veto detector is constructed at the external target experimental terminal to increase the number of effective neutron-rich events.