[13p-B12-1~18] 2.2 Detection systems

△ [13p-B12-3] Development of time-of-flight detector for studying on superheavy nuclei

〇(M2C)Satoshi Ishizawa¹, Kouji Morimoto², Daiya Kaji², Fuyuki Tokanai³ (1.Yamagata Sci Eng., 2.RIKEN Nishina Center., 3.Yamagata Univ.)

Keywords:time-of-flight (ToF), studying on superheavy elements

We have been studying on superheavy elements using a gas-filled recoil ion separator (GARIS) installed at the RIKEN Linear Accelerator (RILAC) facility. Recently, we have been developed a new recoil separator GARIS-II for hot fusion reaction toward search for new elements beyond atomic number greater than 118.At the same time, we are also preparing a focal plane detection system, which consists of a time-of-flight (TOF) detector and a silicon semiconductor detector box (Si box), for GARIS-II. In this report, we will give the performance of the TOF detector in detail. The TOF detector is used for two purposes. One is to measure the time-of-flight of the evaporation residue (ER) into the Si box to obtain the information on the mass numbers of ERs. The other is to distinguish decay signals in Si box used in anti-coincidence mode. The TOF detector consists of an entrance foil to generate secondary electrons, accelerator grid to extract the secondary electrons, and mirror grid to guide to micro-channel plates (MCP) assembly along the electrostatic field. One of the important characteristics of the TOF detector is the detection efficiency for charged particles passing through the TOF detector. The high efficiency can be obtained by achieving the high collection efficiency of the secondary electrons. In order to properly collect the secondary electros to the MCPs, an additional voltage was newly applied to the side panel of the TOF detector. The detection efficiency were checked by impinging α-particle from a ²⁴¹Am standard source into the entrance foil. Nine Si-detectors were set with suitable interval behind the TOF detector and operated in coincidence mode. Thus, the position dependence of detection efficiency was obtained. Moreover, the detection efficiency was measured as a function of applied voltage. As a result, it improved with increasing the applied voltage. For example, the detection efficiency of more than 90% were achieved in the region of ±70 mm from the center of the foil at the applied voltage of 350 V.