Session 2WGB: New Detector Technologies for Radioactive Isotope Beam Facilities II

Gaseous Xe scintillator as a new particle-identification detector for high-intensity heavy RI beams

RIKEN RIBF is one of the accelerator facilities which can provide high-intensity and heavy-ion unstable beams in the world. Because the unstable nuclei are produced by in-flight method in RIBF, particle identification should be performed event by event. However, beam intensity is limited by the radiation hardness and pile-up events of the particle-identification (PID) detectors. Especially, an ion chamber, which is a standard delta E detector in RIBF, is bottleneck due to the slow response.  Therefore, we have been developing a gaseous Xe scintillator. The reason for using a gas detector is that it is good radiation hardness since a gas state is structureless. The scintillation process of Xe gas is about 100 ns which is much faster than the response of the IC (decay time : ~100ns vs ~mu{}s order).  The averaged energy to emit one scintillation photon is about 20 eV. and the gas requires small energy to produce one scintillation photon (~20eV). Thus, the gaseous Xe scintillator is promising as a new standard beam-PID detector, especially as a delta{}E detector.

We have performed beam irradiation tests for the gaseous Xe scintillator several times until now. We have evaluated the performance by performing PID of cocktail beams. As a result, it was found that the Xe scintillator has a good energy resolution and makes it possible to separate atomic number Z of beams in a broad region of Z=20 to around Z=55. In addition, we found that it also has good timing and position resolutions.

In this presentation, we report the details of the test experiments and future prospects.   

Xenon-gas ionization chamber for improving high-Z beam particle identification

RI beams are used in the study of exotic nuclei. The RI Beam Factory (RIBF) at RIKEN is capable of producing a variety of RI beams of 200-300 MeV/nucleon. The in-flight RI beam separator BigRIPS at RIBF has successfully provided RI beams in the Z<70 regions. Recently, the BigRIPS separator started to supply heavier RI beams with Z>70; however, one of the problems was the need for more Z resolution for particle identification.

The insufficient Z resolution is due to the increased energy loss straggling due to charge fluctuations. The Z is estimated from the particle velocity and the energy loss of the ions. The energy loss is measured using an ionization chamber (IC) at BigRIPS. The conventional IC gas, P-10 (argon 90% and methane 10%), causes a large energy loss straggling due to insufficient charge state changes.

To solve this problem, we switched from the conventional P-10 gas to xenon-based gas (xenon 70% and methane 30%) to increase the number of charge state changes. According to the charge fluctuation cross-section, the xenon-based gas increases the number of charge state changes by approximately one order of magnitude compared to P-10 gas. By reducing the energy loss straggling caused by the charge fluctuation, the Z resolution is expected to improve.

The Z resolutions of the xenon-based gas IC and the P-10 gas IC were evaluated using a cocktail beam with Z=40-90 at 200-250 MeV/u. The results of the xenon-based gas IC showed a Z resolution comparable to that of the P-10 gas IC at Z<70 and a remarkable improvement in Z resolution at Z>70. The Z resolution (FWHM) at Z=84-88 was 1.3 for the P-10 gas and 0.8 for the xenon-based gas, which is a 1.7-fold improvement over the P-10 gas. The simulation of the charge fluctuation explained this improvement. In conclusion, the xenon-based gas IC was highly effective for Z identification. The separation and supply of heavy RI beams using the xenon-based gas IC have already been started at RIBF.   

Identification of heavy isotope beams with multiple charge states

One of the primary challenges in performing successful inverse-kinematics measurements with heavy nuclei is the successful identification and tagging of the beam, which often contains many species. For this purpose, the Heavy Isotope Tagger (HEIST) was developed and commissioned at the National Superconducting Cyclotron Laboratory (NSCL).  HEIST utilizes two micro-channel plate timing detectors to measure the time-of-flight, a multi-sampling ion chamber to measure energy loss, and a high-purity germanium detector to identify isomer decays and calibrate the isotope identification system. We discuss the simulation and performance of HEIST using a rare isotope beam centered around ¹⁹⁷Pb at about 75 MeV/A. With heavy nuclei at this energy, the beam is not fully stripped, and multiple charge states of each isotope can be present. This is one of the largest sources of contamination when trying to uniquely identify the beam. In this talk, we examine the simulation of beam production, including charge state distributions, and compare the simulation to the experimentally determined performance of HEIST. We will present the purity of typical experimental cuts on the beam PID and show how our measured charge state distributions compare to charge state models such as GLOBAL.