Development of a new imaging device using a VUV scintillator and a gas photomultiplier with a μ-PIC and GEM

We have developed a new imaging device consisting of a VUV scintillator coupled to a gas photomultiplier (gas PMT) with a CsI photocathode, micro pixel chamber (μ-PIC) and gas electron multipliers (GEMs). Generally, the VUV scintillator has a short decay time of less than ∼ 10 nsec. Thus the new detector could be used under the high rate counting ( ∼ 10 MHz) in the hard X-ray to soft-gamma ray region. Our goal in this paper is to obtain a soft gamma-ray image at 0.1 MeV with our gas PMT combined with the VUV scintillator as a high rate counter, and we have optimized electric fields in the gas PMT and developed a new VUV scintillator with a higher light output. In order to obtain a higher collection efficiency of photoelectrons and suppress the ion feedback in the gas PMT, we first optimized the electric field. Then we decided the electric field in the drift, transfer, and induction region to be 0.25, 1.0 and 3.0 kV/cm, respectively. The total gas gain of the gas PMT was approximately 2 × 10⁵, and the gas PMT was estimated to have a quantum efficiency (QE) of 0.7% at 178 nm. Additionally, as a consequence of new VUV scintillators search, Nd:LuLiF₄ and Nd:LuF₃ with a volume of 10 mm × 10 mm × 5 mm were found to have higher light outputs than Nd:LaF₃, which is a conventional VUV scintillator, by a factor of 2.1, and 2.6, respectively, and the Nd:LuF₃ irradiated with 5.5−MeV alpha−rays had a light output of approximately 400 photons. Finally, we succeeded in obtaining the crystal images upon 5.5 MeV alpha and 0.122 MeV gamma rays excitation from ²⁴¹Am− and ⁵⁷Co sources, respectively, using the gas PMT with the Nd:LuF₃.