M. Mimura

Ratio of Transverse Diffusion Coefficient to Mobility of Electrons in High-Pressure Xenon and Xenon Doped with Hydrogen

We have obtained the ratio of the transverse diffusion coefficient to the mobility of electrons multiplied by the elementary charge, in Xe and Xe+H2 under electric fields at a higher pressure of 1 MPa in comparison with those in preceding experiments. The results show that the density effect (nonlinear effect) of the ratio in both pure Xe and Xe+H2 is ≤15% below 1 MPa over the reduced electric field range from 0.08 to 0.6×10-17 Vcm2. We also found that the diffusion of an electron swarm is suppressed by adding trace amount of hydrogen to high-pressure xenon gas. We discuss the application of Xe+H2 gas to a new gamma-ray camera.

A scintillation response and an ionization yield in pure xenon and mixtures of it with methane

The scintillation response to an α source (241Am) and the ionization yield in pure gaseous xenon and its mixtures with 0.2–2% of methane were measured at a pressure of 26 atm and different values of reduced electric field E/N. Both light and ionization yields in xenon were shown to decrease with increasing percentage of methane.

Average Numbers of Scintillation Photons and Electrons Produced by an Alpha Particle in High-Density Xenon Gas

The average numbers of scintillation photons and electrons produced by an ?-particle with 5.49 MeV were determined in high-density xenon gas in the density range of 0.03?0.12 g/cm3. No significant density dependence was observed for the number of excited atoms or liberated electrons. The average energies expended per excited atom (Wex-value) and liberated electron (W-value) were found to be 34.1?2.4 and 20.9?0.4 eV, respectively. It was also found that some liberated electrons escape from recombination at zero electric field.

Xenon Time Projection Chamber for Next-Generation Planetary Missions

Imaging of γ rays with 1–5 MeV with a xenon time projection chamber (Xe-TPC) has been simulated with the GEANT4 code. The clear image of a γ-ray source was obtained using the Xe-TPC. The dependence of angular resolution and imaging efficiency on xenon density were also investigated over the density range from 0.06 to 0.18 g/cm 3 . It was found that a Xe-TPC is promising for imaging of high-energy γ-rays (∼5 MeV) on planetary surface.

Ultraviolet-Sensitive Windowless Silicon PIN Photodiodes for Alpha-Ray Spectrometry

Energy resolution measurements of ultraviolet-sensitive windowless silicon p–i–n photodiodes were carried out with alpha rays. A resolution of 12.6 keV in full width at half maximum for 5.486 MeV was achieved. The ultraviolet-sensitive silicon p–i–n photodiodes were found to be suitable to alpha-ray spectrometry.

Measurements of Electron Drift Velocity in 3He Gas

Electron drift velocities in 3He and 4He gases were measured at 0.1 MPa and we compared the results in 3He and 4He under the electric field strength normalized by the number density of helium gas, E/N, from 0.04 to 1.0 Td. This is the first experimental confirmation of the fact that the electron drift velocity in 3He gas is larger than in 4He gas. The difference in the electron drift velocities is attributed to the difference in the atomic masses of 3He and 4He.

A Next Generation Gamma-ray Camera for imaging of planetary surfaces

We summarize the present R&D status of high-pressure xenon time projection chamber (HPXe-TPC) which is a new gamma-ray camera for planetary science mission.

Scintillation yield in high pressure xenon and xenon doped with methane

Scintillation light and ionization yields in pure xenon and xenon doped with various concentration of methane (0.2-2%) have been measured as a function of the reduced electric field (E/N) from 0 to 4/spl times/10/sup -18/ Vcm/sup 2/ at a pressure of 2.6 MPa. The measurements of photons and electrons have been made in gaseous xenon excited by /sup 241/Am alpha particles. High purity xenon was used to fill a high-pressure xenon chamber equipped with MgF/sub 2/ window coupled to a photomultiplier tube (PMT). The scintillation light in xenon-methane mixture is observed to decrease as the concentration of methane increases.

Electron diffusion and scintillation in xenon doped with hydrogen for high-pressure xenon time projection chamber

High-pressure xenon time projection chamber (HPXe-TPC) is a sophisticated detector, which can determine not only an energy of gamma ray but also its arrival direction by using Compton scattering. Diffusion of electrons and scintillation yield in Xe and Xe+H/sub 2/ mixture have been investigated to realize a HPXe-TPC. The transverse characteristic energy of electrons in Xe-H/sub 2/ mixture (H/sub 2/ 0.44%) was reduced to about 40% against that of pure Xe at 1.0 MPa. Scintillation yields by alpha-particles (/sup 241/Am) in Xe+H/sub 2/ mixture (H/sub 2/ /spl sim/ 0.23-5.7%) at 2.6 MPa were obtained as a function of E/N. The scintillation yields in Xe+H/sub 2/ mixture at 2.6 MPa and at E/N = 0.3/spl times/10/sup -17/ Vcm/sup 2/ were luminous enough to generate a trigger signal of a HPXe-TPC. It is found that doping hydrogen into xenon is effective to prevent the degradation of angular resolution of a HPXe-TPC.