Mitsuhiro Miyajima

Drift velocities of electrons, saturation characteristics of ionization and W-values for conversion electrons in liquid argon, liquid argon-gas mixtures and liquid xenon

Abstract The drift velocities of electrons, the saturation characteristics of ionization and the W -values for internal conversion electrons emitted from 207 Bi in liquid argon, liquid argon-gas (xenon, nitrogen, methane and ethylene) mixtures and liquid xenon have been measured by using gridded ionization chambers. The drift velocity of electrons in xenon or nitrogen-dopes liquid argon shows little increase compared with that in liquid argon, while that in liquid argon-methane or -ethylene mixtures shows a remarkable increase. From each saturation curve, the so-called “recombination coefficient” k is obtained, assuming that the electric field ( E ) dependence of the collected charge ( Q ) is represented by Q∝( 1+ k E ) −1 , and the values for liquid rare gases as shown above are tabulated. The W -values in liquid argon-xenon mixture and liquid xenon have been determined by comparing with that in liquid argon. It is shown that admixing liquid argon with a small amount of xenon results in the reduction of the W -value and that the W -value in liquid xenon is much smaller than that in the gas phase. On the basis of these experimental results, the applicability of the liquid rare gases to detector medium is discussed.

Absolute number of photons produced by alpha-particles in liquid and gaseous xenon

Abstract The W s which is defined as an average energy expended per scintillation photon, was found to be 16.3 ± 0.3 eV for alpha-particles in liquid xenon, and 49.6 ± 1.1 eV in gaseous xenon, respectively. These results followed from the number of photoelectrons measured with a VUV sensitive photomultiplier tube, which was used as a photodiode. The number of photoelectrons from the photomultiplier photocathode was measured absolutely with a well calibrated charge sensitive amplifier system as a function of distance between the alpha-source and the photomultiplier photocathode. The detection geometries included both reflective and nonreflective walls. The data were well fitted to corresponding curves obtained with a Monte Carlo simulation, and yielded the total number of scintillation photons.

Proportional counter filled with highly purified liquid xenon

Abstract The electron avalanche in highly purified liquid xenon is observed with reproducible gain. The electronegative ion pumping is effective for suppressing spurious discharges rather than for purifying liquid xenon.

Gamma-ray spectrometer (GRS) for lunar polar orbiter SELENE

The high-precision gamma-ray spectrometer (GRS) on the lunar polar orbiter SELENE is designed to measure 200 keV—12 MeV gamma rays in order to determine elemental compositions of the lunar surface. The GRS consists of a large germanium (Ge) crystal as a main detector and a massive bismuth germanate crystal and a plastic scintillator as anticoincidence detectors. The Ge detector is cooled by a Stirling cryocooler with its compressor attached to a passive radiator facing the cold space. The cooling system maintains the Ge detector below 90 K during the observation. The flight model of the GRS has achieved an energy resolution of 3.0 keV (FWHM) at 1333 keV. Energy spectra obtained by the GRS will show sharp gamma-ray lines whose energies identify the elements and whose intensities determine the concentrations of the elements, permitting global mapping of the elemental abundances in the sub-surface of the Moon. The elemental maps obtained by the GRS with such high-energy resolution enable us to study lunar geoscience problems.

Number of photoelectrons from a photomultiplier cathode coupled with a NaI(Tl) scintillator

Absolute numbers of photoelectrons from several PMT photocathodes coupled with NaI(Tl) scintillators were measured for the several energies of gamma-rays. Typically, 6.85 × 103 photoelectrons were obtained with a resolution of 13% for 662 keV gamma-rays of 137Cs.

First Results of High Performance Ge Gamma-Ray Spectrometer Onboard Lunar Orbiter SELENE (KAGUYA)

The high precision gamma-ray spectrometer (GRS) is carried on the first Japans large-scaled lunar explorer, SELENE (KAGUYA), successfully launched by the H-IIA rocket on Sep. 14, 2007. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals and a plastic scintillator as anticoincidence detectors. After a series of initial health check of the GRS, it started a regular observation on December 21, 2007. Energy spectra including many clear peaks of major elements and trace elements on the lunar surface have been measured by the GRS. Global measurement of thorium counting rate on the lunar surface is presented. The region showing the highest count rate of thorium extends from Kepler to Fra Mauro region in the Procellarum. And Apennine Bench and Aristillus region and the northwestern region of Mare Imbrium are high in thorium count rate. Second high count rate region is located in the South Pole-Aitken basin of the farside. Arago and Compton/Belkovich craters are also e...

A self-triggered liquid xenon drift chamber by the use of proportional ionization or proportional scintillation

Abstract A liquid xenon drift chamber with an electron drift space of 13 mm in length has been constructed and the spatial resolution of the drift chamber was investigated by using alpha particles. In this experiment, the scintillation light directly produced by an alpha particle was used as trigger signal. To detect the drifted electrons, a proportional counter was used with proportional ionization or proportional scintillation mode. The best value of the spatial resolution (r.m.s.), achieved in the drift chamber, was about 20 μm for both modes. From analytical considerations it is concluded that the main part of the resolution comes from the position uncertainty caused by the finite range of alpha particles in liquid xenon and the finite size of the alpha source. This shows that, if such a drift chamber is used for minimum ionizing particles, a spatial resolution of less than ±10 μm can be expected.

Complexities of gamma-ray line intensities from the lunar surface

Assuming different abundances of the Apollo lunar soil samples and the average spectrum of galactic cosmic ray protons, energy spectra of neutrons and gamma rays and emission rates of gamma-ray lines from major elements have been estimated by using the reviewed Monte Carlo simulation library Geant4 and nuclear data. Previously, such libraries were not able to reproduce gamma-ray lines properly for the planetary application. Results clearly show that the emission rate of gamma rays heavily depends not only on the chemical abundance but also neutron flux within the lunar subsurface. While the intensities of gamma-ray lines are mostly proportional to elemental abundances, the intensity per unit elemental abundance can vary. Such a complex correlation is attributed to the change in neutron flux within the lunar subsurface and petrological restriction of elemental variation.

Numbers of scintillation photons produced in NaI(Tl) and plastic scintillator by gamma-rays

The W/sub S/-value, which is defined as an average energy expended per scintillation photon, is determined to be 17.2+or-0.40 eV for a NaI(Tl) phosphor and 60.8+or-4.3 eV for a plastic scintillator (NE-102A). These are obtained from the numbers of photoelectrons measured with several combinations of a photomultiplier tube and a NaI(Tl) or a NE-102A scintillator. The number of photoelectrons, which are measured by the photomultiplier tube as a vacuum photodiode, are converted to the number of scintillation photons by using an averaged quantum efficiency of each photomultiplier photocathode and a calculated collection efficiency of the scintillation photons at the photocathode. The above values do not include the uncertainties due to the unknown exact emission spectra and the photomultiplier response curves. >

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.