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The average numbers of scintillation photons and liberated electrons produced by 5.49-MeV α - particles were measured in high-density gaseous xenon. The density range is 0.12–1.32 g/cm3 for scintillation measurements at zero electric field, and 0.12–1.03 g/cm3 for the scintillation and ionization measurements under various electric fields. The density dependence of scintillation yield at zero electric field was observed. The W s - value , which is defined as the average energy expended per photon, increases with density and becomes almost constant in the density range above 1.0 g/cm3. Anti-correlations between average numbers of scintillation photons and liberated electrons were found to vary with density. It was also found that the total number of scintillation photons and liberated electrons decreases with increasing density. Several possible reasons for the variation in scintillation and ionization yields with density are discussed.

The time profile of recombination luminescence of dense xenon gas irradiated by an α -particle was absolutely measured in the density range of 0.027 – 0.112 g / cm 3 with the aim to investigate the intensities of recombination luminescence at zero electric field. It was found that the number of scintillation photons due to the recombination luminescence increases with xenon density up to around 0.05 g / cm 3 , and then becomes nearly constant ( = 2.32 × 10 5 photons for an α -particle with 5.49 MeV). The probability of recombination between electrons and ions at zero electric field is about 88% in the density range of 0.05 – 0.112 g / cm 3 . It was also found that the percentage of slow component ( ∼ sub-milliseconds) in the recombination luminescence decreases with density from 73% to 33% in the density range from 0.027 to 0.112 g / cm 3 .

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.

Fluctuation in ionization, scintillation and gas proportional scintillation (GPS) yields have been measured in gaseous xenon as a function of the reduced electric field strength E / N in Td (townsend), where E is the electric field strength in volts/cm and N the atomic number density of xenon. 1 Td is defined as 1×10 −17 V cm 2 . The measurements are made under the pressures of 0.1 and 0.2 MPa by a plane parallel ionization chamber (PPIC) equipped with a photo-multiplier tube (PMT) sensitive to VUV photons. It is found that the fluctuation in ionization is largely improved in PPIC if alpha-particles were almost vertically emitted to the plane of the electrode and their track lengths were nearly equal to the distance between the electrodes. The fluctuation in the scintillation is 33.5% (full-width at half-maximum (FWHM)) at the pressure of 0.1 MPa and 35.6% (FWHM) at the 0.2 MPa, respectively, including the fluctuation of 10% (FWHM) in the alpha-particle energy. The season may be attributed to the fluctuations in several parameters on PMT. In GPS, it is turned out that the proportional gain of about 17 photons/electron at 3.8 Td–45 photons/electron at 4.6 Td is easily achieved and its fluctuation is about a few percent. It is equal to twice of that value at the same reduced field in the case of 0.2 MPa. The fluctuation in the photon yields in GPS shows just equal to one of the ionization yields.

The high precision gamma-ray spectrometer (GRS) is scheduled to be launched on the lunar polar orbiter of the SELENE mission in 2007. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals as an anticoincidence detector. A Stirling cryocooler was adopted in cooling the Ge detector. The flight model of SELENE GRS has been completed and an energy resolution of 3.0 keV (FWHM) at 1.332 MeV has been achieved. The spectrometer aims to observe nuclear line gamma rays emitted from the lunar surface in a wide energy range from 100 keV to 12 MeV for one year and more to obtain chemical composition on the entire lunar surface. The gamma-ray data enable us to study lunar geoscience problems including crust and mantle composition, and volatile reservoirs at polar regions.

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.

Acoustic signals were observed with a lead–zirconate–titanate (PZT) element that was directly irradiated with a 368 MeV/n xenon beam. Using an array comprising PZT elements, the energy loss in the PZT was studied. These elements are sensitive to an energy deposit of 100 nJ. A series of values of output voltage vs. integrated thickness of PZT was represented along a line similar to the ionization loss calculated by the Bethe–Bloch formula. The induced voltage was attributed to several processes—ionization, thermal, elastic, and piezoelectric processes. This study describes the possible applications of the PZT element as an active medium for calorimeters and a monitor for hypervelocity impact of space dust.

The photon-to-photoelectron conversion efficiency and the reflectance of a photomultiplier have been measured as a function of the photon incident angle. Appreciably large conversion efficiency and reflectance were obtained for a incident angle near 45°. A calculation was made for an optical model of a photomultiplier. The results were consistent with the experimental results.

The average energies required to produce one scintillation photon Ws were determined for 662 keV gamma rays in NaI(Tl) and CsI(Tl) crystals to be 15.0±1.3 and 13.3±1.1 eV, respectively, from the absolute numbers of photoelectrons measured for several combinations of a crystal and a photomultiplier tube (PMT) used as a vacuum photodiode. The numbers of scintillation photons were obtained by calculating the collection efficiency of scintillation photons at the photocathode using Monte Carlo simulations and by determining experimentally the photon-to-photoelectron conversion efficiency at PMT photocathode. The values of Ws determined in the present study are in good agreement with the theoretical values presented recently. The factors affecting energy resolutions were also examined. The calculated resolution agrees well with that obtained experimentally.

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.

The acoustic signals generated by bombarding 400 MeV/n xenon ions in water were studied using an array of piezoelectric lead–zirconate–titanate elements. The observed signal was reduced to a bipolar form through Fourier analysis. The output voltage corresponded to the amount of energy deposit in water, and it tailed off beyond the range of 400 MeV/n xenon in water. This magnitude was explained qualitatively as cumulative processes. Its behavior was consistent with the calculations based on the Bethe–Bloch formula. Possible applications of this detector to radiology and heavily doped radiation detectors are described.

In order to identify a daughter ion of Ba+ with laser-induced fluorescence just after a candidate event of double beta decay of 136Xe is found in a liquid xenon ionization drift chamber, the size of the laser beam directed to the vertex position is determined to be 1.6 mm in diameter by simulating the signal amplitudes of scintillation and ionization due to two beta rays with the EGS4 simulation code. The detection efficiencies of the apparatus and the trigger threshold of scintillation signals, which are obtained with the simulation, are also discussed in detail.

The luminescence yield of gaseous xenon by alpha particles is measured as a function of electric field strength under the pressures of 1.0 and 2.0 atm. The luminescence yield rapidly decreases as increase of the electric field and reaches a constant yield at higher electric field strength. The constant luminescence yield suggests an average energy expended per photon (W/sub s/) is determined from its value. In this study, the W/sub s/ is obtained to be 59.4/spl plusmn/2.4 eV and the ratio of average luminescence yield to average ionization one is calculated to be 0.37/spl plusmn/0.02, which agrees well with Platzman's prediction.

Scintillation and ionization yields in xenon gas for 5.49 MeV alpha-particles were measured in the range of pressure from 0.35 to 3.7 MPa and the electric field strength (E) over the number density of xenon atoms (N), E/N from 0 to 5×10−18 V cm2. When our data are normalized at the data point measured by Saito et al., the number of scintillation photons is 2.3×105 while the number of ionization electrons is 2.0×105 at 2.6 MPa and at 3.7×10−18 V cm2. The scintillation and ionization yields of xenon doped with 0.2% hydrogen, High-Pressure Xenon gas[H2-0.2%], at 2.6 MPa was also measured. Scintillation yield of the Xe–H2 mixture gas is 80% as high as that of pure xenon. It is found that the scintillation yield is luminous enough to generate a trigger pulse of the high-pressure xenon time projection chamber, which is expected as a promising MeV Compton gamma-ray camera.

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.

To apply scintillation material to radiation detectors, a quantity such as the W-value, defined as an average energy expended per ion-pair in gaseous detectors, must be determined experimentally. The authors define the effective W/sub S/-value as E/sub 0//N/sub p/ an average energy dissipated to emit a scintillation photon, if the energy of radiation E/sub o/ is completely absorbed in material and the number of scintillation photons N/sub p/ are emitted. They have attempted to determine N/sub p/ in liquid xenon due to alpha-particles with an energy of 5.303 MeV from /sup 210/Po to estimate W/sub S/ for scintillation in liquid xenon. The absolute scintillation yield was obtained as 3.26*10/sup 5/ for alpha-particles for 5.303 MeV in liquid xenon. This result followed from fitting the absolute photo-electron yields measured with a VUV sensitive photomultiplier, which was used as a photodiode, to the results of Monte Carlo simulation. >

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.

The Naverage energy per scintillation photon in NaI(Tl) crystals of 3" in diameter is determined to be 14.40/spl plusmn/0.14eV using a combination of experimental methods and Monte-Carlo simulations. The factors affecting the energy resolution is also examined from the point of the number of photons. The calculated resolution agrees well with the measured resolution.

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.

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.

Measurements of the mobility and longitudinal diffusion coefficient of electrons under external electric fields were performed in high-density gaseous xenon at room temperature. The xenon density and reduced electric field ranges were from 2.55 ×1020 to 1.73 ×1021 cm-3 and from 0.027 to 0.19 Td, respectively. The electron transport parameters of the density-normalized mobility, density-normalized longitudinal diffusion coefficient, and the ratio of the longitudinal diffusion coefficient to the mobility were found to increase with increasing density for reduced electric fields at around 0.04 Td. This result indicates the density-dependent variation of the effective momentum-transfer cross section for electron–atom scattering. It was also found that the density-dependent variations are different depending on the electron transport parameters, which can be attributed to the difference in parameter sensitivity to the momentum-transfer cross section.

The longitudinal diffusion coefficient of electrons and the ratio of the longitudinal diffusion coefficient to the mobility of electrons were measured for the first time in high-density gaseous xenon in the extensive density range of 4.19×1019–4.82×1020 cm-3. The density dependence of the longitudinal diffusion coefficient was observed in the reduced electric field range of 0.025–0.055 Td.

The average numbers of scintillation photons and electrons produced by an α-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.

Research Institute for Science and Engineering., Waseda Univ., Japan College of Health Science, Saitama Prefectural University,, Japan Inst. of Space and Astronautical Science, JAXA, Japan Nippon Medical School, Japan Dep. of Chemistry, Tokyo Metropolitan Univ., Japan National Institute of Polar Research, Japan Japan Agency for Marine-Earth Science and Technology, Japan Research Institute, Chiba Institute of Technology, Japan Centre d’Etude Spatiale des Rayonnements,Toulouse, France Dep. of Meteorites, Univ. of New Mexico, USA *nhasebe@waseda.jp

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 V·cm2. 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.

Reflectance and photon-to-electron conversion efficiency have been measured for photomultiplier tubes (PMTs; Hamamatsu R878 and 7696) using three-color light-emitting diode (LED) emitting photons with central wavelengths of 470, 525, and 640 nm. The use of a light guide with a refractive index similar to that of a photomultiplier window enabled the measurement at the angle of incidence to a photocathode up to 75°. An appreciably large reflectance and conversion efficiency were obtained at the angles of approximately 45 and 60°, respectively. It was shown that these PMTs work with a higher photon-to-electron conversion efficiency than the quantum efficiency given by the manufacturer in many cases of scintillation counting. The reflectance measured in this study is consistent with that calculated using an optical model, where the photocathode was a thin film with a complex refractive index.

We constructed a parallel plate drift chamber for measuring the ratio of the transverse diffusion coefficient D t to the mobility µ of electrons multiplied by the elementary charge e, e D t/µ, which is called the transverse characteristic energy, in high-pressure xenon gas. The characteristic energies of electrons in Xe were obtained at a high pressure of 1.0 MPa and the reduced electric field, E/N, from 0.77 to 7.7 ×10-18 V·cm2, where E denotes an electric field and N the number density of Xe atoms. At the same E/N, our results agreed well with the data at pressures from 107 to 193 kPa previously obtained by Koizumi et al. [J. Phys. B 19 (1986) 2331]. The characteristic energies of electrons in Xe were found to be constant as a function of E/N from a low pressure to 1.0 MPa.

A single gridded ionization chamber with a movable cathode was constructed in order to measure W -values in highly purified rare gases without ambiguity. The chamber gases were continuously purified with a purifier filled with many pellets of titanium-barium getter. The purifier proved to be so powerful as to reduce impurities in rare gases to the level of 1 ppb or less. Performance tests of the chamber were made by measurements of W -values of argon-methane mixtures relative to that of argon. The measurements were made with a precision of ±0.14%.

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.

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.

Dosimetric measurements of fast neutrons at a level lower than 10 mrem from an Am-Be source have been made by using CR-39 plastic sheets. After being etched in 6.8N NaOH for 18 to 20 h, the CR-39 sheets were scanned under an optical microscope and counted, with the requirement that etch pits have sharp tips. The background pits due to alpha-particles at the surface of sheets are eliminated by this criterion. The density of etch pits as a function of rem-dose shows excellent linearity over the range of 1 to 10 mrem. The sensitivity is 13.4 etch pits/cm mrem.

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.

Iron-liquid argon and lead-liquid argon sampling calorimeters were constructed and tested. The obtained energy resolutions due to the sampling fluctuation σ s / E for 1 GeV electrons were 6.1% and 9.6%, respectively. Our results are compared to those of scintillation sampling calorimeters and what the energy resolution depends on is discussed.

A series of beam stop type experiments was performed with 500 MeV protons. The distributions both of induced radionuclides and of dose rate due to the residual activities were measured. Discussions to explain these distributions were done qualitatively. A clear difference in secondary flux distributions between charged particles and thermal neutrons was observed. Derived thick target cross sections were compared with the previous results. These show the natural energy dependences both in the absolute values and in the depth dependent shapes. The Faraday cup method for the beam intensity measurement was found to be applicable to the 500 MeV energy region.

The alpha particle energy and the half-life of Gd150 have been measured with a double-grid ionization chamber. Backgrounds are minimized by the grid-collector coincidence technique. The alpha particle energy is determined as 2.715±0.018 MeV by referring to that of Sm147 (Eα = 2.23±0.02 MeV) reported by Macfarlane. The half-life is estimated to be (1.4±0.4)×106 y from the observed counting rate and the total number of Gd150 nuclei contained in the specimen. The number is estimated from the observed beta-decay rate of the parent nuclei Eu150, which are produced by a photonuclear reaction Eu151(γ, n)Eu150. The experimental half-life thus obtained agrees within a factor of 4.4 with the theoretical half-life calculated from the observed alpha-particle energy on the basis of a conventional barrier-penetration theory. The results show that Gd150 behaves quite normally in the systematics of rare-earth alpha-decay.

An improved method of measuring the alpha-gamma angular correlation is proposed. The apparatus consists of a gridded ionization chamber and a NaI scintillation counter. In a gridded chamber, generally, the cathode pulse yields information on the emission angle of an alpha-particle, while the collector pulse gives information on its energy. Accordingly, the alpha-gamma angular correlation of mixed alpha-sources, in which some isotopes emitting alpha-particles of different energies and subsequent gamma-rays of almost the same energy are included, can be easily measured from the pulse height distribution of the cathode pulses coincident with the scintillation pulses and the collector pulses in the desired energy intervals, respectively. As examples, the angular correlation for two decay series in the daughter products of ThB, which are 1. ThC′ ∗ (γ)ThC′(α)ThD and 2. ThC(α)ThC″ ∗ (4+)(γ) ThC″(5+), were measured. The results are in good agreement with theoretical prediction and those obtained by other workers.

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.

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.

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.

The high precision gamma-ray spectrometer (GRS) is carried on the first Japan's 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...