Eido Shibamura

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.

Let dependence of scintillation yields in liquid argon

Scintillation yields (scintillation intensity per unit absorbed energy) in liquid argon for ionizing particles are reviewed as a function of LET for the particles. The maximum scintillation yield, which is obtained for relativistic heavy ions from Ne to La, is about 1.2 times larger than that for gamma rays in NaI(Tl) crystal. In the low LET region, the scintillation yields for relativistic electrons, protons and He ions are 10–20% lower than the maximum yield. This tendency can be explained by taking into account the existence of the electrons which have escaped from their parent ions. In the high LET region, a quenching effect due to high ionization density is observed for alpha particles, fission fragments and relativistic Au ions.

Estimation of absolute photon yields in liquid argon and xenon for relativistic (1 MeV) electrons

Abstract Using recent scintillation data of liquid argon and xenon, the numbers of photons emitted in both liquids have been estimated. The ideal photon yields are 5.1 × 10 4 photons/MeV for liquid argon and 6.8 × 10 4 photons/MeV for liquid xenon. The photon yields due to 1 MeV electrons are 4.0 × 10 4 photons and 4.2 × 10 4 photons for respective liquids. These numbers are almost the same as that of a NaI(Tl) crystal.

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.

A liquid xenon proportional scintillation counter

Abstract The characteristics of a liquid xenon proportional scintillation counter have been studied by using conversion electrons from 207 Bi. Pulse shapes of the induced charge and the scintillation in the ionization chamber mode were observed and the variations of relative photon yield of the proportional scintillation against the applied voltage were measured. The energy spectra of the proportional scintillation for 207 Bi were compared with those of the charge. The best resolution of the proportional scintillation was about 18% fwhm for 1 MeV electrons, which was almost the same as that of the charge. The linear relation between the normalized photon yield at constant electric field strength on wire surface and wire radius as expected from analogy with gas was not obtained but the tendency of increase in the normalized photon yield for wire radius was confirmed.

Test of a dual-type gridded ionization chamber using liquid xenon

Abstract A liquid xenon dual-type gridded ionization chamber designed as a gamma-ray spectrometer was constructed and some tests for gamma-rays were made by using highly purified xenon gas. The energy resolution of 8.6% at fwhm was obtained for collimated gamma-rays of 662 keV. The resolutions at fwhm for non-collimated gamma-rays were (9.6 ± 0.4)% at 662 keV, (6.5 ± 0.3)% at 1332 keV and (4.5 ± 0.3)% at 2614 keV. These results show that, for gamma-ray energies above 1600 keV, the energy resolution for the liquid xenon ionization chamber is better than that for a conventionally available 1 3 4 ″ diameter × 2″ NaI(Tl) crystal. This characteristic of the chamber was kept nearly constant for more than 24 h. The resolution obtained experimentally were compared with the theoretical values and the causes of the differences between them are discussed.

Ionization and scintillation signals produced by relativistic La ions in liquid argon

Abstract We have observed simultaneously the ionization and scintillation signals produced by relativistic La ions in liquid argon. The two signals are highly correlated and the sums of these signals are constant with the standard deviation of 1.2% over the range of the electric field from 0 to 7.5 kV/cm. The ratio of the sum signals expressed in unit of the number of species to the value N i + N ex is close to unity, where N i and N ex are the numbers of ion pairs and excitons, respectively, produced by La ions in liquid argon. The pulse height resolution of the sum of the signals is better than that of ionization or scintillation alone. Almost no quenching is found in the scintillation signal from relativistic La ions when compared to signals from lighter ions.

SCINTILLATION YIELDS BY RELATIVISTIC HEAVY IONS AND THE RELATION BETWEEN IONIZATION AND SCINTILLATION IN LIQUID ARGON

Abstract Scintillation yields per unit energy deposited by relativistic heavy ions in liquid argon have been measured. They are (1.41 ± 0.07) for 613 MeV/n Ne ions and (1.39 ± 0.07) for 705 MeV/n Fe ions, if the value is normalized to unity for 5.3 MeV alpha particles. These results show that the scintillation intensity for relativistic heavy ions in liquid argon is proportional to the deposited energy, that is, to Z 1 2 , where Z 1 is the nuclear charge of heavy ions. Furthermore, the variation of ionization and scintillation in liquid argon has been investigated as a function of electric field using relativistic Ne and Fe ions. It is shown that a linear combination of ionization signal I and scintillation signal S , I + aS , is proportional to the deposited energy, independently of electric field strength and types of incident particles. This suggests that if we use both signals, we may construct a massive calorimeter that can be used for nuclear reactions induced by relativistic heavy ions.