A. Hitachi

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.

Estimation of Fano factors in liquid argon, krypton, xenon and xenon-doped liquid argon

Abstract Fano factors in liquid argon, krypton, xenon and xenon-doped liquid argon are estimated from the Fano formula by using the parameters in the energy balance equation for the absorbed energy of ionizing radiation. As a result, it is shown that the values for liquid argon, krypton and xenon are smaller than those in the gas phase and the value for xenon-doped liquid argon is smaller than that for liquid argon as in argon-molecular gas mixture. In particular, the value for liquid xenon is extremely small, i.e. about 0.05, which is comparable to that for Ge(Li) detectors. Using the so obtained Fano factor and an electronic noise level which can easily be achieved, the fwhm in the liquid xenon ionization chamber is estimated to be about 3 keV for 1 MeV electrons.

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.

Photoionization Effect in Liquid Xenon Doped With Triethylamine (Tea) or Trimethylamine (Tma)

Abstract Photoionization effects have been observed in liquid xenon doped with triethylamine (TEA) or trimethylamine (TMA) by using a small multiwire chamber whose anode consists of 9 wires of 10 μm in diameter. Quantum efficiencies for photoionization in both dopants are estimated to be close to unity for photons emitted from liquid xenon. Electron multiplication related to proportional scintillation was observed at wire voltages above about 3 kV. The energy resolution in ionization mode for alpha particles is improved from 15% to 4% (fwhm) by introducing TEA.

Attenuation length measurements of scintillation light in liquid rare gases and their mixtures using an improved reflection suppresser

Abstract The attenuation lengths for scintillation photons in liquid argon, krypton, xenon, argon-xenon (3%) mixture, and krypton-xenon (3%) mixture, have been measured by using a newly designed reflection suppresser. The obtained values are 66±3 cm for argon, 82±4 cm for krypton, 29±2 cm for xenon, 170±23 cm (first experiment) and 118±10 cm (second experiment) for argon-xenon (3%) mixture, and 136±11 cm for krypton-xenon (3%) mixture. The reason why the attenuation lengths in the mixtures are longer than those in the pure liquids is attributed to the reduced Rayleigh scattering in the mixtures.

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.

Ionization and scintillation produced by relativistic Au, He and H ions in liquid argon

Abstract We have measured ionization and scintillation produced by relativistic ions of Au, He and H in liquid argon. The sum of ionization signal and scintillation signal per unit energy deposition is the same for He and H ions, which is also the same as that for relativistic Ne, Fe and La ions previously measured. We have found that quenching occurs when liquid argon is irradiated by relativistic Au ions and that the sum per unit energy deposition for the Au ions is 70–76% of that for the other ions mentioned above.

Photoionization in liquid argon doped with trimethylamine or triethylamine

Abstract Ionization yields for alpha particles and 1 MeV electrons in liquid argon doped with trimethylamine and with triethylamine have been measured. In both liquids, a collected charge larger than Andersons results is observed for alpha particles although that for electrons is similar to Andersons value. By using a new equation for the collected charge, the photoionization quantum efficiencies for scintillation light in liquid argon are estimated to be 0.35±0.05 for trimethylamine and greater than 0.23±0.03 for triethylamine.