Iwao Ogawa

Alpha decay of Gd150

Abstract 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.

Electron cyclotron resonances in a radiofrequency ion source

Abstract An axial magnetic field was applied to a Thonemann type radio-frequency ion source and the resonance-like increase in the probe current at a certain field was observed for various operating frequencies. The resonance was found to occur at a field-strength about four times larger than the field for which the cyclotron frequency of electrons is just equal to the operating frequency, independent of the kind of gases used. It is suggested that the resonance may correspond to one of the parametric resonances of plasma electrons which are expected to occur in an rf-modulated magnetic field. The favoured excitation of the fourth harmonic is tentatively attributed to the suppression of other (higher or lower) harmonics by the collision of accelerated electrons with either a gas molecule or the wall of the discharge tube.

Neutron Radiography at Rikkyo Triga-II Reactor

A vertical diverging collimator is installed in the reactor tank of Rikkyo 100 kW TRIGA-II. The configuration of the collimator and the beam characteristics are summarized. The ASTM E545-81 sensitivity and beam purity indicators have shown that the collimator is qualified to be the category I. Inherent unsharpness for the combination of the 25 pm Gd screen and the KODAK-SR film is determined experimentally and found to be 48.0±0.5 µm. An application is given for the inspection of mold cavities in carburetors.

Carrier multiplication in semiconductor detectors

Abstract On the basis of a slightly modified version of the Townsend-Miller theory of electron avalanches accompanied by “β-mechanism”, it is shown that the observed dependence of the multiplication factor M in an internally amplifying semiconductor detector upon the position x0 of the formation of primary electron-hole pairs should be attributed primarily to the marked difference in “ionization” coefficient between electrons and holes in the material and not to the non-uniformity of the electric field within the depletion region. A formula for M(x0) is presented together with its simplified forms in particular cases. Expressions and estimations are also given of the intrinsic fluctuation of avalanche sizes.

THE DOUBLE-GRID IONIZATION CHAMBER

A double-grid ionization chamber is designed such that the additional grid serves as a screen grid and eliminates a serious loss in the energy resolution. The energy spectrum of the alpha particles from a natural uranium source obtained with this chamber is shown. The grid pulseheight distributions due to U/sup 2//sup 3//sup 4/ and U/sup 2//sup 3//sup 8/ are graphicall y shown, and the separation of the grid pulses is satisfactory in spite of the rather small difference in energy of the alpha particles. (N.W.R.)

State Dependence of Mobilities for Kr++ in Kr at 88 K

An injected-ion drift tube mass spectrometer, whose drift tube can be cooled with liquid nitrogen, was developed for studying the mobilities of ions in low-temperature gases. The mobilities of Kr ++ in Kr were measured at 88 K with this apparatus as a function of the reduced field E / N ranging from 30 to 280 Td. It was found that the Kr ++ ions have four distinct mobilities in the E / N range below about 70 Td. These mobilities were identified as being associated with Kr ++ ions in the four different electronic states, 3 P 2 , 3 P 1 , 3 P 0 and 1 D 2 .

Fission of Uranium-238 Induced by 55 MeV Protons

Distributions of fragment kinetic energies and masses in the 55 MeV proton-induced fission of U 238 have been measured in various directions with respect to the beam by using a back-to-back arrangement of surface-barrier detectors. The observed energy distribution showed a single broad peak with a slight shoulder on the high energy side. This indicates a marked growth of symmetric fission. The most probable value of the total fragment kinetic energy was observed to be 156±5 MeV. The initial total kinetic energy before neutron emission was estimated from the value to be about 161 MeV. The mass distribution obtained in the transverse direction was in fair agreement with the radiochemical result. By making a fine comparison between these results, an attempt was made to obtain some information about neutron yields from individual fragments. The mass distribution in the backward direction was almost the same in shape as in the transverse direction, when corrected for the projectile momentum. Furthermore, the o...

On the limitation of the Owen-Batchelor method of pulse shape discrimination in a photomultiplier against high energy electrons and muons

Abstract A proposal is made on the origin of the high energy limit for the Owen-Batchelor method of pulse shape discrimination in a photomultiplier between recoil protons and energetic electrons or muons. According to the proposed mechanism, the initial negative swing of a voltage pulse on the last dynode becomes so large for a high energy electron or a muon, that the space charge effect between the last dynode and the anode would be appreciably reduced as a result of temporary enhancement in the potential difference across the space. This would give rise to a “saturation” of the negative amplitude of the last-dynode voltage pulse against the linear pulse-height, and would result in a rapid increase in the “Owen pulse-height” Vp (the positive amplitude of a last-dynode pulse), as well as in a rapid decrease in the zero-crossover time t0 of the last-dynode pulse, with increasing linear pulse-height. In consequence, the discriminating power would be progressively deteriorated as the linear pulse-height (or the energy deposit Ee in the scintillator) for an electron or a muon exceeds a certain upper limit. Approximate expressions for Vp and t0 are derived as a function of Ee, together with a relation between Vp and t0. Several experimental evidences are presented for the proposed mechanism, including the improving effect of added capacity in the last-dynode circuit as observed recently by Hiramoto et al. It is remarked that the breakdown of discrimination using Vp is necessarily associated with the failure of another method using t0, owing to the presence of a definite relation between Vp and t0.