Hirokazu Umezawa

Mass distribution and the total fission cross section in the fission of 238U with protons of energies ranging between 13 and 55 MeV

Abstract The fission of 238 U with protons of energy ranging from 13 to 55 MeV was studied radio-chemically. The mass-yield curve was obtained for each of the eleven incident energies, and an analysis was performed, based on the two-mode-fission mechanism. The observed total chain yields were reasonably reproduced by the combination of the three Gaussian curves with partial distortions. The deduced number of neutrons emitted in the fission was found to agree well with the postulated value in the study of the charge distribution systematics. For the three highest incident energies, the formation of the fissile nuclei through a direct process was shown to occur and the analyzing method was discussed. The peak-to-trough ratio and the total fission cross section were obtained from thus-established mass distribution curves.

Radiochemical separation of 95Zr and 95Nb with tri-n-butyl phosphine oxide

Summary 95 Zr and 95 Nb behave differently on extraction from nitric acid solutions with tri- n -butyl phosphine oxide (TBPO). These isotopes can be separated efficiently by extraction with 0.005 M TBPO from 2 N nitric acid solution. 95 Zr was extracted into the organic phase and 95 Nb remained in aqueous phase. Separation from hydrochloric acid solution was not possible.

Non-Destructive Gamma-Ray Spectrometry on Spent Fuels of a Boiling Water Reactor

The spent fuels from the JPDR-I reactor were measured by means of a γ-scanning facility installed in the fuel storage pool. The spatial distributions of the fission products (134Cs and 137Cs) were measured and analyzed in reference to the effects of control rod pattern. The ratios holding between the products of neutron capture and of direct fission (134Cs/137Cs and 154Eu/137Cs) were also examined for its relevance to non-destructive burnup determination. The activity ratios of the fission products can be expressed by a linear function of burnup, provided that corrections are made to account for differences in irradiation history and for spatial variations in the neutron spectrum.

Systematic behaviour of the most probable charge in the medium-energy fission

Abstract The most probable charge {ce:inline-formula}Z p{/ce:inline-formula} observed in the medium energy fission of various nucleides was studied in comparison with the thermal-neutron-induced and the spontaneous fission. In the course of study, the systematics in the numbers of neutrons emitted before and after fission for the former was found to show no saw-tooth structure as found for the latter. It was found that the {ce:inline-formula}Z p{/ce:inline-formula} showed rather clear systematic behaviour with respect to the fragment mass fraction. The number of neutrons evaporated during the competition against fission was calculated using Jackson-Vandenbosch model and was compared with that deduced from the neutron systematics.

Fast Computer Analysis of the γ Spectrum from Ge(Li) Detectors

A method for rapidly analyzing the γ spectrum obtained with Ge(Li) detectors was developed for a medium-size electronic computer. The analysis is based on the first derivative method, associated with a number of peak-shape tests. The effects of data smoothing and of changes in peak width were studied to determine the optimum conditions for spectrum analysis. The resulting computer program was subjected to various quality tests on the predicted values of the peak position and area. The results of the tests showed that the code thus developed works quite satisfactorily. The code requires 10 sec and a memory core of 29 k when using a FACOM 230–60 for the analysis of a 2,047 channel spectrum.

Systematic radiochemical analysis of fission products

A scheme for the sequential separation of fission products has been developed on the basis of ion-exchange techniques. It consists of a main cation-exchange process for group separation and subsidiary processes of cation or anion exchange for further separations or purifications of the individual fission products. By the present method, Zn, Rb, Sr, Y, (Zr), Mo, Pd, Cd, Te, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu and Tb can be separated simultaneously from an irradiated uranium sample. Of these, alkali, alkaline earth and rare earth metal ions are separated quantitatively. A polarographic method was applied to determine the recoveries of Zn, Mo, Pd, Cd and Te, which were not separated quantitatively.

A new technique of target preparation

Abstract A technique for preparing targets of various thicknesses ranging from 0.1 mg/cm 2 to 60 mg/cm 2 was developed by centrifuging powder suspended in liquid paraffin. The uniformity was investigated by measuring the penetrability of a collimated β-ray beam. The local fluctuation in thickness from the average value was found to be less than ±3% except at the very edge of each target.

The absolute measurement of beta- disintegrating nuclides emitting conversion electrons with appreciable proportions

Abstract A new method was proposed for the 4πβ and 4πβ-γ coincidence absolute measurements of nuclides emitting conversion electrons with appreciable proportions. Correction formulas giving the disintegration rate were shown for 153 Sm, which can be easily extended to general forms. The validity of the derived formulas was examined by performing 4πβ, 4πβ-γ-coincidence, and β-γ-coincidence countings for 153 Sm, 156 Eu and 147 Nd, by comparing the resulting values of the disintegration rate with one another and also with those obtained by other methods, and by examining the consistency of the model. It was concluded that the present method gave satisfactory results for the above three nuclides; that is, we could determine the disintegration rate with an uncertainty of only 0·5 per cent.

4β-γ coincidence absolute measurement of nuclides emitting soft conversion electrons in appreciable proportions

Abstract A correction method has been derived for the disintegration-rate measurement of nuclides emitting soft conversion electrons along the lines of the method developed in our previous paper. The resulting correction formulae have been applied to the 4πβ-γ coincidence absolute measurement of 161 Tb and of the 140 Ba- 140 La mixed activity. The resulting disintegration rate has been found to be a reasonable value with an uncertainty of, at most, 1 per cent in either case. The proposed correction formulae have been concluded to be particularly useful for the absolute determination of nuclides emitting conversion electrons of low energies, ⪅30keV.

Radiochemical Determination of Fission Rate in JRR-4

The fission rate in the core of the Japan Research Reactor 4 (JRR-4) was determined by a method based on radiochemical analysis of 99Mo formed in the U samples irradiated in the reactor core. The contribution of epithermal neutron fission to the total fission rate was evaluated from the Cd ratio for U fission. The contribution was several percent. For comparison, the thermal neutron flux also was measured, by Au-foil activation. The fission rate determined from the U samples agreed well with the Au-foil data, except at positions in the peripheral region of the reactor core.