A. De Clercq

Study of the catcherfoil technique with the aid of252Cf s.f. and235U(nth, f)

Abstract The well-known post neutron mass distributions of 235 U(n th , f ) and 252 Cf s.f. were studied with the catcherfoil technique in order to investigate the possibilities of this technique for fission studies. A set of necessary spectroscopic data of the studied fission products was selected from the available literature. In the case of 235 U(n th , f ) and 252 Cf s.f. relative yields of respectively 37 and 43 masses could be determined. The agreement between the obtained results and those given in the literature is very good. In the case of 252 Cf s.f. the relative yields of seven mass chains were determined for the first time by another method than semiconductor detector measurements. At the same time the catcherfoil method was elaborated so that in a few cases we were able to measure fractional independent chain yields. To test this, the fractional independent chain yield of 134 I and 135 Xe was measured in both fissioning systems, mentioned above. Here too our results agree very well with the independent yields given in the literature.

Kinetic energy and fragment mass distributions forPu240(s.f.),Pu239(nth,f), andPu240(γ,f)

Energy correlation measurements were performed for /sup 240/Pu(s.f.), /sup 239/Pu(n/sub th/,f), and the photofission of /sup 240/Pu with 12-, 15-, 20-, and 30-MeV bremsstrahlung. The photofission cross section for /sup 240/Pu was determined up to 30 MeV, which permitted the calculation of the average excitation energy of the compound nucleus. The average total kinetic energy of the fragments for /sup 240/Pu(s.f.) was found to be 1.2 +- 0.5 MeV higher than for /sup 239/Pu(n/sub th/,f). A decrease of , with , d/d = -0.37 +- 0.08, is observed in the photofission of /sup 240/Pu. Fragment shell effects are present in the kinetic energy and mass distributions for /sup 240/Pu(s.f.). Changes in the measured distributions with increasing excitation energy of the compound nucleus /sup 240/Pu are discussed in the framework of the scission point model of Wilkins et al.

The thermal neutron sub-barrier fission of 237Np

Abstract The post-neutron mass distribution for the sub-barrier fission of the odd-odd nucleus 238Np, produced by thermal neutron capture in 237Np, has been measured using the technique of γ-spectrometry of fission product catcher-foils at the Grenoble high-flux reactor. The values of the average light and heavy fragment masses and the peak widths are in very good agreement with the systematics, deduced from even-even above-the-barrier fissioning systems. A peak-to-valley ratio of 510 ± 50 was determined radiochemically. Enhanced yields at the masses 133 and 135, possibly due to shell effects in the pre-neutron mass distribution, are observed. The independent yields of 134I and 135Xe were measured. The neutron emission curve ν(m ∗ ) , deduced from the measured post neutron mass distribution and the fragment energy correlation data of Asghar et al., shows the typical sawtooth structure, as generally observed in low-energy fission. This indicates that the excitation energy of the fragments in the sub-barrier fission of the odd-odd nucleus 238Np is not seriously different from that in above-the-barrier even-even fissioning systems.

A versatile CAMAC based linear PHA system

Abstract A pulse height analysis based on a PDP 11/10-E system with a CA11C CAMAC interface is described. The data are stored in 4096 channels, each containing 24 bits (≈ 16 × 10 6 counts maximum). On-line programming, providing the possibility to record different spectra after each other, and limited on-line data handling is available.

Method for the determination of the yields of short-lived fission products

Abstract A program was written which enables the measurement on a PDP 11/10 system, with CA11C CAMAC interface, of 120 successive spectra of 4096 channels each with counting times down to 200 ms and time intervals between two successive measurements less than 150 us. Off-line a number of successive spectra appropriate to the half-life of the considered fission products is added up. The variation of the dead time during the measuring cycle can be followed with the pulser technique. The method was used for a study of the short-lived fission products in the photofission of 238U with 20 MeV bremsstrahlung. The variation of the dead time during the constructed measuring intervals was taken into account in the expressions for the calculation of the product yields from the peak areas in the spectra. The influence of this variation on the calculated yields in the performed experiments was investigated.

Isotopic Distributions and Element Yields in the Photofission of 235U and 238U with 12-, 15-, 20- and 30-MeV Bremsstrahlung

Using chemical separation techniques, γ-spectrometry of fission product catcherfoils and direct γ-spectrometry of irradiated uraniumsamples, the fractional independent or cumulative chain yields of about 50 fission products were measured for the photofission of 235U and 238U with 12-, 15-, 20- and 30-MeV bremsstrahlung. In addiation a number of fractional independent chain yields were calculated using the charge distribution curves. This enables us to study the isotopic distributions for Kr, Rb, Sr, Sn, Sb, Te, I, Xe, Cs and Ba isotopes for the photofission of 235U and for Rb, Sn, Sb, Te, I and Xe isotopes for the photofission of 238U. The corresponding element yields for all investigated isotopic distributions were calculated. As well for the photofission of 235U as for 238U at all bremsstrahlung end point energies a very low yield of 136I and 134Sb was obtained.

Kinetic energy and fragment mass distributions for Pu-240 (s.f.), Pu-239 (nth, f), and Pu-240 (gamma, f)

Energy correlation measurements were performed for /sup 240/Pu(s.f.), /sup 239/Pu(n/sub th/,f), and the photofission of /sup 240/Pu with 12-, 15-, 20-, and 30-MeV bremsstrahlung. The photofission cross section for /sup 240/Pu was determined up to 30 MeV, which permitted the calculation of the average excitation energy of the compound nucleus. The average total kinetic energy of the fragments for /sup 240/Pu(s.f.) was found to be 1.2 +- 0.5 MeV higher than for /sup 239/Pu(n/sub th/,f). A decrease of , with , d/d = -0.37 +- 0.08, is observed in the photofission of /sup 240/Pu. Fragment shell effects are present in the kinetic energy and mass distributions for /sup 240/Pu(s.f.). Changes in the measured distributions with increasing excitation energy of the compound nucleus /sup 240/Pu are discussed in the framework of the scission point model of Wilkins et al.

Kinetic energy and fragment mass distributions for /sup 240/Pu(s. f. ), /sup 239/Pu(n/sub th/,f), and /sup 240/Pu(. gamma. ,f)

Energy correlation measurements were performed for /sup 240/Pu(s.f.), /sup 239/Pu(n/sub th/,f), and the photofission of /sup 240/Pu with 12-, 15-, 20-, and 30-MeV bremsstrahlung. The photofission cross section for /sup 240/Pu was determined up to 30 MeV, which permitted the calculation of the average excitation energy of the compound nucleus. The average total kinetic energy of the fragments for /sup 240/Pu(s.f.) was found to be 1.2 +- 0.5 MeV higher than for /sup 239/Pu(n/sub th/,f). A decrease of , with , d/d = -0.37 +- 0.08, is observed in the photofission of /sup 240/Pu. Fragment shell effects are present in the kinetic energy and mass distributions for /sup 240/Pu(s.f.). Changes in the measured distributions with increasing excitation energy of the compound nucleus /sup 240/Pu are discussed in the framework of the scission point model of Wilkins et al.