Fission product yields in the fast-neutron fission of238U

The fission yields of 38 fission products in the fast-neutron induced fission of²³⁸U have been determined using a rapid, multiscaling gamma-ray spectroscopic method. To obtain absolute yields for fission products having half-lives ranging from 32 s to 40 d, a total of 56 multi-scaling gamma-ray spectra were collected using various irradiation and cooling periods. Gamma-rays and photopeak areas of interest were assigned to the fission products by their energies and half-lives. Fission product activities were evaluated from spectral data using growth and decay calculations and fission yields were determined by normalizing the¹⁴⁰Ba yield to the average value from reported data. The depleted uranium target, covered with a boron-cadmium thermal neutron shield, was used to keep interference from the fission of²³⁵U minimal. Results for the cumulative fission yields, including 17 mostly short-lived fission products measured for the first time, are compared with previous measurements and with the recommended yields in recent evaluations. The agreement, and some discrepancies, in the comparisons are discussed. No explicit even-odd pairing effects are observed in the fission yield data for fast-neutron induced fission of²³⁸U.     

Spontaneous 238U fission half-life measurements based on fission-track techniques

In the last recommendation of the International Union of Pure and Applied Chemistry (I.U.P.A.C.) on spontaneous fission half-lives for ground-state nuclides, a number of measurements of ²³⁸U based on fission-track techniques were discarded. The arguments given by the authors are not clear. In this work a more detailed discussion of these determinations is given, considering the possible systematical errors inherent in fission-track approaches.     

Spontaneous fission yields for 246Cm

Cumulative yields for 17 fission products in the spontaneous fission of ²⁴⁶Cm have been determined with γ-ray spectrometry. These yields are compared with those determined in the spontaneous fission of ²⁴⁴Cm and ²⁴⁸Cm.     

Proton induced fission of 238U

The proton induced (E_p = 13, 14, 15 and 16 MeV) ²³⁸U nuclear-fission cross-section of formation of several nuclides as well as the independent yields of ⁹⁵Nb, ¹³²I and ¹⁴⁰La have been measured. The most probable charge of the fragments as determined with the assumption of a Gaussian charge distribution appears to be lower than expected from higher energy data, suggesting a difference in the charge distribution below and above 20 MeV.     

Deuteron induced fission of 238U

The deuteron induced (E_d = 14, 15, 16 and 17 MeV) ²³⁸U nuclear-fission cross section of formation has been measured for several nuclides. The valley to peak ratios of the mass yield curves have been analysed using the two-mode-fission-mechanism (TMFM) and a satisfactory agreement with the (d, nf) mode is obtained.     

Nine MeV bremsstrahlung fission of 238U

The mass yield distribution of fission products produced in the photofission of ²³⁸U using 9 MeV bremsstrahlung have been radiochemically measured for 21 mass chains. The absolute activities of the nuclides measured were determined from gross β counting and total chain yields were measured relative to ⁹⁹Mo. The peak to valley ratio was found to be about 310, which is comparable with previous results measured at 10 MeV. Fine structure was found to be a prominent feature and occurred around mass number 133, although mass number 134 could not be measured in the present investigation. The fission yields measured in this work have been compared to all known existing values for work in photon energy range of 9 ± 1 MeV.     

Absolute yields of 99Mo and 111Ag in the reactor neutron induced fission of 238U

The absolute fission yields of ⁹⁹Mo and ¹¹¹Ag have been measured in the reactor neutron induced fission of depleted uranium employing a fission track-cum-radiochemical technique. The absolute disintegrations were determined from beta decay and Ge(Li) gamma spectrometry. The total number of fissions were measured by fission track registration on lexan solid state track detector immersed in fissile material solution. The yields for⁹⁹Mo and ¹¹¹Ag were found to be (6.27 ± 0.30)% and (0.046 ± 0.002)% respectively. The results are compared with the published data and the merits of the present method have been discussed.     

Mass and charge distribution in the fission of 238U

The mass and charge distribution in the fission of ²³⁸U by protons of 15·6 MeV have been determined using Ge(Li) spectroscopy. The total fission cross section was found to be 1005 mb. For 10 shielded or quasi-shielded nuclides, correlation with a Gaussian charge dispersion was investigated using various mass formulae and charge distribution hypotheses. The postulate of minimum potential energy using a shell-effected mass equation was found to best correlate the data.     

Independent yields of 148mPm, 148gPm and 150Pm from fission of 235U and 238U with 14.8 MeV neutrons

We measured the independent fractional chain yields of ^148mPm, ^148gPm, and ¹⁵⁰Pm from fission of ²³⁵U and ²³⁸U irradiated with 14.8-MeV neutrons. The results for ²³⁵U are: ¹⁴⁸Pm, (8.0 ± 0.6) × 10⁻⁶; ^148gPm, (1.05 ± 0.13) × 10⁻⁶; and ¹⁵⁰Pm, (1.79 ± 0.15) × 10⁻³. Those for ²³⁸U are: ^148mPm, ≤ 1.6 × 10⁻⁷; ^148gPm, ≤ 1.3 × 10⁻⁷; and ¹⁵⁰Pm, (2.3 ± 0.7) × 10⁻⁵. We found that the yields are generally smaller than those predicted on the basis of the independent-yield systematics of Nethaway, although they are near the lower end of the predicted range. Values of Z_p inferred from the measured yields average about 0.12 charge unit smaller than the predicted values. The difficulty in estimating yields such as these may be because the actual charge dispersion curve is narrower than assumed, the neutron emission associated with energetically unfavored charge splits is uncertain, or the reference Z_p function is uncertain in this mass region.     

The Spontaneous Fission Decay Constant of 238U Using SSNTD

This work reports the results of 5 measurements of the ²³⁸U decay constant for spontaneous fission, _f carried out using solid state nuclear track detectors (SSNTD), resulting in a mean value of _f = (8.35±0.24)· 10^–17 y^–1. The neutron fluence of the irradiations needed for these measurements were monitored with thin films of natural uranium.     

Determination of the 238U spontaneous fission decay constant without neutron irradiation

We have developed a methodology for measuring the decay constant of the spontaneous fission of ²³⁸U, l_f, using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by spontaneous fission of trans-uranium elements bearing a mass number close to 238 are similar to ²³⁸U spontaneous-fission ones. Loading a thick source of uranium (thickness greater than the fission fragment range) with a small amount of a suitable trans-uranium element (for instance, ²⁴²Pu, which presents a spontaneous fission half-life of 6.75^.10¹⁰ y), it is possible to determine the observation efficiency of a particle-track detector for fission fragments. Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These results make it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a l_f value.     

Absolute yields of the fission products in the neutron induced fission of 232Th and 233U

Absolute yields of 10 fission products in the reactor neutron induced fission of ²³²Th and 17 fission products in the thermal neutron induced fission of ²³³U have been determined using track etch-cum γ-ray spectrometric technique. The number of fissions occurring in the target is obtained by recording the fission events in a mica track detector. The fission product atoms are determined by the direct counting of the irradiated target on a 60 cm³ Ge(Li) detector coupled to a 4096 channel analyser. A one step radiochemical separation was done, before gamma ray counting, for few fission products in the reactor neutron induced fission of ²³²Th. The measured yield values are compared with the available literature data and an analysis of the possible errors are done. The overall uncertainty in the measurements is estimated to be 4–5%.     

Absolute fission yields in the fast neutron induced fission of 233U by track etch combined with gamma-ray spectrometry

The absolute fission yields of twenty seven fission products were determined in the fast neutron induced fission of ²³³U, employing track etch in combination with gamma-ray spectrometry. The total number of fissions was measured by registering the fission tracks on a small strip of lexan, a solid state track detector. The fission products were analysed by gamma-ray spectrometry. The measured yield values were compared to the ENDF/B-VI compilation and show a good agreement.     

Independent yields of 92mNb in the thermal neutron-induced fission of 233U, 235U and 239Pu

Independent yields of ^92mNb were measured in the thermal neutron-induced fission of ²³³U, ²³⁵U, and ²³⁹Pu, using radiochemical techniques. Upper limits of < 2.2 × 10⁻⁸% and < 4.6 × 10⁻⁸% were found for ²³³U and ²³⁵U, respectively. The independent yield for ²³⁹Pu was determined to be (11 ± 6) × 10⁻⁸%. This yield is much higher than values predicted from models of charge and isotopic distributions. It probably reflects the influence of the nearby closed 50 neutron shell.     

Spontaneous-fission decay constant of 238U measured by nuclear track techniques without neutron irradiation

A value of the decay constant for spontaneous fission of ²³⁸U, l_f, determined by particle-track detector techniques, is presented. The main source of systematic error in fission-track measurements of l_f, the so called neutron dosimetry, has been avoided. Instead of calibrating the neutron source and the usual mica detector through tracks of ²³⁵U fission induced by thermal neutrons, spontaneous-fission tracks and alpha-particle tracks of ²³⁸U and ²⁴²Pu were used. A value of l_f = (8.66±0.38)·10^-17 y^-1 has been obtained.     

Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of233U,235U and239Pu

Cumulative yields of short-lived ruthenium isotopes in the thermal neutron induced fission of²³⁵U,²³⁵U and²³⁹Pu have been determined using a fast radiochemical separation technique followed by gamma spectrometry. The cumulative yields of¹⁰⁷Ru and¹⁰³Ru in²³³U (n_th, f) and¹⁰⁷Ru and¹⁰⁹Ru in²³⁹Pu (n_th, f) are determined for the first time. The measured cumulative yields are converted to chain yields assuming normal charge distribution systematics for comparison with the literature data on chain yields.     

Spontaneous fission in the heavy elements

Investigations of mass and kinetic-energy distributions from spontaneous fission have been extended in recent years to an isotope of element 104 and, for half-lives, to an isotope of element 108. The results have been surprising in that spontaneous fission half-lives have turned out to be much longer than expected and mass and kinetic-energy distributions were found to abruptly shift away from those of the lighter actinides, showing two modes of fission. These new developments have caused a re-evaluation of our understanding of the fission process, bringing an even deeper appreciation of the role played by nuclear shell effects upon spontaneous fission properties.