De-excitation Of Fission Fragments Research Articles

Modes of de-excitation of fission fragments

Requirements to be met in adequate descriptions of fission fragment de-excitation by nucleon and γ-ray emission are summarized. In addition to the use of standard evaporation theory adopted to the diversity of fragment configurations, the statistical multi-step compound theory (SMC) including pre-equilibrium effects has been modified to account for nucleon emission from fission fragments after scission as an chaotic process. Results of calculations are compared with experimental energy and angular distributions of neutrons and protons from 252-Cf(sf). It is shown that nucleon emission in fission is mainly due to evaporation from fully accelerated, highly excited fragments. The possible role of particle emission close to scission point is discussed.

Monte Carlo calculation of the deexcitation of fission fragments in the spontaneous fission ofCf252

The consequences of the evaporation of prompt neutrons and $\ensuremath{\gamma}$ rays from primary fission fragments in the spontaneous fission of $^{252}\mathrm{Cf}$ have been examined by a Monte Carlo method. A semiempirical mass equation and experimental values of total kinetic energies are used to obtain the sum of excitation energies of complementary fragments. The division of this energy between complementary fragments is determined by the two-spheroid model for the scission configuration. The prompt neutron multiplicity distribution gives a value of 3.68 for the average number of prompt neutrons in $^{252}\mathrm{Cf}$(sf). Calculated values of the average number of prompt neutrons as a function of fragments mass are in good agreement with the experimental values. Calculated results for the charge dependence of total $\ensuremath{\gamma}$-ray energies show an odd-even effect, whereas no such effect is apparent in the charge dependence of the average number of neutrons. Mass and charge distributions of secondary fission fragments are also well reproduced in the calculations.NUCLEAR REACTIONS, FISSION $^{252}\mathrm{Cf}$(sf); calculated prompt neutrons and $\ensuremath{\gamma}$ rays. Pairing effect on $\ensuremath{\gamma}$-ray energy.

A study of the low-energy transitions arising from the prompt de-excitation of fission fragments

A four-dimensional experiment has been performed in which the energies of coincident complementary fragment pairs, internal conversion electrons, and K X-rays emitted as a result of the spontaneous fission of 252Cf were recorded event by event using a multiparameter analyser. The fission fragment energies were used for mass identification and the X-ray energies supplied precise atomic number identification. The analysis of the conversion electron spectra has resulted in the assignment of numerous transitions to new isotopes, the determination of many transition half-lives, and the procurement of limited information concerning multipolarities. It was concluded, based on the X-ray and conversion electron data, that most of the large number of observed low-energy transitions associated with heavy fragments are attributable to odd-mass or doubly odd nuclei and that surprisingly little contribution to the low-energy spectra is associated with doubly even nuclei in the rare-earth region. Low-energy transitions in doubly even nuclei were found to be more prevalent among the light fragments. Data are presented supporting the assignment of a high intensity 241 keV transition as the 2 + to 0 + transition in 110Ru. An examination of this transition in terms of the energy predicted on the basis of rotational behaviour and with regard to the systematics of neighboring doubly even Ru isotopes reveals evidence which strongly suggests that 110Ru belongs to a new region of stable deformation.

PromptKX Rays as a Function of Fragment Mass and Total Kinetic Energy in the Thermal Fission ofU235

In order to obtain information about the deexcitation of fission fragments and the division of nuclear charge in fission for the case of thermal-neutron fission of ${\mathrm{U}}^{235}$, a measurement of the prompt $K$ x rays (0 to\ensuremath{\sim}1 nsec after fission) was performed. A thin foil of uranium was caused to fission by a beam of neutrons from a beam port of a nuclear reactor. The fission fragments were detected by silicon surface-barrier detectors, and the x rays, by a NaI(Tl) scintillator. A three-parameter analyzer recorded the energies of the two fragments and the energy of the x ray emitted in coincidence with the fragments. The x-ray energies were sorted "off line" according to fragment mass or total kinetic energy in order to obtain x-ray spectra for different mass groups and different energy groups. From these spectra, x-ray yields were found as a function of fragment mass and as a function of total kinetic energy of the fragments. The results are compared, where possible, with those obtained in other laboratories for the thermal-neutron fission of ${\mathrm{U}}^{235}$ and with results for the spontaneous fission of ${\mathrm{Cf}}^{252}$. Differences and similarities are noted. The most probable charge versus mass was also estimated from the spectra, and these results are in agreement with radiochemical analysis.

Angular Momentum Effects in the Gamma-Ray De-Excitation of Fission Fragments

The energy dissipated as $\ensuremath{\gamma}$ rays during the de-excitation of fission fragments (7.2\ifmmode\pm\else\textpm\fi{}0.8 MeV for ${\mathrm{U}}^{235}$ thermal fission) is somewhat higher than has been estimated theoretically (4.9 MeV). It has been suggested that this discrepancy arises from the angular momentum of the fission fragments. We have made a quantitative evaluation of this possibility, taking into account the angular momentum dependence of the level density, the nonexistence of levels of a given angular momentum below some minimum energy (yrast energy), and the competition between neutron and $\ensuremath{\gamma}$-ray emission. The initial angular momentum distribution is that derived from measurements of isomers produced in fission. Initial excitation energies are based on the known de-excitation properties of fission fragments. All other parameters were derived from sources having no direct connection with the fission process. The calculations for an average pair of fragments (${\mathrm{Sr}}^{96}$ and ${\mathrm{Xe}}^{140}$) indicate that 7.1 MeV should appear as $\ensuremath{\gamma}$ rays and that the average photon energy is 0.9 MeV. The calculated average neutron energy and number of neutrons are also in agreement with experiment.

Gamma de-excitation of fission fragments: (II). Delayed radiation

The delayed part of the gamma radiation emitted from fission fragments of Cf 252 has been studied. The fission fragments were detected with semiconductor counters and the gamma radiation with a scintillation spectrometer. Mass-ratio selection was employed so that the properties of the gamma radiation could be studied as a function of fragment mass. The delayed radiation was found to have an intensity of 6% relative to the prompt one. The half-life depends on the mass and on the delay. It varies in the range 15–100 nsec. The mass spectrum of the fragments associated with delayed gamma radiation was found to exhibit a pronounced fine structure with most of the delayed gamma emission concentrated to deformed and magic fragments. The gamma-ray spectra have a shape consistent with a rotational cascade in deformed nuclei and a vibrational cascade in magic nuclei, respectively. Of special interest is the fact that the gamma emission from some of the fragments in the mass range 92–110 has the same characteristics as the emission from the deformed fragments in the rare-earth region. It is therefore concluded that these fragments have a stable deformation. Various theoretical and experimental evidence for the existence of this new region of deformation is discussed.

Gamma de-excitation of fission fragments: (I). Prompt radiation

The gamma radiation emitted in fission of Cf 252 has been investigated in coincidence with the fission fragments. The mass ratio of the fragments was recorded, various properties of the radiation being studied as a function of the mass ratio. This paper deals with the prompt radiation, i.e. radiation with a half-life shorter than 1 nsec. The life-time of the gamma emitting states was measured by the time-of-flight technique. Using a collimator, the gamma radiation was detected after the fragments had travelled different distances. In this way, the decay curve was investigated and the half-life of the radiation was found to be about 1 · 10 −11 sec. Using this information, it was possible to find a collimator setting which permitted a complete separation of the radiation from the two fragments. The properties of the radiation could then be studied as a function of mass. Thi arrangement was used to record gamma spectra for a number of different masses. It was found that there is a pronounced variation in spectrum shape. The spectra from fragments close to magic numbers are shifted to higher energies. With the same arrangement the yield of gamma rays was determined as a function of mass. The yield curve has a saw-tooth shape. The significance of the experimental results is discussed. The characteristic feature of the de-excitation process seems to be that it involves high spin states. It is estimated that the average value of the initial spin is about 10. The results of this work indicate that the radiation mainly consists of vibrational transitions. This fact is explained as a consequence of the conditions at scission.