Excitation Energy Of Fragments Research Articles

Transfer reactions and sequential decays of the projectile-like fragments in the 60 MeV/nucleon 40Ar + natAg, 197Au Reactions

Light charged particles and nuclei (1⩽Z⩽8) have been detected in a large forward plastic multidetector (3° < θ <30°) in coincidence with projectile-like fragments (5⩽Z⩽ 19) in the 40Ar+ natAg and 40Ar + 197Au reactions at 60 MeV/nucleon bombarding energy. It is shown that transfer reactions from the projectile to the target are still present at such a high incident energy. Most of the coincident Z = 2 particles are found to be sequentially emitted by excited primary projectile-like fragments. The sequential decay pattern is also observed with the Z = 1 particles, but the main part of those particles probably arise from other mechanisms such as direct and/or pre-equilibrium emission. An attempt is made to derive the excitation energies of the primary projectile-like fragments from these coincident events. The data are not reproduced by the usual models such as the high-energy-abrasion model, the equal-temperature-limit assumption or the equal partition of the excitation energy.

Searhh of quantum and fragment decay effects on observable emission yields of isotopically resolved fragments

Quantum statistics and Maxwell-Boltzmann statistics are adopted to calculate distributions of isotopically resolved nuclear fragments emitted in the break-up of nuclear sources assumed to be in chemical equilibrium. Effects of unstable energy level decay in changing the composition of exploded nuclides before their detection are taken into account. The calculated nuclide distributions are compared with experimental emission yields, for all the measured single nuclides, isotopically resolved, with mass numberA between ≈6 and ≈30. Results obtained by using quantum statistics are very close to that ones deduced by Maxwell-Boltzmann statistics. A remarkable agreement is obtained when excitation energy of fragments and unstable energy level decay effects are taken into account.

Distribution of excitation energy in the 505 MeV 56Fe+165Ho reaction.

The projectile-like fragment excitation energy in the /sup 56/Fe+/sup 165/Ho reaction at 9 MeV/nucleon has been obtained by applying a light particle evaporation calculation to data obtained from coincidence measurements of projectile-like and target-like fragments. It is found that the excitation energy per nucleon is greater for the projectile-like fragment than for the target-like fragment at energy losses less than 200 MeV and that the excitation energy per nucleon is shifted increasingly toward the target-like fragment as a function of increasing energy loss. This shift is found to depend strongly upon the net amount and direction of mass exchange. The variance of the excitation energy about the mean value was also obtained with the result that this variation is approximately a constant fraction of the total excitation energy above an energy loss of 75 MeV.

Neutron evaporation during fission fragment acceleration

Neutron evaporation during fission fragment acceleration is studied in the framework of a modified cascade emission model for symmetric fission of 252Cf. In particular, the fragment excitation energy is considered as a function of time after scission on the basis of different assumptions. It is shown that the fraction of neutrons evaporated during fragment acceleration and their energy and angular distribution are very sensitive to the time behaviour of the dissipation of deformation energy. The results are discussed with regard to studies of emission mechanisms and investigations of the competition between fission and neutron emission in the case of highly excited fissionable nuclei.

Variation of even-odd proton effects with excitation energy in fission of 252Cf: Confirmation of fine structure in v( A)

The existence of even-odd proton effects (fine structure) in νv( A) of individual fragments from 252Cf(sf) has been confirmed by analysis of previously reported data. The fine structure is seen to increase in amplitude as the fragment excitation energy decreases, a behaviour similar to the well-known behaviour of the mass yield curve. The present mass yield data also show these effects, as expected. Conversely, our total kinetic energy data TKE( A), not previously reported, show very little even-odd proton effect. It is shown that energy balance considerations, also, support the existence of even-odd proton effects in neutron emission from 252Cf(sf).

Variation of even-odd charge effects with excitation energy in ν(a) and mass yield for 252Cf(sf)

Abstract The existence of even-odd charge effects (fine structure) in ν(A) for 252Cf(sf) has been confirmed. The fine structure is seen to increase in amplitude as the fragment excitation energy decreases, a behaviour similar to that of the mass yield curve.

Anomalon Production by Impulsive Excitation in Relativistic Heavy-Ion Collisions

We have calculated the excitation energies of projectile fragments produced when relativistic projectiles make peripheral collisions with target nuclei. We find that the excitation energy of the fragment is much greater when the target nucleus is relatively heavy (such as Ag or Br) than when it is light (C or O). This could explain the difference between the results of anomalon searches in nuclear emulsion and plastic detectors.

Alpha-particle emission in the strongly damped reaction of 165Ho + 56Fe at 8.3 MeV/nucleon

Alpha-particle emission in coincidence with damped reaction fragments is studied for the 165Ho + 56Fe (Elab = 465 MeV) reaction. Statistical emission from the two fully accelerated strongly damped reaction fragments accounts for the dominant sources of the α particles. The α-particle multiplicity Mα depends strongly on the fragment excitation energy and atomic number, with an average multiplicity 〈Mα〉 = 0.2 for Fe-like fragments from damped reaction events with −Q > 100 MeV. Temperatures derived from neutron and α-particle spectra are in good agreement.

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.

Fragment mass and kinetic energy distributions for the photofission ofU235with 12-, 15-, 20-, 30-, and 70-MeV bremsstrahlung

Energy correlation measurements were performed for the photofission of $^{235}\mathrm{U}$ with 12-, 15-, 20-, 30-, and 70-MeV bremsstrahlung. Overall fragment mass and kinetic energy distributions are deduced. The behavior of the total fragment kinetic energy as a function of the fragment mass and excitation energy of the compound nucleus is studied. The results are interpreted in terms of the scission-point model of Wilkins et al.NUCLEAR REACTIONS, FISSION $^{235}\mathrm{U}$($\ensuremath{\gamma}$,$f$), ${E}_{\ensuremath{\gamma}max}=12, 15, 20, 30, \mathrm{and} 70$ MeV; measured fragment energies ${E}_{1}$, ${E}_{2}$; deduced $\frac{N(\ensuremath{\mu},{E}_{K})}{〈{E}_{\mathrm{exc}}({E}_{e})〉}$.

Evidence for shape deformations of fragments in deep inelastic collisions

The number and energy spectra of neutrons evaporated by the heavy and light fragments in deep inelastic collisions have been obtained for the system 240 MeV40Ar +197Au. They indicate that at the scission point the deformation energy is a large part of the total excitation energy espe cially for the light fragment. In the last two years, neutron emission has been studied in deep inelastic reactions1−6. All the results are similar: the incident energy which is lost during the process is transformed into excitation energy of the final fragments and is shared between the two products proportionnally to their masses. This means that the nuclear temperature was uniform in the composite system. The excitation energy equilibration time is really very short since equilibration is achieved after reaction times as small as 10−22 s2,6. Moreover, no preequilibrium emission (direct neutrons-) has been observed.

Magnitude of energy dissipation in fission

It is shown that no definite conclusions can be drawn regarding the magnitude of energy dissipation in fission during the descent from the saddle point, by a comparison of the calculated deformation and interaction energies of fission fragments near scission, with the experimental fragment excitation and kinetic energies. This is contrary to the conclusions drawn by Schultheis and Schultheis on the basis of a similar analysis. The reasons for this discrepancy are also discussed.

Estimation of Neutron Yield from Individual Fragment in Medium-Excitation Fission

A method of calculation is described to estimate the average number of neutrons emitted per fragment in medium-excitation fission from published experimental data on neutron emission in thermal-neutron induced fission, average total kinetic energy as a function of fragment mass and mass yield in low- and medium-excitation fission reactions. Use is made of a relation of fragment excitation energy with internal excitation and deformation energies, and the difference in kinetic energy between the fission reactions at two-excitation energies. A tentative calculation is made for the fission of 238U induced by 12 MeV protons. The results are in good agreement with experimental data. The method developed in the present work may make it possible to predict the average number of neutrons emitted from individual fragment in medium-excitation fission which has not yet been measured experimentally.

Abrasion-ablation calculations of large fragment yields from relativistic heavy ion reactions

Calculations of large mass fragment yields from high-energy heavy-ion reactions are performed based on the abrasion-ablation model. The geometrical picture of the clean-cut fireball model is used to calculate the number of participant nucleons in the abrasion stage and the excitation energy of the spectators (primary residues). A standard statistical evaporation code is used to calculate the ablation stage. Results from this model show an overall agreement with experimental data, although some systematic discrepancies are found and discussed. A frictional spectator interaction is introduced which increases the average excitation energy of primary fragments and improves the results considerably.NUCLEAR REACTIONS Relativistic Heavy-ions, abrasion-ablation model, peripheral reactions, calculated fragmentation cross sections.

Estimation of neutron yield from individual fragment in medium-excitation fission.

A method of calculation is described to estimate the average number of neutrons emitted per fragment in medium-excitation fission from published experimental data on neutron emission in thermal-neutron induced fission, average total kinetic energy as a function of fragment mass and mass yield in low- and medium-excitation fission reactions. Use is made of a relation of fragment excitation energy with internal excitation and deformation energies, and the difference in kinetic energy between the fission reactions at two-excitation energies. A tentative calculation is made for the fission of 238U induced by 12 MeV protons. The results are in good agreement with experimental data. The method developed in the present work may make it possible to predict the average number of neutrons emitted from individual fragment in medium-excitation fission which has not yet been measured experimentally.

A statistical analysis of gamma emission in actinide fission

A statistical analysis makes it possible to obtain the basic properties of the angular momenta of the fission fragments. In particular it is possible to reproduce the average value of j for the various fragments and its saw-tooth dependence on A. The anisotropy observed experimentally in gamma emission is also reproduced as being due to a polarisation effect of the spins in the plane perpendicular to the motion axis. The fragment excitation energy and subsequent neutron and gamma emissions are, however, related to the final stage of the fission process. The model developed by the authors and hereafter referred to as the 'partial equilibrium model' has given a satisfactory evaluation of these excitation energies and has allowed a correct description of the fragment charge distributions.

A statistical approach to the scission mechanism

A statistical model of the scission mechanism is reported; the fission fragments are described as two nuclear droplets at given temperature and distance. The canonical formalism is introduced in order to describe the states of the system; the intrinsic degrees of freedom, representing both proton and neutron repartitions in a given pair A1, A2 and the various possible configurations assumed by the excited nucleons are considered to be in statistical equilibrium. The degrees of freedom related to the collective motion of nucleons, which means to the fragment kinetic energies, are not assumed to be in statistical equilibrium. The partial equilibrium model has been applied to the calculations of fragment excitation energies and to the analysis of the fragment charge distributions. 235U fission induced by thermal neutrons has been analysed.

Fine structures in the mass yield of252Cf fission fragments from a double-energy measurement

We report the results of a measurement of the kinetic energies of the complementary fragments released in the spontaneous fission of252Cf. The pre-neutron emission values of the fragment masses are reconstructed starting from the energies measured. For low excitation energies of the fragments, six fine structures in the mass distribution are observed. These results are discussed referring to previous experimental data, and correlated with preferred even-even charge division in the fission process.

A statistical model for excitation energy distributions in actinide fission

The last stage of the fission process has been studied on the basis of a statistical model. Two fundamental hypotheses are made: (i) the internal degrees corresponding to the fragment nuclear structure are in conditions of statistical equilibrium and (ii) the freedom degrees related to the fragment motion at the scission point are not in statistical equilibrium with the internal degrees. The proposed statistical model reproduces the general properties of the excitation energies of the fission fragments in actinide fission.

Calculation of charge vibration in fission with Strutinsky shell correction

The Strutinsky shell correction method has been applied to the two spheroid model to study charge vibrations in fission. The investigation is carried out by calculating the potential energy surface with respect to three degrees of freedom: charge vibration from the uniform value and the deformations of the two fragments. The results suggest that the effect of shells at Z = 50 and N = 82 do not cause large deviations from the liquid drop model charge density around mass 132; their effect is much more pronounced in the fragment excitation energy. The results also suggest that the fragment excitation and kinetic energies for a given mass ratio are markedly charge density dependent. Some features inherent to this treatment with respect to characteristic periods of individual degrees of freedom have been discussed.