R.P. Schmitt

Temperatures, barriers, and level densities of highly excited nuclei with A ≈ 160☆

Abstract From coincidence measurements between heavy residues, light particles, and γ -rays, the excitation excitation energy dependence of the temperatures, barriers, and nuclear level density parameters for nuclei with A ≈ 160 has been determined. The temperature increases with excitation energy in the range of 100 to 400 MeV consistently with a nuclear level density parameter a increasing from essentially a = A /8 to a = A /13. The emission barrier is lower than predicted by either spin dependent or temperature dependent theoretical calculations.

Production and characterization of hot nuclei in the reactions of 19 and 35 MeV/u 14N with 145Sm

Abstract Hot nuclei produced in the reactions of 261 MeV and 490 MeV 14 N with 154 Sm have been studied. The initial properties of these nuclei; excitation energies, angular momenta and temperatures, have been characterized through measurements of residue velocities, gamma ray multiplicities and α-particle energy spectra. Nuclei with excitation energies as high as 400 MeV and temperatures as high as 6 MeV are produced. Determinations of the variation of temperature with excitation energy for nuclei of A ⋍ 160 indicate that the apparent level density parameter a , defined as E ∗ |T 2 changes from A /8 at low energies to ≈ A /13 at 400 MeV excitation energy. Effective particle emission barriers suggest large shape fluctuations during the de-excitation cascade. At 35 MeV/u the variation of angular momentum transfer with linear momentum transfer in incomplete fusion reactions is in reasonable agreement with values calculated using a geometric overlap model.

Determination of the temperatures of hot nuclei from “first chance” emission spectra

Abstract The “first chance” spectra of light charged particles and neutrons emitted from 160Yb produced in the 60Ni+100Mo reaction have been isolated. The same initial temperature for the different evaporated particles and a decrease of the Coulomb barriers for the charged particles are observed. The inverse level density parameter K= A/ a reaches a value of 13.8±0.7 MeV at ETH≈236 MeV, confirming the decrease of the parameter a at excitation energies above 150 MeV.

A flexible 4π neutron detector for in-beam studies: the Texas A&M neutron ball

Abstract A 4π “neutron ball” is described. This detector operates on the same principles as the liquid scintillator tanks traditionally employed in low energy fission studies. However, the current device has been specifically designed for in-beam experiments. The system features a comparatively large scattering chamber and a segmented tank. The response of the detector to 252 Cf neutrons is discussed and compared to Monte Carlo simulations. Some results from in-beam studies are also presented.

Angular momentum transfer in 12C-, 20Ne- and 40Ar-induced fission

Abstract Angular momentum transfer in a variety of 12C-, 20Ne- and 40Ar-induced fission reactions has been investigated using γ-ray multiplicity techniques. Fission fragments were detected in coincidence using a pair of solid-state detectors. The fragment masses were deduced from the kinetic energies and emission angles using two-body kinematics. The γ-ray multiplicities (Mγ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, Mγ is nearly independent of the exit-channel mass asymmetry. The strongest dependence on mass is observed in the systems 154sm + 240 MeV 40Ar, where a minimum exists at symmetry, and 197Au + 164 MeV 20Ne, where nuclear structure effects are suggested by the data. For all the reactions the quantity Mγ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of Mγ generally appears to be larger than expected on the basis of rigid rotation, suggesting a spin enhancement effect. The data are compared with a simple model which assumes the statistical excitation of a variety of angular momentum bearing collective modes. Reasonable agreement is obtained with the experimental results. The roles of other collective effects, such as shape fluctuations and angular momentum fractionation, are also considered.

Continuous distribution of αt relative kinetic energies in 7Li breakup reactions

Abstract A newly developed technique has been used to probe a previously inaccessible region of the continuum in the at channels of 7 Li breakup reactions. This was used to investigate reactions of 63 MeV 7 Li with 58 Ni, 120 Sn, 144 Sm, and 208 Pb targets. Breakup pairs of α-particles and tritons emitted within a finite opening angle were analyzed according to their magnetic rigidity using a K = 150 Enge split-pole spectrograph. The αt coincidences were taken with two single-wire proportional counters mounted at different positions along the focal plane of the spectrograph. The large energy acceptance ( E max / E min ≈ 8) of the spectrograph is crucial to the present coincidence measurements involving two particles with different q / m values. Very small relative energy components were studied without any detection threshold. A pronounced bump was observed immediately above the αt particle threshold. No strong indication of the post Coulomb acceleration was observed. The excitation of the continuum states as well as the 7 − 2 state in 7 Li is discussed in terms of the distorted-wave Born approximation. The astrophysical implications of the data are also discussed regarding the Coulomb excitation of the continuum. The present technique offers the promise of very high sensitivity at small relative energies if it were applied in conjunction with ray-tracing, a thin target, and a dispersion-matched beam.

Observation of α-t continuum states with relative energies of 0–2 MeV in7Li breakup reactions

Abstract The breakup of 7 Li at 63 MeV was studied in an α-t coincidence experiment using an Enge split pole spectrograph. For the first time, the entire spectrum of the α-t continuum states of 7 Li was observed in the range of the relative energy ɛ from zero to 2 MeV. A pronounced bump was found at ɛ ≈ 0.4MeV. The astrophysical implications of the data are discussed.

Strongly-suppressed post-Coulomb acceleration in non-resonant breakup of 7Li

Abstract Cross sections were measured at energies of astrophysical relevance for the non-resonant breakup of 42 MeV– 7 Li by 6 targets. It was found that post-acceleration of the α and the triton in the target Coulomb field is vanishingly small, independent of the target nucleus, compared to that for prompt breakup. The transition probabilities were evaluated for the astrophysical continua in 7 Li which decay by tunneling through the Coulomb barrier. They are consistent with the present observations.

Thermal properties and dynamics of hot nuclei

Abstract The heat capacities and the dynamics of hot medium mass nuclei have been explored by studies of light particle emission. The variation in heat capacity with temperature, expressed in terms of an effective Fermi gas level density parameter appropriate to the statistical description of nuclear de-excitation, corresponds to a decrease in a from A/8 to A/13 as the excitation energy increases from 1 to 2 MeV/nucleon. This result is compared to recent theoretical predictions. An apparent sudden change in the heat capacity observed at slightly higher excitation energies per nucleon is also discussed. Prescission and post-scission particle multiplicities and temperatures in coincidence with evaporation residues and fission fragments provide further information on the decay dynamics of hot nuclei. Possible means of isolating presaddle emission and of determining the temperature dependence of the fission barrier are discussed.

Probing the tilting mode in fission with γ-ray multiplicity techniques

Abstract The γ-ray multiplicities of the fragments from a number of 4 He-induced fission reactions have been measured as a function of fragment emission angle. The value of M γ is found to vary with angle in qualitative agreement with the predictions of statistical models of fission-fragment angular distributions. The observed variation is rather weak, on the order of 5%. The data are compared with several models. Calculations assuming a rigidly rotating, transition-state nucleus predict a much stronger angular dependence of the fragment spin than indicated by the data. The agreement is significantly improved if a fragment-spin enhancement effect is included in these calculations. The fragment spins are explored within the framework of the statistical scission model and the collective mode model. Both of these models predict large fragment spins and thus a relatively weak angular dependence of the total fragment spin. Neither model provides a completely satisfactory explanation of the data. Depending on the assumptions made in the calculations, the models either predict too strong an angular dependence of the total fragment spin or spins which seem incompatible with M γ .