190 research outputs found
Particle-hole state densities with non-equidistant single-particle levels
The correct use of energy-dependent single-particle level (s.p.l.) densities
within particle-hole state densities based on the equidistant spacing model
(ESM) is analysed. First, an analytical expression is obtained following the
convolution of energy-dependent excited-particle and hole densities. Next, a
comparison is made with results of the ESM formula using average s.p.l.
densities for the excited particles and holes, respectively. The Fermi-gas
model (FGM) s.p.l. densities calculated at the corresponding average excitation
energies are used in both cases. The analysis concerns also the density of
particle-hole bound states. The pairing correlations are taken into account
while the comparison of various effects includes the exact correction for the
Pauli exclusion principle. Quantum-mechanical s.p.l. densities and the
continuum effect can also match a corresponding FGM formula, suitable for use
within the average energy-dependent partial state density in multistep reaction
models.Comment: 29 pages, ReVTeX, 11 postscript figures, submitted to Phys.Rev.
Nucleon-induced reactions at intermediate energies: New data at 96 MeV and theoretical status
Double-differential cross sections for light charged particle production (up
to A=4) were measured in 96 MeV neutron-induced reactions, at TSL laboratory
cyclotron in Uppsala (Sweden). Measurements for three targets, Fe, Pb, and U,
were performed using two independent devices, SCANDAL and MEDLEY. The data were
recorded with low energy thresholds and for a wide angular range (20-160
degrees). The normalization procedure used to extract the cross sections is
based on the np elastic scattering reaction that we measured and for which we
present experimental results. A good control of the systematic uncertainties
affecting the results is achieved. Calculations using the exciton model are
reported. Two different theoretical approches proposed to improve its
predictive power regarding the complex particle emission are tested. The
capabilities of each approach is illustrated by comparison with the 96 MeV data
that we measured, and with other experimental results available in the
literature.Comment: 21 pages, 28 figure
Equilibrium and pre-equilibrium processes in the 55Mn(6Li,xp) and 57Fe(a,xp) reactions
Spectra of outgoing neutrons and protons from the 6Li+55Mn reaction and
protons from the a+57Fe reaction have been measured with beams of 15 MeV 6Li
ions and 30 MeV alpha-particles. These reactions proceed through the same 61Ni
nucleus at the same excitation energy, thus allowing the difference in reaction
mechanism to be studied. It is shown that spectra from the first reaction
measured at backward angles are due to emission from a traditional compound
nucleus reaction, in which the intermediate nucleus has reached statistical
equilibrium; the spectra from the second reaction contain a significant
fraction of pre-equilibrium emission at all angles. Level density pa- rameters
of the residual nucleus 60Co have been obtained from the first reaction. Both
emission spectra and angular distributions have been measured for the second
reaction. It was found that the pre-equilibrium component exhibits a
forward-peaked angular distribution, as expected, but with a steeper slope than
predicted and with an unusual slight rise at angles above 120\deg. The
backward- angle rise is explained qualitatively by the dominance of the
multi-step compound mechanism at backward angles
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Phenomenology of preequilibrium angular distributions
The systematics of continuum angular distributions from a wide variety of light ion nuclear reactions have been studied. To first order, the shape of the angular distributions have been found to depend only on the energy of the outgoing particle and on the division of the cross section into multi-step direct and multi-step compound parts. The angular distributions can be described in terms of Legendre polynomials with the reduced polynomial coefficients exhibiting a simple dependence on the outgoing particle energy. Two integer and four continuous parameters with universal values are needed to describe the coefficients for outgoing energies of 2 to 60 MeV in all the reaction types studied. This parameterization combined with a modified Griffin model computer code permits the calculation of double differential cross sections for light ion continuum reactions where no data is available
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