278 research outputs found
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Utilization of Baeten's Dead-Time Correction Formalism for Multiplicity Counting
We evaluate a dead-time correction formalism for double and triple coincidences in multiplicity counting. We will describe the accuracy by which S (singles), D (doubles), and T (triples) (and consequently, the mass, multiplication, and alpha) of fissile material samples can be determined using this formalism
Quasi Free 238U (e,e'f)-Cross Section in Macroscopic-Microscopic Approach
We present the result of a theoretical study of inclusive quasi free
electrofission of U. The off-shell cross sections for the quasi free
reaction stage have been calculated within the Plane Wave Impulse Approximation
(PWIA), using a Macroscopic -Microscopic description of the proton and neutron
single particle momentum distributions. Electron wave function distortion
corrections were included using the effective momentum approximation, and the
Final State Interaction (FSI) effects were calculated using an optical
potential. The fissility for the proton single hole excited states of the
residual nucleus Pa was calculated both without and with contributions
of the pre-equilibrium emission of the particles. The fissility for
residual nuclei was calculated within the compound nucleus model.
The cross sections thus obtained were compared with available
experimental data.Comment: 26 pages, 7 figure
Evidence for transfer followed by breakup in 7Li + 65Cu
The observation of a large cross-section for the alpha + d channel compared
to breakup into the alpha + t channel from an exclusive measurement for the
7Li+65Cu system at 25 MeV is presented. A detailed analysis of the angular
distribution using coupled channels Born approximation calculations has
provided clear evidence that the observed alpha + d events arise from a two
step process, i.e. direct transfer to the 2.186 MeV (3+) resonance in the alpha
+ d continuum of 6Li followed by breakup, and are not due to final state
interaction effects.Comment: 12 pages, 3 figures, To be published in Phys. Letts.
Enhanced Fusion-Evaporation Cross Sections in Neutron-Rich Sn on Ni
Evaporation residue cross sections have been measured with neutron-rich
radioactive Sn beams on Ni in the vicinity of the Coulomb
barrier. The average beam intensity was particles per second
and the smallest cross section measured was less than 5 mb. Large subbarrier
fusion enhancement was observed. Coupled-channels calculations taking into
account inelastic excitation and neutron transfer underpredict the measured
cross sections below the barrier.Comment: 4 pages including 1 table and 3 figure
Spectroscopy of Po
Prompt, in-beam rays following the reaction Yb + 142 MeV
Si were measured at the ATLAS facility using 10 Compton-suppressed Ge
detectors and the Fragment Mass Analyzer. Transitions in Po were
identified and placed using -ray singles and coincidence data gated on
the mass of the evaporation residues. A level spectrum up to
J10 was established. The structure of Po is more
collective than that observed in the heavier polonium isotopes and indicates
that the structure has started to evolve towards the more collective nature
expected for deformed nuclei.Comment: 8 pages, revtex 3.0, 4 figs. available upon reques
Angular anisotropy of the fusion-fission and quasifission fragments
The anisotropy in the angular distribution of the fusion-fission and
quasifission fragments for the O+U, F+Pb and
S+Pb reactions is studied by analyzing the angular momentum
distributions of the dinuclear system and compound nucleus which are formed
after capture and complete fusion, respectively. The orientation angles of
axial symmetry axes of colliding nuclei to the beam direction are taken into
account for the calculation of the variance of the projection of the total spin
onto the fission axis. It is shown that the deviation of the experimental
angular anisotropy from the statistical model picture is connected with the
contribution of the quasifission fragments which is dominant in the
S+Pb reaction. Enhancement of anisotropy at low energies in the
O+U reaction is connected with quasifission of the dinuclear
system having low temperature and effective moment of inertia.Comment: 17 pages 8 figures. Submitted to Euro. Phys. Jour.
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Radiation imaging technology for nuclear materials safeguards
Gamma-ray and neutron imaging technology is emerging as a useful tool for nuclear materials safeguards. Principal applications include improvement in accuracy for nondestructive assay of heterogeneous material (e.g., residues) and wide-area imaging of nuclear material in facilities (e.g., holdup). Portable gamma cameras with gamma-ray spectroscopy are available commercially and are being applied to holdup measurements. The technology has the potential to significantly reduce effort and exposure in holdup campaigns; and, with imaging, some of the limiting assumptions required for conventional holdup analysis can be relaxed, resulting in a more general analysis. Methods to analyze spectroscopic-imaging data to assay plutonium and uranium in processing equipment are being development. Results of holdup measurements using a commercial, portable gamma-cameras are presented. The authors are also developing fast neutron imaging techniques for NDA, search, and holdup. Fast neutron imaging provides a direct measurement of the source of neutrons and is relatively insensitive to surroundings when compared to thermal or epithermal neutron imaging. The technology is well-suited for in-process inventory measurements and verification of materials in interim storage, for which gamma-ray measurements may be inadequate due to self-shielding. Results of numerical simulations to predict the performance of fast-neutron telescopes for safeguards applications are presented
Very high rotational frequencies and band termination in 73Br
Rotational bands in 73Br have been investigated up to spins of 65/2 using the
EUROBALL III spectrometer. One of the negative-parity bands displays the
highest rotational frequency 1.85 MeV reported to date in nuclei with mass
number greater than 25. At high frequencies, the experimental dynamic moment of
inertia for all bands decrease to very low values, indicating a loss of
collectivity. The bands are described in the configuration-dependent cranked
Nilsson-Strutinsky model. The calculations indicate that one of the
negative-parity bands is observed up to its terminating single-particle state
at spin 63/2. This result establishes the first band termination case in the A
= 70 mass region.Comment: 6 pages, 6 figures, submitted to Phys. Rev. C as a Rapid
Communicatio
Neutrino-induced neutron spallation and supernova r-process nucleosynthesis
In order to explore the consequences of the neutrino irradiation for the
supernova r-process nucleosynthesis, we calculate the rates of charged-current
and neutral-current neutrino reactions on neutron-rich heavy nuclei, and
estimate the average number of neutrons emitted in the resulting spallation.
Our results suggest that charged-current captures can be important in
breaking through the waiting-point nuclei at N=50 and 82, while still allowing
the formation of abundance peaks. Furthermore, after the r-process freezes out,
there appear to be distinctive neutral-current and charged-current
postprocessing effects. A subtraction of the neutrino postprocessing effects
from the observed solar r-process abundance distribution shows that two mass
regions, A=124-126 and 183-187, are inordinately sensitive to neutrino
postprocessing effects. This imposes very stringent bounds on the freeze-out
radii and dynamic timescales governing the r-process. Moreover, we find that
the abundance patterns within these mass windows are entirely consistent with
synthesis by neutrino interactions. This provides a strong argument that the
r-process must occur in the intense neutrino flux provided by a core-collapse
supernova.Comment: 34 pages, 4 PostScript figures, RevTe
Prompt dipole radiation in fusion reactions
The prompt gamma ray emission was investigated in the 16A MeV energy region
by means of the 36,40Ar+96,92Zr fusion reactions leading to a compound nucleus
in the vicinity of 132Ce. We show that the prompt radiation, which appears to
be still effective at such a high beam energy, has an angular distribution
pattern consistent with a dipole oscillation along the symmetry axis of the
dinuclear system. The data are compared with calculations based on a collective
bremsstrahlung analysis of the reaction dynamics
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