266 research outputs found
Multiple scattering and attenuation corrections in Deep Inelastic Neutron Scattering experiments
Multiple scattering and attenuation corrections in Deep Inelastic Neutron
Scattering experiments are analyzed. The theoretical basis is stated, and a
Monte Carlo procedure to perform the calculation is presented. The results are
compared with experimental data. The importance of the accuracy in the
description of the experimental parameters is tested, and the implications of
the present results on the data analysis procedures is examined.Comment: 19 pages, 8 figure
Uncertainties In Direct Neutron Capture Calculations Due To Nuclear Structure Models
The prediction of cross sections for nuclei far off stability is crucial in
the field of nuclear astrophysics. For spherical nuclei close to the dripline
the statistical model (Hauser-Feshbach) approach is not applicable and direct
contributions may dominate the cross sections. For neutron-rich, even-even Sn
targets, we compare the resulting neutron capture cross sections when
consistently taking the input for the direct capture calculations from three
different microscopic models. The results underline the sensitivity of cross
sections calculated in the direct model to nuclear structure models which can
lead to high uncertainties when lacking experimental information.Comment: 4 pages, using espcrc1.sty, Proc. Intl. Conf. "Nuclei in the Cosmos
IV", Univ. Notre Dame 1996, Nucl. Phys. A, in press. A postscript version can
also be obtained from http://quasar.physik.unibas.ch/research.htm
Test of nuclear level density inputs for Hauser-Feshbach model calculations
The energy spectra of neutrons, protons, and alpha-particles have been
measured from the d+59Co and 3He+58Fe reactions leading to the same compound
nucleus, 61$Ni. The experimental cross sections have been compared to
Hauser-Feshbach model calculations using different input level density models.
None of them have been found to agree with experiment. It manifests the serious
problem with available level density parameterizations especially those based
on neutron resonance spacings and density of discrete levels. New level
densities and corresponding Fermi-gas parameters have been obtained for
reaction product nuclei such as 60Ni,60Co, and 57Fe
Parity-Violating Interaction Effects in the np System
We investigate parity-violating observables in the np system, including the
longitudinal asymmetry and neutron-spin rotation in np elastic scattering, the
photon asymmetry in np radiative capture, and the asymmetries in deuteron
photo-disintegration d(gamma,n)p in the threshold region and
electro-disintegration d(e,e`)np in quasi-elastic kinematics. To have an
estimate of the model dependence for the various predictions, a number of
different, latest-generation strong-interaction potentials--Argonne v18, Bonn
2000, and Nijmegen I--are used in combination with a weak-interaction potential
consisting of pi-, rho-, and omega-meson exchanges--the model known as DDH. The
complete bound and scattering problems in the presence of parity-conserving,
including electromagnetic, and parity-violating potentials is solved in both
configuration and momentum space. The issue of electromagnetic current
conservation is examined carefully. We find large cancellations between the
asymmetries induced by the parity-violating interactions and those arising from
the associated pion-exchange currents. In the np capture, the model dependence
is nevertheless quite small, because of constraints arising through the Siegert
evaluation of the relevant E1 matrix elements. In quasi-elastic electron
scattering these processes are found to be insignificant compared to the
asymmetry produced by gamma-Z interference on individual nucleons.Comment: 65 pages, 26 figures, submitted to PR
Direct radiative capture of p-wave neutrons
The neutron direct radiative capture (DRC) process is investigated,
highlighting the role of incident p-wave neutrons. A set of calculations is
shown for the 12-C(n,gamma) process at incoming neutron energies up to 500 keV,
a crucial region for astrophysics. The cross section for neutron capture
leading to loosely bound s, p and d orbits of 13-C is well reproduced by the
DRC model demonstrating the feasibility of using this reaction channel to study
the properties of nuclear wave functions on and outside the nuclear surface. A
sensitivity analysis of the results on the neutron-nucleus interaction is
performed for incident s- as well as p-waves. It turned out that the DRC cross
section for p-wave neutrons is insensitive to this interaction, contrary to the
case of incident s-wave neutrons.
PACS number(s): 25.40Lw,21.10Gv,23.40.HcComment: 16 pages, REVTeX file, PostScript file, .dvi fil
Spin measurements for 147Sm+n resonances: Further evidence for non-statistical effects
We have determined the spins J of resonances in the 147Sm(n,gamma) reaction
by measuring multiplicities of gamma-ray cascades following neutron capture.
Using this technique, we were able to determine J values for all but 14 of the
140 known resonances below En = 1 keV, including 41 firm J assignments for
resonances whose spins previously were either unknown or tentative. These new
spin assignments, together with previously determined resonance parameters,
allowed us to extract separate level spacings and neutron strength functions
for J = 3 and 4 resonances. Furthermore, several statistical test of the data
indicate that very few resonances of either spin have been missed below En =
700eV. Because a non-statistical effect recently was reported near En = 350 eV
from an analysis of 147Sm(n,alpha) data, we divided the data into two regions;
0 < En < 350 eV and 350 < En < 700 eV. Using neutron widths from a previous
measurement and published techniques for correcting for missed resonances and
for testing whether data are consistent with a Porter-Thomas distribution, we
found that the reduced-neutron-width distribution for resonances below 350 eV
is consistent with the expected Porter-Thomas distribution. On the other hand,
we found that reduced-neutron-width data in the 350 < En < 700 eV region are
inconsistent with a Porter-Thomas distribution, but in good agreement with a
chi-squared distribution having two or more degrees of freedom. We discuss
possible explanations for these observed non-statistical effects and their
possible relation to similar effects previously observed in other nuclides.Comment: 40 pages, 13 figures, accepted by Phys. Rev.
Gamma-ray strength function and pygmy resonance in rare earth nuclei
The gamma-ray strength function for gamma energies in the 1-7 MeV region has
been measured for 161,162-Dy and 171,172-Yb using the (3-He,alpha gamma)
reaction. Various models are tested against the observed gamma-ray strength
functions. The best description is based on the Kadmenskii, Markushev and
Furman E1 model with constant temperature and the Lorentzian M1 model. A
gamma-ray bump observed at E_gamma=3 MeV is interpreted as the so-called pygmy
resonance, which has also been observed previously in (n,gamma) experiments.
The parameters for this resonance have been determined and compared to the
available systematics.Comment: 11 pages, including 4 figures and 2 table
Evaluation of the mean intensity of the P-odd mixing of nuclear compound states
A temperature version of the shell-optical-model approach for describing the
low-energy compound-to-compound transitions induced by external single-particle
fields is given. The approach is applied to evaluate the mean intensity of the
P-odd mixing of nuclear compound states. Unified description for the mixing and
electromagnetic transitions allows one to evaluate the mean intensity without
the use of free parameters. The valence-mechanism contribution to the mentioned
intensity is also evaluated. Calculation results are compared with the data
deduced from cross sections of relevant neutron-induced reactions.Comment: LaTeX, 10 page
Non-resonant direct p- and d-wave neutron capture by 12C
Discrete gamma-rays from the neutron capture state of 13C to its low-lying
bound states have been measured using pulsed neutrons at En = 550 keV. The
partial capture cross sections have been determined to be 1.7+/-0.5,
24.2+/-1.0, 2.0+/-0.4 and 1.0+/-0.4 microb for the ground (1/2-), first (1/2+),
second (3/2-) and third (5/2+) excited states, respectively. From a comparison
with theoretical predictions based on the non-resonant direct radiative capture
mechanism, we could determine the spectroscopic factor for the 1/2+ state to be
0.80 +/- 0.04, free from neutron-nucleus interaction ambiguities in the
continuum. In addition we have detected the contribution of the non-resonant
d-wave capture component in the partial cross sections for transitions leading
to the 1/2- and 3/2- states. While the s-wave capture dominates at En < 100
keV, the d-wave component turns out to be very important at higher energies.
From the present investigation the 12C(n,gamma)13C reaction rate is obtained
for temperatures in the range 10E+7 - 10E+10 K.Comment: Accepted for publication in Phys. Rev. C. - 16 pages + 8 figure
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Collimator design for the NSNS accumulator ring
Collimators are used to remove halo or off-momentum particles from the main proton beam. Off-momentum particles are removed by situating collimators in high dispersion areas of the beam. In addition to removing halo particles collimators will also act as shielding for the remainder of the accelerator structures. Thus, collimators reduce uncontrolled losses around the ring and reduce activation of the accelerator components. Requirements and performance goals for the collimator are summarized. In order to meet these goals a self-shielding collimator configuration will be designed. An arrangement consisting of a layered structure will be considered. The initial layers (in the direction of the proton beam) are transparent to protons, and become progressively less transparent (blacker) with depth into the collimator. In addition, a high density (iron) shield will be added around this structure, particularly in the backward direction, to attenuate any reflected protons. The protons are stopped in the approximate center of the collimator, and thus the bulk of the secondary particles will also be generated there. Since these secondary particles are primarily produced isotropically their leakage path length will be maximized in this manner (high probability of capture or attenuation). In the case of neutrons a black layer is included at each end in order to further minimize their leakage in the direction of the beam. This design will therefore minimize the activation of surrounding accelerator components
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