2,289 research outputs found
Non-Statistical Effects in Neutron Capture
There have been many reports of non-statistical effects in neutron-capture
measurements. However, reports of deviations of reduced-neutron-width
distributions from the expected Porter-Thomas (PT) shape largely have been
ignored. Most of these deviations have been reported for odd-A nuclides.
Because reliable spin (J) assignments have been absent for most resonances for
such nuclides, it is possible that reported deviations from PT might be due to
incorrect J assignments. We recently developed a new method for measuring spins
of neutron resonances by using the DANCE detector at LANSCE. Measurements made
with a 147Sm sample allowed us to determine spins of almost all known
resonances below 1 keV. Furthermore, analysis of these data revealed that the
reduced-neutron-width distribution was in good agreement with PT for resonances
below 350 eV, but in disagreement with PT for resonances between 350 and 700
eV. Our previous (n,alpha) measurements had revealed that the alpha strength
function also changes abruptly at this energy. There currently is no known
explanation for these two non-statistical effects. Recently, we have developed
another new method for determining the spins of neutron resonances. To
implement this technique required a small change (to record pulse-height
information for coincidence events) to a much simpler apparatus: A pair of C6D6
gamma-ray detectors which we have employed for many years to measure
neutron-capture cross sections at ORELA. Measurements with a 95Mo sample
revealed that not only does the method work very well for determining spins,
but it also makes possible parity assignments. Taken together, these new
techniques at LANSCE and ORELA could be very useful for further elucidation of
non-statistical effects.Comment: 8 pages, 3 figures, for proceedings of CGS1
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.
The Ni(n,) cross section measured with DANCE
The neutron capture cross section of the s-process branch nucleus Ni
affects the abundances of other nuclei in its region, especially Cu and
Zn. In order to determine the energy dependent neutron capture cross
section in the astrophysical energy region, an experiment at the Los Alamos
National Laboratory has been performed using the calorimetric 4 BaF
array DANCE. The (n,) cross section of Ni has been determined
relative to the well known Au standard with uncertainties below 15%.
Various Ni resonances have been identified based on the Q-value.
Furthermore, the s-process sensitivity of the new values was analyzed with the
new network calculation tool NETZ.Comment: 11 pages, 13 page
Genome size and organization in the blacklegged tick, \u3ci\u3eIxodes scapularis\u3c/i\u3e and the Southern cattle tick, \u3ci\u3eBoophilus microplus\u3c/i\u3e
Genome sizes and the organization of repetitive DNA were determined in the hard ticks Ixodes scapularis and Boophilus microplus using reassociation kinetics. The I. scapularis genome contains ~ 2.15 pg (2.1 × 10 3 Mbp) of DNA and consists of no foldback (FB), 27% highly repetitive (HR), 39% moderately repetitive (MR), and 34% unique DNA. The B. microplus genome contains 7.5 pg (7.1 × 10 3 Mbp) DNA, and consists of 0.82% FB, 31% HR, 38% MR, and 30% unique DNA. In both species, repetitive sequences occur in a mixture of long and short period interspersion but most (65–80%) of the DNA follows a pattern of short period interspersion. Genome size and organization in the three tick species so far examined are distinct from other arthropods in having a greater proportion of MR, a lower proportion of unique and HR DNA of very low sequence complexity
- …