146 research outputs found
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
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.
Starting on the Right Foot: Carryover Effects of Larval Hydroperiod and Terrain Moisture on Post-metamorphic Frog Movement Behavior
Changing patterns of precipitation and drought will dramatically influence the distribution and persistence of lentic habitats. Pond-breeding amphibians can often respond to changes in habitat by plastically shifting behavioral and developmental trait response. However, fitness tradeoffs inherent in life history strategies can carry over to impact development, behavior, and fitness in later life stages. In this experiment, we investigated carryover effects of hydroperiod permanence on the movement behavior of newly-metamorphosed juvenile Northern Red-legged Frogs (Rana aurora). Frogs were raised through metamorphosis in mesocosms under either permanent or ephemeral hydroperiod conditions. After metamorphosis, individuals were removed from the mesocosms, measured, uniquely tagged with elastomer, and moved to holding terrariums. Movement behavior was quantified under two terrain conditions: a physiologically-taxing, dry runway treatment, or a control, moist runway treatment. Individuals were given 30 min to move down the 1 Ă— 20m enclosed structure before distance was measured. We applied a hurdle model to examine two distinct components of movement behavior: (1) the probability of moving away from the start location, and (2) movement distance. We found that hydroperiod condition had an indirect carryover effect on movement via the relationship between individual size and the propensity to move. Individuals from ephemeral mesocosm conditions metamorphosed at a smaller size but showed increased growth rates as compared to individuals from permanent hydroperiod conditions. Individual snout-vent length and runway condition (moist or dry) were significant predictors of both aspects of movement behavior. Larger individuals were more likely to move down the runway and able to move a farther distance than smaller individuals. In addition to the influence of size, dry runway conditions reduced the probability of individuals moving from the start location, but increased the distance traveled relative to the moist runway. The demonstrated cumulative impact of stressors suggests the importance of addressing direct, indirect, and carryover effects of stressors throughout ontogeny
Characteristics of a Lead Slowing-Down Spectrometer Coupled to the LANSCE Accelerator
Abstract A description is given of a lead slowing-down spectrometer (LSDS) installed at the 800-MeV proton accelerator of the Los Alamos Neutron Science Center (LANSCE). The LSDS is designed to study neutron-induced fission on actinides that can only be obtained or used in small quantities. The characteristics of this LSDS (energy-time relation, energy resolution, neutron flux) are presented through simulations with MCNPX and measurements with several different methods. Results on neutron-induced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, additional MCNPX calculations have been performed to simulate the measurement of the cross-section for 235m UĂ°n; fĂž using different target quantities and different initial isomer-to-ground state compositions
First Measurements with a Lead Slowing-Down Spectrometer at LANSCE
Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described
First Measurements with a Lead Slowing-Down Spectrometer at LANSCE
Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described
First Measurements with a Lead Slowing-Down Spectrometer at LANSCE
Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described
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