[Show abstract][Hide abstract] ABSTRACT: A new low-mass multiple gas-filled parallel-plate avalanche counter for the fission-fragment detection has been developed to mark the fission occurrence in measurements of the prompt fission neutron energy spectrum as a function of incident neutron energy. It was used successfully for the neutron-induced fission of 235U and 239Pu with a total mass near 100 mg each and the spontaneous fission of 252Cf. Both the incident neutron energy and the prompt fission neutron energy are measured by using the time-of-flight method. The design and performance of this avalanche counter are described.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 09/2015; 794:76-79. DOI:10.1016/j.nima.2015.05.010 · 1.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In an experiment at the A1900 spectrometer of the National Superconducting Cyclotron Laboratory at Michigan State University the new isotope Ge59 was identified and the cross section for production of the most neutron-deficient Ge59-62 isotopes in the fragmentation of a Kr78 beam at 150 MeV/nucleon measured. This experimental information is relevant for the hunt of new two-proton emitters in the region above the doubly-magic Ni48.
Physical Review C 07/2015; 92(1). DOI:10.1103/PhysRevC.92.014622 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The parity doublet in 20Ne at 11.26 MeV is a good candidate to study parity violation in nuclei. However, its energy splitting is known with insufficient accuracy for quantitative estimates of parity violating effects. To improve on this unsatisfactory situation, nuclear resonance fluorescence experiments using linearly and circularly polarized γ-ray beams were used to determine the energy difference of the parity doublet and the ratio of their integrated cross sections . Shell-model calculations predict a parity-violating matrix element having a value in the range 0.46–0.83 eV for the parity doublet. The small energy difference of the parity doublet makes 20Ne an excellent candidate to study parity violation in nuclear excitations.
[Show abstract][Hide abstract] ABSTRACT: The IS475 collaboration conducted Coulomb-excitation experiments with postaccelerated radioactive 220Rn and 224Ra beams at the REX-ISOLDE facility. The beam particles (Ebeam ≈ 2.83 MeV/u) were Coulomb excited using 60Ni, 114Cd, and 120Sn scattering targets. De-excitation γ-rays were detected employing the Miniball array and scattered particles were detected in a silicon detector. Exploiting the Coulomb-excitation code GOSIA for each nucleus several matrix elements could be obtained from the measured γ-ray yields. The extracted 3−||Ê3||0+ matrix element allows for the conclusion that, while 220Rn represents an octupole vibrational system, 224Ra has already substantial octupole correlations in its ground state. An observation that has implications for the search of CP-violating Schiff moments in the atomic systems of the adjacent odd-mass nuclei.
Journal of Physics Conference Series 09/2014; 533(1):012007. DOI:10.1088/1742-6596/533/1/012007
[Show abstract][Hide abstract] ABSTRACT: The electromagnetic transition strength between the two bound states
were measured in the one-neutron halo nucleus 11Be from
Coulomb excitation on 196Pt at projectile energies of 1.727
and 2.086 MeV/nucleon at TRIUMF. A B(E1) strength of 0.102(2)
e2fm2, deduced from the forward-scattering data,
is consistent with previous Coulomb excitation measurements at
intermediate projectile energies with a model-dependent analysis.
[Show abstract][Hide abstract] ABSTRACT: The total prompt γ-ray energy distributions for the neutron-induced fission of U235 and Pu239,241 in the neutron energy range of 0.025 eV – 100 keV and the spontaneous fission of Cf252 were measured using the Detector for Advanced Neutron Capture Experiments (DANCE) in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. DANCE is a highly segmented, highly efficient 4π γ-ray calorimeter. Corrections were made to the measured distribution by unfolding the two-dimension spectrum of total γ-ray energy vs multiplicity using a simulated DANCE response matrix generated with a geometrical model of the detector arrays and validated with the γ-ray calibration sources. The mean values of the total prompt γ-ray energy, determined from the unfolded distributions, are ∼20% higher than those of early measurements for all the fissile nuclei studied. The implication for the γ heating in nuclear reactors is discussed.
Physical Review C 07/2014; 90(1). DOI:10.1103/PhysRevC.90.014602 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electromagnetic dipole strength in Be-11 between the bound states has been measured using low-energy projectile Coulomb excitation at bombarding energies of 1.73 and 2.09 MeV/nucleon on a Pt-196 target. An electric dipole transition probability B(E1; 1/2(-) -> 1/2(+)) = 0.102(2) e(2) fm(2) was determined using the semi-classical code Gosia, and a value of 0.098(4) e(2) fm(2) was determined using the Extended Continuum Discretized Coupled Channels method with the quantum mechanical code FRESCO. These extracted B(E1) values are consistent with the average value determined by a model-dependent analysis of intermediate energy Coulomb excitation measurements and are approximately 14% lower than that determined by a lifetime measurement. The much-improved precisions of 2% and 4% in the measured B(E1) values between the bound states deduced using Gosia and the Extended Continuum Discretized Coupled Channels method, respectively, compared to the previous accuracy of similar to 10% will help in our understanding of and better improve the realistic inter-nucleon interactions. (C) 2014 The Authors. Published by Elsevier B.V.
Physics Letters B 05/2014; 732:210-213. DOI:10.1016/j.physletb.2014.03.049 · 6.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Higher quality measurements of outgoing prompt neutron spectra from neutron-induced fission as a function of the incoming neutron energy are needed. These data can be used in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security applications. As part of the program to measure the prompt fission neutron spectra (PFNS) from the fission of 239Pu at the Los Alamos Neutron Science Center, we are developing a new array of liquid-scintillator detectors. This array will be used to measure the PFNS over a range of outgoing neutron energies from approximately 600 keV to 12 MeV and incident neutron energies from 0.5 to 30 MeV. A complete characterization of the detectors and the array as a whole will be carried out, targeted at understanding the light-output curves, efficiencies, and the neutron multiple-scattering backgrounds.
Nuclear Data Sheets 05/2014; 119:371-372. DOI:10.1016/j.nds.2014.08.102 · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The prompt neutron spectrum from neutron-induced fission needs to be known in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security application. A program to measure this spectrum for neutron-induced fission of 239Pu is underway at the Los Alamos Neutron Science Center. The goal is to obtain data on the shape of the spectrum with a small uncertainty over the emitted neutron energy range of 100 keV to 12 MeV with additional data below and above this range. The incident neutron energy range will be from 0.5 to 30 MeV. The status of this program including results of initial experimental measurements is described here.
Nuclear Data Sheets 05/2014; 119:205-208. DOI:10.1016/j.nds.2014.08.057 · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The prompt γ-ray spectra from 235U(n,f) at incident energies of 1 to 20 MeV and for 235Cf(s.f.) were measured up to 4 MeV in coincidence with two parallel plate avalanche counters and the liquid scintillator array FIGARO. The unfolded γ-ray spectra from 1–4 MeV using the single value decomposition and iterative Bayesian techniques for the incident neutron energy regions corresponding to the first, second and third chance fissions were found to nearly identical and similar to the distribution from the spontaneous fission of 252Cf. General agreement with the γ-ray distributions from fission was also found with previous measurements.
Nuclear Data Sheets 05/2014; 119:221-224. DOI:10.1016/j.nds.2014.08.061 · 4.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background: Knowing the energy dependence of the (γ,n) cross section is mandatory to predict the abundances of heavy elements using astrophysical models. The data can be applied directly or used to constrain the cross section of the inverse (n,γ) reaction.Purpose: The measurement of the reaction Ce142(γ,n)141Ce just above the reaction threshold amends the existing experimental database in that mass region for p-process nucleosynthesis and helps to understand the s-process branching at the isotope Ce141.Method: The quasi-monoenergetic photon beam of the High Intensity γ-ray Source (HIγS), TUNL, USA, is used to irradiate naturally composed Ce targets. The reaction yield is determined afterwards with high-resolution γ-ray spectroscopy.Results: The experimental data are in agreement with previous measurements at higher energies. Since the cross-section prediction of the Ce142(γ,n) reaction is exclusively sensitive to the γ-ray strength function, the resulting cross-section values were compared to Hauser-Feshbach calculations using different γ-ray strength functions. A microscopic description within the framework of the Hartree-Fock-BCS model describes the experimental values well within the measured energy range.Conclusions: The measured data show that the predicted (γ,n) reaction rate is correct within a factor of 2 even though the closed neutron shell N =82 is approached. This agreement allows us to constrain the (n,γ) cross section and to improve the understanding of the s-process branching at Ce141.
Physical Review C 02/2014; 89(3). DOI:10.1103/PhysRevC.89.035803 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The 239Pu(n ,γ) cross section has been measured over the energy range 10 eV to 1 keV using the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center as part of a campaign to produce precision (n ,γ) measurements on 239Pu. Fission coincidences were measured with a parallel-plate avalanche counter and used to measure the prompt fission γ-ray spectrum in this region to accurately characterize background. The resulting (n ,γ) cross section is generally in agreement with current evaluations. The experimental method utilizes much more detailed information than past measurements on 239Pu and can be used to extend the measurement to higher incident neutron energies.
Physical Review C 02/2014; 89(3). DOI:10.1103/PhysRevC.89.034610 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to investigate the discrepancy in the available sets of data on 239Pu, Chi-Nu is a program to measure prompt-fission-neutron spectra at the Los Alamos Neutron Science Center (LANSCE). To meet the required accuracy based on the compilations among different theoretical models and sets of data, two different types of neutron detectors, liquid scintillators and 6Li-glass scintillators, are used in conjunction to the actinide Parallel Plate Avalanche Counter by measuring a time of flight. Monte Carlo simulations are used to study detector responses. In this manuscript, we discuss the status of the Chi-Nu project including the commissioning in 2012 summer.
Fission and Properties of Neutron-Rich Nuclei: Proceedings of the Fifth International Conference on ICFN5; 01/2014
[Show abstract][Hide abstract] ABSTRACT: The dipole response of 3476Se in the energy range
from 4 to 9 MeV has been analyzed using a
(γ⃗,γ') polarized photon scattering technique,
performed at the High Intensity γ-Ray Source facility at Triangle
Universities Nuclear Laboratory, to complement previous work performed
using unpolarized photons. The results of this work offer both an
enhanced sensitivity scan of the dipole response and an unambiguous
determination of the parities of the observed J=1 states. The dipole
response is found to be dominated by E1 excitations, and can reasonably
be attributed to a pygmy dipole resonance. Evidence is presented to
suggest that a significant amount of directly unobserved excitation
strength is present in the region, due to unobserved branching
transitions in the decays of resonantly excited states. The dipole
response of the region is underestimated when considering only ground
state decay branches. We investigate the electric dipole response
theoretically, performing calculations in a three-dimensional (3D)
Cartesian-basis time-dependent Skyrme-Hartree-Fock framework.
Physical Review C 12/2013; 88(6):64308-. DOI:10.1103/PhysRevC.88.064308 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Prompt γ-ray multiplicity and energy distributions in the neutron-induced fission of 235U, 239Pu, and 241Pu, and spontaneous fission (SF) of 252Cf, were measured using a highly segmented 4π γ-ray calorimeter in coincidence with a gas-filled parallel-plate avalanche counter for fission tagging. The multiplicity and energy distributions were then unfolded according to the γ-ray detector response that was numerically simulated by a Monte Carlo model and validated with γ-ray calibration sources. The multiplicity distributions were measure directly for the first time. Our measurement  of 252Cf is compared against Brunson’s double-Poisson fit  in Fig. 1. In the γ-ray multiplicity distributions, the mean value increases and width gets wider as the fissile nucleus mass increases. In the γ-ray energy distributions, differences are observed above 4 MeV. To study correlations between γ-ray multiplicity and total energy, prompt γ cascades were simulated through the detector response by randomly selecting number of γ rays and then randomly selecting γ energies. The experimental data, results of this study, and a detailed description of the experiment will be presented. Recent technical developments as well as new results will be presented…
Proceedings of the Fifth International Conference on ICFN5; 11/2013