arXiv subjects

W. Loveland

Publications and source records attributed to W. Loveland.

At least 19 recordsLinked to original sources

arXiv2025

Remeasurement of the $^{239}$Pu(n,f)/$^{235}$U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber Using Vapor-deposited Targets

The NIFFTE fission Time Projection Chamber (fissionTPC) has been used to measure the $^{239}$Pu(n,f)/$^{235}$U(n,f) cross-section ratio for neutron-induced fission in the range of 0.1 - 100 MeV, with high precision. A white neutron source was provided by the Los Alamos Neutron Science Center, where the experiment was conducted as a remeasurement to evaluate a roughly 2% discrepancy of the previous fissionTPC results with ENDF/B-VIII.0. A detailed accounting of measurement uncertainties was performed, based on the fissionTPC's novel ability to provide three-dimensional reconstruction of fission-fragment ionization profiles. Current results obtained using a vapor-deposited, highly uniform $^{239}$Pu target, in comparison to the measurement published in 2021, where a $^{239}$Pu electroplated target was used, are presented and discussed. The remeasurement presented here is in agreement with the previous fissionTPC result within measurement uncertainties.

nucl-ex↗

arXiv2024

Meaurement of spin vs. TKE of $^{144}$Ba produced in spontaneous fission of $^{252}$Cf

We measure the average spin of $^{144}$Ba, a common fragment produced in $^{252}$Cf(sf), as a function of the total kinetic energy (TKE). We combined for the first time a twin Frisch-gridded ionization chamber with a world-class $γ$-ray spectrometer that was designed to measure high-multiplicity $γ$-ray events, Gammasphere. The chamber, loaded with a $^{252}$Cf(sf) source, provides a fission trigger, the TKE of the fragments, the approximate fragment masses, and the polar angle of the fission axis. Gammasphere provides the total $γ$-ray yield, fragment identification through the tagging of decay $γ$ rays, and the feeding of rotational bands in the fragments. We determine the dependence of the average spin of $^{144}$Ba on the fragments' TKE by correlating the fragment properties with the distribution of discrete levels that are fed. We find that the average spin only changes by about $0.5$ $\hbar$ across the TKE range of 158-203 MeV. The virtual independence of the spin on TKE suggests that spin is not solely generated through the statistical excitation of rotational modes, and more complex mechanisms are required.

nucl-ex↗

arXiv2022

Instrumentation for correlated prompt $n$-$γ$ emission studies in coincidence with fission fragments

Recent theoretical and experimental results have brought renewed interest and focus on the topic of fission fragment angular momentum. Measurements of neutrons and $γ$ rays in coincidence with fission fragments remain the most valuable tool in the exploration of fission physics. To achieve these scientific goals, we have developed a system that combines a state-of-the-art fission fragment detector and $n$-$γ$ radiation detectors. A new twin Frisch-gridded ionization chamber has been designed and constructed for use with a spontaneous fission source and an array of forty \textit{trans}-stilbene organic scintillators (FS-3) at Argonne National Laboratory. The new ionization chamber design we present in this work aims at minimizing particle attenuation in the chamber walls, and provides a compact apparatus that can be fit inside existing experimental systems. The ionization chamber is capable of measuring fission fragment masses and kinetic energies, whereas the FS-3 provides neutron and gamma-ray multiplicities and spectra. The details of both detector assembly are presented along with the first experimental results of this setup. Planned event-by-event analysis and future experiments are briefly discussed.

physics.ins-det↗

arXiv2022

Relativistic effects for the reaction Ubq + 6 CO = Ubq (CO)6 or Ubq (OC)6:Prediction of the existence, atomization energy, and isomerization energy of the isomers Ubq (CO)6 and Ubq (OC)6 of element Ubq ( Z=124, eka-uranium)

Our ab initio all-electron relativistic Dirac*Fock (DF) calculations for the octahedral (Oh) Ubq(CO)6 and Ubq(OC)6 predict atomization energies (Ae) of 50.25 (47.93) and 43.43 (44.88 ) eV, respectively where the corresponding non-relativistic calculated Ae's are given in parenthesis. Our calculated DF and NR isomerization energies (E iso) for Ubq (CO) 6 = Ubq (OC) 6 are 6.83 and 3.05 eV, respectively. Our calculated DF (NR) energy for the reaction Ubq + 6CO = Ubq (CO)6 is -4.31 (-4.30) eV, while the energy of the isomeric reaction Ubq + 6CO = Ubq(OC)6 is 2.52 ( -1.25 ) eV, respectively. The relativistic effects increase the Eiso of Ubq (CO)6 by ~3.78 eV , decrease the Ubq-C bond distance by 0.12 angstroms and have a negligible effect on the C-O bond distance as expected. These are the first results of relativistic effects for isomerztion energy and atomization energy of the superheavy Ubq (CO)6 and Ubq(OC)6. The bond distances Ubq-C and Ubq-O optimized at the DF level of theory for Ubq(CO)6 and Ubq(OC)6 are 2.572 and 2.559 angstroms, while the corresponding optimized bond distances Ubq-C and Ubq-O at the NR level of theory are 2.691 and 3.616 angstroms, respectively. Both our DF and NR calculations clearly predict the formation of both the isomers Ubq(CO)6 and Ubq(OC), and the former is predicted to be more stable at the DF level of theory by ~7 eV than the latter.No such calculations have been reported before for systems of such superheavy elements.

physics.chem-ph↗

arXiv2022

Relativistic and magnetic Breit effects for the isomerization of Sg(CO)6 and Sg(OC)6

Abstract Our ab initio all-electron relativistic Dirac-Fock (DF) calculations for seaborgium hexacarbonyl Sg(CO)6 and seaborgium hexaisocarbonyl Sg(OC)6 predict atomization energies of 68.80 and 64.30 eV. Our Dirac-Fock-Breit-Gaunt (DFBG) calculations for Sg(CO)6 and Sg(OC)6 yield atomization energies of 69.18 and 64.77 eV. However, our calculated non-relativistic (NR) Ae for Sg (CO)6 and Sg(OC)6 are 68.46 and 62.62 eV. The calculated isomerization energies at the DFBG, DF, and NR levels are 4.41,4.50 and 5.83 eV. The contribution of relativity to the Eiso is - ~1.33 eV. The optimized bond distances Sg-C and C-O for octahedral Sg(CO)6 using our DF (NR) calculations are 2.151 ( 2.318 and 1.119 ( 1.114 (ang}). The optimized six Sg-O and C-O bond distances for octahedral Sg(OC)6 at the DF level are equal to 4.897 and 1.108 {ang}. However, the optimized four Sg-O bond distances for the octahedral Sg(OC)6 at the NR level are 5.160 {ang} each, and two Sg-O bonds of 2.721 {ang} each, but all six C-O bonds are 1.108{ang} each. The energies at the DF level of theory for the reaction Sg+6CO to yield Sg (CO)6 and Sg(OC)6 are calculated as -7.30 and -2.80 eV. Moreover, the energies of the reaction at the DFBG level to yield Sg(CO)6 and Sg(OC)6 are very close to those predicted at the DF level of theory of -7.17 and -2.76 eV. However, the NR energies of the above-mentioned reaction are -6.99 and -1.15 eV. The mean bond energies predicted for Sg(CO)6 with our DF, DFBG, and NR calculations are 117.40 , 115.31, and 112.41 kJ/mol, whereas the mean bond energies calculated for the isomer Sg(OC)6 at DF, DFBG and NR levels are 45.03 ,44.39 and 18.49 kJ/mole. The predicted existence of both the isomers with Eiso of ~ 4.50 and ~ 5.80 eV, may cause problems for experimental identification of seaborgium hexacarbonyl.

physics.chem-ph↗

arXiv2022

The fast neutron induced fission of $^{240}$Pu and $^{242}$Pu

We report the measurement of the TKE release in the fast neutron induced fission of $^{240}$Pu and $^{242}$Pu. The results are compared to the predictions of the GEF model, the CGMF model, and the model of Denisov and Sedykh as well as previous experimental work on these reactions. Our absolute measurements of the TKE release are in good agreement with the previous measurements of Nethaway \textit{et al.} for the interaction of 14.8 MeV neutrons with $^{240}$Pu \cite{Nethaway} and of Winkelmann and Aumann for the interaction of 15 MeV neutrons with $^{242}$Pu \cite{Winkelmann}. The general trends of the measured TKE values agree with phenomenological models but the variances of the TKE distributions are significantly less than predicted by various models. The mean post neutron emission TKE release decreases non- linearly with increasing neutron energy and can be represented as TKE(MeV) = 175.8 $\pm$ 0.3 - (2. 4 $\pm$ 0.8)log$_{10}$E$_n$ - (1.4 $\pm$0.4)log$_{10}$E$_{n}^2$ for $^{240}$Pu and TKE(MeV) = 177.1 $\pm$ 0.3 - (1.2 $\pm$ 0.9)log$_{10}$E$_n$ - (1.8 $\pm$0.5)log$_{10}$E$_{n}^2$ for $^{242}$Pu.

nucl-ex↗

arXiv2021

Measurement of material isotopics and atom number ratio with alpha-particle spectroscopy for the NIFFTE fission Time Projection Chamber actinide target

We present the results of a measurement of isotopic concentrations and atomic number ratio of a double-sided actinide target with alpha-spectroscopy and mass spectrometry. The double-sided actinide target, with primarily Pu-239 on one side and U-235 on the other, was used in the fission Time Projection Chamber (fissionTPC) for a measurement of the neutron-induced fission cross-section ratio between the two isotopes. The measured atomic number ratio is intended to provide an absolute normalization of the measured fission cross-section ratio. The Pu-239/U-235 atom number ratio was measured with a combination of mass spectrometry and alpha-spectroscopy with a planar silicon detector with uncertainties of less than 1%.

nucl-ex↗

arXiv2021

Measurement of the $^{239}$Pu(n,f)/$^{235}$U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber

The $^{239}$Pu(n,f)/$^{235}$U(n,f) cross-section ratio has been measured with the fission Time Projection Chamber (fissionTPC) from 100 keV to 100 MeV. The fissionTPC provides three-dimensional reconstruction of fission-fragment ionization profiles, allowing for a precise quantification of measurement uncertainties. The measurement was performed at the Los Alamos Neutron Science Center which provides a pulsed white source of neutrons. The data are recommended to be used as a cross-section ratio shape. A discussion of the status of the absolute normalization and comparisons to ENDF evaluations and previous measurements is included.

nucl-ex↗

arXiv2020

Total kinetic energy release in the fast neutron-induced fission of 237Np

The total kinetic energy (TKE) in the fast neutron induced fission of 237Np was measured for neutron energies from En = 2.6 - 100 MeV at the LANSCE-WNR facility. The post TKE release decreases non-linearly with increasing incident neutron energy and can be represented as TKE(MeV) = (174.38 +- 0.72) - (5.11 +- 0.5821) log10 En for En > 1 MeV. Analysis of the fragment mass distributions indicates that the decrease in TKE with increasing En is a consequence of two factors; shell effects fade out at high excitation energies, resulting in the increasing occurrence of symmetric fission, and TKEasym decreases rapidly at high En.

nucl-ex↗

arXiv2020

Multi-nucleon transfer in the interaction of 977 MeV and 1143 MeV $^{204}$Hg with $^{208}$Pb

A previous study of symmetric collisions of massive nuclei has shown that current models of multi-nucleon transfer (MNT) reactions do not adequately describe the transfer product yields. To gain further insight into this problem, we have measured the yields of MNT products in the interaction of 977 (E/A = 4.79 MeV) and 1143 MeV (E/A = 5.60 MeV) $^{204}$Hg with $^{208}$Pb. We find that the yield of multi-nucleon transfer products are similar in these two reactions and are substantially lower than those observed in the reaction of 1257 MeV (E/A = 6.16 MeV) $^{204}$Hg + $^{198}$Pt. We compare our measurements with the predictions of the GRAZING-F, di-nuclear systems (DNS) and improved quantum molecular dynamics (ImQMD) models. For the observed isotopes of the elements Au, Hg, Tl, Pb and Bi, the measured values of the MNT cross sections are orders of magnitude larger than the predicted values. Furthermore, the various models predict the formation of nuclides near the N=126 shell, which are not observed.

nucl-ex↗

arXiv2020

Neutron Induced Fission Fragment Angular Distributions, Anisotropy, and Linear Momentum Transfer Measured with the NIFFTE Fission Time Projection Chamber

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measurements. Angular anisotropy is an important experimental observable for understanding the quantum mechanical state of the fissioning nucleus and vital to determining detection efficiency for cross section measurements. Neutron linear momentum transfer to fissioning $^{235}$U, $^{238}$U, and $^{239}$Pu and fission fragment angular anisotropy of $^{235}$U and $^{238}$U as a function of neutron energies in the range 130 keV--250 MeV are presented.

nucl-ex↗

arXiv2019

Fission Fragment Angular Anisotropy in Neutron-Induced Fission of $^{235}$U Measured with a Time Projection Chamber

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

nucl-ex↗

arXiv2019

1H(n,el) as a Cross Section Reference in a White Source Neutron Beam With the fissionTPC

We provide a quantitative description of a method to measure neutron-induced fission cross sections in ratio to elastic hydrogen scattering in a white-source neutron beam with the fission Time Projection Chamber. This detector has measured precision fission cross section ratios using actinide references such as $^{235}$U(n,f) and $^{238}$U(n,f). However, by employing a more precise reference such as the H(n,el) cross section there is the potential to further reduce the evaluation uncertainties of the measured cross sections. In principle the fissionTPC could provide a unique measurement by simultaneously measuring both fission fragments and proton recoils over a large solid angle. We investigate one method with a hydrogenous gas target and with the neutron energy determined by the proton recoil kinematics. This method enables the measurement to be performed in a white-source neutron beam and with the current configuration of the fissionTPC. We show that while such a measurement is feasible in the energy range of 0.5 MeV to $\sim$10 MeV, uncertainties on the proton detection efficiency and the neutron energy resolution do not allow us to preform a fission ratio measurement to the desired precision. Utilizing either a direct measurement of the neutron time-of-flight for the recoil proton or a mono-energetic neutron source or some combination of both would provide a path to a sub-percent precision measurement.

nucl-ex↗

arXiv2018

Measurement of the normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio from threshold to 30 MeV with the fission Time Projection Chamber

The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$β$5 at 14.5 MeV.

nucl-ex↗

arXiv2018

Predicting the production of neutron rich heavy nuclei in multi-nucleon transfer reactions using GRAZING-F

Background: Multi-nucleon transfer reactions have recently attracted attention as a possible path to the synthesis of new neutron-rich heavy nuclei. Purpose: We study transfer reactions involving massive nuclei with the intention of understanding if the semi-classical model GRAZING coupled to an evaporation and fission competition model can satisfactory reproduce experimental data on transfer reactions in which fission plays a role. Methods: We have taken the computer code GRAZING and have added fission competition to it (GRAZING-F) using our current understanding of $Γ_n/Γ_f$, fission barriers and level densities. Results: The code GRAZING-F seems to satisfactory reproduce experimental data for $+1p$, $+2p$ and $+3p$ transfers, but has limitations in reproducing measurements of larger above-target and below-target transfers. Nonetheless, we use GRAZING-F to estimate production rates of neutron-rich $N=126$ nuclei, actinides and transactinides. Conclusions: The GRAZING code, with appropriate modifications to account for fission decay as well as neutron emission by excited primary fragments, does not predict large cross sections for multi-nucleon transfer reactions leading to neutron-rich transactinide nuclei, but predicts opportunities to produce new neutron-rich actinide isotopes.

nucl-ex↗

arXiv2017

Total kinetic energy release in the fast neutron-induced fission of $^{235}$U

We have measured the total kinetic energy (TKE) release for the $^{235}$U(n,f) reaction for $E_{n}$=2-100 MeV using the 2E method with an array of Si PIN diode detectors. The neutron energies were determined by time of flight measurements using the white spectrum neutron beam at the LANSCE facility. To benchmark the TKE measurement, the TKE release for $^{235}$U(n$_{th}$,f) was also measured using a thermal neutron beam from the Oregon State University TRIGA reactor, giving pre-neutron emission $E^*_{TKE}=170.7\pm0.4$ MeV in good agreement with known values. Our measurements are thus absolute measurements. The TKE in $^{235}$U(n,f) decreases non-linearly from 169 MeV to 161 MeV for $E_{n}$=2-100 MeV. The multi-modal fission analysis of mass distributions and TKE indicates the origin of the TKE decrease with increasing neutron energy is a consequence of the fade out of asymmetric fission, which is associated with a higher TKE compared to symmetric fission. The average TKE associated with the superlong, standard I and standard II modes for a given mass is independent of neutron energy. The widths of the TKE distributions are constant from $E_{n}$=20-100 MeV and hence show no dependence with excitation energy.

nucl-ex↗

arXiv2017

Total kinetic energy release in the fast neutron-induced fission of $^{235}$U

We have measured the total kinetic energy (TKE) release for the $^{235}$U(n,f) reaction for $E_{n}$=2-100 MeV using the 2E method with an array of Si PIN diode detectors. The neutron energies were determined by time of flight measurements using the white spectrum neutron beam at the LANSCE facility. (To calibrate the apparatus, the TKE release for $^{235}$U(n$_{th}$,f) was also measured using a thermal neutron beam from the OSU TRIGA reactor). The TKE decreases non-linearly from 169.0 MeV to 161.4 MeV for $E_{n}$=2-90 MeV. The standard deviation of the TKE distribution is constant from $E_{n}$=20-90 MeV. Comparison of the data with the multi-modal fission model of Brosa indicates the TKE decrease is a consequence of the growth of symmetric fission and the corresponding decrease of asymmetric fission with increasing neutron energy. The average TKE associated with the Brosa superlong, standard I and standard II modes for a given mass is independent of neutron energy.

nucl-ex↗

arXiv2017

Characterizing the mechanism(s) of heavy element synthesis

A review of the current state of our understanding of complete fusion reaction mechanisms is presented, from the perspective of an experimentalist. For complete fusion reactions, the overall uncertainties in predicting heavy element synthesis cross sections are examined in terms of the uncertainties associated with the calculations of capture cross sections, fusion probabilities and survival probabilities.

nucl-ex↗