Triangle Universities Nuclear Laboratory Research Articles

The contribution of the ${}^3\vec He(\vec \gamma ,n)pp$ reaction to the GDH integrand below pion production threshold

The first measurements of the three-body phototdisintegration of 3 He polarized parallel and anti-parallel to a circularly polarized γ -ray beam were carried out at the High Intensity γ -ray Source (HIγ S) facility located at Triangle Universities Nuclear Laboratory (TUNL). A high pressure 3 He target, polarized via spin-exchange optical pumping with alkali metals, was used in the experiments. The neutrons from the three-body photodisintegration were detected with sixteen 12.7 cm diameter liquid scintillator detectors. The spin-dependent cross sections and the contributions from the three-body photodisintegration to the 3 He Gerasimov-Drell-Hearn sum rule integrand were extracted and compared with state-of-the-art three-body calculations at the incident photon energies of 12.8, 14.7, and 16.5 MeV. The calculations, which include the Coulomb interaction are in good agreement with the results of the measurements at 12.8 and 14.7 MeV but deviate from the results at 16.5 MeV.

Measurement of the doubly-polarized [formula omitted] reaction at 16.5 MeV and its implications for the GDH sum rule

We report new measurements of the double-polarized photodisintegration of 3He at an incident photon energy of 16.5 MeV, carried out at the High Intensity γ-ray Source (HIγS) facility located at Triangle Universities Nuclear Laboratory (TUNL). The spin-dependent double-differential cross sections and the contribution from the three-body channel to the Gerasimov–Drell–Hearn (GDH) integrand were extracted and compared with the state-of-the-art three-body calculations. The calculations, which include the Coulomb interaction and are in good agreement with the results of previous measurements at 12.8 and 14.7 MeV, deviate from the new cross section results at 16.5 MeV. The GDH integrand was found to be about one standard deviation larger than the maximum value predicted by the theories.

Neutron radiative capture reactions on nuclei of relevance to 0νββ, dark matter and neutrino/antineutrino searches

A program is underway at the Triangle Universities Nuclear Laboratory (TUNL) to measure the neutron capture cross section in the 0.5 to 15 MeV energy range on nuclei whose radioactive daughters could potentially create backgrounds in searches for rare events. Here, we refer to neutrino-less double-beta decay and dark-matter searches, and to detectors built for neutrino and/or antineutrino studies. Neutron capture cross-section data obtained by using the activation method are reported for 40 Ar , 74,76 Ge, 128,130 Te and 136 Xe and compared to model calculations and evaluations.

Spin-dependent cross sections from the three-body photodisintegration ofHe3at incident energies of 12.8 and 14.7 MeV

The first measurement of the three-body photodisintegration of polarized $^{3}\mathrm{He}$ using a circularly polarized photon beam has been performed at incident energies of 12.8 and 14.7 MeV. This measurement was carried out at the high-intensity $\ensuremath{\gamma}$-ray source located at Triangle Universities Nuclear Laboratory. A high-pressure $^{3}\mathrm{He}$ target, polarized via spin exchange optical pumping with alkali metals, was used in the experiment. The spin-dependent double- and single-differential cross sections from $\stackrel{P\vec}{{}^{3}\text{He}}(\stackrel{P\vec}{\ensuremath{\gamma}},n)pp$ for laboratory angles varying from ${30}^{\ensuremath{\circ}}$ to ${165}^{\ensuremath{\circ}}$ are presented and compared with state-of-the-art three-body calculations. The data reveal the importance of including the Coulomb interaction between protons in the three-body calculations.

Measurement of the elastic scattering cross section of neutrons from argon and neon

Background: The most significant source of background in direct dark matter searches are neutrons that scatter elastically from nuclei in the detector's sensitive volume. Experimental data for the elastic scattering cross section of neutrons from argon and neon, which are target materials of interest to the dark matter community, were previously unavailable. Purpose: Measure the differential cross section for elastic scattering of neutrons from argon and neon in the energy range relevant to backgrounds from (alpha,n) reactions in direct dark matter searches. Method: Cross-section data were taken at the Triangle Universities Nuclear Laboratory (TUNL) using the neutron time-of-flight technique. These data were fit using the spherical optical model. Results: The differential cross section for elastic scatting of neutrons from neon at 5.0 and 8.0 MeV and argon at 6.0 MeV was measured. Optical-model parameters for the elastic scattering reactions were determined from the best fit to these data. The total elastic scattering cross section for neon was found to differ by 6% at 5.0 MeV and 13% at 8.0 MeV from global optical-model predictions. Compared to a local optical-model for 40Ar, the elastic scattering cross section was found to differ from the data by 8% at 6.0 MeV. Conclusions: These new data are important for improving Monte-Carlo simulations and background estimates for direct dark matter searches and for benchmarking optical models of neutron elastic scattering from these nuclei.

First measurements of spin-dependent double-differential cross sections and the Gerasimov-Drell-Hearn Integrand from 3He(γ,n)pp at incident photon energies of 12.8 and 14.7 MeV.

The first measurement of the three-body photodisintegration of longitudinally polarized (3)He with a circularly polarized γ-ray beam was carried out at the High Intensity γ-ray Source facility located at Triangle Universities Nuclear Laboratory. The spin-dependent double-differential cross sections and the contributions from the three-body photodisintegration to the (3)He Gerasimov-Drell-Hearn integrand are presented and compared with state-of-the-art three-body calculations at the incident photon energies of 12.8 and 14.7 MeV. The data reveal the importance of including the Coulomb interaction between protons in three-body calculations.

Decay pattern of the pygmy dipole resonance in60Ni

Spin-1 states in ${}^{60}$Ni were excited with the $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction, exploiting the High Intensity $\stackrel{P\vec}{\ensuremath{\gamma}}$-ray Source at Triangle University Nuclear Laboratory. This facility is capable of providing fully linearly polarized, quasimonochromatic, Compton-backscattered photons in the entrance channel of the reaction. The depopulation of low-lying levels in an energy region far below the incident quasimonochromatic photons allows us to obtain average branching ratios of the excited spin-1 states. Levels within the energy region associated with the PDR showed regular behavior and $\ensuremath{\approx}$75$%$ of their decays are direct ground-state decays. The levels in the energy region above the PDR exhibit a statistical decay behavior to a large number of low-lying excited states and have only $\ensuremath{\approx}$50--60$%$ branches to the ground state. Within the framework of the quasiparticle phonon model this feature can be explained with the number of quasiparticles contributing to the wave functions of the excited spin-1 states. Quasimonochromatic photon beams provide a new method to test the microscopic nature of ${1}^{\ensuremath{-}}$ levels.

Nuclear physics detector technology applied to plant biology research

The ability to detect the emissions of radioactive isotopes through radioactive decay (e.g. beta particles, x-rays and gamma-rays) has been used for over 80 years as a tracer method for studying natural phenomena. More recently a positron emitting radioisotope of carbon: 11C has been utilized as a 11CO2 tracer for plant ecophysiology research. Because of its ease of incorporation into the plant via photosynthesis, the 11CO2 radiotracer is a powerful tool for use in plant biology research. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Presently there are several groups developing and using new PET instrumentation for plant based studies. Thomas Jefferson National Accelerator Facility (Jefferson Lab) in collaboration with the Duke University Phytotron and the Triangle Universities Nuclear Laboratory (TUNL) is involved in PET detector development for plant imaging utilizing technologies developed for nuclear physics research. The latest developments of the use of a LYSO scintillator based PET detector system for 11CO2 tracer studies in plants will be briefly outlined.

Measurements of the48Ca(γ,n)reaction

The 48Ca({\gamma},n) cross section was measured using {\gamma}-ray beams of energies between 9.5 and 15.3 MeV generated at the Triangle Universities Nuclear Laboratory (TUNL) high-intensity {\gamma}-ray source (HI{\gamma}S). Prior to this experiment, no direct measurements had been made with {\gamma}-ray beams of sufficiently low energy spread to observe structure in this energy range. The cross sections were measured at thirty-four different {\gamma}-ray energies with an enriched 48Ca target. Neutron emission is the dominant decay mechanism in the measured energy range that spans from threshold, across the previously identified M1 strength, and up the low-energy edge of the E1 giant dipole resonance (GDR). This work found B(M 1) = 6.8 \pm 0.5 {\mu}N2 for the 10.23 MeV resonance, a value greater than previously measured. Structures in the cross section commensurate with extended random-phase approximation (ERPA) calculations have also been observed whose magnitudes are in agreement with existing data.

Discovery of low-lyingE1andM1strengths inTh232

Properties of low-energy dipole states in $^{232}\mathrm{Th}$ have been investigated with the nuclear resonance fluorescence technique. The present work used monoenergetic $\ensuremath{\gamma}$-ray beams at energies of 2--4 MeV from the high-intensity $\ensuremath{\gamma}$-ray source at Triangle Universities Nuclear Laboratory. Over 40 transitions corresponding to deexcitation to the ground state and first excited state were observed for the first time. Excitation energies, integrated cross sections, decay widths, branching ratios, and transition strengths for those states in $^{232}\mathrm{Th}$ were determined and compared with quasiparticle random-phase-approximation calculations. A large number of $E1$ transitions were observed for the first time in actinide nuclei with summed strength of $3.28(69)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {e}^{2}\phantom{\rule{0.3em}{0ex}}{\mathrm{fm}}^{2}$. The observed summed $M1$ strength of 4.26(63)${\ensuremath{\mu}}_{N}^{2}$ is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei.

High energy gamma-ray spectroscopy with LaBr 3 scintillation detectors

Lanthanum bromide scintillation detectors produce very high light outputs (∼60,000 ph/MeV) within a very short decay time (typically ∼20 ns) which means that high instantaneous currents can be generated in the photocathode and dynode chain of the photomultiplier tube (PMT) used for the scintillation readout. The net result is that signal saturation can occur long before the recommended PMT biasing conditions can be reached. In search of an optimized light readout system for LaBr 3, we have tested and compared two different PMT configurations for detection of gamma-rays up to 15 MeV. This range was chosen as being appropriate for gamma-ray remote sensing and medium energy nuclear physics applications. The experiments were conducted at two facilities: the Laboratori Nazionali del Sud (LNS) in Catania, Italy [1] and the High Intensity Gamma-ray Source (HIγS) at Triangle University Nuclear Laboratory, in Durham, North Carolina, USA [2]. The PMT configurations we have tested are (1) a standard dynode chain operated under-biased; (2) a 4-stage reduced chain operated at nominal inter-dynode bias. The results are that shortening the number of active stages, as in configuration (2), has advantages in preserving energy resolution and avoiding PMT saturation over a large energy range. However, the use of an under-biased PMT, configuration (1), can still be considered a satisfactory solution, at least in the case of PMTs manufactured by Photonis. The results of this study will be used in support of the Mercury Gamma-ray and Neutron Spectrometer (MGNS) on board of BepiColombo, the joint ESA/JAXA mission to Mercury, scheduled for launch in 2014.

Proton-3He elastic scattering at low energies and the “AyPuzzle”

The Kohn variational principle and the hyperspherical harmonic technique are applied to study p − 3He elastic scattering at low energies. Preliminary results obtained using several interaction models are reported. The calculations are compared to a recent phase shift analysis performed at the Triangle University Nuclear Laboratory and to the available experimental data. Using a three-nucleon interaction derived from chiral perturbation theory at N2LO, we have found a noticeable reduction of the discrepancy observed for the Ay observable.

Cross sections forU238(n,n'γ)andU238(n,2nγ)reactions at incident neutron energies between 5 and 14 MeV

Precision measurements of $^{238}\mathrm{U}$$(n,{n}^{'}\ensuremath{\gamma})$ and $^{238}\mathrm{U}$$(n,2n\ensuremath{\gamma})$ partial cross sections have been performed at Triangle Universities Nuclear Laboratory (TUNL) to improve crucial data needed for testing nuclear reaction models in the actinide mass region. A pulsed and monoenergetic neutron beam was used in combination with high-resolution \ensuremath{\gamma}-ray spectroscopy to obtain partial cross sections for incident neutron energies between 5 and 14 MeV. \ensuremath{\gamma}-ray yields were measured with high-purity germanium clover and planar detectors. Measured partial cross-section data are compared with previous results using white and monoenergetic neutron beams and calculations from the GNASH and TALYS Hauser-Feshbach statistical-model codes. Present experimental results are in fair to good agreement with most of the existing data for the $^{238}\mathrm{U}$$(n,{n}^{'}\ensuremath{\gamma})$ reaction. However, significant discrepancies are observed for the $^{238}\mathrm{U}$$(n,2n\ensuremath{\gamma})$ reaction.

SU‐FF‐T‐372: Novel Neutron Dosimetry Using Radiochromic Films for 10 MeV Neutrons

The purpose of this paper is to estimate neutron doses using radiochromic XRQA films. Five radiochromic films, placed at varying distances from the neutron source and a water bottle, were exposed to a flux of 10 MeV neutrons produced via the d(d, n)He reaction using the Tandem accelerator at Triangle Universities Nuclear Laboratory at Duke University. An estimation of the neutron flux was obtained by neutron activations of two sets of aluminum and gold activation foils. One foil of each type was placed at the front and back sides of the water target. The flux calculation was based on gamma spectroscopy of the irradiated foils using HPGe detectors. A Monte Carlo (MC) simulation based on FLUKA2008 code was also used to estimate the 10 MeV neutron dose for the same geometry. The accuracy of the results from MC was verified by the comparison of dose on water target from MC with the dose calculated on the same target using flux and the ICRU data tables. Based on these results, a calibration curve was obtained for optical density of the film vs. absorbed neutron dose in the range of 0–20 Gy. We have demonstrated the potential application of radiochromic films for neutron dosimetry. This research was, in part, supported by National Institute of Allergy and Infectious Diseases Grant 5U19 AI067798‐03

SU‐FF‐T‐436: Comparison of Neutron Doses to a Water Target Exposed to a 10 MeV Neutron Beam: Foil Activation Method Vs. Monte Carlo Simulations

We developed a mouse dosimetry model for neutron beam, and compared the calculated and simulated neutron doses for a water target (mouse) exposed to a 10 MeV neutron beam. We also estimated the longitudinal neutron dose profile for the water target. A water bottle (a simulated mouse, 6 cm × 1.3 cm radius) was exposed to a flux of 10 MeV neutrons produced via the d(d, n)He reaction using the Tandem accelerator at Triangle Universities Nuclear Laboratory at Duke University. An estimation of the neutron flux was obtained by neutron activations of two sets of aluminum and gold activation foils. One foil of each type was placed at the front and back sides of the water target. The flux was then calculated based on gamma spectroscopy of the irradiated foils using HPGe detectors. A Monte Carlo (MC) simulation based on FLUKA2008 code, was also used to estimate the 10 MeV neutron dose for the same geometry. The highest neutron flux measured using the activation technique was (8.538×106 +/− 6.89×104) n.cm−2.s−1 at the front end of the target. Using neutron kerma factors from ICRU data tables, the calculated dose rate to a water target at the location of the foil was (1.890+/− 0.015) Gy.hr−1. Using the data from the MC simulation, the calculated neutron dose to the front end of the water target was (1.870 +/− 0.001) Gy.hr−1. The dose calculated using the foil activation technique was in good agreement with the MC results. The slight discrepancy in the measured flux was likely the result of the closer position of the activation foils to the neutron source than the water bottle target, and also absorption within the activation foils. Assuming these results are in good agreement, we estimated the dose profile for the simulated mouse.

Neutron detection efficiency determinations for the TUNL neutron–neutron and neutron–proton scattering-length measurements

The methods employed and the results obtained from measurements and calculations of the detection efficiency for the neutron detectors used at Triangle Universities Nuclear Laboratory (TUNL) in the simultaneous determination of the 1S 0 neutron–neutron and neutron–proton scattering lengths a nn and a np , respectively, are described. Typical values for the detector efficiency were 0.3. Very good agreement between the different experimental methods and between data and calculation has been obtained in the neutron energy range below E n = 13 MeV .

Pulsed and monoenergetic beams for neutron cross-section measurements using activation and scattering techniques at Triangle Universities Nuclear Laboratory

In support of the Stewardship Science Academic Alliances initiative, an experimental program has been developed at Triangle Universities Nuclear Laboratory (TUNL) to measure (n,xn) cross-sections with both in-beam and activation techniques with the goal of improving the partial cross-section database for the NNSA Stockpile Stewardship Program. First experimental efforts include excitation function measurements on 235,238U and 241Am using pulsed and monoenergetic neutron beams with En=5–15MeV. Neutron-induced partial cross-sections were measured by detecting prompt γ rays from the residual nuclei using various combinations of clover and planar HPGe detectors in the TUNL shielded neutron source area. Complimentary activation measurements using DC neutron beams have also been performed in open geometry in our second target area. The neutron-induced activities were measured in the TUNL low-background counting area. In this presentation, we include detailed information about the irradiation procedures and facilities and preliminary data on first measurements using this capability.

Neutron-deuteron breakup experiment atEn=13MeV: Determination of the1S0neutron-neutron scattering lengthann

We report on results of a kinematically complete neutron-deuteron breakup experiment performed at Triangle Universities Nuclear Laboratory using an ${E}_{n}=13$ MeV incident neutron beam. The ${}^{1}{S}_{0}$ neutron-neutron scattering length ${a}_{\mathit{nn}}$ has been determined for four production angles of the neutron-neutron final-state interaction configuration. The absolute cross-section data were analyzed with rigorous three-nucleon calculations. Our average value of ${a}_{\mathit{nn}}=\ensuremath{-}18.7\ifmmode\pm\else\textpm\fi{}0.7$ fm is in excellent agreement with ${a}_{\mathit{nn}}=\ensuremath{-}18.6\ifmmode\pm\else\textpm\fi{}0.4$ fm obtained from capture experiments of negative pions on deuterons. We also performed a shape analysis of the final-state interaction cross-section enhancements by allowing the normalization of the data to float. From these relative data, we obtained an average value of ${a}_{\mathit{nn}}=\ensuremath{-}18.8\ifmmode\pm\else\textpm\fi{}0.5$ fm, in agreement with the result obtained from the absolute cross-section measurements. Our result deviates from the world average of ${a}_{\mathit{nn}}=\ensuremath{-}16.7\ifmmode\pm\else\textpm\fi{}0.5$ fm determined from previous kinematically complete neutron-deuteron breakup experiments, including the most recent one carried out at Bonn. However, this low value for ${a}_{\mathit{nn}}$ is at variance with theoretical expectation and other experimental information about the sign of charge-symmetry breaking of the nucleon-nucleon interaction. In agreement with theoretical predictions, no evidence was found of significant three-nucleon force effects on the neutron-neutron final-state interaction cross sections.

Predictions for polarized-beam and/or vector-polarized-target observables in elastic Compton scattering on the deuteron

Motivated by ongoing developments at High-Intensity Gamma Source (HI\ensuremath{\gamma}S) at Triangle Universities Nuclear Laboratory (TUNL) that include increased photon flux and the ability to circularly polarize photons, we calculate several beam-polarization and/or target-spin-dependent observables for elastic Compton scattering on the deuteron. This is done at energies of the order of the pion mass within the framework of heavy-baryon chiral perturbation theory. Our calculation is complete to $\mathcal{O}({Q}^{3})$ and at this order there are no free parameters. Consequently, the results reported here are predictions of the theory. We discuss paths that may lead to the extraction of neutron polarizabilities. We find that the photon/beam polarization asymmetry is not a good observable for the purpose of extracting ${\ensuremath{\alpha}}_{n}$ and ${\ensuremath{\beta}}_{n}$. However, one of the double polarization asymmetries, ${\ensuremath{\Sigma}}_{x}$, shows appreciable sensitivity to ${\ensuremath{\gamma}}_{1n}$ and could be instrumental in pinning down the neutron spin polarizabilities.

The HIγS Facility: A Free-Electron Laser Generated Gamma-Ray Beam for Research in Nuclear Physics

The High Intensity Gamma-ray Source (HIγS) is a joint project between Triangle Universities Nuclear Laboratory (TUNL) and the Duke Free Electron Laser Laboratory (DFELL). This facility utilizes intra-cavity back-scattering of the FEL photons in order to produce a γ-flux enhancement of approximately 103 over the existing sources. At present, gamma-ray beams with energies ranging from 2 to 50 MeV are available with intensities of 105–107 γ/ s , energy spreads of 1% or better (with lower intensities), and 100% linear polarization. An upgrade is presently underway which will allow for the production of γ rays up to an energy of about 225 MeV having intensities in excess of 108 γ/ sec . The primary component of the upgrade is a 1.2 GeV booster-injector which will provide for efficient injection at any chosen operating energy of the storage ring from 300 MeV to 1.2 GeV. In addition, an upgrade of the present OK-4 FEL to a helical undulator system (OK-5) is underway. This new system has many advantages over the present one, including making switchable linear and circularly polarized beams available, an increase in power and a decrease in mirror-damaging radiation. The full system, including the booster injector, is expected to be ready for use by 2006. TUNL researchers, in collaboration with outside theoretical and experimental colleagues, have proposed a broad based research program in nuclear physics which is designed to exploit the unique flux, energy resolution and polarization of the HIγS beams. A description of the presently available facility and the anticipated facility following the present upgrades will be given in this review, along with a description of recent and planned experiments.