E. Kwan

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.

Discovery of low-lying E-1 and M-1 strengths in Th-232

Properties of low-energy dipole states in {sup 232}Th have been investigated with the nuclear resonance fluorescence technique. The present work used monoenergetic {gamma}-ray beams at energies of 2-4 MeV from the high-intensity {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 {sup 232}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)x10{sup -3} e{sup 2} fm{sup 2}. The observed summed M1 strength of 4.26(63){mu}{sub N}{sup 2} is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei.

Pygmy and core polarization dipole modes in 206Pb: Connecting nuclear structure to stellar nucleosynthesis

Abstract A high-resolution study of the electromagnetic response of 206Pb below the neutron separation energy is performed using a ( γ → , γ ′ ) experiment at the HI γ → S facility. Nuclear resonance fluorescence with 100% linearly polarized photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in 206Pb from 4.9 to 8.1 MeV. The extracted Σ B ( E 1 ) ↑ and Σ B ( M 1 ) ↑ values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9 ± 0.2 e 2 fm 2 and 8.3 ± 2.0 μ N 2 , respectively. These measurements are found to be in very good agreement with the predictions from an energy-density functional (EDF) plus quasiparticle phonon model (QPM). Such a detailed theoretical analysis allows to separate the pygmy dipole resonance from both the tail of the giant dipole resonance and multi-phonon excitations. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of 206Pb of α D = 122 ± 10 mb / MeV . When compared to predictions from both the EDF+QPM and accurately calibrated relativistic EDFs, one deduces a range for the neutron-skin thickness of R skin 206 = 0.12 – 0.19 fm and a corresponding range for the slope of the symmetry energy of L = 48 – 60 MeV . This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross section Pb 205 ( n , γ ) Pb 206 at 30 keV to be σ = 130 ± 25 mb . The astrophysical impact of this measurement—on both the s-process in stellar nucleosynthesis and on the equation of state of neutron-rich matter—is discussed.

Two detector arrays for fast neutrons at LANSCE

The 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. The experimental data base of fission neutron spectra is very incomplete and most present evaluated libraries are based on the approach of the Los Alamos Model. To validate these models and to provide improved data for applications, a program is underway to measure the fission neutron spectrum for a wide range of incident neutron energies using the spallation source of fast neutrons at the Weapons Neutron Research (WNR) facility at the Los Alamos Neutron Science Center (LANSCE). In a double time-of-flight experiment, fission neutrons are detected by arrays of neutron detectors to increase the solid angle and also to investigate possible angular dependence of the fission neutrons. The challenge is to measure the spectrum from low energies, down to 100 keV or so, to energies over 10 MeV, where the evaporation-like spectrum decreases by 3 orders of magnitude from its peak around 1 MeV. For these measurements, we are developing two arrays of neutron detectors, one based on liquid organic scintillators and the other on 6Li-glass detectors. The range of fission neutrons detected by organic liquid scintillators extends from about 600 keV to well over 10 MeV, with the lower limit being defined by the limit of pulse-shape discrimination. The 6Li-glass detectors have a range from very low energies to about 1 MeV, where their efficiency then becomes small. Various considerations and tests are in progress to understand important contributing factors in designing these two arrays and they include selection and characterization of photomultiplier tubes (PM), the performance of relatively thin (1.8 cm) 6Li-glass scintillators on 12.5 cm diameter PM tubes, use of 17.5 cm diameter liquid scintillators with 12.5 cm PM tubes, measurements of detector efficiencies with tagged neutrons from the WNR/LANSCE neutron beam, and efficiency calibration with 252Cf spontaneous fission neutrons. Design considerations and test results are presented.

Development of Neutron Detector Arrays for Neutron-Induced Reaction Measurements

The outgoing neutron energy spectra from neutron-induced fission of various actinides are important for basic understanding of the fission process near the scission point as well as playing a large role in neutron transport codes, which are heavily relied upon in the design of advanced nuclear reactors and simulations of critical assemblies. The reliability of the results of neutron transport models is a strong function of the quality of the nuclear data used as input. Currently, the worlds experimental database of fission neutron spectra is severely incomplete (especially for higher incident neutron energies) with large uncertainties in key portions of the outgoing energy spectra. Many transport codes use evaluated data libraries, which are based on the approach of the Los Alamos model. Other theoretical models have been developed, but the available data cannot distinguish the results of different models (as is the case for 239Pu). Better measurements are needed for all incident and outgoing neutron energies, but most urgently in the low-energy (below 1 MeV) and high-energy (above 6 MeV) portions of the outgoing spectra where theoretical model results differ greatly. We present the design considerations (and some characterization results) of the two Chi-Nu neutron detector arrays: one array of 6Li-glass detectors and one array of liquid-scintillator detectors. These detector arrays are being constructed to meet the challenge of measuring the prompt fission neutron spectra (for a few common actinides) to a higher accuracy and precision than achieved previously and over a larger incident energy range than has been covered by previous experimenters. We see a significant reduction in neutron-scattering backgrounds with our new array designs.

Observation of the Isovector Giant Monopole Resonance via the Si 28 (Be 10, B∗ 10 [1.74 MeV]) Reaction at 100 AMeV

The (^{10}Be,^{10}B^{*}[1.74  MeV]) charge-exchange reaction at 100  AMeV is presented as a new probe for isolating the isovector (ΔT=1) nonspin-transfer (ΔS=0) response of nuclei, with ^{28}Si being the first nucleus studied. By using a secondary ^{10}Be beam produced by fast fragmentation of ^{18}O nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in ^{28}Al, and performing high-resolution γ-ray tracking with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) to identify the 1022-keV γ ray associated with the decay from the 1.74-MeV T=1 isobaric analog state in ^{10}B, a ΔS=0 excitation-energy spectrum in ^{28}Al was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole resonance and isovector giant monopole resonance (IVGMR) were identified. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of 2, which likely indicates deficiencies in the reaction calculations based on the distorted wave Born approximation.

Dipole‐Strength Distributions Below the Giant Dipole Resonance in the Stable Even‐Mass Molybdenum Isotopes

Dipole‐strength distributions in the stable even‐mass molybdenum isotopes up to the neutron‐separation energies have been studied in photon‐scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE at the Research Center Dresden‐Rossendorf, Germany, and with mono‐energetic photon beams at the High Intensity Gamma‐ray Source facility at Triangle Universities Nuclear Laboratory. In order to determine the dipole‐strength distribution, statistical methods were developed for the analysis of the measured spectra. The data obtained for the stable even‐mass molybdenum isotopes from the present (γ,γ’) experiments are combined with (γ,n) cross sections from the literature resulting in a photoabsorption cross section covering the full range from about 4 to 15 MeV, which is of interest for nuclear structure as well as for nuclear astrophysics network calculations. Novel information about the low‐energy tail of the Giant Dipole Resonance and the energy spreading of its strength is der...

Determination of the B(E3, 0+ → 3−)-excitation strength in octupole-correlated nuclei near A ≈ 224 by the means of Coulomb excitation at REX-ISOLDE

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−||E3||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.

Energy separation of the 1 +/1 - parity doublet in 20Ne

The parity doublet of 1+/1− states of Ne-20 at 11.26 MeV excitation energy is one of the best known test cases to study the weak part of the nuclear Hamiltonian. The feasibility of parity violation experiments depend on the effective nuclear enhancement factor (RN/|E(1+) − E(l−)|) which amplifies the impact of the matrix element of the weak interaction on observables indicating parity mixing. An extreme large value of Rn/|E(1+) − E(l−)| = (670 ± 7000) MeV−1 was reported for the doublet in 20Ne. The large uncertainty depends amongst others on the large uncertainty of |E(1+) − E(l−)| = 7.7±5.5 keV of the parity doublet. Nuclear resonance fluorescence (NRF) experiments with linearly and circularly polarized photon beams were performed at the High Intensity Gamma-Ray Source at Duke University, Durham, NC, USA, to determine the energy difference of the parity doublet with higher precision. The different angular distributions for 0+ → 1− → 0+ and 0+ → 1+ → 0+ NRF cascades in polarized γ-ray beams were used to determine the energy difference of the parity doublet to 2.9(13) keV.