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Now showing items 1 - 16 of 71556

  • Applications of (CLYC)-L-7 scintillators in fast neutron spectroscopy

    Brown, T.   Chowdhury, P.   Doucet, E.   Jackson, E. G.   Lister, C. J.   Mitchell, A. J.   Morse, C.   Rogers, A. M.   Wilson, G. L.   D'Olympia, N.   Devlin, M.   Fotiades, N.   Gomez, J. A.   Mosby, S. M.   Nelson, R. O.  

    The capabilities of Li-7-enriched Cs-2 (LiYCl6)-Li-7 ((CLYC)-L-7) scintillation detectors for fast neutron spectroscopy are explored in benchmark experiments that exploit its excellent pulse-shape discrimination between neutrons and gamma rays, and its unprecedented approximate to 10% energy resolution for fast neutrons in the few MeV range, obtained through the Cl-35(n,p) reaction. Energy- and angle-resolved elastic and inelastic neutron scattering cross-section measurements of Fe-56(n,n') were performed at Los Alamos National Laboratory with a pulsed white neutron source and an array of 1'' x 1'' (CLYC)-L-7 crystals. The results convincingly establish the utility of this dual n/gamma scintillator for fast neutron spectroscopy. Intrinsic efficiency measurements of both 1'' x 1'' and the first ever 3'' x 3'' C7LYC crystal have been initiated, using mono-energetic fast neutron beams at UMass Lowell generated via the Li-7(p,n) reaction. The spectroscopic capabilities and potential of (CLYC)-L-7 are discussed in the context of developing this emerging scintillator for targeted science applications.
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  • Machine learning n/gamma discrimination in CLYC scintillators

    Doucet, E.   Brown, T.   Chowdhury, P.   Lister, C. J.   Morse, C.   Bender, P. C.   Rogers, A. M.  

    Two machine learning techniques, one supervised (Artificial Neural Network) and the other unsupervised (k-means++) have been applied to the task of n/gamma discrimination in Li-7-enriched CLYC detectors, and compared to traditional charge-comparison methods. The results show that a very basic artificial neural network can provide very good discrimination in the energy range investigated, and the k-means++ algorithm is capable of separating neutrons and gamma-rays in CLYC scintillators as well as suggesting reasonable window parameters for charge comparison methods.
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  • Sensitivity of GRETINA position resolution to hole mobility

    Prasher, V. S.   Cromaz, M.   Merchan, E.   Chowdhury, P.   Crawford, H. L.   Lister, C. J.   Campbell, C. M.   Lee, I. Y.   Macchiavelli, A. O.   Radford, D. C.   Wiens, A.  

    The sensitivity of the position resolution of the gamma-ray tracking array GRETINA to the hole charge-carrier mobility parameter is investigated. The chi(2) results from a fit of averaged signal ("superpulse") data exhibit a shallow minimum for hole mobilities 15% lower than the currently adopted values. Calibration data on position resolution is analyzed, together with simulations that isolate the hole mobility dependence of signal decomposition from other effects such as electronics cross-talk. The results effectively exclude hole mobility as a dominant parameter for improving the position resolution for reconstruction of gamma-ray interaction points in GRETINA.
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  • Ameliorating neutron damage in orthogonal-strip planar germanium detectors

    Jackson, E. G.   Hull, E. L.   Lister, C. J.   Pehl, R. H.  

    The segmentation of the electrodes of germanium detectors facilitates gamma ray imaging and tracking. Replacing the traditional n-type lithium drifted contact is a key to Finer segmentation. Amorphous-germanium is a promising alternative contact technology, and large, highly segmented detectors have been fabricated. One factor in adopting any new detector technology is its robustness in hostile environments. Therefore, to explore the effects of neutron damage on position sensitive amorphous-contact germanium gamma-ray detectors and investigate methods for mitigation and repair or damage, two detectors were intentionally exposed to a non-uniform neutron fluence of greater than 4(1) x 10(9) n/cm(2) produced in the Li-7(p, n)Be-7 reaction at the UMass Lowell Van-de-Graati accelerator. Post-irradiation tests were made on the counters by varying the electric field, the charge deposition rate, the operating temperature, and utilizing various annealing cycles in order to ascertain the robustness of their performance after irradiation. (C) 2014 Elsevier B.V. All rights reserved.
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  • Coupling Gammasphere and ORRUBA

    Ratkiewicz, A.   Pain, S. D.   Cizewski, J. A.   Bardayan, D. W.   Blackmon, J. C.   Chipps, K. A.   Hardy, S.   Jones, K. L.   Kozub, R. L.   Lister, C. J.   Manning, B.   Matos, M.   Peters, W. A.   Seweryniak, D.   Shand, C.  

    The coincident detection of particles and gamma rays allows the study of the structure of exotic nuclei via inverse kinematics reactions using radioactive ion beams and thick targets. We report on the status of the project to couple the high-resolution charged-particle detector ORRUBA to Gammasphere, a high-efficiency, high-resolution gamma ray detector.
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  • Isomers and excitation modes in the gamma-soft nucleus Os-192

    Dracoulis, G. D.   Lane, G. J.   Byrne, A. P.   Watanabe, H.   Hughes, R. O.   Kondev, F. G.   Carpenter, M.   Janssens, R. V. F.   Lauritsen, T.   Lister, C. J.   Seweryniak, D.   Zhu, S.   Chowdhury, P.   Shi, Y.   Xu, F. R.  

    New spectroscopic results for high-spin states in Os-192 populated in deep-inelastic reactions include the identification of a 2-ns, 12(+) isomeric state at 2865 key and a 295-ns, 20(+) state at 4580 keV and their associated Delta J = 2 sequences. The structures are interpreted as manifestations of maximal rotation alignment within the neutron i(13/2) and proton h(11/2) shells at oblate deformation. Rotational band members based on the long-lived, K-pi =10(-) isomer are also identified for the first time. Configuration-constrained, potential-energy-surface calculations predict that other prolate multi-quasiparticle high-K states should exist at low energy. (c) 2013 Elsevier B.V. All rights reserved.
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  • Study of the fusion reaction C-12+C-12 at low beam energy

    Jiang, C. L.   Albers, M.   Almaraz-Calderon, S.   Alcorta, M.   Back, B. B.   Bertone, P.   Bucher, B.   Collon, P.   Courtin, S.   Deibel, C. M.   DiGiovine, B.   Esbensen, H.   Fang, X.   Greene, J.   Haas, F.   Henderson, D. J.   Janssens, R. V. F.   Lauritsen, T.   Lefebvre-Schuhl, A.   Lister, C. J.   Marley, S. T.   Pardo, R.   Paul, M.   Rehm, K. E.   Seweryniak, D.   Tang, X. D.   Ugalde, C.   Zhu, S.  

    In this article we discuss two aspects related to the C-12 + C-12 fusion reaction at low energies for carbon burning in supermassive stars. First we present plausible arguments for the notion that the observed resonance structures at the lowest measured energies arise from the relatively large spacing and narrow width of Mg-24 compound levels at the corresponding excitation energy region. We thus point out that the Incoming Wave Boundary Condition is inappropriate for calculating the fusion cross section under these situations. Secondly, we report on a particle-gamma coincidence technique that has been used for the first time to measure the fusion cross section in the system C-12 + C-12 at low beam energies. Based on these results, it should be possible to measure this important fusion cross section down to the 10 pb level within a reasonable length of time.
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  • Optimizing Cs2LiYCl6 for fast neutron spectroscopy

    D'Olympia, N.   Chowdhury, P.   Guess, C. J.   Harrington, T.   Jackson, E. G.   Lakshmi, S.   Lister, C. J.   Glodo, J.   Hawrami, R.   Shah, K.   Shirwadkar, U.  

    Cs2UYCl6 (CLYC) has generated recent interest as a thermal neutron detector due to its excellent n/gamma-ray pulse-shape discrimination and energy resolution. Here, the capabilities of CLYC as a fast neutron detector and spectrometer are reported. A 1 in. x 1 in. CLYC detector was used to measure the response of mono-energetic neutrons over a range of 0.8-2.0 MeV produced via the Li-7(p,n) reaction at the University of Massachusetts Lowell 5.5 MV Van de Graaff accelerator. A broad continuum from the Li-6(n, alpha) reaction was observed, as well as additional peaks below the thermal capture peak. Based on possible reactions in CLYC, the additional peaks are determined to be due to the Cl-35(n,p)S-35 reaction, with a Q-value of +615 keV, and corroborated in simulations using MCNPX. The average resolution of 9% for these peaks makes CLYC a promising candidate for a fast neutron spectrometer. Published by Elsevier B.V.
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  • Projectile Coulomb Excitation of the nucleus Pt-194

    Moeller, T.   Bauer, C.   Janssens, R. V. F.   Lister, C. J.   McCutchan, E. A.   Pietralla, N.   Rainoyski, G.   Seweryniak, D.   Stahl, C.   Zhu, S.  

    Low-lying collective excited states of Pt-194 have been studied via the C-12(Pt-194, Pt-194*) projectile Coulomb excitation reaction at 85% of the Coulomb barrier(850 MeV) using the Gamma sphere Ge-detector array at Argonne National Laboratory. Absolute E2 transition strengths have been obtained from the Coulex crosssections that were deduced from the relative gamma-ray yields. They are discussed with respect to the structure suggested by the O(6) symmetry of the Interacting Boson Model.
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  • Isomers and alignments in Ir-191 and Os-192

    Dracoulis, G. D.   Lane, G. J.   Byrne, A. P.   Watanabe, H.   Hughes, R. O.   Palalani, N.   Kondev, F. G.   Carpenter, M. P.   Seweryniak, D.   Zhu, S.   Janssens, R. V. F.   Lister, C. J.   Lauritsen, T.   Chowdhury, P.   Shi, Y.   Xu, F. R.  

    Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in Os-190, Os-192, Os-194, Ir-191 and Ir-193. These include a 2 ns 12(+) state at 2865 keV and a 295 ns, 20(+) state at 4580 keV in Os-192. Although a number of multi-quasiparticle states arising from prolate and triaxial deformations are expected in these nuclei, the main structures in Os-192 can be interpreted as a two-stage alignment of (i)(13/2) neutrons at oblate deformation, in close analogy with similar structures in the isotones Pt-194 and Hg-196. The isomers are attributed to low-energy E2 transitions at the point of the alignment gains. The isomer observed in Ir-191 is long-lived (tau(m) similar to 8 s) and probably arises from coupling of the h(11/2) proton to the 10(-) v9/2(-)[505]11/2(+)[615] prolate configuration that gives rise to long-lived isomers in Os-190 and Os-192, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial.
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  • Decay of a three-quasiparticle isomer in the neutron-rich nucleus Ta-183

    Palalani, N.   Lane, G. J.   Dracoulis, G. D.   Kondev, F. G.   Byrne, A. P.   Carpenter, M. P.   Chiara, C. J.   Chowdhury, P.   Hughes, R. O.   Janssens, R. V. F.   Lauritsen, T.   Lister, C. J.   McCutchan, E. A.   Seweryniak, D.   Stefanescu, I.   Watanabe, H.   Zhu, S.  

    Excited states in neutron-rich tantalum isotopes have been studied with deep-inelastic reactions using Xe-136 ions incident on a W-186 target. New transitions observed below the tau=1.3 mu s isomer in Ta-183 have enabled the establishment of its energy and put limits on the spin and parity. On the basis of the reduced hindrances for the depopulating transitions, a 3-quasiparticle configuration of nu 1/2(-)[510]11/2(+)[615] circle times pi 9/2(-)[514] is suggested.
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  • Nuclear structure studies along the proton dripline at ATLAS

    Seweryniak, D.   Lotay, G.   Woods, P. J.   Carpenter, M. P.   Chiara, C. J.   David, H. M.   Davinson, T.   Hoffman, C.   Janssens, R. V. F.   Khoo, T. L.   Lauritsen, T.   Lister, C. J.   Liu, Z.   McCutchan, E. A.   Rogers, A. M.   Walters, W. B.   Zhu, S.  

    Nuclei far from the line of stability are a focal point of contemporary nuclear physics. Nuclear structure studies along the proton dripline remain an important component of the scientific program at the ATLAS facility. The Gammasphere gamma-ray array and the Fragment Mass Analyzer offer the unprecedented sensitivity and selectivity required to study nuclei at and beyond the proton dripline. Recent results in proton decay studies, in-beam.-ray spectroscopy around Sn-100 and studies of proton resonances in light nuclei involved in the astrophysical hydrogen burning are presented. Future prospects in the context of planned and on-going upgrades of ATLAS and experimental apparatus are also briefly discussed.
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  • The deformed 0(+) state in (34)Si

    paschalis, S.   Fallon, P.   Macchiavelli, A. O.   Petri, M.   Bender, P. C.   Carpenter, M. P.   Chen, X.   Chiara, C. J.   Clark, R. M.   Cromaz, M.   Gros, S.   Hamilton, L.   Hoffman, C. R.   Janssens, R. V. F.   Lauritsen, T.   Lee, I. Y.   Lister, C. J.   McCutchan, E. A.   Phair, L.   Reviol, W.   Sarantites, D. G.   Seweryniak, D.   Tabor, S. L.   Toh, Y.   Wiedeking, M.   Zhu, S.  

    The energy of the lowest deformed 2-particle 2-hole (2p2h) 0(+) state in even-even N=20 nuclei is a key observable directly related to the size of the neutron N=20 shell closure. (34)Si, with 14 protons and 20 neutrons, lies at the boundary of the "island of inversion", where the deformed 2p2h 0(+) state is the ground state in even-A nuclei. In (34)Si, the 2p2h 0(+) state is expected to be particularly low lying - in some theories it is even predicted to lie below the first 2(+) state. While there have been a number of attempts, using various techniques, no experiment to date has been able to firmly locate the (34)Si 2p2h 0(+) state although a number of candidates have been suggested. Here we present, for the first time, data obtained from a fusion-evaporation reaction (18)O((18)O, 2p) to produce (34)Si. Gammasphere and Microball were used to detect gamma-gamma coincidences and charged particles (two protons), respectively. The increased sensitivity of this experiment using gamma-gamma coincidences and a high charged-particle detection efficiency helped to exclude previously reported candidates and provided a stringent limit on the anticipated gamma decay from the first 2(+) state to the 2p2h 0(+) state
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  • Cross-shell excitations in Al-30 and Si-30 at high spin

    Steppenbeck, D.   Deacon, A. N.   Freeman, S. J.   Janssens, R. V. F.   Carpenter, M. P.   Hoffman, C. R.   Kay, B. P.   Lauritsen, T.   Lister, C. J.   O'Donnell, D.   Oilier, J.   Seweryniak, D.   Smith, J. F.   Spohr, K. -M.   Tabor, S. L.   Tripathi, V.   Wady, P. T.   Zhu, S.  

    Yrast and near-yrast states in Al-30 and Si-30 have been populated to high spin with the O-18+ C-14 fusion-evaporation reaction in inverse kinematics. The level schemes for these two isobars have been extended up to J similar to 9h at 9.4 and 15.5 MeV, respectively. Their decay schemes indicate that cross-shell excitations dominate at high spin, where negative-parity structures exist. Positive-parity states are compared to the results of shell-model calculations using the USD, USDA, and USDB effective interactions. The negative-parity levels are compared to predictions of the W BP interaction and the recently-developed WBP-a Hamiltonian, by allowing 1p-1h excitations to fp-shell orbitals. The results suggest that single-neutron excitations to the 0f(7/2) orbital play a significant role at high spin. (C) 2010 Elsevier B.V. All rights reserved.
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  • Pulse-shape analysis of CLYC for thermal neutrons, fast neutrons, and gamma-rays

    D'Olympia, N.   Chowdhury, P.   Lister, C. J.   Glodo, J.   Hawrami, R.   Shah, K.   Shirwadkar, U.  

    Cs2LiYCl6:Ce (CLYC) has been demonstrated to be sensitive to thermal neutrons via the Li-6(n, alpha)t reaction, and recently to fast neutrons via the Cl-35(n,p) reaction. The scintillation properties of CLYC have been investigated in more detail to further understand its capabilities. Pulses from thermal neutron, fast neutron, and gamma-ray induced excitations were captured, digitized over a 16 mu s time range, and analyzed to identify the scintillation mechanisms responsible for the observed shapes. Additionally, the timing resolutions of CLYC crystals of different sizes were measured in coincidence with a fast CeBr3 scintillator. The effect of high count rates on fast neutron energy resolution and pulse-shape discrimination was investigated up to 45 kHz. Published by Elsevier B.V.
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  • A target vacuum interlock system for Gammasphere

    McCutchan, E. A.   Lister, C. J.   Greene, J. P.  

    An interlock system is described that allows volatile targets to be prepared in an evaporator, sealed under vacuum, transported to the gamma-array Gammasphere, and inserted into the reaction chamber for experiments, while maintaining a vacuum better than 10(-5) torr. The performance of the system is demonstrated for lithium metal targets. A comparison is made between lithium targets covered by a gold evaporation to reduce oxidation and open metal targets transferred in the interlock. (C) 2009 Elsevier B.V. All rights reserved.
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