Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session R9: Nuclear Instrumentation II |
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Sponsoring Units: DNP Chair: Nick Fotiades, Los Alamos National Laboratory Room: 250A |
Monday, April 18, 2016 10:45AM - 10:57AM |
R9.00001: Examining Signal Decomposition in Ge Tracking Detectors through Source-Based Coincidence Measurements M. Cromaz, C.M. Campbell, R.M. Clark, H.L. Crawford, P. Fallon, I.Y. Lee, A.O. Macchiavelli, A. Wiens, L. Riley, R. Taniuchi The performance of a gamma-ray tracking detector, such as those used in the GRETINA spectrometer [1], is dependent on its ability to accurately locate multiple interaction points in the Ge crystal. Interactions are located by observing both net and induced charge as a function of time on the detector’s segmented contact. As multiple interactions are likely, linear combinations of basis signals, a set of simulated signals with unit charge deposited on a grid that spans the detector volume, are fit against the observed signal yielding the interaction positions. While the location of the primary interaction point was found to be good ($\sigma_{pos} \le 2$mm) the location of secondary, lower energy interactions appear less reliable. To investigate this issue, we carried out a series of source-based coincidence measurements. These employed a collimated source and a secondary detector by which we could select single interaction events. Given these events originate from known positions, we can take them in combination to directly test the efficacy of the signal decomposition procedure. We will present a description of the method and preliminary results with a GRETINA quad detector. [1] S. Paschalis, I.Y. Lee, et. al., Nucl. Inst. and Meth. in Phys. Res. A, 709 (2013) 44 [Preview Abstract] |
Monday, April 18, 2016 10:57AM - 11:09AM |
R9.00002: Measuring gamma-ray linear polarization with Gretina? Samuel Tabor Determining the parity of nuclear states is critical to understanding of the shell or band structure but difficult to determine with confidence for states with higher spin or complex structure. Tracking arrays such as GRETA/GRETINA or AGATA offer the highest sensitivity and efficiency for measuring gamma-ray linear polarization from Compton scattering, which indicates whether the transition involves a parity change. The polarization sensitivity of GRETINA has been demonstrated in an ideal case with one only one gamma line in the spectrum coming from the first excited state in $^{24}$Mg aligned to 97{\%}. It is another matter to learn how well GRETINA works in a typical fusion-evaporation reaction with many nuclei formed, many gamma lines emitted, and limited nuclear alignment. The present experiment ($^{18}$O $+ \quad^{18}$O at Elab $=$ 28.7 MeV) at ANL-ATLAS was the first to combine GRETINA with the PhoswichWall for reaction channel selection. Results for transitions in $^{30}$Si as a test case will be presented. [Preview Abstract] |
Monday, April 18, 2016 11:09AM - 11:21AM |
R9.00003: Exploring the spatial resolution of position-sensitive microchannel plate detectors Blake Wiggins, Davinder Siwal, Romualdo deSouza High amplification and excellent timing make microchannel plate (MCP) detectors excellent devices for detection of photons, electrons, and ions. In addition to providing sub-nanosecond time resolution MCP detectors can also provide spatial resolution, thus making them useful in imaging applications. Use of a resistive anode (RA) is a routinely used approach to make an MCP position-sensitive. The spatial resolution of the RA associated with detection of a single incident electron was determined. Factors impacting the spatial resolution obtained with the RA will be discussed and the achieved spatial resolution of 64 $\mu $m (FWHM) will be presented. Recently, a novel approach has been developed to provide position-sensitivity for an MCP detector. In this approach, namely the induced signal approach, the position of the incident particle is determined by sensing the electron cloud emanating from a MCP stack. By utilizing the zero-crossing point of the inherently bipolar signals, a spatial resolution of 466 $\mu $m (FWHM) has been achieved. Work to improve the spatial resolution of the induced signal approach further will be presented. [Preview Abstract] |
Monday, April 18, 2016 11:21AM - 11:33AM |
R9.00004: Toward Direct Reaction-in-Flight Measurements Jerry Wilhelmy, Todd Bredeweg, Malcolm Fowler, Matthew Gooden, Anna Hayes, Gencho Rusev, Joseph Caggiano, Robert Hatarik, Eugene Henry, Anton Tonchev, Charles Yeaman, Megha Bhike, Krishi Krishichayan, Werner Tornow At the National Ignition Facility (NIF) neutrons having energies greater than the equilibrium 14.1 MeV value can be produced via Reaction-in-Flight (RIF) interactions between plasma atoms and upscattered D or T ions. The yield and spectrum of these RIF produced neutrons carry information on the plasma properties as well as information on the stopping power of ions under plasma conditions. At NIF the yield of these RIF neutrons is predicted to be 4-7 orders of magnitude below the peak 14 MeV neutron yield. The current generation of neutron time of flight (nTOF) instrumentation has so far been incapable of detecting these low-yield neutrons primarily due to high photon backgrounds. To date, information on RIF neutrons has been obtained in integral activation experiments using reactions with high energy thresholds such as $^{169}$Tm(n,3n)$^{167}$Tm and $^{209}$Bi(n,4n)$^{206}$Bi. Initial experiments to selectively suppress photon backgrounds have been performed at TUNL using pulsed monoenergetic neutron beams of 14.9, 18.5, 24.2, and 28.5 MeV impinging on a Bibenzyl scintillator. By placing 5 cm of Pb before the scintillator we were able to selectively suppress the photons from the \textunderscore -flash occurring at the production target and enhance the n/\textunderscore signal by \textasciitilde 6 times. [Preview Abstract] |
Monday, April 18, 2016 11:33AM - 11:45AM |
R9.00005: Demonstrating a directional detector based on neon for characterizing high energy neutrons Allie Hexley MITPC is a gas-based time projection chamber used for detecting fast, MeV-scale neutrons. The standard version of the detector relies on a mixture of 600 torr gas composed of 87.5{\%} helium-4 and 12.5{\%} tetrafluoromethane for precisely measuring the energy and direction of neutron-induced nuclear recoils. I describe studies performed with a prototype detector investigating the use of neon, as a replacement for helium-4, in the gas mixture. My discussion focuses on the advantages of neon as the fast neutron target for high energy neutron events (100 MeV) and a demonstration that the mixture will be effective for this event class. I show that the achievable gain and transverse diffusion of drifting electrons in the neon mixture are acceptable and that the detector uptime lost due to voltage breakdowns in the amplification plane is negligible, compared to 20{\%} with the helium-4 mixture. [Preview Abstract] |
Monday, April 18, 2016 11:45AM - 11:57AM |
R9.00006: Measuring scattering lengths of gaseous samples M.G. Huber, T.C. Black, R. Haun, D.A. Pushin, C.B. Shahi, F.E. Weitfeldt Neutron interferometry represents one of the most precise techniques for measuring the coherent scattering lengths ($b_c$) of particular nuclear isotopes. Currently $b_c$ for helium-4 is known only to 1 \% relative uncertainty; a factor of ten higher than precision measurements of other light isotopes. Scattering lengths are measured using a neutron interferometer and by comparing the phase shift a neutron acquires as it passes through a gaseous sample relative to that of a neutron passing through vacuum. The density of the gas is determined by continuous monitoring of the sample’s temperature and pressure. Challenges for these types of experiments include achieving the necessary long-term phase stability and accurate determination of the phase shift caused by the aluminum cell used to hold the gas; a phase shift many times greater than that of the sample. The present status on the effort to measure the n-4He scattering length at the NIST center for Neutron Research will be given. [Preview Abstract] |
Monday, April 18, 2016 11:57AM - 12:09PM |
R9.00007: Precision neutron flux measurements and applications using the Alpha Gamma device Eamon Anderson The Alpha Gamma device [1] is a totally-absorbing ${}^{10}B$ neutron detector designed to measure the absolute detection efficiency of a thin-film lithium neutron monitor on a monoenergetic neutron beam. The detector has been shown to measure neutron fluence with an absolute accuracy of 0.06%. [2] This capability has been used to perform the first direct, absolute measurement of the ${}^6Li(n,t){}^4He$ cross section at sub-thermal energy, improve the neutron fluence determination in a past beam neutron lifetime measurement by a factor of five, and is being used to calibrate the neutron monitors for use in the upcoming beam neutron lifetime measurement BL2 (NIST Beam Lifetime 2) [3]. The principle of the measurement method will be presented and the applications will be discussed. Particular focus will be given to the proposed measurement of the ${}^{235}U(n,f)$ cross section for thermal neutrons. [1] D. M. Gilliam, G. L. Greene, and G. P. Lamaze, Nucl. Instrum. Methods A 284, 220 (1989) [2] A.T. Yue et al, Phys. Rev. Lett. 111, 222501 (2013) [3] http://arxiv.org/abs/1410.5311 [Preview Abstract] |
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