Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session KC: Nuclear Physics II |
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Chair: Henning Back, North Carolina State University Room: Holiday Inn Brownstone Jefferson |
Friday, October 31, 2008 3:45PM - 3:57PM |
KC.00001: A Diamond Multistrip Detector for the JLab Hall C Compton Polarimeter Amrendra Narayan Precision polarimetry is essential for any precision asymmetry measurement. The QWeak experiment at JLab will use parity violating electron scattering from the proton to perform a precision measurement of the weak charge of the proton (Q$^{P}_{\small{{\mbox{Weak}}}}$). This experiment requires the knowledge of the electron beam polarization at a level of $\sim$1\%. To achieve this, a Compton Polarimeter is under construction in JLab Hall C. The Polarimeter includes a recoil electron detector. The QWeak experiment plans to use a 180 $\mu$A polarized electron beam, in order to get the highest luminosity possible at JLab. At these luminosities, the typically used silicon detectors are rendered unsuitable due to rapid radiation damage. Thus, Chemical Vapor Deposited (CVD) diamond was chosen for the recoil electron detector. CVD Diamond detectors are well known for their radiation hardness. A prototype diamond multi-strip detector was characterized at Mississippi State University. We will present preliminary spectra obtained from this detector. The status of the full-size detector currently under construction will also be presented. [Preview Abstract] |
Friday, October 31, 2008 3:57PM - 4:09PM |
KC.00002: Novel Likelihood PID Method for Hypernuclear Spectroscopy Pavlo Baturin, Joerg Reinhold Jefferson Lab experiment E01-011 (HKS), undertaken in Fall 2005, measured the electroproduction of $\Lambda$-hypernulcei in the (e,e'K$^+$) reaction with a resolution of 400 keV (FWHM), a record for reaction spectroscopy. The experiment employed time-of-flight and several layers of aerogel and water \v Cerenkov detectors for particle identification (PID). At the current moment, the entire analysis has been done by applying hard cuts to the corresponding distributions. Here we present an alternative PID approach that employs a likelihood method. Probability density functions were obtained for each detector distribution, which then were combined to likelihood values for each possible particle. This improves the efficiency for kaon identification and should also reduce background due to misidentification of protons and pions. The expected increase in signal to background ratio will improve the statistical significance of the observed excitation spectra, especially for the core exited states with poor statistics. It might also improve the energy resolution. The presentation will explain the new likelihood PID approach, compare it to the standard one, and give a brief outline of the method's benefits. [Preview Abstract] |
Friday, October 31, 2008 4:09PM - 4:21PM |
KC.00003: Electroproduction of Hyperons at Low Momentum Transfer Armando Acha, Pete Markowitz A H(e,e'K) measurement was performed at Hall A, TJNAF as part of the hypernuclear experiment E94-107. E94-107 hypernuclear spectroscopy measurements on $^{9}$Be, $^{12}$C and$^{16}$O targets allow the study of the $\Lambda $-N interaction. However, one important ingredient to the hypernuclear cross section calculation is the elementary cross section for production of hyperons, $\Lambda $ and $\Sigma ^{0}$. This was measured using a hydrogen (i.e. a proton) target. In addition, there is not much data available for electroproduction of hyperons at low Q$^{2}$ and $\Theta _{CM}$ and the available theoretical models differ a lot in this kinematical region of W. The measurement of the elementary cross section will help not only in the hypernuclear spectroscopy studies but also in constraining existing theoretical models for the elementary reaction. Measurements of the differential cross sections will be reported as well as their results binned in Q$^{2}$ ,W and $\Theta _{CM}$ to understand the dependence on these variables. To extract the cross sections a Hall A Monte Carlo simulation (MCEEP) was used in comparison, assuming a smooth dependence of these variables. Details of the calculations and results will be shown. [Preview Abstract] |
Friday, October 31, 2008 4:21PM - 4:33PM |
KC.00004: The Padua Algorithm for the Computation of Deuteron wave functions, Binding Energy and Other Properties Mesgun Sebhatu The deuteron is a bound state of a neutron and proton in $^{3}$S$_{1}$ and $^{3}$D$_{1}$ states. The wavefunctions that represent the $^{3}$S$_{1}$(u) {\&} $^{3}$D$_{1}$(w) are obtained by solving a Rarita-Schwinger (RS) equation. In this presentation, an algorithm pioneered by a Padua group [1] for solving the RS equation is described. The Padua algorithm yields accurate results with fewer mesh points (N=300 or more) compared to standard methods [2]. The algorithm relies on the transformation x= tan$^{-1}$(r). This truncates the infinite integration domain 0$<$r$<\infty $ to 0$<$x$<$ $\pi $/2. This enables a grid that is uniform in x to explore the inner most region of r with a finer mesh than the asymptotic region. After second-order central difference approximations are used to replace the derivatives in the modified RS equation, it reduces to a standard eigenvaue problem of the form AY = E$^{2}$Y, where A is a 2Nx2N matrix, N the maximum number of steps, E the binding energy and Y is u(n) (1$<$n$<$N) and w(n) (N+1$<$n$<$2N). n is an index for individual steps. A FOTRAN program with a subroutine from IMSL is used to solve the RS equation for the Reid Soft Core Potential to illustrate the algorithm. [1] T.A. Minnelli, A.Pacolini, and C. Villi, Nuovo Cimento, 101, (1991) p.1626 [2] W.H.Press et al. Numerical Recipes, 1986, Ch. 16 [Preview Abstract] |
Friday, October 31, 2008 4:33PM - 4:45PM |
KC.00005: The $^{3}$He injection test for the search of neutron electric dipole moment (nEDM) Xiaofeng Zhu The proposed nEDM experiment at the Spallation Neutron Source (ORNL) aims at a sensitivity of 10$^{-28}$ e-cm. The experimental strategy is to form a three-component fluid of ultracold neutrons and $^{3}$He atoms in a bath of superfluid $^ {4}$He. Polarized $^{3}$He serves as a co-magnetometer and an ultracold neutron spin precessing frequency analyzer, using the spin-dependent nuclear reaction: $\vec{n}+\vec{^{3}He} \rightarrow p + t + 764$ keV. The injection test described in this talk is to study methods of injecting polarized $^{3}$He from an Atomic Beam Source (ABS) into the superfluid $^{4}$He and demonstrate that the polarization loss is acceptable for the nEDM experiment. In this presentation, I describe the design of the magnet system, a pulsed NMR system for polarization measurement, and cryogenic issues associated with the injection test apparatus. [Preview Abstract] |
Friday, October 31, 2008 4:45PM - 4:57PM |
KC.00006: $^3$He Relaxation Time Measurements at $\sim$400mK for the neutron electric dipole moment (nEDM) experiment Qiang Ye, Haiyan Gao, Robert Golub, Dipangkar Dutta, Paul Huffman, Franklin Dubose In the new nEDM experiment planned to be carried out at the SNS, the measurement cell will be made of dTPB-dPS (wavelength shifting material) coated acrylic and filled with superfluid $^4$He. NMR technique will be used to measure the neutron precession frequency by comparing with that of the polarized $^3$He using the spin-dependent nuclear reaction: $\vec{n}+\vec{^{3}He} \rightarrow p + t + 764$ keV. The polarized $^3$He will be used as a comagnetometer to monitor the B field \emph{in situ} during the experiment. Understanding the relaxation mechanism of polarized $^3$He under the experimental conditions and maintaining $^3$He polarization is crucial. Following our earlier study of the $^3$He relaxation time in a dTPB-dPS coated cylindrical acrylic cell at a temperature of 1.9K in the presence of superfluid $^4$He with a magnetic holding field of 21 G, similar measurements at $\sim$400 mK (the proposed nEDM experimental temperature) have been carried out using a dilution refrigerator in TUNL at $\sim$7 G. Preliminary results will be presented. [Preview Abstract] |
Friday, October 31, 2008 4:57PM - 5:09PM |
KC.00007: Cryogenic Engineering for the Neutron Electric Dipole Measurement at the Spallation Neutron Source D.G. Haase, P.R. Huffman, J. Boissevain, E.I. Ihloff, C. Vidal A planned experiment at the SNS at ORNL will increase the precision of present limits on the electric dipole moment of the neutron by almost two orders of magnitude. Neutrons from the Fundamental Neutron Physics Beamline will enter a container of liquid helium at 0.45 K and become trapped by losing their kinetic energy in collisions with phonons in the superfluid helium. The experiment requires a large insulated cryovessel, several containers of liquid helium including a 1000 l chamber, a high cooling power dilution refrigerator and a dedicated helium liquefier/refrigerator. The final design must include limited use of magnetic, conducting and neutron activated materials. The time for cooling and warming the cryovessel must be minimized to facilitate the testing process. We will describe the design of this system, and an analysis of the heat flows and experimental constraints of this large cryogenic experiment. [Preview Abstract] |
Friday, October 31, 2008 5:09PM - 5:21PM |
KC.00008: Purification of $^{4}$He through Differential Evaporation F. DuBose, D.G. Haase, P.R. Huffman The neutron electric dipole moment (nEDM) experiment, to be housed at the Spallation Neutron Source at Oak Ridge National Laboratories, will probe for a dipole moment at the level of 10$^{-28}$ e cm. As part of the measurement process, neutrons precess in an environment of isotopically pure helium, doped with polarized $^{3}$He. After this $^{3}$He depolarizes it must be removed. We are developing an evaporative purification technique for this removal, lowering the concentration of $^{3}$He in $^{4}$He from 10$^{-8}$ to 10$^{-10}$, at an operating temperature of 300 -- 350 mK. Because the vapor pressure of $^{3}$He is enhanced at temperatures below 500mK, $^{3}$He atoms can be preferentially removed from the solution. The purifier requires a large liquid surface area, while minimizing superfluid film flow. The evaporated atoms are adsorbed on activated charcoal. We have built a device to measure $^{3}$He/$^{4}$He ratios using a leak detector mass spectrometer and a residual gas analyzer. [Preview Abstract] |
Friday, October 31, 2008 5:21PM - 5:33PM |
KC.00009: A thermal model for cooling the nEDM $^{3}$He services D.P. Kendellen, D.G. Haase, P.R. Huffman The neutron electric dipole moment (nEDM) experiment proposed for the Spallation Neutron Source is a precision test of time reversal symmetry, probing the same physics believed to be responsible for the matter-antimatter imbalance in the universe. In the experiment, polarized neutrons and polarized 3He atoms suspended in a bath of superfluid $^{4}$He at 0.35 K precess in a weak magnetic field. When a strong electric field is applied parallel or antiparallel to the B-field, a change in the neutron precession rate signifies a nonzero nEDM. The polarized $^{3}$He, which acts as a co-magnetometer, must be replenished every 1000--2000 seconds. Electrical heaters produce heat flows to sweep $^{3}$He in and out of the measurement cells. The heat is transferred to a dilution refrigerator through plastic or sintered metal heat exchangers. We have modeled the heat flows needed for $^{3}$He transport, to determine the heat load to the refrigerator and to guide the design and placement of the heat exchangers. [Preview Abstract] |
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