Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session FE: Electroweak Interactions I |
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Chair: John Hardy, Texas A&M University Room: 103AB |
Thursday, October 27, 2011 4:00PM - 4:12PM |
FE.00001: High-precision digital $\beta$ counting for superallowed $\beta$-decay studies Lixin Chen, John C. Hardy Superallowed $\beta$-decay ft values must be measured to high precision in order to test the Electroweak Standard Model and probe new physics beyond it. To establish a more robust $\beta$ counting system for high precision half-life measurements, we have developed a new digital $\beta$ counting system, which we have now tested on-line. An 8-bit digitizer with 1 GS/s sampling rate was used to record the waveforms from our gas proportional counter, and a software filter applied to discriminate and count genuine decay events. The software filter uses pulse-shape analysis to separate genuine $\beta$-decay events from proportional-counter spurious pulses. The digital counting method and the results obtained from our recent on-line test experiments will be presented in detail. This work demonstrates the first successful application of a high-speed digitizer and off-line digital-signal-processing techniques to high precision nuclear $\beta$-decay lifetime measurements. [Preview Abstract] |
Thursday, October 27, 2011 4:12PM - 4:24PM |
FE.00002: Superallowed Branching Ratio in the $\beta $ Decay of $^{34}$Ar V.E. Iacob, J.C. Hardy Precise \textit{ft}-values for superallowed $0^+\to 0^+ \quad \beta ^{+}$-decays contribute to the most demanding unitarity test of the Cabibbo-Kobayashi-Maskawa matrix. The decays from T$_{z}$=-1 nuclei, like $^{34}$Ar, are particularly valuable because they can constrain the isospin symmetry-breaking corrections that must be applied to the measured \textit{ft}-values. This requires their branching ratios to be determined to $\sim $0.1{\%} or better. We report here on our continuing quest to reach this goal, which most recently has involved better control of our detection geometry (with laser ranging) and continuous monitoring of dead-time. We produced a pure $^{34}$Ar beam at the exit of the MARS recoil separator using a 25A MeV $^{35}$Cl beam from the Texas A{\&}M cyclotron to bombard a hydrogen gas target. The $^{34}$Ar beam was extracted into air, degraded and implanted into the Mylar tape of our fast transport system. In repeated cycles, each $^{34}$Ar sample was collected for 2s and then moved in 175 ms to the center of a well-shielded $\beta -\gamma $ counting station, where $\beta $ singles and $\beta -\gamma $ coincidences were recorded for 2s. The $\beta $'s were detected in a1-mm-thick plastic scintillator, while the $\gamma $'s were detected by our precisely efficiency-calibrated HPGe detector. With the laser sensor we read the tape-to-HPGe distance to a precision better than 0.1mm and recorded it for each cycle. Branching-ratio results will be reported. [Preview Abstract] |
Thursday, October 27, 2011 4:24PM - 4:36PM |
FE.00003: High-Precision Branching Ratio Measurement for the Superallowed $\beta^+$ emitter $^{74}$R\lowercase{b} Ryan Dunlop Precision measurements of superallowed Fermi beta decay allow for tests of the Cabibbo-Kobayashi-Maskawa matrix (CKM) unitarity, the conserved vector current hypothesis, and the magnitude of isospin-symmetry-breaking effects in nuclei. A high-precision measurement of the branching ratio for the $\beta^+$ decay of $^{74}$Rb has been performed at the Isotope Separator and ACcelerator (ISAC) facility at TRIUMF. The $8\pi$ spectrometer, an array of 20 close-packed HPGe detectors, was used to detect gamma rays emitted following the decay of $^{74}$Rb. PACES, an array of 5 Si(Li) detectors, was used to detect emitted conversion electrons, while SCEPTAR, an array of plastic scintillators, was used to detect emitted beta particles. In this talk, the importance of the branching ratio measurement of the $^{74}$Rb superallowed decay will be discussed and preliminary results from the recent measurements at ISAC will be presented. [Preview Abstract] |
Thursday, October 27, 2011 4:36PM - 4:48PM |
FE.00004: High Precision Determination of the $^6$He Half-life A. Knecht, B.G. Delbridge, A. Garcia, G.C. Harper, R. Hong, R.G.H. Robertson, H.E. Swanson, S. Utsuno, D.I. Will, C. Wrede, D.W. Zumwalt, P. Mueller, W. Will We performed a high precision measurement of the half-life of $^6$He. The motivation for this experiment lied not only in resolving a long-standing discrepancy between the previous most precise measured values of $806.7 \pm 1.5$~ms and $798.1 \pm 1$~ms, respectively, but also in serving as a solid basis for the extraction of the axial coupling constant $g_A$ by comparing the result to ab initio calculations. The measurement took place at the tandem accelerator of the Center for Experimental Nuclear Physics and Astrophysics of the University of Washington where we used the $^7$Li($^2$H,$^3$He)$^6$He reaction to produce $^6$He. We impinge a 10~$\mu$A, 18~MeV deuteron beam on a liquid lithium target station that delivers $\sim$$10^9$ atoms/s to a low-background experimental area. Here we present the design of the lithium target station and the results of the $^6$He half-life determination. [Preview Abstract] |
Thursday, October 27, 2011 4:48PM - 5:00PM |
FE.00005: High Precision Measurement of the Lifetime of $^{19}$Ne Leah Broussard The T=$\frac{1}{2}$ mirror transitions, such as the $\beta^+$ decay of $^{19}$Ne, have been identified as excellent candidates for high precision studies of the weak interaction in the Standard Model, complementary to the well-studied $0^+\rightarrow0^+$ decays and neutron decay measurements. The Triangle Universities Nuclear Laboratory (TUNL) has developed an experiment to perform high precision lifetime measurements using the Trapped Radioactive Isotopes Microlaboratories for fundamental Physics (TRI$\mu$P) facility at the Kernfysich Versneller Instituut (KVI) in Groningen, the Netherlands. We use a custom tape drive system to collect and transport the purified isotope from the TRI$\mu$P isotope separator to the HPGe clover detector array, where annihilation radiation is detected in coincidence. We will discuss analysis of systematic uncertainties due to contamination, diffusion, and rate-dependent effects and present measurement results. [Preview Abstract] |
Thursday, October 27, 2011 5:00PM - 5:12PM |
FE.00006: A Magneto-Gravitational Trap for the Measurement of the Free Neutron Lifetime Daniel Salvat Recent measurements of the free neutron beta-decay lifetime using trapped Ultra-Cold Neutrons (UCN) have yielded results with high precision ($\sim $ 1 s), but with central values significantly lower than the previous world average. To resolve this controversy, we initiated a new effort to measure the neutron beta-decay lifetime using UCN in a magneto-gravitational trap at the Los Alamos Neutron Science Center. The trap eliminates wall interactions which lead to additional UCN loss. We will present the design of the trap, and R{\&}D results. Extensive Monte Carlo techniques are in development to investigate the time dependence of the neutron phase space within the trap, and systematically study marginally trapped UCN. We also investigate a novel method for UCN detection, wherein a vanadium foil absorbs neutrons within the trap, and the foil activation is measured using a beta-gamma coincidence technique. This provides a clean signal proportional to the number of trapped UCN, and avoids the process of emptying UCN from the trap. The current experimental status, and plans for a proof-of-principle 1 second measurement of the lifetime will be discussed. [Preview Abstract] |
Thursday, October 27, 2011 5:12PM - 5:24PM |
FE.00007: A T-odd Momentum Correlation in Radiative Beta Decay Susan Gardner, Daheng He A triple-product momentum correlation in the neutron or nuclear radiative $\beta$-decay rate isolates the pseudo-Chern-Simons term found by Harvey, Hill, and Hill as a consequence of the baryon vector current anomaly and SU(2)$_{L}$$\times$U(1)$_Y$ gauge invariance at low energies. The correlation appears if the imaginary part of the coupling constant is nonzero, so that its observation potentially probes sources of CP violation beyond the Standard Model. The effect can be mimicked by electromagnetic final-state interactions in the Standard Model; we have computed the induced T-odd triple-momentum correlation in the decay rate in ${\cal O}(\alpha)$ in the absence of recoil effects. We retain the parametric dependence on masses and coupling constants throughout so that our results serve as a template for the evaluation of the asymmetry in allowed nuclear radiative $\beta$-decays as well. We discuss the role nuclear processes can play in discovering the effect. [Preview Abstract] |
Thursday, October 27, 2011 5:24PM - 5:36PM |
FE.00008: Status of the Neutron Radiative Decay Experiment Benjamin ONeill The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and antineutrino should be accompanied by a continuous spectrum of photons.~ We previously reported detection of photons from neutron beta decay with a branching ratio of (3.09$\pm $ 0.32)$\times $10$^{-3}$ in the energy range of 15keV to 340keV. This was achieved by prompt coincident detection of an electron and photon, in delayed coincidence with a proton. The photons were detected using a single bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode (APD). Our most recent experiment employs an array of twelve of these detectors, as well as three large area APD detectors.~ We anticipate that our improved measurement of the branching ratio will have an uncertainty of 1 percent.~ In addition, we have extended the detectable energy range down to $\approx $250 eV and up to the endpoint.~ We will present an overview of the apparatus and an update of the status of the analysis on the branching ratio and the photon energy spectrum with a focus on the large area APDs. [Preview Abstract] |
Thursday, October 27, 2011 5:36PM - 5:48PM |
FE.00009: The n-3He Parity Violation Experiment Christopher Crawford We are proposing an experiment at the SNS Fundamental Neutron Physics Beamline to measure the parity violating longitudinal neutron spin asymmetry from the reaction n + $^3$He $\to$ p + $^3$H. Recent calculations of 4-body wave functions and P-odd operators have shown that this observable is sensitive to both the I=1 long range and I=0 components of the hadronic weak interaction. The experimental setup and expected sensitivity will be discussed. [Preview Abstract] |
Thursday, October 27, 2011 5:48PM - 6:00PM |
FE.00010: ABSTRACT WITHDRAWN |
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