6,298 research outputs found

    Improving the Limits of Detection of Low Background Alpha Emission Measurements

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    Alpha particle emission, even at extremely low levels, is a significant issue in the search for rare events (e.g., double beta decay, dark matter detection). Traditional measurement techniques require long counting times to measure low sample rates in the presence of much larger instrumental backgrounds. To address this, a commercially available instrument developed by XIA uses pulse shape analysis to discriminate alpha emissions produced by the sample from those produced by other surfaces of the instrument itself. Experience with this system has uncovered two residual sources of background: cosmogenics and radon emanation from internal components. A development program is underway to enhance the system and extend the pulse shape analysis technique further, so that these residual sources can be identified and rejected as well. In this paper, we review the theory of operation and pulse shape analysis techniques used in XIA`s alpha counter, and briefly explore data suggesting the origin of the residual background terms. We will then present our approach to enhance the system`s ability to identify and reject these terms. Finally, we will describe a prototype system that incorporates our concepts and demonstrates their feasibility.Comment: 8 pages, 13 figures, presented at LRT-201

    Exchange Current Corrections to Neutrino--Nucleus Scattering

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    Relativistic exchange current corrections to neutrino--nucleus cross sections are presented assuming non--vanishing strange quark form factors for the constituent nucleons. For charged current processes the exchange current corrections can lower the impulse approximation results by 10\% while these corrections are found to be sensitive to both the nuclear density and the strange quark axial form factor of the nucleon for neutral current processes. Implications on the LSND experiment to determine this form factor are discussed.Comment: 11 pages, 2 figures, revtex 3.0, full postscript version of the file and figures available at http://www.nikhefk.nikhef.nl/projects/Theory/preprints/preprints.html To appear in Phys. Rev. Lett

    Muon capture on nuclei with N > Z, random phase approximation, and in-medium renormalization of the axial-vector coupling constant

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    We use the random phase approximation to describe the muon capture rate on 44{}^{44}Ca,48{}^{48}Ca, 56{}^{56}Fe, 90{}^{90}Zr, and 208{}^{208}Pb. With 40{}^{40}Ca as a test case, we show that the Continuum Random Phase Approximation (CRPA) and the standard RPA give essentially equivalent descriptions of the muon capture process. Using the standard RPA with the free nucleon weak form factors we reproduce the experimental total capture rates on these nuclei quite well. Confirming our previous CRPA result for the N=ZN = Z nuclei, we find that the calculated rates would be significantly lower than the data if the in-medium quenching of the axial-vector coupling constant were employed.Comment: submitted to Phys. Rev.

    Order-disorder transition in nanoscopic semiconductor quantum rings

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    Using the path integral Monte Carlo technique we show that semiconductor quantum rings with up to six electrons exhibit a temperature, ring diameter, and particle number dependent transition between spin ordered and disordered Wigner crystals. Due to the small number of particles the transition extends over a broad temperature range and is clearly identifiable from the electron pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small systems see http://www.smallsystems.d

    Scaling Of Chiral Lagrangians And Landau Fermi Liquid Theory For Dense Hadronic Matter

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    We discuss the Fermi-liquid properties of hadronic matter derived from a chiral Lagrangian field theory in which Brown-Rho (BR) scaling is incorporated. We identify the BR scaling as a contribution to Landau's Fermi liquid fixed-point quasiparticle parameter from "heavy" isoscalar meson degrees of freedom that are integrated out from a low-energy effective Lagrangian. We show that for the vector (convection) current, the result obtained in the chiral Lagrangian approach agrees precisely with that obtained in the semi-phenomenological Landau-Migdal approach. This precise agreement allows one to determine the Landau parameter that enters in the effective nucleon mass in terms of the constant that characterizes BR scaling. When applied to the weak axial current, however, these two approaches differ in a subtle way. While the difference is small numerically, the chiral Lagrangian approach implements current algebra and low-energy theorems associated with the axial response that the Landau method misses and hence is expected to be more predictive.Comment: 39 pages, latex with 4 eps figure, modified addresses and reference

    Predicting total reaction cross sections for nucleon-nucleus scattering

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    Nucleon total reaction and neutron total cross sections to 300 MeV for 12C and 208Pb, and for 65 MeV spanning the mass range, are predicted using coordinate space optical potentials formed by full folding of effective nucleon-nucleon interactions with realistic nuclear ground state densities. Good to excellent agreement is found with existing data.Comment: 10 pages, 4 figure

    Three-body Faddeev Calculation for 11Li with Separable Potentials

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    The halo nucleus 11^{11}Li is treated as a three-body system consisting of an inert core of 9^{9}Li plus two valence neutrons. The Faddeev equations are solved using separable potentials to describe the two-body interactions, corresponding in the n-9^{9}Li subsystem to a p1/2_{1/2} resonance plus a virtual s-wave state. The experimental 11^{11}Li energy is taken as input and the 9^{9}Li transverse momentum distribution in 11^{11}Li is studied.Comment: 6 pages, RevTeX, 1 figur

    Neutrino induced transitions between the ground states of the A=12 triad

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    Neutrino induced reactions on 12^{12}C, an ingredient of liquid scintillators, have been studied in several experiments. We show that for currently available neutrino energies, EΜ≀E_{\nu} \le 300 MeV, calculated exclusive cross sections 12^{12}Cgs(Îœ,l)_{gs}(\nu,l)12^{12}Ngs_{gs} for both muon and electron neutrinos are essentially model independent, provided the calculations simultaneously describe the rates of several other reactions involving the same states or their isobar analogs. The calculations agree well with the measured cross sections, which can be therefore used to check the normalization of the incident neutrino spectrum and the efficiency of the detector.Comment: 9 pages REVTEX, 2 postscript figures, text and figures available at http://www.krl.caltech.edu/preprints/MAP.htm

    Polarization transfer in the 16^{16}O(p,pâ€Č)(p,p') reaction at forward angles and structure of the spin-dipole resonances

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    Cross sections and polarization transfer observables in the 16^{16}O(p,pâ€Č)(p,p') reactions at 392 MeV were measured at several angles between Ξlab=\theta_{lab}= 0∘^\circ and 14∘^\circ. The non-spin-flip (ΔS=0{\Delta}S=0) and spin-flip (ΔS=1{\Delta}S=1) strengths in transitions to several discrete states and broad resonances in 16^{16}O were extracted using a model-independent method. The giant resonances in the energy region of Ex=19−E_x=19-27 MeV were found to be predominantly excited by ΔL=1{\Delta}L=1 transitions. The strength distribution of spin-dipole transitions with ΔS=1{\Delta}S=1 and ΔL=1{\Delta}L=1 were deduced. The obtained distribution was compared with a recent shell model calculation. Experimental results are reasonably explained by distorted-wave impulse approximation calculations with the shell model wave functions.Comment: 28 pages RevTex, including 9 figures, to be published in Phys. Rev. C.; a typo in Eq. (3b) was correcte
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