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Register a new userDetection System for Neutron $beta$ Decay Correlations in the UCNB and Nab experiments
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We describe a detection system designed for precise measurements of angular correlations in neutron $beta$ decay. The system is based on thick, large area, highly segmented silicon detectors developed in collaboration with Micron Semiconductor, Ltd. The prototype system meets specifications for $beta$ electron detection with energy thresholds below 10 keV, energy resolution of $sim$3 keV FWHM, and rise time of $sim$50 ns with 19 of the 127 detector pixels instrumented. Using ultracold neutrons at the Los Alamos Neutron Science Center, we have demonstrated the coincident detection of $beta$ particles and recoil protons from neutron $beta$ decay. The fully instrumented detection system will be implemented in the UCNB and Nab experiments, to determine the neutron $beta$ decay parameters $B$, $a$, and $b$.
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Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.
The Nab experiment will measure the ratio of the weak axial-vector and vector coupling constants $lambda=g_A/g_V$ with precision $deltalambda/lambdasim3times10^{-4}$ and search for a Fierz term $b_F$ at a level $Delta b_F<10^{-3}$. The Nab detection system uses thick, large area, segmented silicon detectors to very precisely determine the decay protons time of flight and the decay electrons energy in coincidence and reconstruct the correlation between the antineutrino and electron momenta. Excellent understanding of systematic effects affecting timing and energy reconstruction using this detection system are required. To explore these effects, a series of ex situ studies have been undertaken, including a search for a Fierz term at a less sensitive level of $Delta b_F<10^{-2}$ in the beta decay of $^{45}$Ca using the UCNA spectrometer.
Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $lambda = g_A / g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $delta a / a = 10^{-3}$ and the Fierz interference term $b$ to $delta b = 3times10^{-3}$ in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio $lambda$ with a precision of $delta lambda / lambda = 0.03%$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.
We investigate several Pb$(n,ngamma$) and Ge$(n,ngamma$) reactions. We measure $gamma$-ray production from Pb$(n,ngamma$) reactions that can be a significant background for double-beta decay experiments which use lead as a massive inner shield. Particularly worrisome for Ge-based double-beta decay experiments are the 2041-keV and 3062-keV $gamma$ rays produced via Pb$(n,ngamma$). The former is very close to the ^{76}Ge double-beta decay endpoint energy and the latter has a double escape peak energy near the endpoint. Excitation $gamma$-ray lines from Ge$(n,ngamma$) reactions are also observed. We consider the contribution of such backgrounds and their impact on the sensitivity of next-generation searches for neutrinoless double-beta decay using enriched germanium detectors.
Cryogenic bolometers, with their excellent energy resolution, flexibility in material, and availability in high purity, are excellent detectors for the search for neutrinoless double beta decay. Kilogram-size single crystals of TeO_2 are utilized in CUORICINO for an array with a total detector mass of 40.7 kg. CUORICINO currently sets the most stringent limit on the halflife of Te-130 of T > 2.4x10^{24} yr (90% C.L.), corresponding to a limit on the effective Majorana neutrino mass in the range of < 0.2-0.9 eV. Based on technology developed for CUORICINO and its predecessors, CUORE is a next-generation experiment designed to probe neutrino mass in the range of 10 - 100 meV. Latest results from CUORICINO and overview of the progress and current status of CUORE are presented.
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