Session PL: Neutron Physics II: Precision Neutron Decay

Simulations of Electron Detection Systematic Errors for the UCNA+ Experiment

The Ultracold Neutron Asymmetry UCNA+ experiment at Los Alamos National Laboratory is an upgrade to the original UCNA experiment which used polarized ultracold neutrons (UCN) to extract the electron asymmetry from neutron beta decay. The Asymmetry parameter, A₀, is the angular correlation between the neutron spin and the direction of the momentum of the decay electron. The angular correlation coefficient A₀ determines the ratio of the weak axial-vector and vector coupling constants, g_A/g_V . The UCNA experiment measured the coefficient A₀ to a precision of 0.60% using a 2 × 2π magnetic spectrometer. Upgrades to several subsystems are being considered, including the electron detection system. We present an exploratory Monte Carlo analysis of the UCNA+ experimental setup. We have simulated different experimental geometries in an effort to explore systematic uncertainty arising primarily from beta particles scattering from the trap endcap foils and detectors. One of the goals of this work is to identify a set of calibration measurements that can benchmark the modeling precision for scattering effects in the upgraded experiment. Results from these simulations will be discussed.   

Probing Nab's proton time-of-flight reconstruction accuracy through detailed pulse shape simulation

Precise measurements of neutron beta decay observables are one of the most promising avenues for testing the electroweak Standard Model at the TeV scale. In particular, a more precise determination of the vector to axial vector admixture in neutron beta decay will shed light on the current tension in the Cabibbo-Kobayashi-Maskawa matrix top-row unitarity requirement without any of the nuclear structure corrections that limit the current global average. The Nab experiment at the Spallation Neutron Source at Oak Ridge National Lab will measure the beta-antineutrino angular correlation following neutron decay inside a hermetic, magnetic spectrometer with an anticipated relative precision of approximately 0.1%. An event-by-event angular reconstruction occurs via a time-of-flight measurement of the outgoing proton, which requires excellent control on timing distortions due to field transport and detector effects. We will present an overview of sophisticated pulse shape simulation efforts, and highlight results of detailed plasma effects and entrance window studies.   

Prototype testing for the UCNProBe experiment

Recent measurements of the free neutron lifetime using ultracold neutrons (UCNs) continue to disagree with results from cold neutron beam experiments by four to five standard deviations. The UCNProBe experiment will measure the neutron decay lifetime using UCNs and address this discrepancy. To measure the beta decay lifetime, we will count the number of UCNs in the trap as well as the number of beta decay electrons to 0.1 percent precision. The apparatus consists of a layer of deuterated scintillator for confining UCNs and an outer veto plastic scintillator layer. In this talk, we will present the new detection method for collecting light from the scintillating layers using PMTs, as well as expected backgrounds from neutron capture and mitigation, and ongoing prototype testing.   

Closed Loop Helium Gas Cooling System for the Nab Experiment

The Nab experiment at the Spallation Neutron Source will use an unpolarized neutron beam to measure the electron-neutrino correlation coefficient, 'a', and the Fierz interference term, 'b.’ These decay correlation parameters will be used to extract λ, the ratio of the axial-vector to vector coupling constants, to provide a measurement of V_ud and test CKM matrix unitarity free of nuclear corrections. The protons and electrons from neutron beta decay will be measured by pixelated silicon detectors. To minimize noise and optimize energy and timing resolution, the detector will be actively cooled to below 200K. Temperature regulation will be achieved by a closed loop helium gas cooling system. This talk will focus on the performance of an upgrade to the detector cooling system that incorporates a counter-flow heat exchanger into the loop and outline future improvements.   

A Detector Testing Chamber for the Nab Experiment

The Nab experiment is being performed to study the neutron decay correlations in the search for BSM physics and verification of CKM unitarity.  It will use pixelated silicon detectors to detect the decay electron energy and proton time of flight for complete kinematic reconstruction of each event.  It is important to characterize timing and energy response of the Nab detectors and the impact of charge sharing among detector pixels to understand the systematic errors.  This includes the timing response to the decay protons and electrons, which must be measured under controlled conditions.  We will test the detector response to protons as a function of energy, position, and angle of impact.  We will also study multipixel events, charge sharing, individual channel trigger thresholds and their impact on total measured energy, and the recovery of particle energies in multipixel events.

Sealed sources will be used for energy calibration.  We report on the design of a detector testing chamber with a calibration source and timing detector assembly.  We will move a collimated source over detector pixel edges to study the response of the pixel and its neighbors.  I will present the requirements, current design and implementation of the detector testing chamber and this source positioner.   

A Cooling System for the Nab Detector Preamplifiers

The decay of free neutrons enables precise studies of CKM unitarity, by measuring V_ud, and searches for beyond the Standard Model physics via weak scalar and tensor interactions. The goal of the Nab experiment, located at the Spallation Neutron Source at Oak Ridge National Laboratory, is to measure the electron-neutrino correlation coefficient, a, and the Fierz interference term, b, to a precision δa/a = 10^-3 and δb = 3×10^-3, respectively, via beta decay of cold free neutrons. The Nab experiment employs two identical, segmented silicon detectors and uses 22 printed circuit boards (PCBs) each with six channels that amplify and shape incoming detector signals. The combined power dissipation from all cards is estimated to be up to 100 W. Fully powering all preamplifier cards for extended periods of time will thus result in high temperatures, significantly reducing the efficiency, speed, and longevity of the operational amplifiers. Therefore, a self-regulating, reliable cooling system is necessary for continuous operation. I will present a design for a custom preamplifier liquid cooling loop, implementation into the Nab detector system, and preliminary cooling power test results.   

Neutron flux measurement for beam polarization study at Fundamental neutron Physics Beamline of SNS

    The goal of the Nab experiment at Spallation Neutron Source (SNS) in Oak Ridge National Lab is to measure the beta neutrino correlation co-efficient a_βν and Fierz interference term (b) in unpolarized free neutron decay with the relative uncertainty of ∆a/a < 10^-3 and ∆b/b < 3 × 10^-3 respectively. One of the major sources of systematic uncertainty in such a measurement is the degree of polarization of the neutron beam. Other properties of the neutron beam such as time of flight spectrum, beam profile, and beam stability impact our ability to measure the polarization, as well as the expected counting rate and fiducial area for the decay product detectors.

    In the current setup ¹⁴N based counting-mode and ³He based current-mode detectors are installed upstream and downstream of the Nab spectrometer to monitor the neutron beam flux, but our scheme for the beam polarization measurement requires that the flux be measured as an instantaneous current in both the detector. We have developed a novel approach using a transimpedance amplifier as a pulse charge integrator, thus making it a ratemeter with a high dynamic range to be used with a neutron counting detector. In this talk, I will present an overview of the neutron monitor circuit along with its performance for beam flux measurement, and the results of studies of the properties of the neutron beam used in Nab.