Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session RG: Mini-Symposium: Precision Beta Decay IV |
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Chair: Elizabeth (Mae) Scott, UMD |
Sunday, November 1, 2020 8:30AM - 8:42AM |
RG.00001: Statistical Biases in the UCNtau Experiment Chen-Yu Liu Statistical biases come to the fore as the UCNtau experiment pushes the uncertainty of neutron lifetime measurements toward $0.01$\% precision. In UCNtau, we store ultracold neutrons (UCN) in a lossless magneto-gravitational trap and count the surviving neutrons after various storage times. The reduction of the UCN population is described by the exponential radioactive decay law; the observed decay constant is the inverse of the neutron lifetime. The Poisson statistics of particle counting -- due to its asymmetric distribution -- leads to varying degrees of bias in the extracted lifetime, depending upon the treatment and combination of individual measurement cycles. I will explain how these biases manifest in the context of the multi-step neutron counting scheme used in UCNtau. These effects must be quantified to control the systematic shift that results from the phase-space evolution of neutrons stored in the trap. [Preview Abstract] |
Sunday, November 1, 2020 8:42AM - 8:54AM |
RG.00002: Spin Dynamics in the UCN$\tau$ Magneto-gravitational Ultracold Neutron Trap Adam Holley The UCN$\tau$ experiment measures the free neutron lifetime $\tau_\mathrm{n}$ by counting surviving ultracold neutrons (UCN) following storage in a combined magnetic and gravitational potential. This approach eliminates a significant non-$\beta$-decay disappearance channel associated with UCN-matter interactions evident in previous material ``bottle'' measurements, which is replaced by a considerably smaller systematic effect associated with the spin dynamics of UCN during storage. The depolarization rate can be estimated by comparing empirical measurements of the trap lifetime as the strength of the polarization-preserving ``holding'' field is varied to a model$^{2}$ based on calculations using an idealized field configuration. This constrains the associated systematic effect to 0.008\% of $\tau_\mathrm{n}$, well below the current, but not the foreseeable, measurement precision. We will present results from our simulation effort that incorporates empirically determined magnetic field profiles and detailed spin tracking to enhance the fidelity of this constraint. \\ \\ $^{2}$A. Steyerl et al., Phys. Rev. C 95, 035502 (2017) [Preview Abstract] |
Sunday, November 1, 2020 8:54AM - 9:06AM |
RG.00003: Probing UCN$\tau$ Systematic Effects Through Neutron Tracking Simulations Francisco Gonzalez The UCN$\tau$ experiment at Los Alamos National Laboratory measures the neutron lifetime by storing ultracold neutrons (UCN) in a magneto-gravitational trap for variable holding times. Potential UCN loss mechanisms besides $\beta$-decay lead to systematic uncertainties. In particular, UCN with energies above the trapping potential could escape during storage; this effect is minimized through the use of a “cleaner” lowered into the trap prior to the storage period, and cleaned neutrons are counted using a new "active" UCN cleaner. Possible time dependent changes in the phase-space distribution of UCN could lead to changes in the detection efficiency or exacerbate over-threshold neutron losses. This effect is reduced through in-situ detection, and quantified by lowering the primary detector in steps to probe various UCN energies. A custom Monte Carlo simulation of UCN trajectories has been developed on Indiana University’s Big Red 3 supercomputing cluster to model UCN dynamics and constrain systematic effects. We will present results of these simulations as part of an effort to reduce UCN$\tau$’s total uncertainty to 0.2s. [Preview Abstract] |
Sunday, November 1, 2020 9:06AM - 9:18AM |
RG.00004: UCN$\tau$+: an upgrade to the UCN$\tau$ experiment Alexander Saunders The UCN$\tau$ experiment measures the free neutron lifetime by counting the surviving polarized ultracold neutrons (UCNs) after storing them in an asymmetric magneto-gravitational storage volume built from a Halbach array of permanent magnets. The current apparatus will achieve a statistical uncertainty of about 0.3 s and has established a systematic uncertainty of 0.28 s; it is expected to ultimately reach a total uncertainty of about 0.2 s, limited primarily by the efficiency with which UCN$\tau$ utilizes the neutrons supplied by the Los Alamos UCN facility. In this talk, we will describe the "UCN$\tau$+" upgrade, which will improve the statistical reach by increasing the efficiency of loading UCNs into the storage volume ten-fold. The current loading technique, using a removable section of the magnet array, induces depolarization which reduces the number of trappable UCNs. Here we describe a dedicated, insertable transfer volume that avoids significant sources of depolarization. This technique can potentially reduce the total uncertainty on the neutron lifetime to below 0.1 s. [Preview Abstract] |
Sunday, November 1, 2020 9:18AM - 9:30AM |
RG.00005: Ultra-Cold Neutron measurement of Proton branching ratio in neutron Beta decay (UCNProBe) Nick Floyd, Md Taufique Hassan, Zhaowen Tang The free neutron lifetime can be measured using one of two methods: measuring the decay products of neutrons in a well-calibrated neutron beam (beam experiment), or counting the number of surviving neutrons stored in a UCN trap over time (bottle experiment). The lifetime results from the two different methods differ by ~10 seconds or five standard deviations. Our goal is to resolve the difference between the two measurements by measuring the proton branching ratio of neutron decay using UCNs. Detecting a proton branching ratio of less than one will indicate new physics beyond the Standard Model of particle physics. The experiment is realized by storing the neutrons in a material trap made from deuterated scintillators. To measure the beta decay lifetime, we will attempt to measure the absolute number of UCNs inside the trap and the absolute number of electrons from beta decay to 0.1 percent precision. In this talk, we will describe the concept of the experiment and report the characterization of the deadlayer of the scintillator. [Preview Abstract] |
Sunday, November 1, 2020 9:30AM - 9:42AM |
RG.00006: BL3: Next generation beam experiment to measure the neutron lifetime Nadia Fomin Neutron beta decay is an archetype for all semi-leptonic charged-current weak processes. A precise value for the neutron lifetime is required for consistency tests of the Standard Model and is needed to predict the primordial 4He abundance from the theory of Big Bang Nucleosynthesis. An effort is under way for an in-beam measurement of the neutron lifetime that is able to evaluate the systematic uncertainties at the 0.3 s level. This effort is part of a phased campaign of neutron lifetime measurements based at the NIST Center for Neutron Research, using the Sussex-ILL-NIST technique. Recent advances in neutron fluence measurement techniques as well as new large area silicon detector technology address the two largest sources of uncertainty of in-beam measurements, paving the way for a new measurement. The experimental design, schedule, and projected uncertainties for the main subsystems will be discussed. [Preview Abstract] |
Sunday, November 1, 2020 9:42AM - 9:54AM |
RG.00007: A New Neutron Fluence Calibration Monitor for the BL3 Experiment Geoffrey Greene, Chen-Yu Liu The BL3 experiment determines the neutron lifetime by measuring the rate of decay protons emerging from a beam of cold neutrons at the NIST research reactor. To reach the precision goal of less than 0.03\%, the fluence of the neutron beam passing through the decay volume needs to be determined to a precision better than accuracy attained in the previous calibration[A. Yue, et al., Metrologia 55, 460 (2018)]. To improve the performance beyond the current statistical limit, we will implement a new “Alpha-Gamma device” (AG) to accept a larger neutron beam ($\sim$35~mm vs 10~mm for BL2) with higher detection efficiency. The new device will employ the previous the 4-step measurement procedure based on geometrical efficiency determination, but will also employ an alternative, independent, coincidence method that provides a “first principles” calibration [Gilliam, Greene, Lamaze G P 1989 Nucl. Instrum. Meth., A 284, 220, (1989)]. In this talk, we will describe the working principles of the new AG device that includes optimized detector positioning to reduce systematic effects from the extended beam profile and angular correlations between the reaction particles as well as precision in-situ target positioning for rapid target alternation. [Preview Abstract] |
Sunday, November 1, 2020 9:54AM - 10:06AM |
RG.00008: Optimization of BL3 Neutron FluxDetectors Austin Nelsen, Emily Ballantyne, Rebecca Calvert, Sarah Vickers, Chris Crawford The recent measurement of the lifetime of the free neutron using the beam method has an 8.7s (4$\sigma$) discrepancy with UCN measurements. The goal of the BL3 experiment is to improve the statistical error of this measurement and help rule out systematic uncertainties as an explanation for the discrepancy. A well-characterized neutron flux detector with flat response is essential, since the neutron flux enters linearly into the neutron lifetime. I will present a new detector geometry optimization with uniform acceptance up to sixth order in neutron position. We have determined the optimal position, orientation, and shape of one and two rings of detectors through a series of analytical and numerical calculations. [Preview Abstract] |
Sunday, November 1, 2020 10:06AM - 10:18AM |
RG.00009: Simulations of Proton Detector Performance in the BL3 Experiment Wouter Deconinck, T.T. Trang Bui, Jason Fry, Robert Pattie The BL3 experiment at the NIST Center for Neutron Research aims to improve the systematic precision of neutron lifetime measurements in order to resolve the 8.7 s discrepancy between beam-type and bottle-type measurements of the neutron lifetime. In the experiment, the recoil protons from neutron decay in a quasi-Penning trap with a solenoidal magnetic field and axial electrodes propagate to a segmented silicon detector. The BL3 experiment will use higher neutron flux in a wider beam which requires a larger proton detector than the BL2 experiment. The BL3 collaboration is using a combination of simulation tools to assess the anticipated performance. A geant4-based simulation has been developed which uses an external magnetic and electric field map and a dedicated low-energy physics list for semiconductor interactions. A Kassiopeia-based simulation uses a multipole expansion for the electric and magnetic fields calculated from their current distributions, which allows more easily for changes in field geometries. An SRIM-based simulation is used to determine energy deposition, dead layer effects, and charge sharing in the proton detector. This talk will compare the results obtained with these three different approaches in achieving the performance parameters for the BL3 experiment. [Preview Abstract] |
Sunday, November 1, 2020 10:18AM - 10:30AM |
RG.00010: A New Proposal of Beam Neutron Lifetime Experiment in Superfluid Helium-4 Scintillator Wanchun Wei A large discrepancy in the measured neutron lifetime between the beam and bottle methods remains as a puzzle. More beam experiments of high precision, but with a different set of systematic effects, can greatly help in resolving the puzzle. In this report, we propose a new beam experiment at a sensitivity goal of 0.1% or sub-1 second by counting electrons from neutron beta decays in a superfluid helium-4 scintillation detector at 0.5 K, and monitoring the neutron flux simultaneously by the helium-3 captures in the same volume. In order to eliminate scattering with superfluid helium and suppress background events, the cold neutron beam must be of wavelength $\lambda$>16.5 {\AA}. [Preview Abstract] |
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