arXiv subjects

K. Leung

Publications and source records attributed to K. Leung.

5 recordsLinked to original sources

arXiv2025

From Commissioning to Precision Data-Taking: Resolving Operational Challenges in the Nab Detector Systems

Our understanding of the weak mixing of quarks, described by the Cabibbo Kobayashi Maskawa (CKM) matrix, currently presents an anomaly. Thanks to major strides in both theory and experiment, improved precision in determinations of the first row of matrix elements has revealed disagreement with the expectation of unitarity. The Nab experiment at the Spallation Neutron Source is designed to precisely extract the first matrix element $V_{ud}$ and shed light on experimental tensions within the neutron beta decay dataset. Nab's asymmetric spectrometer allows coincident reconstruction of the decay proton and electron energies, which will be used to determine the electron-neutrino correlation coefficient, and thus (with the neutron lifetime) determine $V_{ud}$. This unique approach has provided a more comprehensive view of neutron beta decay, including a first observation of the full momentum phase space of the decay above detector thresholds and limits on exotic neutron states. Recent upgrades to the Nab detector system have improved the robustness and stability of the detector performance in terms of proton detection efficiency, noise performance, and detector segment availability, setting the stage for high precision physics data-taking.

physics.ins-det↗

arXiv2025

Ultra-sensitive radon assay using an electrostatic chamber in a recirculating system

Rare event searches such as neutrinoless double beta decay and Weakly Interacting Massive Particle detection require ultra-low background detectors. Radon contamination is a significant challenge for these experiments, which employ highly sensitive radon assay techniques to identify and select low-emission materials. This work presents the development of ultra-sensitive electrostatic chamber (ESC) instruments designed to measure radon emanation in a recirculating gas loop, for future lower background experiments. Unlike traditional methods that separate emanation and detection steps, this system allows continuous radon transport and detection. This is made possible with a custom-built recirculation pump. A Python-based analysis framework, PyDAn, was developed to process and fit time-dependent radon decay data. Radon emanation rates are given for various materials measured with this instrument. A radon source of known activity provides an absolute calibration, enabling statistically-limited minimal detectable activities of 20 $\mu$Bq. These devices are powerful tools for screening materials in the development of low-background particle physics experiments.

physics.ins-det↗

arXiv2024

The Systematics and Operational Studies (SOS) Apparatus as a testbed for nEDM@SNS experiment

The nEDM experiment at the SNS (nEDM@SNS) is the first measurement of the neutron EDM to directly measure the precession frequency of the neutron spin due to magnetic and electric fields. Previous measurements have inferred the precession frequency by measuring the residual polarization of neutrons after a long period of free precession. This difference provides independent information on potential unknown systematic uncertainties compared to previous and on-going measurements. It is precisely because nEDM@SNS is using a number of novel techniques, that it is essential to perform detailed studies of these techniques in order to optimize the statistical sensitivity and minimize the systematic uncertainty. The Systematic and Operational Studies Apparatus (SOSA) was one of the main efforts of nEDM@SNS collaboration, concentrated on development of a cryogenic test-bed for learning precise manipulation of spins and studing systematic effects of the proposed new technique. The test bed does not need an electric field but designed to have full NMR capability to the same level as nEDM@SNS.

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arXiv2018

Solid deuterium surface degradation at ultracold neutron sources

Solid deuterium (sD_2) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD_2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD_2 material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (`conditioning') of the sD_2. We show that, in addition to the quality of the bulk sD_2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D_2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD_2 frost formation on initially transparent sD_2 in offline studies with pulsed heat input at the North Carolina State University UCN source results in a consistent description of the UCN yield decrease.

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arXiv2015

A comparison of two magnetic ultra-cold neutron trapping concepts using a Halbach-octupole array

This paper describes a new magnetic trap for ultra-cold neutrons (UCNs) made from a 1.2 m long Halbach-octupole array of permanent magnets with an inner bore radius of 47 mm combined with an assembly of superconducting end coils and bias field solenoid. The use of the trap in a vertical, magneto-gravitational and a horizontal setup are compared in terms of the effective volume and ability to control key systematic effects that need to be addressed in high precision neutron lifetime measurements.

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