Session KK: Mini-Symposium on Probing Fundamental Symmetries with Nuclei, Neutrons, Muons, and Atoms IV

Ultracold Neutrons in Canada and Japan

When neutrons are taken out of atomic nuclei and cooled down, they have weird properties: they bounce off walls, they can be stored in magnetic bottles, and they form quantized energy levels in Earth's gravitational field. Once they've been trapped using such methods, their properties can be studied very carefully to search for deviations from expectations based on the standard model of particle physics. If a deviation is found, it would signify new physics beyond the standard model. We are building a new source of ultracold neutrons in Canada (at TRIUMF, Vancouver, BC) with the help of collaborators from Japan (Masuda, et al). The source is projected to provide the highest density of ultracold neutrons ever produced in the world, and we'll use the neutrons to push some very interesting physics experiments to unprecedented levels of precision, as I'll describe.   

Progress on the Construction of the PULSTAR Solid Deuterium Ultracold Neutron Source

An ultracold neutron (UCN) source utilizing solid deuterium is being constructed at the 1MW PULSTAR nuclear reactor on the campus of North Carolina State University. The final stages of assembly and commissioning are underway. The overall design, status of construction, and benchmarking measurements will be discussed. The UCN source design is based on detailed simulations including MCNP, UCN transport Monte Carlo, and CFD of the cryogenic systems. The source will be available for developing general UCN experiment technology, such as guides and detectors in support of current neutron EDM and UCNA projects. Other plans include fundamental physics measurements such as neutron beta decay and gravity measurements, as well as development of new techniques to use UCN in material and surface physics studies. The expected experimental density of UCN will be competitive with currently available sources, including those at significantly more powerful reactors. This work is supported in part by NSF grant {\#}0314114 and funds from the DOE INIE program.   

A New Ultra-Cold Neutron Source Available at Los Alamos

A new source of ultra-cold neutrons (UCNs) has been constructed at Los Alamos National Lab and is now in operation. Although the primary purpose of the source is to supply UCNs to the UCNA experiment, a second test beam port is also available and can be used to supply UCNs to another experiment simultaneously. During source operation, the neutron density at the test port has been estimated to be approximately 5 UCN/cc. During UCNA data acquisition, the UCN beam is available at the test port with a duty factor of about 1/6; that is, 10 minutes out of each hour, with the balance being delivered to UCNA. Experiments that have used and/or intend to use the test port beam include nEDM engineering tests, guide tests for UCN transport and depolarization, and a new measurement of the neutron lifetime using magnetically trapped UCNs. Measurements of the density, flux, and velocity distribution of the neutrons at the test port will be presented.   

Suppression of Ultracold Neutron Depolarization on Material Surfaces with Magnetic Holding Fields

Experiments involving polarized Ultracold Neutrons (UCN) for high precision measurements require the use of high Fermi potential materials with a low spin flip probability per bounce. Previous studies show that the spin flip probability for materials vary on the order of 10${-3}$ to 10$^{-6}$. In this study, the depolarization of UCN was measured within 1-m long, 2 3/4" diameter bare copper, electropolished copper, diamond-like carbon-coated copper, and stainless steel guide tubes as a function of the magnetic holding field. The UCN were trapped between a 6 Tesla solenoidal magnet and a copper plate. A series of Helmholtz coils produced a magnetic holding field over the length of the test guide at 10, 100, or 250 Gauss. The surface depolarization was observed to be suppressed at higher holding fields. These measurements will aid in the determination of an upper limit on depolarization of UCN in the UCNA beta asymmetry measurement at LANL and in understanding the mechanisms for depolarization in non-magnetic guides.   

Detailed Characterization of Copper Guide Polishing Methods for use in UCN Transport

The UCNA experiment at Los Alamos National Lab (LANL) employs polarized ultracold neutrons (UCN) to measure the beta-asymmetry in free neutron decay. The 2009 beamline makes use of electro-polished stainless steel and copper guides. In the depolarization region of the experiment, hydrogen-free Diamond-like-Carbon (DLC) coated copper guides are utilized. A target-biased, pulsed laser deposition technique was used to produce an adhered coating on these copper guides with an expected Fermi potential of 240neV. A series of guide tests performed last December indicate that these DLC coated copper guides have very low depolarization per bounce and a higher Fermi potential than bare copper. However, transmission results, for both the coated and uncoated copper guides, suggest that the underlying polish was not optimum. An investigation of the mechanical and electro-polishing processes has been conducted utilizing profilometry and atomic force microscopy. These data have been used to simulate the effects of these polishes on UCN transport and indicate that the final mechanical polish direction can have a big impact. Results of this study will be presented along with its impact on making higher Fermi potential DLC coated copper guides.   

Precision UCN Polarimetry and the UCNA Experiment

The goal of the UCNA experiment is to determine the angular correlation between the electron momentum and the neutron spin (the beta-asymmetry) in free neutron decay using polarized ultracold neutrons (UCN). The experimental strategy is to transport UCN into a decay volume through a 7T static magnetic field, allowing the magnetic potential to polarize the UCN. The neutron polarization can then be reversed via an rf adiabatic spin flipper which sits between the 7T polarizing field and the decay volume. This spin flipper also allows an \textit{in situ} measurement of the depolarized contamination that develops during a constant-polarization measurement cycle. Since the neutron polarization multiplies the beta-asymmetry in the expression for the polarized neutron decay rate, precision measurements of the beta-asymmetry require at least a commensurate precision in the UCN polarization. Details of the polarimetry techniques utilized for UCNA and the limits they place on the spin contamination present in the experiment will be discussed and compared to expectations from Monte Carlo transport models. Plans to enhance the sensitivity of our polarimetry measurements will also be discussed in relation to the goal of measuring the beta-asymmetry to better than $0.5\%$.   

Current mode wire chambers for cold neutron detection at the SNS FNPB

A $^3$He chamber is a multi-wire proportional counter for detecting neutrons. A $^3$He nucleus that captures a neutron will break up by the reaction $n+^3He\rightarrow p+T+765\ keV$ which is detected by gas ionization inside the chamber caused, by the reaction products. The 765 keV is released as kinetic energy of the proton and triton, allowing a consistent signal from each capture. The chamber gas is a mixture of gases with a fraction of a $^3He$, the amount of which is used to adjust the neutron thickness; the fraction of beam that is captured in the monitor. I will report on the design, construction, and testing of a new set of beam monitors for the Spallation Neutron Source Fundamental Neutron Physics Beam line (FNPB), which use this technology. The $^3$He chambers will be used to monitor the neutron flux at various positions along the neutron beam, as it passes through cold neutron experiments planned at the SNS. In addition, I will report on the design of a $^3$He wire chamber that will be used in the n$^3$He experiment at the SNS. This chamber uses the same neutron detection process as described above, but will be black to neutrons (high $^3He$ content) with a small amount of ionization gas, to allow the protons to range out over as long a distance as possible. This chamber will be used to measure the parity violating longitudinal asymmetry in the number of protons emitted in the capture reaction.   

Proton source for Silicon detector tests

In neutron beta decay a neutron decays through the weak force into an electron, proton and antineutrino. The detection of the decay protons is an important aspect of certain neutron beta decay experiments. One particular experiment of this type is the Nab experiment. In the Nab experiment decay protons are electrostatically accelerated to 30keV in a flat magnetic field region of a magnetic spectrometer before striking silicon detectors. To calibrate the silicon detectors for such experiments a low intensity proton accelerator is being designed and built at the University of Manitoba. Progress in proton source and accelerator development will be discussed. The results of the silicon detector tests would be important for experiments detecting post-accelerated recoil protons using silicon detector technology.   

Calibration of the UCNA Beta Spectrometer

The UCNA experiment measures the angular correlation between the polarization direction of a neutron and the momentum of the electron emitted in beta decay. Polarized ultracold neutrons decay in a 1T magnetic field, which conducts the emitted betas to detectors on either side of the decay region. Measuring the energy spectrum of the electrons is necessary due to energy dependence of the observed asymmetry and electron backscattering corrections. The beta detectors consist of a wire-chamber for position tracking and a plastic scintillator calorimeter. This talk describes how conversion electron sources and the beta spectrum endpoint are used to calibrate the position-dependent energy response of the scintillators, and how the calibration of the detectors is monitored and stabilized throughout the 2008 run.   

Measurement of UCN Energy Spectrum of He-II Spallation UCN source

Ultra cold neutron (UCN) is a very low energy neutron ($\sim$200neV), which can be confined in a material bottle. UCN can be used for experiments on fundamental physics such as precise measurements of neutron EDM. A higher intensity UCN source is required to do such precise measurements. We have developed a He-II spallation UCN source at RCNP, Osaka Univ.. In this source, fast neutrons obtained by spallation reactions are moderated by RT D$_2$O and 10K solid D$_2$O, then scattered by phonons in superfluid helium (He-II) to be UCN. The obtained UCN density was 15 UCN/cm$^3$. We measured the energy spectrum of UCN at the exit of this source by storing them in a cylindrical bottle in order to understand the performance of this source. A polyethylene disk installed at the height {\it h} in this bottle absorbs UCN with energy larger than {\it mgh}. The UCN energy spectrum was deduced by differentiating the UCN counts detected as a function of the disk height {\it h}. The obtained spectrum is well reproduced by the Monte Carlo simulation.   

Search for Violation of Time Reversal Invariance at J-PARC

The Time Reversal Experiment with Kaons (TREK) at J-PARC aims to find New Physics beyond the Standard Model by measuring the T-violating transverse polarization $P_T$ of muons in the $K^{+}_{\mu3}$ decay of stopped kaons. TREK will use a high-intensity kaon beam and the upgraded apparatus of the E-246 experiment from KEK-PS. The sensitivity for $P_T$ of 10$^{-4}$ at J-PARC is improved by a factor of 20 compared to the current E-246 limit, well in the allowed range of various models involving New Physics from exotic scalar interactions. An overview of the planned experiment and the status of the detector upgrade will be presented.