Session ME: Ultracold Neutron Sources and Measurement

LANL UCN source status and upgrade plans

The Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center (LANSCE) took commissioning beam in 2004 and has produced UCN each beam cycle since. The UCN production medium is solid deuterium with neutrons derived from proton driven spallation. Although the UCN source has had several changes over the past ten years the cold moderator and deuterium volume have remained unchanged. During the past year a new UCN source has been designed with a new cold moderator and deuterium volume based on lessons learned and new Monte Carlo work (this work is discussed by T. Ito in the talk ``Modeling the LANL ultracold neutron source''). The design goal for this UCN source is a factor of 2 increase in a UCN production with other gains in UCN density from changes in proton beam structure, increased proton current and improved UCN transport. This talk will cover the current status of the running UCN source with increased beam during the 2013 accelerator cycle and planned upgrades for the 2015 cycle.   

Modeling the LANL Ultracold neutron source

The ultracold neutron (UCN) source at LANL uses a solid deuterium UCN converter and is driven by pulsed spallation neutrons. This UCN source has been and is being used for various science experiments. An effort to further upgrade the UCN source is currently underway. An extensive source modeling has been performed. In this talk, the method and results of the modeling of the current and the upgraded sources, the insight gained by the modeling work, and the expected performance of the upgraded source will be presented.   

The second generation superthermal Ultra-Cold Neutron Source at RCNP

The project of a second generation superthermal ultra-cold neutron (UCN) source is currently going on at RCNP, Osaka University, Japan. It is aiming to produce the world's highest density of polarized UCNs using down-scattering of spallation-produced and moderated cold neutrons in superfluid helium (He-II) at 0.6 K. This project is developed in collaboration between KEK (Tsukuba, Japan) and RCNP. The first generation UCN source was using a vertical extraction and was optimized from 2002 to 2012 to increase its density of UCN from 0.7 UCN/cc to 26 UCN/cc. We have built a second generation UCN source which use a horizontal extraction system thanks to the energy boost induced by the field of a superconducting polarizer magnet (SCM). The SCM allows only one spin state to pass through, which make our UCN source a source of polarized UCN. Polarization is kept thanks to new UCN guides. The first experimental results, the performances and the future improvements of this second generation source will be presented in this talk.   

Design and construction of the UCN facility at TRIUMF

The future ultracold neutron (UCN) source at TRIUMF will employ a new spallation source of neutrons coupled to a cryogenic UCN converter containing superfluid helium. Its flagship experiment will be a measurement of the neutron electric dipole moment (EDM). The UCN facility at TRIUMF began its installation in earnest in January 2014. Key components of a new proton beamline, and substantial modifications to the radiation shielding for the facility were completed this year. The installation will be completed in 2016. This presentation will cover the status of the design and installation, and plans for the facility's successful completion. This will include design studies for a high-intensity cold source to surround the He-II bottle of the super-thermal UCN converter being developed in Japan, and studies of UCN transport from the source to the experiment.   

Development of UCN rebuncher for nEDM experiments with pulsed sources

We are planning to construct a spallation ultracold neutron (UCN) source and carry out the searches of the neutron electric dipole moment at J-PARC. It produces high-density pulsed UCNs. However, UCNs are diffused in guide tubes during long-range transport. In order to focus UCNs on the experimental bottle, we have developed a neutron accelerator named ``UCN rebuncher.'' This apparatus consists of a large electromagnet and a resonance spin flipper. The kinetic energy of neutron changes when it flies into static magnetic field and the change is retained due to spin-flip in the field. By accelerating slower neutrons or decelerating faster neutrons suitably this apparatus controls the diffusion of UCNs. We succeeded in the proof-of-principle experiment of the first rebuncher in 2011. It can change the kinetic energy in the range from 72 neV to 118 neV. At present we are developing the second rebuncher in order to increase the controllable range of kinetic energy by two times. It will be able to change the kinetic energy in the range from 33 neV to 124 neV.   

Guide coating and evaluation techniques for Ultracold Neutrons transport

Ultracold neutrons (UCN) are produced when a cold neutron flux down-scatters in a solid deuterium source in UCN Facility of Los Alamos National Laboratory (LANL) and are then transported to the experimental decay volume of the UCNA experiment and to other UCN experiments through a sequence of guide. These tubes are coated with diamond-like carbon (DLC) films to maintain UCN polarization and maximize material potential. We will briefly review the guide system at UCN Facility of LANL, the requirements for UCN guides, and the pulsed-laser deposition (PLD) process we use to produce diamond-like carbon (DLC) films. Different characterization methods (AFM,XPS,Profilometry etc.) and the results obtained by applying them to our coatings will be discussed, as will data from a series of UCN guide tests performed earlier this year on guides coated with our approach. We will also present future research and development in UCN guide coating techniques and materials.   

Development of Techniques for a Precision Neutron EDM Measurement at RCNP

A non-zero neutron electric dipole moment (nEDM) breaks the time-reversal symmetry. A precision measurement of the nEDM is expected to be a good probe to search for theories beyond the standard model. We have been developing techniques for a nEDM measurement, using a high intensity ultra-cold neutron (UCN) source developed by the collaboration between KEK and RCNP. We have succeeded to polarize UCNs by a super conducting polarizer, and stored them in a cell. This cell will be installed in static magnetic and electric fields for a nEDM observation by the Ramsey separated-oscillatory-field magnetic resonance method. The homogeneity of the magnetic field is being improved aiming to increase the transverse relaxation time $T_{2}$. A multilayered magnetic shielding and a compensation coil system was developed to cancel the geomagnetic field. Some materials around the cell which were not completely non-magnetic were replaced. We are developing a $^{129}$Xe co-magnetometer for the high precision field monitoring, and a high voltage system including electrodes with minimum UCN losses. In this talk, the present status of these apparatuses will be discussed.   

Magnetic Field R\&D for the neutron EDM experiment at TRIUMF

The neutron EDM experiment at TRIUMF aims to constrain the EDM with a precision of $1\times 10^{-27}$~e-cm by 2018. The experiment will use a spallation ultracold neutron (UCN) source employing superfluid helium coupled to a room-temperature EDM apparatus. In the previous best experiment, conducted at ILL, effects related to magnetic field homogeneity and instability were found to dominate the systematic error. This presentation will cover our R\&D efforts on passive and active magnetic shielding, magnetic field generation within shielded volumes, and precision magnetometry.   

Development of an optical co-magnetometer for a neutron EDM experiment at TRIUMF

TRIUMF is now constructing a new facility that will produce high density ultracold neutrons (UCN). One of the important experiments for the new facility is the measurement of the neutron electric dipole moment (nEDM), an experiment that exploits the fundamental symmetries of nature to search for new physics beyond the Standard Model. In order to improve the present world's best experimental result for the nEDM by more than an order of magnitude, it is indispensable to develop an extremely sensitive co-magnetometer, which measures the magnetic field inside the nEDM cell while the nEDM measurement is being conducted. For this purpose, our group is proposing to use a dual-species comagnetometer with $^{199}$Hg and $^{129}$Xe. In this method, polarized $^{199}$Hg and $^{129}$Xe atoms will be introduced into the nEDM cell at the same time as the neutrons, and the spin-precession frequencies of both species are measured simultaneously. The Xe and Hg atoms are probed continuously by observing the modulation of transmitted probe light, at 253.7 nm, for Hg, and emission in the near infrared (823 nm and 895 nm) for Xe by exciting a two-photon transition at 252.4 nm. This talk will present our progress on the development of the dual-species comagnetometer.   

Spin Relaxation and Geometric Frequency Shifts in the SNS nEDM Experiment

The search for the neutron electric dipole moment (nEDM) is a promising search for physics beyond the Standard Model. The nEDM violates time reversal (T), and the size of T violation predicted by the Standard Model is incompatible with present ideas concerning the creation of the observed baryon-antibaryon asymmetry. A measurement of a nEDM requires the detection of a shift in the Larmor precession in proportion to an applied electric field. The most recent measurement of the nEDM is limited by a systematic effect termed the geometric phase [C. A. Baker, et al. Improved Experimental Limit on the Electric Dipole Moment of the Neutron. \textit{Physical Review Letters}, 97(13):131801(4), 2006.], a frequency shift linear in the applied electric field. The nEDM planned for the Spallation Neutron Source(SNS) at Oak Ridge National Laboratory will use polarized $^{3}$He as a co-magnetometer and detector, it is also subject to the geometric phase. I will present recent work aimed at understanding and mitigating this effect. This discussion will emphasize the trajectory correlation functions of the neutron and $^{3}$He modeled by continuous time random walks with arbitrary scattering times. The same tools for predicting the geometric phase can also be used to predict polarization decay.   

Interior Vector Magnetic Field Monitoring for the SNS Neutron EDM Experiment

A concept has been developed which provides for a real-time determination of the spatial dependence of the vector components of the magnetic field (and, hence, the $\partial B_i / \partial x_j$ field gradients) within the interior fiducial volume of the SNS neutron EDM experiment solely from exterior measurements at fixed discrete locations. This technique will be especially important during the operation of the experiment, when direct measurements of the field gradients present within the fiducial volume will not be physically possible. Our method, which is based on the solution to the Laplace Equation, is completely general and does not require the field to possess any type of symmetry. We describe the concept and our systematic approach for optimizing the locations of these exterior measurements. We also present results from prototyping studies of a field monitoring system deployed within a half-scale prototype of the experiment's magnetic field environment.   

Magnetic field uniformity for the nEDM experiment

The nEDM experiment at the Spallation Neutron Source (SNS) will search for a neutron electric dipole moment (EDM) with a sensitivity of \textless 5*10$^{\mathrm{-28}}$ e-cm. Neutrons will precess in a constant magnetic field and variable electric field, and non-zero neutron EDM will appear as a variation in the precession frequency correlated with the changing electric field. Geometric phase and neutron polarization lifetime effects constrain the allowed magnetic field gradient to below 0.1 uG/cm. Gradients nearly satisfying this requirement have been achieved using a cos($\theta )$ coil inside an open-ended superconducting lead shield operated at cryogenic temperatures and using the design electric fields. I will describe efforts to further improve the magnet design using a superconducting endcap.   

$\textit{In Situ}$ Detection of Trapped Ultracold Neutrons Using a Vanadium Foil

The UCN$\tau$ experiment at Los Alamos National Laboratory (LANL) employs a novel $\textit{in situ}$ detector to measure the free neutron lifetime. Ultracold Neutrons (UCN) are confined in a magneto-gravitational trap, held for times on the order of their lifetime, and then counted. The $\textit{in situ}$ detector works by activating a vanadium foil inside the trap with UCN and then raising it into a counter. Vanadium was chosen for its negative material potential (-7 neV), 4 minute half-life, and $\beta$/$\gamma$ coincidence. The activity is measured using a plastic scintillator backed by a NaI array to detect coincidences and reject background. This $\textit{in situ}$ detector has advantages compared to emptying the trap to an external counter: higher activation efficiency, less sensitivity to phase space dependent efficiency, faster absorption time, and the ability to probe phase space evolution in situ. The detector can currently absorb trapped UCN with a time constant of less than 10 seconds and counts with a 19\% efficiency. In this talk we will present data taken using this detector during the 2013 run cycle at LANL along with simulations which together characterize the performance of the detector.