P.R. Huffman

Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid 4He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an 833−63+74s trap lifetime, consistent with the currently accepted value of the free neutron lifetime.

Fundamental neutron physics beamline at the spallation neutron source at ORNL

Abstract We describe the Fundamental Neutron Physics Beamline (FnPB) facility located at the Spallation Neutron Source at Oak Ridge National Laboratory. The FnPB was designed for the conduct of experiments that investigate scientific issues in nuclear physics, particle physics, astrophysics and cosmology using a pulsed slow neutron beam. We present a detailed description of the design philosophy, beamline components, and measured fluxes of the polychromatic and monochromatic beams.

Invited Article: Development of high-field superconducting Ioffe magnetic traps

We describe the design, construction, and performance of three generations of superconducting Ioffe magnetic traps. The first two are low current traps, built from four racetrack shaped quadrupole coils and two solenoid assemblies. Coils are wet wound with multifilament NbTi superconducting wires embedded in epoxy matrices. The magnet bore diameters are 51 and 105 mm with identical trap depths of 1.0 T at their operating currents and at 4.2 K. A third trap uses a high current accelerator-type quadrupole magnet and two low current solenoids. This trap has a bore diameter of 140 mm and tested trap depth of 2.8 T. Both low current traps show signs of excessive training. The high current hybrid trap, on the other hand, exhibits good training behavior and is amenable to quench protection.

Chaotic scattering and escape times of marginally trapped ultracold neutrons

We compute classical trajectories of Ultracold neutrons (UCNs) in a superconducting Ioffe-type magnetic trap using a symplectic integration method. We find that the computed escape time for a particular set of initial conditions (momentum and position) does not generally stabilize as the time step parameter is reduced unless the escape time is short (less than approximately 10 s). For energy intervals where more than half of the escape times computed for UCN realizations are numerically well determined, we predict the median escape time as a function of the midpoint of the interval.

Precision neutron interferometric measurements of the n–p, n–d, and n–3He zero-energy coherent neutron scattering amplitudes

Abstract We have performed high-precision measurements of the zero-energy neutron scattering amplitudes of gas phase molecular hydrogen, deuterium, and 3He using neutron interferometry. We find b np = ( - 3.7384 ± 0.0020 ) fm [K. Schoen, D.L. Jacobson, M. Arif, P.R. Huffman, T.C. Black, W.M. Snow, S.K. Lamoreaux, H. Kaiser, S.A. Werner, Phys. Rev. C 67 (2003) 044005], b nd = ( 6.6649 ± 0.0040 ) fm [T.C. Black, P.R. Huffman, D.L. Jacobson, W.M. Snow, K. Schoen, M. Arif, H. Kaiser, S.K. Lamoreaux, S.A. Werner, Phys. Rev. Lett. 90 (2003) 192502, K. Schoen, D.L. Jacobson, M. Arif, P.R. Huffman, T.C. Black, W.M. Snow, S.K. Lamoreaux, H. Kaiser, S.A. Werner, Phys. Rev. C 67 (2003) 044005], and b n 3 He = ( 5.8572 ± 0.0072 ) fm [P.R. Huffman, D.L. Jacobson, K. Schoen, M. Arif, T.C. Black, W.M. Snow, S.A. Werner, Phys. Rev. C 70 (2004) 014004]. When combined with the previous world data, properly corrected for small multiple scattering, radiative corrections, and local field effects from the theory of neutron optics and combined by the prescriptions of the particle data group, the zero-energy scattering amplitudes are: b np = ( - 3.7389 ± 0.0010 ) fm , b nd = ( 6.6683 ± 0.0030 ) fm , and b n 3 He = ( 5.853 ± .007 ) fm . The precision of these measurements is now high enough to severely constrain NN few-body models. The n–d and n–3He coherent neutron scattering amplitudes are both now in disagreement with the best current theories. The new values can be used as input for precision calculations of few body processes. This precision data is sensitive to small effects such as nuclear three-body forces, charge-symmetry breaking in the strong interaction, and residual electromagnetic effects not yet fully included in current models.

High-sensitivity measurement of He 3 − He 4 isotopic ratios for ultracold neutron experiments

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of ^3He to ^4He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10^(−14) level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of ^3He/^4He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

Survival analysis approach to account for non-exponential decay rate effects in lifetime experiments

In a variety of neutron lifetime experiments, in addition to β−decay, neutrons can be lost by other mechanisms including wall losses. Failure to account for these other loss mechanisms produces systematic measurement error and associated systematic uncertainties in neutron lifetime measurements. In this work, we construct a competing risks survival analysis model to account for losses due to the joint effect of β−decay losses, wall losses of marginally trapped neutrons, and an additional absorption mechanism. We determine the survival probability function associated with the wall loss mechanism by a Monte Carlo method. We track marginally trapped neutrons with a symplectic integration method that assumes neutrons are classical particles. We model wall loss probabilities of Ultracold Neutrons (UCNs) that collide with trap boundaries with a quantum optical model. Based on a fit of the competing risks model to a subset of the NIST experimental data, we determine the mean lifetime of trapped neutrons to be approximately 700 s – consid∗Corresponding author Email address: kevin_coakley@nist.gov (K. J. Coakley) Preprint submitted to NIMA August 12, 2015 ar X iv :1 50 8. 02 13 7v 1 [ nu cl -e x] 1 0 A ug 2 01 5 erably less than the current best estimate of (880.1 ± 1.1) s promulgated by the Particle Data Group [1]. Currently, experimental studies are underway to determine if this discrepancy can be explained by neutron capture by He impurities in the trapping volume. We also quantify uncertainties associated with Monte Carlo sampling variability and imperfect knowledge of physical models for neutron interactions with materials at the walls of trap as well as beam divergence effects. Finally, in a Monte Carlo experiment, we demonstrate that when the trapping potential is ramped down and then back up again, systematic error due to wall losses of marginally trapped neutrons can be suppressed to a very low level. Monte Carlo simulation studies indicate that this ramping strategy is more efficient than an alternative ramping scheme where UCNs are produced when the field is fully ramped, and then increased to its maximum value after the trap is filled. The survival probability formalism developed here should be applicable to other experiments where neutron (or other particle) loss mechanisms are non-trivial (i.e., where the associated survival probability function with the loss mechanism is non-exponential).

Detecting scintillations in liquid helium

We review our work in developing a tetraphenyl butadiene (TPB)-based detection system for a measurement of the neutron lifetime using magnetically confined ultracold neutrons (UCN). As part of the development of the detection system for this experiment, we studied the scintillation properties of liquid helium itself, characterized the fluorescent efficiencies of different fluors, and built and tested three detector geometries. We provide an overview of the results from these studies as well as references for additional information.

Progress Towards a Precision Measurement of the Neutron Lifetime Using Magnetically Trapped Ultracold Neutrons

As part of an on‐going program utilizing magnetically trapped ultracold neutrons (UCNs), we are developing a technique that offers the possibility of improving the precision of the neutron lifetime by more than an order of magnitude. The experiment works by loading an Ioffe‐type superconducting magnetic trap with UCNs through inelastic scattering of 0.89 nm neutrons with phonons in superfluid 4He. Trapped neutrons are detected when they decay; charged decay electrons ionize helium atoms in the superfluid resulting in scintillation light that is recorded in real time using photomultiplier tubes. At present, we are installing a larger and deeper superconducting magnetic trap into our apparatus, implementing techniques to reduce background events, and working to increase the neutron decay detection efficiency. We report the status of the construction of the improved apparatus.