Production and Storage of Ultra Cold Neutrons in Superfluid Helium

This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL) concerning the investigation of a new method for the experimental exploitation of ultra-cold neutrons. The production and storage of ultra cold neutrons in superfluid helium has been suggested as a tool for the production of high densities of ultra cold neutrons for f’undamental nuclear physics as well as for sensitive measurements for condensed matter. A particular application of this technique has been suggested by Doyle and Lamoreaux [l] that involves the trapping of neutrons in a magnetic field within the superfluid helium volume. Neutron decays within the trap volume are detected by the scintillation light produced in the liquid helium. A cryostat and magnetic trap have been constructed as well as a prototype light detection system. This system was installed on a cold neutron beam line at the NIST Cold Neutron Research Facility in the summer of 1997. Preliminary results indicate the detection of helium scintillation light from the detection vessel. Background and Research Objectives The beta decay lifetime of the neutron is a parameter of great importance in nuclear physics, particle physics, astrophysics and cosmology. An improvement in the experimental determination of the neutron lifetime would have important implication for tests of the “Standard Model” of particle physics. To date, the most accurate determinations of the neutron lifetime have used “ultra-cold” neutrons (UCN) [2] confined in “bottles” having material walls. Such measurements are ultimately limited by the ability to measure the loss rate of UCN on the wall. The current effort represents a new approach in which the UCN are created and stored in a magnetic bottle, which employs the interaction between inhomogeneous magnetic fields and the neutron’s magnetic moment to confine the neutrons without material walls. The purpose of the funded activities was to carry out initial tests of this concept. Ultimately it is hoped that this approach will lead to measurement of the neutron lifetime with significantly reduced uncertainty. *Principal Investigator, email address: glg @lanl.gov


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Production and Storage of Ultra Cold Neutrons in Superfluid Helium
Geoffrey L. Greene," and Steve Lamoreaux Abstract This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL) concerning the investigation of a new method for the experimental exploitation of ultra-cold neutrons.The production and storage of ultra cold neutrons in superfluid helium has been suggested as a tool for the production of high densities of ultra cold neutrons for f'undamental nuclear physics as well as for sensitive measurements for condensed matter.A particular application of this technique has been suggested by Doyle and Lamoreaux [l] that involves the trapping of neutrons in a magnetic field within the superfluid helium volume.Neutron decays within the trap volume are detected by the scintillation light produced in the liquid helium.A cryostat and magnetic trap have been constructed as well as a prototype light detection system.This system was installed on a cold neutron beam line at the NIST Cold Neutron Research Facility in the summer of 1997.Preliminary results indicate the detection of helium scintillation light from the detection vessel.

Background and Research Objectives
The beta decay lifetime of the neutron is a parameter of great importance in nuclear physics, particle physics, astrophysics and cosmology.An improvement in the experimental determination of the neutron lifetime would have important implication for tests of the "Standard Model" of particle physics.To date, the most accurate determinations of the neutron lifetime have used "ultra-cold" neutrons (UCN) [2] confined in "bottles" having material walls.Such measurements are ultimately limited by the ability to measure the loss rate of UCN on the wall.The current effort represents a new approach in which the UCN are created and stored in a magnetic bottle, which employs the interaction between inhomogeneous magnetic fields and the neutron's magnetic moment to confine the neutrons without material walls.The purpose of the funded activities was to carry out initial tests of this concept.Ultimately it is hoped that this approach will lead to measurement of the neutron lifetime with significantly reduced uncertainty.

Importance to LANL's Science and Technology Base and National R&D Needs
Nuclear physics is one of LANL's core competencies and neutron science is one of its strategic goals.The effort to determine the neutron lifetime is thus directly pertinent to the Laboratory mission.It should also be noted that activities related to the determination of the neutron lifetime have been endorsed and supported by both the NSF and the DOE Division of Nuclear and Particle Physics.

Scientific Approach and Accomplishments
The free neutron is an unstable elementary particle that beta decays into a proton, an electron and an anti-neutrino with a mean lifetime of about 10 minutes.The beta decay of the neutron may be considered to be the archetype for all nuclear beta decay and its rate (i.e. the reciprocal of the mean life) provides a fundamental measurement of the strength of the weak nuclear force.An accurate value of the neutron lifetime is of importance to a wide variety of disciplines including nuclear theory, particle physics, nuclear astrophysics and cosmology.
Currently, the most accurate determinations of the neutron lifetime have employed so-called UCN "bottles" having material walls.Very low energy neutrons (kinetic energies on the order of 100 neV) can be confined in such bottles due to the coherent nuclear scattering from the material wall.In principle, the lifetime measurement is made by filling the bottle, waiting a known time (typically a few minutes to perhaps an hour), and then opening the bottle and counting the number of remaining neutrons.While in principle quite simple, this method is ultimately limited by systematic errors associated with the loss of UCN on the walls of the bottle.The ideal UCN bottle would involve a method whereby the UCN are totally confined without the use of material walls."magnetic" bottle, which confines the UCN through the interaction of an inhomogeneous magnetic field and the neutron's magnetic dipole moment.Such a bottle has no material wall and all possible loss mechanisms are well understood.Thus it should be possible to attain a significant improvement in the experimental measurement of the neutron lifetime.magnetic dipole trap.The liquid helium serves two important functions for the experiment.First it serves as the medium for the production of ultra cold neutrons.Secondly it serves as the detection medium for neutron decay.In the experiment, UCN are produced by down scattering of neutrons having energies of about 1 meV (approximately 10,000 times Recently, Doyle and Lamoreaux [ 11 proposed a method to trap neutrons in a The actual approach used in this work involved the use of a liquid-helium-filled 2