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Magnetically Trapped Neutron Lifetime Experiment


The decay of the free neutron is the simplest nuclear beta decay and is the prototype for all charged current semi-leptonic weak interactions. The decay parameters, the neutron lifetime in particular, provide essential inputs to investigations of the weak interaction.


A precise value for the neutron lifetime is required for several internal consistency tests of the SM including searches for right-handed currents and tests of the unitarity of the CKM mixing matrix. Measurements of neutron decay coefficients provide information on the vector and axial-vector coupling constants gv and ga. The neutron lifetime is also an essential parameter in the theory of Big Bang Nucleosynthesis. As well as in determinations of the expected neutrino flux from nuclear reactors.

At present there is a 4 σ discrepancy between determinations of the neutron lifetime coming from UCN bottles when compared to those coming from neutron beam techniques. This discrepancy is not understood. Thus it is essential to resolve this disagreement, a goal that can best be accomplished through measurements using systematically different techniques. As beam-type experiments are limited by measurements of the neutron flux, and material bottle experiments are complicated by wall interactions, magnetic trapping techniques offer a powerful approach for both solving this discrepancy and improving the precision of the neutron lifetime.

This program is a collaborative effort between NIST and NC State. It is designed to measure the neutron beta-decay lifetime using a substantially new technique. Our method confines Ultra Cold Neutrons (UCN) within a three-dimensional magnetic trap. Cooling of the neutrons occurs within the conservative trap when 12 K neutrons (0.89 nm) down-scatter in superfluid 4He to near rest via single phonon emission (superthermal production). The UCN then interact only with the magnetic field via their magnetic moment and when the spin is anti-parallel to the magnetic field, they will seek to minimize their potential energy by moving towards low field regions. By cooling the trap to temperatures of approximately 100 mK, the population of UCN becomes thermally detached from the helium bath allowing accumulation of UCN. Neutron decay is detected by turning off the cold neutron beam and observing the scintillation light resulting from the beta-decay electrons. When an electron moves through liquid helium, it ionizes helium atoms along its track. These helium ions quickly recombine into metastable He*2 molecules. About 35 % of the initial electron energy goes into the production of extreme ultraviolet (EUV) photons from singlet decays, corresponding to approximately 22 photons/keV. These EUV photons are frequency down-converted to blue photons using the organic fluor tetraphenyl butadiene (TPB) coated onto a diffuse reflector surrounding the trapping region. This light is transported via non-imaging optics to room temperature and detected by two photomultiplier tube (PMT)s operating in coincidence. This unique trapping and detection method allows us to observe neutron decay events in situ, and therefore directly measure the decay curve.

There have been multiple stages in the development of this technique, most of which have involved successively larger prototype apparatus. Several years ago we completed construction of a significantly upgraded apparatus that was projected to reach a sensitivity competitive with the best measurements at that time. Data was collected from fall 2010 through spring 2011. Backgrounds were significantly higher than anticipated driving a significant effort devoted to pulse-shape discrimination. This was very successful, allowing us to reduce backgrounds by roughly 50%. Over the last several years, much work has been put into understanding the two dominate systematic effects, marginally trapped neutrons (neutrons that remain in the trap for long periods in spite of have a total energy above the trap depth) and absorption on helium-3. The former has necessitated the development of a detailed Monte Carlo that is proving useful to other collaborations, while the latter has pushed the capability of Accelerator Mass Spectroscopy (AMS) several orders of magnitude in sensitivity.

Data analysis is currently ongoing with preliminary results expected later this year. In parallel, we have constructed a new helium purifier to address helium contamination issues uncovered with AMS. The purifier has been commissioned and several test purification runs should allow us to demonstrate that our unique technique holds the potential to reach sensitivities significantly beyond what has currently been achieved by other techniques.

trapped neutron graph
Figure: Lifetime corrected and background sbtracted data from 17 run pairs, showing the decay curve associated with the trap lifetime.

Operating Apparatus
Operating Apparatus (Photograph by: Neutron Physics Group)

End Date:


Lead Organizational Unit:



Neutron Interactions and Dosimetry Group

Hans (Pieter) Mumm
Alan K. Thompson

NIST Associates:

Paul R. Huffman
North Carolina State University

Karl Schelhammer
North Carolina State University


Hans (Pieter) Mumm