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3.3.1 Magnetic Trapping of Ultra Cold Neutrons and Determination of the Mean Lifetime of the Neutron

K.J.Coakley and G.L.Yang
Statistical Engineering Division, ITL

P.R.Huffman, M.S.Dewey and D.Gilliam
Ionizing Radiation Division, PL

The present experimental value of the mean lifetime of the neutron is 887.4 s with a corresponding uncertainty standard deviation of 1.7 s. Along with other experimental data, a measurement of the mean lifetime of the neutron allows one to test the consistency of the standard model of electroweak interactions. Further, the mean lifetime of the neutron is an important parameter in astrophysical theories.

During the last year, in collaboration with researchers from Harvard University, Los Alamos National Laboratory, and University of Berlin, NIST successfully produced and confined polarized Ultra Cold Neutrons (UCN) in a magnetic trap. Data from this breakthrough experiment yields a lifetime estimate of 750 s. The 68 percent confidence interval for this estimate is (550 s, 1080 s). Based on this new trapping technique, a planned second generation experiment should yield a neutron lifetime estimate with a precision up to 10 times better than the current value. Furthermore, systematic errors should be much lower than in other kinds of neutron lifetime experiments.

In the lifetime experiment, there are may runs of a two stage experiment. In the first stage of each run, neutrons from the NIST Cold Neutron Research Facility are guided into a superfluid 4He bath where they dissipate almost all their energy by inelastic scattering. These UCN are confined in a magnetic trap. After filling the trap to some level, the neutron beam is blocked from entering the trap. During the decay stage of each run, decay events, as well as background events, are recorded.

During the last year, we completed a manuscript on likelihood models for two stage neutron lifetime experiments. We studied the relative performance of two maximum likelihood estimates of the mean lifetime and participated in the statistical analysis of data from a first generation magnetic trapping experiment. We are involved in planning studies for the next generation experiment. In a current study, we are quantifying the relative improvement in the lifetime estimate due to correcting data for background. Because of background correction, a simpler model is fit to the data. However, because of time spent collecting background only runs, fewer overall runs are collected.

The figure on the next page is from Magnetic Trapping of Neutrons, by P.R.Huffman et al. which appears in the January 6, 2000 issue of Nature.

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Figure 10: Upper: observed data and associated $\pm 1 \sigma$ interval for two experiments. For each experiment, the predicted count rate is shown as solid line. Bottom: schematic diagram of the magnetic trap which confines ultra cold neutrons.

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Date created: 7/20/2001
Last updated: 7/20/2001
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