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

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

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

In collaboration with researchers from Harvard University, Los Alamos National Laboratory, and University of Berlin, NIST plans to produce and confine polarized Ultra Cold Neutrons (UCN) in a magnetic trap. The present experimental value of the mean lifetime is 887.4s with a standard deviation of 1.7 s. Based on this new technology, the neutron lifetime should be determined at a precision from 10 to 100 times better than the current value. 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.

There will be many run cycles 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 have extended our work on stochastic modeling of the observed data. In particular, we have studied the relative performance of two maximum likelihood estimates of the mean lifetime. We constructed two likelihood models. One model describes the actual sequence of detection event times. The other model applies to binned event time data. In a simulation study, we examine the possible loss of information as a result of using binned decay data instead of the exact decay times. We compute the bias and variance of the two methods. Further, we compare the performance of the likelihood methods to the performance of a weighted least squares method which we developed earlier.

Figure 16: In the Figure above, we plot the fractional bias and fractional standard error for an estimate of the mean lifetime of the neutron. For this estimate, we use a likelihood model which requires the full sequence of events. We can not distinguish whether an event is due to neutron decay or a background process. In the simulation, the initial number of neutrons is a Poisson random variable with parameter $\lambda$. The background is a uniform Poisson process with rate parameter $\lambda / 20$. In the simulations, we denote the time spent observing events as T_decay. We express this observing time as a multiple of the true mean lifetime ($\tau$). At time T_decay, the expected fraction of neutrons which have decayed is $1 - \exp ( - T_{decay} / \tau )$.

       

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