A new magnetic trap-based method of measuring the lifetime ($\tau_\beta$) of the free neutron has recently been proposed [1]. The success of the proposed metho depends on assuring that the only significant neutron loss from the trap is beta decay. One potential spurious loss mechanism is the delayed escape of those neutrons with energy higher than the trap depth. Computer simulations of neutron tra jectories in two proposed trap configurations, linear quadrupole and “multi-helmholtz,” were performed. Results suggest that escape times for particles with energy higher than the trap depth are exponentially distributed. Over one p ercent of neutrons with energy 119-120% of the trap depth escape between 10 and 100 seconds after they are trapped. In order to achieve the accuracy of $10^{-5}\tau_\beta$, this systematic error will need to be eliminated or carefully characterized.
The proposed method for detecting the trapped neutron decays relies on fluorescent materials for down-converting the UV scintillations generated by the beta-decay electrons passing through liquid helium. The fluorescent decay behavior of ultraviolet down-converting phosphors was experimentally measured. The time response of the fluorescence of several possible materials, tetraphenyl-butadiene, sodium salicylate, and diphenylstilbene was studied. Using a chopped 243 nm coherent light beam, the decay time of fluorescence of these three compounds was measured at room temperature and at liquid-helium temperatures. Time constants of 2.3 ms and 9.0 ms were determined for TPB and NaSal, respectively, at 4K. The combination of TPB having the shortest measured time constant and a previously-measured 200% quantum efficiency makes it the most promising candidate for further exploration.