Neutron lifetime experiment based on an accordion-like UCN storage volume coated with "low temperature Fomblin"

A fresh type of per-fluorinated polymer, "Low Temperature Fomblin," has been proofed as a wall coating in an ultracold neutron (UCN) storage experiment using a gravitational storage combination of parts to form a whole The data show a UCN reflection los coefficient [eta] as depressed as [approximately equal to] 5 X [10sup-6] in the temperature range 105 K to 150 K We plan to use this oil in a novel type of neutron lifetime measurement, where a bellows combination of parts to form a whole ("accordion") enables to vary the trap size in a wide range while the total surface area and distribution of surface area above height remain constant. These unique characteristics, in combination with application of the scaling technique unraveled by W. Mampe et al. in 1989 make sure exact linearity for the extrapolation from inverse storage lifetimes to the inverse neutron lifetime. Linearity clutchs for any energy dependence of los coefficient [mu](E) Using the UCN source at the Institut Laue Langevin we wait for to achieve a lifetime precision below [+ or -]1 s

Keywords: neutron lifetime; ultracold neutrons

1 Introduction

Particle decay data indicate that the Cabibbo-Kobayashi-Maskawa matrix may deviate from unitarity (presently at the 2.7-sigma horizontal [1]). This question depends critically upon the up-down quark mixing amplitude [Vsubud] which is determined greatest in quantity sensitively by the neutron life-time [[tau].sub.n] and the neutron decay asymmetry coefficient A. A reliable, precise value of [[tau].sub.n] will also help to refine designs of astrophysics [2] and cosmology [3] The rife world average is [[tau].sub.n] = (8857 [+ or -] 08) s [4]. We propose a of recent origin [[tau].sub.n] measurement with a precision below 1 s using UCN storage. Wall losse are minimized through the use of "low-temperature Fomblin," and the notoriously difficult extrapolation from storage lifetimes to [[tau].sub.n] is made more reliable by the agency of the novel use of an accordion-like storage utensil In this system, the surface area and its distribution above height remain constant while the turn is changeable in a wide range. Combination with the scaling technique of Mampe et al. [5] render certains that the extrapolation function becomes strictly linear for any shape of UCN representation in the trap and for any efficiency dependence of reflection loss coefficient [mu] Moreover, no correction for gravity is required. These unique features distinguish this rule from all previous [[tau].sub.n] experiments based upon UCN storage in material traps [5-10]

2 Basic Considerations

In UCN-storage based [[tau].sub.n] experiments it is crucial that any non-decay los to be paid to wall collisions, gaps and the residual gas are reliably subtracted from the total los rate. In the elementary theory of wall reflection los the interaction with the wall atoms is described by dint of a step-function barrier determined by the agency of the optical (or mean Fermi) potential U-iW. For an isotropic UCN distribution the mean los probability by bounce is given by [mu](E) = 2[eta]{(U/E)arcsin(E/U)[.sup.1/2] - [(U - E)/E][sup1/2]} where [eta] = W/U and E is the neutron kinetic activity at the impact point. The wall los probability by second is

[[tau].sub.w.sup.-1] = <[nu][mu](E)> (1)

where [nu] is the wall collision rate for a UCN and the average is taken above the UCN spectrum and trap surface. Neglecting gravity, [nu] = v/[lambda] can be press outed by the mean UCN velocity v and the mean unrestrained path [lambda] = 4V/S. [lambda] is independent of v and determined single by the total trap surface s and volume V. As shown in [5] this gas-kinetic be the effect is also valid under gravity provided the trap geometry has a horizontal plane of regularity (as in the "accordion-trap" discussed below) and all UCN have enough intensity to reach the highest point(s) in the trap. For gravitationally bent paths we use the straightforward definition 1/[lambda] = total wall collision rate N[nu] divided by dint of the volume-integrated UCN flux [PHI] in the trap (see Sec 4)

However, gravity does induce an important difference. For our trap geometry the total UCN number N is not exactly proportional to turn V, even for identical spectra. As a end we will plot storage data v [nu] not 1/[lambda], to obtain a linear dependence

If we are positive that a single constant ([eta]) can be factored on the outside of the function [mu](E), Eq (1) may also be written in the form [[tau].sub.w.sup.-1] = [eta][gamma], where [gamma] is defined as [gamma] = <(v/[lambda])([mu]/[eta])>, averaged above spectrum and surface. However, this is strictly justified alone for a smooth, uniform wall without surface contamination and/or microstructure (cracks, roughnes etc) which can be exhibited by a potential step function.

In an experiment we measure the numbers N([tsub1]) N([tsub2]) of UCN numbered after storage times [t.sub.1] and [tsub2] Although for a broad UCN image the decay curve is nonexponential single can define a mean storage lifetime for time interval [tsub1] [tsub2] in the form