Neutron lifetime measurements using gravitationally trapped ultracold neutrons

Abstract

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Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

“…We have also introduced for the first time in situ detection of the surviving neutrons, to eliminate uncertainties associated with transporting the neutrons to an ex situ detector. Recently published storage experiments used storage traps with variable volumes to extrapolate to infinite volume in an attempt to reduce uncertainties associated with losses of neutrons due to interactions with the material walls [5,[10][11][12][13][14]. The present experiment had no detectable losses of neutrons due to interactions with the magnetic and gravitational "walls" of the trap and thus required no extrapolation.…”

On the Neutron Transition Magnetic Moment

“…The existing difference between the neutron lifetime measurements by the beam (appearance) experiments [46] and the UCN trap (disappearance) experiments [38,41,42,43,47] can be explained by n − n conversions due to small transition magnetic moment. The magnitudes of nTMM extracted from the difference in neutron lifetime results can be measured in rather simple experiments, such as one proposed in [32] for the GP-SANS beamline at HFIR.…”

“…Lowering the temperature of the trap walls, although reducing the loss coefficient (per single collision), does not resolve the discrepancy between experiment and theoretical calculations. Only in one of a few experiments using fomblin-oil coated trap [43] the measured (≈ 2 × 10 −6 ) and expected (≈ 1 × 10 −6 ) loss factors per wall collision were in good agreement. (In all other experiments the measured losses were significantly exceeding the theoretical predictions.)…”

“…In the UCN gravitational trap experiments measuring the neutron lifetime the Earth magnetic field B 0.5 G is usually considered as a non-essential factor. The number of wall collisions is experimentally extrapolated to the "zero number of collisions," e.g., in [41,42,43]. A magnetic field non-uniformity can produce a similar disappearance effect in the volume of UCN gravitational traps, but this effect is not removable by extrapolation to the zero number of wall collisions.…”

Precise Neutron Lifetime Measurement with a Solenoidal Coil

“…There are two types of lifetime measurements which have been carried out so far. One is the bottle method [2], in which neutrons are stored in a special material or magnetic bottle, and neutrons which survive some time are counted. The other is the beam method [3] where neutrons are counted with a flux monitor while protons from neutron beta decay are counted using another detector.…”