Storage of ultracold neutrons in the magneto-gravitational trap of theUCNτexperiment

Abstract

The UCNτ experiment is designed to measure the lifetime τn of the free neutron by trapping ultracold neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry provide a background "holding" field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time τstore of the trap by storing UCN for various times, and counting the survivors. The data are consistent with a single exponential decay, and we find τstore = 860 ± 19 s: within 1σ of current global averages for τn. The storage time with the holding field deactiveated is found to be τstore = 470 ± 160 s; this decreased storage time is due to the loss of UCN which undergo Majorana spin-flips while being stored. We discuss plans to increase the statistical sensitivity of the measurement and investigate potential systematic effects.

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

“…Each NdFeB magnet was 2.54 cm x 5.08 cm x 1.27 cm, with a surface field of about 1.0 T. The magnets were installed along the surface of two intersecting tori, one with a major radius of 100 cm and a minor radius of 50 cm, the other with the radii interchanged, and cut off at a height of 50 cm from the bottom of the trap, thus forming an asymmetric trap with a trapping potential of about 50 neV (corresponding to neutron temperature ≲ 0.58 mK) and a fiducial volume of 420 l. An additional externally applied holding field of up to 10 mT, approximately perpendicular to the Halbach field, was used to maintain the neutron polarization during the storage period. The performance of the trap was described in Salvat et al [9].…”

“…The present experiment was designed to reduce systematic uncertainties by using ultracold neutrons (UCN) trapped in a storage volume closed by magnetic fields on the bottom and sides and by gravity on top, as previously demonstrated by Ezhov et al [7]. In this work, we have used an asymmetric trap to reduce the population of longlived closed neutron orbits with kinetic energies over the storable energy threshold in the trap [8,9]. 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.…”

Phenomenology ofn−n¯oscillations revisited

“…We have shown through explicit example that the removal of magnetic fields is not necessary for the observation of n-n oscillations; this opens new possibilities for their experimental discovery. For example, it becomes possible to study n-n oscillations by confining neutrons in magnetic traps, or bottles; such are under development for improved measurements of the neutron lifetime [34][35][36]. In a gravitomagnetic trap a single spin state is confined; we suppose, in addition, that a transverse rf field at resonance is applied.…”

Cold and Ultra-cold Neutrons as Probes of New Physics

“…In order to finally resolve the discrepancy, not only existing 'bottle' and 'beam' projects are continued, but also alternative methods of measuring the neutron lifetime are being developed worldwide: A new 'beam' experiment at J-PARC will detect the decay electrons rather than the decay protons [35], and new 'bottle' experiments at ILL [78,79], PNPI [23], LANSCE [80,81,82], FRM II [83,84], and TRIGA Mainz will confine UCNs with magnetic fields rather than material walls. These experiments aim at accuracies of 1 down to 0.1 s.…”