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Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force 1 and the creation of matter during the Big Bang 2 . Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap 3 .Neutral and charged particle traps are widely used tool for studying both composite and elementary particles, because they allow long interaction times and isolation from perturbing environments 4 . Here we report the magnetic trapping of neutrons.The trapping region is filled with superfluid 4 He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Static magnetic traps are formed by creating a magnetic field minimum in free space.The confining potential depth (D) of such a trap is determined by the magnetic moment of the trapped species (µ) and the difference (∆B) between the magnitude of the field at the edge of the trap and at the minimum, D = µ∆B. A particle in a low-field-seeking state (one with its magnetic moment anti-parallel with the local magnetic field vector) is pushed towards the trap minimum. Low-field-seeking particles with total energy less than D are energetically forbidden from leaving the trapping region. For atoms and molecules with a magnetic moment of one Bohr magneton it is possible to produce trap depths of ∼ 1 K. The trap depth for a neutron in the same trap would be only ∼ 1 mK, because of its much smaller magnetic moment. Despite this difficulty, magnetic trapping of the neutron was proposed as early as 1961 by Vladimirskiȋ 11 .His proposed technique was later used to confine neutrons using a combination of gravity and magnets 12 . A separate effort to trap neutrons using a similar loading method to our work (but different detection scheme) was unsuccessful because of the high temperature of the helium 2 during the loading phase 13 .Crucial to the utility of traps are the techniques used to load them. In order to catch a particle in a static conservative trap, its energy must be lowered while it is in the potential well. Atoms and molecules have been cooled and loaded into magnetic traps by scattering with either cryogenic surfaces 14, 15 , cold gases 16 or photons from a laser beam (laser cooling) 17 .Neutrons, however, cannot be loaded by such methods because they cannot be excited optically and interact too weakly with atoms to be effectively cooled by a gas. Direct thermalization with a cold solid or liquid is generally precluded by the high probability for neutron absorption in the vast majority of materials.Our trapping of neutrons relies on a loading technique that employs the "superthermal process" 18 . A neutron with kinetic energy near 11 K (where the free neutron and superfluid helium dispersion curves cross) that passes through the helium-filled trapping region can lose nearly all of its energy through the creation of a single phonon. N...

The time dependence of extreme ultraviolet ͑EUV͒ fluorescence following an ionizing radiation event in liquid helium is observed and studied in the temperature range from 250 mK to 1.8 K. The fluorescence exhibits significant structure including a short (ϳ10 ns) strong initial pulse followed by single photons whose emission rate decays exponentially with a 1.6-s time constant. At an even longer time scale, the emission rate varies as ''1/time'' ͑inversely proportional to the time after the initial pulse͒. The intensity of the ''1/time'' component from ␤ particles is significantly weaker than those from ␣ particles or neutron capture on 3 He. It is also found that for ␣ particles, the intensity of this component depends on the temperature of the superfluid helium. Proposed models describing the observed fluorescence are discussed.

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