Ultracold Neutron
a very slow neutron with a velocity of ≲ 5 m/sec. Such neutrons are called ultracold because the molecules of a gas at a temperature below 10–2°K would move with approximately that velocity.
Ultracold neutrons have kinetic energies of the order of 10–7electron volt (eV), which are too low to overcome the weak repulsion of the nuclei of most chemical elements. Consequently, such neutrons are totally reflected from the surface of many materials. The value of the repulsive potential is
where h is Planck’s constant, m is the mass of the neutron, Ni is the number of nuclei of type i per unit volume of a substance, and ai is the neutron scattering length of the nuclei. For copper, U = 1.7 x 10”7 eV; for glass, U = 10–7 eV. For 1H, 7Li, 48Ti, and 186W nuclei, U < 0; that is, ultracold neutrons are attracted by such nuclei. To a certain extent, the reflection of ultracold neutrons may be compared to the reflection of light from a metal mirror; for a neutron wave within the reflecting medium, the reflection may be described by an imaginary refractive index.
The total reflection of ultracold neutrons from walls makes it possible to store the neutrons for several minutes within closed, evacuated spaces. This property of ultracold neutrons was first pointed out by Ia. B. Zel’dovich in 1959. The first experiments on the detection and storage of ultracold neutrons were performed by F. L. Shapiro and coworkers in 1968. The storage time for ultracold neutrons in closed vessels is limited by the lifetime of a free neutron before beta decay, by neutron capture by nuclei, and by the inelastic scattering of neutrons by nuclei in a surface layer with a thickness of (4πNa)–½ ~ 10–6cm.
Like a rarefied gas, ultracold neutrons can flow through ducts of arbitrary shape; such ducts for ultracold neutrons are called neutron guides. Bent neutron guides are used to remove ultra-cold neutrons from nuclear reactors and to separate the ultracold neutrons from the thermal neutron flux, in which the fraction of ultracold neutrons is only 10–11. Consequently, the number of ultracold neutrons per unit volume that is actually obtainable is s 1 neutron/cm3.
Magnetic and gravitational fields have a substantial effect on the motion of ultracold neutrons. The properties of ultracold neutrons have not yet been adequately studied, but the neutrons apparently can provide a sensitive means for detecting a possible neutron electric charge or electric dipole moment.
REFERENCES
Gurevich, I. I., and L. V. Tarasov. Fizika neitronov nizkikh energii. Moscow, 1965.Vlasov, N. A. Neitrony, 2nd ed. Moscow, 1972.
V. I. LUSHCHIKOV
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