Transmitted positron reemission from a thin single-crystal Ni(100) foil

We have investigated the reemission of positrons from a 1500-{}-thick unsupported Ni(100) foil irradiated with a collimated beam of positrons of variable energy 0 to 6 keV. At 5 keV, (19 ±{} 3)% of the positrons emerge with low energy from the transmission side of the foil; {} 55% of the slow-positron spectrum is concentrated in a 0.2-eV-wide peak at 1.1 eV. The Ni(100) foil remoderator is as efficient as the W(100) foils investigated by Chen et al. [Phys. Rev. B 31, 4123 (1985)] and had only one-third of the spread in energy. Foils of this type will thus be very useful for brightness-enhancement stages using transmission remoderation.