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Efficient Cryosolid Positron Moderators
Final rept. 1 Oct 2005-1 Aug 2012
AIR FORCE RESEARCH LAB EGLIN AFB FL MUNITIONS DIRECTORATE
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This report documents an in-house experimental effort to improve upon the 949 1 efficiency of state-of-the-art cryogenic solid neon positron moderators. We detail our rationale for why we expected an order-of-magnitude higher positron moderation efficiency from Rapid Vapor Deposited cryogenic parahydrogen solids. We describe our integration of a Matrix Isolation Spectroscopy capability into our Positron Moderation apparatus, which enables spectroscopic characterization of the cryogenic moderators. Despite all our hopes and expectations to the contrary, solid parahydrogen is not a positron moderator. Indeed, a 8776 15-nm-thick layer of solid normal hydrogen is sufficient to kill half the flux of slow positrons emerging from a working solid neon moderator. We also report results from experiments on other rare gas solid positron moderators, on moderators overcoated with layers of other molecular solids, and on moderators containing deliberately included isolated dopants. We present our methods and data in great detail to facilitate the continuation and extension of these experiments by other researchers.
APPROVED FOR PUBLIC RELEASE