Matrix Isolation Spectroscopy Applied to Positron Moderatioin in Cryogenic Solids
Abstract
We report results of Matrix Isolation Spectroscopy (MIS) experiments performed on working cryogenic rare gas solid (RGS) positron (e+) moderators. The e+ is the antiparticle to the electron,1-3 and positrons are produced by energetic processes that result in very broad kinetic energy distributions (KEDs) - typically spanning hundreds of thousands of electron volts (eV). Trapping and manipulating e+ with electromagnetic fields requires narrowing these KEDs below a few eV, which is accomplished via velocity-dependent interactions in a normal matter "moderator". A fast e+ entering a wide bandgap dielectric RGS moderator slows rapidly within the first ~ picosecond, producing a track of ionized and electronically excited species. However, once KEe+ drops below the ~ 10 eV minimum required for generating such excitations, these interactions abruptly switch off, and the e+ enters a phase of "hyperthermal diffusion" that can last for nanoseconds. Positrons that reach a free surface of the moderator before annihilating with an electron may escape into vacuum where they can be manipulated. The best known e+ moderator is cryogenic solid Ne, which still only delivers efficiencies < 1 %; the other 99+ % of the nascent fast e+ are wasted. Additionally, the RGS moderator efficiency is known to decrease during operation, which is attributed variously to the buildup of radiation damage, and/or to contamination of the moderator surface by residual gas deposition. We constructed a novel apparatus that permits optical access to a working cryogenic solid moderator. Our original motivation was to test our hypothesis that solid parahydrogen (pH2) should be an even better e+ moderator than solid Ne, while simultaneously monitoring the condition of the moderators by infrared (IR) absorption spectroscopy. Unfortunately, the performance of our ortho/para hydrogen converter (o/p convert
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 01, 2011
- Accession Number
- ADA547542
Entities
People
- C. M. Lindsay
- Christopher D. Molek
- Mario E. Fajardo
Organizations
- Air Force Research Laboratory