AFRL Develops Polarization Spectroscopy Technique That Probes Cryogenic Moderators for Radiation Damage Published March 13, 2008 By Dr. C. Michael Lindsay AFRL/RW EGLIN AIR FORCE BASE, Fla. -- Determining the properties of exotic materials can be challenging, especially when samples are cooled to temperatures near absolute zero and exposed to harsh, ionizing radiation. Accordingly, AFRL scientists developed a technique to overcome the significant obstacles unique to such cryogenic materials research. The technique, which uses polarized infrared light, is currently providing new insights regarding the physical and chemical properties of solid parahydrogen as a quantum solid, which exists for these experiments at temperatures near 2 K. The capacity to monitor the properties of this cryogenic solid throughout its exposure to beta and gamma radiation is critical to Air Force efforts geared towards improving the efficiency of positron moderators. These moderators are a key component of positron-based defect-characterization apparatus and represent an incremental step towards future antimatter energy storage devices. Solid parahydrogen is known to exhibit long-range order, powerful self-annealing behavior, and very low defect concentrations, making it a promising candidate for positron moderation. By studying the infrared absorption spectra of impurities intentionally doped into the solid, researchers can obtain information about the solid's temperature variations, homogeneity, and crystal packing structure. The AFRL-developed polarization spectroscopy technique not only examines the dependence of these absorption spectra on the orientation of linearly polarized infrared light, but is a particularly helpful mechanism for grasping poorly understood phenomena such as energy dissipation processes, the distribution of charges produced in a moderator during irradiation, and the alignment of polar clusters that self-assemble in doped moderators. Moderators reduce the energy of + particles from relativistic to thermal velocities, at which they are easily manipulated--via electric and magnetic fields--for a variety of applications. Today's state-of-the-art moderators have efficiencies of less than .5%, a deficiency widely attributed to defects in the moderator and damage to the sample sustained during irradiation. Controlling and monitoring the crystal and its imperfections should lead to more efficient moderators, and polarization spectroscopy is proving its critical role in this endeavor. AFRL introduced its newly established polarization technique at an international conference on matrix isolation and is currently working to transfer the technology to several academic laboratories around the world.