A calculation of ultra-cold neutron (UCN) upscattering rates in molecular deuterium solids has been carried out, taking into account intra-molecular exictations and phonons. The different moelcular species ortho-D2 (with even rotational quantum number J) and para-D2 (with odd J) exhibit significantly different UCN-phonon annihilation cross-sections. Para- to ortho-D2 conversion, furthermore, couples UCN to an energy bath of excited rotational states without mediating phonons. This anomalous upscattering mechanism restricts the UCN lifetime to 4.6 msec in a normal-D2 solid with 33% para content.
@article{arxiv.nucl-ex/0004003,
title = {UCN Upscattering rates in a molecular deuterium crystal},
author = {C. -Y. Liu and A. R. Young and S. K. Lamoreaux},
journal= {arXiv preprint arXiv:nucl-ex/0004003},
year = {2008}
}