Temperature-dependent ultracold neutron transmission in $^2$H$_2$ gas: A test of the Young-Koppel model

The Young-Koppel (YK) model describes comprehensively the interaction of slow neutrons with diatomic gases such as H$_2$ and $^2$H$_2$. This paper reports on the first experimental results of ultracold neutron (UCN) scattering over a wide temperature range vindicating the YK model for gaseous $^2$H$_2$ and showing an important difference in the temperature dependence to a low-energy low-temperature approximation (LETA). LETA is confirmed, however, to be valid for monoatomic gases such as Ne. Calculated cross sections for other noble gases were also confirmed for ultracold neutrons. Finally, the total cross section of UCNs in H$_2$ gas was measured and analyzed applying the Young-Koppel model, however, in a more limited temperature range, confirming the theoretical prediction.