The Upscattering of Ultracold Neutrons from the polymer $[C_6 H_{12}]_n$

TL;DR

This study measures the average energy of ultracold neutrons upscattered from hydrogen in a polymer, challenging previous experimental results and supporting theoretical models of neutron heating.

Contribution

It provides the first measurement of upscattered neutron energy from hydrogen in semicrystalline polymer PMP, using MCNP simulations to clarify previous contradictory data.

Findings

Measured average upscattered neutron energy: 26±3 meV.

Results contradict previous experimental values of 10-13 meV.

Findings support theoretical neutron heating models in MCNP.

Abstract

It is generally accepted that the main cause of ultracold neutron (UCN) losses in storage traps is the upscattering to the thermal energy range by hydrogen adsorbed on the surface of the trap walls. However, the data on which this conclusion is based are poor and contradictory. Here, we report a measurement, performed at the Los Alamos National Laboratory UCN source, of the average energy of the flux of upscattered neutrons after the interaction of UCN with hydrogen bound in semicrystalline polymer PMP (tradename TPX), [C$_{6}$H$_{12}$]$_n$. Our analysis, performed with the MCNP code based on the application of the neutron scattering law to UCN upscattered by bound hydrogen in semicrystalline polyethylene, [C$_{2}$H$_{4}$]$_n$, leads us to a flux average energy value of 26$\pm3$ meV in contradiction with previously reported experimental values of 10 to 13 meV and in agreement with the theoretical models of neutron heating implemented in the MCNP code.