Development of closed -cycle dynamic nuclear polarization system for small -angle neutron scattering and neutron reflectometry

Abstract

We develop a closed-cycle dynamic nuclear polarization (DNP) system for spin-contrast-variation (SCV) small-angle neutron scattering (SANS) and neutron reflectometry (NR) to increase the use of these techniques. Compared with our cryogen-filled DNP system, the closed-cycle system is made compact and can change the sample in a comparable time-scale, but achieved proton-polarization is only 11.2 and 20$\pm$3% for bulk and thin-film samples, respectively, whereas 32-37% is achieved with the cryogen-filled system. This is because cooling power of the recycling cold helium gas in the closed-cycle system is lower than that of superfluid helium in the cryogen-filled one. In order to improve the polarization with the closed-cycle system, we have to improve thermal contact between the sample and cold helium gas to minimize microwave heating of the sample.