"m=1" coatings for neutron guides

C P Cooper-Jensen,P M Bentley,E Klinkby,O Kirstein,H Wilkens,B Hjörvarsson,D Rats,V Kapaklis,A Vorobiev

doi:10.1088/1742-6596/528/1/012005

Abstract

A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section.

Highlights

Neutron guides were introduced in 1963 [1] to increase the neutron flux at the sample position
Gamma radiation that is emitted when a neutron is absorbed in the guide has to be attenuated for the safety of the people working in the area and because the gamma radiation can be problematic for some experiments at the end of the neutron guide
Both by reducing the amount of gamma radiation and/or by reducing the energy of the emitted gamma radiation, one will reduce the amount of shielding needed

Summary

Introduction

Neutron guides were introduced in 1963 [1] to increase the neutron flux at the sample position. To reflect neutrons at still higher angles than that corresponding to m=1, one could use the 58Ni isotope This has a higher neutron scattering length density compared to Ni, equivalent to m=1.18. Thereafter come Ni and Be, which have nearly the same scattering length density Both C and Be have a very low absorption cross section compared to Ni, and when a neutron is absorbed both emit gamma radiation at lower photon energies than Ni. The coating for neutron guides is mainly produced by DC magnetron sputtering, since large areas of optical components can be sputtered efficiently with the quality needed. All data are corrected for variations in the incoming neutron flux by scaling the data to a beam monitor

Results and discussion

Be diamond Ni cal 58Ni cal

Conclusion and future work