Abstract
Metal tubes plated with nickel-phosphorus are used in many fundamental physics experiments that use ultracold neutrons (UCN) because of their ease of fabrication. These tubes are usually polished to an average roughness of 25-150nm. However, there is no scattering model that accurately describes UCN scattering on such a rough guide surface with a mean-square roughness greater than 5nm. We, therefore, developed a scattering model for UCN in which scattering from random surface waviness with a size larger than the UCN wavelength is described by a microfacet Bidirectional Reflectance Distribution Function model (mf-BRDF model), and scattering from smaller structures by the Lambert's cosine law (Lambert model). For the surface waviness, we used the statistical distribution of surface slope measured by an atomic force microscope on a sample piece of guide tube as an input of the model. This model was used to describe UCN transmission experiments conducted at the pulsed UCN source in J-PARC. In these experiments, a UCN beam collimated to a divergence angle smaller than ±6° was directed into a guide tube with a mean-square roughness of 6.4 to 17nm at an oblique angle, and the UCN transport performance and its time-of-flight distribution were measured while changing the angle of incidence. The mf-BRDF model combined with the Lambert model with scattering probability pL=0.039±0.003 reproduced the experimental results well. We have thus established a procedure to evaluate the characteristics of UCN guide tubes with a surface roughness of approximately 10nm.
Original language | English |
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Article number | 034605 |
Journal | Physical Review C |
Volume | 108 |
Issue number | 3 |
DOIs | |
State | Published - Sep 2023 |
Externally published | Yes |
Funding
This research was supported by JSPS KAKENHI Grants No. 18H05230, No. 20KK0069, and No. 21K13940. The neutron experiment at the Materials and Life Science Experimental Facility of the J-PARC was performed under user programs (Proposals No. 2021B0272 and No. 2021B0309) and S-type project of KEK (Proposal No. 2019S03).