A Novel Nuclear Emulsion Detector for Measurement of Quantum States of Ultracold Neutrons in the Earth's Gravitational Field

• URL: http://arxiv.org/abs/2201.04346v2
• Date: Thu, 26 May 2022 09:09:19 GMT
• Title: A Novel Nuclear Emulsion Detector for Measurement of Quantum States of Ultracold Neutrons in the Earth's Gravitational Field
• Authors: Naoto Muto, Hartmut Abele, Tomoko Ariga, Joachim Bosina, Masahiro Hino, Katsuya Hirota, Go Ichikawa, Tobias Jenke, Hiroaki Kawahara, Shinsuke Kawasaki, Masaaki Kitaguchi, Jakob Micko, Kenji Mishima, Naotaka Naganawa, Mitsuhiro Nakamura, St\'ephanie Roccia, Osamu Sato, Ren\'e I. P. Sedmik, Yoshichika Seki, Hirohiko M. Shimizu, Satomi Tada, Atsuhiro Umemoto
• Abstract summary: Hypothetical short-range interactions could be detected by measuring the wavefunctions of ultracold neutrons (UCNs) on a mirror bounded by the Earth's gravitational field. We are developing a UCN detector for the with a high spatial resolution.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Hypothetical short-range interactions could be detected by measuring the wavefunctions of ultracold neutrons (UCNs) on a mirror bounded by the Earth's gravitational field. The Searches require detectors with higher spatial resolution. We are developing a UCN detector for the with a high spatial resolution, which consists of a Si substrate, a thin converter layer including $^{10}$B$_{4}$C, and a layer of fine-grained nuclear emulsion. Its resolution was estimated to be less than 100 nm by fitting tracks of either $^{7}$Li nuclei or $\alpha$-particles, which were created when neutrons interacted with the $^{10}$B$_{4}$C layer. For actual measurements of the spatial distributions, the following two improvements were made: The first was to establish a method to align microscopic images with high accuracy within a wide region of 65 mm $\times$ 0.2 mm. We created reference marks of 1 $\mu$m and 5 $\mu$m diameter with an interval of 50 $\mu$m and 500 $\mu$m, respectively, on the Si substrate by electron beam lithography and realized a position accuracy of less than 30 nm. The second was to build a holder that could maintain the atmospheric pressure around the nuclear emulsion to utilize it under vacuum during exposure to UCNs. The intrinsic resolution of the improved detector was estimated by evaluating the blur of a transmission image of a gadolinium grating taken by cold neutrons as better than 0.56 $\pm$ 0.08 $\mu$m, which included the grating accuracy. A test exposure to UCNs was conducted to obtain the spatial distribution of UCNs in the Earth's gravitational field. Although the test was successful, a blurring of 6.9 $\mu$m was found in the measurements, compared with a theoretical curve. We identified the blurring caused by the refraction of UCNs due to the roughness of the upstream surface of the substrate. Polishing of the surface makes the resolution less than 100 nm.

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