TY - JOUR
T1 - A novel nuclear emulsion detector for measurement of quantum states of ultracold neutrons in the Earth's gravitational field
AU - Muto, N.
AU - Abele, H.
AU - Ariga, T.
AU - Bosina, J.
AU - Hino, M.
AU - Hirota, K.
AU - Ichikawa, G.
AU - Jenke, T.
AU - Kawahara, H.
AU - Kawasaki, S.
AU - Kitaguchi, M.
AU - Micko, J.
AU - Mishima, K.
AU - Naganawa, N.
AU - Nakamura, M.
AU - Roccia, S.
AU - Sato, O.
AU - Sedmik, R. I.P.
AU - Seki, Y.
AU - Shimizu, H. M.
AU - Tada, S.
AU - Umemoto, A.
N1 - Publisher Copyright:
© 2022 IOP Publishing Ltd and Sissa Medialab.
PY - 2022/7/1
Y1 - 2022/7/1
N2 - Hypothetical short-range interactions could be detected by measuring the wavefunctions of gravitationally bound ultracold neutrons (UCNs) on a mirror in the Earth's gravitational field. Searches for them with higher sensitivity require detectors with higher spatial resolution. We developed and have been improving an UCN detector with a high spatial resolution, which consists of a Si substrate, a thin converter layer including 10B4C, and a layer of fine-grained nuclear emulsion. Its resolution was estimated to be less than 100 nm by fitting tracks of either 7Li nuclei or α-particles, which were created when neutrons interacted with the 10B4C layer. For actual measurements of the spatial distributions, the following two improvements were made. The first improvement was to establish a method to align microscopic images with high accuracy within a wide region of 65 mm × 0.2 mm. We created reference marks of 1 μm and 5 μm diameter with an interval of 50 μm and 500 μm, respectively, on the Si substrate by electron beam lithography and realized a position accuracy of less than 30 nm. The second improvement was to build a holder for the detector that could maintain the atmospheric pressure around the nuclear emulsion to utilize it under a vacuum during exposure to UCNs. The intrinsic resolution of the improved detector was estimated to be better than 0.56(8) μm by evaluating the blur of a transmission image of a gadolinium grating taken by cold neutrons. The evaluation included the precision of the gadolinium grating. A test exposure was conducted to obtain the spatial distribution of UCNs in the quantized states on a mirror in the Earth's gravitational field. The distribution was obtained, fitted with the theoretical curve, and turned out to be reasonable for UCNs in quantized states when we considered a blurring of 6.9 μm. The blurring was well explained as a result of neutron refraction due to the large surface roughness on the upstream side of the Si substrate. By using a double-side-polished Si substrate, a resolution of less than 0.56 μm is expected to be achieved for UCNs.
AB - Hypothetical short-range interactions could be detected by measuring the wavefunctions of gravitationally bound ultracold neutrons (UCNs) on a mirror in the Earth's gravitational field. Searches for them with higher sensitivity require detectors with higher spatial resolution. We developed and have been improving an UCN detector with a high spatial resolution, which consists of a Si substrate, a thin converter layer including 10B4C, and a layer of fine-grained nuclear emulsion. Its resolution was estimated to be less than 100 nm by fitting tracks of either 7Li nuclei or α-particles, which were created when neutrons interacted with the 10B4C layer. For actual measurements of the spatial distributions, the following two improvements were made. The first improvement was to establish a method to align microscopic images with high accuracy within a wide region of 65 mm × 0.2 mm. We created reference marks of 1 μm and 5 μm diameter with an interval of 50 μm and 500 μm, respectively, on the Si substrate by electron beam lithography and realized a position accuracy of less than 30 nm. The second improvement was to build a holder for the detector that could maintain the atmospheric pressure around the nuclear emulsion to utilize it under a vacuum during exposure to UCNs. The intrinsic resolution of the improved detector was estimated to be better than 0.56(8) μm by evaluating the blur of a transmission image of a gadolinium grating taken by cold neutrons. The evaluation included the precision of the gadolinium grating. A test exposure was conducted to obtain the spatial distribution of UCNs in the quantized states on a mirror in the Earth's gravitational field. The distribution was obtained, fitted with the theoretical curve, and turned out to be reasonable for UCNs in quantized states when we considered a blurring of 6.9 μm. The blurring was well explained as a result of neutron refraction due to the large surface roughness on the upstream side of the Si substrate. By using a double-side-polished Si substrate, a resolution of less than 0.56 μm is expected to be achieved for UCNs.
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U2 - 10.1088/1748-0221/17/07/P07014
DO - 10.1088/1748-0221/17/07/P07014
M3 - Article
AN - SCOPUS:85134405429
SN - 1748-0221
VL - 17
JO - Journal of Instrumentation
JF - Journal of Instrumentation
IS - 7
M1 - P07014
ER -