A novel nuclear emulsion detector for measurement of quantum states of ultracold neutrons in the Earth's gravitational field

N. Muto, H. Abele, T. Ariga, J. Bosina, M. Hino, K. Hirota, G. Ichikawa, T. Jenke, H. Kawahara, S. Kawasaki, M. Kitaguchi, J. Micko, K. Mishima, N. Naganawa, M. Nakamura, S. Roccia, O. Sato, R. I.P. Sedmik, Y. Seki, H. M. ShimizuS. Tada, A. Umemoto

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3 Citations (Scopus)


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.

Original languageEnglish
Article numberP07014
JournalJournal of Instrumentation
Issue number7
Publication statusPublished - 2022 Jul 1


  • Neutron detectors (cold, thermal, fast neutrons)
  • Particle tracking detectors (Solid-state detectors)


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