A novel nuclear emulsion detector for measurement of quantum states of ultracold neutrons in the Earth's gravitational field
Résumé
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 $^{10}$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 α-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 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.
Mots clés
Neutron detectors (cold
thermal
fast neutrons)
Particle tracking detectors (Solid-state detectors)
interaction: short-range
n: interaction
n: detector
detector: design
nuclear emulsion
gravitation
detector: surface
spatial resolution
nucleus: tracks
helium: tracks
lithium
silicon
boron: carbon
measurement methods
fabrication
performance