High-resolution measurement of hypernuclear events in a nuclear emulsion with hard X-ray microscopy

Abstract

A high-resolution measurement method based on X-ray microscopy was developed to analyze double-strangeness hypernuclear events with a complex topology in a nuclear emulsion. In a feasibility study performed on $\alpha$-decay events in emulsions, the resolution of the X-ray microscopy in the focal plane was found to be 0.2 $\upmu {\mathrm{m}}$, which shows an improvement by $\sim 2.4$ times to that of the optical microscopy used for conventional analysis. The extent to which the emulsion underwent modification as a result of X-ray irradiation was also evaluated. The modification mainly occurred in the form of a change in its thickness; however, this affection was adequately small to perform X-ray imaging if the duration of the irradiation was sufficiently short. Stereo imaging with X-ray microscopy improved the resolution by $\sim 2.5$ times to 0.28 $\upmu {\mathrm{m}}$ along the optical axis compared with the depth of field of the optical microscope, 0.7 $\upmu {\mathrm{m}}$. We applied the developed method to the study of a double-strangeness hypernuclear event. The uncertainty on the position of the vertex point and the binding energy of the $\Xi ^-$ and $^{14}$N system was improved from 3 $\upmu {\mathrm{m}}$ to 0.04 $\upmu {\mathrm{m}}$ and $\pm 3$ MeV to $\pm 0.86$ MeV, respectively. The binding energy was deduced to be $-1.23 \pm 0.86~{\mathrm{MeV}}$, and this result indicates that a $\Xi ^-$ atomic state is produced in the observed event.