Speaker
Description
The overall objective of this study is to reveal the proton-neutron pair correlation in neutron-rich nuclei to measure proton-neutron pair transfer. For this purpose, we are aiming at the evaluation of the transition strength of proton-neutron transfer between the ground states of tin and indium via proton-neutron pair transfer reactions such as (p,3He), (d,4He), and (4He,6Li) measured with Grand Raiden magnetic spectrometer. For instance, to resolve the ground state of 114In from its first excited state (190 keV), an energy resolution of approximately 75 keV (FWHM) is required. Furthermore, our ultimate goal is to resolve the ground and first excited states (separated by 60 keV) in the 120Sn(4He,6Li)118In reaction. Energy resolution is primarily determined by the beam spread and energy loss difference relative to the reaction position within the target. The required target thickness for 116Sn(p,3He)114In reaction is less than 2 \mu\mathrm{m}. For the ultimate goal of 60 keV resolution, a target thickness of approximately 0.5 \mu\mathrm{m} is required. Since rolling methods have thickness limitations, vacuum deposition method was adopted. While standard deposition typically maintains a source-to-substrate distance of 10–30 cm to ensure uniformity, the high cost of enriched isotopes poses a financial challenge. Therefore, we adopted a "short-distance deposition" technique to improve collection efficiency, subsequently measuring thickness at multiple points to evaluate the thickness distribution. In practical, the target thickness can be simultaneously measured by using energy loss difference in the physics measurement. To eliminate the background events, the target should be self-supported without any backing. In this paper, we discuss the fabrication method of self-supported thin-film tin targets and the results of the high-resolution measurements obtained using them. Tin has a low melting point and is prone to structural changes, requiring sophisticated techniques for high-quality film formation [1]. We utilized a tungsten boat as the evaporation stage and glass slides as the substrate. The process began by evacuating the chamber, followed by the deposition of a NaCl thin layer. Subsequently, tin metal was deposited onto the NaCl layer. The NaCl layer served as a sacrificial layer. After deposition, it was dissolved in water. The tin foil was carefully scooped up from the water surface using a target holder.
The parameters for 116Sn deposition were: a source-to-substrate distance of 4.5 cm, 100 mg of 116Sn source material, 30 mg of NaCl, a deposition time of 11 min, an initial current of 37.5 A, and a vacuum level of 5\times{10}^{-3}\ Pa. We observed that using approximately 0.9 g of NaCl caused the metallic luster to disappear on both sides, whereas 30 mg of NaCl successfully preserved the luster on the substrate side. In March 2026, we performed the 116Sn(p,3He)114In reaction measurement using this target. To evaluate the target thickness, we utilized the energy loss difference observed in the two-nucleon transfer reaction. The resulting resolution was approximately 57 keV, successfully meeting our goal and allowing for the clear separation of the ground state peak from the first excited state. Our results indicate that excessive thickness of the NaCl sacrificial layer negatively impacts the quality of tin deposition. Through energy loss analysis, a thickness of 1.6 \mu\mathrm{m} is measured, achieving our target of a thickness below 2 \mu\mathrm{m}. This study successfully established a cost-effective, short-distance vacuum deposition method for enriched isotope targets, enabling high-resolution nuclear spectroscopy measurements.
Reference
[1] V. Borra et al., Thin Solid Films, 616, 311 (2016).