A comparative study of target fabrication strategiesfor microgram muonic atom spectroscopy

Muonic atom spectroscopy is a method that can determine absolute nuclear charge radii with typical relative precision of 10-3. Recent developments have enabled to extend muonic atom spectroscopy to microscopic target quantities as low as 5 µg. This substantial reduction from the traditional limit of the order of 100 mg is based on a transfer mechanism in a high-pressure hydrogen gas cell, which transports the muon to the surface of the target material rather than stopping it over a broad depth range. This approach enables the measurement of absolute nuclear charge radii of long-lived radioactive isotopes (half-life above ~20 years), but the production of appropriate targets for the technique has presented some major challenges, such as the formation of organic layers on the substrate. This study presents a systematic investigation of the stopping efficiency for different target preparation methods: ion implantation, drop-on-demand printing, and molecular plating. Notable differences between the three methods were discovered in terms of their performance allowing to further fine tune the method of choice for future target preparations. Our findings show that implantation provides appropriate targets for our method with negligible losses. This achievement opens the landscape of potential measurements to isotopes where high mass separation is required not achievable with other methods. Furthermore, molecular plated targets performed substantially better than those prepared using drop-on-demand printing.