Ion implantation of the electron-capture nuclide 55Fe for measurements by means of metallic microcalorimeters

Precise measurements of fundamental decay data such as energies and transition probabilities of radioactive isotopes are important for the development of corresponding nuclear modelling, activity determination and various applications in science and technology. The EMPIR project PrimA-LTD -"Towards new Primary Activity standardisation methods based on Low-Temperature Detectors" - aims to measure the electron-capture decay of ⁵⁵Fe very precisely using Metallic Microcalorimeters (MMCs) with outstandingly high energy resolution. Using a high-statistics measurement, electron-capture probabilities shall be precisely determined and higher-order effects such as electron shake-up and shake-off shall be examined with unprecedented precision. A key to success for this project is sample preparation. This work reports on the implantation of ⁵⁵Fe into the 140 μm × 140 μm gold absorbers of the MMCs as a proof of principle for scalability. Building up on preparatory laser-spectroscopic studies on stable ⁵⁶Fe, laser resonance ionization at the RISIKO mass separator was used to produce a monoisotopic ⁵⁵Fe ion beam with the required specifications. Successful implantations of this isotope (i) into 32 test absorbers with about 0.7(2) Bq each and (ii) into various on-chip absorbers with an activity close to the requested 5 Bq per absorber are presented. The impact of the implantation on the quality of spectra is highlighted on the basis of first MMC test measurements.