A new energy scale for X- and gamma rays below 100 keV using high-resolution metallic magnetic calorimeters

The X and gamma rays emitted by radionuclides can be used to calibrate the energy scale of energy-dispersive detectors. This is particularly the case for cryogenic detectors, which offer excellent resolution but have non-linearities that need to be corrected. In order to calibrate these detectors accurately below 200 keV, it is necessary to have X-rays and gamma rays with an uncertainty of the order of 0.1 eV. However, recommended gamma-ray photon energies meeting this criterion are rare or based on a single experimental measurement.

The main objective of this thesis is to measure gamma photons below 200 keV with excellent resolution in order to improve the uncertainties on their energies to around 0.1 eV.

To meet this objective, a new cryogenic detector has been designed. It is a Metallic Magnetic Calorimeter (MMC) with eight 50 µm-thick gold absorbers covering a total surface area of 8.4 mm².

An assembly was built to house: four MMC chips, two SQUID chips for reading-out the MMCs, and the printed circuits to connect them to the cables of the dilution refrigerator. The set-up also includes a cryogenic radioactive source sampler, specially designed to operate at very low temperatures and to measure up to four sources sequentially.

The sources measured are mixtures of several radionuclides: three standard radionuclides used to correct the non-linearity of the MMC (¹⁶⁹Yb, ⁵⁷Co and ¹⁵³Gd) and seven radionuclides used to measure their X and gamma energies with very low uncertainty (¹⁰⁹Cd, ¹³³Ba, ¹⁵⁵Eu, ²¹⁰Pb, ²³⁹Np, ²⁴¹Am and ²⁴³Am). Mixtures of radionuclides in the sources were chosen by Monte Carlo simulations to minimise spectral interference. Ytterbium-169 was produced by irradiating a thulium foil with deuterons at the Arronax cyclotron.Two independent measurement sessions on two set of four sources were carried out at around 16 mK during two weeks, with different temperature control conditions.

The energy resolution of the detector is 15 eV to 36 eV for gamma-rays between 0 keV and 200 keV. The non-linearity of each absorber was corrected using the lines of the standard radionuclides and a second-degree polynomial.

The X-ray and gamma-ray energies measured were analysed and combined using various methods. Regardless of the measurement session and analysis method used, the energies obtained are consistent with each other, with only the uncertainties differing. In the end, the energies of 15 gamma-ray lines were measured and for 14 of them, the uncertainty obtained, around 0.2 eV, is lower than the recommended values. Eight Kα X-ray photon energies were also measured; for these well-known lines, very good agreement was observed with the recommended values, thus validating the methodology used in the work presented.

ionizing radiation metrology, photon spectrometry, cryogenic detectors

Full document (FR) : TEL-04833258