1. Metallic magnetic calorimeters (MMC) for high resolution x-ray spectroscopy Loredana GASTALDO, Markus LINCK, Sönke SCHÄFER, Hannes ROTZINGER, Andreas BURCK, Sebastian KEMPF, Jan-Patrick PORST, Andreas FLEISCHMANN, Christian ENSS, George M. SEIDEL

2. Inverse Temperature T  1 [K  1 ] Magnetization M [A/m] Au:Er 300 ppm Temperature T [mK] Specific heat C [10  4 J mol  1 K  1 ] Au:Er 300 ppm Detector setup Thermodynamic properties of interacting spins (RKKY) can be calculated with confidence by mean field approximations or Monte Carlo simulations optimization by numerical methods is possible ++ + + 50 mK 4.2 K 300 K B Very stable material suitable for long lasting measurements

3. Fluctuations of energy in a canonical ensemble Magnetic Johnson noise (thermal currents in the metallic sensor ) Flux noise of the SQUID-magnetometer energy sensitivity close to quantum limit required, 1/f noise sensor electron SQUID (order of magnitude: 1eV for a 10 keV x-ray detector) C abs C spins Noise & energy resolution

4. Aluminum thin window Lead Collimator Brass holder Circuit board Field coil Superconducting shield (lead) Detector set-up Detector SQUID: KSUP-10-50 (IBM) Amplifier SQUID: CCBlue (IPHT Jena) Sensor  Au:Er 600 ppm  x (12.5) 2 x 8  m 3 Absorber  Au 180 x 180 x 5  m 3 stopping power above 98% @ 6 keV Pulses acquired at different temperatures and at different magnetic fields Detector SQUID

5. Magnetization and chip temperature Magnetic Flux T bath [K] T chip [K] Dissipation on the SQUID chip leads to a decoupling of the chip temperature from the bath temperature

6. Pulse analysis Rise time 100  s fast decay time 650  s Decay time slow decay time 8.1 ms Time t [ms] The amplitude of pulses saturates at low temperatures and high fields Temperature T [mK]

7. 55 Fe energy spectrum Counts / 2 eV Counts / 15 eV Energy E [keV] Very low background A  3.695 keV B  3.775 keV Energy [keV] Transition

8. Energy resolution and linearity Counts / 0.24 eV Counts / 0.12 eV Energy Energy E [keV] Relative pulse amplitude A Measured energy E exp [keV] Difference [eV] Energy E [keV] A - A i TFN  0.38 eV + SQUID  1.14 eV + 1/f  1.6 eV ???? 1.85 eV still missing! 1/f 2 due to temperature fluctuations of the chip

9. Conclusion and future plans 2,7 eV energy resolution is a good result but it does not rapresent the limit of magnetic calorimeters Improvements of our detector will follow the good results we are obtaining with microstructuring techniques - 166 Er enriched Au:Er sputter target -Sputtered Au:Er sensor -Overhanging absorber