C. Enss

Metallic magnetic calorimeters

Metallic magnetic calorimeters (MMC) are calorimetric particle detectors, typically operated at temperatures below 100 mK, that make use of a paramagnetic temperature sensor to transform the temperature rise upon the absorption of a particle in the detector into a measurable magnetic flux change in a dc‐SQUID. During the last years a growing number of groups has started to develop MMC for a wide variety of applications, ranging from alpha‐, beta‐ and gamma‐spectrometry over the spatially resolved detection of accelerated molecule fragments to arrays of high resolution x‐ray detectors. For x‐rays with energies up to 6 keV an energy resolution of 2.7 eV (FWHM) has been demonstrated and we expect that this can be pushed below 1 eV with the next generation of devices. We give an introduction to the physics of MMCs and summarize the presently used readout schemes as well as the typically observed noise contributions and their impact on the energy resolution. We discuss general design considerations, the micro‐fabrication of MMCs and the performance of micro‐fabricated devices. In this field large progress has been achieved in the last years and the thermodynamic properties of most materials approach bulk values allowing for optimal and predictable performance.

Metallic magnetic calorimeters for particle detection

The principles and theory of operation of a magnetic calorimeter, made of a dilute concentration of paramagnetic ions in a metallic host, is discussed in relation to the use of such a device as a detector of x-rays. The response of a calorimeter to the absorption of energy depends upon size, heat capacity, temperature, magnetic field, concentration of spins and interactions among them. The conditions that optimize the performance of a calorimeter are derived. Noise sources, especially that due to thermodynamic fluctuations of the electrons in the metal, are analyzed. Measurements have been made on detectors in which Er serves as the paramagnetic ion and Au as the host metal. The measured resolution of a detector with a heat capacity of 10−12 J/K was 12 eV at 6 keV. In a detector suitable for use with hard x-rays up to 200 keV a resolution of 120 eV was obtained. Calculations indicate that the performance of both detectors can be improved by an order of magnitude. At temperatures below 50 mK, the time response of the Au : Er calorimeters to an energy deposition indicates the presence of an additional heat capacity, which we interpret as arising from the quadruple splitting of the Au nuclei in the electric field gradients introduced by the presence of the Er ions.

Direct Measurement of the Mass Difference of

The atomic mass difference of (163)Ho and (163)Dy has been directly measured with the Penning-trap mass spectrometer SHIPTRAP applying the novel phase-imaging ion-cyclotron-resonance technique. Our measurement has solved the long-standing problem of large discrepancies in the Q value of the electron capture in (163)Ho determined by different techniques. Our measured mass difference shifts the current Q value of 2555(16) eV evaluated in the Atomic Mass Evaluation 2012 [G. Audi et al., Chin. Phys. C 36, 1157 (2012)] by more than 7σ to 2833(30(stat))(15(sys)) eV/c(2). With the new mass difference it will be possible, e.g., to reach in the first phase of the ECHo experiment a statistical sensitivity to the neutrino mass below 10 eV, which will reduce its present upper limit by more than an order of magnitude.

^{163}

Dielectric measurements at very low temperature indicate that in a glass with the eutectic composition BaO-Al

Ho and

_2

^{163}

O

Dy Solves the

_3

Q

-SiO

-Value Puzzle for the Neutrino Mass Determination

_2

Evidence for a Phase Transition in Glasses at Very Low Temperature: A Macroscopic Quantum State of Tunneling Systems?

a phase transition occurs at 5.84 mK. Below that temperature small magnetic fields of the order of 10