aza 6 – vorhabenbeschreibung...right: sem (scanning electron microscope) pictures of...

SPARC TDR maXs Cryogenic Micro-Calorimeter Arrays

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maXs

Cryogenic Micro-Calorimeter Arrays for

High Resolution X-ray Spectroscopy Experiments at FAIR

Technical Design Report 2014


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Figures on cover page: Left: Cryogenic platform with long side arm housing the micro-calorimeter array

behind IR-blocking x-ray windows (D. Hengstler, C. Pies, Kirchhoff Institut für Physik, Uni Heidelberg) Right: SEM (scanning electron microscope) pictures of micro-fabricated x-ray absorber array

(A. Pabinger, Kirchhoff Institut für Physik, Uni Heidelberg )


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Summary

The new Facility for Antiproton and Ion Research (FAIR) [1, 2] will be a unique tool to study the physics of matter in extreme states. Within the SPARC collaboration [3,4], highly-charged, heavy ions can be investigated at kinetic energies ranging from the relativistic regime in accelerators and storage rings down to almost rest in ion traps. For atomic structure studies of such systems high precision x-ray spectroscopy using the well-established but still challenging techniques of crystal spectrometers is a very important tool. However, the very low efficiency combined with the small accessible spectral bandwidth which is typical for crystal spectrometers prove also that their application in experiments is rather limited to very dedicated cases. The recent development of novel metallic magnetic calorimeters such as the maXs detector (cryogenic micro-calorimeter arrays for high-resolution X-ray spectroscopy) opens unique possibilities to perform high-precision x-ray spectroscopy combined with a broad bandwidth. More specifically, cryogenic micro-calorimeters offer various advantages, such as providing simultaneously stopping power and spectral acceptance range comparable to standard semiconductor x-ray detectors combined with an energy resolution competitive to crystal spectrometers. In particular, the spectral range vastly exceeding the one of crystal spectrometers enables the simultaneous detection of a variety of spectral lines resulting in a significant reduction of potential systematic uncertainties. At the same time the small active area of individual detector pixels, typically 1mm2 and smaller, can be compensated by the use of arrays of such pixels. In the start-up version, maXs will be based on 64 pixels. The combination of energy resolution, the fast intrinsic rise time, the flexibility in the choice of absorber materials, which allows for the fabrication of detectors with large quantum efficiency and area, makes magnetic calorimeters a promising detector concept for a multitude of precision experiments within the SPARC collaboration at FAIR.


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Table of Contents I. Collaboration II. Introduction III. Physics case IV. State of the art 1. Components of maXs 1.1. System overview

1.2. maXs-20 --- Detector array for 0-20 keV photons



1.5. Quantum efficiencies

1.6. Possible absorber array designs

1.7. X-ray optics

1.8. Read-out electronics and DAQ

1.9. Common cryogenic platform

1.10. Dimensions

1.11. Beam line requirements

1.12. Future extensions of maXs

2. General infra-structure 2.1. Access to and area at the beamline

2.2. Media requirements

2.2.1. Electricity 2.2.2. Cooling water 2.2.3. Pressurized gases 2.2.4. Cryogenics 2.2.5. Exhausts

2.3. Safety

2.3.1. Radiation Protection 2.3.2. Electrical Protection 2.3.3. Cryogenics 2.3.4. Fire prevention

3. Work plan

4. References


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I. Collaboration and institutions Daniel Hengstler Christian Schötz Matthäus Krantz Jeschua Geist Dr. Andreas Fleischmann Dr. Loredana Gastaldo Prof. Dr. Christian Enss

Kirchhoff-Institut für Physik (KIP), Universität Heidelberg

Tobias Gassner Dr. Renate Märtin Dr. Günter Weber Prof. Dr. Thomas Stöhlker Friedrich-Schiller-Universität Jena Helmholtz-Institut Jena Dr. Robert Lötzsch Dr. Ingo Uschmann Prof. Dr. Eckhart Förster Friedrich-Schiller-Universität Jena On behalf of the SPARC Collaboration Contact: Dr. Andreas Fleischmann

Kirchhoff-Institut für Physik Universität Heidelberg Im Neuenheimer Feld 227 D-69120 Heidelberg Tel: ++49 6221 54-9880, -9860 Fax: ++49 6221 54-9869 Email: [email protected]

mailto:[email protected]


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II. Introduction

The new Facility for Antiproton and Ion Research (FAIR) [1, 2] will be a unique tool to study the physics of matter in extreme states. Within the SPARC collaboration [3,4], highly-charged, heavy ions can be investigated at kinetic energies ranging from the relativistic regime in accelerators and storage rings down to almost rest in ion traps. FAIR will provide beams of stable as well as instable heavy nuclides with unprecedented intensities. Combined with the strong electric fields in highly charged ions this opens the door to push atomic physics precision tests of fundamental laws to extreme limits.Highly-charged heavy ions with only one electron or few electrons left represent unique systems to study relativistic and quantum-electrodynamic (QED) effects. Precise information on such phenomena can be revealed by high precision spectroscopy of x-ray photons emitted during recombination and/or de-excitation processes, which occur when heavy ions interact with a super-imposed electron beam or a gas jet target [5]. Such studies were successfully performed in the ESR storage ring of the existing GSI facility (see e.g. Ref. [6]) and are planned to be extended at the future FAIR facility. Here new physics phenomena at very high relativistic energies can be addressed at the HESR [7,8] while at the same time measurements at low kinetic energies down to rest can be performed at the newly implemented CRYRING [9] and the HITRAP facility [10]. X-ray spectroscopy experiments at these facilities are an important part of the research program of the atomic physics collaboration SPARC. A special emphasis is given to high-precision measurements of K-shell transitions in few-electron, high-Z systems which emit hard x-rays ranging up to 100 keV, as well as the L-shell and intra-shell transitions with photon energies between a few keV up to 30 keV.While precision x-ray spectroscopy became a standard tool in many atomic and solid state physics experiments, where intense sources and samples at rest are used, measurements in the context of stored ions at relativistic energies are still challenging and in many cases hardly accessible by conventional detection schemes. This is in particular due to two facts: Firstly, the comparably low intensity of the emitted photons combined with the typically small solid angle covered by conventional high-resolution detectors results in very low count rates and careful coincidence measurements are needed to discriminate events of interest from background. Secondly, in case that the ions are stored at relativistic velocities in a storage ring, the observed photons have a direction dependent Doppler shift, and the geometry of the ion beam, the target and the detector geometry needs to be chosen carefully in order to minimize Doppler-broadening [11,12].

In this regard the recent development of novel metallic magnetic calorimeters such as the maXs detector (cryogenic micro-calorimeter arrays for high-resolution X-ray spectroscopy) discussed in this report promise to be particularly well suited for these studies. These cryogenic micro-calorimeters can meet the various demands on the x-ray detectors in a unique way, as they can provide simultaneously stopping power and spectral acceptance range comparable to standard germanium detectors combined with an energy resolution competitive to crystal spectrometers. The relatively small area of single micro-calorimeters, which is of the order of 1 mm2, will be compensated by the use of arrays of 64 such pixels. In addition, the use of x-ray focussing optics is envisaged which can improve the solid-angle coverage for a given energy interval by up to two orders of magnitude. Moreover, it is possible to extract also timing information which allows for coincidence measurement schemes and the dark count-rate is expected to be negligible in most of the planned experiments. One particular benefit for the experiments will be that magnetic calorimeters cover at any instant the complete energy range from zero up to a design dependent maximum energy. This spectral range vastly exceeds the one of crystal spectrometers and allows for the simultaneous detection of a variety of spectral lines and therefore for a significant reduction of potential systematic uncertainties. Summarizing, the combination of energy resolution, the fast intrinsic rise time, the flexibility in the choice of absorber materials, which allows for the fabrication of detectors with large quantum efficiency and area, makes magnetic calorimeters a promising detector concept for a multitude of precision experiments within the SPARC collaboration at FAIR.


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III. Physics case At the accelerator facility FAIR (Facility for Antiproton and Ion Research) the investigation of extreme atomic conditions becomes accessible using highly-charged, heavy ions over an energy range from the highly relativistic regime down to rest. FAIR will provide the highest intensities of relativistic beams of both, stable and unstable heavy nuclei, which in combination with the strongest electromagnetic fields of high-Z ions will allow the extension of atomic spectroscopy up to the limits of atomic matter. The wide range of ion energies that will become available is demonstrated in Fig. 1. It can be seen, that in the different accelerator structures of the planned facility the ions, after having stripped off all or most of their electrons, can be decelerated basically to rest.

Fig. 1. The left hand scale gives the ion energies for bare uranium (U92+) and the corresponding Lorentz factors γ that can be obtained with the different components of the FAIR facility [3]. The electric field strengths reaches in collisions of bare heavy ions up to 1020 V/cm, in bound states between 1010 and 1016 V/cm while in intense laser fields about 1012 V/cm.

At FAIR, different scenarios for spectroscopic experiments of the atomic structure and dynamics of heavy ions with highest resolution are foreseen. One class of experiments will utilize high-energy ion beams in the HESR, colliding with high-density targets at maximum luminosity, however at the price of strong influences and corrections due to the Doppler-effect. Another class of experiments will utilize highly-charged ions, which after deceleration in several components of FAIR are trapped and cooled almost at rest in the ion-trap facility HITRAP. Under these circumstances, the influence of the Doppler Effect can be neglected, however at the price of luminosity and thus of count-rate.

The detector system maXs will be particularly interesting for precision atomic structure studies of few-electron heavy ions, a field being still largely unexplored. Despite the enormous success of Quantum Electrodynamics (QED) in predicting the properties of electrons in weak electro-magnetic fields, a precise test in the strong-field limit where novel phenomena might show up, is still pending. Accurate measurements of electron binding energies are very well suited to deduce characteristic QED phenomena in strong fields. For example, recent crystal spectrocopy studies by the FOCAL collaboration [13] as well as measurments using microcalarimeters [14] are both aiming for an estimate of the 1s lamb shift in hydrogen-like heavy ions with an uncertainty of less than 1 eV to probe second-order QED corrections at high-Z. Moreover, recent theoretical calculations have shown that from a comparison of the experimental results for hydrogen-like and helium- or lithium-like ions, the uncertainty of the nuclear size correction can drastically be reduced and the full experimental accuracy can be exploited for QED tests aiming for very high precision [12,15].

One prominent example for high-precision experiments is the study of parity violation in atomic systems which can uniquely be performed using helium-like ions, because calculations show, that for isotopes of several nuclear charges Z levels of opposite parity are almost degenerated. The most advantageous situation occurs in heavy helium-like ions near Z = 64 and Z = 92, due to almost degenerated 2 3P0 and 2 1S0 states with opposite parity. In atoms with non-zero nuclear spin the hyperfine and weak quenching effects are mixed. For


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polarized ions, this leads to an unusually large asymmetry of the emission pattern as well as to modified linear polarization properties of the delayed decay photons which can be measured in beam-foil type experiments.The potential of the new GSI facility for these studies is obvious since the energy splitting between the 3P0 und 1S0 states depends on the nuclear size and can be minimized by selecting the appropriate isotope. Thereby the parity violating effects can be amplified strongly so that it exceeds the usual influence by more than four orders of magnitude [16, 17, 18]. The prerequisite for this type of experiments is a polarized ion beam. Quite recently, promising schemes for the polarization of stored highly-charged ions and of the measurement of the degree of the polarization have been presented [19, 20]. This scenario will enable high-accuracy experiments in the realm of atomic and nuclear physics, as well as highly-sensitive tests of the Standard Model.

For laser spectroscopy, the new accelerator complex at GSI will enable an important step by the large increase of the photon frequency range due to the large Doppler shift (2γ for counter propagating laser beams) and by allowing spectroscopy for a wide variety of radioactive beams that is not available otherwise. A singular opportunity is given by the combination of the SIS 12, SIS 100 or the HESR with the PHELIX laser facility. In contrast to the typical experimental situation in conventional single pass accelerator experiments using solid targets (see e.g. Ref. [21]), the storage ring provides precise control of the initial ion species and diagnostics of the final states of the ions on the single-event level. This will enable research at truly undisturbed single-ion condition, where the only interacting partners will be the laser field, the highly charged ion, and the detached electron. One may note, that currently the development of novel XUV laser sources with high repetition rate is rapidly progressing which will be ideally suited for combined laser/ion experiments at the storage rings at FAIR.

In general, the FAIR facility with its intense heavy ion beams, in combination with novel experimental techniques such as excitation by X-ray or laser photons, mono-energetic electron beams and high-resolution spectrometers gives world-wide unique opportunities for atomic spectroscopy. This will enable the exploration of the fundamental QED corrections to binding energies, magnetic moments, and the magnetic interactions in strong fields.

All the experiments mentioned so far would benefit greatly from precision x-ray spectrometers with the following properties:

• Capability for simultaneous photon spectroscopy in the energy range between a few keV and more than 100 keV

• High energy resolution, ranging from about a few eV at energies below 5 keV to about 50 eV at energies above 60 keV

• Capability to measure fast coincidences between photons and down- or up-charged ions

• High compactness for a fast and easy installation at the different locations of the FAIR facility

In principle three type of detector/spectrometer systems are suitable for x-ray spectroscopy of highly charged ions.

Firstly, standard solid state detectors like Ge(i) or Si(Li) detectors provide a relatively large active area and a reasonable time resolution (about 20 ns). However, the moderate energy resolution does only allow for accurate spectroscopy of x-ray lines which are well separated from neighboring transitions. Usually, one has to deal with a line blend which prevents an accurate determination of line centroids. Secondly, crystal spectrometers, with a superior resolving power by at least an order of magnitude compared to standard solid state detectors. Because in case of crystal spectrometers the photon detection is accomplished by solid state detectors (e.g. position sensitive germanium detectors), they combine excellent energy resolution and good time sensitivity. However, the acceptance range for the wavelength regime is very narrow. As a


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consequence only a narrow energy range around the transition of interest can be accessed. Also, the geometrical restriction of such spectrometers prevents a flexible use. Thirdly, micro-calorimeters have an energy resolving power comparable to the one of crystal spectrometers. These systems combine the advantage of a high energy resolution with the one of a large wavelength acceptance range. This is of particular relevance when dealing with fast ion beams because it allows for an internal calibration of the Doppler shift. As an example, the Lyman transitions in H-like systems are in particular suited for a measurement of QED and nuclear correction effects on the electronic binding energies. Simultaneously one always observes Balmer lines with strong transitions into the 2p3/2 state which are much less affected by QED corrections or nuclear effects. Therefore, high resolution measurements of these shifted, but known, lines can be used to determine the Doppler correction with high precision. Compared to other micro-calorimeter concepts, the intrinsic signal rise-time of metallic magnetic calorimeters (MMC) of τ0 < 100 ns is very fast [27], allowing for coincidence measurements similar to Ge or Si(Li) detectors to suppress background contributions.

IV. State of the art The development of calorimetric particle detectors operated at temperatures below 1 Kelvin started about 30 years ago. Ever since, one of the driving forces for this development work has been to combine the excellent resolution of crystal spectrometers (which have a very limited wavelength acceptance range) with the large acceptance range of Ge- or Si(Li)-detectors (which have poor energy resolution).

The presently leading micro-calorimeter concepts make use 3 different types of temperature sensors: Superconducting transition-edge sensors (TES), highly-doped semiconductor thermistors (NTD, and implanted silicon) and paramagnetic alloys (MMC). The detection principle and the properties of these three techniques are reviewed in detail in [23]. With all three concepts an energy resolution below 3eV for soft x-rays has been demonstrated so far. Recently TES and MMC based recently left NTD based ones behind by demonstrating e.g. an energy resolution of 1.6 eV (FWHM) for x-rays up to 6 keV.

Micro-calorimeters based on metallic paramagnetic temperature sensors differ from TES- and NTD-based in particular in three points which are of relevance for the proposed x-ray spectrometer. i) The intrinsic rise time is 1 to 3 orders of magnitude faster compared to presently discussed TES or NTD detectors. ii) There is no abrupt saturation at the upper end of the acceptance range, as exists in the case of TES sensors when the temperature exceeds the transition range. iii) The properties of the detectors are well explained by standard thermo-dynamics [24-26], which allows for numerical optimization of detector designs for photons of different energy [23, 27].

Magnetic calorimeters consist of a metallic absorber for the particles to be detected and a metallic paramagnetic temperature sensor that is placed in a weak magnetic field and coupled to a thermal bath, as depicted in figure 2. By monitoring the magnetization of the sensor with a low noise, high bandwidth SQUID magnetometer one obtains an accurate measure of the temperature and therefore of the energy content of the calorimeter.


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Fig. 2: Scheme of a magnetic calorimeter. The detector consists of a metallic absorber for the particles to be detected and a metallic paramagnetic temperature sensor that is placed in a weak magnetic field and coupled to a thermal bath

Operated at low enough temperatures (T < 50 mK) the sensitivity of the set-up is well suited to detect single photons with high energy resolution. At the same time the response time of the detector (signal rise time) can be arranged to be below 100 ns, because sensor and absorber are normal metals, allowing for fast thermalization even at milli-Kelvin temperatures.

Most of our presently developed MMCs consist of an x-ray absorber made of electro-deposited Au and a paramagnetic temperature sensor made of sputtered Au:Er. A superconducting meander-shaped pickup coil made of a sputter-deposited niobium is used to generate the magnetic field in the sensor volume and to pickup the change of magnetization upon the absorption of a photon. The meander is connected to the input coil of a SQUID, forming a completely superconducting circuit. An on-chip persistent current switch is used to inject the field generating current into the superconducting circuit. Reference [27] summarizes details of the presently used micro-fabrication processes.

The performance of micro-fabricated MMCs depends strongly on the thermodynamical properties of the deposited thin film materials. Of particular importance are the critical current of the niobium structures, the paramagnetic behavior of the Au:Er temperature sensor as well as the specific heat and the thermal conductivity of the x-ray absorbers. The involved deposition techniques, i.e. sputtering and electro-plating, have been continuously improved during the last decade and all relevant thermo-dynamical properties are reaching values close to the ones of the corresponding bulk materials [27], allowing for a detector performance close to optimum.

In figure 3 the SEM pictures (a-d) show some details of the presently developed detectors for soft x-rays. In most of these devices the niobium lines of the meander-shaped pickup coil are 2.5 µm wide as shown in (a). The critical current density is about a factor of two larger than required for optimal operation. The persistent current switch, labelled 'SW' in (b), is formed by a U-shaped extension of the meander-circuit that can be heated above Tc by a AuPd heater. The Au:Er sensors on top of the meanders are co-sputtered form a pure gold target and Au:Er750ppm target to adjust the Er concentration. They are connected to the thermal bath via metallic thermal links made of gold ('G' in fig. b) to tune the thermalization time (signal decay time) to 1 ms. The 5 µm thick, overhanging x-ray absorbers (fig. c and d) are produced by electroplating Au into a two-layer photo-resist mold. In recent devices the absorber is connected to the sensor only through 5 to 24 stems with 10 µm diameter, to reduce the loss of athermal phonons to the solid substrate, which otherwise would cause low energy tails of lines in x-ray spectra. The imprint of the stems in the absorber surface can be seen (d).


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Fig. 3: The SEM (scanning electron microscope) pictures (a-d) of the presently developed detectors

for soft x-rays. For details compare text

Pickup coil and Au:Er sensor of MMCs for photon energies up to 200 keV are fabricated similarly, but cover a larger area of 0.5 mm × 2 mm. The electro-deposition of the 200 µm thick Au absorbers had been a challenge for a while, which was first solved using molds formed by tall AZ-125-NXT or SU-8 photo-resist walls as shown in (e) and described in [28]. Fig. (f) shows three Au absorbers of the first 1×8 array.

Fig. 4: a) Kα line of 55Mn, as measured by a prototype MMC detector system. b) The corresponding

detector response to 4 single x-rays. The fast signal rise amounts 90 ns.

One of the MMCs for soft x-rays described above was tested with x-rays from a 55Fe source. Fig. 4a shows the measured Kα line of 55Mn and, superimposed, the natural line shape convoluted with a 2.8 eV (FWHM) wide Gaussian instrumental line shape. The detector response to 4 single x-rays is shown in figure 4b. The fast signal rise, τ0 ≈ 90 ns, is well described by the expected Korringa-relaxation time of Er in Au at the given temperature as discussed in [27].

Recently we tested first 1x16 detector arrays with improved sensor and pickup coil geometry, where the magnetic flux coupling is maximized by sandwiching the paramagnetic temperature sensor between a pancake spiral coil and a superconducting groundplane. The instrumental linewidth of these devices was found to be 1.6 eV (FWHM) for x-ray energies up to 6 keV, which represents a world record that is only matched by TES-based calorimeters of the NASA/GSFC group.


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In 2014 we performed the first two measurements at the internal gas target of the Experimental Storage Ring at GSI with a metallic magnetic calorimeter of type maXs-200 as shown in fig. 9. In one of these beam times we observed a projectile beam of bare Xe ions interacting with a Xe gas target (see preliminary spectrum in the figure below. Here we achieved an energy resolution below 60 eV from 0 keV to 60 keV. We were able to detect K-lines from differently charged Xe ions, including the Lyman series involving high n levels and could fully resolve the Ly-α-doublet in hydrogen-like Xe. The K-α doublet (as well as the K-α hyper-satellite transitions) produced by double-electron capture events are also clearly visible in the spectrum.

A description of the experimental setup and a discussion of the results is provided in [33].


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1. Components of maXs 1.1. System overview

The detector system maXs for high resolution x-ray spectroscopy consists of a number of components, which are sketched in fig. 5 below and which will be described in detail in the following sections.

Fig. 5: Sketch of the components of the detector system maXs

• 64 pixel micro-calorimeter array, one of various types (maXs-20, maXs-200,..), each being optimized for a different energy range or for mixed energy ranges. All arrays use a standardized mechanical and electrical interface to the cryogenic platform cooled by a dry 3He/4He dilution refrigerator.

• Pulse tube cooled 3He/4He dilution refrigerator (LD-250, Bluefors Oy, Fi) with

o side access port, in which the detector platform is located (compare Fig. 6). o x-ray windows in the side access-port flange of the vacuum can and in all

radiation shields (70K, 4K, < 1K) in the line of sight of the detectors.

o rack mounted 3He/4He-gas-handling system. o remotely located rotary valve mounted on the cryostat support frame to

reduce vibrations of the detector platform caused by that valve

o 4He compressor for the pulse tube cooler that provides 70K and 4K for shields and 3He pre-cooling

o temperature regulation of detector platform o slow control via TCP/IP of all parameters for standard operation

• Readout-electronics and data-acquisition mounted on cryostat support, with o 32 two-stage SQUIDs inside the cryostat o 32 low-noise high-bandwidth (6 MHz) SQUID electronics o 32 channel 16-bit ADC (125MHz for timestamp,

typ. down sampled to 1-10MHz for digital shaping / optimal filtering) o slow-control via TCP/IP of all parameters during standard operation


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8x8 pixels

Fig: 6: Cryogenic platform of maXs (see text for details).

24 cm

80 mm

40 cm


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1.2. maXs-20 --- Detector array for 0-20 keV photons Our first detector array prototype for soft x-rays was the 1×8 detector array shown in the SEM-micrograph below (Fig. 7), which, as already mentioned, performed extremely well. It is fabricated on 2'' silicon wafers in our class-100 clean-room at the KIP.

Fig. 7: Present 1×8 detector array for soft x-rays.

maXs-20 will copy the pixels of this successful design and have:

• high quantum efficiency for x-ray energies up to 20 keV.

• The energy resolution will be ∆EFWHM < 2.5 eV in the full energy band. • 8×8 x-ray absorbers

• made of gold

• active area: 250 µm × 250 µm each

• thickness of 5 µm • quantum efficiency larger than 80 % (overall) and > 98 % for E < 6keV

In our present fabrication process the x-ray absorbers are made of high-purity electro-deposited gold to provide high stopping power and fast internal thermalization.

• The analog signal rise time will be below 100 ns to allow for coincidence measurements.

• The temperature of the absorbers is monitored by planar sputter-deposited paramagnetic Au:Er sensors coupled to planar superconducting pickup coils.

• The magnetic flux signal in the pickup coil is read out by a 2-stage SQUID setup with low-noise high-bandwidth flux-locked-loop electronics.

• The signal decay time will be tuned by a metallic thermal link between the temperature sensor and the thermal bath to be between 1 ms and 3 ms.

The detector is optimized to be operated at a cryostat temperature of 30 mK. The detector chip holder, its wiring and pin-layout is made to fit to the detector platform of the cryostat. It is magnetically shielded by vacuum can of the side arm, which is made of mu-metal, and a superconducting can mounted on the cold detector platform that works reliably for external magnetic stray fields up to 5 mT.


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1.3. maXs-30 --- Detector array for 0-30 keV photons maXs-30 will be the first 2-dimensional maXs array to be available. It is presently being micro-fabricated and expected to yield in October 2014

Fig. 8: Superposition of the design of the photolithography masks of maXs-30 with (left) and

without (center) the x-ray absorber layer. The photograph on the right shows a 2’’ silicon wafer with 16 maXs-30 chips during fabrication.

maXs-30 will have:

• high quantum efficiency for x-ray energies up to 30 keV.

• The energy resolution will be ∆EFWHM < 10 eV in the full energy band. • 8×8 x-ray absorbers

• made of gold

• active area: 500 µm × 500 µm each

• thickness of 30 µm • quantum efficiency larger than 80 % (overall) and > 98 % for E < 20keV


• The analog signal rise time will be artificially slowed down to 1 µs by introducing stems and a thermal bottleneck between the absorber and the sensor in order to suppress the dependence of the detector response on absorption position. With improving the quality of the deposited absorber material, i.e. its thermal diffusivity, we will adjust this rise time closer to the intrinsic rise time of below 100ns.



• The signal decay time will be tuned by a metallic thermal link between the temperature sensor and the thermal bath to be 3 ms.



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1.4. maXs-200 --- Detector array for 0-200 keV photons The detector array maXs-200 was studied by fabricating a 1×8 detector prototype array for hard x-rays. The left side of Fig. 9 below shows a schematic drawing of this prototype, including the superconducting pickup coils, the paramagnetic temperature sensors (Au:Er) and the on-chip persistent current switches. The SEM-micrograph on the right side of Fig. 8 shows three of the 200 µm thick x-ray absorbers made of electro-deposited gold.

Fig. 9 (left): Schematic drawing of this prototype, including the superconducting pickup coils, the paramagnetic temperature sensors (Au:Er) and the on-chip persistent current switches. (right): The SEM-micrograph shows three of the 200 µm thick x-ray absorbers made of electro-deposited gold. MaXs-200 is fabricated on 2'' sapphire wafers in our class-100 clean-room at the KIP. The sapphire substrate is stronger than silicon and provides a somewhat better match to the thermal expansion of the gold absorbers leading to a reliable configuration to survive the thermal stress during cool-down.

• maXs-200 has high quantum efficiency for x-ray energies up to 200 keV

• The energy resolution is ∆EFWHM < 50 eV in the full energy band

• 8×8 x-ray absorbers

• made of gold

• active area of 1 mm × 1 mm

• thickness of 200 µm • quantum efficiency larger than 30 % for E < 150keV

and > 90 % for E < 50 keV.


• The analog signal rise time will be artificially slowed down to 5 µs by introducing stems and a thermal bottleneck between the absorber and the sensor in order to suppress the dependence of the detector response on absorption position. With improving the quality of the deposited absorber material, i.e. its thermal diffusivity, we will adjust this rise time closer to the intrinsic rise time of below 100ns.


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• The signal decay time will be tuned by a metallic thermal link between the temperature sensor and the thermal bath to be 5 ms.


1.5. Quantum efficiency As already mentioned the thickness of the x-ray absorbers of the different detector arrays maXs-20,-30,200 was chosen to allow for a large quantum efficiency at differently high x-ray energies. Fig. 10 shows the full energy stopping power of maXs-20 and -200 as simulated with PENELOPE for the designed thicknesses.

Fig. 10: (left) Full energy stopping power of maXs-20 with a 5 μm thick x-ray absorber made of gold and (right) the one of maXs-200 with 200 μm thick gold, as calculated with PENELOPE, from [32].

For experiments that aim for intensities or intensity ratios of lines, those monte-carlo simulations should be performed with the dimensions of used devices. In particular at energies below about 1 keV the absorption of all x-ray windows and IR-filters in the line of view needs to be characterized and taken into account. At present the x-ray window in the vacuum can of the cryostat is planned to be of type Moxtek AP-5, and there are two IR-filters made of 200 nm thick Aluminum at 50 K and 4 K, which have high and well characterized x-ray transmission. The absorption of windows on the source side and the air between source and detector needs to be taken into account in addition.


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1.6. Possible absorber array designs In order to increase the solid angle covered by the detector and to reach a reasonable overall quantum efficiency and count rate, maXs makes use of an array of 64 x-ray absorbers which are read-out by 32 electronic channels. This array size is large enough for a variety of planned experiments and can be increased, if required by future experiments. The detector arrays of maXs will be operated in a pulse-tube-cooled 3He/4He-dilution refrigerator. This "dry" cryostat does not need any maintenance with cryogenic liquids. In standard operation all slow control concerning the cryostat and the detectors can be done remotely via TCP-IP, which is essential for measurements in restricted areas such as storage rings.

The future maXs detector system has the great advantage of a flexible/variable detector array design, which can be adapted to the specific needs of various experimental scenarios. The main investment of the complete system is the cryostat working without cryogenic liquids which can be transported to different experimental areas. The detector array itself can be mounted on a holder which provides the temperature of 30mK and allows the use of different array designs. For example a possible design would be an array maXs which consist of two sub-arrays, of the prototype maXs-20 and maXs-200, in order to make it a versatile instrument for a large number of spectroscopy experiments, where x-ray energies can reach up to 150 keV for K-shell transitions, while the transitions to higher shells e.g. in few-electron systems, can be well below 20 keV. The existing prototype detector arrays maXs-20 and maXs-200 are optimized for two different energy ranges and have different absorber thicknesses to provide quantum efficiency close to unity in both cases.

A possible design for an initially feasible experiment e.g. at the ESR or CRYRING is a combined detector array of two different kinds of pixels allowing the study of Balmer and inner-shell transitions in highly charged one or few electron ions as discussed in [29]. Fig. 11 (left) shows a sketch of such an array, where maXs-20 type pixels are located in the centre of the detector array to cover the focus area of an x-ray lens for soft x-rays. These high resolution pixels will be surrounded by medium sized pixels with an resolution of approx. 14 eV. These more efficient pixel types will be the main working horse for energies up to 30 keV. They are similar to the discussed maXs-30 pixels, but have a 4 times larger area of 1mm×1mm. Figure 10 (right) shows amongst others the spectra simulated for a possible experimental scenario using H-like Uranium. In this example the photon spectra are observed at medium ion energy (68 MeV/u) using the typical target and ion beam geometries at the current ESR target chamber.


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Fig. 11. Left: maXs detector array with mixed pixel sizes, as described in the text. Right: Photon spectra measured with a standard single germanium detector (grey spectra) and corresponding identified Balmer transition lines taken from [30]. The red photon spectra was simulated for the same experimental parameters at the ESR internal gas jet target for a medium sized micro-calorimeter pixel with a resolution of 14 eV.

Another currently discussed micro-calorimeter system is a novel high precision Compton polarimeter for the energy range from 10 to 30 keV. This kind of detector system is subject of a recently accepted BMBF grant. The so-called polar-maXs detector consists e.g. of a cylindrical active low-Z scatterer and around a ring of individual high-Z ( Au) absorber plates (see Fig. 12).

Fig. 12: Possible geometry of micro-calorimeter used as a Compton polarimeter.


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1.7. X-Ray Optics In some of the planned experiments the photon flux is expected to be extremely low. Such experiments could benefit greatly from refocusing x-ray optics. At present several options are discussed for the study of x-ray transition in the energy range from 7-15 keV using cylinder rings or ellipsoid crystals. An array of grazing incidence mirrors with small curvature was already tested in our detector system for the Heidelberg EBIT. Moreover, an x-ray optic based on a HAPG crystal ring has just been developed at the University of Jena which provides an enhancement of solid-angle coverage by up to two orders of magnitude for x-ray energy range between 7 and 12 keV [31].

Fig. 13: Left side: HAPG focusing x-ray optic as has been developed at the University of Jena. Right side: Principle of an x-ray optics for the efficient transport

of x-ray radiation from a source to the detector [31]. 1.8. Read out electronics and DAQ All discussed micro-calorimeter arrays fit into the same detector platform of the cryostat and can be read out by the same 2-stage SQUID setup, which will be installed on the detector sidearm at the mixing chamber of the cryostat.

The magnetic flux signal of the 64 micro-calorimeters is read out by 32 electronic channels. In each channel, two micro-calorimeters with the corresponding pick-up coils for a thermal gradiometer, where the pickup coils are connected in parallel to the matched input coil of a dc-SQUID. The signal of this primary SQUID is amplified at low temperature by a series-SQUID-array and serves as input signal of a low-noise high-bandwidth flux-locked-loop (FLL) electronics at room temperature with flux feedback to the primary SQUID.

The analog output signals of the 16 FLL electronics are proportional to the magnetic flux in the 16 corresponding pickup coil pairs and are digitized by a 16-channel 16-bit ADC with a sampling-rate of 125 MHz and bipolar digital triggering capability. The triggered, digitized and down-sampled traces are transferred to a standard PC, where an algorithm based on the idea of 'optimal/matched filtering' assigns a photon energy to each trace.

The initialization of the detectors, i.e. in particular the tuning of the SQUIDs and the initialization of the optimal filter will be automized as far as possible. The supervision of the initialization, manual initialization and the slow control during standard operation can be done remotely via TCP/IP.


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1.9. Common cryogenic platform All proposed micro-calorimeter arrays will be mounted and operated in the same cryostat, which provides the necessary temperature of 30 mK and cooling power (> 1µW).The cryostat has a side access port that houses the side arm with detector platform.

Among the presently commercially available cooling techniques, pulse-tube cooled (PTC) 3He/4He-dilution refrigerators (DR) fulfil the requirements on the detector platform of maXs best. The closed-cycle pulse-tube cooler of this type of cryostat provides a 50K-stage and a 4K-stage to cool radiation shields and to pre-cool the 3He of the dilution unit, which allows for long continuous measurement times without maintenance and refilling of cryogens, as necessary when operated at the storage ring.

The first x-ray spectrometer of type maXs will be installed in an LD-250 cryostat from Bluefors Oy, Finland, which was provided to the project by the Helmholtz Gesellschaft. Fig. 14 shows a technical drawing of the cryostat without side arm, as produced by the company. It provides a base temperature below 10 mK and a large enough continuous cooling power of several micro-Watt at the operational temperature of the detectors.

The pulse-tube is driven by a 10 kW water-cooled He-compressor. The distance between compressor and rotary valve at the cryostat can be as long as 20 meters. Other pumps and electronics for operation and temperature regulation, all rack-mounted, amount to another 10 kW. It can be controlled remotely. The time for cool-down is about 24 hours.

All necessary wiring for the detectors, the installation of the SQUIDs and x-ray windows as well as the design and the installation of the detector platform including the superconducting shields is done within one of the present BMBF projects. Special care will be taken for mechanical isolation and for the filtering of cables that enter the cryostat, in order to minimize emi with high-frequency signals in the vicinity of the storage ring.


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Fig. 14: Technical drawing of a LD-250 dilution cryostat from BlueFors, Finland, with custom designed side access port and support frame for maXs.


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1.10. Dimensions

Cryostat-frame:

110cm×82cm×172cm (width×depth×height)

mass < 400 kg

Gas handling and control


mass < 400 kg 4He compressor


mass < 150 kg

max. distance from cryostat frame: 10 m

1.11. Beamline requirements MaXs is a versatile detector system and foreseen to be used at different facilities within FAIR including the internal target. The design of the interaction chamber will take the geometrical requirements of maXs into account. It would be particularly useful, if no static magnetic stray fields larger than 5mT are present at the position of the detector platform and if fluctuating magnetic stray fields (power supplies, turbo pump controllers) could be avoided in close vicinity, conditions which can ideally be fulfilled at the internal target sections of the storage rings CRYRING, ESR, HESR.

1.12. Future extensions of maXs maXs as presented so far already opens the door to a new era in high resolution x-ray spectroscopy. But we believe, that the present fast progress in micro-fabrication of the micro-calorimeters as well as the superconducting readout-electronics will allow for substantial changes of a number of essential parameters like pixel-count, active area, resolving-power and time resolution. Precision atomic physics and benchmark tests of QED in high fields will benefit greatly from the following list of extensions to maXs, where each listed project is already backed by promising pre-tests that suggest that the proposed R&D projects will successfully lead to a substantial upgrade of the detector infrastructure at FAIR.

• Microwave-SQUID-multiplexer to reach pixel-counts beyond 1000 The proposed read-out scheme will use only 2 coaxial cables, one HEMT amplifier at 4 Kelvin and fast analog/digital-electronics to read out 1000 detectors, each being coupled through a RF-SQUID and a micro-fabricated λ/4 resonator with unique frequency coupled to a single transmission line. The project will require the collaboration of two strong, the group of C. Enss (U Heidelberg) for the superconducting electronics and the group of U. Kebschull (U Frankfurt) for the analog/digital read-out at room temperature. […] Financial details were removed in this public version of the TDR.


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• Microcalorimeters with massive calorimeters for photon energies up to 500 keV FAIR will provide a number of experimental settings for high resolution x-ray spectroscopy that have not existed before. Among these is the possibility to do spectroscopy on ions stored in the HESR at relativistic velocities, such that the Doppler-shift cam boost the energy of photons emitted in forward direction up to 500keV in lab frame. A magnetic micro-calorimeter with massive photon absorbers will have superior resolving power. The absorbers of such a detector array will most likely be made of bulk material and the processes to include those into the micro-fabricated detectors needs to be developed. The project will require: […] Financial details were removed in this public version of the TDR.

• maXs@0°, detector arrays under 0°/180° in CRYRING for high resolution x-ray

spectroscopy

Doppler-broadening can be minimized and effective solid angle can be increased when detecting the photons that are emitted in forward or backward direction. However this arrangement comes along with additional challenges in cryogenics and detector design. So far, most experiments did not benefit fully from the excellent energy resolution of micro-calorimeters, as the accessible detection angle between 45° and 135° came along with a Doppler-broadening larger than the instrumental linewidth and inacceptably large systematic Doppler-shift uncertainties. The proposed project will help to overcome all these issues. The compact size of CRYRING and its segments will allow to place cryogenic micro-calorimeters at the first dipole magnet before and after the interaction zone, i.e. under 180° and 0°, while keeping the distance between source and detector at a tolerable level. Fig.15 shows a sketch of the relevant region around the electron cooler together with the dry 3He/4He-Dilution fridge of maXs and its cooling finger, both redesigned for an installation perpendicular to the beam.

Fig. 15: Electron-cooler of CRYCRING together with the

micro-calorimeter maXs-20/200 to be developed.

The project will require: […] Financial detailswere removed in this public version of the TDR.


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2. General infra-structure 2.1. Access to and area at the beamline During installation of maXs at the internal target access to this region is mandatory. During normal operation / data taking all necessary slow control can be done remotely via TCP/IP and no access is foreseen. At all locations where maXs is to be used, space for the components listed in 1.9 is required, most of which within a circle of about 5m around the cryostat. 2.2. Media requirements 2.2.1. Electricity 4He compressor 3-phase, 10 kW, maintenance interval 30 000 h gas handling and pumps 220 V, 2×16A single phase electronics, DAQ 220 V, 1×16A, single phase 2.2.2. Cooling water 4He compressor (CryoMech CP2880 for PT410RM) temperature non-condensing to 25 °C

flow 7-9 l/s, typ 15L/min at 3.5bar pressure drop

max power 10 kW 3He turbo pump (Pfeiffer HiPace400)

temperature non-condensing to 25 °C flow typ 2L/min at 3.5bar pressure drop

max power 500 W 2.2.3. Pressurized gases Pressurized air pressure: 5 bar

flow: < 10 liters per day, just used to actuate valves and operate air springs

The pulse tube cooler is operated with a closed 4He circuit at a pressure of 25 bar. 2.2.4. Cryogenics The pulse tube cooled dilution refrigerator is a closed cycle cooler and does not require the transfer of cryogenic liquids. In case of subtle leaks in the 3He circuit, a liq. N2 cold trap might serve as temporary solution. 2.2.5. Exhausts

During standard operation no exhausts are required. During cool-down (24h-48h) the system gets evacuated and an exhaust line for the vacuum pump is desirable but not neccessary.


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2.3. Safety 2.3.1. Radiation Protection No lasers involved. No radioactive sources required or presently planned. However, depending on the photon flux in one of the future experiments a weak x-ray source in the line of sight of the detector might prove useful for long time drift corrections.

2.3.2. Electrical Protection No high voltage components involved. The observance of VDE or EU regulations is self-evident. 2.3.3. Cryogenics No handling of cryogenic liquids required. The total volume of cryogenic liquids in the system during normal operation is less than 1 liter and the cryostat is equipped with over-pressure valves at all necessary positions. 2.3.4. Fire prevention

For fire prevention it is recommended to use only flame resistant materials. Especially the cables should be flame resistant and should not contain any PVC


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3. Work plan A preliminary time schedule for the fabrication and installation, test and first experiments of combined detector array design is summarized in the following table. Experiments at the first storage ring of FAIR, CRYRING, can be already performed by the end of 2017. Before first experimental runs at the ESR storage ring are most probable. Tabel 1: Workplan


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Group members at KIP and there responsibility within the project: Dr. A. Fleischmann (scientific leader), Dr. Loredana Gastaldo (scientific leader), Prof. Dr. C. Enss (scientific leader),

Daniel Hengstler (PhD student), Christian Schötz (PhD student), Matthäus Krantz (PhD student), Jeschua Geist (PhD student), Several diploma/master students:

Numerical optimization of sensor parameters, designing mask layouts for the micro fabrication, micro-fabrication processes in the cleanroom, design of mechanical supports, system integration, operation of cryostats, test measurements, detector characterization, software development, data taking and data analysis.

Dipl. Ing. T. Wolf (Cleanroom facilities): Operation of all micro-fabrication facilities (Sputtering, thermal evaporation, laser-lithography, plasma-etching, wafer saw, etc.)

The project will in addition be supported by manpower from the institute’s technical support groups, i.e. helium liquification, the machine shop for mechanical parts, the electronics lab and the IT group.

Available equipment / infrastructure at the KIP, that is essential for the project One 3He/4He-dilution refrigerator and one adiabatic demagnetization cryostat with cables for SQUID operation and high quality temperature stabilization to test the detectors at milli-Kelvin temperatures before the full installation of the ordered cryostat is finished. Class-100-cleanroom with laser lithography system for mask writing, five-gun UHV DC/RF-sputtering system, plasma etching system, e-beam evaporator, wafer saw, wedge bonder, ball bonder, spin-coater, scanning electron microscope, high temperature oven. All listed cleanroom components will be needed for the micro-fabrication of the detector arrays.


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4. References [1] W.F. Henning, The future GSI facility, Nucl. Instr. Methods Phys. Res. B 214, 211 (2004). [2] FAIR Green Paper: The Modularized Start Version 2009 (www.fair-center.de/fileadmin/fair/publications FAIR/FAIR GreenPaper 2009.pdf). [3] R. Schuch al., 2005 SPARC Technical Proposal (www.gsi.de/fileadmin/SPARC/documents/sparc-technical-proposal_print.pdf). [4] Th. Stöhlker et al., SPARC: The Stored Particle Atomic Research Collaboration At FAIR, AIP Conf. Proc. 1336, 132–7 (2011). [5] G. Grisenti et al., Design of the Target@HESR (2013) (https://www.gsi.de/fileadmin/SPARC/documents/HESR/[email protected]). [6] J. Eichler and Th. Stöhlker, Phys. Rep. 439, 1–99 (2007). [7] Th. Stöhlker et al., SPARC Experiments at the HESR: A Feasibility Study (2012) (www.gsi.de/fileadmin/SPARC/documents/SPARC@HESR FS V26.pdf). [8] Th. Stöhlker et al., SPARC experiments at the high-energy storage ring, Phys. Scr. T156, 014085 (2013). [9] M. Lestinsky et al., CRYRING@ESR: A study group report, (2012) (https://www.gsi.de/fileadmin/SPARC/documents/Cryring/ReportCryring_40ESR.PDF). [10] H. J. Kluge et al., HITRAP: A facility at GSI for highly charged ions", Advances in Quantum Chemistry, Vol 53 53, 83-98 (2008). [11] Th. Stöhlker et al., 1s lamb shift in hydrogenlike uranium measured on cooled, decelerated ion beams, Phys. Rev. Lett. 85, 3109-3112 (2000). [12] A. Gumberidze et al., Quantum electrodynamics in strong electric fields: The ground-state lamb shift in hydrogenlike uranium", Phys. Rev. Lett. 94, 223001 (2005). [13] S. Chatterjee et al, The FOCAL spectrometer for accurate X-ray spectroscopy of fast heavy ions. Nucl. Instr. Meth. Phys. Res. B. 245, 67–71 (2006). [14] S. Kraft-Bermuth et al., High-precision x-ray spectroscopy of highly charged ions with microcalorimeters. Phys. Scr. 014022 (2013). [15] V. M. Shabaev et al., g factor of high-Z lithiumlike ions Phys. Rev. A 65, 062104 (2002). [16] L.N. Labzowsky et al.,Parity-violation effect in heliumlike gadolinium and europium Phys. Rev. A 63, 054105 (2001). [17] R. W. Dunford, Parity nonconservation in high-Z heliumlike ions Phys. Rev. A 54, 3820 (1996). [18] A. Schäfer et al., Prospects for an atomic parity-violation experiment in U90+, Phys. Rev. A 40, 7362 (1989).

http://www.fair-center.de/fileadmin/fair/publications%20FAIR/FAIR%20GreenPaper%202009.pdf

http://www.fair-center.de/fileadmin/fair/publications%20FAIR/FAIR%20GreenPaper%202009.pdf

http://www.gsi.de/fileadmin/SPARC/documents/sparc-technical-proposal_print.pdf

https://www.gsi.de/fileadmin/SPARC/documents/HESR/[email protected]

https://www.gsi.de/fileadmin/SPARC/documents/Cryring/ReportCryring_40ESR.PDF


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[19] J. Gunst et al., Parity-nonconservation effects on the radiative recombination of heavy hydrogenlike ions, Phys. Rev. A 87, 032714 (2013). [20] R. Märtin et al., Polarization Transfer of Bremsstrahlung Arising from Spin-Polarized Electron, Phys. Rev. Lett. 108, 264801 (2012). [21] R. Anholt and W.E. Meyerhof, Atomic collisions with relativistic heavy ions. V. The states of ions in matter, Phys. Rev. A 33, 1556–1568 (1986). [22] L. Fleischmann et al., Metallic Magnetic Calorimeters for X-ray Spectroscopy, IEEEm European Supercond. News Forum 7 (2008). [23] Cryogenic Particle Detection, (ed. C. Enss), Topics Appl. Phys. 99 (2005) [24] A. Fleischmann et al., Low Temperature Properties of Erbium in Gold, J. Low Temp. Phys. 118, 7-21, (2000). [25] T. Herrmannsdörfer, R. König, C. Enss, Properties of Er-doped Au at Ultralow Temperatures, Physica B 284-288, 1698-1699 (2000). [26] C. Enss, et al., Metallic Magnetic Calorimeters for Particle Detection, J. Low Temp. Phys. 121, 137-177 (2000). [27] A. Fleischmann et al., Metallic magnetic calorimeters, AIP Conf. Proc. 1185, 571 (2009). [28] C. Pies et al., MMCs for high precision QED tests at GSI/FAIR AIP Conf. Proc. 1185, 603 (2009). [29] E. Silver et al., Using a microcalorimeter to measure the Lamb shift in hydrogenic gold and uranium on cooled, decelerated ion beams. Nucl. Inst. Meth. Phys. Res. A 520, 60–62 (2004). [30] P.H. Mokler et al., The x-ray spectrum of H-like uranium. Zeitschrift für Physik D 35, 77-80 (1995). [31] R. Lötzsch, I. Uschmann, E. Förster et al., to published. [32] C. Pies et al., maXs: Microcalorimeter Arrays for High-Resolution X-ray Spectroscopy at GSI/FAIR, Journal of Low Temperature Physics 167 (3/4), 269 (2012) [33] D. Hengstler, M. Keller, C.Schötz, J. Geist, M. Krantz, S. Kempf, L. Gastaldo, A. Fleischmann, T. Gassner, G. Weber, R. Maertin, T. Stoehlker, C. Enss, Towards FAIR: first measurements of metallic magnetic calorimeters for high-resolution x-ray spectroscopy at GSI Physica Scripta T 166, 014054 (2015)

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