Silicon Microcalorimeters for Atomic Physics Experiments at FAIR V. Andrianov2,4, A. Bleile2, A. Echler1,2,3, P. Egelhof2,3, P. Grabitz2,3, C. Kilbourne5, S. Kraft-Bermuth1,2, D. McCammon6, P. Scholz1 1Institut für Atom- und Molekülphysik, Justus-Liebig-Universität Gießen, Germany 2GSI Darmstadt, Darmstadt, Germany 3Institut für Physik, Johannes-Gutenberg-Universität Mainz, Germany 4Lomonosov Moscow State University, Moscow, Russia 5NASA/Goddard Space Flight Center, Greenbelt, USA 6University of Wisconsin, Madison, USA

Outline Motivation Microcalorimeters: Detection Principle SiM-X: Silicon Microcalorimeters for X-rays The Lamb Shift Experiment at the Experimental Storage Ring (ESR) Conclusion: Status Perspectives S. Kraft-Bermuth Outline

I. Motivation X-ray Spectroscopy on Highly-Charged Ions Z=1 Eb = 13.6 eV Z·a « 1 hydrogen uranium ion Z=92 Eb = 132 keV Z·a  1 Physics: precise test of QED for Hydrogen atom: tested with highest precision (10-6)  Excellent agreement between experiment and theory for high Z: strong Coulomb fields  Zα  1: sensitive test of QED still missing S. Kraft-Bermuth Motivation

I. Motivation X-ray Spectroscopy on Highly-Charged Ions Lamb shift of 238U91+ idea of the experiment: measure transition energy of the Lyman-α line for hydrogen-like Au, Pb, U  energy resolution determines precision theoretical prediction for 238U91+: 464.3  0.5 eV (Yerochin et al., Phys.Rev.Lett 91 (2003) 073001) experimental value for 238U91+ (measured at GSI with conventional Ge detectors): 460.2  4.6 eV (Gumberidze et al., Phys. Rev. Lett. 94 (2005) 223001) to improve experimental accuracy: use detector concept of microcalorimeters (P. Egelhof et al., NIM A 370 (1996) 263) S. Kraft-Bermuth Motivation

Energy Detection: Standard Method Conventional detection method: Ionization +V Incoming particle Voltage signal - - - - + neg. electrons pos. ions + + Germanium Detector, T. Stöhlker et al. Energy loss of incoming particle caused by ionization  detection of charge Typical excitation energies: Eex ~ 1 – 10 eV S. Kraft-Bermuth Motivation

Energy Detection: Microcalorimeter Charges recombine and produce heat Some part of the particle energy is deposited directly as heat without creating charge! For high charge densities: substantial signal losses due to recombination before charges can be collected! Idea: detect heat directly Detection of phonons Heat sink Temperature signal Phonons Calorimetric detectors Typical excitation energies for phonons: Eex ~ 1 meV Incoming particle S. Kraft-Bermuth Motivation

II. Microcalorimeters: Detection Principle rise time determined by electronics signal height DT = E/C decay time: teff = C/k: 500 µs – 5 ms potential advantage: excitation energy of one phonon ~ 1 meV better statistics of detected quanta better energy resolution as compared to crystal spectrometers: large dynamic range large detection efficiency by using many detectors in parallel Microcalorimeters S. Kraft-Bermuth

Optimization of Detector Parameters Absorber material: Temperature signal DT = E/C: small heat capacity C = c ·m Debye law for insulators: c ~ (T/QD)3 for superconductors: c ~ exp(T/TC) low temperatures high Debye temperature Otherwise, absorber can be adapted to experimental requirements. TC Thermometer: Resistance thermometer: DT  DR  DU for high sensitivity: high dR/dT specially doped semiconductor (large dynamic range) or superconductor in phase transition (very high dR/dT) S. Kraft-Bermuth Microcalorimeters

III. Silicon Microcalorimeters for X-rays Energies 10 – 100 keV: TA  60 mK Absorbers: superconductors for small c high Z material Pb, Sn absorber thickness: 50 - 100 µm Thermistors: large energy range Si doped with P and B 36 pixel detector array pixel area 2 x 0.5 mm² developed and fabricated by Madison / Goddard group (A. Bleile et al., AIP Conf. Proc. 605, 2002) S. Kraft-Bermuth Silicon Microcalorimeters for X-rays

Performance of Prototype Array: Energy Spectrum Energy spectrum obtained for a 241Am source with an Sn absorber (0.3 mm² x 66 µm (A. Bleile et al., AIP Conf. Proc. 605, 2002)) DE = 65 eV energy resolution at E = 59.5 keV: DE = 60 – 65 eV for Sn and Pb absorbers for comparison: theoretical limit for a conventional semiconductor detector: ΔEFWHM  380 eV S. Kraft-Bermuth Silicon Microcalorimeters for X-rays

Comparison Between Different Detection Schemes Germanium detector Crystal spectrometer Calorimeter Energy resolution 400 eV 60 eV 60 eV Total efficiency 10-3 – 10-4 10-7 10-5 Dynamic range large small large In principle, with arrays of several hundred pixels, also for calorimeters an efficiency of 10-3 – 10-4 can be obtained. S. Kraft-Bermuth Silicon Microcalorimeters for X-rays

IV. The Lamb Shift Experiment at the Experimental Storage Ring (ESR) gas jet target Au78+ Au79+ injection of fully stripped ions, beam cooling and deceleration electron capture in the gas target detection of the Lyman-α-radiation transformation of measured Lyman-a energy into laboratory frame joint experiments with crystal spectrometer FOCAL cryostat S. Kraft-Bermuth The Lamb Shift Experiment

Experiment for Lamb Shift Measurement on 197Au78+ (2012) beam: 197Au78+ at 125 MeV/u  Doppler broadening DED = 110 eV Full array with 32 pixels  overall detection efficiency: 2.5 x 10-6 usable data from 14 pixels  effective detection efficiency: 1.1 x 10-6 Preliminary achieved energy resolution: DE = 190 eV Result: E(Ly-a1) = (71568  4stat  10syst) eV in good agreement with theoretical prediction E(Ly-a1) = 71569.7(5) eV Uncertainties limited by determination of Doppler correction S. Kraft-Bermuth The Lamb Shift Experiment

V. Conclusion: Status Silicon microcalorimeters provide excellent energy resolution of DE/E = 1 – 4 x 10-3 for the detection of X-ray photons at E = 50 – 100 keV Two arrays successfully applied in experiments at the ESR for the determination of the 1s Lamb Shift in hydrogen-like ions: overall detection efficiency ~ 10-6 energy resolution DE ~ 190 eV, mostly due to Doppler broadening precision dominated by systematic uncertainties In addition: new cryogen-free cryostat under comissioning in Gießen obtained energy resolution DE = 100 eV @ 60 keV optimization of experimental setup and energy resolution ongoing S. Kraft-Bermuth Conclusion: Status

VI. Perspectives: Improvement of Precision to improve systematic uncertainities: intrinisic Doppler correction use Balmer transition energies at E ~ 5 – 10 keV no QED corrections transition energies determined by Doppler shift add pixels with thinner absorbers optimized for energies below 10 keV decrease Doppler broadening new gas-jet target with smaller jet diameter decelerated beams (CRYRING facility at GSI) trapped ions at HITRAP facility practically at rest disadvantage: very low intensities as compared to ESR (factor 105) investigation of X-ray lenses (difficult for high X-ray energies) necessary spectroscopy at lower X-ray energies with thinner absorbers to improve statistical uncertainity and lateral sensitivity: new setup with three arrays (96 pixels total) S. Kraft-Bermuth Perspectives

Improvement of Statistical Uncertainties: New Setup with 3 Arrays 1st step: test of a new, more compact design for 32 pixels addition of low-energy pixels separated load resistor boards investigation of heat load and noise performance assembly in progress first tests expected in summer 2015 2nd step: expand this design to 3 x 32 = 96 pixels design studies in progress expected start of production in fall 2015 expand readout electronics and DAQ new JFET boards based on standard PCBs: easy design and production Technical Design Report currently under review S. Kraft-Bermuth Perspectives

First Experiment at ESR/CRYRING Decay of 1s2s2 2S1/2 in U89+ “Two Electron One Photon (TEOP)” decay predicted to be dominant decay channel M1 ~7% Auger ~21% E1(TEOP) ~71% E1 <1% 1s22s 2S1/2 1s2s2 2S1/2 1s22p 2P3/2 1s2 1S0 1s22p 2P1/2 M1 96.1 keV Auger 63.1 keV E1(TEOP) 95.8 keV E1 91.6 keV Up to now: no experimental proof of line intensities! Energy resolution not sufficient to separate neighbouring lines! (S. Trotsenko et al., J. Phys. Conf Ser. 58, 141 (2007)) S. Kraft-Bermuth Perspectives

Expected Advantage of Microcalorimeter Simulated spectrum of Li-like uranium for a Germanium detector and a microcalorimeter at an ion energy of 90 MeV/u: DEGe = 600 eV DEMC = 100 eV TEOP and M1 only with microcalorimeter clearly separated Determination of relative line intensities with high precision S. Kraft-Bermuth Perspectives

Thank you all for your attention! Summary We have a new detector array with larger solid angle and intrinsic Doppler correction in preparation. We expect first experiments on atomic physics in 2017. SiM-X will be a perfect tool for new and exciting atomic physics at FAIR. Thank you all for your attention! S. Kraft-Bermuth Summary