1. 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

2. 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

3. 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

4. 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+:  0.5 eV (Yerochin et al., Phys.Rev.Lett 91 (2003) )
experimental value for 238U91+ (measured at GSI with conventional Ge detectors):  4.6 eV (Gumberidze et al., Phys. Rev. Lett. 94 (2005) )
to improve experimental accuracy: use detector concept of microcalorimeters
(P. Egelhof et al., NIM A 370 (1996) 263)
S. Kraft-Bermuth
Motivation

5. 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

6. 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

7. 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

8. 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

9. 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: 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

10. 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

11. 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

12. 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

13. 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) = (5) eV
Uncertainties limited by determination of Doppler correction
S. Kraft-Bermuth
The Lamb Shift Experiment

14. 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 = keV
optimization of experimental setup and energy resolution ongoing
S. Kraft-Bermuth
Conclusion: Status

15. 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

16. 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

17. 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
E 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

18. 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

19. 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