1. Neutron Transmutation Doped Germanium Thermistors for Cryogenic Detector Applications J. W. Beeman and E.E. Haller

2. J.W. Beeman 2 Topics to be discussed Thermal Detection Compared to Traditional Approaches Thermistors: Operation and Figures of Merit Neutron Transmutation Doped (NTD) Germanium Used as Thermistors

3. J.W. Beeman 3 Thermal Detection Compared to Traditional Approaches Typical High-Purity Ge p-i-n Diode Detector OR 

4. J.W. Beeman 4 Semiconductor Nuclear Radiation Detectors Reverse biased p-n or p-i-n junction devices Signal is the charge Q produced through ionization, Q = Ne = (E/  pair ) e and collected at the contacts FWHM is dominated by fluctuations in Q;  N =, F = Fano factor, F Ge  0.1 Only 1/3 to 1/4 of the total energy is converted into electron-hole pairs How can the resolution be further improved?  total energy measurement

5. J.W. Beeman 5 Take it from Sir Isaac Newton “The increase in the internal energy of a system is equal to the amount of energy added by heating the system minus the amount lost as a result of the work done by the system on its surroundings.” Translation:Use the heat, man!

6. J.W. Beeman 6 Energy Conversion into Heat Far infrared and radio astronomy and astrophysics have led the way: semiconductor bolometers operated at milliKelvin temperatures are used as detectors in telescope focal planes There is no easy “direct” energy conversion into charge at meV and  eV energies: composite bolometers are the sensors of choice The incredible improvements in resolution begin to influence nuclear and high energy physics instrumentation

7. J.W. Beeman 7

8. J.W. Beeman 8 Composite Bolometer for Far IR and  Wave Astronomy Courtesy P. L. Richards, UCB

9. J.W. Beeman 9 Cal Tech Spiderweb Bolometers Photo Courtesy PLANCK HFI

10. J.W. Beeman 10 The Ideal Bolometer Material Large  (thermistor temp. coefficient) appropriate R B (bolometer resistance) small H (heat capacity)  heavily doped semiconductor

11. J.W. Beeman 11 Neutron Transmutation Doped (NTD) Germanium Thermistors

12. J.W. Beeman 12 Starting Material: Ultra-pure Germanium Single crystals (7 cm , 25 cm long) have been developed for  -ray spectroscopy Most crystals are grown by the Czochralski method from a melt contained in a silica crucible and in an H 2 atmosphere The crystals have low dislocation densities (≤ 10 3 cm -2 ) The net-dopant concentration is |N A –N D | ≤ 2  10 10 cm -3 Electrically inactive residual impurities are: H (  10 14 cm -3 ), O (  10 14 cm -3 ) and Si (  10 14 cm -3 )

13. J.W. Beeman 13 Neutron Doping Process Acceptor 70 Ge (21%) + n  71 Ge 71 Ge  EC) 71 Ga (  T = 3.43±0.17 b,  R = 1.5 b) Donor 74 Ge (36%) + n  75 Ge 75 Ge  75 As +  -  (  T = 0.51±0.08 b,  R = 1.0±0.2 b) Double Donor 76 Ge (7.4%) + n  77 Ge 77 Ge  77 As +  - 77 As  77 Se +  - (  T = 0.16±0.014 b,  R = 2.0±0.35 b) Nominal neutron dose 4x10 18 n/cm 2 Nominal concentrations Ga:1 x 10 17 /cm 3 As: 3 x 10 16 /cm 3 Se:2 x 10 15 /cm 3 Net dopant concentrations |N A –N D | ≤ 2  10 10 cm -3 Nominal neutron dose 4x10 18 n/cm 2 Nominal concentrations Ga:1 x 10 17 /cm 3 As: 3 x 10 16 /cm 3 Se:2 x 10 15 /cm 3 Net dopant concentrations |N A –N D | ≤ 2  10 10 cm -3 Homogeneous impurity distribution reflects uniform neutron flux and uniform isotopic distribution Reproducibility: Ultra-pure starting material: neutron doping levels >> residual impurities

14. J.W. Beeman 14 Doping Homogeneity is Inherent to NTD Neutron field is much larger than any semiconductor crystal  homogeneous neutron flux Thermal neutron cross sections are small (  1 barn (10 -24 cm 2 ))  Little “self shadowing,” (if we do our job properly) Isotopes in the semiconductor crystal are distributed homogeneously (and have been for quite some time!) Bottom line: homogeneous doping

15. J.W. Beeman 15 Hopping Conduction and Compensation ++N D << N A CB = neutral acceptor; – ionized acceptor CB ++++++N D < N A NANA VB CB + + + + + + + + + + + +N D ~ N A NANA VB NANA low comp. medium comp. high comp.

16. J.W. Beeman 16 Resistivity of NTD Germanium Temperature (mK) T -1/2 (K -1/2 ) Resistivity (Ohm-cm)

17. J.W. Beeman 17 The LABOCA Array – APEX Telescope, Chile Photos: E. Kreysa

18. J.W. Beeman 18 Silicon Nitride Micromesh ‘Spider-web’ Bolometers Silicon Nitride Micromesh ‘Spider-web’ Bolometers Courtesy J. Bock, NASA-JPL Spider-web architecture provides low absorber heat capacity minimal suspended mass low-cosmic ray cross-section low thermal conductivity = high sensitivity Sensitivities and heat capacities achieved to date: NEP = 1.5 x 10 -17 W/  Hz, C = 1pJ/K at 300 mK NEP = 1.5 x 10 -18 W/  Hz, C = 0.4 pJ/K at 100mK Detectors baselined for ESA/NASA Planck/HFI Arrays baselined for ESA/NASA FIRST/SPIRE Planned or operating in numerous sub-orbital experiments: BOOMERANGCaltechAntarctic balloon CMB instrument SuZIE Stanford S-Z instrument for the CSO MAXIMA UC Berkeley North American balloon CMB instrument BOLOCAM CIT/CU/CardiffBolometer camera for the CSO ACBAR UC Berkeley Antarctic S-Z survey instrument BICEP Caltech CMB polarimeter MAT UPenn CMB experiment for Chile POLATRON Caltech CMB polarimeter for OVRO Archeops CNRS, France CMB balloon experiment BLASTU. PennSubmillimeter balloon experiment Z-SPECCaltechmm-wave spectrometer QUESTStanfordCMB polarimeter PRONAOS IAS, France Submillimeter balloon experiment

19. J.W. Beeman 19 Region in the “Southern Cross” constellation. Composite includes images from bolometers running at <100 mK in the Herschel Space Observatory. Photo: ESA

20. J.W. Beeman 20 “Energetic Event” Detection EDELWEISS CUORE/Cuoricino Smithsonian Astrophysical Observatory

21. J.W. Beeman 21 For E = 1 MeV:  T = E/C  0.1 mK Signal size: 1 mV Time constant:  = C/G = 0.5 s Heat sink: Cu structure (8 mK) Thermal coupling: Teflon (G = 4 pW/mK) Thermometer: NTD Ge-thermistor  dR/dT  100 k  K) Absorber: TeO 2 crystal (C  2 nJ/K  1 MeV / 0.1 mK) Heat sink: Cu structure (8 mK) Thermal coupling: Teflon (G = 4 pW/mK) Thermometer: NTD Ge-thermistor  dR/dT  100 k  K) Absorber: TeO 2 crystal (C  2 nJ/K  1 MeV / 0.1 mK) TeO 2 Bolometer: Source = Detector CUORE/Cuoricino Bolometer Single pulse example Time (ms) Amplitude (a.u.) 1000 2000 3000 4000 Energy resolution (FWHM) achieved in CUORICINO: ~ 5-10 keV at 2.5 MeV

22. J.W. Beeman 22 CUORE/CUORICINO

23. J.W. Beeman 23 Conventional Solid State Detector (Ge, Si(Li) 2.5 eV FWHM Semiconductor Materials Analysis Source: E. Silver SAO

24. J.W. Beeman 24 oxygen iron nickel magnesiu m aluminum silicon phosphor us sulfur carbon calcium Allende Meteorite KT6158 Photo: E. Silver, SAO

25. J.W. Beeman 25 Shapes, Sizes, Impedance

26. J.W. Beeman 26 Summary Thermal detectors offer significant advantages for “small energy” experiments –Excellent energy resolution –Stable, robust, reliable, and well understood –Slow but speed not necessary for rare decay searches Experiments with detector arrays show excellent detector uniformity and long term stability