Outline. Introduction, motivation Readout electronics, Peltier cooling Input J-FETsJ

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1 Progress in low energy X-rayX spectroscopy using semi-insulating insulating GaAs detectors F. Dubecký 1, B. Zaťko 1, P. Boháček 1, L. Ryć 2, E. Gombia 2, and V. Nečas 3 1 IEE SAS, Bratislava, Slovakia 2 IPP&LM, Warsaw, Poland 3 IMEM-CNR, Parma, Italy 4 FEE&IT SUT, Bratislava, Slovakia

2 Outline Introduction, motivation Readout electronics, Peltier cooling Input J-FETsJ and PA evaluation SI GaAs detector for low energy X-raysX Current-voltage measurement Detection of X and γ-rays Summary, conclusion

3 Introduction Semi-insulating GaAs as a material of radiation detector Wide bandgap energy E g = 1.42 ev Relative high atomic number 31 & 33: efficient detection of X- and gamma rays Fast reaction rate: high drift mobility High resistance to damage by irradiation Drawback: high leakage current at RT for application in soft X-ray spectroscopy

4 Motivation Energy gained from a fusion reaction could solve problem of energy Fusion has many attractive features in terms of safety, fuel reserves, and minimal damage to the environment The primary disadvantage being associated with difficult scientific and engineering challenges that are inherent in the fusion process Problem that must be solved: On-line hot plasma diagnostics Application in detection of Extreme UV photons

5 10 8 Attenuation coefficent µ, cm x 23x Photon energy, kev GaAs Si InP CdTe V D (cms -1 ) Electrons in GaAs Holes in GaAs Detector operation 10 5 region E (Vcm -1 )

6 Measuring spectrometric system Battery voltage source U 0 COOLING NECCESSARY Required performance: Noise: < 100 e - rms SDD: < 140 ev FWHM 13 e - rms R b γ-photon SI GaAs Detector Charge sensitive preamplifier Eurorad PR-307 Shaping amplifier CREMAT CR-200 Analog-to-digital converter ORTEC 800 ADC e - h + J-FET ohmic contact pasive region depletion layer Schottky contact M2D multichannel analyzer MCA + PC

7 J-FETs evaluation S K 152 G k ev 2N kev B F kev Counts per channel C h a nnel Transistor Yfw, ms Ciss, pf Noise, nvhz-1/2 Package Notice 2SK ,2 1,20 TO92 obsolete, SMD unavailable BF ,8 TO236, SOT23 Noise at 100 khz, SMD 2N ,20 6 TO72, TO236 SMD package available MX TO72 Chip available, Ciss at -4 V MX ,7 1,2 TO72 Chip available, Ciss at -4 V NJ14AL 5,5 2,3 4 Chip Chip U SMD package available SMPJ ,5 6 SMD package available SMPJ ,5? SMD package available

8 Preamplifiers testing and evaluation Counts per channel Fe: 5.9 kev Si3-8 V, E = 1.50 kev, ST = 1 µs, CR 102B Si3-8 V, E = 1.68 kev, ST = 1 µs, CR 110 (1) Si3-8 V, E = 1.65 kev, ST = 1 µs, CR 110 (2) Fe: 5.9 kev Si3-8 V, E = 1.78 kev, ST = 1 µs, Eurorad PR 307 (1) 1800 Si3-8 V, E = 1.62 kev, ST = 1 µs, Eurorad PR 307 (2) Si3-18 V, E = 1.67 kev, ST = 1 µs, battery source Energy, kev 241 Am: 59.5 kev Counts per channel Energy, kev 241 Am: 59.5 kev Hybride Preamp, separated FET CR-110 CREMAT 1.5 kev CR-102B CREMAT, obsolete 1.2 kev PR307 EURORAD 0.85 kev A-250 AMPTEC 1 kev ASICS various < 300 ev * e.g. Bertuccio, G., Caccia, S., NIM A 579 (2007) 243 Noise FET

9 Blocking scheme of developed charge sensitive preamplifier Part cooled by the Peltier cooler Charge sensitive preamplifier with cooled first stage and SI GaAs detector

10 Cooled stage detailed layout Temperature Sensor (AD590 chip) Testing capacitor SMD J-FET Substrate < 200 µm: Al 2 O 3 AlN Be-based ceramic SI GaAs detector Working resistor Feedback resistor Feedback capacitor 6x6 mm 2

11 Developed low noise preamplifiers Input J-FET Charge sensitive preamplifier based on CR-102B hybrid circuit and external input J-FET 2SK152 Noise: 135 e - rms at RT Detector SI GaAs detector is directly coupled to the input of the charge sensitive preamplifer

12 Charge sensitive preamplifier with Peltier cooler PR307 SI GaAs detector Cooled J-FET stage with detector SMD BF862 J-FET

13 New solution: 3 stage Peltier cooler Additional tasks neccessary to solve: input thin window with a low absorption collimator micro-temperature sensor efficient cooling of the heat sink compact bias source improvement of detector radiation and neutron damage resistance PR-307 CERAMIC SUPPORT with detector and J-FET

14 Compact DC biasb source Rechargable batteries (AA) 1k2 V 1 + C L TMR dc-dc V 0 +V in V in MHV Series 180, 300, 350 V out V ref 4 3 V adj 5 HV out C pf V out Com 6-12 AA V 2 V 3 + C + C L L 2 6 TMR TMR k2 1k2 +12 V Output 12 V +12 V Output 12 V R1 R2 VR = 5kΩ +/- 12 V bias for charge sensitive preamplifier and shaping amplifier High voltage output for detector

15 Simple SI GaAs detector for low energy X-raysX square 0.4 mm Ti/Pt/Au (10/40/70 nm) Schottky contact SI GaAs wafer (130 µm) AuGeNi/Au (50/70 nm) contact bulk VGF SI GaAs wafer (100) - detector grade producer: CMK Ltd. Žarovica, Slovakia RT resistivity: Ωcm RT Hall mobility: 7060 cm 2 V -1 s -1

16 Current-voltage characteristic of SI GaAs RT 100n DC current decrease: 10 with each 20 deg. of temperature lowering: 260 K I sat 10 pa Current, A 10n 1n 100p Optimal operation area SI GaAs detector T = 300 K φ (Schottky) = 0.5 mm Bias, V

17 Detection of X-X and RT 160 µm Counts per channel Np X Lα, Lβ, Lγ kev kev escape Ag K α 33.2 kev FWHM 7.7 kev γ 59.5 kev Pb K α and K β LEC SI GaAs 160 µm, φ = 0.5 mm 241 Am, 57 Co, RT Bias = V escape γ kev 0 K α Ga & As *7 γ kev Channel

18 RT pulse-height spectrum of 241 Am Counts Np X 241 Lα, Lβ, Lγ Am 17.8 kev 20.8 kev 13.9 kev 26.3 kev escape FWHM 5.11 kev U bias = V τ = 0.5 µs T = 295 K γ 59.5 kev 130 µm Energy, kev

19 LT: Pulse-height spectrum of 241 Am Np X Lα, Lβ, Lγ 17.8 kev 241 Am U bias = V τ = 3 µs T = 255 K 130 µm Counts kev 20.8 kev 26.3 kev escape FWHM 4.64 kev γ 59.5 kev Energy, kev

20 RT&LT LT: Pulse-height spectrum of 241 Am using optimized electronics 4000 Np X Lα, Lβ, Lγ 17.8 kev 241 Am Spectra: RT and LT OPERATION 130 µm Counts kev 20.8 kev 26.3 kev escape U bias = V τ = 0.5 µs T = 295 K τ = 3 µs T = 255 K FWHM 4.3 kev 4.0 kev γ 59.5 kev Energy, kev

21 Energy resolution Temperature, K Energy resolution, % FWHM (kev) improved improved 17.8 kev peak < 25 (< 4.5) < 23 (< 4.2) < 22 (3.9) < 19 (< 3.6) < 16 (< 3.0) < 14 (< 2.8) 59.5 kev peak 8.6 (5.0) 12.9 (7.7) 7.2 (4.3) 7.8 (4.6) 7.2 (4.3) 6.8 (4.0)

22 Two-line SI GaAs detector module One detection mini-module with 2 64 pixel detectors Zaťko, B., Dubecký, F., Přibil, J., Boháček, P., Frollo, I., Ščepko, P., Mudroň, J., Grybos, P., and Nečas, V.: On the development of portable X-ray CT mini-system using semi-insulating GaAs radiation imaging detectors, Nuclear Instr. and Methods in Phys. Res. A 607 (2009) IF: COST MP601 Meeting

23 Block diagram of single channel Grybos et al.: IEEE Nucl. Sci. Sym. Conf. Record (2006) 693. Photograph of SI GaAs radiation detectors coupled to ASIC DX64 readout chip. Current (A) 1µ 100n 10n 1n 100p Forward Reverse T = 300 K 10p Diameter 0.75 mm 0.5 mm 0.3 mm 0.2 mm Detector R Detector L Voltage (V) I-V characteristics of SI GaAs detectors measured at RT. Total counts 100k 10k 1k 100 Diameter, Bias = 220 V 0.75 mm mm 0.3 mm 0.2 mm Threshold Integral spectra of SI GaAs detectors measured using ASIC DX-64 readout chip. Total counts RT 3.8 kev Energy (kev) Calculated differential spectra of 241 Am measured using ASIC DX-64 with SI GaAs detector 0.3 mm in diameter

24 New choice: electrodes technology 1µ Forward 35,0n 300 V Noise rms A Noise rms A Current (A) 100n 10n 1n Reverse AuGeNi In Mg Gd Current (A) 30,0n 25,0n 20,0n 200 V Noise rms A Noise rms A AuGeNi In Mg Gd Voltage (V) Time (min) Counts per channel noise SI GaAs: Schottky Ti/Pt/Au φ = 0.5 mm AuGeNi In Mg Gd Counts per channel SI GaAs: Schottky Ti/Pt/Au φ = 0.5 mm AuGeNi In Mg Gd Channel Channel

25 Conclusions We developed and tested charge sensitive preamplifiers with external J-FET operated at RT and lowered temperature. SI GaAs detector dc coupled to the developed front-end electronics gives detectable energy threshold of 4.5 kev at 295 K and < 3 kev at 265 K. Fabricated and evaluated surface SI GaAs Schottky barrier detector has an active base width of 130 µm and gives energy resolution of about 2.8 kev FWHM for 17.8 kev peak from 241 Am source at 265 K. The results show that first of all the noise of the front-end electronics chain must be reduced to <100 e - rms. Future plan toward spectrometric detector technology: electrodes, topology, material aspects, neutron damage

26 Acknowledgements This work was done within the Center of Excellence CENAMOST (Contract No. VVCE ) With parial support from grants: No. APVV (Slovak Research and Development Agency), No. 2/0163/09, 1/0689/09, 2/0192/10 (Grant Agency for Science), EURATOM/CU (Project P3) and COST MP0601

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