I n s t i t u t e of H i g h E n e r g y P h y s i c s И н с т и т у т Ф и з и к и В ы с о к и х Э н е р г и й Influence of cooling on the working parameters of GaAs detectors. S. Golovnia a*, S. Gorokhov a, Y. Tsiupa a, A. Vorobiev a O. Koretskaja b, L. Okaevich b, O. Tolbanov b a Institute of High Energy Physics, Protvino, Russia b Tomsk State University, Tomsk, Russia

The following semiconductor detector parameters is testing to find the influence of cooling on it. Detector’s Dark Current vs. Bias Voltage Detector’s Capacitance vs. Bias Voltage Detector’s Noise vs. Bias Voltage Detector’s Response on β-particles and γ-rays Detector’s Charge Collection Efficiency Detector’s γ -Ray Detection Efficiency & Thickness

Detectors geometry and types Resistive, doped Cr. Sample dimensions 3,3x3,4 mm, thickness 780 mkm. initial indot of GaAs is ~ 0,8 – 1,2 * 10 ^17 [1/cm^3], bulk resistivity 0,5 – 0,8 *10^9 Epitaxial grown and Cr compensated. Sample dimensions 1,1 x 6,6 mm, thickness 500 mkm initial indot is 1*10^17 [1/cm^3] Concentrations ratio is Cr/Sn = 6.8

The Dark Current temperature dependence vs. High Voltage The typical I-V characteristic of resistive type detector sample. The characteristic is independent from the voltage polarity and forward and reverse branches is equal. The dark current decreases in ten times if the detector temperature is decreasing on 20 °C

The detector capacitance dependence from the detector temperature at different modulation frequencies The typical C-V characteristic of resistive type GaAs detector samples. The temperature changes is not affected on the detector capacitance if the modulation frequency higher then 10 kHz

Where: λ = T dif /T int ~1 k = 1.380* J/K T =~300 temperature ( 0 K) g = ~20 C = ~ total capacitance I = ~ total current V f = flicker noise voltage The detector noise can be predicted in case of 2 stage integration The contribution from different noise sources can be easy separated 1.Shot noise is proportional ~ C 2 /T int 2.I and Thermal noise ~T int 3.Flicker noise does not depend on Tint Shot noiseFlicker noiseCurrent & Thermal noise

The detector system noise can be predicted in case of 2 stage integrating Where: λ = T dif /T int ~1 k = 1.380* J/K T =~300 temperature ( 0 K) g = ~20 C = ~ total capacitance I = ~ total current V f = flicker noise voltage The contribution from different noise sources can be easy separated 1.Shot noise is proportional ~ C 2 /T int 2.I and Thermal noise ~T int 3.Flicker noise does not depend on Tint Shot noiseFlicker noiseCurrent & Thermal noise

The LED system setup for Noise vs. Current measurements The schematic view of the test system setup. Both the detector and the LED put into shielded box filled with dry gas mixture to prevent condensate. The temperatures from -40 to +50 °C can be reached. By changing the voltage in the LED chain the photocurrent in the test sample can be easily changed.

The dependence of Sigma noise (in ADC channels) vs. detector current at different shaping times is given for resistive type detector sample.

The dependence of Sigma noise (in ADC channels) vs. detector current is given for resistive type detector sample. dSigma/dI for both 50 and 100 high voltages is equal. It is mean, that noise is put together additive, without influence on each other and noise linked with high voltage only.

The dSigma/dI [e 2 /nA] vs. Shaping time is given for resistive type detector samples. The dSigma/dI = 5768,9 [e 2 /nA] that much more that predicted value of 1171,8 [e 2 /nA] calculated before.

The epitaxial detector response on γ-rays from 241 Am source This spectrum from the radioactive source 241 Am on the epitaxial type detector show us that Charge Collection Efficiency is ~ 86-90%.

The epitaxial detector response on beta particles from 90Sr radioactive source

Charge collection efficiency vs. high voltage at different temperatures measured on Diffusion Samples

Charge collection efficiency vs. temperature at different high voltage measured on Epitaxial Samples

Detection Efficiency vs. GaAs Detector thickness for γ-Ram Energy Region from 20 to 60 keV

The epitaxial detector active area thickness vs. detector temperature for high voltages 50 & 100 V

The resistive detector active area thickness vs. high voltage at temperature +20 o C

Conclusion The presented experimental results show, that cooling both GaAs resistive & epitaxial detectors can increase their working parameters The high charge collection efficiency of epitaxial detectors together with their active area thickness up to 100 mkm make us possible to us them as a detector for X-Ray imaging application The resistive type detectors have the ability to operate with both polarity of high voltage have been presented