Fast GaAs Photoconductor Responses to Subnanosecond Proton Pulses
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FAST GaAs PHOTOCONDUCTOR RESPONSES TO SUBNANOSECOND PROTON PULSES B. BRULLOT*, R. GALLI*, X. LECAT*, C. RUBBELYNCK*, T. POCHET** *Centre d'Etudes Nucl~aires, DAMICEM, B.P. 12, 91680 Bruy~re-le-Chatel, France **LETI (CEA-Technologies Avanc~es), DEIN/SPE, Centre d'Etudes Nucl~aires de Saclay, 91191 Gif-sur-Yvette Cedex, France
ABSTRACT GaAs photoconductors -have been tailored to detect ultrafast proton pulses having energies ranging between 4 and 9 MeV. The sensitivity, the linearity and the speed of response of the devices are analyzed as a function of their neutron pre-irradiation treatment. The dependence of the sensitivity on the proton energy and the applied polarization is also studied. Finally, the experimental results are compared with a simple theoretical model.
1- INTRODUCTION
Fast gamma ray photoconductors are currently designed in our Laboratory. Our work is particularly focused on neutron irradiated GaAs(LEC) photoconductors [1-3]. Over the past years the response of these devices to fast gamma ray pulses has been studied. The influence of the electronic properties of the material, the carrier lifetimes and mobilities and the neutron induced defect density on the sensitivity of the detectors has been reported [1]. Our experimental results were shown to be in good agreement with a theoretical model describing
the signal formation in the device. More recently, our work focused on the detection of fast proton pulses with energies ranging from 4 to 9 MeV. After a brief description of the detectors and the experimental set-up, the time responses of the devices are analyzed as a function of the proton pulse energies and the electronic characteristics of the material.
2- THE DETECTORS
The geometry of the photoconductors is optimized in order that they operate as dE/dx detectors. In this regime, the linear density of charges created in the devices is relatively low and constant throughout the active volume. GaAs detectors are parallelepipedic with a surface of 3 x 3 mm and a thickness of 80 microns. Metallic contacts are formed on two 3 x 0.08 mm opposite sides by evaporating Au/Ge/Ni. The ohmicity of the contacts is then checked by Current- Voltage measurements. Afterwards the material is irradiated with fission neutrons generated by a research nuclear power reactor located in Saclay. The neutron dose was in the range 5.1014 to 3.1015 n/cm 2 . This procedure generates defects in the bulk of the crystal that results in a decrease of its carrier lifetime and a subsequent improvement of the response speed of the device. Table I displays the actual sizes and irradiation doses for each sample used.
Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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Figure 1: The experimental Set-Up.
G.aAs Photodetector
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AmplifierI
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• Dolay Generator
3- EXPERIMENTAL SET-UP
The experimental set-up is shown in Figure 1. Protons are produced by a 14 MV Van de Graaff Tandem accelerator located in the Nuclear Center of Bruy~re-le-Chatel
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