Experimental Results in Picosecond and Subpicosecond Range of IIa Type Diamond Detector in X-UV, Visible and IR Fields
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EXPERIMENTAL RESULTS IN PICOSECOND AND SUBPICOSECOND RANGE OF Ila TYPE DIAMOND DETECTOR IN X-UV, VISIBLE AND I1 FIELDS
M NAIL", Ph. GIBERT*, J.L. MIQUEL*, M. CUZIN**
*Commissariat Al'Energie Atomique, Centre d'Etudes de Limeil-Valenton 94195 Villeneuve-Saint-Georges C6dex, France **LETI, C.E.A. Technologie Avanc6e, 38041 Grenoble C6dex, France
ABSTRACT Type Ila diamonds present photoconductive properties as well in X-ray detection range, as in near UV, visible, IR light range, in spite of the large bandgap of the material. Experimental results in these four fields, time measurements in picosecond and subpicosecond ranges are presented.
I - INTRODUCTION
Diamond photoconductive detectors have already been used to measure the burn time on laser fusion experiments [1] [2]. They can also be devoted to measure X-ray radiation emitted from laser induced plasmas [31 [41 . Due to the 5.5 eV band gap of diamond intrinsic conductivity, we can expect they will not be sensitive to scattered laser radiation. Unfortunately, natural diamond presents a lot of impurities and its conductivity is nonnegligible for photon energies below the intrinsic band gap. One of the aims of this paper is to present the diamond sensitivities to 1064 nm (co), 532 nm (2 o)) and 355 nm (3 co) laser radiation wavelengths and to compare them with the X-ray radiation sensitivity. Another is to show the fast temporal response of such detectors. 2 - DETECTOR DESIGN 2.1 - Geometry
Photoconductive devices have been fabricated from Ila natural diamond (Dricker International Netherlands). They consist of two different sizes small cylinders : 3 mm diameter by 2 mm height for the first one and 5 mm in diameter by 3 mm high for the second one. They have been set by LETI (CEA, Grenoble, France) at the end of a 50 0l KS 2 or KS 3 semi rigid coaxial cable.as shown on Figure 1. Top and bottom electrodes are made of Ti/Pt/Au coating. This geometry is not so convenient to measure X-ray radiation but these detectors were previously designed for neutron measurement [2].The electrodes are realized on the plane faces of the crystal . The upper electrode is connected to the outer conductor of the coaxial cable and the bottom electrode to its inner conductor. To allow the input of the laser light or the X-ray radiation, a 1mm wide slit is made in the cable outerdacket, along the diamond crystal thickness. The detector bandwidth depends on the carrier life times and its capacity, LETI calculations [5] predict capacity values of 0.2 pf and 0.36 pf (figure 2) for KS2 and KS3 cable geometries.The corresponding risetime values x are respectively 22 ps and 40 ps (with Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
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t = 2.2 RC and R = 50 02). Five detectors are tested : two are of KS2 type, KS2-1 and KS2-2 and two of KS3, type, KS3-1 and KS3-2. A cubic diamonqpdetector has been set by LETI (CEA Saclay, France). Moreover the KS2-2 detector is 2.10 thermal neutron pre-damaged, to try to improve temporal characteristics of detector as pre-damaged GaA
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