Hybrid Superconducting Pixel Detector

Abstract

A new position sensitive cryogenic detector for high energy particles based on Josephson Tunnel Junctions (JTJs) is proposed. This consists of a semiconductive substrate to transduce the energy of particles in charge and of a JTJ as a current driven sample and hold circuit. The possibility of coupling to high-speed Josephson logic families and the sensitivity to magnetic fields are also addressed. Preprint Bern BUHE-97-01 to appear in Nucl. Instr. and Meth. in Phys. Res. A (1997) 2 Josephson Tunnel Junctions (JTJs) have since long time been proposed as fast position and threshold particle detectors [1]. Besides the possible picosecond response of these devices when considering the hot-spot mechanism [2], the main interest lies in the radiation hardness of the Josephson devices as recently demonstrated in the Gigarad region [3]. We propose an hybrid approach which combines the efficiency of a semiconductor detector in detecting minimum ionising particles (mips) with the high radiation hardness of JTJs based read-out integrated electronics in a simple and reliable scheme. Since presently the poor radiation hardness of semiconductive pixel detectors [4] stems essentially from the read-out electronics, this approach results in a dramatic improvement of the detector performances. Moreover, radiation hard semiconductors (e.g. CVD diamond) are fully compatible with the proposed scheme and eventually can bring the radiation hardness of the whole device at the level of the Josephson electronics itself. A JTJ is a complex system in which peculiar effects due to “weak superconductivity” arise. A comprehensive treatment of the Josephson junction can be found in ref. [5]. The I-V characteristic of such a device is depicted in Fig. 1. The device is characterised by two well defined states, one at zero voltage and the second at a voltage corresponding to the energy gap of the superconducting material constituting the electrodes. Increasing the value of the bias current results eventually in a very fast switching to the gap voltage (Vg) state when the critical current value (Ic) is exceeded. Moreover, the zero voltage state being metastable, noise driven switching can occur when the bias is too close to the critical value Ic. We will then define a noise threshold as the upper bias limit. Exceeding this limit can cause random switching. Let us now consider one of such junctions deposited on a high purity Si wafer with both surfaces processed in order to create an ohmic contact (semiconductors other than Si can be considered). The structure realises the electric circuit of Fig. 2, where the Si wafer, acting as a detector, is represented by an ideal current generator due to its very high internal resistivity. Corrections due to finite resistance and capacitance are also indicated as dashed connected elements (Rs and Cs). Such a system, once properly biased, realises a Si detector with nonblocking electrodes, coupled to a current sensitive sample and hold JTJ element. At the liquid He temperature (4.2 K), the Si resistivity has dramatically increased [6] keeping the leakage current at very low values without the necessity of using a reversed biased diode configuration. Moreover, at such a low temperature, the saturated electron drift velocity is achieved for a drastically reduced voltage (30 V in the case of 111 crystals [7]). This velocity does not greatly differ from the value at room temperature, but in this case also the holes gain a large mobility and, under the same bias condition, their velocity is only one half of that of the electrons [6]. This leads to a non negligible contribution of the holes to the total signal. The time dependence of the collected charge can then be straightforwardly calculated [8] under the assumption of constant energy loss through the entire thickness and constant electric field throughout the wafer: Preprint Bern BUHE-97-01 to appear in Nucl. Instr. and Meth. in Phys. Res. A (1997) 3 Q(t) = Ne t td e 1 − t 2td e       + t td h 1− t 2td h       

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