Eschke, J.

Ghosh, P.

GSI Repository

Characterization of double sided silicon micro-strip sensors with a pulsedinfra-red laser system for the CBM experiment∗Pradeep Ghosh†1,2 and Ju¨rgen Eschke2,31Goethe-Universita¨t, Frankfurt am Main, Germany; 2GSI Darmstadt, Germany; 3FAIR GmbH, Darmstadt, GermanyThe Silicon Tracking System (STS) of the CompressedBaryonic Matter (CBM) experiment at FAIR is composedof 1292 double-sided silicon micro-strip sensors. For thedevelopment and the quality assurance of produced sensorsa laser test system has been built up. The main aim of thesensor scans with the pulsed infra-red laser system is todetermine the charge sharing between strips and to measurethe uniformity of the sensor response over the whole activearea. The prototype sensors CBM02 tested so far with thelaser system have 256 strips with a pitch of 50 μm on eachside [1]. They are read out by the self-triggering n-XYTERprototype read-out electronics.Figure 1: Schematic representation of the laser setup.The laser system is intended to measure the sensor re-sponse in an automatized procedure at several thousand po-sitions across the sensor with focused infra-red laser light(σspotsize ≈ 15 μm). The duration (∼5 ns) and power (fewmW) of the laser pulses are selected such that the absorp-tion of the laser light in the 300 μm thick silicon sensorsproduces about 24k electrons, which is similar to the chargecreated by minimum ionizing particles (MIP) there.Figure 2: Dependence of number of fired strips on the dis-tance to focuser from the sensor surface.The wavelength of the laser was chosen to be 1060 nmbecause the absorption depth of infra-red light with this∗Work supported by HGS-HIRe, H-QM and HIC-for-FAIR† Pr.Ghosh@gsi.de, Tel. +49-6159-71-2586wavelength is of the order of the thickness of the siliconsensors [2]. Figure 1 shows the measurement setup in aschematic view. The laser light is transmitted through a6 μm (inner diameter) thick optical fibre to a two-lens fo-cusing system, which focuses the light to a spot size ofabout 15 μm diameter; working distance is about 10 mm.Figure 2 shows the dependence of the number of fired stripsas a function of the distance of the laser to the sensor sur-face. The best focusing was obtained at position 7 w.r.t thereference level.Figure 3: Distribution of hits per strips on n-side.Figure 4: ADC amplitude distribution (n-side).We have illuminated a prototype sensor with a focusedpulsed laser and could achieve a spot size of a little morethan one strip [See Figure 3]. The preliminary resultsdemonstrate that we are successful in inducing charge sim-ilar to 1 MIP (24k electrons). Figure 4 shows that the num-ber of single-strip clusters is about an order of magnitudehigher than that of two-strip clusters. The next step of ourwork will be to investigate the charge sharing function be-tween the strips.References[1] J.M. Heuser et al., CBM Progress Report 2011, Darmstadt.2012 p.17[2] P.O’Connor et al., Proc. of SPIE Vol. 6276 62761W-1, p.2PHN-NQM-EXP-19 GSI SCIENTIFIC REPORT 201248

Characterization of double sided silicon micro-strip sensors  with a pulsed infra-red laser system for the CBM experiment

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Characterization of double sided silicon micro-strip sensors with a pulsed infra-red laser system for the CBM experiment

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