Laser Testing of Silicon Detectors Rhorry Gauld University of Saint Andrews IPM program – PPD Mentor: Ronald Lipton 30/07/08 1

Introduction Science involved Aim of the project First attempts Development of technique Application of technique Future applications Conclusion Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 2

How Does A Silicon Detector Work? Charged particle passes through detector and creates ionisation in the bulk silicon This creates electron/hole pairs Holes drift in an applied electric- field towards the negatively charged p-type strips Induces readable charge on the Aluminium strips Thus by recording which read-out channel is fired, it is possible to determine where the charged particle passed through the detector Image 1: strip detector showing ionization as a charged particle passes through Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 3

Testing the Detectors Why is it important? - ALICE experiment will use ~25000 detectors - Need to know each detector works efficiently - No defects ( make sure strips are working) - Properties of the detectors ( doping/electric field penetration depth etc.) Benefits of technique? - compactness - ease of control of amount and location of deposited charge - allows detectors with no read-out system to be tested Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 4

First Attempts & Problems Setup 1 Problems; - Very difficult to position laser whilst probing detector - No scope reading for laser - Picoprobe not working Solutions; - Use a different type of detector which allows probing and laser pulsing at different points - Replace oscilloscope - New Detector also had built resistor and contacts ( so the ‘probe’ no longer required) Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 5

Experiment Set-up Setup 2 -Vertical laser height fix -Laser attenuation fix -Pulse rate at which base line fell before next pulse -Measurement of detector spacing -Laser pulse duration Problems come from picoprobe tip & connections in circuit Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 6

Co-ordinate system Measured detector spacing using Pixel Detector Matched laser scan( number of turns) with strip scan ( um) Measured the effect that changing the voltage bias has on the amplitude of our scan 1 Turn : 800 um Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 7

8 Errors Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 -Trying to over lap laser scan vs. strip scan not completely accurate Estimated ~5% Error in overlap of graphs. 1 Turn : 800um +/- 40um - Easily fixed by installing coupled dial gauge - Error found when measuring output signal from oscilloscope - Fluctuations caused 1.2% read-out error

Set-up Check Charge deposited on 20um x 10cm strip( at x = 0) per pulse is Q = C V C = 1pF/cmV = 188mV => Q = 1.88 x 10^-12 C Calculate expected current average produced by laser I = Q total / t Integrate curve fit for strip width / entire curve Charge deposited for strip at x = 0 : Q = 2.98% => Q total = 6.31 x 10^-11 C Pulse Duration = 5.6 ns Pulse Generator Period = 40 us => I = 1.13 x 10^-2 A & I ave = 1.56 uA agrees with observed current produced by laser Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 9

Results d = 300 um and V = 175V d =   1/2 e N D 2 ε ε 0 V d = depletion width V = voltage bias at full depletion ε ε 0 = permittivity for silicon = 11.9 x … × 10^−14 F/cm e = electric charge of electron = x10^-19 C N D = doping level => N D = x 10^12 cm^-3 Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 10

More Results Fall time ~1/ (V^1/2) leveling off at 175V Agrees with C ~ 1/ (V^1/2) Agrees with RC time constant suggests a C 1pF/cm Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 11 Full calculations still to come

Application of technique Using technique to detect efficiency, defects and properties of detectors Currently testing a small strip detector which we do not know the properties of Difficult as I am scanning for an area which has < 0.1% area of a 2x2cm detector. Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 12 Currently Testing rise/fall times at different values of Voltage Bias Will calculate depletion width then doping concentration next

13 Improvements Set-up laser very accurately Measure beam properties Calculate electric field within detector at different BIAS Relate the velocity of charge carriers to the rise time of out-put voltage peaks Set-up coupled dial gauge Use a very accurate current meter Use Laser with different wavelength to see how penetration depth of the photons effects results Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

References Semiconductor Detector System – Helmuth Spieler – Oxford Science Publications – TK9180.S C2 Prototyping of Silicon Strip Detectors for the Inner Tracker of the ALICE Experiment / Oleksiy Sokolov - [S.l.] : [s.n.], Tekst. - Proefschrift Universiteit Utrecht Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Volume 573, Issues 1-2, 1 April 2007, Pages Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 14