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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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. 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

9. 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

10. 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 8.8541… × 10^−14 F/cm e = electric charge of electron = 1.602 x10^-19 C N D = doping level => N D = 2.561 x 10^12 cm^-3 Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 10

11. 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

12. 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. 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

14. References http://hepwww.rl.ac.uk/OpenDays98/Detectors/silicon.htm http://www.physik.rwth-aachen.de/institut-ib-new-version/forschung/cms/ Semiconductor Detector System – Helmuth Spieler – Oxford Science Publications – TK9180.S68 2005 C2 http://ece-www.colorado.edu/~bart/book/book/chapter3/ch3_3.htm#fig3_3_2 http://www.tf.uni-kiel.de/matwis/amat/semi_en/kap_2/backbone/r2_2_2.html Prototyping of Silicon Strip Detectors for the Inner Tracker of the ALICE Experiment / Oleksiy Sokolov - [S.l.] : [s.n.], 2006 - 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 12-15 Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08 14