Marko Miku ž University of Ljubljana & Jo ž ef Stefan Institute Ljubljana, Slovenia

1. Study of Polycrystalline and Single Crystal Diamond Detectors Irradiated with Neutrons up to 1016 cm-2 Marko Mikuž University of Ljubljana & Jožef Stefan Institute Ljubljana, Slovenia IEEE NSS’07 N44-5 Hawaii, November 1, 2007

2. Collaboration • JSI & Univ. Ljubljana, Slovenia • M. Mikuž, V. Cindro, I. Dolenc, A. Gorišek, G. Kramberger, I. Mandić, M. Zavrtanik • Ohio State University, USA • H. Kagan, S. Cline, S. Smith • University of Toronto, Canada • W. Trischuk Work performed as part of CERN RD-42 programme

3. Word of caution • Irradiations and measurements were all performed during the last two months, most of them even in the last two weeks • All results strictly preliminary • No time yet for a real systematic study • 1016 n/cm2 data not available yet – up to 3x1015 n/cm2 • scCVD data just starting • But still lots of interesting data available

4. Aim of study • Diamonds are proposed as sensor material for the innermost tracker layers at sLHC • Fluence benchmark up to 3x1016 particles/cm2 • Mosty pions, ~10 % neutrons • Most irradiations use protons from CERN PS • Thought to be representative for charged particle damage • NIEL in Si as scaling factor taken for granted • NIEL violations observed in Si • Neff in oxygenatedSi • Trapping p vs. n • NIEL for diamonds much smaller • Is NIEL representative of damage ? W. de Boer et al. arXiv:0705.0171v1 Si C

5. Diamond as sensor material

6. Polycrystalline Chemical Vapour Deposition (pCVD) Grown in μ-wave reactors on non-diamond substrate Exist in Φ = 12 cm wafers, >2 mm thick Small grains merging with growth Grind off substrate side to improve quality → ~500 μm detectors Base-line diamond sensor material Diamond sensor types - pCVD Surface view of growth side All photographs courtesy of Element Six & OSU Side view Test dots on 1 cm grid

7. Single Crystal Chemical Vapour Deposition (scCVD) Grown on diamond substrate RD-42 has research contract with E6 to develop this material Exist in ~ 1 cm2 pieces, max 1.4 cm x 1.4 cm, thickness > 1 mm A true single crystal Size limit - not a real option for sensors at this moment After heavy irradiations expect similar properties to pCVD Clean environment for study of basic material properties Diamond sensor types - scCVD

8. Signal from pCVD diamonds • No processing: put electrodes on, apply electric field • Trapping on grain boundaries and in bulk • much like in heavily irradiated silicon • Parameterized by Charge Collection Distance, defined by • CCD = average distance e-h pairs move apart • Coincides with mean free path in infinite (t ≫ CCD) detector  mean not most probable CCD measured on recent 1.4 mm thick pCVD wafer

9. Radiation Damage - Basics • Charge trapping the only relevant radiation damage effect • NIEL scaling questionable a priori • Egap in diamond 5 times larger than in Si • Many processes freeze out • Typical emission times order of months • Like Si at 300/5 = 60 K – Boltzmann factor • Lazarus effect ? • Time dependent behaviour • A rich source of effects and (experimental) surprises !

10. Samples and irradiations • 7 pCVD and 2 scCVD acquired from Diamond Detectors Ltd. (ex. Element Six) • All 5x5 mm2 • Metallization and initial test at OSU • CCD between 205 and 250 μm • Leakage currents < 10 pA @ 1 kV • 1 scCVD ok, 1 exhibits large polarization • Suspect surface problems →return to DDL • 2 pCVD to PSI pion irradiation in Sep’07 • Thanks to Maurice Glaser • 200 MeV π+ beam • Most representative of LHC • Plan for 1 and 2x1015π/cm2 • Obtained 3.2 and 6.1 x1014π/cm2 • Beam problems at PSI OSU CCD measurements PSI irradiation site

11. Neutron irradiations • TRIGA Mark II research reactor at JSI • Irradiation channel in reactor core • Flux ~2x1012 neq/cm2 @ full power • Scalable down with power • 1016reached in a good hour ! • Dosimetry well established • Extensively used by RD-48, 50 • Threshold activation • Core simulations • Standard Si diodes • Scales perfectly with time at stable reactor power • Spectrum with two dominant components • Thermal – important for activation • Fast – exclusive contribution to NIEL in Si • 2 samples irradiated to same fluences in log steps • One pair: 1014, 1015,1016 (done, ½ done, to be done) • Second: 3x1014, 3x1015 • Programme in progress, next step when data understood Thermal Fast

12. Tex2440 Amplifier +shaper pad detector 90Sr thermal insulation cold plate scintillator Peltier cooler water cooled heat sink Sample evaluation • Charge integrating TCT set-up • DAQ chain (rate to disk ~ 50 Hz) • ORTEC 142B preamplifier • custom made 25 ns shaping amp. • Tex 2440 oscilloscope connected to PC • Triggered only by electrons fully traversing diode • ~98% purity assures good measurements also at low S/N<1 • Features • Peltier cooling up to T=-30°C (stable to 0.1°C) • HV (bias) up to 5 kV • Full computer control (automatic scans) • Routinely used in Si detector studies • Gain calibrated on 241Am photo-peak • Absolute charge collection measurement

13. First results • Non-irradiated samples • CCD of sensors as received agrees within 10 % with OSU data • Signal drops in first hour, then stable on days scale • CCD/V can exhibit polarization effects • Proceed to irradiations • 1014 and 3x1014 n/cm2 • Results initially depressing • Large CCD drop observed • Signal decreases with time • First pion irradiated sample • 3.2x1014 π/cm2 • CCD droped by ~20 % • Signal increases with time • Are neutrons really that much more damaging than pions ?? • Thought of 1015π/cm2 3x1014 n ~2h ~1h 3x1014π

14. Time heals all wounds • “Discovered” pumping • 37 MBq 90Sr source too well collimated • Pumping slow – takes days to saturate • After ~3 days complete CCD recovery for 3x1014 n/cm2 ! • Need about 10 kRad ionization • Automatic in experiment – 10 kRad = 3x 1011π/cm2 • Stable but don’t shine UV on it ! • Pumping method • Take 90Sr source out of collimator and put it 4 mm over detector • Pump & check CCD until saturated • Move source to collimator • Start measurement • Possible qualitative explanation (same as for Lazarus) • Traps get filled by ionization current • Cannot trap same carrier any more • Hole and electron trap densities roughly match • Saturated space charge compensated • In reality plenty of traps with different properties • gt, Et, charge states, capture σ, …. • Happy with results, proceed to 1015, 3x1015 n/cm2 ~3 days ~2 hours Q-TCT signal [mV] Pumping step ~ 20 min

15. Neutrons strike back • At 1015&3x1015n/cm2 pumping hits its limits, CCD decreases • Residual trapping starts to dominate over trapping on grain boundaries • CCD vs. voltage not saturated @ 1000 V • For mean free path in infinite detector expect • With CCD0initial trapping on grain boundaries, k a damage constant 1015 n 3x1015 n Not saturated @ 1 kV !

16. Results summary • Summary of all CCD measurements • Neutron curve drawn by hand to match data according to expected CCD dependence • Parameters used • CCD0 = 210 μm • k = 3.5x10-18μm-1cm-2 • Definitely too simplistic • Material not infinite • Errors to be understood • Pion data look overshooting • Lack high fluence data to pin down pion curve • Confront with proton data Preliminary

17. To do list • Finish irradiations • Carefully re-check procedures • Check contacts • Exchange samples with OSU • Add proton data • Evaluate scCVD sample, now at 1014 n/cm2 • Most apporpriate for basic material study • What’s going on in diamond ? • TCT with α particles • Large signal enables auto-triggering • Can in principle resolve space charge and trapping • TCT of non-irradiated scCVD gives reasonable data on drift velocity saturation scCVD TCT Current Transient time