Radiation damage in the STAR environment and performance of MAPS sensors

1. Radiation damage in the STAR environment and performance of MAPS sensors Compilation of different test results mostly from Michael’s thesis

2. Outline • What can we expect at STAR? • Figure of merit – S/N • Ionizing radiation effects • Non-ionizing radiation effects • Simple noise model • Some solutions and issues

3. What can we expect at STAR? Based on H.W. estimates (http://rnc.lbl.gov/~wieman/radiation dose straus oct 2007 HW.ppt) • For the radius of 2.5 cm: • Ionizing radiation: • Total dose: 155 kRad • TLD projection: 300 kRad • Non-ionizing radiation • average pion count for 1 Yr: 3x1012 cm-2 • TLD projection (pion assumption): 12x1012 cm-2

4. Figure of merit for high detection efficiency is S/N • Test results of MimoSTAR2 show that • Measured with cuts on clusters • Similar performance can be expected from single threshold algorithm for S/N>12 detection efficiency >99.6%

5. Ionizing radiation effects • Increased leakage current • Increase of shot noise • Faster discharge time of the self-biased structure that in extreme cases could lead to signal losses

6. Time constant for discharge of the self-biased node from Michael’s thesis for the radiation tolerant diode layout and the operating temperature of 40 °C

7. Discharge time • At LBL  =130 ms at 30 °C and about 11 ms at the unknown (uncontrolled “ambient”) temperature for a non-irradiated sensor • the charge collection efficiency (CCE) at the level of 77%, the sensor integration time was 1.7 ms • at IPHC for the integration time of 4 ms CCE of 75% • short discharge time constant on the order of 10 ms (6 x integration time) did not affect signal collection Degradation of the discharge time after integrated ionizing dose of a few hundred krad should not be a problem.

8. Non-ionizing radiation • Charge losses due to bulk damage • Increased leakage current (not negligible) • Increase of shot noise

9. Charge losses Mimosa-9 -20 °C Tint 0.7 ms Mimosa-9 -20 °C Tint 0.7 ms 0.7 signal Half of signal Mimosa-15 -20 °C Tint 0.7 ms pixel pitch 20 μm. -20 °C – minimized contribution from shot noise (negligible for fluences up to 2x1012 neq/cm2) Half of signal irradiation inherently added ionizing background that is approximately 100 krad for the fluence of 1x1013 neq/cm2. This background can be expected to scale linearly with the dose.

10. Black: Ref Blue: 4.7x1011 Purple: 2.1x1012 Red: 5.8x1012 Green: 1.1x1013 Charge collection efficiency (CCE) • Beam Test Calibration of the Mimosa-15 prototype (S. Amar-Youcef, M. Deveaux, M. Goffe – 05.2006) At -20 °C and +20 °C: • 20 μm pitch and 2.1x1012 neq/cm2 the CCE decrease to about 0.5 • 30 μm pitch (MimoSTAR-like) CCE decreases to • 0.66 (4.7 x 1011 neq/cm2) • 0.39 (2.1 x 1012 neq/cm2) MimoSTAR2 pixel type

11. Consequences • The charge losses after 2.1 x 1012 neq/cm2 will lead to the S/N reduction: • 28 to 14 for the 20 μm pitch • 20 to 8 for the 30 μm pitch • This results in an unacceptable degradation of S/N for the MimoSTAR2-like pixel. MIP detection efficiency of irradiated prototypes was measured only at 0 °C due to the limited duration of the beam tests. This limits the estimation of the detection efficiency achievable at STAR to theoretical considerations of the evolution of S/N.

12. Leakage current (noise) from bulk damage 3T pitch 20 μm SB pitch 30 μm. after 2x1012 neq/cm2 the noise degrades from 15 to 18 e- Chip 3 ref Chip 4 4.7x1011 Chip 6 2.1x1012 Chip 10 5.8x1012 Chip 11 1.1x1013 Chip 5 ref Chip 7 23 krad γ Chip 8 20 krad X Chip 9 1 Mrad X

13. Simple model for noise performance Mimosa-11(rad-hard) • Ni(I,Tint) = A × √I × √Tint • N=√(N02+Ni2) • Temperature • Rule of thumb 2 x I every 10°C • Ionizng radiation • At 40 °C shot noise contribution is • 0 krad => 10 e- • 500 krad => 20 e- • 1000 krad => 30 e- • linear fit: (0.002 x Dose(krad)+1) • After tuning: (0.003 x Dose(krad)+1) • Model fits data well (within 10%) • Non-ionizing radiation • At 20 °C shot noise contributions is • 0 => 11 e- (incompatible with results for ionizing radiation) • 2.1 x 1012 => 15 e- • 5.8 x 1012 => 21 e- • 1.1 x 1013 => 29 e- • Linear fit: (0.14 x Dose(1012)+1) • Model fits data well but noise incompatible with the measurements on Mimosa-11 (leakage current needs to be x4 to fit the results) A × √I = 10 at 40 °C N=√(N0^2+ (A × √I × √Tint × (0.003 x Dose(krad)+1) × (0.14 x Dose(1012)+1))^2)

14. Simple model for charge losses Exponential dependency?

15. MimoSTAR2 pixel • Single detector replacement – not sufficient • Full elimination of shot noise – not sufficient • Annealing ?

16. MimoSTAR2 pixel PHASE1 integration time

17. Solutions • Decrease pixel pitch to 20 μm • # columns x 1.5 => power dissipation x 1.5 • column length x 1.5 => integration time unchanged • SUZE can not run faster than designed (160 ns) • Cluster size? • Signal decrease to 0.5 from non-ionizing radiation damage (2.1 x 1012 neq/cm2) • And noise increase 1.15 @ 0.2 ms integration time (assumption for 155 krad @ 40 °C) • S/N goes down to about 0.44 (28 =>12) it looks OK but what is the accuracy of these estimations? (detector replacement S/N~16)

18. Other solutions • Smaller pixel pitch (15, 18 μm) (power dissipation?) • 30 μm pitch with multiple charge collecting diodes? • Graded substrate (!) • Deep P implants • Latch up in SUZE and Mimosa22 needs to be investigated • Hot pixels after non-ionizing irradiation (Function of dose? Annealing?) • How to mask them for SUZE? • Irradiations with pions? Other issues

19. Noise from Mimosa15

20. Distribution of discharge time constant (MimoSTAR2)