1 Charge collection in Si detectors irradiated in situ at superfluid helium temperature E. Verbitskaya, V. Eremin Ioffe Physical-Technical Institute, St. Petersburg, Russian Federation B. Dehning, C. Kurfürst, M. Sapinski, M. R. Bartosik CERN, Geneva, Switzerland N. Egorov Research Institute of Material Science and Technology, Zelenograd, Russian Federation J. Härkönen Helsinki Institute of Physics, Helsinki, Finland RESMDD14 Florence, Oct 8-10, 2014

2 Outline  Motivation  In situ irradiation test  Experimental methods and results  Simulation of collected charge in irradiated Si detectors  TCT results  Analysis  Current status and plans  Conclusions E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

3 Motivation Task: Upgrade of Beam Loss Monitoring (BLM) system of LHC with high luminosity of the proton beams based on semiconductor detectors Current position of BLMs (gaseous detectors) – outside the cryostat of the triplet magnets  measurement of the energy deposition into the magnet coils is limited because of the collision debris, which are masking the beam loss signals Semiconductor detectors will be placed inside the cold mass as close as possible to the superconducting coils (CryoBLMs)  measured dose corresponds more precisely to the dose deposited into the coil (allows preventing quench-provoking damage). Possible candidates: silicon and diamond detectors Question: what is the radiation hardness at T~(2-4)K? E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

4 Requirements to detector operation  T = 1.9 K (superfluid helium);  Integrated dose of about 1x10 16 p/cm 2 (~2 MGy in 20 year);  Linear detector response between 0.1 and 10 mGy/s (the range of signals expected close to the quench), and faster than 1 ms;  Magnetic field of 2 T and a pressure of 1.1 bar;  Stability within operational time of several years. E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

5 Interstitials – mobile at T~4K Vacancies (V+, V-) - mobile at: T~70K (standard n-Si) T~150K (standard p-Si) T~ 200K (high resistivity Si) Radiation damage in Si at low T G. D. Watkins, Defects and diffusion in silicon processing, Ed. T. D. De la Rubia, et al.; MRS Sypm. Proc. Vol. 469, Pittsburgh (1997) 139. G. D. Watkins, EPR of Defects in Semiconductors: Past, Present, Future, Phys. of Solid State, 41 (1999) Radiation damage at LHe T: formation of secondary vacancy- related defects critical for degradation is expected to be suppressed E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

6 Earlier results on in situ irradiation at low T Irradiation at 83 K by 450 GeV protons After p/cm 2 CCE ~ 55% at 200V (before annealing) 1 st annealing: 160 K, 1 h, CCE ~95% 2 nd annealing: 207 K, 1 h 3 rd annealing: RT, 1 year, CCE~55%. CCE recovery by low temperature operation is not affected by the temperature of irradiation and by reverse annealing. G. Ruggiero, et al., Silicon detectors irradiated ‘‘in situ’’ at cryogenic temperatures, NIM A 476 (2002) 583 E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, x10 15 p/cm 2

7 In situ irradiation test  Special system for cooling down to 1.9 K  Irradiation at CERN PS, about six weeks totally  23 GeV protons, FWHM of beam diameter ~1 cm at the detector location  Beam intensity 1.3×10 11 proton/cm 2 per 400 ms spill (1×10 10 p/s on detectors)  Maximum fluence 1×10 16 p/cm 2  p + -n-n + silicon pad detectors designed and processed by Ioffe Physical- Technical Institute, St. Petersburg, and Research Institute of Material Science and Technology, Zelenograd, both Russia  : 10 k  cm, 500  cm and 4.5  cm; 300  m  Collected charge Q c is determined by integrating the detector output DC current over 400 ms spill, averaging within each 16 ms range in each of 20 spills.  TCT, LeCroy WavePro 7300A, 3 GHz bandwidth, 630 nm laser, width 45 ps width Thermal history: irradiation at 1.9K; heating to 80K (meas.) and then to RT (meas.); deactivation - kept in the fridge at -18°C in M. Moll Lab at CERN E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

8 Q c (F) dependences 1×10 14 – 1×10 16 p/cm 2 Q c ~ F -0.5 Q c ~ F -1 Currrent Injected Detector (CID)  Different slope in Q c (F) dependences  Larger Q c in CID at F< 1x10 16 p/cm 2  The same Q c at  V and different  at F = 1x10 16 p/cm 2 E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014 Error ±15%

Procedure and main parameters ♦ EXCEL worksheet, numerical calculation/simulation ♦ Poisson equation combined with the rate equation ♦ one-dimensional approach for detector geometry, E = E(x) ♦ Variable: F, T, V ♦ Deep levels: DA E c – 0.53 eV; DD E v eV (fit 1) or E v eV (fit 2) ♦  e,h =  e,h F eq ;  – variable parameters 9 Approximation of Q c (F) using Hecht equation E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

10 Q c (F,V) dependences ♦ At LHe T the model gives uniform E(x); real E is nonuniform (?) ♦ Reasonable agreement in Q c (F) dependences ♦ Fit requires essential reduction of  tr E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014 fit 1: DD E v eV; fit 2: DD E v eV

11 TCT results  Si detector 1  1 mm 2  Laser 630 nm, width 45 ps, f = 10 kHz LeCroy WavePro 7300A, 3 GHz bandwidth  Bias voltage up to  400 V (randomly 500 V)  Before irradiation: signals from both sides  Cooling to 1.9K without irradiation Pulse width: RT – 4 ns, 1.9 K – 2 ns (v dr increases)  In situ irradiation – signal from n + side Before irradiation : RT, signal from p + V rev = 30 V E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

12 Current pulse response vs. bias voltage mode 5  p/cm 2 Laser at n + -side P+-n-n+: Low E at n + contact E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

13 Influence of spill charge and frequency at V rev E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014 Signal increase due to spill charge j = en o f / S o No influence at frequency reduction High E at n+

14 Influence of spill charge and frequency at V forw  DP signal at 5  p/cm 2 and V rev ;  SCSI at F comparable with RT;  Degradation rate is rather high at both bias polarities;  CID is advantageous for charge collection;  Signal value and its shape are affected by frequency and spill (carrier injection and trapping) E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

15 Analysis: correlation to defect formation at LHe T Expected:  Minimum of vacancy-related defects (vacancies are immobile at LHe temperatures)  Carrier freezing even at shallow levels  Reduction of N eff, uniform electric field, saturation of drift velocity at higher V. E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014 E mean = V/d

16 Results give rise to imply:  Concentration of radiation-induced defects and N eff are not so small as expected;  No full compensation of donors and acceptors;  Trapping is essential Analysis E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

17 I-V characteristics after reverse annealing  I-V characteristics of detectors irradiated at LHe, heated to RT and then stored in the fridge are the same as after RT irradiation  reverse annealing is insensitive to irradiation temperature. E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

18 Current status of semiconductor BLM development April 2014: Si detector modules are installed on the magnets December 2014: test of a new setup (new beam area, new cryostat, new acquisition system) 2015 (plan): New irradiation test E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

19  The results of in-situ irradiation showed that Si detectors are appropriate for BLM application at 1.9K after 1  p/cm 2 irradiation.  The rate of signal degradation rate is higher than at RT irradiation.  The possible reason may be due to a prevailing formation of hole traps (no compensation of donors and acceptors) and corresponding trapping.  Operation in CID mode is more preferential for charge collection.  New tests are planned. Conclusions E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014

20 This work was performed: - within the framework of Agreement on Scientific Collaboration between CERN-BE-BI-BL group and Ioffe Physical-Technical Institute, - in the scope of the CERN-RD39 collaboration program, and supported in part by the Fundamental Program of Russian Academy of Sciences on collaboration with CERN. Great thanks to Maurice Glaser and Federico Ravotti for irradiation! Thank you for attention! Acknowledgments E. Verbitskaya, et al., RESMDD14, Florence, Oct 8-10, 2014