1. Jim Brau, Amsterdam, April 2, 2003 1 Nikolai Sinev and Jim Brau University of Oregon April 2, 2003 Radiation Damage Studies of Vertex Detector CCDs First studies of radiation damage to spare CCD ladders of the SLD VXD3 were reported in –IEEE Trans. Nucl. Sci. 47, 1898 (2000) Now we are renewing these studies with –passive annealing investigation (of old damage) –post-mortem measurements of damage in VXD3 itself –new exposures with electrons

2. Jim Brau, Amsterdam, April 2, 2003 2 Radiation Hardness Surface Damage from ionizing radiation hard to > 1 Mrad (acceptable for LC) (however, SLD VDX3 damage!) Bulk Damage results in loss of charge-transfer efficiency (CTE) ionizing radiation damage suppressed by reducing the operating temperature hadronic radiation (neutrons) damage clusters  complexes Temperature (K) 100 140 180 220 260 300 Charge Transfer Ineff. MS Robbins

3. Jim Brau, Amsterdam, April 2, 2003 3 SLD Experience during VXD3 commissioning, An undamped beam was run through the detector, causing radiation damage in the innermost barrel. The damage was observed as the detector was operating at an elevated temperature (  220 K). Reducing to 190 K ameliorated the damage VXD3 Experience on Radiation Damage Temperature (K) 100 140 180 220 260 300 Charge Transfer Ineff.

4. Jim Brau, Amsterdam, April 2, 2003 4 Background estimates for the next Linear Collider have varied from 10 7 n/cm 2 /year to 10 11 n/cm 2 /year - 2.3 x 10 9 n/cm 2 /year (Maruyama) Expected tolerance for CCDs in the range of 10 9-10 Increase tolerance to neutrons can be achieved through improve understanding of issues and sensitivity engineering advances flushing techniques supplementary channels bunch compression & clock signal optimization other ideas Neutron Damage

5. Jim Brau, Amsterdam, April 2, 2003 5 Theory of Bulk Damage The most important radiation damage in CCDs caused by heavy particles is displacement in the bulk silicon. 1 MeV neutrons can transfer up to 130 keV to PKA. Only 15 eV is needed to displace an atom from the lattice. Example of simulated tracks of knock-out silicon atoms from a primary knock-out energy of 40 keV. (V.A.J.Van Lint, NIM A253, 453 (1987).) Vacancy (V) and interstitial silicon (I) pairs are created as a result of atom displacement. More than 90% of such pairs recombine immediately. Those which are not recombined diffuse until they form complexes of two or more vacancies (V2 or V3) or vacancy-impurity (VP, V2O and so on). Such complexes are usually not mobile. Some of them are able to bind electrons, and the bound energy for some of these is about 0.35 - 0.5 eV below the conduction band. These may act as electron traps when empty. If the bound energy is close to the conduction band, (shallow traps) the lifetime of the bound state is so short, that the trapped electron will be released quickly and re-join the charge packet before the packet passes through the trap region. In this case no charge transfer inefficiency will be introduced by the defect.

6. Jim Brau, Amsterdam, April 2, 2003 6 Theory of Bulk Damage (cont.) However, for the deeper levels (close to 0.5 eV below the conduction band) the lifetime of the bound state, which is: is larger than the inter-pixel transfer time, so trapped electrons are removed from the charge packet and released after the packet passes through the trap region. This leads to charge transfer inefficiency. Such inefficiency may be cured, however, by cooling the CCD to a low enough temperature, that the lifetime of the bound electrons in the trap becomes very long, so that the filled traps remain occupied when the next charge packet passes. Filled trap can't capture more electrons, so this trap will not lead to charge transfer inefficiency.

7. Jim Brau, Amsterdam, April 2, 2003 7 R&D Plan 1998-99 - first neutron damage study IEEE Trans. Nucl. Sci. 47, 1898 (2000) 2003 - investigation of status of old irradiations (passive annealing) - study of the nature of the damage in SLD VXD3 (in progress) - exposure of test CCD ladders to moderate energy electrons (planning)

8. Jim Brau, Amsterdam, April 2, 2003 8 History of Neutron Exposures Nov 98~ 2  10 9 n/cm 2 room temperature Pu(Be)  4 MeV Dec98-Jan 99Annealing study100  C for 35 days Mar 99~ 3  10 9 n/cm 2 room temperature reactor  neutrons  1 MeV ( ~1  10 9 n/cm 2 lower energy) Apr 99~ 1.5  10 9 n/cm 2 dry ice cooled (~190K) reactor  neutrons  1 MeV ( ~1  10 9 n/cm 2 lower energy) Total exposure ~ 6.5  10 9 n/cm 2 mix of source and reactor * UC Davis (G. Grim et al)

9. Jim Brau, Amsterdam, April 2, 2003 9 Images of damaged sites Neutron Damage Study Individual traps are identified by charge losses during readout IEEE Trans. Nucl. Sci. 47, 1898 (2000) T=187K, after dose of 2x10 9 n/cm 2 T=187K, after dose of 5x10 9 n/cm 2

10. Jim Brau, Amsterdam, April 2, 2003 10 Readout timing diagram 600 e/pxl25 e/pxl Filling traps Clearing pixels Filling traps Clearing pixels

11. Jim Brau, Amsterdam, April 2, 2003 11 Image of damaged sitesImage of damaged sites after flushing Neutron Damage and Amelioration Study Basic concept demonstrated; future work will involve charge injection to keep traps filled. IEEE Trans. Nucl. Sci. 47, 1898 (2000)

12. Jim Brau, Amsterdam, April 2, 2003 12 Pulse height distribution

13. Jim Brau, Amsterdam, April 2, 2003 13 Damage Results Defect Results from Exposures # defect (> 6 e - ) # defect (>20e - ) 800,000 pixels 800,000 pixels Prior to exposure 12524 Nov 98 exposure 916 160 ~ 2  10 9 n/cm 2 source Mar 99 exposure 5476 442  + ~ 3  10 9 n/cm 2 reactor Apr 99 exposure 7036 298  + ~ 1.5  10 9 n/cm 2 reactor * this surprising decrease is not understood Signal Loss Results from Exposures Exposure(10 9 n/cm 2 ) ~ 2 ~ 6.5 T = 185K, cluster sum 4.05% 29.1% no flushing light T = 185K, cluster sum 1.5% 18.0%  with flushing light T = 178K 11.0%  Note (  ) - flush is only partially effective due to required delay between flush and readout (1 second) In LC detector – better result expected

14. Jim Brau, Amsterdam, April 2, 2003 14 Measurements repeated in 2003 Damage sites remain

15. Jim Brau, Amsterdam, April 2, 2003 15 Comparison of loss 1999/2003 Electrons (1999) Electrons (2003) Electrons (2003)

16. Jim Brau, Amsterdam, April 2, 2003 16 Post-mortem tests of VXD3 Have removed VXD3 from SLD and will do measurements to establish the nature of the damage

17. Jim Brau, Amsterdam, April 2, 2003 17 Summary of new studies 1. Test stand for testing radiation damage of the CCDs has been fully operational for about 2 months now. 2. The 1998-1999 measurements of neutron induced radiation damage has been confirmed. 3. Some of the damage (~30%) has annealed in the 4 years, during which the CCDs were stored at room temperature. 4. We are now collecting high statistics on the damage in these detectors, after which we plan to irradiate the same CCDs with relatively high energy (tens of Mev) electrons, to compare the characteristics of damage inflicted by electrons and neutrons. We are looking for the beam to do the electron irradiation. 5. VXD3 detector has been extracted from SLD and is waiting of disassembling.

18. Jim Brau, Amsterdam, April 2, 2003 18 Conclusion

19. Jim Brau, Amsterdam, April 2, 2003 19 Extras

20. Jim Brau, Amsterdam, April 2, 2003 20 Vertex Detectors Design CCD’s for –Optimal shape ~2 x 12 cm –Multiple (~20) ReadOut nodes for fast readout –Thin -≤ 100 µ –Improved radiation hardness –Low power Readout ASIC –No connectors, cables, output to F.O. –High reliability –Increased RO speed from SLD VXD3 –Lower power than SLD VXD3

21. Jim Brau, Amsterdam, April 2, 2003 21 Vertex Detectors, continued Mechanical –Eliminate CCD supports, “stretch” Si. –Very thin beampipes?? –Cooling Simulation –Quantify/justify needs SLD VXD3 has been removed from SLD for damage analysis of CCD’s.

22. Jim Brau, Amsterdam, April 2, 2003 22

23. Jim Brau, Amsterdam, April 2, 2003 23

24. Jim Brau, Amsterdam, April 2, 2003 24 (100 o C after 2 x 10 9 ) (days)

25. Jim Brau, Amsterdam, April 2, 2003 25