Doses in the SVT until 2009 Brian Aagaard Petersen Stanford University

1. Doses in the SVT until 2009 Brian Aagaard Petersen Stanford University For BaBar Silicon Vertex Detector Group September 2003

2. SVT Doses Until 2009 Motivation: SVT performance degrades with integrated radiation dose Limit from irradiation tests: ~5 Mrad Currently trying to enforce 4 Mrad budget Have seen unexpected damage to read out after 1-1.5 Mrad Real budget might be significantly lower! Need to know how much radiation to expect Prediction Method: Instantaneious dose rates depend mainly on beam currents Fit dose rates as function of currents and extend into the future using beam currents predicted by John Seeman Given assumptions on running time, efficiency and injections, we can predict integrated doses in the SVT

3. Radiation Sensors Radiation measured by PIN-diodes placed near the SVT Two sensors per SVT module One forward, one backward of interaction point Sees 100-150% of the dose in SVT Silicon wafers Naming Convention: Forward Diodes Backward Diodes BE:TOP/ FE:TOP BW:TOP/ FW:TOP Most radiation comes in the mid-plane In general west diodes mainly sensitive to HER, while east diodes are sensitive to LER BW:MID/ FW:MID BE:MID/ FE:MID BW:BTM/ FW:BTM BE:BTM/ FE:BTM

4. Diodes versus SVT Dose rates proportional to SVT occupancy Scales with ~1% occupancy per 5 mrad/s except in FE:MID From DQ limits, the dose rates should be below 50-100 mrad/s Not understood why dose rate is higher here at low occu-pancy!

5. MID Radiation Doses Until Now Budget is set to reach 4 Mrad by 7/1-2005 (to be lowered?) FW:MID is consistently overestimated in Run 3

6. Non-MID Radiation Doses TOP diodes Bottom diodes At the moment all diodes are within 4 Mrad budget Need to make sure they will continue to be

7. 2003 Doses in Numbers Radiation doses from January to June 2003: Numbers are in krad, (%) is of dose in stable beams Dose during stable beam is almost completely in MID plane Radiation comes from lost particles Injections tend to have more uniform radiation pattern Mainly coming from HER injection and into west diodes

8. Modeling Dose Rates Model: Parameterize dose rate as a function of beam currents Dose rate = h1·IHER + h2·(IHER)2 + l1·ILER + l2·(ILER)2 In principle could also be cross terms and luminosity terms In most fits a simple linear model describes the data No attempt has been made to add the extra terms Fitting the Model: Fit model to data recorded with single beam Tried fitting collision data, but beam currents are too correlated to give meaning full results Data sets: 2001 data where PEP-II is coasting with one beam for hours Dedicated data from February 2002 single beam runs Part of the data is affected by annealing effects

9. 2001 Single Beam Data Data is taken from several periods with just one beam All fits are to second order in beam currents Results in good agreement with fit to 2000 data Dose rate in mrad/s FE:MID 2001 FE:MID 2000 LER current in mA

10. 2002 Single Beam Data MID diodes cannot be fitted due to annealing effects The non-MID diodes are fitted with only linear terms After offset correction, most diodes agree with 2001 model Linear Model to 2002 data Second order model from 2001 Offsets from zero are ignored in model - They are caused by imprecisions in the diode calibration

11. Model Parameters For MID diodes use fits to 2001 data with second order terms Non-MID diodes use fits to 2002 data with only linear terms Only significant second order term

12. Comparison to Run 3 Compared measured dose in 2003 with prediction from model With a few exceptions the average ratio is 1±0.3 for each diode Exceptions are diodes with known systematics Some diodes see higher than predicted rates from May 2003

13. MID Dose Rates until 2009 Use Seeman model for beam currents to predict dose rates: Dose rates do not looking forbiddingly high Rates peak at roughly the current soft abort limits - DQ should still be reasonable

14. Non-MID Dose Rates until 2009 For TOP and BTM diodes both 2002 and 2001 models were used The 2001 models with second order terms are up to 100% Effect on integrated doses will be less than 30% Use 2002 model for prediction non-MID diode doses

15. Integrated Doses Estimating Integrated Doses: Currents and number of running days taken from Seeman model Assume average dose rate is 80% of peak Accounts for downtime, injections etc. Injection dose obtained by scaling with 2003 injection fraction Hopefully a pessimistic assumption Assumed MID modules will be exchanged in 2005 Put in a possible ±60º rotation in 2007 Caveats: There are many uncertainties in the prediction: Future beam currents, injections, efficiency, effects of high beam currents, effects of dust events, the IR change... Predictions are probably only good to about 50%?

16. Midplane Doses until 2009 Module exchange in 2005 looks well timed with 4 Mrad budget One rotation in 2007 should be able to keep MID modules installed in 2005 below 5 Mrad A rotation will move the high dose in FE:MID to other module

17. TOP Module Doses until 2009 TOP modules look OK, except if FE:TOP becomes MID module BW:TOP and FW:TOP doses are probably overestimated 85-90% of the dose is supposed to come from injection

18. BTM doses until 2009 Either BW:BTM or FE:BTM will go above 5 Mrad after rotation Otherwise BTM diodes should be fine, unless budget is much lower than 4 Mrad

19. Summary SVT dose rates have been fitted to beam currents Used to predict rates at future currents Dose rates look reasonable enough for data quality With exchange in 2005 and rotation in 2007, most modules can be kept below 5 Mrad If budget is only 2-2.5 Mrad, most modules will have problems before 2009 Effect of different rotation strategies needs to be explored