doses in the svt until 2009 brian aagaard petersen stanford university
DESCRIPTION
Doses in the SVT until 2009 Brian Aagaard Petersen Stanford University For BaBar Silicon Vertex Detector Group September 2003. SVT Doses Until 2009. Motivation: SVT performance degrades with integrated radiation dose Limit from irradiation tests: ~5 Mrad - PowerPoint PPT PresentationTRANSCRIPT
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Doses in the SVT until 2009
Brian Aagaard PetersenStanford University
For BaBar Silicon Vertex Detector Group
September 2003
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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
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Radiation SensorsRadiation 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
Forward Diodes
Backward DiodesBE:TOP/
FE:TOP
BE:MID/FE:MID
BE:BTM/FE:BTM
BW:TOP/FW:TOP
BW:MID/FW:MID
BW:BTM/FW:BTM
Naming Convention:
Most radiation comes in the mid-planeIn general west diodes mainly sensitive to HER, while east diodes are sensitive to LER
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Diodes versus SVTDose 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!
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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
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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
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2003 Doses in Numbers
FW FE BW BETOP 81 (14%) 52 (41%) 100 (9%)MID 374 (63%) 333 (67%) 488 (66%) 215 (65%)BTM 90 (30%) 59 (38%) 82 (20%) 58 (45%)
25 (55%)
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
Radiation doses from January to June 2003:
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Modeling Dose Rates
Model:Parameterize dose rate as a function of beam currents
Dose rate = h1·IHER + h2·(IHER)2 + l1·ILER + l2·(ILER)
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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 termsFitting the Model:Fit model to data recorded with single beam
Tried fitting collision data, but beam currents are too correlated to give meaning full resultsData 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
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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
FE:MID 2001
FE:MID 2000
LER current in mA
Dose
rate
in m
rad/s
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2002 Single Beam DataMID 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
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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 termsDiode
FW:TOP 1.67 - .96 -FW:MID 8.38 .70 0 0FW:BTM 1.59 - .78 -FE:TOP .64 - .91 -FE:MID 0 0 12.70 1.60FE:BTM .94 - 1.38 -BW:TOP 1.40 - .65 -BW:MID 24.60 .91 0 0BW:BTM 1.73 - .69 -BE:TOP 1.24 - .67 -BE:MID 0 0 7.67 0BE:BTM .97 - .45 -
h1 h2 l1 l2
Only significant second order term
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Comparison to Run 3Compared 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
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MID Dose Rates until 2009Use Seeman model for beam currents to predict dose rates:
Dose rates do not looking forbiddingly highRates peak at roughly the current soft abort limits- DQ should still be reasonable
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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
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Integrated DosesEstimating 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%?
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Midplane Doses until 2009Module 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:MIDto other module
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TOP Module Doses until 2009TOP 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
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BTM doses until 2009Either 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
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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