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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