studies of heating issues with silicon sensors during

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STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING IRRADIATION AT THE BIRMINGHAM FACILITY IoP Meeting (Manchester April 2015) Matthew Baca University of Birmingham On behalf of Irradiation Team

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Page 1: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING IRRADIATION AT THE

BIRMINGHAM FACILITY

IoP Meeting

(Manchester April 2015)

Matthew Baca

University of Birmingham

On behalf of Irradiation Team

Page 2: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

THE BIRMINGHAM IRRADIATION FACILITYMC40 Cyclotron

Originally from Veterans Affairs Medical Centre in Minneapolis

Active at University of Birmingham since 2004

Provides p, d and He ion beams with range of energies

Proton energy range: 3 – 38 MeV (use 27.5 MeV for these irradiations)

Range of beam currents – typically use 1µA for sensor irradiations

Fluences of 1015 1MeV neq cm-2 in about a minute –Equivalent to the amount strip sensors are required to withstand at HL-LHC (3000 fb-1)

To date, 217 LHC upgrade components have been irradiated at the facility

See K. Parker’s talk for more details about the Irradiation procedure!

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Page 3: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

ANNEALING THEORYArrhenius Equation gives an equivalence factor for temperature dependent reaction rates.

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𝐹 =𝑒−𝐸𝐴𝑘𝐵𝑇1

𝑒−𝐸𝐴𝑘𝐵𝑇2

[‘Annealing effects in the n+ -p strip detectors irradiated with high neutron fluences’ I.

Mandic, V. Cindro, G Kramberger, M. Mikuz, Nuclear Instruments and Methods in Physics

Research A, Nov 2010]

For an activation energy EA = 1.31 eV, the acceleration factor between 20°C and 60°C is 500 (e.g. 1 minute at 60°C = 500 minutes at 20°C)

Therefore 1 minute at ~80°C is the equivalent of ~5 days* at 20°C!Understanding of temperature in irradiation is important.

*[‘Effects of accelerated annealing on p-type silicon micro-strip detectors after very high does of proton radiation’ G. Casse, P.P. Allport, A. Watson, Nuclear Instruments & Methods in Physics Research A, June 2006]

Page 4: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

TESTING OF SENSORS

Sensors irradiated to 1x1015 1MeV neq cm-2 at Birmingham and tested at Liverpool using ALiBaVa

Exposed to a 90Sr source at varying applied voltage

Collected charge recorded

Compared to results before/after annealing from KEK and Los Alamos.

Annealed at 60°C for 80 minutes. Equivalent to ~28 days at 20°C

Behaviour of Birmingham data similar to post-controlled annealing – Motivation to study and understand the temperature of the sensor during irradiation!

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[Data from P. Dervan, University of Liverpool]

Page 5: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

INITIAL COOLING SYSTEM AND BOX MOVEMENT

Box is mounted on an robotic x-y stage that steps the sensors through the beam.

Default:

Vertical speed = 20mm/s (fixed)

Horizontal speed = 1mm/s

Alternative horizontal speeds: 2mm/s, 4mm/s, 8mm/s and 10mm/s (speed selected when calculating path)

Glycol based cooling system. Operating at approximately -15°C Electric fan circulates air Constructed by University of Sheffield

Temperature readings are very high: dT ~ 100°C! Equivalent to roughly 10 weeks at 20°C!

Extreme annealing is likely. Need to understand temperature better! 5

Page 6: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

LIQUID NITROGEN COOLING SYSTEM

To combat dramatic temperature rises, a more effective cooling system was introduced

Norhof 915 System

Pumps liquid nitrogen from dewar into the box

Active temperature control system (monitor temperature/regulate liquid N2 flow)

Operated at about -48°C

Liquid nitrogen falls on to a metal ‘evaporator’ which has a large surface area

Fan blows air through the ‘teeth’ of the evaporator and allows it to circulate

Slope on opposite side of evaporator specifically directs airflow towards sample

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Dry Nitrogen

inputElectric fan

SlopeEvaporator

Silica gel

packs

Page 7: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

ACTIVE COOLING

Cooling system attempts to maintain ambient temperature of -48°C

In reality fluctuations are observed – temperature data recorded by the system.

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Cool

down

Beam on Warming up

Page 8: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

MEASURING THE SENSOR TEMPERATURE DURING IRRADIATION

The Pt-1000:

Platinum temperature sensor

1kΩ at room temperature

Resistivity depends on temperature

Traditionally Pt-1000 would be mounted on the sensor. That would mean Pt-1000 is irradiated and reading not the temperature of the sensor.

‘Finger’ of silicon irradiated in parallel to sensor

Pt-1000 on finger remains out of beam path

Extrapolate information from finger to infer sensor temperature

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Pt-1000Sensor

Finger

Page 9: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

VARYING SCANNING SPEED

Additional way to reduce temperature peaks

Scan faster and repeat more times to reach same fluence

Faster scans = more temperature peaks, each with lower maximum

Annealing rate is exponentially dependent on temperature – lower temperature peaks are more desirable.

Moving too fast risks damaging the mounted samples

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Page 10: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

CONSTRUCTING A MODEL FOR HEAT TRANSFER

Total Power = Power from beam on module – Power loss to air

PBeam = Input power from beam to module at given time (beam position and object surface area and volume dependent)

A = surface area of convection to air

TS – TA = difference in temperature between object and air

m = Mass of module

C = Specific Heat Capacity of module

h = Convection coefficient

Obtain value of h from finger data, and use it to obtain TSfor sensor

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𝑑𝑇

𝑑𝑡=𝑃𝐵𝑒𝑎𝑚 − ℎ𝐴(𝑇𝑆 − 𝑇𝐴)

𝑚𝐶 Calculating PBeam:

dE/dx (27.5MeV protons) = 15.7 MeV g-1 cm2

Density of Silicon = 2.33 g cm-3

dE/dx = 15.7 x 2.33 = 36.6 MeV cm-1

Each proton deposits 1.1 MeV in 300µm of silicon

At beam current of 1µA, PBeam = 1.1W

Note: This is for case where beam is aligned with sensor (maximum power)

Page 11: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

MODELING THE BEAM MOVEMENTSoftware looks at the programmed scan path and outputs a histogram showing fraction of total power from the beam (when centred on the sensor) for each object as a function of time

Model consults this histogram on each time step

Allows us to experiment with different beam paths and beam speeds and generate a prediction of how the temperatures will change

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Page 12: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

RESULTS EXTRAPOLATED FOR SENSOR CONFIGURATION

4mm/s

@ 0.5µA

4mm/s

@ 1µA

8mm/s

@ 0.5µA

Four datasets used 4mm/s @ 0.5µA

4mm/s @ 1µA

8mm/s @ 0.5µA

8mm/s @ 1µA

Differential equation solved numerically and model fitted to finger data with a χ2

minimisation.

Finger model fit to finger data and returned value for ‘h’ parameter

Average h = 64 ± 8 W m-2 K-1

Extrapolate this model to sensor dimensions and mass to model sensor temperature.

The inferred sensor temperature remains below -10°C.

SCT module specification required sensors remain below -7°C.

8mm/s

@ 1µA

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Page 13: STUDIES OF HEATING ISSUES WITH SILICON SENSORS DURING

SUMMARY

Model for heat loss via convection is consistent across all taken data sets. This can be used to estimate the temperature of the sensor during irradiations.

These results imply that the sensors remained below -10°C – in that case thermal annealing should not be an issue.

Thanks to:

Upgraded cooling system

Increased scan velocity

Using this model, we can estimate the temperature profiles reached for different scan paths at different speeds to check in advance whether annealing will be an issue or not.

Model is currently a work in progress in order to validate the assumptions made in the model’s construction.

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