studies of heating issues with silicon sensors during
TRANSCRIPT
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
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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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]
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]
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
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
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
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
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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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)
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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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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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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