Investigation of radiation hard

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Investigation of radiation hard sensor
materials
.

K. Afanaciev on behalf of FCAL collaboration
The Xth International School-Seminar The Actual
Problems of Microworld Physics Gomel, Belarus 2009

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Very Forward Region of the ILC Detector
Interaction point
BeamCal

• EM calorimeter with sandwich structure
• 30 layers of 1 X0
• 3.5mm W and 0.3mm sensor
• Angular coverage from 5 mrad to 45 mrad
• Moliére radius RM 1cm
• Segmentation between 0.5 and 0.8 x RM

• The purpose of the instrumentation of the very
  forward region is
• Hermeticity increase the coverage to polar
  angles gt 5mrad
• Fast beam diagnostics

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The Challenges for BeamCal
Creation of beamstrahlung at the ILC
e-
e
Interaction
Bethe-Heitler process
ee- pairs from beamstrahlung are deflected into
the BeamCal 15000 ee- per BX gt Edep ? 10
20 TeV 5 MGy per year strongly dependent on
the beam and magnetic field configuration gt
radiation hard sensors Detect the signature of
single high energetic particles on top of the
background. gt high dynamic range/linearity
1 MGy/y
5 MGy/y
For 1 layer, per cell
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Irradiation facility
Superconducting DArmstadt LINear
ACcelerator Technical University of Darmstadt
Irradiation up to several MGy using the injector
line of the S-DALINAC 8 and 10MeV electrons,
beam currents from 2 to 100 nA corresponding to
doserates about 10 to 600 kGy/h
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Investigated materials

• Gallium arsenide (GaAs),
• Polycrystalline CVD (chemical vapour deposition)
  Diamond (pCVD)
• Single crystall CVD Diamond (sCVD)
• Silicon (common detector-grade Si for comparison)

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GaAs Si Diamond Density 5.32 g/cm3
2.33 3.51 ? Pair creation E 4.3 eV/pair
3.6 13 ? Band gap 1.42 eV 1.14 5.47 ?
Electron mobility 8500 cm2/Vs 1350 2200 Hole
mobility 400 cm2/Vs 450 1600 ? Dielectric
const. 12.85 11.9 5.7 ? Radiation
length 2.3 cm 9.4 18.8 Ave. Edep/100
?m (by 10 MeV e-) 69.7 keV 53.3 34.3
Ave. pairs/100 ?m 13000 9200
3600 Structure p-n or insul. p-n insul.

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Methodology. Irradiation

• Irradiation under bias voltage
• Monitoring of beam and sample currents, sample
  temperature

exit window of beam line
Faraday cup (IFC, TFC)
collimator (IColl)
sensor box (IDia, TDia, HV)
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Methodology. CCD Setup
Sr90
sample
ADC
delay
Sr90 source
Scint.
discr
PM1

Gate
discr
PM2
Preamplifier
Sensor box
Trigger box
typical spectrum of an E6 sensor
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pCVD Diamond Detector
(courtesy of IAF)

• pCVD diamonds are an interesting material
• radiation hardness
• nice properties like high mobility, low eR
  5.7, thermal conductivity
• availability on wafer scale
• Solid-state ionisation chamber (no p-n junction)
• Samples from two manufacturers were investigated
  
• Element SixTM (ex-DeBeers)
• Fraunhofer Institute for Applied Solid-State
  Physics IAF
• 1 x 1 cm2
• 200-500 µm thick (typical thickness 300µm)
• Ti(/Pt)/Au metallization

(courtesy of IAF)
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pCVD Diamond. Irradiation
A number of samples from two producers were
irradiated
Typical behaviour Increase in CCD at low dose
gt pumping - i.e. filling of the traps Then
gradual decrease of efficiency with dose
After absorbing 7MGy CVD diamonds still
operational.
CCD of the pumped value vs dose
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pCVD Diamond. Irradiation results
E6_4 sample from Element 6, 500 µm
-80
Signal decreased by ? 80 after absorbed dose of
about 7 MGy But signal is still visible. Slight
increase in current, but still in pA range
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pCVD Diamond. Irradiation results
FAP5 sample from IAF, 500 µm
Signal from sample decreases with time after
irradiation further decrease after purging
trapping levels with UV light Signal increases
back after irradiation with small dose - pumping
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sCVD Diamond detector
Single crystal CVD (chemical vapour deposition)
diamond
CVD growth on top diamond substrate Low
defect content, very good detector properties
- Small area (up to 5x5 mm), very high price
Sample produced by Element Six 5x5 mm, 320µm
thickness initial charge collection efficiency
about 100 (CCD 320µm)
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sCVD Diamond. Irradiation results
Irradiation to 10 MGy CCE dropped to 10 of the
initial value
No visible annealing in 18 month
No significant increase in the dark current after
the irradiation (still in pA range)
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sCVD Diamond. Switched HV
Charge trapping leads to the decrease in the
efficiency due to nonuniform charge
distribution gt switched HV supply gt more
uniform charge distribution
CCD converge to about 1.5x higher and no time
dependence
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Diamond Detector. Linearity
Testbeam at CERN Proton Synchrotron. 4 GeV
hadrons Response to 10 ns spills of variable
intensity
linear response for particle fluxes from 2x104
MIP/cm2 up to 3x106 MIP/cm2
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GaAs Detector
Supplied by FCAL group at JINR Produced by
Siberian Institute of Technology, Tomsk
Sample is semi-insulating GaAs doped by
Sn (shallow donor) and compensated by Cr (deep
acceptor). This is done to compensate electron
trapping centers EL2 and provide i-type
conductivity.
Sample works as a solid state ionisation
chamber Structure provided by metallisation
(similar to diamond)
500 ?m thick detector is divided into 87 5x5 mm
pads and mounted on a 0.5mm PCB with fanout
Metallisation is V (30 nm) Au (1 ?m)
Same material - simple 4 pad structure

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GaAs. Signal
S8 pad4 ring 4
Clear separation of peaks from Sr90 source
S8 pad4 ring 6
Quite homogeneous response over different pads
Saturation of signal _at_ about 200V bias
Collection efficiency ? 60
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GaAs. Irradiation results
? Sample 7
? Sample 8
Preliminary
Samples 78, pad4, ring6 _at_ 200V
Results CCE dropped to about 6 from 60 (by
90) after 1.5 MGy this corresponds to signal
size of about 2000 e-
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GaAs. Irradiation results
Dark current increased ? 2 times (from 0.4 to 1
?A _at_ 200V)
Signal is still visible for an absorbed dose of
about 1.5 MGy
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GaAs. Second testbeam
A set of GaAs samples with different doping
concentrations was irradiated
Batch Shallow donor type Concentration, cm-3
1 Te
(1-1.5)1017 2 Te
(5-6)1016 3 Sn
(1-3)1016
Thicknesses 150 200 µm
Metallization V (30 nm) Au (1 µm) from both
sides
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GaAs. Irradiation results
A set of GaAs samples with different doping
concentrations was irradiated
Batch 3 - 90 drop in CCE _at_ 1.2 MGy
Batch with lowest shallow donor concentration
shows best radiation hardness
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Silicon. Comparison
For comparison purposes two silicon detectors
were irradiated (detector grade by Micron)
Degradation of signal starts at doses ? 40
kGy At higher doses dark current and noise
level increases dramatically (from ? 10 nA to ?A
range). No clear signal visible
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Conclusion

• For Electromagnetic irradiation
• pCVD Diamond operational up to 7 MGy
• sCVD Diamond operational up to 10 MGy
• Semiinsulating GaAs operational up to 1.5 MGy

Diamond detector have linear response for
particle fluxes from 2x104 MIP/cm2 up to 3x106
MIP/cm2
At the moment CVD diamond and GaAs prove to be
sufficiently radiation hard for application in
BeamCal Inconsistent results from different
diamond samples could be a problem Pumping-depump
ing behavior could be a problem for calorimetry
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Future work
A new testbeam is planned for this year New GaAs
samples are being produced by JINR FCAL group We
are going to test samples with different donor
and acceptor concentrations. More tests are
needed to understand the possible application
of switched HV to improve the diamond detector
efficiency Possible new materials? Thank you
for your attention
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Backup slides
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Irradiation facility