Diapositiva 1

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RESMDD02 July 10-12, 2002 Florence
SiC and Diamond dosimetry for radiotherapy
applications
F. Nava INFN Bologna University of Modena, Italy
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RESMDD02 July 10-12, 2002 Florence
Silicon Dosimeters low radiation
hardness Sensitivity dependence on the
accumulated dose
S(D) c1(1-aD)c2exp(-bD)
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RESMDD02 July 10-12, 2002 Florence
Advanced Dosimeters ? High bandgap semiconductors
CVD Diamond
Advantages
tissue equivalent ( Z 6 ) high radiation
hardness Non toxic High density (high spatial
resolution)
Schottky barrier Epitaxial SiC 
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RESMDD02 July 10-12, 2002 Florence
Advanced Materials for radiation detection
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RESMDD02 July 10-12, 2002 Florence
Crystal Structure of Silicon Carbide 
Silicon carbide is known as a wide band gap
semiconductor existing in many different
polytypes. All polytypes have a hexagonal frame
with a carbon atom situated in the center of a
tetrahedral of Si atoms.
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RESMDD02 July 10-12, 2002 Florence
SiC polytypes
Different arrangements with variable percentage
of hexagonality depending on the stacking of Si
and C atoms along the c-axis the structure can
be cubic, hexagonal or rhomboedric
3CZincBlende
2HWurtzite
The electronic properties of the crystal are
affected by the polytypes forms
Gap depends on the percentage of hexagonality h
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RESMDD02 July 10-12, 2002 Florence
IRRADIATION FACILITY Radiotherapy Unit -
Clinical Fisiopathology Department - Florence

• 22 MeV electrons
• from linear accelerator
• dose-rates 0.5-4 Gy/min
• 6 MV photons
• from linear accelerator
• dose-rates 2-10Gy/min
• ?-radiation
• from Co60 source
• dose-rates .1-.3Gy/min

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RESMDD02 July 10-12, 2002 Florence
DOSIMETRIC CHARACTERIZATION
Dose measurement cylindrical ionization chamber
Farmer NE257 Source-Surface Distance SSD 70-100
cm Field 10x10 cm2
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SiC diode and diamond devices can be embedded in
tissue equivalent epoxy resin to avoid the
contribution of air to the signal response
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RESMDD02 July 10-12, 2002 Florence
CVD DIAMOND

• The bulk of un-doped CVD diamond films is
  characterized by a complexly structured
  distribution of deep levels, mainly dominated by
  the A band (2.5 eV) plus several deep level
  distributed in the range 0.8-1.4 eV above the
  valence band. These traps are quite effective in
  reducing
• Carrier lifetime ? worsening CCE
• Dynamic response speed
• Reproducibility of the current response

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RESMDD02 July 10-12, 2002 Florence
Pre-irradiating the diamond dosimeter with fast
neutron we achieved a clear improvement of the
device performance
Dosimetric performance of CVD diamond films
significantly improved by a pre-irradiation
procedure with fast neutrons up to 5x1014cm-2.
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After neutron irradiation the released charge vs
absorbed dose is linear Co60 g dose rate
0.21Gy/min Vrev 100 Volt
Device sensitivity ? slope of the linear fit
Sensitivity 490 nC/Gy
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RESMDD02 July 10-12, 2002 Florence
Current response of diamond vs. dose rate
6MV photon beam

• 0.9
• R 10 nA/(Gymin)
• Idark 3.5nA

The quasi-linear dependence of the current
response on the dose rate suggests that, after
neutron irradiation, a significant residual
concentration of traps is still present
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RESMDD02 July 10-12, 2002 Florence
4H-SiC Epitaxial wafer
The silicon carbide sample used in this study is
a Schottky diode fabricated on 4H-SiC epitaxial
wafers, provided by CREE research. The active
layer tickness is a n-type epitaxial layer, 30 mm
thick, grown on a n-type substrate of 4H-SiC.
The Schottky contact is circular with a diameter
of 2 mm and has been formed on the silicon
surface of the epitaxial layer by deposition of
100nm of gold.
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RESMDD02 July 10-12, 2002 Florence
Active Depth in Schottky barrier epitaxial SiC
The active depth of the SiC dosimeter during
irradiation is determined by adding to the
depletion depth due to the applied reverse
voltage the minority carrier diffusion length in
the neutral region of the epitaxial layer. The
built-in voltage of the Schottky barrier is
1.7Volt, corresponding to a depletion depth of
about 1mm at 0V bias. So the active volume
results 13mm.
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Characteristics of a Schottky contact
Dark current
Direct current
Capacity
Depletion depth
Resistance
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RESMDD02 July 10-12, 2002 Florence
Dark current of the SiC dosimeter embedded in the
epoxy resin and placed in the PMMA finger before
irradiation
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RESMDD02 July 10-12, 2002 Florence
Vrev 150 Volt Isignal 1nA Idark 0.1pA
Stable signal - High signal/noise - no priming
effects
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Released charge vs absorbed dose Co60 g dose
rate 0.21Gy/min Vrev 0 Volt
Sensitivity 14.1 nC/Gy
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RESMDD02 July 10-12, 2002 Florence
IV characteristics during Co60 g-irradiation
The signal depends linearly on the
dose-rate. Device sensitivity ?slope of the
linear fit
The current response depends on the square root
of the reverse voltage
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RESMDD02 July 10-12, 2002 Florence
Co60 g-irradiation
The sensitivity increases linearly with the
total voltage across the diode
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RESMDD02 July 10-12, 2002 Florence
RADIATION DAMAGE
After heavy irradiations the diffusion length can
degrade, due to the creation of deep levels in
the semiconductor bandgap, which enhance the
carrier recombination. The sensitivity as a
function of the accumulated dose has been
measured with a 137Cs g-source up to 10 kGy.
A significant degradation is occurring during the
first irradiation step, up to the dose of 200 Gy
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RESMDD02 July 10-12, 2002 Florence
Comparison of SiC and diamond with standard
dosimeters
 
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The absolute sensitivity of the diamond device is
quite the highest one, while its sensitivity per
unit volume is lower than those of silicon and
SiC. This is because, in the calculation of the
active volume, the entire thickness of the
diamond sample is taken into account, whereas for
Si and SiC the sum of the depletion depth and the
diffusion length is considered, which is much
lower than the total thickness of the
devices. Presumably, the active thickness of the
diamond sample is lower than its thickness and
can be considered of the order of the ccd in
this case 50-100mm. The diamond sensitivity is
240-400nC/Gymm3 .
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RESMDD02 July 10-12, 2002 Florence
CONCLUSIONS

• Diamond and SiC have shown to have, in on-line
  configuration, charge and current responses which
  are linear respectively with the dose and
  dose-rate with sensitivities comparable to those
  of standard dosimeters.

• The tissue equivalence of diamond makes this
  material more suitable for clinical dosimetry
  applications. Nevertheless, the presence of deep
  levels in high concentrations may give rise to
  priming effects which can influence the
  measurement during irradiation. This problem can
  be partially overcome by performing
  pre-irradiation procedures.

• SiC is a promising material for dosimetry,
  nevertheless its radiation hardness must be
  studied in more detail to definitely assess its
  potential in alternative to silicon diodes.