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TRANSIENT VISIBLE-UV ABSORPTION IN BETA
IRRADIATED SILICA
S. Agnello 1 and B. Boizot 2 1 INFM and
Department of Physical and Astronomical Sciences,
Palermo, Italy 2 Laboratoire des Solides Irradiés
Ecole Polytechnique, Palaiseau, France
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Outline

• Introduction
• Experimental details
• Results Discussion
• Conclusions

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Introduction
Modification of the optical properties of a-SiO2
by ionizing radiation
Transmittance degradation Fiber gratings
Point defects generation mechanisms
Stabilization and destruction of defects
Precursors Unperturbed sites
Diffusing species Electrons - holes
We report experimental study of the optical
absorption induced by ?-ray irradiation in high
purity silica during and after the irradiation
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Experimental details
4 commercial silica samples (50 x 5 x 0.5 mm3) ?
Natural dry EQ906 (20 ppm OH) and wet Herasil
1 (150 ppm) ? Synthetic dry Suprasil 300 (lt1
ppm) and wet Suprasil 1 (1000 ppm)
? irradiation at T 300 K with 2.5 MeV
electrons (Dose rate 51?67 kGy/min Van de Graaff
accelerator, Palaiseau) In-situ prism-based
detection system Sources halogen or deuterium
Detection 1024 photodiode array
(PDA) Spectral range 1.24 ? 6.2 eV (1000 ? 200
nm) Spectral resolution 0.2 nm Ex-situ
absorption measurements (from 2 to 5 days after
irr.) Jasco V-570 double beam spectrophotometer
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In-situ optical absorption set-up
to prism and PDA (1024)
e- beam (2.5 Mev)
e- counter
He
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from lamp (2D, halogen)
Single beam system difference absorption spectra
with reference to not irradiated sample
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Irradiation effects
Natural dry
Synthetic dry
absorption from ? 3 eV
absorption from ? 1.3 eV
Higher irradiation resistance in the synthetic
material
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Post-Irradiation effects
Natural dry
Synthetic dry
absorption decays with profile changes
absorption decays with unchanged profile
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Natural dry
Synthetic dry
in-situ
ex-situ
OA 4.7 eV (NBOHC ? Si-O) L.Skuja, R.Silin,
Phys. Stat. Sol (a) 56 (1979) 11 OA 5.8 eV
(E-Si ? Si) R.A. Weeks, C.M. Nelson, J. Appl.
Phys. 31 (1960) 1555
OA 2.3 eV (Al centers ?) OA 4.2 eV (Ge centers
?) H. Nishikawa in Silicon-Based Materials and
devices, Ed. H.S. Nalwa (2001) OA 5.6 eV (E-Si
? Si)
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Natural dry (9.2 MGy)
Synthetic dry (52 MGy)
Irradiation induced modifications tend to a limit
value in all the spectral ranges activation of
precursor defects Post-irradiation kinetics are
non single exponential and last more than 1 h
diffusion limited process Different relaxation
rates are mainly evidenced for the spectral
features observed in the natural sample
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Natural wet (10 MGy)
Synthetic wet (59 MGy)
As for dry materials in the wet ones larger
spectral dependence of the relaxation rates is
found in the natural sample
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abs (max dose)- abs (1 h after irradiation)
Difference absorption spectra in the
post-irradiation relaxation evidence another
transient band peaked at 4.1 eV in all the
materials. It can be related to intrinsic defects
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Conclusions

• We evidenced irradiation induced absorption in
  natural and synthetic silica arising from
  precursor defects.
• In natural samples two bands are evidenced around
  2.3 and 4.2 eV and are related to impurities.
  Intrinsic defects are responsible for absorption
  above 5 eV
• Transient effects are found in all the materials
  with decay rates of the order of hours. They can
  be attributed to diffusion limited relaxation
  processes.
• A novel transient absorption band at 4.1 eV is
  found and is guessed to arise from intrinsic
  defects being present in all silica types.

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ACKNOWLEDGMENTS
Palermos group R. Boscaino M. Cannas F.M.
Gelardi M. Leone Discussion H.
Nishikawa People at LSI laboratory J.
Ardonceau and S. Guillous