Advances in Ma terials Physics and Che mist ry, 2012, 2, 115-118
doi:10.4236/ampc.2012.24B031 Published Online December 2012 (htt p://
Copyright © 2012 SciRes. AMPC
Evaluation of UV Optical Fibers Behavior under
Neutron Irradiation
Dan Spore a1, Adelina Sporea1, Mirela Ancuta2, Du mitru Barbos2, Maria Mihalach e2, Mirea Mladin2
1Laser Metrology Laboratory, National Institute for Laser, Plasma and Radiation Physics, Magu rele, Roma nia
2Institute for N uclear Researc h, Pitesti, Romania
Received 2012
Degradation of UV transmitting optical fibers under nuclear reactor neutron exposure is reported. Four type of optical fibers (solari-
zation resistant, H2-loaded; UV transmission standard OH; UV enhanced transmission, high OH, H2-loaded; high OH, deep UV en-
hanced) were exposed to neutron fluences up to 4 x 1017 n/cm2. The optical transmission was measured off-line over the 200 nm
900 nm spectral range and the build-up of color centers was monitored.
Keywords: Irradiation Effects; Neutron; Optical Attenuation; UV Optical Fibers
1. Introduction
In the last 30 years optical fibers were extensively studied in
order to assess their possible use in radiation environments for
communications, sensing, remote control, light guides, robotics
etc. [1-3]. Optical fibers and, by extension, optical fiber-based
systems h ave a great p otential for such applications consider-
ing their advantages such as: capabilities to work under strong
electromagnetic fields; possibility to carry multiplexed signals
(time, wavelength multiplexing); small size and low mass; abil-
ity to handle multi-parameter measu rements in di stributed con-
figuration; possibility to monitor sites far away from the con-
troller; their availability to be incorporated into the monitored
structure; wide bandwidth for communication applications. In
addition, these systems are free of hazards such as fire, explo-
sion, and contamination. All these facts recommend them for
space or terrestrial applications (spacecraft on board instru-
mentation, nuclear facilities, future fusion installations, medical
treatment and diagnostics premises, medical equipment sterili-
zation), embedded into various all-fiber or hybrid sensors or as
light-guides for control and diagnostics. In these implementa-
tions optical fiber systems accept real-time interrogation capa-
bilities, provide spatially resolved answers (the capability to
build array detectors), make possible on-line/ real time investi-
gations [4].
Authors acknowledge the financial support through the grant
PN 09 39 03 01/2012 - Program NUCLEU and grant 12084/
2008 Program “Parteneriate”, both awarded by the National
Autho r ity for Scient ific Research.
Different types of optical fibers (silica-based, plastic, sap-
phire) and glass types were investigated under various irradia-
tion conditions: gamma-ray, X-ray, electron beam, neutron,
proton, alpha particles [4-7]. Exposed to ionizing radiation,
silica optical fibers exhibit effects such as: radiation induced
absorption (RIA), radiation induced luminescence (RIL), increase
of the optical radiation scattering as it propagates over the fiber
length, thermo-luminescence, change of the waveguide refrac-
tive ind ex. Attempt s were made to reformulate the problem and
to use these effects as a measure of the dose rate/total dose of
the radiation to which the optical fiber is exposed [4,8]. A spe-
cial situation appears in the case of multimode optical fibers
proj ected for UV-visible transmission, when the degradation of
the optical transmission is more evident [9-11].
The present paper reports for the first time, according to our
knowledge, the evaluation of UV optical fibers degradation
under neutron irradiation, exposed in a research nuclear reactor .
2. Experiment
The optical fiber samples we investigated fall into four different
categories: solarization resistant, H2-loaded; UV transmission
standard OH; UV enhanced transmission, high OH, H2-loaded;
high OH, deep UV enhanced. Th ey ar e all commerciall y avail-
able products, from two manufactures. In Table 1 the characte-
ristics of the irr adiated optical fiber s amples are specified .
The optical transmission of optical fibers samples was meas-
ured in the Laser Metrology Laboratory, at the National Insti-
tute for Laser, Plasma an d Radiatio n Physics (NILP RP), before
the irra di a tion proce s s.
Neutron irradiation was performed at TRIGA SSR research
reactor, operated by the Institute for Nuclear Research. In this
investigation channel J7 belonging to the beryllium reflector of
the reactor was used. The TRIGA-SSR reactor is a nuclear
reactor whose active area is supplied with LEU fuel in zirco-
nium hydride matrix type.
This fuel feeds stainless steel bars, these bars being grouped
in boxes of 5 x 5 pins. The total length of fuel pellets is 57.5 cm.
In the active zone the axial flux distribution is not uniform, and
can be represented by a cosine function. This distribution is
maintained outside the active area, so it follows the neutron
source distribution. For this reason, it is necessary to know this
flux distribution. The irradiation process run in two phases, for
the neutron flux of 3x1013 n∙cm-2s-1 calculated for th e lower end
of the optical fibers, and a neutron flux of 7x1013 n∙ cm-2s-1
Copyright © 2012 SciRes. AMPC
calculated for the upper end of the optical fibers, and different
exposure times were employed (from 60 s to 4 h) to obtain
different fluences ( 1015 n /cm2, and respectively 1017 n/cm2).
The characterization of the neutron irradiation channel was
done by neutron activation analysis applied to a set of activa-
tion foil detectors, followed by the deconvolution of the neutron
spectrum deduced from the measured reaction rates. This cha-
racterization was performed by measuring the neutron flux
density and the spatial distribution, relative to the information
provided by the monitoring system. The axial profile of the J-7
channel as resulted from the above mentioned characterization
is given in Figure 1.
To character ize the J-7 neutron channel, was developed a set
of flux monitors consisting of: An5%-Al-Al Dy5%, Lu5%-Al-
Al Mn1%, In100%, Fe100% Al100% Ni100%, Mg100%. The
set was completed with two additionally irradiated monitors
coated by a 1mm thick cadmium layer: Au5%-Al-Al and
Mn1%, in order to evaluate the contribution of intermediate
neut rons at th e respect ive moni tors’ reso nances. The irrad iation
time for each flux monitor was selected to obtain a sufficient
activity level to enable the measurements precisely without
extra handling precautions. Figure 2 illustrates the integral
spectrum obtained for channel J-7 at the end the measurements.
Figure 1. T he axial profile corresponding to the irradiation c hannel
Figure 2. The integral spectrum of neutrons corresponding to the
irradiation channel used.
After the radiation exposure, the irradiated optical fibers
were transferred and stored in Post Irradiation Examination
Laboratory for "cooling"/ disintegration of the radioactive
products, to allow handling and safe transfer to NILPRP for
further investigations. At this stage, the samples were subjected
to gamma spectrometry measurements for all isotopes emitting
gamma radiation with energy of 60 keV and 2.5 MeV, with
gamma-ray line intensity greater than 4% range for the deter-
mination of activation products. Measurements were per formed
using a chain of high-resolu tion ga mma spectro metr y cali brat ed
in efficiency for different distances and consisting of an HPGe
detector and a multichannel analyzer with 8192 channels,
coupled to the data acquisition system. Following gamma spec-
trometry measurements some gamma radioactive impurities
were found: 124 Sb, Sc 46, Zn 65, I 152 and 137 Cs. The gen-
eral set-up for the off-line optical absorbance measurements is
similar to that we used previously [10, 11] but, for the purpose
of this investigation, it has a better S/N ratio (1,000:1 full sig-
nal), 16 bits A/D conversion resolution, a dynamic range of
25,000:1, a greater quantum efficiency in the UV range (65 %
at 250 nm), spectral resolution 1.2 nm, a sensitivity of 0.065
counts/e-, and a minimum OD detection level of 0.4 [4].
For the optical set-up used (this value is determined by two
factors: first, the core of the connecting optical fibers and the
core of the samples ar e different, and second, the sample optical
fibers have no fixed connectors, hence, a biasing level which
limits the set-up lowest detectable OD). Such a detecting
scheme makes possible a better tracking of the color centre
development in the UV spectral range and enables a higher
range of absorption levels to be detected (O.D. of 4.4). For the
reported optical absorption curves, the signal was averaged
over three detected acquisitions with a value of 2 for the box
car parameter. Irradiation and off-line measurements were car-
ried ou t at room temperature.
3. Results
Figures 3 to 6 represent the results of the spectral optical ab-
sorption measurements for the tested optical fibers after they
were irradiated with neutrons in a research reactor. For com-
parison the curves corresponding to the non-irradi ated case and
for two fluencies (1.8 x 1015 and 4.3 x 1017 n/cm2) are super-
posed. According to data from the literature [12,13], we were
interested to observe the change of the optical transmission at
specific wa vel engths, correspon ding t o expected color centers:
Figure 3. Th e optical spectral absorp t i on f or s ample NP 1.
Copyright © 2012 SciRes. AMPC
Figure 4. Th e optical spectral absorp t i on f or s ample NP 2.
Figure 5. The op t ic al spe ct ra l absorp t ion for sample NP 3.
Figure 6. Th e optical spectral absorp t i on f or s ample NP 4.
a) λ = 248 nm, ODC(II) twofold coordinated silicon (or ac-
cording to some authors an oxygen deficiency center),
b) λ = 265 nm, non bridging-oxygen hole color center,
c) λ = 320 nm, bound chlorine center,
d) λ = 330 nm, molecular chlorine center,
or peroxy linkage,
e) λ = 630 nm, non bridging-oxygen hole color center,
Based on the available post irradiation information, two of
the optical fibers (samples NP 1 and NP 2) exhibit a degrada-
tion of the optical transmission at λ = 630 nm, for the high neu-
tron flux. At a low flux value this phenomenon is not present.
For sample NP 3 the influence of the chlorine related color
center can explain the increase of the optical absorption in the
300 nm 600 nm spectral range, may be combined with the
effect due to the non bridging-oxygen hole color center.
The molecular chlorine center (λ = 330 nm) is less present in
sample NP 4, at higher neutron flux. At the lower neutron flu x,
samples NP 1 and NP 2 proved to be more sensitive to neutron
irradiation for ODC(II) and non bridging-oxygen hole color
center (λ = 248 nm and λ = 265 nm).
The presence of the non bridging-oxygen hole color center is
confirmed by the degradation of the optical transmission at the
two characteristic wavelengths (λ = 265 nm and λ = 630 nm).
Less vulnerable at shorter wavelengths seems to be the opti-
cal fiber NP 3, as the optical ab sorption is lower in this case, at
the highest neutron flux.
It is known that high neutron fluxes induce mechanical de-
gradation of the irradiated glass. In order to check this assup-
tion electron microscopy investigations were carried out on
opt ical fiber samples befo re and after irardi ation. Figure 7 illu-
strates th e deffects induced in the glass as this was subjected to
4. Conclusions
The study of the UV optical fibers exposed to high neutron
fluxes from a nuclear reactor is reported for the first time. The
effect of this irradiation on the formation of color center in the
UV spectral range was studied along with the mechanical de-
gradation of the optical fiber s sampl es .
Figure 7. Electron microscopy image of an irradiated optical fiber
Copyright © 2012 SciRes. AMPC
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