Plasma and Fusion Research: Regular Articles

Volume 14, 3405089 (2019)

Gamma Ray Irradiation Eﬀects on Optical Elements of ITER
Divertor Impurity Monitor∗)
Sin-iti KITAZAWA, Toshihiro OIKAWA, Hiroaki OGAWA, Tomohiro YOKOZUKA,
Toshiyuki MARUYAMA and Suguru TANAKA
Naka Fusion Institute, National Institutes for Quantum and Radiological Science and Technology (QST),
Naka 311-0193, Japan(Received 10 January 2019 / Accepted 13 March 2019)
The ITER divertor impurity monitor (DIM) is a diagnostic system that directly observes 200 - 1000 nm lightemissions from nuclear fusion plasma. Because the DIM observes a wide range of light from infrared to ultraviolet using a single optical system, high optical performance, such as spatial resolution and aberration, is required
for the DIM optical elements. The DIM must be highly robust against external environmental factors, such astemperature, humidity, vibration, and magnetic field. Components having high radiation resistance must be usedbecause of the high radiation environment in ITER. In this study, DIM optical elements installed in the interspaces
and the port cells are investigated because gamma ray irradiation can impact their optical performance. The irradiation experiments were performed at QST’s gamma-ray irradiation facilities in Takasaki. The attenuation oftransmittance due to gamma irradiation were evaluated for silica, fluorite, and LiCaAlF6 as the glass material in
the spectrometers to be installed in the port cell. The influence of irradiation on silica polka dot beam splitters
was found to be negligible; however, the transmittance of high OH concentration fibres for measuring UV lightdecreased significantly due to irradiation and will thus require countermeasures. The information necessary forproceeding with the ITER DIM design was obtained from these experimental results.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: ITER, divertor impurity monitor (DIM), diagnostic system, nuclear fusion, gamma ray irradiation,
LiCAF, LiCaAlF6 , optical fibres, optical density
DOI: 10.1585/pfr.14.3405089

1. Introduction
ITER is an international project in which Japan, theUSA, the EU, Russia, China, South Korea, and India
are cooperating to build and operate a thermonuclear fusion experiment reactor [1]. ITER plasma diagnostic systems are used for plasma control, machine protection, andphysics research. Approximately 50 diagnostic systems
are procured by the participating parties. In the tokamakbuilding, equipment for ITER diagnostics is installed in
the vacuum vessel (VV), the interspaces (ISs) between thevacuum boundary and the biological shielding, and the port
cells (PCs) on the outside of the biological shielding. Datais transferred to an acquisition system installed in the diagnostic building. The ITER diverter impurity monitor(DIM) system observes light from fusion plasma in therange from UV to near IR (200 - 1000 nm) in a single optical system. The DIM system has five optical elements in
the Upper Port (UP) #01, Equatorial Port (EP) #01, andLower Port (LP) #02 [2], where the optical systems in
LP#02 consists of the one under the divertor dome and theones at the gap between adjacent divertor cassettes. Lightauthor’s e-mail: kitazawa.siniti@qst.go.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

observed is guided from the front-end optical system in the
VV through ISs to PCs by using mirrors. The DIM system
specifications are 50 mm for spatial resolution and 1 ms
for time resolution, thus, very high optical performanceis required, such as increasing the yield of light observed
and suppressing aberration [3]. The DIM system observes
200 - 400 nm UV light, which is aﬀected by the reductionof light transmittance due to the formation of a color center under a high gamma ray radiation environment. During
the ITER engineering design activities (EDA) phase, radiation eﬀects on the diagnostic instruments were studied [4].
However, as the design progressed, it became necessary totest and verify the latest equipment expected to be used in
the actual equipment of ITER. The DIM equipment thatwas tested and verified in this study is from the present design and is summarized in Table 1, although the equipment
may be updated as the design continues to progress.
For the DIM optical elements, front-end mirrors andreflectors are installed in the VV [5,6], steering mirrors andcollection lenses are installed in the ISs, and steering mirrors, beam splitters, lenses and bandpass filters for spectroscopes and optical fibres are installed in the PCs. In thisstudy, the vacuum window is excluded as part of the VV. In
the case of optical transmission by optical fibres from thelight collecting system to the spectrometers, light longer

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

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Table 1 DIM equipment for irradiation tests.

beam splitters, and optical fibres.

2. Gamma Ray Irradiation Facility
Gamma ray irradiation experiments were performedat the gamma ray (60 Co) irradiation facilities of QST’s
Takasaki Advanced Radiation Research Institute. Irradiations at dose rates of 0.4 to 10 kGy/h were done in the 2ndirradiation building capable of schedule irradiation, while
those at lower dose rates were performed in the food irradiation building. The specimens were set at appropriate positions for their target dose rates based on a space dose rate
map obtained by measuring using an ionization chamberand calibration using a polymethyl methacrylate (PMMA)
dosimeter (Radix W) [7].

3. Gamma Irradiation on Lens Materials

Table 2 Estimated dose for DIM install space.

than 400 nm has low attenuation and can be introduced tothe spectrometers installed in the diagnostic building morethan 100 m away from the PCs. Light in the 200 - 400 nm
region, however, attenuates significantly. Hence, the spectrometers to measure UV light have to be installed near thecollection optics in the PCs to shorten the optical fibres asmuch as possible. The UV signals are converted to electrical signals in the PCs.
Typical gamma ray doses in the ISs and the PCs at
ports having no equipment or shielding where DIM equipment is installed are summarized in Table 2. These doses
are calculated from the Radiation Map for the entire ITER
tokamak. The minimum and maximum dose values in eachspace are also shown in Table 2. The doses were calculated by multiplying the dose rate by the total ITER operation time of 4,700 h. The doses are higher near the plasma
because gamma rays are generated by radiation from theplasma or activation by neutrons. Here, the maximum
value of the dose rate in the LP #02 PC is large because
the piping for activated cooling water for divertors passes
nearby. In this study, the maximum dose to be evaluated istaken as tolerances, 10 MGy in the ISs, and 200 kGy in thePCs.
In this study, we report the results of experiments on
gamma ray irradiation eﬀects on lens materials, polka-dot

The preliminary design of the DIM employs achromatic lenses in ISs and PCs and lens groups for spectroscopes in PCs [3]. Silica (SiO2 ), fluorite (CaF2 ), and LiCAF (LiCaAlF6 ) were chosen as vitreous materials for theirradiation experiments. SiO2 and CaF2 are standard materials available on the market for optical elements whosetypes vary depending on their constituents and manufacturing processes. Materials having high radiation resistancehave also been developed and the eﬀects of gamma ray irradiation on them are well known. Therefore, the eﬀects
of gamma rays on materials from candidate manufacturersthat are likely to be used in the actual equipment of the
DIM system must be confirmed, which will contribute to
the design of the shielding. LiCAF is a promising inorganic material for neutron scintillators and lasers and has
undergone fundamental and application studies [8, 9]. LiCAF is also an advantageous material to use in the UV
wavelength region due to its high Abbe number and thusly
its high capability of chromatic aberration correction in the
UV region. The eﬀects of gamma ray irradiation on LiCAF
must be investigated for LiCAF to be used in ITER radiation environments. The glass material specimens were procured from Bunkoukeiki Co., Ltd. [10], a candidate manufacturer for the spectrometers. The dimensions of specimens are 10 × 7 × 4.1 mm for SiO2 , 10 × 7 × 4.0 mm for
CaF2 , and 10 × 7 × 5.2 mm for LiCaAlF6 .

3.1 Gamma irradiation eﬀects on silica
The transmittance spectra of SiO2 specimens were
measured using a spectrophotometer (JASCO V-750) at
each step of gamma ray irradiation, as shown in Fig. 1.
The small peak at 400 nm was caused by a filter in the
spectrometer, the step at 340 nm was caused by switching
the light source. The gamma ray induced color centers of
SiO2 are dominated by non-bridging oxygen hole centers
NBOHC (≡Si-O ·) with a peak at 260 nm and E’-center(≡Si ·) with a peak at 215 nm. In the region above 400 nm
where these eﬀects are absent, the decrease in transmit-

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Fig. 1 Transmittance spectra of silica glass specimen (4.1 mm
thickness) after 60 Co gamma ray irradiation.

tance at 10 MGy irradiation is about 2%. However, thetransmittance decreases with the doses in the region below
300 nm due to the color centers and becomes extremely
low near 215 nm at 10 MGy.
Irradiation-induced degradation of transmittance has
to be minimized as much as reasonably achievable. In theISs, when assuming the transmittance of a SiO2 lens having a thickness of 4 mm after 10 MGy of irradiation has
to be kept above 80% over the entire wavelength region of
DIM measurements, the 10 MGy dose must be reduced toabout 250 kGy by shielding. This reduction is equivalent
to 6cm-thick tungsten, 10-cm thick lead, or 22-cm thickstainless steel for shielding 16 N gamma rays. However,shielding of the optical elements may not be easy because
gamma rays can also reach along the optical path. In thePCs, the transmittance needs to be as high as possible because many silica materials are used for lenses, band-passfilters, and other elements in the spectrometers. In the region of 200 - 300 nm, the transmittance drop of an opticalfibre is relatively large (5 - 10%) even without the eﬀects ofirradiation and the gamma irradiation-induced attenuationof transmittance (about 10%) was observed for a 4.1-mm
thick silica specimen at a dose of 200 kGy. Therefore, thedesign of PC optics must be improved by taking into account a simple optical configuration, choice of glass materials, layout of optical elements having diﬀerent radiation
resistance, and reasonable and realistic shielding that fits
in the narrow assigned space in the PCs.

3.2

Gamma irradiation eﬀects on fluorite

The transmittance spectra of CaF2 specimens after
gamma ray irradiation are shown in Fig. 2. The measurements were performed in the same manner as with SiO2 .
Neither major color center nor irradiation-induced attenuation of transmittance were observed. No special consideration needs to be taken for CaF2 as regards the design of
the DIM optical system because CaF2 is used together with
SiO2 to correct the chromatic aberration of SiO2 , thus, theradiation shielding for SiO2 is suﬃcient for CaF2 .

Fig. 2 Transmittance spectra of CaF2 specimen (4.0 mm thickness) after 60 Co gamma ray irradiation.

Fig. 3 Transmittance spectra of LiCAF glass specimen (5.2 mm
thickness) after 60 Co gamma ray irradiation. Absorption
due to the F center at 262 nm and transition at 245 nm areapparent.

3.3 Gamma irradiation eﬀects on LiCAF
The transmittance spectra of LiCaAlF6 specimens after gamma ray irradiation are shown in Fig. 3. Comparedwith SiO2 , the transmittance decreases in the wavelength
region lower than 400 nm even at low doses, but saturation
occurs at doses of several MGy. It would be diﬃcult to use
LiCaAlF6 in an optical system in a gamma ray environment because a 5% reduction near 250 nm was observedat an irradiation of only 10 Gy. The advantage of LiCAF
glass is its correction of chromatic aberration in the UV
region. However, its superiority would be lost due to its
low transmittance in the UV region in a radiation environment. LiCaAlF6 glass has a color center of F-centerat 262 nm [8], which can be observed in terms of its optical density (absorbance). The optical density per unit
length OD (λ) / d [cm−1 ] is calculated from the formula(1), where transmittance is T (λ), reflectance is R (λ), andsample thickness is d (cm). The reflectance measured istaken into consideration.

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OD(λ)/d = − log10 [T (λ)/(1 − R(λ))] /d.

(1)

The optical density of a LiCAF specimen at the F-

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Fig. 4 Optical density of LiCAF glass specimen at 262 nm after60
Co gamma ray irradiation. Below 10 kGy, the contribution from the F center at 262 nm is dominant, and theinfluence by transition of electron trapped at F center isapparent above 20 kGy.

center (262 nm) is shown in Fig. 4. The error bars on the
optical density were added as measurement error. The plotincreases with the same slope up until about 10 kGy, after
which the slope becomes steeper. This diﬀerence in slope
is owing to the dominant eﬀect of F-center when the number of color centers is small, however, the transition of asingle electron trapped in the F-center F 0 → F + (245 nm)
increases with the number of color centers. Here, F + , F 0 ,and F − mean the states that the F center can trap 0, 1, and2 electrons, respectively. At low doses, the contributions
from the formation of the F-center (absorption at 262 nm)
is dominant, and over 10 kGy, the contributions by the generated F-center are apparent.

4. Gamma Irradiation on Beam Splitters
The DIM has three kinds of spectrometers, a filter spectrometer for observation of a fixed wavelength ata high time resolution, a survey spectrometer that measures over a wide wavelength region simultaneously, anda high dispersion spectrometer at a high wavelength resolution [11]. Therefore, it is necessary to distribute theobserved light to spectrometers without wavelength dependence in the measurement range (200 - 1000 nm) by using
beam splitters. Several methods for splitting light havebeen studied, such as branching using optical fibres [4].
A simple optical system to divide incident light is necessary because optical components are installed in the narrow assigned space in the PCs. A polka-dot mirror, which
is formed by vapor depositing small, circular mirrors ona glass form is considered to be a promising candidate.
The eﬀects of gamma ray irradiation on transmittance, reflectance, and morphology were investigated.
The specimen is a Shimadzu T50-20S fused silica
base Al dotted mirror with MgF2 protection coating, which
is a so-called half mirror having approximately 50% reflection. Its dimensions are 50.8 × 50.8 × 1.5 mm and its dot

Fig. 5 Transmittance spectra of polka-dot half mirror after 60 Co
gamma ray irradiation.

Fig. 6 Reflectance spectra of polka-dot half mirror after 60 Co
gamma ray irradiation.

pitch is about 0.3 mm [12].
The transmittance spectra of polka-dot half mirrors after gamma irradiation are shown in Fig. 5. A small peak at
400 nm that was caused by an internal filter of the spectrophotometer was removed to avoid any misinterpretations. The measurement conditions were the same as thoseof the measurements of the lens materials. Since the polkadot mirror is designed as a 50% reflecting half mirror, it
shows characteristics similar to those of SiO2 in Fig. 1 having a maximum transmittance of 50%.
The reflectance spectra of polka-dot half mirrors areshown in Fig. 6. The reflections of the front coated surface and the back substrate surface were measured using a
JASCO V-780 spectrophotometer with a SLM-907 specular reflectance accessory having a 5◦ incident angle. The
multi surface reflections were not excluded. For back reflection, an absorption peak of silica (≡Si - OH) was observed near 1390 nm. No significant eﬀects from irradiation were observed in the range up to 200 kGy on both thesurface reflection and the back reflection.
SEM observations were carried out to investigate themorphology of the polka-dot half mirrors after irradiation.
SEM images before and after 200 kGy of irradiation are
shown in Figs. 7A and 7B, respectively. The accelerating voltage of the electron beam was 2 kV. The circles are

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Fig. 7 SEM images of polka-dot half mirror. A) Before irradiation, B) 200 kGy irradiation.

the Al-deposited surface on the fused silica substrate. The
electric conductance of the protective coat of MgF2 is low
so that the electron charge-up produces some noise. Any
visible damage due to gamma ray irradiation is not observed along the edges of the aluminum circles.
From these results, it was found that the impact on
optical performance by gamma ray irradiation of approximately 200 kGy would be negligible if a polka-dot beam
splitter is used in the PCs.

Fig. 8 Optical system used for measuring transmittance of optical fibres. (a) UV measurement (b) Vis / IR measurement.

5. Gamma Irradiation on Optical Fibres
Optical fibres are widely used in various gamma rayenvironments. Research was also carried out in the engineering design phase of ITER [13, 14]. In most of
this research, the wavelength region evaluated was mostly
longer than 400 nm, while the DIM system measures 200 1000 nm. Hence, it is necessary to evaluate the opticalcharacteristics after irradiation in the UV range of 200 400 nm. The DIM system employs optical fibres to transmit collected light by a telescope to the UV spectrometersin the PCs and to the Vis/IR spectrometers in an externalbuilding. The length of UV optical fibres in the PCs areless than 5 m and the length of Vis/IR is about 10 m in thePCs and over 100 m in non-radiation areas [11]. We conducted preliminary experiments on the eﬀects of gamma
ray irradiation on high OH concentration optical fibres andlow OH concentration fibres, which are used for measurements in the UV region and the Vis/IR region, respectively.
Optical systems were set up as shown in Figs. 8 (a)and 8 (b) to measure the change in transmittance due togamma ray irradiation of optical fibres. In both systems,
the gamma-irradiated optical fibres were connected to alight source and a polychrometer via connection fibres
to adjust the amount of light and convert FC connectors
to SMA connectors. The light source for UV was a D2
lamp (Hamamatsu L 10290) and the light source for Vis/IR
was an integrating sphere light source (HELIOS). Avantec
AvaSpec-2048L polychrometers were used to measure UV(200 - 450 nm) and Vis/IR (350 - 1100 nm) using diﬀerent
gratings. The optical fibre specimens were Fujikura FFC2P-2M-S 200/220 high OH type for UV and FFC-2P-2MS, and 200/220B low OH type for Vis/IR. The core diameters were 200 µm, the lengths were 2 m, and FC connec-

Fig. 9 Radiation-induced attenuation spectra of high OH fibres
for UV in UV region by 60 Co gamma ray irradiation.

tors were attached at both ends [15]. The technical specification shows the spectral attenuation are 0.1 dB/m in theregion longer than 400 nm, and spectral attenuation drops
sharply as the wavelength becomes shorter.
The observed radiation-induced attenuation (RIA)
spectra in the UV region of the high OH fibre is shownin Fig. 9. The horizontal axis is wavelength, and the vertical axis indicates the ratio of the transmittance of the irradiated fibres to that of unirradiated fibres. Because thesignal of the transmitted light in the region under 250 nm
was quite small, that area was not included in the figure.
The observed decrease in the region under 300 nm is significant even at 1 kGy of irradiation. In particular, in theregion around 250 nm, the transmittance is very weak. As
the irradiation increases, the transmittance on the shorter
wavelength becomes lower. Consequently, the attenuationof the light transmittance in the UV range due to gamma
ray irradiation is very large and some remedial measures
should be considered.
The results of RIA spectra in the Vis/IR region of the
low OH fibres are similar to the results from former experiments [6]. The decrease of transmittance of the low OH
fibre in the Vis/IR region is negligible for its use in PCs
with reasonable radiation shielding. The next step for op-

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tical fibres is to conduct gamma ray irradiation on the fibre
bundle assemblies.

6. Summary
Gamma ray irradiation tests for DIM optical elementsthat are installed in the ITER radiation environment werecarried out for the final design. For the lens materials,
we found that the use of silica and LiCAF requires moreshielding for 10 MGy irradiation in the ISs. For LiCAF,
it was observed that the absorption due to the generated
F-centers becomes prominent over 1 MGy. For polka-dot
beam splitters, it was found that the eﬀects of gamma ray
irradiation on the reflectance and transmittance was negligible for its use in the PCs. For optical fibres, it was
found that the attenuation of the transmittance of the highOH fibre for UV measurement is significant and hence it isnecessary to make design improvements in terms of transmission of light in the PCs, such as enhanced radiation resistance of a fibre combined with shielding or an alternative optical configuration without fibres. Further gamma
ray irradiation tests on the ITER DIM system design arenecessary to improve and secure the performance of each
optical/mechanical/structural/electrical component.

Acknowledgments

agnostic Group for their valuable advice in carrying out
this study. We would like to thank Naotsugu Nagasawa and Hajime Seito for the implementation of the
gamma ray irradiation experiments. We would like tothank Shunya Yamamoto, Hiroshi Koshikawa, and Masaki
Sugimoto for their cooperation during the observation including Nathaniel Duncan for his help with making thismanuscript.
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We appreciate the members of the QST Plasma Di-

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