The Radiation Tolerance of Chalcogenide Glasses
Masataka Naitoha, Haruyoshi Katayamaa, Masatomo Haradaa, Masahiro Suganumaa, Yoshihiko
Okamuraa, Yoshio Tangea, Kenny Rogersb, Yann GuimondcaEarth Observation Research Center, Japan Aerospace Exploration Agency(JAXA/EORC),
Sengen, Tsukuba, Ibaraki, Japan, 305-8505;b
Umicore Coating Services LTD, Kinnoull Street, Dundee DD2 3ED, United Kingdom;cUmicore IR Glass, ZA du Boulais, Acigne 35960, France ;ABSTRACT
Chalcogenide glasses are compounded from chalcogen elements, such as sulphur, selenium, and tellurium. These
glasses are applied to commercial applications, e.g., night vision, because they transmit infrared in the spectral range of
0.8-16μm. Chalcogenide glasses have greater advantages over germanium (Ge), i.e., their wide spectral range of hightransmissivity and their small temperature dependence of the refractive index.
We have developed the Compact Infrared Camera (CIRC) with an uncooled infrared array detector (microbolometer)
for space applications. The CIRC has been scheduled to launch in 2013 to demonstrate the usability of a microbolometeras a space application. The optics of the CIRC adopts two different kinds of materials for athermal optics. One isgermanium, and the other is GASIR1® which is a chalcogenide glass (Ge22As20Se58) developed by Umicore. However,
the radiation tolerance of GASIR® has not been investigated in the past.
We carried out irradiation tests to investigate the radiation tolerance of GASIR1®. We irradiated GASIR1® withgamma-rays (Co60, 1.17 MeV and 1.33 MeV) up to 3Mrad. We measured the transmissivity and refractive index in theinfrared range before and after irradiation. In this paper, we report the results of the irradiation tests of GASIR1®.
Keywords: chalcogenide glass, GASIR, radiation tolerance, irradiation test

1. INTRODUCTION
Chalcogenide glasses are compounded from chalcogen elements (16th group of elements), such as sulphur, selenium,
and tellurium. Because they are oxygen-free, chalcogenide glasses are capable of high infrared transmission in the wide
spectral range of 0.8-16μm. The most widely distributed glasses, such as silicates, are transparent only in the nearinfrared up to several micrometers because the vibration frequency of oxygen bonds with any other element
approximates these very wavelengths[1].
Today, infrared transparent crystals are usually used in thermal infrared optics such as single crystalline germanium. But
chalcogenide glasses have certain advantages over crystalline germanium as the materials for infrared optics. The
manufacturing of chalcogenide glass is comparatively easy and more available for mass production. Thus, it enables tobe lower cost and manufacture large size production[2][3]. Crystalline germanium, on the other hand, is rare andexpensive. In addition, Chalcogenide glasses have small temperature dependence of the refractive index compared togermanium. Therefore, chalcogenide glasses are applied to commercial applications such as night vision.
We have developed the Compact Infrared Camera (CIRC) with an uncooled infrared array detector (microbolometer) for
space application[4][5][6]. The optics of the CIRC adopts two different kinds of materials for athermal optics. One isgermanium, and the other is GASIR1®, which is a chalcogenide glass (Ge22As20Se58) developed by Umicore. However,
the radiation tolerance of chalcogenide glasses has not been investigated in the past. So, we examined the radiation
tolerance of a chalcogenide glass, GASIR1®.

Infrared Technology and Applications XXXVI, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton,
Proc. of SPIE Vol. 7660, 766028 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.850146

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2. COMPACT INFRARED CAMERA FOR SPACE APPLICATIONThe Compact Infrared Camera (CIRC) is an infrared camera equipped with an uncooled infrared array detector(microbolometer). The main mission of the CIRC is to demonstrate the technology of wildfire detection using a
microbolometer. The CIRC is a small, lightweight, and low-cost thermal infrared imager for space applications. We
employ athermal optics and shutter-less image correction to reduce the size, weight, and cost of the CIRC.

181mm

206mm
Figure 1. Schematic view of the CIRC.

2.1 Hardware Design
Figure 1 shows a schematic view of the CIRC. The CIRC is based on a commercial infrared camera developed by
Mitsubishi Electric Corporation (MELCO). However, to utilize it for space applications, we added some modifications to
the hardware design.
The specifications of the CIRC are shown in Table 1. We set the baseline specifications to meet requirements for wildfire
detection [4][5]. The detector uses a large format (640×480) to obtain a wide field of view. The spatial resolution is an
important factor for wildfire detection. The baseline specification of the spatial resolution is 200 m from the altitude of
600 km. The Noise Equivalent Differential Temperature (NEDT) is related to the contrast between a background area
and a burning area. If the NEDT is smaller, detectability of the wildfire increases. Considering the detector performanceand the optical design, we set the NEDT is 0.2K at 300K background.
To reduce the size, weight, and cost, we minimized the functions of the CIRC. The optics unit of the CIRC uses f/1.2
optics. We employ athermal optics to maintain optical performance in a wide range of temperatures. This will be anadvantage of the CIRC for small satellites, because we do not need an active thermal control or a focus mechanism forthe optics.
We use a 640×480 pixel SOI diode uncooled IR FPA of which pixel size is 25 μm2. The NEDT is 40 mK with f/1 optics.
Drive and readout circuits are almost the same as those of a commercial infrared camera. For space applications, we areplanning to perform a radiation damage test, and a screening of commercial devices.

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Table 1. Main specification of the CIRC

Item

Characteristics

Detector

Uncooled infrared detector(SOI diode)

Number of pixels

640 × 480

Wavelength

8 - 12 μm

Size

20×18×10cm

Mass

2.6kg

Power Consumption

20W

Spatial Resolution

200m observed from 600km(0.33 rad)Field of View

12° × 9°(128km×96km)

Exposure

33ms

Dynamics Range

180K - 400K

NEDT

0.2K @ 300K

2.2 Athermal Optics
The optics of the CIRC is f/1.2 refractive optics. The focal length of the optics is 78 mm.
The temperature of the CIRC is changed on orbit. The temperature change of the optics will cause a defocus because
refractive indices of lens materials are highly dependent on temperature. To compensate for this defocus, we have toemploy a focus mechanism or a heater to keep the optics temperature constant. However, such mechanisms increase
sensor resources. Athermal optics can compensate for the defocus due to temperature change without such mechanisms.
Figure 2 shows the optical design of the CIRC. The athermal optics of the CIRC compensate for the defocus by utilizing
a combination of different lens materials and diffractive lenses. The optics of the CIRC uses germanium andchalcogenide glass (GASIR®1). Figure 3 shows the calculated MTF of the CIRC optics versus the ambient temperaturefrom -15°C to 50°C. The MTF is constant over the wide range of temperatures.

Figure 2. Schematic view the athermal optics of the CIRC

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.
Figure 3. Calculated MTF of the CIRC optics in the ambient temperature from -15°C to 50 °C.

3. IRRADIATION TEST WITH GAMMA-RAYS
As shown in section 2.2, the GASIR1® is a key material for the CIRC optics. However, the radiation tolerance of
GASIR1® has not been investigated in the past. So, we examined the radiation tolerance of GASIR1®.
We carried out irradiation tests at Tokyo Metropolitan Industrial Technology Research Institute. In order to compare theradiation tolerance of GASIR1® with that of other infrared lens materials, Ge and ZnSe are added to samples. We
irradiated the samples with gamma-ray (Co60, 1.17 MeV and 1.33 MeV) up to 3 Mrad.
We carried out the irradiation test twice. The first irradiation test was performed on March 5, 2009, and the second test
was performed on October 13, 2009. The irradiance level in first test was up to 100krad, and its level in second test was
up to 3Mrad.
3.1 Irradiance Level
Figure 4 shows the depth-dose curve in a sun-synchronous orbit at an attitude of 600km from January 1, 2010 to
December 31, 2010. The horizontal axis represents an aluminum shield depth in millimeters. The vertical axis represents
the total dose radiation in rad. The absorption material is GaAs. Although the radiation environment of the CIRC is still
unknown, we can estimate the irradiation level of GASIR1® lens on orbit from this depth-dose curve. If the aluminum
shield depth is 1mm, the total dose level is about 10krad per year. However, if the aluminum shield depth is 0.1mm(almost no shield), the total dose level increase up to 180krad per year. The GASIR1® lens of the CIRC is exposed tospace. It means that we cannot expect the shield effect by a lens holder or other lenses. The 3Mrad irradiation
corresponds to the total dose radiation of about 17 years, which is enough longer than a typical satellite lifetime in a low
earth orbit.
We first performed the irradiance test up to 100krad to confirm the radiation tolerance in a low dose level. Afterconfirming the radiation tolerance up to 100krad, we performed a further irradiation test up to 3Mrad.

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Figure 4. Depth-dose curve in a sun-synchronous orbit at an attitude of 600km from January 1, 2010 to December 31,
2010. The horizontal axis represents an aluminum shield depth in millimeters. The vertical axis represents thetotal dose radiation in rad.

3.2 Samples
We prepared samples for irradiation test, anti-reflective (AR) coated GASIR1®, uncoated GASIR1®, germanium(Ge),and ZnSe. Table 2 shows the specifications of each sample. We prepared five samples for each material. Theiridentification labels are from A to E. Table 3 shows the irradiance level of each sample. We carried out the irradiation
test twice. In the first irradiation test, the irradiation level was from 5krad to 100krad. In the second irradiation test, theirradiation level was from 300krad to 3Mrad. Figure 5 shows the picture of the samples used in the irradiation test:
GASIR1®, uncoated GASIR1®, Ge, and ZnSe attached to lens holders.
Table 2. The list of samples for gamma-ray irradiation test

Samples

AR coat

DiameterThickness

GASIR1®

Nothing

20mm

3mm

GASIR1®

Both Faces

20mm

2mm

Ge

Both Faces

20mm

1mm

ZeSe

Both Faces

20mm

3mm

Table 3. The irradiance level of five test samples in the first and the second irradiation test.

A

B

C

D

E

st

1 test (krad)

100

50

10

5

0

nd

3000

1000

0

300

0

2 test (krad)

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Figure 5. The samples used in the second irradiation test: ZnSe, Ge, Uncoated GASIR, and AR coated
GASIR1®.
3.3 Irradiation Facility
Irradiation tests were carried out at Tokyo Metropolitan Industrial Technology Research Institute. Table 4 shows abrief overview of the irradiation test room. Figure 6 shows a picture of the Irradiation Room 1.

Table 4. Irradiation test facility at Tokyo Metropolitan Industrial Technology Research Institute

Date of Irradiation Test

Irradiation Room 1

Irradiation Room2

October 13,2009

March 5,2009

60

Radiation SourceCo (1.17MeV, 1.33MeV)

Radiation Capacity

185TBq129.5TBq

Weight of Radiation Source
Container

About 630kg with Pb shield

About 760kg with 4 ton of Pb
ShieldIrradiation Roomabout 8m3

about 60m3

Safety System

Automatic display and interlock by dosemeter

Control System

Hand operation

Drive System

Floor-standing slow hoist with motor

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Figure 6. Tokyo Metropolitan Industrial Technology Research Institute irradiation room 1.

3.4 Configuration of the Irradiation Test
Figure 7 and Figure 8 show the pictures of the configuration of the irradiation test. Figure 7 is a picture of the first
irradiation test. Figure 8 is a picture of the second irradiation test. We controlled the irradiation level by changing the
distance from the radiation source Co60 to the test samples. The test samples were attached to lens holder and settled at
the location calculated by the absorbed dose rate.

Figure 7. The configuration of the first irradiation test.

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Figure 8. The configuration of the second irradiation test.

3.5 Measurement
Before and after irradiation, we measured transmission of each sample by Fourier Transform Infrared Spectroscopy. We
used Spectrum One manufactured by Perkin Elmer. The wavenumber range of measurement, is 7800~350 cm-1 This
corresponds to 2.5 ~ 22.5 μm in wavelength. The wavenumber resolution is 4 cm-1.
In addition, we measured the refractive index of uncoated GASIR1® at 1.55 μm before and after irradiation at Umicore’s
facility.

4. RESULTS
4.1 Transmission
The measurement results are shown in Figure 9 and Figure 10. Figure 9 shows the transmissions of AR coated
GASIR1®, uncoated GASIR1®, Ge and ZnSe before and after irradiation. Left figures show the transmissions before
irradiation test. Right ones show those of after irradiation test. These figures show the transmission of AR coated
GASIR1®, uncoated GASIR1®, Ge, ZnSe starting from the top.
Figure 10 shows the ratio of transmission after irradiation divided by those of before irradiation in order to investigate
the effect of radiation.

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3000 krad
1000 krad
300 krad10 krad0 krad3000 krad
1000 krad
300 krad10 krad0 kradFigure 9. Left figures show the transmissions before irradiation test. Right one show those of after irradiation test. The
horizontal axis represents wavelength, and the vertical axis represents transmission in percent. These figures show thetransmission of AR coated GASIR1®, uncoated GASIR1®, Ge, ZnSe starting from the top.

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We can see there is a slight change (less than 1%) of transmission after irradiation in Figure 10. The change can be seen
especially at the wavelength of more than 11μm. However, a clear relationship between the degradation of transmission
and the irradiance level can not be seen. Therefore, we confirmed that GASIR1® has a high radiation tolerance likeother infrared optics materials such as Ge and ZnSe. In addition, there is not much difference in radiation tolerance of thetransmission between AR coated GASIR1® and uncoated GASIR1®.
We can see some features like absorption lines around 7μm, 8μm, 9μm, and 15μm in ZnSe transmission. These features
are not related with the irradiation level. We consider these are influenced by absorption of air at the time of
measurements or contamination of the samples

3000 krad
1000 krad
300 krad10 krad0 kradFigure 10. The ratio of transmission after irradiation divided by those of before irradiation. The horizontal axis is Wavelength. The
upper left is the transmission of ARcoated GASIR1®, the upper right is that of uncoated GASIR, the lower left is that of Ge, andthe lower right is that of ZnSe.

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4.2 Refractive index
We measured the refractive index of uncoated GASIR1® at 1.55 μm before and after irradiation at Umicore’s facility.
We measured three points at each sample and three times at each measurement point. Table 5 shows the result of themeasurement of uncoated GASIR1®. The values of the refractive index are averages of nine measurements, (threemeasurements at each of the three points). The tolerance on this measurement is +/- 0.0005. So, a difference of 0.0001 is
negligible. Therefore, we can conclude that there is no influence of the gamma-ray irradiation on GASIR1®’s refractive
index.
Table 5. Measurement results of refractive index of uncoated GASIR1®

GASIR1 AGASIR1 BGASIR1 CGASIR1 DGASIR1 E

100

50

10

5

0

2.54242.54222.54242.54222.5422

Irradiance Level of 2 test [krad]

3000

1000

0

300

0

After 2nd Irradiation Test

2.54242.54222.54242.54232.5423st

Irradiance Level of 1 test [krad]stAfter 1 Irradiation Testnd

5. SUMMARY
We carried out the irradiation tests in order to investigate the radiation tolerance of GASIR1®. We irradiatedGASIR1® with gamma-rays (Co60, 1.17MeV and 1.33MeV) up to 3Mrad. We measured the transmission andrefractive index in the infrared range before and after irradiation.
There is a slight change (less than 1%) of the transmission of GASIR1® after irradiation. We also confirmed there is
no change of refractive index of GASIR1® at 1.55 μm. Therefore, the GASIR1® turned out to be tolerant of highirradiation up to 3Mrad. We confirmed that GASIR1® is an infrared optics material able to be utilized for space
applications.

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