Cesium-iodide-based thin films for the detection of ionizing radiation

Cesium-iodide-based thin films for the detection of ionizing radiation
Azadeh Farzaneh1 , Mohammad Reza Abdi*,1 , Khadijeh Rezaee Ebrahim Saraee2 , Mojtaba
Mostajabaldaavati2 . A.Quaranta3
1
Department of Physics, Faculty of Science, University of Isfahan, Isfahan 81746-73441, Iran
Department of Nuclear Engineering, Faculty of Advance Sciences and Technologies, University of Isfahan, Isfahan
81746-73441, Iran
3
University of Trento, Dipartimento di Ingegneria dei Materiali e delle Tecnologie Industriali – DIMTI, Via
Mesiano 77, I-38050 Povo, Trento, Italy
2
Abstract
Thin film of CsI 440 nm thickness range was deposited on glass substrate using the sol gel
technique. The thin films represented a columnar growth behavior with orientation along (110).
The different surface morphology of sol- gel films with respect to evaporated films can be
assumed as the main cause of the more QE (quantum efficiency) of sol- gel CsI films. Also the
light yield of CsI film will be compared with CsI bulk. The thin films were characterized by
scanning electron microscopy (SEM), Atomic force microscopy (AFM), X-ray diffraction
(XRD), steady-state UV-visible optical absorption and transmittance and photoluminescence
spectroscopy. To determine the refractive index (n), extinction coefficient (k) and absorption
coefficient (α) of CsI thin film the optical transmission method was used. In this research optical
energy band gap of CsI thin films was also estimated. Scintillation tests were performed on the
CsI thin film by photo multiplier tube (PMT) based material exposed to low- level gamma
irradiation.
Key words: CsI, Photoluminescence, Sol gel, Thin film
*
Corresponding authors: Mohammad Reza Abdi; email: r.abdi@phys.ui.ac.ir, Tel: +989131026744; Fax:
+983117934800
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1. Introduction
Depending on specific requirements for each detector application, for example the type of
radiation, speed requirement and spatial resolution in the case of imaging applications,
scintillators are used. Among scintillators, alkali halide materials due to their excellent electronemitting properties in X-ray energy ranges are important [1]. CsI has high quantum efficiency
and relatively high stability to ambient air and gas environment [2, 3] so it is a best choice as an
alkali halide. The thin films have various application in different fields, such as biomedical and
energy [4, 5]. Among all these halides, the CsI is a generally preferred material owing to its
higher quantum efficiency arising due to a lower electron affinity and large electron escape
length. Further, it is less hygroscopic and the films on glass substrate have a continuous structure
for a given thickness compared to other materials. CsI films have potential applications including
display devices, X-ray tubes, charged particle accelerators and high power microwave devices
[6-11].
There are different methods which can employ to deposit CsI as thin film such as thermal
evaporation, electron-beam evaporation, ion-beam sputtering and sol-gel [12]. Comparison with
results obtained for evaporated and ion-beam sputtering CsI films indicated that the effective
photo-emissive surface area, is one of the important parameters in influencing the QE (quantum
efficiency). The use of sol- gel processes for the properties of scintillating materials in the form
nanocrystal films seems to be a very interesting method [13]. By this method we can produce
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transparent, homogeneous, multi component oxide thin films of many compositions on various
substrates at low cost and it allows the tuning of the refractive index and thickness of the film by
varying synthesis parameters [14]. In fact, various compositions, different type and concentration
of the doping ion can be reached easily by the sol- gel process. The different surface morphology
of sol-gel films with respect to evaporated films can be assumed as the main cause of the more
QE of sol- gel CsI films.
Due to three-dimensional confinement and much better overlap of electron and hole
wavefunctions, the optical transitions are expected to have higher efficiency and faster rate than
bulk scintillators, which should eliminate the major problem of relatively slow response of
scintillator detectors. Large single-crystal inorganic scintillators are very fragile, expensive to
grow, and the size of high-quality crystals is limited. Particulate inorganic scintillators of
micrometer size are scalable and robust but they have low solubility in organic and polymeric
matrices [13]. When they prepared in inorganic matrices, such as sol- gel, they produce an
optically opaque gel, which significantly reduces scintillation output. Using composites of
nanocrystalline materials is one way to overcome these limitations. Nanocrystalline materials
due to their small size have better solubility in organic polymers or inorganic sol-gel host
materials and to cause much less scattering, which should result in higher efficiency of the
scintillator [15]. This study is aimed to present preliminary results on the structural, chemical,
morphological and photoluminescence properties of CsI films deposited by sol- gel.
2. Material and method
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The CsI films were synthesized by the sol- gel method. The chemicals used were 98% pure
tetraethylorthosilicate (TEOS) and 99.9% pure CsI powder purchased from Sigma-Aldrich
Chemie, Germany. Other reagents were 98% pure ethanol (EtOH), polyethylene glycol (PEG)
(Merck) and concentrated HCL. The synthesis of the hybrid matrix was performed at 30 ˚C,
under continuous magnetic stirring of the TEOS ethanol solution with HCL (cac = 0.01 mol/L)
to prepare sol A in the first container. In second container the solution of PEG, ethanol and H 2 O
were dissolved under continuous magnetic stirring at 30˚C, the n CsI powder will be added to get
a transparent and homogeneous sol to prepare sol B. Finally sol A will be added dropwise to sol
B under stirring for 30 minutes to get a final homogeneous and transparent [16, 17]. The molar
ratio of EtOH: TEOS: H2O: PEG is 10:1:5:2. The hydrolysis of TEOS were done under acidic
catalyst conditions. Figure 1 summarized the procedure of making the silica sols. Then the silica
sols were dropped on 1 × 1 cm glass substrates. The glass substrates were dip-deposited using a
commercial dip coater at room temperature. Dipping the glass substrate in the sol and withdrawing it
at a rate of 0.9 cm/min at room temperature prepared the gel films .The films were deposited with 9
coatings and dried at 70 °C for 15 min after each successive coating. The final gel films onto glass
substrate were annealed at 200 °C for 60 min in order to obtain the CsI films.
The resulting samples were studied by means of scanning electron microscopy (SEM), X-ray
diffraction (XRD), steady-state UV- visible optical absorption and photoluminescence
spectroscopy.
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Fig. 1. Scheme of the modified sol- gel process
3. Result and discussion
3.1. Structural and morphological studies
Figure 2 shows the XRD pattern of 440 nm thin films [18].This pattern confirmed the crystalline
nature of films (PCPDF data #06-0311). In the XRD pattern of the 440 nm thick film, the (110)
is the most intense peak, however, the relative intensities of (200) and (310) peaks were greater
than those reported for the powder XRD pattern [2]. This can be attributed to a preferential
orientation of crystalline grains along (200) and (310) directions in the film. The average grain
sizes are reported 90.52 nm, evaluated from the full- width-at-half- maximum (FWHM) of the
mean Bragg peak using the Scherer's equation [19].
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The XRD patterns of CsI films deposited by the sol - gel method were compared to the XRD
patterns of sputtered and thermal evaporated CsI films in literatures [10]. The XRD patterns of
sputtered CsI films feature an intense peak at 2Θ of about 27.6◦ which corresponds to the (110)
preferred orientation of CsI and the evaporated film shows a (200) preferred crystallographic
orientation, whereas, its XRD patterns by sol- gel method show an intense peak at 2Θ of about
27.6◦ which corresponds to the (110) preferred orientation. However, the relative intensities of
(200) and (310) peaks were greater than those reported for the powder XRD pattern [2].
Fig.2. XRD pattern of CsI of film
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Fig.3. SEM micrograph of CsI thin film
Fig.3 shows the SEM images of CsI thin film at different magnifications. SEM studies showed
that, the particles are distributed uniformly and mostly of homogeneous morphology as can be
seen from the higher magnification micrograph. It shows the surface morphology of the CsI thin
film annealed at 200ºC.
The AFM image of sol gel CsI film with thickness of 440 nm is shown in Figure 4. The mode of
AFM image is contact mode that landing force and scan rate are 0.1nN and 2µm/s. The surface
showed the formation of nano islands (growth around a nuclei) having diameters of about 100nm
and average height of 26.2 nm and 8.15 nm. The density of islands increases with deposits rates
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and the grain growth proceeds through a grain boundary diffusion process at room temperature
(Fig. 4).
(b)
(a)
Fig.4. Typical AFM surface images of a CsI film: a) CsI thin film with 4 time deposited and b) CsI thin
film with 10 time deposited
According to the variation of the volume ratio of EtOH:TEOS, the surface morphology in the
film samples formed by the sol–gel method was measured by AFM, as shown in Figure 4. The
surface morphology of two film samples by sol–gel deposition in which the EtOH:TEOS volume
ratio is 4:1 and 3:1 is rough, as shown in Fig. 5a and b, respectively. In addition, the number and
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the size of the pores in the film sample in Figure 5b formed using a sol with EtOH:TEOS volume
ratio of 3:1 are larger and smaller, respectively, than that in the film sample in Figure 5a formed
using a sol with EtOH:TEOS volume ratio of 4:1. Perhaps with increasing ethanol content, the
effect of PEG on pore size decreased and the morphology of pore became connective. It can be
referred to the nanocomposites with covalent bonds between the inorganic (siloxane) and organic
(polymer) phases, in which large quantities of CsI can be dissolved.
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Fig.5. The surface morphology of sol- gel films. The films were processed using the following
conditions: (a) EtOH:TEOS volume ratio of 4:1, (b) EtOH:TEOS volume ratio of 3:1
3.2. Optical properties
Figure 6a and b shows the transmittance and absorption spectra of the CsI films deposited by the
sol-gel method. The appearance of the spectrum in the range of 300-800 nm in Fig. (6b) indicates
the smooth reflecting surface of the film and low scattering loss at the surface. The films
annealed at 200ºC exhibit good transparency in the visible region (>85%). A sharp increase in
transmittance was observed at wavelength λ = 340 nm and CsI films was found to transparent in
the spectral region 340 nm to 900 nm.
10
g
Fig.6. a) Absorption spectra of CsI thin film. b) Transmittance spectra of the CsI thin film
The absorption coefficient data were used to determine the optical band gap, Eg, using the
relation αhv ≈ (hv − Eg )n/2 where hν is the photon energy and n=1 for direct transition, n = 4
for indirect transition. In this case the transitions are direct and the absorption coefficient α was
obtained from the transmittance data using the relation α= (1/d) ln(1/T), where d and T are the
thickness and the transmittance of the films. The absorption coefficient (α) is shown in Fig.7.
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Fig.7. Absorption coefficient of CsI thin film
The estimated band gap from the plot of (αhν)2 vs. hν for CsI thin film can be seen in Fig. 8. The
band gap „Eg‟ is determined by extrapolating the straight portion to the energy axis at α=0. The
calculated band gap value of the CsI thin film was 3.7 eV.
Fig.8. Plot of (αhν)2 vs. hν of CsI thin film
The extinction coefficient (k) was calculated from the absorption coefficient (α) using the
formula:
k=
Where λ is the wavelength of the absorption spectrum. The variation of refractive index n and
extinction coefficient k with photon energy for CsI film annealed at 200ºC is shown in Fig. 9 and
10.
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Fig.9. Extinction coefficient (k) as function of photon energy
In Fig.10 the refractive index increases with increasing photon energy, with peak at about
3.00eV. This can be attributed to the band gap of CsI ~3.7 eV. The refractive index of the
coating film increased from 2.01 to 2.13 when it went through surface modification using TEOS.
This phenomenon can be explained by the filling model with an acidic catalyst [20]. In fact, the
growth of silica sol tends to form linear chains and the pore volume of the formed film is
extremely low causing the film with high refractive index [16].
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Fig. 10. Plot of refractive index n as a function of Photon energy for CsI film
Fig. 11(a) shows the PLE spectra of CsI thin film, in which the main peaks at 270 and 330 nm
indicate the band-gap energy Eg. The band-gap values (Eg=3.7) of our CsI thin film sample
suggest the formation of a cubic structure [20, 21]. Consequently, the theoretical correlation
between lattice constants of the thin films and the optical band gap can predict satisfactorily the
decrease of Eg with respect to bulk values [23]. The band gap can be used to get the total
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scintillation light output that is the number of photons emitted by the scintillator per unit of
energy absorbed (usually photons/MeV) and it is given by
β is a constant that appears approximately 2.5. For the ideal situation, the transfer efficiency S
and the quantum efficiency Q of the activator ion are 100% and then with Eg=3.7 eV the light
yield will be 108108 ph/MeV. So, The higher light output is reported for the thin film of pure
CsI which have the smaller band gap with respect to bulk pure CsI [24].
Other weaker peaks at 245, 395 nm are also observed in the PLE spectra, probably due to other
allowed electronic transitions between various energy levels (orbits) in the valence bands and the
conduction bands in the CsI thin film. Several other observations can be made from Fig. 11(a)
and (b). First, there are large Stokes shifts (1.23eV) between the band-edge absorption peaks
(330nm) and the main luminescence peaks (430nm) for CsI thin film, which result in good
optical transparency for its own emitted light. CsI thin film also has fast, near-band-edge
emission that is interest for scintillation. The near-band-edge emitting scintillators have excited
states that extend over many atoms, and this involves negligible lattice relaxation. This emission
may arise from band-to-band or free exciton recombination transitions [25]. If the transition is
direct (such as CsI film) and parity allowed, the radiative lifetime may be short, a nanosecond or
less.
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Fig.11(a). Emission spectra for thin film of CsI
Fig.11 (b). Excitation spectra of thin film of undoped CsI
4. Scintillation test with CsI thin film
To assure that the energy dose not lose as ionized electrons traverse non-scintillating glass, glass
plate alone is exposed to sources. Its spectrum shows no scintillator pulse amplitude which
reflects deposited energy with glass (Fig.12).
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There have also been papers on microcolumner growth of CsI films for x ray imaging. As a
scintillator x rays, it seems a 440 nm film is useless because it does not stop a significant number
x rays while this thin film which made of nanocrystalline could appears the peaks of gamma rays
from low activity Cs source with the energy of 662 kev and Zn source with the energy of 1.11
Mev (Fig.13 and 14).
For charged particles, however, thin films are useful so we discuss that in the paper. To test CsI
thin film for scintillation, low-activity α Amersham sources were used, the source was a
241
Am
disk source with the energy of 5.486 Mev. Scintillation events were detected with a Hamamatsu
R1894 quartz window bi-alkali photomultiplier tube (PMT) biased at 1250 V. The electronic
signal from the PMT was processed using IAP (Institute Applied Physics) 3600 preamplifier,
IAP 3001 amplifier with a gain of 100 and a shaping time of 1.5 µs and IAP 4110 multichannel
analyzer. All measurements were taken over a live time of 2000 s. Scintillation tests were
performed on the as-synthesized CsI thin film. Optical grease was applied between the thin film
and the PMT detector to decrease interface scattering. The thin film was placed with its
uncovered face right in front of the PMT photocathode. The 1-inch diameter disk sources were
placed at the center of the thin film, touching the back face of the thin film. The α particles from
the Am-241 source create two peak with CsI thin film and there is a significant amount of counts
in low energy. This suggests that a large number of electrons escape from the scintillator. When
the layer thickness is comparable to the range of electron, some electrons deposit their full
energy, creating a peak in a pulse height spectrum. Fig.15 shows the two peaks at the channels
rang 1020 and 1040 is referred to the spectrum for α particle detection. We conclude that the
concentration of nanocrystals and the thickness of thin film in this experiment was too enough to
observe their scintillation.
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Fig.12. Results of scintillation tests on glass exposed to sources
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Fig.13. Results of scintillation tests on CsI thin film exposed to 137Cs
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Fig.14. Results of scintillation tests on CsI thin film exposed to 65 Zn
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Fig.15. Results of scintillation tests on CsI thin film exposed to 241 Am
5. Conclusion
To conclude, CsI thin film with thickness of 440 nm has been deposited on a glass substrate by a
sol-gel technique. The synthesis temperature could be brought down to 2000 C for the formation
of a single phase sample. The thin film was characterized by AFM, XRD and SEM. XRD studies
confirmed the formation of a crystalline compound with a preferred orientation along the 110
direction. The emission spectra of the sample showed characteristic peaks corresponding to
strong transition at 430 nm. Photon absorbance CsI films on glass substrate were performed in
the spectral region 190 nm to 900 nm, which varies in between 0 to 2.5. One strong absorption
peak was observed at a wavelength smaller than 225 nm. Optical energy band gap for CsI films
are determined from absorbance data are found to 3.7 eV. Optical transmittance derived from
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absorbance of CsI film in the spectral range 190 nm to 900 nm shows CsI films are opaque in the
spectral range 190 to 290 nm, while in the spectral range 225 nm to 900 nm films were found to
be transparant, and having more than 80% transmittance. Also scintillation tests were carried out
on CsI thin film by PMT.
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