Scintillators are high-density luminescent materials that convert X-rays to visible light. Thallium doped cesium iodide (CsI:Tl) scintillation materials are widely used as converters for X-rays into visible light, with very high conversion efficiency of 64.000 optical photons/MeV. CsI:Tl crystals are commercially available, but, the possibility of developing these crystals into different geometric shapes, meeting the need for coupling the photosensor and reducing cost, makes this material very attractive for scientific research. The objective of this work was to study the feasibility of using radiation sensors, scintillators type, developed for use in imaging systems for X-rays. In this paper, the CsI:Tl scintillator crystal with nominal concentration of the 10 -3 M was grown by the vertical Bridgman technique. The imaging performance of CsI:Tl scintillator was studied as a function of the design type and thickness, since it interferes with the light scattering and, hence, the detection efficiency plus final image resolution. The result of the diffraction X-ray analysis in the grown crystals was consistent with the pattern of a face-centered cubic (fcc) crystal structure. Slices 25 × 2 × 3 mm 3 (length, thickness, height) of the crystal and mini crystals of 1 × 2 × 3 mm 3 (length, thickness, height) were used for comparison in the imaging systems for X-rays. With these crystals scintillators, images of undesirable elements, such as metals in food packaging, were obtained. One-dimensional array of photodiodes and the photosensor CCD (Coupled Charge Device) component were used. In order to determine the ideal thickness of the slices of the scintillator crystal CsI:Tl, Monte Carlo method was used.
X-rays were first discovered by Wilhelm Roentgen in Germany, in 1895 [
Detectors for X-ray application cover a broad range including Si detectors, compound detectors and single crystal detectors. In the low energy X-ray region from a few hundred eV to about 20 KeV, direct detectors such as Si PIN photodiodes, Si APDs, and CCD (Charge Coupled Device) area image sensors are utilized [
Indirect digital X-ray imaging requires scintillator material and some imaging sensor, such as photodiodes, CCD and CMOS imaging devices, for diagnostic and industrial applications. Thus, among the CsI:Tl inorganic scintillators crystal was elected in our research on the basis of the common favorable characteristics as a scintillator: (a) good mechanical strength, (b) it is slightly hygroscopic and (c) its emission spectrum light coincides with the visible region: it is, therefore, suitable to use of CCD and photodiodes system [
Our objective was to study the feasibility of using radiation sensors, type scintillators, developed in our laboratories, for use in imaging systems for X-rays. In this paper, the CsI:Tl scintillator crystal with nominal concentration of the 10−3 M, grown by the vertical Bridgman technique was used in order to convert the X radiation into light photons. For this purpose, one-dimensional array of photodiodes and the photosensor CCD component, mentioned in the previous paragraph, was used. Monte Carlo method exclusive software was used [
The crystal growth was accomplished by means of the vertical Bridgman technique [
The crystals were cut in the desired dimensions for each type of experiment with a disk saw with diamond edge (Buehler ISOMET 11-1180). The cutting was done slowly, avoiding mechanical shock. The crystals were polished with ethylene glycol p.A. (C2H6O2). The side surfaces were unpolished, to enhance internal reflection. The CsI:Tl crystals were cut and polished in the following dimensions: (a) Slices of 25 × 2 × 3 mm3 (length, thickness, height); (b) mini crystals of 1 × 2 × 3 mm3 (length, thickness, height); the mini crystals were introduced into the16 holes of the mini crystals support.
The diffraction analysis of X-ray CsI:Tl crystals in, order to determine the crystal structure, was carried out by the powder diffraction method, at room temperature, using a diffractometer (Rigaku RINT2000 model) in following conditions: radiation: k-alpha Cu, scan rate: 2˚ min−1, voltage: 40 kV, current: 30 mA.
In order to determine the ideal thickness of the slices of the scintillator crystal CsI:Tl, Monte Carlo method was used. The absorption of the X-ray energies in the scintillator was estimated by the Monte Carlo method. This method simulates all occurrences of gamma radiation interaction in the material media. The following interactions of the electromagnetic radiation taken into account were: Photoelectric effect, Compton scattering and Rayleigh scattering. Based on these interactions, a program was developed to simulate the interactions in the scintillator block and to estimate the level of absorption of the radiation X, as a function of the dimensions of the scintillator.
The photodiodes dimensional arrangement units were mounted in a 200 mm line to cover the entire width of the mat of drag the sample to be analyzed. Another alternative used was the use of the CCD chip type, Sony model ILX551B 2048-pixel Linear Sensor (B/W) which gives the work more simple electronic circuitry.
Among the X- ray and gamma-ray converters in light photons, known as scintillators, the most efficient emits photons with wavelengths close to 400 nm.
Particularly among them, the CsI:Tl crystal has the best fit between the light emission spectrum (peak at 540 nm) and the quantum sensitivity curve of the photodiodes. This explains the renewed interest in using this crystal as a scintillator.
X-ray diffraction analysis demonstrated that the crystalline lattice of the crystal grown by the Bridgman technique resulted in a centered face cubic crystalline structure (cfc). This evidence can be established by comparing the X-ray diffraction pattern with that described by the tables (US Joint Committee on Powder Diffraction Standards) [
The main purpose of the CsI:Tl crystal is its use in converting high energy photons from gamma rays and X-rays into light photons capable of sensitizing the photosensors. In order to meet this objective, the CsI:Tl crystal offers adequate conditions mainly because it consists of chemical elements with a high atomic masses (Zcesium = 133, Ziodine = 127) and, therefore, favouring interactions of X radiation with the multiple electron layers (photoelectric effect and Compton). The results of the simulation analysis using the Monte Carlo method are a convincing demonstration of the use of these crystals.
The results simulated by the Monte Carlo method relate the scintillator crystal thickness and the energy absorption percentage of the gamma photon energies of an Americium-241 source (241Am). This source was used because it has the emission spectrum of energy (
The
For the CsI:Tl crystals, at each thickness, the increase of 0.0874 mm doubles the absorption of the X-rays.
Two electronic boards were specially designed for the purpose of using the scintillation crystals, which convert light produced into electrical signals and, subsequently, through these plates the data is digitized and acquired by the PC computer.
241Am Gamma photon Frequency of emission (Yield) (%) | Energy (keV) |
---|---|
42.7 | 13.9 |
2.4 | 26.3 |
0.106 | 33.2 |
35.9 | 59.5 |
0.18 | 69.2 |
Scintillator thickness (mm) | Absorption (%) | CsI:Tl* |
---|---|---|
Mini Crystals | Slice | |
6 | 100 | 100 |
3 | 100 | 100 |
2 | 99.6 | 100 |
1 | 99.5 | 100 |
0.5 | 93.5 | 94.2 |
0.25 | 79.9 | 78.6 |
0.125 | 59.8 | 60.1 |
0.05 | 37.4 | 37.8 |
0.025 | 22 | 24.2 |
0.01563 | 17.4 | 17.7 |
0.008 | 9.6 | 9.7 |
0.004 | 5.7 | 5.1 |
0.0025 | 3.6 | 3.4 |
0.002 | 2.7 | 2.8 |
0.001 | 1.3 | 1.5 |
0.0005 | 0.6 | 0.7 |
0.0001 | 0.1 | 0.1 |
*Absorption of each gamma ray reaching the crystal.
made: (1) CsI:Tl crystal slice and (2) CsI:Tl mini crystals inserted into the spaces of a one-dimensional arrangement, to the photodiode array.
Electronic board for data acquisition with CCD-type component, using simpler electronic circuitry,
Experiments were conducted with the imager (scanner) using array of photodiodes.
The result of the imager driven experiment with CCD technology, directly
coupled to the CsI:Tl scintillator crystal, is shown in
The good quality of the images obtained in experiments with X-rays imaging using the CsI:Tl mini-crystals and slices of the CsI:Tl crystal, coupled to one-dimensional photodiodes arrangement or coupled photosensor CCD, indicated that this system can be used in quality control in food industry.
The experiments demonstrated that the crystal of cesium iodide doped with thallium grown in our laboratory showed good results, when used in imaging equipment with X-rays.
The result of the analysis of X-ray diffraction on the crystal grown by the vertical Bridgman technique showed a structure compatible with the standard cubic crystalline face centered (fcc).
Slices of the CsI:Tl crystal, with a thickness of 2 mm and mini crystals in the dimensions of 1 × 2 × 3 mm3 may be successfully used in imaging.
By means of estimates obtained by the Monte Carlo method, it has been shown that 2 mm slices are capable of absorbing 100% of the energy of the incident X-rays.
Imaging with one-dimensional array of photodiodes and imaging with scanner in CCD version, used with CsI:Tl scintillator crystal, showed efficiency in image quality.
CsI:Tl crystals, although are commercially available, there is the possibility of development of these crystals in our laboratory, which allows obtaining the crystals in different geometric shapes, to meet the need for coupling the photosensor, such as, one-dimensional array of photodiodes and CCD photosensor, with goal of the X-ray imaging.
In continuation of the research will be used the CCD with dimensions A3 (297 mm) directly coupled to a long slice of the crystal scintillator of CsI:Tl covering all its sensitive face, so that it is possible to obtain images of large bodies.
The authors thank FAPESP (Foundation Research of the State of São Paulo) for financial support.
Pereira, M. da C.C., Filho, T.M., Berretta, J.R. and de Mesquita, C.H. (2018) Characteristics of the CsI:Tl Scintillator Crystal for X-Ray Imaging Applications. Materials Sciences and Applications, 9, 268-280. https://doi.org/10.4236/msa.2018.92018