Summary of TECHNICAL REPORT 2.1.b «A study of the non linearity of scintillators intrinsic conversion efficiency» Authors: 1. Nektarios Kalyvas 2. Ioannis Valais 3. Christos Michail 4. George Fountos 5. Stratos David 6. Panagiotis Liaparinos 1
Contents 1. Introduction.3 2. Materials and Methods...3 2.1 Theory.....3 2.2 Methodology for estimating n C....5 3. Results.5 3.1 GdAlO 3 :Ce...5 3.2 YAP:Ce 6 3.3 Gd 2 O 2 S:Eu...7 3.4 Lu 2 O 3 :Eu..8 4. Discussion.9 5. Conclusion......11 6. References..12 2
1. INTRODUCTION Scintillators are used in X-ray imaging detectors in conjunction with appropriate photoreceptors like TFT, CCD, CMOS and GaAs. [1,2]. The sensitivity of a photoreceptor is usually considered in terms of the number of optical photons escape per incident X-rays or per incident exposure or Air KERMA. Another parameter characterizing the sensitivity of the scintillator is absolute efficiency. Absolute efficiency is defined as the emitted optical photon power per incident X-ray exposure. It depends upon X-ray absorption efficiency, intrinsic conversion efficiency (generated optical photon power per absorbed X-ray power) and optical photon escape efficiency [3,4,5]. It is customary to consider a constant number for the intrinsic conversion efficiency (usually calculated through available light yield values), however in literature it has been reported that the production of optical photons is not a linear phenomenon specially when the scintillator is excited with MeV energies. [6-9]. In X-ray diagnosis the incident X-ray spectrum is in the order of KeV however there are thousands of X-ray photons per incident exposure thus the energy imparted in the scintillator may as well be in the order of MeV. The scope of this report is to examine the intrinsic conversion efficiency of granular scintillators by comparing theoretical models to absolute efficiency experimental data. 2. MATERIALS AND METHODS 2.1 Theory 2.1.1 Intrinsic Conversion Efficiency When energy, E, is absorbed in a scintillator electron-hole pairs are created. The average energy necessary to create an electron-hole pair is beg, where Eg is the energy gap of the scintillator and b is a constant characterizing the excess energy required for electron-hole pair generation. B has been reported to take a value equal to 1.5, but in literature higher values can be found [10]. Therefore the total number of electron-hole pairs created equals to Ε/ beg. A fraction, R, of the produced pairs will reach the activator (R Ε/ beg) and subsequently a fraction q will ultimately absorbed by it and excite to optical wavelengths. Therefore the total number of electron-hole pairs that produce optical photons equals to RqΕ/ beg. If E λ is the optical photon energy, expressed in ev, then n C can be calculated as [10]: 3
n C RqE beg RqE E E be (1) g By inspecting equation (1) we deduce that n C calculation is based upon parameters that may not constant but are depended either by the availability of the scintillator energy levels (R, q) or the existence of electrons in the valence band (b). An indicative n C value can be obtained be using the quantum yield (L) which is provided by the manufacturer in units photons/mev as [11] n C L E (2) 1000000 In order equation 2 to be valid the value of L provided by the manufacturer should minimize the effect of optical photon absorption in the scintillator 2.1.2 Absolute Efficiency (AE). Absolute efficiency depends upon (i) the X-ray absorption efficiency, ( E, T), by intrinsic conversion efficiency n C and the probability of optical photon escape (,,, T), which id a function of optical absorption and scatter (, ) in the phosphor, the reflectivity of the inner and output interfaces (ρ) and the thickness of the scintillator (Τ). G l n Q A complete formula for calculating AE is the following [3,4,5]: nc ( E) tr( E)(1 ) e AE 2 2 2( ( E) ) ( E) T T ( E) T ( ( E) )(1 ) e 2( ( E) ) e ( ( E) )(1 ) e T T (1 )( ) e (1 )( ) e T (3) where (E) is the X-ray total mass attenuation coefficient of the scintillator [12, 13], (E) translates the energy flux (W/m 2 ) in exposure rate (mr/s), and t, is the transparency of the substrate on which the scintillator has been deposited. If the energy spectrum f(e) is considered [14] then ΑΕ can be calculated as [3,4,5]: 4
AE tot E f ( E) AE E f ( E) (4) 2.2 Methodology for estimating n C A retrospective analysis of published and measured AE results of different scintillators has been performed (YAlO 3 :Ce, GdAlO 3 :Ce, Gd 2 O 2 S:Eu and Lu 2 O 3 :Eu) [11, 15-17]. The materials were in the form of screens constructed with the sedimentation method. For each phosphor an initial bibliographic value of n C was considered. For the case of Lu 2 O 3 :Eu high surface density pills were constructed by high pressure application. The scintillator detectors were irradiated with tube voltages ranging from 50 kvp to 140 kvp, tube current 64 ma and exposure time 1 sec. The exposure rate had been measured by means of an X-ray exposure meter. The measurement reproducibility was below 5% for YAlO 3 :Ce, GdAlO 3 :Ce, Gd 2 O 2 S:Eu and below 10% for Lu 2 O 3 :Eu. For each material and for each phosphor thickness the optical properties (σ) was calculated. Since the optical properties are not affected by the absorbed X-ray energy the derived values were considered constant per X-ray thickness. The other parameters used in equation 3 were set equal to t r =0.964 and β=0.03 [3, 4, 5]. 3. RESULTS 3.1 GdAlO 3 :Ce GdAlO 3 :Ce was purchased in powder form (Phosphor Technology Ltd,England,code:UM58#9438). Five phosphor screens with the sedimentation method were constructed with surface densities 14.7 mg/cm 2, 31 mg/cm 2, 53.7 mg/cm 2, 67.2 mg/cm 2 and 121.1 mg/cm 2.. 5
Fig 1: The dependence of GdAlO 3 :Ce intrinsic conversion efficiency (n C ) for different X-ray tube voltages and surface densities In Figure 1 the variations of intrinsic conversion efficiency is presented. It may be observed that n C values are within 0.013 to 0.02. These values corresponds to σ values ranging from 900 cm 2 /g to 1700 cm 2 /g. These values differs from the average values pair that has been stated in literature (n C =0.00009 and σ=67 cm 2 /g) and was found by fitting equation 3 to the experimental data. The reason for the higher n C in this report is the utilization of equation 2 and the adoption of literature values for light yield [11, 15] 3.2 YAP:Ce YAlO 3 :Ce was purchased in powder form (Phosphor Technology Ltd,England,code:QM58/S- S1). Five phosphor screens with the sedimentation method were constructed with surface densities 34.5 mg/cm 2, 53.7 mg/cm 2, 70.7 mg/cm 2, 88 mg/cm 2, 110 mg/cm 2 and 110 mg/cm 2. [11] 6
Fig.2 The dependence of YAP:Ce intrinsic conversion efficiency (n C ) for different X-ray tube voltages and surface densities. In Figure 2 the variations of intrinsic conversion efficiency is presented. It may be observed that n C values are within 0.0029 to 0.04. These values corresponds to σ values ranging from 96 cm 2 /g to 135 cm 2 /g. These values are comparable with the reported literature value for σ (σ=104.3 cm 2 /g) [11]. 3.3 Gd 2 O 2 S:Eu Gd 2 O 2 S:Eu was purchased in powder form (Phosphor Technology Ltd,England,code:UKL63/N- R1). Five phosphor screens with the sedimentation method were constructed with surface densities 33.1 mg/cm 2, 46.4 mg/cm 2, 63.1 mg/cm 2 78.3 mg/cm 2 and 139.8 mg/cm 2. 7
Fig.3 The dependence of Gd 2 O 2 S:Eu intrinsic conversion efficiency (n C ) for different X-ray tube voltages and surface densities In Figure 3 the variations of intrinsic conversion efficiency is presented. It may be observed that n C values are within 0.045 to 0.12. These values correspond to σ values ranging from 30.6 cm 2 /g to 82 cm 2 /g. 3.4 Lu 2 O 3 :Eu Lu 2 O 3 :Eu nanophosphor was synthesized by the research team of Prof. Ε. Zych. Three different pills, of surgace densitites 222 mg/cm 2, 262 mg/cm 2 and 468 mg/cm 2 were constructed by applying high pressure to the nanophosphor. 8
Fig.3 The dependence of Lu 2 O 3 :Eu intrinsic conversion efficiency (n C ) for different X-ray tube voltages and surface densities In Figure 4 the variations of intrinsic conversion efficiency is presented. It may be observed that n C values are within 0.055 to 0.08. These values corresponds to σ values ranging from 26.5 cm 2 /g to 35.5 cm 2 /g. These values are comparable with the reported literature value for σ (σ=33 cm 2 /g) [17]. 4. DISCUSSION From Figures 1 to 4 it may be observed that intrinsic conversion efficiency do not change significantly for the Ce 3+ activated phosphors under investigation, a result already mentioned in literature [18]. In addition it may be observed that n C was found to decrease with X-ray energy for Eu 3+ activated phosphors. In order to better study n C in terms of the absorbed X-ray energy the Quantum Detection Efficiency, (QDE) was calculated according to QDE E f ( E)1 e E f ( E) T (5) Where Τ is the surface density of the scintillator. 9
Then QDE was multiplied with the Air-Kerma, (K) measured in mgy. The result of the multiplication is an indication of the energy absorbed in the material. Therefore the ratio n C /(KxQDE) is an indication of n C variation per absorbed air kerma. In Figures 5,6,7,8 the normalized n C with respect to air kerma for GdAlO 3 :Ce, YAP:Ce, Gd 2 O 2 S:Eu and Lu 2 O 3 :Eu is presented. From these figures we deduce that normalized n C is decreased with increasing X-ray tube voltage. Fig 5: Normalized n C values for GdAlO 3 :Ce for different X-ray tube voltages and surface densities Fig 6: Normalized n C values for ΥAlO 3 :Ce for different X-ray tube voltages and surface densities 10
Fig 7: Normalized n C values for Gd 2 O 2 S:Eu for different X-ray tube voltages and surface densities Fig 8: Normalized n C values for Lu 2 O 3 :Eu for different X-ray tube voltages and surface densities 5. CONCLUSION The n C values calculated in the report depends upon (ι) statistical errors in absolute efficiency measurements, which are always less than 10% and (ιι) the accuracy of the X-ray spectra f(e) used for calculating equations 4 and 5 [14]. Despite the statistical errors, an tendency for n C 11
decrease with respect to X-ray tube voltage for some scintillators is observed. This behavior should be considered in scintillator theoretical studies. 6. REFERENCES 1. Del Guerra A Ionizing Radiation Detectors for Medical Imaging ISBN 9812386742 World Scientific Pub Co Inc. ISBN 9812386742. 2. Van Eijk CWE, Inorganic scintillators in medical imaging Phys. Med. Biol. 47, R85, 2002. 3. Kandarakis Ι., Cavouras D., Panayiotakis G.S., Agelis T., Nomicos C.D., and Giakoumakis G.: "X-ray induced luminescence and spatial resolution of La2O2S:Tb phosphor screens". Physics in Medicine and Biology, 41, 297-307, 1996. 4. Panayiotakis G.S., Cavouras D., Kandarakis I., Nomicos C.D.: A study of X-ray luminescence and spectral compatibility of europium-activated yttrium-vanadate (YVO 4 :Eu) screens for medical imaging applications. Applied Physics. A (Materials Science and Processing) 62, 483-486, 1996. 5. Cavouras D., Kandarakis I., Panayiotakis G.S., Evangelou E., and Nomicos C.D.: An evaluation of they 2 O 3 : Eu 3+ scintillator for application in medical X-ray detectors and image receptors. Medical Physics, 23, 1965-1975, 1996. 6. Nikl M.: Scintillation detectors for X-rays, Meas. Sci. Technol. 17, R37, 2006. 7. Moses W.W., Payne S.A., Choong W-S., Hull G., Reutter B.W.: Scintillator Non- Proportionality: Present Understanding and Future Challenges IEEE TNS, 55, pp1049-1053, 2008 8. Khodyuk I.V., de Haas J.T.M. and Dorenbos P.: Nonproportionality Response Between 0.1-100keV Energy by Means of Highly Monochromatic Synchrotron X-rays IEEE TNS 57, 1175-1181, 2010. 9. Dorenbos P.: Fundamental Limitations in the Performance of Ce 3+ -, Pr 3+ -, and Eu 2+ - Activated Scintillators. IEEE TNS, 57, 1162-1167, 2010. 10. Kalivas N., Costaridou L, Kandarakis I., Cavouras D., Nomicos C.D. Panayiotakis G.: Effect of intrinsic gain fluctuations on quantum noise of phosphors used in medical x-ray imaging detectors. Applied Physics A 69, 337-341, 1999. 11. Kalivas N., Valais I., Nikolopoulos D., Konstantinidis A., Gaitanis A., Cavouras D., Nomicos C.D., Panayiotakis G. and Kandarakis I. et al.: Light emission efficiency and 12
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