A scintillation dual-detector system featuring active Compton suppression

Transcription

1 University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2007 A scintillation dual-detector system featuring active Compton suppression Dean Inwood University of Wollongong Recommended Citation Inwood, Dean, A scintillation dual-detector system featuring active Compton suppression, MSc-Res, Department of Engineering Physics, University of Wollongong, Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

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3 A SCINTILLATION DUAL-DETECTOR SYSTEM FEATURING ACTIVE COMPTON SUPPRESSION A thesis submitted in partial fulfilment of the requirements for the award of the degree MASTER OF SCIENCE - RESEARCH from UNIVERSITY OF WOLLONGONG by DEAN INWOOD, B. Med. Rad. Phys. Department of Engineering Physics 2007

4 I, Dean Inwood, declare that this thesis, submitted in partial fulfilment of the requirements for the award of Master of Science, in the Department of Engineering Physics, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Dean Inwood November

5 Section Title Page List of Figures 4 List of Tables 7 List of Equations 7 Abstract Introduction Background Physics of Scintillation/Solid-State Radiation Detectors 12 a. Scintillators 12 b. Optimum Scintillator Light Collection 14 c. Photodiodes 14 d. Collimation Concept of Compton Suppression Single Detector Intraoperative Probes Dual-Detector Intraoperative Probe Summary and Conclusions Material and Methods Principle of the New Dual-Detector System 26 a. Coincidence and Traditional Compton Suppression 26 b. Light Sharing and Anti-Compton Suppression 28 c. Active collimation Prototype Device Designs 31 a. Prototype 1: CD1 31 b. Prototype 2: CD Outline of Investigation 34 a. Photodiode Experiments 34 b. Photodiode/Scintillator Experiments Experimental Procedure Detector I-V Characterisation 36 a. Aim 36 b. Method 36 c. Results 38 d. Discussion Noise Threshold 39 a. Aim 39 b. Method 39 c. Results 43 d. Discussion Crosstalk 44 a. Aim 44 b. Method 44 c. Results 46 d. Discussion Shaping Time 48 2

6 a. Aim 48 b. Method 48 c. Results 50 d. Discussion Energy Resolution 54 a. Aim 54 b. Method 54 c. Results 55 d. Discussion Possibility of Compton Suppression 61 a. Aim 61 b. Method 61 c. Results 64 d. Discussion Conclusions I-V Measurements Noise Measurements Crosstalk Measurements Effect of Shaping Time on Energy Resolution System Energy Resolution Compton Suppression Advantage of Dual Photodiode-Dual Scintillator Probe Design Further Work References 77 3

7 List of Figures Figure Diagram of the energy states within an inorganic scintillator. 13 Figure Diagram of a typical reverse biased p-n junction. 15 Figure Diagram of the dual beta probe used by Daghighian et al. 20 Figure Diagram of the scintillation detector configuration reported by Hickernell et al. 21 Figure Diagram of the stacked IISD unit used by Raylman et al. 21 Figure Diagram of the detector used by Watabe et al. 22 Figure Block diagram of the dual detector system used by Watabe et al. 23 Figure Diagram of the major features of a typical gamma-ray spectrum. 26 Figure Diagram of interactions of use in anti-compton spectrometry. 27 Figure Diagram showing the effects of light sharing on energy spectra. 29 Figure Schematic diagram of the CD1 photodiode. 31 Figure Schematic diagram of the CD1 prototype. 32 Figure Schematic diagram of the CD2 photodiode. 33 Figure Schematic diagram of the CD2 prototype. 33 Figure Circuit diagram for the I-V measurements. 36 Figure Circuit diagram for the I-V measurements. 37 Figure Results of I-V measurements for CD1#6. 38 Figure Results of I-V measurements for CD2#1. 38 Figure Block diagram showing the setup for the noise measurements. 40 Figure Photograph showing setup inside the die-cast box. 41 Figure Photograph showing the NIM modules, die-cast box and oscilloscope used. 41 Figure Photograph showing the PC and MCA unit used. 42 Figure Results of the noise measurements for CD1#8. 43 Figure Results of the noise measurements for CD2#1. 43 Figure Block diagram showing the setup used for the crosstalk measurements. 45 4

8 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Crosstalk for CD1#8 using I-125 as measured from the inner photodiode. 46 Crosstalk for CD1#8 using I-125 as measured from the outer photodiode. 46 Crosstalk for CD2#1 using I-125 as measured from the inner photodiode. 47 Crosstalk for CD2#1 using I-125 as measured from the outer photodiode. 47 Block diagram showing the setup used for the resolution measurements. 49 Shaping times versus resolution for CD1#2 as measured from inner photodiode. 50 Shaping times versus resolution for CD1#2 as measured from outer photodiode. 50 Shaping times versus resolution for CD2#5 as measured from outer photodiode. 51 Shaping times versus resolution for CD2#5 as measured from inner photodiode. 51 Block diagram showing the setup used for the resolution measurements. 55 Figure Results from CD1#7 using I Figure Results from CD2#1 using I Figure Results from CD2#5 + CsI(Tl) using Co Figure Results from CD1#2 + CsI(Tl) using Na Figure Results from CD2#5 + CsI(Tl) using Na Figure Results from CD1#2 + CsI(Tl) using Cs Figure Results from CD2#5 + CsI(Tl) using Cs Figure Block diagram showing the setup used for Compton suppression measurements. 62 Figure Equipment setup inside the die-cast box for both single and double-sources. 62 5

9 Figure Figure Figure Results showing the effect of changing LLD on suppression for CD1#2 + CsI(Tl). Results showing the effect of changing LLD on suppression for CD1#2 + CsI(Tl). Results showing the effect of changing ULD on suppression for CD1#2 + CsI(Tl) Na-22 spectra with and without suppression for CD1#2 + CsI(Tl). 65 Figure Suppression factor versus energy for. 66 Figure Cs-137 spectra with and without suppression for CD1#2 + CsI(Tl). 66 Figure Suppression factor versus energy for Figure Figure Cs-137 spectra with and without suppression for CD2#5 + CsI(Tl). 67 Figure Suppression factor versus energy for Figure Figure Na-22/Cs-137 spectra with and without suppression for CD1#2 + CsI(Tl). 68 Figure Suppression factor versus energy for Figure Figure Co-57/Cs-137 spectra with and without suppression for CD2#5 + CsI(Tl). 69 Figure Suppression factor versus energy for Figure

10 List of Tables Table 1.1 Properties of common inorganic scintillators. 13 Table 1.2 Characteristics of commercially available single-detector IOPs. 19 Table 1.3 Characteristics of prototype dual-detector IOPs available in the literature. 24 Table 3.1 Table of FWHM resolution versus shaping time for both detector prototypes using Na Table 3.2 Table of FWHM resolution using various radioisotopes for both detector prototypes. 59 Table 3.3 Truth table for MCA logic gate. 61 List of Equations Equation 1 Maximum energy of a photon Compton scattered through π radians. 27 Equation 2 Definition of the suppression factor. 27 Equation 3 Energy of the detector photon scattered from the inner to the outer detector 29 Equation 4 Number of photons arriving at the inner photodiode with no light sharing. 29 Equation 5 Number of photons arriving at the outer photodiode with no light sharing. 30 Equation 6 Number of photons arriving at the inner photodiode with light sharing. 30 Equation 7 Number of photons arriving at the outer photodiode with light sharing. 30 7

11 Acknowledgements I would like to thank Prof Anatoly Rozenfeld, Dr Mike Lerch, Terry Braddock and Lakshal Perera for all their help, guidance and the lending of computer equipment. Without their invaluable assistance, this project would never have been completed. I would also like to thank Dr Mark Reinhard and all the folk at ANSTO for their help with the IV measurements. In particular, I would like to acknowledge that the design of the anti-compton probe with dual annular detectors is the intellectual property of Prof. Anatoly Rozenfeld and belongs to the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong. 8

12 Abstract This thesis focuses on the performance characterisation of two prototype scintillation dual-detector radiation monitors that feature active gamma-ray collimation and the suppression of events caused by Compton scattering. This system is currently under development at the Centre for Medical Radiation Physics (CMRP) for radio-guided surgery and lymphoscintigraphy procedures involving low and high-energy gamma ray emitting isotopes. The two CMRP custom-designed configurations of dual-scintillator CsI(Tl) crystals were optically coupled to custom designed Si photodetectors. One configuration, nicknamed CD1, consists of an N - -type Si base with P + -type circular inner and square outer regions, producing a p-n junction, while the other, CD2, consists of an N - -type base with smaller P + -type circular inner and outer regions. Both CD1- and CD2-type configurations were investigated for their current-voltage characteristics, noise, crosstalk levels and system energy resolution for various radioisotopes. Both types of device were then integrated into a Compton-event suppression system. The devices were found to require a reverse bias voltage of not more than -60V for the CD1 type device and 30V for the CD2 type. Noise prevents the resolving of low energy peaks below around 150 kev for the CD1 type and below 100 kev for the CD2 type and crosstalk levels were found to be <2% on both types of device. Energy resolution of the CD1-type inner detector was found to be approximately 13% for Cs-137 s 662 kev peak and 8% for the CD2-type inner detector, while the resolution on the outer detector was found to be 21% for Cs-137 s 662 kev peak for the CD1 type and 19% for the CD2 type. Both devices were successfully integrated into a Compton event suppression system and achieved significant levels of suppression of Compton events that would otherwise contribute to the Compton continuum. These results indicate that the dual-detector system will be satisfactory for radio-guided surgery and lymphoscintigraphy using both low and highenergy radioisotopes. 9