CHARACTERIZATION OF A SODIUM IODIDE DETECTOR USING MCNP

Transcription

1 CHARACTERIZATION OF A SODIUM IODIDE DETECTOR USING MCNP Muhammad Abdulrahman Mushref A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in [ Nuclear Engineering ] FACULTY OF ENGINEERING KING ABDULAZIZ UNIVERSITY JEDDAH - SAUDI ARABIA MOHARAM 1427 (H) FEBRUARY 2006 (G)

2 توصيف لكاشف يود الصوديوم باستخدام برنامج MCNP محمد عبد الرحمن عبد الغني مشرف المستخلص تستخدم الكواشف الا شعاعية للتعرف على نوع النوى المشعة وتحديد تركيزها. وتعتبر ا شعة جاما ا حد ا هم ا نواع الا شعاعات النووية التي تنبعث من نواة بعض الذرات غير المستقرة. ويتم تحليل هذا النوع من الا شعة باستخدام منحنى الطاقة الذي يوضح عدد الفوتونات المسجلة من الكاشف عند طاقات مختلفة. نقوم هذا البحث في بتحليل وحساب التفاعلات الفوتونية في كاشف يود الصوديوم مع مصدر جاما مشع على شكل دورق مارنيلي باستخدام برنامج مونت كارلو على الحاسب الا لي. هذا البرنامج يقوم بعمل محاكاة مماثلة لحركة الفوتونات وتفاعلا ا الممكنة في التجربة المعملية ومطابقة لما يتم الحصول عليه من النتاي ج مع القياسات المعملية لعدة مصادر عيارية مشعة باستخدام كاشف يود الصوديوم. تعتمد كفاءة الكاشف الا شعاعي على عوامل متعددة مثل طاقة ا شعة جاما وحجم بللورات يود الصوديوم في الكاشف وسماكة الطبقة الخارجية ومستوى الطاقة التي تعمل بها الا نظمة الا لكترونية المساعدة والمواد المحيطة والمسافة التي تفصل الكاشف عن المصدر المشع والشكل الخارجي للدورق المحتوي على المصدر المشع. و ق د تم في هذا البحث حساب الكفاءة المطلقة لوعاء على شكل دورق مارنيلي وتمت بعد ذلك مقارنة النتاي ج مع الكفاءة المطلقة المحسوبة على برنامج مونت كارلو. وقد وجدنا ا ن النتاي ج المحسوبة نظريا باستخدام برنامج مونت كارلو متوافقة مع النتاي ج التجريبية التي تم قياسها معمليا. ج

3 CHARACTERIZATION OF A SODIUM IODIDE DETECTOR USING MCNP Muhammad Abdulrahman Mushref ABSTRACT Gamma ray spectrometers are used to identify radio nuclides and determine their concentrations. Gamma ray analysis is performed by obtaining an energy spectrum (i.e., the number of photons per energy interval observed by the detector). This thesis analyzes and quantifies the photoelectric interactions in sodium iodide (NaI) detector under various geometric conditions using Monte Carlo simulation and confirms the results by measuring several radioactivity standards. MCNP5 is used to simulate the entire possible interactions in real situations. Detector response depends on many parameters such as gamma energy, detector volume, energy collection, associated electronics, surrounding material, source distance, and source shape. In order to accurately determine the absolute efficiency of the detector, several parameters of the detector were estimated. The absolute efficiency of the detector was found for the Marenilli beaker geometry and was compared to that of a known concentration standard sources and previously documented results. Results were found in agreement with experiments and literature. vii

4 TABLE OF CONTENTS ACKNOWLEDGMENT ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS Page vi vii viii x xi xiv Chapter I INTRODUCTION 1 Chapter II THE SODIUM IODIDE DETECTOR 2.1 GAMMA RAY INTERACTIONS 2.2 SCINTILLATION MATERIALS 2.3 SODIUM IODIDE Chapter III THE EXPERIMENT SOURCE PREPARATION EFFICIENCY MEASUREMENTS EXPERIMENTAL RESULTS 35 Chapter IV MCNP SIMULATION PROCEDURE ERROR ESTIMATION SIMULATION RESULTS PULSE HEIGHT SPECTRA GAUSSIAN ENERGY BROADENING 50 viii

5 4.6 CURVE FITTING OPTIMIZATION 56 Chapter V DISCUSSION OF RESULTS 5.1 COMPARISON OF DATA 5.2 ERROR PROPAGATION 5.3 UNCERTAINTY OF RESULTS 5.4 SPECTRA RESULTS 5.5 FITTING RESULTS 5.6 GAUSSIAN SHAPING 5.7 OPTIMIZATION RESULTS Chapter VI RECOMMENDATIONS AND CONCLUSIONS THE RESEARCH METHODS PROJECT EVALUATION FUTURE WORK CONCLUSIONS 91 REFERENCES 93 APPENDIX A MCNP5 INPUT FILE # 1 99 APPENDIX B MCNP5 OUTPUT FILE # APPENDIX C MCNP5 INPUT FILE # APPENDIX D MCNP5 OUTPUT FILE # APPENDIX E MATLAB Program muhammad.m 122 APPENDIX F MATLAB Function mushref.m 123 APPENDIX G MCNP5 INPUT FILE # APPENDIX H MCNP5 OUTPUT FILE # APPENDIX I MCNP5 INPUT FILE # APPENDIX J MCNP5 OUTPUT FILE # APPENDIX K MCNP5 INPUT FILE # APPENDIX L MCNP5 OUTPUT FILE # APPENDIX M MCNP5 INPUT FILE # APPENDIX N MCNP5 OUTPUT FILE # ix

6 CHAPTER I INTRODUCTION Radiation detection and measurement is a fundamental subject in nuclear physics and engineering. Various instruments were developed in the last few decades to detect and measure particles with greatest accuracy. Sodium Iodide (NaI) detectors are devices used in gamma ray detection and activity measurements [1]. Several researchers employ the Monte Carlo method for estimating the radioactivity concentration. In 1948, Hofstadter first demonstrated that crystalline sodium iodide 1 produced an exceptionally large scintillation light output compared with the organic materials [2]. The most notable properties of NaI(Tl) are; the excellent yield of light generated due to gamma ray absorptions and the small nonproportionality of the scintillation response with deposited electron energy. However, NaI(Tl) is hygroscopic and will deteriorate due to water absorption if exposed to the atmosphere. For this reason, it is canned in an air tight container for ordinary use. Several studies were completed in several applications until 1988 when Drndarevic, et al. considered a high counting rate signal processor for prompt gamma neutron activation using NaI(T1) detectors. The application was a low rate producing pile up at the energy of interest. Several methods for improving high counting rate performance 2 have been investigated and implemented. The processor was experimentally evaluated at 4x10 5 counts per second without significant distortion of the spectrum and with a minimum degradation of resolution. Through the use of an effective pulse width 1 With a trace of thallium iodide doping, NaI(Tl). 2 Such as shortening of the detector signal, base line inspection and pile-up rejection and filtering. 1

7 2 sensitive pile up rejector, the suppressed pile up effect has been reduced by a factor of three while rejecting less than 10% of valid counts. High data throughput made the processor applicable to online process control Performance has been tested at total counting rates of 4x10 5 counts per second [3]. In 1988, Dyer, et al. studied the radioactivity induced in detectors by protons and secondary neutrons that limit the sensitivity of spaceborne gamma ray spectrometers. Three dimensional Monte Carlo transport codes have been employed to simulate particle transport of cosmic rays and inner belt protons in various representations of the Gamma Ray Observatory Spacecraft and the Oriented Scintillation Spectrometer Experiment. Results were used to accurately quantify the contributions to the radioactive background, assess shielding options and examine the effect of detector and spacecraft orientation in anisotropic trapped proton fluxes [4]. Schotanus, et al. presented more details about recent developments in scintillator research in A description of the scintillation mechanism and the influence of doping on the scintillation characteristics were also obtained [5]. Miyajima, et al. in 1992 estimated the number of scintillation photons emitted in NaI(Tl) and plastic scintillators by gamma rays. The number of photoelectrons from the photomultiplier photocathode was absolutely measured with several combinations of photomultipliers and scintillators. The photomultipliers were used as a vacuum photodiode. The number of scintillation photons was found by using the effective quantum efficiency and the collection efficiency of photons by photocathode [6]. Large two dimensional position sensitive NaI(Tl) crystals used in positron emission tomography normally have gaps or inactive, unusable areas at the edges. Experiments aimed at reducing these edge effects have been performed in 1993 by Freifelder, et al.. Unencapsulated crystals have been used to test the feasibility of optically coupling crystals together to decrease gap size. Other experiments increased the sampling of the scintillation light at the edges in order to obtain better position sensitivity. Work was

8 3 also performed to treat the edges to reduce unwanted reflections and increase the position sensitive area. Finally, experiments were performed for improving the position resolution throughout the crystal as well as at the edges [7]. In 1993, Shepherd, et al. discussed the preparation and performance of thin film NaI(T1) scintillators using two different techniques: 1. Standard BLE 3 in which a single resistance heated boat contains the total source quantity. 2. Standard PFE 4 in which the film s characteristics can be easily reproduced and the relative light yields can be predicted by controlling the substrate temperature, the boat temperature and the composition of the source. The PFE films were made with homogeneous thallium concentrations which surpass the highest light yield measured for two commercially available single crystal NaI(T1) scintillators and for BLE films with optimized light yield [8]. Also in 1993, the average energy expended per scintillation photon was determined to be 17.2 ± 0.40 ev for a NaI(Tl) phosphor. This was obtained from the numbers of photoelectrons measured with several combinations of a photomultiplier tube and a NaI(T1) scintillator. The number of photoelectrons 5 was converted to the number of scintillation photons by using averaged quantum efficiency of each photomultiplier photocathode and a calculated collection efficiency of the scintillation photons at the photocathode. However, the above values do not include the uncertainties due to the not known exact emission spectra and the photomultiplier response curves. This work was completed by Miyajima, et al. [9]. Lju, et al. investigated the position sensitive detectors in In 1 D position sensitive 3 Bulk Load Evaporation. 4 Powder Flash Evaporation. 5 This was measured by the photomultiplier tube as a vacuum photodiode.

9 4 bar detectors, multiple internal reflections and edge effects of scintillation light cause excessive light spread and lead to poor intrinsic spatial resolution along the length of the detector. Surface treatment techniques have been used to limit light spread in 2 D position sensitive scintillation detectors and improve resolution performance. Since it is costly and time consuming to rely on custom manufacturers to build a NaI(T1) detector for each design variation, a practical model to predict and compare the potential effectiveness of each technique would be useful to guide detector design. An empirical model was developed such to evaluate surface treatment techniques in containing light spread. In this model, a LED 6 is used to simulate light propagation and refraction in a NaI(T1) bar detector. Light sensors were placed above the exit window with an air gap for spatial recording of light refracted out of the bar. Among the eleven different combinations of surface treatments evaluated, three side transverse sanding turned out to be the compromise between light spread and total light output. These results were later verified by experiments performed on NaI(T1) bar detectors. This model could be a valuable tool to guide the design of position sensitive NaI(T1) bar detectors [10]. In 1993, Ramana et al. carried out experiments for studying the resolution of 10 cm x 10 cm x 40 cm NaI(T1) detectors used in high sensitivity airborne gamma ray spectrometric surveys. Change of resolution with source detector geometry emphasized the inadequacy of the existing specifications for resolution in explaining the detector response. A new additional specification for resolution measurement namely overall resolution was proposed. Measurement and applications of overall resolution of the detector were also discussed [11]. Dorenbos, et al., in 1995, presented a review and new data on the absolute photon yield emitted by classical NaI(Tl + ) scintillation crystals after absorption of X and gamma rays of energies ranging from 5 kev to 1 MeV. Factors influencing the energy resolution with which high energy photons can be detected with scintillator photomultiplier combinations were reviewed. Attention was focused on the effects of nonproportionality in the scintillation response on the energy resolution [12]. 6 Light Emitting Diode.

10 5 In 1996, Rooney, et al. designed and implemented a Compton coincidence experiment to study the light yield nonproportionality of inorganic scintillation materials. The coincidence technique was used to measure the nearly monoenergetic scintillator electron response by recording events only when energetic electrons are produced by gamma rays that are Compton scattered through a specific angle. This technique provides the ability to accurately determine the light yield nonproportionality of scintillation materials as a function of electron energy while minimizing the potential complicating effects of surface interactions and X ray escape. To benchmark the CCT 7 the electron response for NaI(Tl) has been measured for electron energies from 2 to 450 kev and compared to previously published analytical and measured light yield electron responses. These CCT results were believed to be the most accurate NaI(T1) electron response measurements to date for energies below 20 kev where the light yield nonproportionality was most pronounced [13]. In 1997, Leutz, et al. reviewed several articles on the scintillation response of NaI(Tl). Those publications 8 match within a ±0.9% standard deviation of a linear scintillation response between 3MeV and 3keV. Several authors who placed their sources outside the crystals measured non linear responses of NaI(Tl) with up to ±7% of standard deviations between 700 and 4.5 kev. The presented analysis suggested that the observed non linearity is not an intrinsic effect of NaI(Tl) but could be caused by effects occurring at the radiation entrance surfaces of the crystals [14]. In 1997, Rooney, et al. developed a technique 9 to calculate photon response. A discrete convolution of measured electron response and the electron energy distribution for a particular scintillator yields the photon response. By establishing the ability to accurately calculate photon response, the experimental implications of scintillator light yield nonproportionality and geometry effects was studied without the requirement for experimental measurements. Also, this technique provided a detailed characterization of photon response than experimental techniques that rely on the use of multiple gamma and X ray sources. To demonstrate this technique, NaI(Tl) photon responses have been 7 Compton Coincidence Technique. 8 Which report results obtained with radioactive sources either dispersed through out the NaI(Tl) lattice or located inside the wells of NaI(Tl) crystals. 9 To study the effects of scintillator light yield nonproportionality.

11 6 calculated. This technique was validated by comparing calculated results to both measured photon responses and previously published photon responses for this scintillator [15]. Cho, et al. developed an electronic dose conversion technique in 1997 to assess the exposure dose rate due to environmental radiation from terrestrial sources. For a cylindrical NaI(TI) scintillation detector, pulse height spectra were obtained for gamma rays of energy up to 3 MeV by Monte Carlo simulation. Based on the simulation results and the experimentally fitted energy resolution, dose conversion factors were calculated by a numerical decomposition method. These calculated dose conversion factors were electronically implemented to a developed DCU 10 which is a microprocessor controlled SCA with variable discrimination levels. The simulated spectra were confirmed by measurement of several monoenergetic gamma spectra with a MCA 11. The converted exposure dose rates from the implemented dose conversion algorithm of DCU for the field test in the vicinity of the nuclear power plant at Kori as well as for standard sample sources were also evaluated and the results were in good agreement with separate measurements by a HPIC 12 within 6.4% [16]. The scintillator energy resolution component 13 has been characterized for NaI(Tl) by Valentine, et al. in Results were based on a discrete convolution of measured electron response data and the electron energy distribution resulting from full energy absorption events. The behavior of this energy resolution component as a function of energy was observed to be strongly dependent on the shape of the electron response. In some energy regions, the light yield nonproportionality component was observed to be larger than the resolution predicted by assuming Poisson photoelectron statistics. Characterization of this energy resolution component can facilitate deconvolution of other components from the total energy resolution [17]. NaI(T1) position sensitive detectors have been used in medical imaging for many years. 10 Dose Conversion Unit. 11 SCA = Single Channel Analyzer, MCA = Multi Channel Analyzer. 12 High Pressure Ionization Chamber. 13 This is due to light yield nonproportionality.

12 7 For PET applications without collimators, the high counting rates place severe demands on such large area detectors. In 1998, Freifelder, et al. read out the NaI(T1) detectors in the scanners via photomultiplier tubes and preamplifiers. Those preamplifiers use a delay line clipping technique to shorten the characteristic 240ns 14 fall time of the NaI(T1) signal. As an alternative, a pole zero networks were investigated to shorten the signal followed by a multi pole shaper to produce a symmetric signal suitable for high counting rates. This has been compared to the current design by measuring the energy and spatial resolution of a single detector as a function of different preamplifier designs. Data were taken over a range of integration times and count rates. The new design showed improved energy resolution with short integration times. Effects on spatial resolution and dead time were reported for large position sensitive detectors at different count rates [18]. A detailed model of the response of large area NaI(T1) detectors and their triggering and data acquisition electronics has been developed in 1998 by Wear, et al.. This allowed examining the limitations of the imaging system performance due to degradation in the detector performance from light pile up and dead time from triggering and event processing. Comparisons of simulation results to measurements from the scanner have been performed to validate the Monte Carlo model. The model was then used to predict improvements in the high count rate performance of the scanner using different signal integration times, light response functions, and detectors [19]. Two of the significant conclusions that have been made from former studies 15 are the NaI(T1) scintillation response is nearly linear 16 and the NaI(T1) scintillation response has significant nonproportionalities. While these conclusions may at first appear to be inconsistent with each other, upon further consideration these conclusions are not mutually exclusive. In fact, these conclusions are fully consistent with each other. Published results typically confirm only one of these conclusions 17 and are not always presented such that direct comparisons can be made with other results. Consequently, it 14 ns = nano second. 15 With respect to using NaI(T1) to detect gamma and X rays. 16 Especially over limited energy ranges. 17 As it was likely the intention to study either linearity or proportionality but not both.

13 8 is critical that both the intent of the study and the means with which the results are presented be appropriately considered. Otherwise, it is possible to make inaccurate conclusions about both linearity and proportionality. To facilitate accurate conclusions and meaningful comparisons, Valentine, et al. in 1998 defined a fully consistent nomenclature, examined some of the subtleties which must be considered when comparing scintillation response results and used a review of the literature to demonstrate both the definitions and the subtleties [20]. Measurements of the light output response of NaI(T1) to X rays were taken by Waynea, et al. in These measurements cover the energy range kev. Radionuclides and fluorescent sources were used to provide a coarse sampling of this energy range and a tunable X ray monochromator was used to provide a fine scale mapping around the iodine K edge at kev where the light output response of NaI(T1) varies rapidly with energy. The light output response to X rays was measured to an accuracy of +0.7%. In addition to presenting the results of these X ray calibrations, the data were used to infer the response of NaI(T1) to electrons in the energy range kev [21]. Single crystalline plates of 430 mm x 430 mm x 17 mm were prepared by directional solidification of NaI in a graphite mold. This was prepared by Berthold, et al. in The setup was heated in a standard chamber furnace under nitrogen gas at atmospheric pressure. The entire mold bottom was cooled with air. At high freezing rates the plates were polycrystalline and interspersed with bubbles. Low rates yielded single crystalline plates without bubbles. Only three days were needed for a complete run of melting, crystallization and cooling down. The dopant was homogeneously distributed in the plate except for the last to freeze zones [22]. In 2000, Ghanem calculated the response features for sodium iodide detector for a monoenergetic gamma ray source because of the importance of this type of detector in industry and research. The response features calculated were full energy peak, single scattered escape, single escape peak, double scattered escape, double escape peak, besides single scatter, double scatter and triple scatter continua. These features were compared for different sizes and geometric efficiencies of the detector. The Monte Carlo simulation provided the weighting factors of the previously mentioned eight