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Zachariadou Katerina et al

2 nd SENS-ERA WORKSHOP on “Advanced sensor systems & networks” TEI Piraeus, 6-7 December 2012. Simulation studies on the localization of radioactive sources using a portable detector based on pixelated CdTe crystals. Zachariadou Katerina et al

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Zachariadou Katerina et al

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  1. 2nd SENS-ERA WORKSHOP on “Advanced sensor systems & networks” TEI Piraeus, 6-7 December 2012 Simulation studies on the localization of radioactive sources using a portable detector based on pixelated CdTe crystals Zachariadou Katerina et al TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology

  2. gamma-ray imaging technologies Radiation detectors that use gamma-ray imaging technologies in order to identify radioactive sources are of great scientific interest because of their wide range of applications: Compton imaging based on the interactions of the emitted gamma-rays with the detector’s sensitive elements via the Compton scattering process • nuclear medicine • astrophysics • waste monitoring • counter terrorism • the growing global interest for accurate detection of radioactive sources • rapid advances in detector technologies (both in terms of material fabrication and electronics) great impetus to the R&D of Compton imaging detectors with enhanced detection capability. the objective of our research is to evaluate the radioactive source imaging performance of a Compton imaging detector under development (COCAE) , based on pixelated CdTe crystals Introduction

  3. Design- Functionality • The COCAE detector Principle of Compton Imaging Technique Monte Carlo simulation procedure • Reconstructed Image Resolution • Two radioactive source discrimination • Algorithms for the localization of point-like radioactive sources Source orientation estimation Source-to-detector distance estimation Outline

  4. COCAE 1. TEI of Halkis (Greece)  pixel electronics 2. Greek Atomic Energy Commission (Greece) Monte Carlo Simulations 3. Institute of Nuclear Physics, Demokritos (Greece) Monte Carlo Simulations 4. Oy Ajat Ltd (Finland) bump bonding of the pixel electronics 5. Freiburger Materialforschungszentrum(Germany) growth of crystals 6. Universidad Autonoma de Madrid Departemento de Fisica deMateriales(Spain) growth of crystals 7. Riga Technical University(Latvia) Shottky diode structures 8. Lashkaryov Institute of Semiconductor Physics, NationalAcademy of Sciencesof Ukraine(Ukraine)  growth of crystals +fabrication of pixel detectors 9. Chernivtsi Yuri Fedkovych National University(Ukraine) radiation detectorcharacterization Web Site: www.cocae.eu

  5. The COCAE detector COCAE is a portable detector aimed to be used for the accurate localization & identification of radioactive sources, in a broad energy range up to 2 MeV • Security inspections at the borders (airports, seaports etc) • At recycling factories for the detection & determination of the strength of possible radioactive sources into scrap metals

  6. The COCAE Detector 2D array of radiation sensing pixels (100x100) (400μm pitch) bump-bonded in a 2D array of readout CMOS circuits (300μm thick) Pixelated 2mm thick CdTe crystals

  7. Gamma interactions with matter: • Compton scattering: The direct interaction of a gamma with an electron of the sensing material • The energy and the momentum of the scattering are conserved Principle of Compton Reconstruction COCAE exploits the Compton imaging technique to deduce: • the energy • the origin (within a cone) • of the incident gamma photons emitted by a radioactive source

  8. Successive interactions of the emitted gamma rays create overlapping cones and the source location is the intersection of all measured cones Radioactive source source Projection imaging plane θ z y x Principle of Compton Imaging Deduce the energy of the incident gamma photons as well as their origin within a cone In principle, three cones are sufficient to reconstruct the image of a point source. In practice, due to measurement errors & incomplete photon absorption, a large number of reconstructed cones are needed to derive the source location accurately.

  9. The design of the COCAE detector Crucial parameters that determine the design of the detector: Influence the event statistics Detection efficiency Strong absorption of gamma photons Energy resolution affects the evaluation of the compton scattering angle

  10. The design of the COCAE detector • Detection efficiency CdTe semiconductor crystals have higher detection efficiency (compared to Ge and NaI detectors) due to the higher atomic number (48, 52) and density (~6gr/cm3) To achieve even better efficiency, a thick CdTe detector of several mm would be needed an increase of the crystal thickness would deteriorate the detector energy resolution (due to incomplete charge collection of CdTe semiconductors) BUT Bypass: The instrument is designed as a system of ten 2mmstacked sensors, instead of one thick mono-crystal.

  11. The design of the COCAE detector • Energy Resolution Affects the determination of Compton scattering angle • The challenge for the COCAE instrument is to achieve high energy resolution without the need of cryogenics • (CdTe semiconductors can be operated at room temperatures due to their high energy bandwidth ) p-n , Schottky diodes (CdTe, CdZnTe) have been investigated The energy resolution achieved is better than 1% FWHM @662keV

  12. A simulated gamma-ray interacts with the COCAE detector. Three energy depositions are recorded. Monte Carlo Simulation steps • model the exact detector geometry in order to ensure an accurate simulation of the real detectors’ performance : • Accurate geometric and physical description of the detector’s passive materials. • implementation of the correct isotopic composition of all detectors’ materials • Implementation of all the corresponding cross sections of the particle interactions • MEGAlib Geant4 • Geant4: A toolkit for the simulation of the passage of particles through matter, http://geant4.web.cern.ch/geant4/ • model point-like isotropicγ- sources placed at different distances emitting a large number of photons (~2x109) in an energy range [60keV - 2000keV ] output : collection of hits (hit=energy deposition + position of each interaction)

  13. Event reconstruction • group together the individual simulated hits into events • identify their original interaction process • (Compton scattering or photo-effect event) • and the associated information (energy and direction) Monte Carlo Simulation steps-cont Compton sequence reconstruction (identification of the sequence of Compton interactions) The incident gamma photon can interact with the detector’s sensitive materials via multiple Compton scatterings before the scattered photon is ideally fully absorbed in the detector’s volume.

  14. φ1 φ2 Monte Carlo Simulation steps-contCompton Sequence Reconstruction the distance between the layers is very small 10 cm it is impossible to have a timing tag for each hit 2-hit event The correct time ordering of the sequence of the Compton interactions affects the efficiency of estimating the incident photons’ orientation

  15. Principle of Compton Reconstruction-cont if N hits are recorded in the detector, there are N! possible combinations 3-hit event 3 != 6 combinations The algorithms for the Compton scattering sequence reconstruction identify the hit ordering by exploiting the kinematical and geometrical information of the event as well as statistical criteria

  16. Overall efficiency Sequence Reconstruction techniques-contResults ~90% @ E=200keV down to ~70% @E>600keV

  17. Photo-peak count information from each detecting layer Quality of reconstructed image Triangulation technique Localization of Point-like Radioactive Sources Source orientation estimation Source-to-detector distance estimation Reconstructed Image Resolution Resolve two sources tested on a large number of Monte Carlo simulated gamma photons (~2x109) interacting with the detector’s model, emitted from point-like sources in an energy range [200keV, 2MeV] located at various orientation and distances from the detector’s model

  18. Image reconstruction of radioactive sources The image of a source is reconstructed by applying the List Mode Maximum Expectation Maximization imaging (LM-MLEM) algorithm • The image of a point-like source is generated • by projecting each Compton event cone into an imaging plane • by performing successive iterations on the back-projected images in order to find the source distribution with the highest likelihood of having produced the observed data.

  19. Reconstructed Image Resolution z θ φ COCAE LM-MLEM algorithm, 50 iterations: 800keV point-like radioactive source (x,y,z=25,0,50 cm)

  20. Reconstructed Image Resolution z φ θ θ φ COCAE Reconstructed Image resolution: the combined FWHM of the azimuth (φ) & inclination (θ) profiles of the source image

  21. Reconstructed Image Resolution-cont OFF the detector’s symmetry axis ON the detector’s symmetry axis less than ~4x10-3sr ~2.5x10-3 sr (for source-to-detector distances ~50cm) ~0.5x10-3 sr for point-like sources located at distances greater than ~1m

  22. Reconstructed Image Resolution-cont Study the dependence of the detector’s ability to reconstruct images on the number of events used to reconstruct the image of a radioactive source. Azimuth (φ) of the source : 180o minimum number of triggered events required to reconstruct the source’s image : ~ 5x103

  23. Minimum detectable source activity • Given that 5000 events are sufficient to reconstruct the source image and that one photon is emitted per disintegration) • given the evaluated total efficiency of the detector (~5-7x10-5 for point-like sources located @ z=120cm emitting photons @E:[ 400keV, 1250keV]) Minimum detectable source activity vs the data acquisition time, for various gamma energies: For a data acquisition time of 60s the system is able to detect 50 μCi radioactive sources @ z=1,2m

  24. Multiple Source Discrimination

  25. Source-to-detector distance estimation Photo-peak count information from each detecting layer Quality of reconstructed image Triangulation technique Localization of Point-like Radioactive Sources Source direction estimation

  26. Source Direction Estimation z θ φ COCAE azimuth & inclination source’s coordinate is estimated within less than one degree for inclination angles up to 50o

  27. Quality of reconstructed image Triangulation technique Localization of Point-like Radioactive Sources Source-to-detector distance estimation Photo-peak count information from each detecting layer

  28. Source-to-Detector Distance The Photo-peak Count Information Technique g k d z the estimation of the source-to-detector distance (d) is based on the number of the fully absorbed photons (via a photoelectric effect) (Ni) in each detecting layer absorption by the front layers of the detector ratio of the solid angle of the ith detecting layer over the solid angle of the first detecting layer.

  29. Source-to-Detector Distance The Photo-peak Count Information Technique z θ φ COCAE this method can estimate source-to-detector distances within 2σup to ~2m, for incident photon energies up to ~2MeV Higher statistics is necessary in order to reduce the errors for the case of incident photon energies > ~1000keV emitted by sources located at distances > ~1m.

  30. Photo-peak count information from each detecting layer Triangulation technique Localization of Point-like Radioactive Sources Source-to-detector distance estimation Quality of reconstructed image

  31. Source-to-Detector Distance The Reconstructed Image Technique Projection imaging planes Real source position cocae the quality of the image should be better (the FWHM of the x and y-distribution of the image has the lowest value) if the projection imaging plane is placed on the real source-to-detector-distance rather than in other distances • The image of each point source has been reconstructed by the LM-MLEM imaging algorithm (200 iterations) at various projection imaging planes • the combined FWHM of the x- and y- coordinate distributions is evaluated

  32. Source-to-Detector Distance The Reconstructed Image Technique this method can accurately estimate onlythe distance of point sources being in the near field of the COCAE detector (distances up to z=30cm)

  33. Photo-peak count information from each detecting layer Quality of reconstructed image Localization of Point-like Radioactive Sources Source-to-detector distance estimation Triangulation technique

  34. The Triangulation Technique To test the method, two models of the COCAE instrument at a given distance (d) have been used in order to identify the direction of the photons emitted by point-like sources located on the first COCAE’s symmetry axis, at a distance of 50cm from its center. The source position is estimated as the middle of the minimum distance vector of the two 3D skew lines defined by the estimated source directions e1 and e2 and the detector position.

  35. The Triangulation Technique The method can estimate the position of point-like sources within few centimeters

  36. Summary -I Functionality-Design • The COCAE instrument Principle of Compton reconstruction Monte Carlo simulation studies

  37. Summary -II • Reconstructed Image Resolution less than ~4x10-3sr • Minimum detectable source activity as a function of the data acquisition time  60s for 50μCi • Two Source Discrimination

  38. Summary-III evaluated within 1o by source image reconstruction using the LM-MLEM imaging algorithm, for inclination angles up to 50o Direction estimation can estimate distances (within 2σ) up to ~2m for point-like sources of energies up to 2ΜeV Distance estimation Distance estimation Photo-peak count information Distance estimation Quality of reconstructed image can estimate only distances in the near field triangulation Can estimate the position of point-like sources within few cm

  39. 2nd SENS-ERA WORKSHOP on “Advanced sensor systems & networks” TEI Piraeus, 6-7 December 2012 Simulation studies on the localization of radioactive sources using a portable detector based on pixelated CdTe crystals Zachariadou Katerina et al TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology

  40. Spares Zachariadou Katerina et al TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology International Scientific Conference eRA-7 ,TEI of Piraeus, 27-30 September 2012

  41. Typical energy deposition spectrum for 200 keV incident gamma rays energy illustrating the photo-peak and the Compton plateau Monte Carlo Simulation studies -cont • The full energy peak originates from two different types of events: • one cluster events (photoelectric effect) • sequence of Compton scatterings followed by a photoelectric interaction • Reminder: only Compton scattering events that are fully absorbed are useful for the Compton Imaging Principle technique

  42. First step: Apply Compton kinematics to reject (if possible) the one of the two orderings 2-cluster event Compton Sequence Reconstruction techniques for incident photon energies below 200keV all of the dual cluster Compton events in the photo-peak have a unique ordering

  43. For handling the ambiguous ordering events, three algorithms have been evaluated DCS-A: The sequence with the higher Klein-Nishina cross-section is assumed to be the correct one DCS-B: calculates the Klein-Nishina differential cross-section multiplied to the probability for absorption via a photoelectric effect and assumes that the sequence with the higher product probability is the correct one DCS-C: the cluster that has the larger energy deposition is assumed to be the first Compton scattering Sequence Reconstruction techniques- contDual cluster events (DCS) algorithms CDS-B and DCS-C have similar performance, being able to identify the correct Compton sequence with an efficiency of about 95% for incident gamma energies above 800keV

  44. φ0 φ1 z Sequence Reconstruction techniques-contMultiple cluster events (MCS) For Multiple cluster events there are N! possible sequences Compton scatter angles are calculated by the measured energy depositions Ideally the quality factor equals zero for the correct cluster sequence of Compton events, in the case where the photon is fully absorbed. Although measurement errors result in a quality factor greater than zero, the correct sequence is still most likely to correspond to its minimum value Compton scatter angle calculated by positions of the photons before and after the scattering

  45. Overall efficiency Sequence Reconstruction techniques-contResults

  46. Point-like source @ z=80cm Monte Carlo Simulation studies Overall Event Reconstruction efficiency ~80% @ E=200keV down to ~55% @E>600keV

  47. Reconstructed image resolutionON axis

  48. Reconstructed image resolutionOFF axis

  49. Monte Carlo Simulation studies Recorded events in the photo-peak over the total number of generated events Detecting efficiency Sources located @ ~70cm from the first layer of the detector

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