Double sided strip planar Germanium detectors used for
Double sided strip planar Germanium detectors used for gamma-ray imaging Paul Nolan (p. j. nolan@liv. ac. uk) University of Liverpool
Outline AGATA - the development of pulse shape analysis and detector scanning Characteristics of Double Sided Strip Planar Germanium Detectors Detector Properties Energy resolution Position and Timing response Interaction position determination Uses of the detectors in Medical Imaging and Homeland Security (John Simpson) PET SPECT Compton Camera
AGATA: g-ray tracking Ge Tracking Array eph Ndet ~ 50% ~ 100 ~80% q ~ 1º Combination of: • segmented detectors • digital electronics • pulse processing • tracking the -rays
Scanning and pulse shape analysis • Scanning system developed for AGATA detectors • Collimated gamma –ray beam (1 mm diameter) scanned across detector • Pulse shapes recorded at many points • Pulse shapes then related to position • Comparison with electric field simulations Andy Boston - Thursday Use same methods for planar detectors scan in X – Y - Z
Planar Double sided strip Ge detectors Two Ge detectors active area thickness guard ring thin contacts strips on each face strip width strip gaps 60 x 60 mm 20 mm 7 mm all round 0. 3µm, 50µm 12 5 mm 180µm, 300µm Detectors originally designed for a PET system Smart. PET
Smart. PET Detectors
Detector geometry • The crystal is grown along the <100> axis, the detector is cut so that <100> axis aligns with depth.
The Smart. PET DSGSD detectors Detector Specification • Depletion at -1300 V, Operation at -1800 V • 12 x 12 Segmentation, 5 mm strip pitch • 1 mm thick Aluminium entrance window • Warm FET configuration, 300 m. V/Me. V pre-amps • Average energy resolution ~ 1. 5 ke. V FWHM @ 122 ke. V
Energy Response / Contact thickness 17. 8 ØAC 0. 3 microns Strip gap 300 microns ØDC 50 microns Strip gap 180 microns 26. 3 20. 8 DC 06 60 17. 8 20. 8 26. 3 60 Energy resolution @ 60 ke. V ØAC ØFWHM = 1. 45 ke. V ØDC ØFWHM = 1. 1 ke. V
Energy resolution results Electron trapping? Teresa Underwood DC face AC face
Position dependence / scanning • 1 mm Tungsten collimation (9 cm) • 1 mm step positions • Sources • 1 GBq 241 Am annular source – 40 cps • 1. 8 GBq 57 Co source (pellet) – 150 cps • 70. 21 MBq 137 Cs – 35 cps Ge crystal Collimator • Scans Source • 241 Am 120 seconds at each position (AC and DC) • 57 Co 60 seconds (AC and x-y position table DC) 120 seconds (side) • 137 Cs 180 seconds
Am-241 source AC face AC DC AC 01 AC 12 DC 1 AC face – Energy 60 ke. V Fold (1, 1) The results are for 2 minutes of data per position
Charge Sharing • What is the charge sharing mechanism? • A B g e- h+ Single site interaction • Field lines split between strips • Charge cloud shared between strips • Real charge observed on both A and B • Incorrect assignment of fold
The influence of charge sharing Source AC side Difference 10. 5 M – 9. 4 M = 1. 1 M (~10%) AC 01 DC 12 AC 12
241 Am Source DC face AC DC AC 01 AC 12 DC 01 DC 12 DC face – Energy 60 ke. V Fold (1, 1)
The influence of charge sharing Source DC side Difference 9. 6 M – 9. 2 M = 0. 4 M (~4. 5%)
Image Charge Response Image charge asymmetry varies as a function of lateral interaction position - Calibration of asymmetry response h e
Co-57 side surface intensity distribution The results for 122 ke. V with 1 minute of data per position.
Co-57 side surface T 30 rise time distribution T 30 AC T 30 DC
Smart. PET detector depth response “superpulse” pulse shapes for versus depth DC signals 137 Cs events AC signals
Electric Field Simulations : MGS I Geometry II Potential Electric field III Drift velocities Smart. PET detector response IV Weighting fields • Electric field simulations have been performed and comparisons have been made with experimental pulse shape data.
MGS detector simulation • MGS pulse shapes vs depth
Position resolution achieved: X-Y 1 mm x 1 mm segmentation, image charges Z 1 -2 mm rise times, pulse shape analysis
Point source imaging - PET FBP reconstruction 5 mm No PSA FWHM = 9. 5 mm Andy Mather FWHM = 1. 2 mm Andy Mather 60 mm 1 mm 60 mm
Conclusions • Detectors show excellent response • Positions determined to 1 x 1 -2 mm • Charge sharing ~5 -10% (AC+DC) • Cross talk ~0. 6% x Gamma energy • Two 20 mm thick detectors have been used for imaging Over to John Simpson Note references at the end of John’s talk
- Slides: 25