Digital Gamma-ray Spectroscopy & Imaging with Semiconductor detectors Frontiers of gamma-ray spectroscopy AGATA GRETA Dr Andy Boston ajboston@liv.ac.uk And its applications
The AGATA Concept Without Compton suppression shields Compton continuum. => Large peak to total ratio With BGO shielding Less solid angle coverage => Big drop in efficiency With highly segmented detectors E ' γ Eγ = 2 1+ ( E / mc )(1 cosθ ) γ Path of γ-ray reconstructed to form full energy event => Compton continuum reduced => Excellent efficiency ~% @1MeV => Greatly improved angular resolution (~1 0 ) to reduce Doppler effects
AGATA : Two candidate configurations Ge crystals size: length mm diameter mm 1 hexagonal crystals 3 shapes triple-clusters all equal Inner radius (Ge) 22 cm Amount of germanium 310 kg Solid angle coverage % Singles rate ~ khz 64 segments Efficiency: % (M γ =1) 25% (M γ =) Peak/Total: 65% (M γ =1) % (M γ =) 120 hexagonal crystals 2 shapes triple-clusters 2 shapes Inner radius (Ge) 17 cm Amount of germanium 220 kg Solid angle coverage 74 % Singles rate ~ khz 4320 segments Efficiency: 38% (M γ =1) 21% (M γ =) Peak/Total: 63% (M γ =1) 47% (M γ =)
The Standard Germanium Shell Idealized configuration to determine maximum attainable performance R i = 15 cm R o = 24 cm 2 kg of Ge A high multiplicity event E γ = 1.33 MeV M γ = M γ = 1 ε ph = 65% P/T = 85% M γ = ε ph = 36% P/T = % Assuming 5 mm Position Resolution good 27 gammas detected -- 23 in photopeak 16 reconstructed -- 14 in photopeak bad
AGATA Design 3 different asymmetric hexagonal shapes are used Completed array (64 segments) with support structure Triple cluster modular units in a single cryostat The AGATA demonstrator: 5 triple clusters, 5 segments. Scheduled for completion 2008 2π of complete d array
AGATA 1 st symmetric cluster
Automated Scanning Tables Liverpool System Parker linear positioning table Pacific scientific stepper motors 920MBq 137 Cs 1 collimator Singles/coincidence system Precise position calibration How well your basis fits your real data
Liverpool Detector Scanning System Data recorded using 14 bit Mhz digital electronics to disk. Pulse shape analysis of detector response as a function of position.
Charge pulse response
Cs-137 Singles Fine scan 60 920MBq Cs-137 source sec per position 1mm steps 1mm diameter collimator 00 50 00 00 00 20 2000 10 0 0 Max count rate = 720 cps Background = cps 0keV CFD threshold Accepted triggers = 120 cps 1GB pre-sorted data
Core 662keV Ring gated 0 0 0 0 0 200 0 5 0 4 0 3 0 2 200 1 0 0 0 0 0 0 200 0 0 0 0 0 0 0 0 0 200 0 0 0 0 0 0 0 0 0 200 0 0 0 0 0 0 0 0 200
Core 662keV & Fold 1, Core T 20 10 20 10 75 65 55 45 35 25 20 15 10 20 10 20 10 20 10
Core 662keV & Fold 1, Core T 200 1 1 1 120 20 200 1 1 1 120 20 2 220 200 1 1 1 120 20 0 2 2 220 200 1 1 1 120 20 0 2 220 200 1 1 1 120 20 0 2 220 200 1 1 1 120 20 0
Coincidence scan: set up Physical segmentation depth: Collimation gap centred on (w.r.t. crystal base): mm 72mm 54mm 36mm 21mm 8mm 0mm 83.7mm Div = 2.1mm 65mm 48mm 29mm 13.5mm 2mm Div = 1.5mm
Coincidence scan: set up
Which grid to choose? advantages drawbacks r,θ cst. values of t 10- cylindrical not homogenous r,θ cst. values of t 10- homogen., cylindr. not the same x/y accuracy x,y,z homogenous simple not cylindrical large distances to grid hexagon, z hexagonal compact Adaptive grid cylindrical compact cylindrical maximum compacity optimum conditioning of the problem not compact in z not homogenous less standard not homogenous
Cartesian Grid Different colors show active regions for the different segments
New Quasi-cylindrical Grid Different colors show active regions for the different segments Spacing is equidistant in sensitivity
New Quasi-cylindrical Grid Different colors show active regions for the different segments Spacing is equidistant in sensitivity
Coincidence scan: plan Azimuth 4 4 3 2 1 6 7 Azimuth 1 Azimuth 2 5 13 Azimuth 7 A B 12 11 10 F C E D 14 8 16 15 9 17 Azimuth 3 Azimuth 5 Azimuth 6
Segment pulses 6.0 mm 15.5 mm 28.5 mm 45.5 mm Centre contact pulses
Electric Field Simulations : MGS I Geometry II Potential Elec field III Drift velocities IV Weighting fields Electric field simulations have been performed and details comparisons have been made with experimental pulse shape data. AGATA symmetric crystal simulation
The SmartPET Project Andy Boston ajboston@liv.ac.uk
SmartPET December 2005
SmartPET detector depth response superpulse pulse shapes for 137 Cs events versus depth DC signals AC signals DC signals AC signals
Image Charge Response Image charge asymmetry varies as a function of lateral interaction position - Calibration of asymmetry response Asymmetry = Area Area left left + Area Area right right 0 AC03 0 AC04 0 AC05 0 AC06 0 AC07 0 0 0 0 0 0 0 0 0 0 Magnitude (kev) 0 0 200 0 0 0 2000 00 Time (ns) 0 0 200 0 0 0 2000 00 Time (ns) e h 0 0 200 0 0 0 2000 00 Time (ns) 0 0 200 0 0 0 2000 00 Time (ns) 0 0 200 0 0 0 2000 00 Time (ns)
Image Charge Analysis Interaction near AC06 Interaction near AC04 One approach to applying simple PSA is to split this distribution equally into five regions
Image Charge Analysis Asymmetry distribution as a function of scan (collimator) position (AC05) Near AC06 Near AC04
Reconstructed Images Simple PSA techniques applied event-by-event Filtered Back Projection 22 Na source No PSA PSA FWHM = 9.5mm FWHM = 1.2mm Andy Mather mm Andy Mather mm
Compton Camera 10μCi 152 Eu mm from SPET1 Source rotated Zero degrees in 15º steps up to º Detector separation 3 11cm in 2cm steps Gates set on energies 779, 18keV 2 22 Na sources at different x and y
Analytical method 0 degrees 5 cm separation degrees 5 cm separation J Gillam, Monash
Iterative Method 0 degrees 3 cm separation degrees 3 cm separation J Gillam, Monash
Compton Camera measurements (Ge/Ge) E = 18 kev 6 cm source to crystal J Gillam, Monash 3 cm crystal to crystal
Digital Gamma-ray Spectroscopy & Imaging with Semiconductor detectors Frontiers of gamma-ray spectroscopy AGATA GRETA Dr Andy Boston ajboston@liv.ac.uk And its applications