Digital Gamma-ray Spectroscopy & Imaging with Semiconductor detectors Frontiers of gamma-ray spectroscopy

Transcription

1 Digital Gamma-ray Spectroscopy & Imaging with Semiconductor detectors Frontiers of gamma-ray spectroscopy AGATA GRETA Dr Andy Boston And its applications

2 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 => Greatly improved angular resolution (~1 0 ) to reduce Doppler effects

3 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 =)

4 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 in photopeak 16 reconstructed in photopeak bad

5 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 π of complete d array

6 AGATA 1 st symmetric cluster

7 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

8 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.

9 Charge pulse response

10 Cs-137 Singles Fine scan MBq Cs-137 source sec per position 1mm steps 1mm diameter collimator Max count rate = 720 cps Background = cps 0keV CFD threshold Accepted triggers = 120 cps 1GB pre-sorted data

11 Core 662keV Ring gated

12 Core 662keV & Fold 1, Core T

13 Core 662keV & Fold 1, Core T

14 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

15 Coincidence scan: set up

16 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

17 Cartesian Grid Different colors show active regions for the different segments

18 New Quasi-cylindrical Grid Different colors show active regions for the different segments Spacing is equidistant in sensitivity

19 New Quasi-cylindrical Grid Different colors show active regions for the different segments Spacing is equidistant in sensitivity

20 Coincidence scan: plan Azimuth Azimuth 1 Azimuth Azimuth 7 A B F C E D Azimuth 3 Azimuth 5 Azimuth 6

21 Segment pulses 6.0 mm 15.5 mm 28.5 mm 45.5 mm Centre contact pulses

22 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

23 The SmartPET Project Andy Boston

24 SmartPET December 2005

25 SmartPET detector depth response superpulse pulse shapes for 137 Cs events versus depth DC signals AC signals DC signals AC signals

26 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 AC Magnitude (kev) Time (ns) Time (ns) e h Time (ns) Time (ns) Time (ns)

27 Image Charge Analysis Interaction near AC06 Interaction near AC04 One approach to applying simple PSA is to split this distribution equally into five regions

28 Image Charge Analysis Asymmetry distribution as a function of scan (collimator) position (AC05) Near AC06 Near AC04

29 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

30 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

31 Analytical method 0 degrees 5 cm separation degrees 5 cm separation J Gillam, Monash

32 Iterative Method 0 degrees 3 cm separation degrees 3 cm separation J Gillam, Monash

33 Compton Camera measurements (Ge/Ge) E = 18 kev 6 cm source to crystal J Gillam, Monash 3 cm crystal to crystal

34 Digital Gamma-ray Spectroscopy & Imaging with Semiconductor detectors Frontiers of gamma-ray spectroscopy AGATA GRETA Dr Andy Boston And its applications