High-Sensitivity Gamma-Ray Imaging With Double Sided Strip Detectors

Transcription

1 High-Sensitivity Gamma-Ray Imaging With Double Sided Strip Detectors Thomas Niedermayr, Morgan Burks, Lucian Mihailescu, Karl Nelson, David Lange, John Valentine, Kai Vetter Lawrence Livermore National Laboratory, Livermore, CA, USA 1

2 Motivation: Counter Nuclear Threats 2

3 Countering Radiological and Nuclear Threats Homeland Security: Counter nuclear threats with new and improved capabilities in the detection, identification, and localization of nuclear materials Nuclear threats: Radiological Dispersive Devices (RDDs, dirty bombs) Improvised Nuclear Devices (INDs) Nuclear weapons components Particularly: Special Nuclear Materials (SNMs): 235 U (Highly Enriched Uranium, HEU) 239 Pu (Weapons Grade Plutonium, WGPu) 3

4 How can Gamma-Ray Imaging Improve Sensitivity? Source Source Reduced background Background Observables: Count rate Energy spectrum Omnidirectional detector can be overwhelmed by background radiation Imaging detector can isolate weak signals from the background Observables: Count rate Energy spectrum Source shape Source location 4

5 4π gamma-ray imaging Laboratory test (indoors): Combine 2x19 segmented DSSD HPGe detector and 4π photo camera: 5

6 4π gamma-ray imaging In-field test: Mount gamma-ray imager and photo camera on fieldportable cart: Sky-ground asymmetry visible; more statistics needed 6

7 Concept of Compton Gamma-Ray Imaging Gamma rays interact several times with detector via Compton interaction (e.g. until it is stopped by the photo-electrical effect) Measuring positions and energies of individual interactions enables to determine pathway of gamma ray in detector (tracking) Energies and positions of first two interactions define cone of incident angles (electron path is not measured) Cones are projected on plane or sphere (one circle per event) for 2D or into cube (one cone per event) for 3D imaging 3 components critical for Compton imaging: Position Resolution (distance between first two interactions) Energy Resolution (energy deposition/ scattering angle) Intrinsic Electron Momentum (scattering angle, gamma-ray energy) r1 r ρ r2 12 r3 E 1 θ r4 E γ The Compton scattering formula gives θ: source E m c cosθ = 1 E ( E E ) γ γ E2 + E3 E4 E = E + γ source 7

8 How to generate the image? Cone -projection on plane, 20 events Cone -projection on plane, 1000 events image2 image2 8

9 Our approaches to realize high-sensitivity Compton imagers Implementation of three-dimensional position sensitive Ge and Si detectors: Full-volume imagers Hybrid imager Two-dimensionally segmented, coaxial HPGe detector Planar DSSD HPGe detector Planar DSSD HPGe + planar DSSD HPSi detector 40-fold segmented coaxial HPGe detector manufactured by ORTEC. 2x19 and 2x39-fold segmented DSSD HPGe detectors manufactured by Ethan Hull (LLNL). 2x20 and 2x32-fold segmented DSSD HPSi detectors 9

10 Planar Double Sided Strip Detectors One 2x19-fold and one 2x39-fold segmented DSSD HPGe detector (built by Ethan Hull, LLNL): 38x38 mm 2 and 78x78 mm 2 area, 11mm thickness 2mm pitch size Custom-made, very compact preamplifiers mounted on rectangular motherboard close to feedthroughs equipped with warm FETs. Energy resolution: 1.4keV at 60keV and 2.5keV at 1332keV 2x20 and 2x32-fold and segmented DSSD Si(Li) detectors (Paul Luke, LBNL, and Davor Protic, Germany): 2mm pitch size, 6,10mm thickness Custom-made, compact preamplifiers Energy resolution: ~1.6keV at 60keV Data acquisition: 8-channel waveform digitizer modules built by Struck Innovative Systems (optical VME-PCI readout) 10

11 CCI-1 prototype, a transportable Compton imager system Si-Ge Compton imager mounted on custom made cart which comprises detectors, data acquisition system and data analysis and display computer Power supply and trigger logic Amplifiers 18 8-channel 100 MHz/14 bit VME digitizer boards Si Ge 11

12 How to determine positions DSSD HPGe detector? y2 y1 X-Y: Strip number + pulse-shape analysis (PSA) (amplitudes of transient signals) γ Z: Timing ( t in risetime of opposite strips) - + x1 0.1 Image Waveforms X x2 Y (orthogonal direction) X 1 X E field t Y X 1 X X 2 12

13 Three-Dimensional Position Resolution in DSSD Detectors With pulse-shape analysis we obtain a position resolution of ~0.5mm at 122keV in all three dimensions. 13

14 Demonstration of resolution Shadow image of a 12 mm diameter hole: No PSA With PSA The shadow of a hole collimator on the detector surface (E=122keV, FWHM interp =440µm). 14

15 Impact on interpolation procedure 22 Na point source images: Without Interpolation ( ~2mm) image1 shortimage1 With Interpolation -No cuts made in data ( ~0.5mm) image2 shortimage2 Improvement of 2.5 in image resolution due to interpolation 15

16 Secret of a successful imager: efficiency, efficiency, efficiency The signals are weak in a large, fluctuating background A factor 2-10 can be gained in sensitivity by using a larger set of events Improving position resolution will increase the angular resolution, which in turns helps sensitivity We need to do signal decomposition using an algorithm that optimizes efficiency 16

17 Our goal: implementing D. Radford s code y2 y1 Implementing D. Radford s code developed for GRETINA: γ Move from 3D geometry to 2 x 2D geometry x1 Our initial volume: 2mm x 2mm x 11mm x2 Instead of (x,y,z) we get (x,z) and (y,z) 100 Status: modified code to 2D, does not work with many positions. ( Signals) i, k i 17

18 Conclusions Gamma-ray imaging will play an important role with small scale system, e.g. for secondary inspection but particularly with largescale system for wide area search and surveillance tasks. Signal decomposition will be crucial to increase the sensitivity of the systems and keep the cost down 18