Exploitation of geometric occlusion and covariance spectroscopy in a gamma sensor array

Transcription

1 DOE/NV/ SPIE ' # Optics+Photonics Exploitation of geometric occlusion and covariance spectroscopy in a gamma sensor array Sanjoy Mukhopadhyay, Richard Maurer, Ronald Wolff, Stephen Mitchell, Paul Guss, and Clifford Trainham National Security Technologies, LLC This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946 with the U.S. Department of Energy. H1B9r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

2 Goals 1. Optimize the number of elements needed in a close-packed array to best exploit the gamma-ray occlusion effect ensure that geometry and size of detectors allow Compton scattering to be utilized for better definition of the gamma source. 2. Use one array where the relative orientation of detectors can be changed without changing design like using two carousels rotating independently. 3. Use at least another array where the relative position and orientation is fixed but the detection system is position sensitive. 4. Compare angular resolution; perform identification by embedding commercially available software in the data acquisition protocol. Managed and Operated by National Security Technologies, LLC N ational S ecu rity Technoloc Vision Service Partnership

3 Overview Important Physics Concepts: 1. Careful placement and orientation of an individual detector with reference to other detectors in an array can provide improved angular resolution in determining the source position by occlusion mechanism. It is the same concept as that of "Active Masking." 2. The spectral coincidence technique often known as covariance spectroscopy analyzes the correlations and fluctuations in data, which contain valuable information about radiation sources, transport media, and detection systems. 3. Covariance spectroscopy enhances radionuclide identification techniques, provides directional information, and makes weaker gamma-ray emissions detectable, which are not detectable by common spectroscopic analysis. a Managed and Operated by National Security Technologies, LLC N ational S ecu rity Technologies Vision Service Partnership

4 Various means of light collection PMT: low dark current, high gain, Vacuum, large form factor, low QE, high voltage [Xmax ~415 nm for Nal:TI, 375 nm for LaBr3:Ce], Photodiode: small form factor, low power, low gain, high noise [Xmax ~470 nm]. CCD: high QE, high fill factor, low gain, slow readout, not CMOS compatible [Xmax ~ 630 nm]. APD: medium gain, large area, small form factor, medium noise, excess noise, dark current, high voltage [Xmax ~450 nm]. SSPM: high gain, small form factor, CMOS compatible, low power, dark noise, excess noise, fill factor lim its[ Xmax ~480 nm]. d a N a t i o n a l Managed and Operated by National Security Technologies, LLC National Security Technologies1-1-0-

5 Project Activities 1. Built experimental setup, and procured multiple (10) x Nal:Tl. Used newer MCAs designed by the Special Technologies Laboratory, Santa Barbara. 2. Performed MCNPX simulation for several arrays and configurations of scintillators and semiconductors, for angular response determination 3. Performed covariance spectroscopy, which enhances radionuclide identification techniques, provides directional information, and makes weaker gamma-ray emission detectable, which is not detectable by common spectroscopic analysis B _5_ N a tio n a l S e c u rity T e c h n o lo g ie s 1 M anaged and Operated by N ational Security 1 ecnnologies, LL C vision service Partnership

6 Project Activities 1. Procured CskTl crystals on SSPM (from SensL Corp.) to build an array of nine-element CskTl crystals being viewed by a 9 x 9 array of solid-state photomultiplier tubes 2. Putting together four CskNa crystals with Hamamatsu position sensitive multiplier tubes for angular position determination 3. Testing STL MCA and ensuring that spectral data can be written at user-defined short time intervals (milli-seconds order) M g g jg lj I Managed and Operated by National Security Technologies, LLC National Security Technologies1-1-0-

7 Results Three-Element System - Experimental Angle vs. Asymmetry X R2 = Angle vs. Asymmetry Linear (Angle vs. Asymmetry) (0.200) (0.100) B 1E3r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

8 Results A Four-Element System - MCNPX 90 degrees Asymmetry vs. Angle n > 0 degrees Asymmetry vs. Angle 270 degrees (0.30) (0.20) (0. 10) Linear (Asymmetry vs. Angle) y = X R2 = Seriesl H 1E3r National Security Technologies1-1-0-

9 Results Four-Element System - Experimental Source Angle a vs. asymmetry Source Angle a function of asymmetry Linear (Source Angle a function of asymmetry) (0.200) d a N a t i o n a l Managed and Operated by National Security Technologies, LLC

10 Results Six-Element System - Experimental 100 Angle vs. Asymmetry Angle vs. Asymmetry Linear (Angle vs. Asymmetry) -20 H1E3r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

11 Results Nine-Element System - MCNPX 90 degrees Angle as a function asymmetry -Linear (Angle as a of asymmetry) > 0 degrees y = x R2 = Angle as a function of asymmetry V 270 degrees H1E3r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

12 Results: Nine-element Csl: Tl crystals coupled to SSPM array Sample readout from matrix9 The nine-element segmented scintillator on the pixellated readout. The 137Cs button source is positioned to the right side on the detector. One can see the intensity profile I reflects the proximity to the source. 100 o> Nine-Element Asymmetry vs. Angle y = x R2 = N ine E lem ent A s y m m e try vs. A ngle o> -L in e a r (Nine E lem ent A s y m m e try vs. A ngle) Experimental Setup A s y m m e try N e v a d a N a t i o n a l Managed and Operated by National Security Technologies, LLC National Security Technologies1-1-0-

13 Description of the activities - Vertilon and RSLA- Four-element Csl: Tl crystals coupled to position-sensitive multi-anode photomultiplier tubes Blue slabs - 4 Csl:Na sensors 0.5" x 1.5" x 2" ea. Hamamatsu channel multi-anode PMT PS-PMT used for Compact Gamma camera - 2-D Radiation Monitoring Number of Anode pixel 256 (16 x 16 matrix) Pixel Size/Pitch at the center 2.8 x 2.8 / 3.04 mm Effective area 49 mm x 49 mm Packing density (effective area/external size) ~ 90% Weight 177 gm B 1 r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

14 Technical Accomplishments Gone through a large number of design configurations and MCNPX simulations (100 ^ici of 241 Am, 137Cs, and 60Co at 30" away) Source is at 90 deg O c t 12 iii; "Jh-hiw O c t d Det2 ODtQIVH DH10 O c t f r Oct 9 Managed and Operated by National Security Technologies, LLC _14_ National Security TechnologiesLLC-

15 Gamma Energy Spectrum from a combination source (at 90 deg) 1.00E-05 Det E-06 Det 4 oo L t i I"- 1.00E-07 x, </> Q. O 1.00E-08 Det 6 Det 8 Det E-09 Det E (MeV) B 1 r National Security Technologies1-1-0-

16 Gamma Energy Spectrum from individual detectors (2,4, 6,.. 12) Source at 90 degrees, 30" away from center 1.00E E E E E E E-09 DET 2 DET 4 DET 6 WWVr 1.00E E E-05 DET E-06 G am m a 1.00E-07 E nergy in MeV 1.00E E-09 Det E E E E E E-08 Det E E E-04 DET E E-04 DET E E-04 DET E E-06 Det E E-06 Det E E-06 Det E E E E E-09 Gamma Ener in MeV 1.00E E E E-09 H1^3r M anaged and Operated by N ational Security Technologies, L L C National Security TechnologiesLLC-

17 Summed Count Rate Response G am m a G ross C o u nt rates o W r V A o i h i r M anaged and Operated by N ational Security Technologies, L L C N ational S ecu rity Tech Vision Service Partnership

18 Source Angle vs. Asymmetry - MCNPX Angle vs. Asymmetry y = X R2 = Angle vs. Asymmetry Linear (Angle vs. Asymmetry) H 1 E 3 r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

19 Source Angle vs. Asymmetry - Measured 70 Asymmetry Asymmetry -Linear (Asymmetry) y = X R2 = B 1 r National Security Technologies1-1-0-

20 Covariance Spectroscopy Looking at energy response in two adjacent detectors in a six-element setup (0 & 5). Geometry as shown before. I P Q> C 1 V W i : MO 10M Energy (KeV) It shows sharing of energy between the two detectors. (The one-dimensional plots are not projection; they are a guide). Source is Cs-137; the lower peak is at 200 kev (backscatter): good for isotopic identification Managed and Operated by National Security Technologies, LLC National Security Technologies1-1-0-

21 Crosstalk 7000 H D D average sig, first det average sig, second det lull crosstalk Positive crass talk Angularly separated detectors will have 5000 less and less crosstalk 4QDD a measure of occlusion and Counts 3000 Compton scattering, both D 2D0Q Energy (kgv) Managed and Operated by National Security Technologies, LLC National Security Technologies1-1-0-

22 Crosstalk Asymmetry CD U Es 60 LE 3 50 g EE 40 -i r crosstalk asymmetry Asymmetry can be mapped into relative angle between the source and the detectors. ej Energy difference (kev) Am L M im L H 1E3r M anaged and Operated by N ational Security Technologies, L L C National Security Technologies1-1-0-

23 Summary We have worked on multiple systems of small sensor arrays to determine the optimized angular response from gammaemitting sources at different angular positions. Three of them produce comparable and useful tools for determining the direction/location of a radioactive point-like source. 1. Six element 2" x 2" Nal:Tl cylindrical occluded sensor using covariance spectroscopy. 2. Four element 0.5" x 1.5" x 2" Csl:Tl using position sensitive multi-anode photomultiplier tube (PS MA-PMT). 3. Nine element Csl:TI using solid-state photomultiplier tubes from SensL. B 1 r National Security Technologies Vision Service Partnership