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

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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 o f Energy.

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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.

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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 Nevada National Security Site

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Various means of light collection

• PMT:

low dark current, high gain,

Vacuum, large form factor, low

QE, high voltage [Xmax ~415 nm fo r Nal:TI,

375 nm fo r LaBr3:Ce],

d a N a t i o n a l

• 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].

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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

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Nevada National Security Site

_5 _ — N a tio n a l S e c u r it y T e c h n o lo g ie s 1 M anaged and Operated by N a tio n a l Security 1 ecnnologies, L L C vision • service • Partnership

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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)

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Results

Three-Element System - Experimental

Angle vs. Asymmetry

265.01X + 28.594 R2 = 0.9871

Angle vs. Asymmetry

— Linear (Angle vs. Asymmetry)

(0 .200) (0 .100) 0.100 0.200 0.300

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Results

Four-Element System - MCNPX

Asymmetry vs. Angle

(0.30) (0.20) (0.10)

1.00

0.10 0.20 0.30

Asym m etry vs. Angle -Linear (Asymmetry vs. Angle) y = 357.63X- 0.0506 R2 = 0.988 A 90 degrees

n

>

2 7 0 degrees

0

degrees -100 50 100 S eriesl

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Results

d a N a t i o n a l

Four-Element System - Experimental

Source Angle a vs. asymmetry

♦ Source Angle a function of asymmetry

Linear (Source Angle a function of asymmetry)

(0.200) 0.000 0.200 0.400 0.600 0.800

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Results

Six-Element System - Experimental

Angle vs. Asymmetry

100

Angle vs. Asym m etry

Linear (Angle vs. Asymmetry)

-20

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Results

Nine-Element System - MCNPX

90 degrees

-0.3 0.3 Angle as a function asymmetry -Linear (Angle as a of asymmetry) y = 375.1 x - 0.4493 R2 = 0.9881

>

0 degrees

Angle as a function of asymmetry

V

270 degrees

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Results:

Nine-element Csl: Tl crystals coupled to SSPM array

Sam ple 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

reflects the proximity to the source.

Nine-Element Asymmetry vs. Angle

I

N e v a d a N a t i o n a l

100

y = 339.12 x - 21.875

R2 = 0.9892

o> o> 0.2 0.3 0.4 -20 N in e E le m e n t A s y m m e try vs. A n g le -L in e a r (N ine E le m e n t A s y m m e try vs. A n g le ) 0 4 A s y m m e try

Experimental Setup

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Description of the activities

Vertilon and R SLA

-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 9500 256 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

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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

O c t d

DH10

iii; "Jh-hiw

Det2

ODtQIVH

O c t f r

Oct 9

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Gamma Energy Spectrum

from a combination source (at 90 deg)

1.00E-05

1.00E-06

oo

L t i

I"-”

1.00E-07

x , </>

Q.

O

1.00E-08

1.00E-09

Det 2

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15

-

1-1-0-Vision ■ S e rvice • P a rtn e rs h ip — —

Det 4

Det 6

Det 8

Det 10

Det 12

1

0.0

0.5

1.0

1.5

2.0

E (MeV)

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Gamma Energy Spectrum

from individual detectors (2,4,

6,..

12)

Source at 90 degrees, 30" away from center

DET 2

DET 4

DET 6

1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09

WWVr

1.00E-03 1.00E-04 1.00E-05

—•— DET 2

1.00E-06 G a m m a 1.00E-07 E n e rg y in M eV 1.00E-08 1.00E-09

— Det 4

DET 8

1.00E-03 1.00E-04 1.00E-05

— Det 8

1.00E-06 1.00E-07 1.00E-08

Gamma Ener

1.00E-09

in MeV

DET 10

1.00E-03 0.5 2.0 1.00E-04 1.00E-05

Det 10

1.00E-06 1.00E-07 1.00E-08 1.00E-09 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09

Det 6

DET 12

1.00E-03 1.00 2.00 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09

Det 12

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Summed Count Rate Response

G am m a G ro s s C o u n t ra te s o

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r

A

V

o i---1--- 1--- 0 1 1 1 -160 -100 -40 20 80 140 h

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Source Angle vs. Asymmetry - MCNPX

Angle vs. Asymmetry

y = 198.91X + 3.6399 R2 = 0.986 -0.4 -0.3 -0.2 -0.1 0.2 0.3 0.4 Angle vs. Asymmetry

-Linear (Angle vs. Asymmetry)

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Source Angle vs. Asymmetry - Measured

Asymmetry

70 60 50 40 30 20 y = -204.53X + 68.583 R2 = 0.9924 10

0

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Asymmetry -Linear (Asymmetry)

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Covariance Spectroscopy

6 5

■

10 12 14 16

I

P

Q> C

111

V

W i

:

MO

10M 1200 1400

Energy (KeV)

M anaged and Operated by National Security Technologies, LLC

Looking at energy

response in tw o adjacent

detectors in a six-element

setup (0 & 5).

Geometry as shown

before.

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

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C o u n ts

Crosstalk

7000

H D D

5000

4QDD

3000

£000

1000

-1000

average sig, firs t det average sig, second det lull crosstalk Positive crass talk

500

1000

Energy (kgV) 150D

Angularly

separated

detectors

will have

less and less

crosstalk—

a measure of

occlusion and

Compton

scattering, both

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Crosstalk Asymmetry

100 90 00 70 CD U Es 60 LE 3 ₅₀ ■g E E ₄₀ ej 30 20 10 0 -i---r crosstalk asymmetry Am L MiM L 400 600

Asymmetry

can be

mapped into

relative

angle

between the

source

and the

detectors.

-800 -600 -400 -200 0 200

Energy difference (keV)

800

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Summary

W e have worked on multiple systems of small sensor arrays to

determine the optimized angular response from

gamma-emitting 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.

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