We describe a gamma-ray imaging camera (GIC) for active interrogation of explosives being developed by NASA/GSFC and NSWCICarderock. The GIC is based on the Three-dimensional Track Imager (3-DTI) technology developed at GSFC for gamma-ray astrophysics. The 3-DTI, a large volume time-projection chamber, provides accurate, approx.0.4 mm resolution, 3-D tracking of charged particles. The incident direction of gamma rays, E, > 6 MeV, are reconstructed from the momenta and energies of the electron-positron pair resulting from interactions in the 3-DTI volume. The optimization of the 3-DTI technology for this specific application and the performance of the GIC from laboratory tests is presented

Barbier, L. M.

deNolfo, G. A.

Floyd, S. R.

Guardala, N.

Hunter, S. D.

Link, J. T.

Skopec, M.

Son, S.

Stark, B.

NASA Technical Reports Server

GAMMA-RAY IMAGING FOR EXPLOSIVES DETECTION 
G.A. de Nolfo*", S. D. ~ u n t e r * ~ ,  L.M. ~ a r b i e r ~ ,  J.T. Linkc, S. Sona, 
S.R. ~ l o ~ d ~ ,  N. ~ u a r d a l a ~ ,  M. skopecd, B. starkd 
aNASA/Goddard Space Flight Center, Code 66 1, Greenbelt, MD, USA 2077 1 ; 
b ~ ~ ~ ~ / ~ R E ~ ~ ~ ~ ~ ~ ~ ,  Code 66 1, Greenbelt, MD USA 2077 1 ; 
CUSRA/CRESST/NASA, Code 66 1, Greenbelt, MD USA 2077 1 ; 
d ~ a v a l  Surface Warfare Center, Carderock, MD USA 208 14 
ABSTRACT 
We describe a gamma-ray imaging camera (GIC) for active interrogation of explosives being developed by 
NASAIGSFC and NSWCICarderock. The GIC is based on the Three-dimensional Track Imager (3-DTI) technology 
developed at GSFC for gamma-ray astrophysics. The 3-DTI, a large volume time-projection chamber, provides 
accurate, -0.4 mm resolution, 3-D tracking of charged particles. The incident direction of gamma rays, E, > 6 MeV, are 
reconstructed from the momenta and energies of the electron-positron pair resulting from interactions in the 3-DTI 
volume. The optimization of the 3-DTI technology for this specific application and the performance of the GIC from 
laboratory tests is presented. 
Keywords: Micro-well detector (MWD), Gamma-ray Imaging Camera (GIC), WGPu, HEU 
1. INTRODUCTION 
The Department of Defense has been tasked to develop means of standoff interrogation to detect small quantities of 
WGPu, HEU, and high explosives (HE). WGPu is readily detected passively by its fast neutron emission (Hunter et al. 
these proceedings). HEU can also be detected using neutron imaging. HEU detection requires active interrogation with 
neutrons to increase the low, natural neutron emission rate by induced fission. Detection of HE can also be achieved by 
employing active interrogation with neutrons, however, the most promising method requires imaging gamma rays from 
thermal neutron capture on nitrogen, 1 4 ~ ( n , y ) 1 5 ~ * .  De-excitation of the 15N* results in several gamma-ray lines with 
energies from 4.48 to 10.82 MeV (Kinsey et al., 1950). Fifteen percent of the de-excitations result in the release of a 
10.82 MeV gamma ray (Mernagh, et al. 1977). 
Measurement of the gamma rays for imaging a source of HE is based on their dominant interactions in matter, Compton 
scattering and pair production. For both processes one detects and measures the tracks of resulting recoil electron or 
electron-positron pair. Our gamma-ray imaging technology, developed for astrophysics in the 5 MeV to 50 GeV energy 
range, not only can be used here, but can be optimized for the detection of the gamma rays resulting from neutron 
capture on nitrogen. 
The Gamma-ray Imaging Camera (GIC) is based on the NASAIGSFC Three-Dimensional Track Imager (3-DTI) 
detector technology being developed for medium- and high-energy gamma-ray telescopes. The 3-DTI, a large volume 
time-projection chamber, provides accurate and precise (-0.4 mm resolution) three-dimensional tracking of charged 
particles. 
The GIC is sensitive to gamma rays that interact via Compton scattering or pair production. The relative strengths of 
these two channels is a strong function of energy. Restricting ourselves to pair production and its characteristic 'V' 
formed by the electron-positron pair results in a detection mode with properties of strong background rejection. Thus, 
the 3-DTI detector for GIC has been optimized as a gamma-ray pair telescope, i.e., to measure the momentum of the 
electron-positron pair. The incident directions and energies of the gamma rays are reconstructed from these momentum 
measurements. 
https://ntrs.nasa.gov/search.jsp?R=20080044052 2019-08-30T05:38:58+00:00Z
We describe the development of a 10x10~15  cm3 GIC prototype detector, Figure 1, and subsequent tests in the 
laboratory using radioactive sources. The goal of the GIC design is to provide a large cubic meter, high-resolution 
camera, with full 360" azimuth sensitivity, that is capable of passively detecting HE at reasonable stand-off distances. 
The development of GIC will take place in three stages, starting with the already built and tested 10x10~15  cm3 
prototype, followed by a scale up 50x50~30  cm3 prototype, and finally a 1 m3 GIC camera that will be ready for realistic 
2. TECHNICAL DESCRIPTION 
The underlying principle of the Gamma-ray Imaging Camera 
(GIC) is to measure the energy and position in three dimensions of 
charged particles traversing the camera medium (xenon gas 
mixture at 3 atm). Gamma-rays, entering the active volume, 
interact with the detector gas via Compton scattering and pair 
production. While GIC is capable of identifying Compton-scatter 
events, particularly in the low background environments expected 
from naturally occurring high-energy gamma radiation, we focus 
in this paper on the identification of pair-production events. At 
high energies (E > 30 MeV) pair production dominates, although a 
significant number of pair events will be observed down to lower 
energies. The resulting electron and positron leave a trail of 
ionization as they traverse the active volume. The characteristic 
vertex formed from pair production serves to identify incoming 
gamma rays from other charged particle interactions. The incident 
direction and energy of the incoming photon is determined by 
accurately measuring the momenta and energies of the electron 
and positron. Precise measurements of the kinematics of the 
incoming gamma rays provides a powerful tool for source 
identification and for minimizing background. 
GIC is comprised of a large volume gas time projection chamber 
(TPC) with two-dimensional, gas micro-well detector (MWD) 
readout, see Fig. 1. The TPC is filled with a xenon gas mixture 
that acts as a large volume, low density, homogenous tracker. 
Incident high-energy gamma-rays enter the TPC and interact via 
Compton scattering and pair production. For energies greater than 
- 30 MeV, pair production dominates in xenon and the resulting 
electron and positron travel through the chamber leaving an 
ionization trail along their trajectory. The TPC measures the 
position of the charge particle track and the energy loss by 
ionization in the gas. It is bounded at the top with a drift electrode 
and an array of MWDS at the bottom. A cage of electric-field 
shaping biased wires defines the sides of the active volume and 
provides a uniform drift field (-900VIcm). The ionization charge 
along the tracks drifts toward the MWDs at the bottom. The 
MWDs form a two-dimensional array of gas proportional counters. 
The MWDs are orthogonal electrode strips rigidly affixed to an 
insulating substrate. The wells themselves are defined by holes in 
the cathode strips and in the insulators that expose the orthogonal 
anode strips below. The micro-wells are maintained at a bias 
Ionization 
e lechn 
I I 
Fig. 1. Schematic of GIC. The TPC volume, 
MWD, and micro-well operation. The TPC drift 
field is defined by the cathode plane and the 
drift electrode. An energetic charged particle 
traversing the TPC produces ionization 
electrons that drift into the micro-wells, where, 
in the intense electric field, they produce an 
avalanche. The avalanche charge is collected on 
the anode and an equal and opposite charge is 
collected on the cathode. 
voltage such that the electric field is lo4-10' Vlcm. Thus, the I 
charge, as it enters the wells, produces an ionization avalanche. The electrons are collected on the anode while equal 
opposite mirror charges are set up on the cathodes. The orthogonal strips, thus, provide 2D imaging. 
To achieve 3-D reconstruction of a track, the z-coordinate of the ionization charge is determined by recording the time 
structure of the avalanche charge signals on each anode and cathode. The drift velocity of the ionization charge in the 
gas determines the translation from time to spatial coordinate. The drift velocity of the ionization charge (free electrons) 
in xenon is on the order of 30 mmlys, but depends strongly on additive gases ( C K ,  C02, CH3N02, etc.) and drift field 
(Piesart & Sauli, 1984). Furthermore, because this velocity is significantly higher than the thermal velocity of the gas, 
electron drift has significant longitudinal and transverse diffusion that smears out the spatial structure of the ionization 
charge (track structure) after only a few cm of drift. The drift velocity of the ionization charge can be substantially 
reduced by the addition of gaseous CS2, a molecule with moderate electron affinity that efficiently scavenges the 
ionization electrons forming negative anions. The anions drift in thermal equilibrium with the gas and their drift 
velocity, -0.1 mmlps, is about of that for free electrons. Drifting in thermal equilibrium also substantially reduces 
the transverse and longitudinal diffusion (Martoff et al. 2000), greatly increasing the maximum drift distance. These 
anions drift towards the anodes where, in the strong electric field of the micro-well, the electrons are stripped off and an 
electron cascade is produced in the well. In addition, the slow drift velocity reduces the required sampling rate of the 
transient digitizers that record the arrival time of the ionization charge. 
The signals from each anode and cathode are processed though front-end electronics and a transient digitizer. Each FEE 
consists of a charge sensitive pre-amplifier and a pulse shaper with gain. The charge avalanche in a well induces the 
same and opposite polarity signal and both signals are readout through an anode and a cathode, respectively. The 
Transient Digitizer (TD) consists of a differential receiver, 12-bit ADC, and a 
- 
common control FPGA. The TD operates at a sampling rate of 2.5 Msamplesls (i.e., 
the FEE is read out every 400 ns). After the analogue signal is digitized, the value is 
stored in a circular buffer with a size of 32,000 Sampleslchannel. Events, satisfying 
- - 
a trigger threshold, are captured by a PC via USB interface. 
3. MANUFACTURING AND PERFRMANCE CHARACTERISTICS 
3.1 MWD Manufacturing and Performance 
We have developed an in-house UV laser ablation technique for fabrication of 
5x5cm2 MWDs (Deines-Jones 2002b) and demonstrated stable proportional 
operation at gas gains up to 3x104, spatial resolution of -200 pm (Black et al. 2000, 
200 - - 
i D kc\ \ - ra t  
120 
- *
g I00 
d 
30 
0 L - 
0 500 1000 1500 2000 
ID( '  ( Iin111ic1 
Fig. 3. Energy Spectrum from 5 5 ~ e  obtained from a portion 
of NIC. The 6 keV and argon escape peak (-3keV) are 
readily visible. 
- - 
~ e i n e s i ~ o n e s  et al. 2002a), and FWHM energy 
resolution of 10% at 20 keV in P-10 (90% Ar I 10% 
CH4) and 18% resolution at 6 keV in 95% Xe/5% C02. 
We are currently setting up our laboratory for the 
continue in-house fabrication of MWDs in order to 
fine-tune the fabrication process. 
We have been working with several commercial 
vendors to develop viable fabrication processes that can 
be scaled to 50x50 cm2 areas (Hunter, 2006). MWDs 
with 10x10 cm2 areas have been produced and tested, 
see Figure 2. The micro-wells are 200 pm in diameter 
and 200 pm deep, on 400 ym x 400 pm center-to- 
center pitch. We have integrated these MWDs into a 
1 Ox 1 Ox 15 cm3 GIC prototype. We demonstrate an 
energy resolution of -18% using an 5 5 ~ e  source in the 
lab with P-10/CS2. Figure 3 shows the 5 5 ~ e  spectrum 
obtained with the 5.9 keV x-ray peak and the argon 
escape peak at -3 keV. 
3.2 GIC "Proof of Principle" Performance 
Characterization of GIC has been achieved using a "Proof of Principle" detector (Figure 4) that incorporates 5 x 5 x 9 
cm3 MWDs. This detector configuration, sized to fit in a 6" (15 cm) diameter vacuum chamber, supports a MWD and 
drift electrode with 2.5 or 9 cm drift distance, and interface to 
96 channels of FEE. A vacuum pump and multi-gas manifold 
allows this chamber to be easily evacuated and filled with a 
variety of gas mixtures up to 3 atm. 
Using the proof-of-principle 3-DTI detector, we have 
demonstrated thresdimensional track imaging of electrons 
fiom gamma-ray interactions in a variety of gas mixtures: P- 
10, helium, and xenon, with CS2. We have measured an 
energy resolution (AEE) of -20% at 5.9 keV in P-10/CS2 at a 
gas gain of lo3. Good angular resolution of a pair tracker 
such as GIC is a trade off between the number of samples 
along the track and the effects of Coulomb scattering. 
Essentially, one must make many measurements of the track 
within the first few milli-radiation lengths (milli-RL), before 
multiple Coulomb scattering begins to dominate. The key to 
improving angular resolution is to reduce the granularity 
The high granularity at a fixed TD sample rate is, in part, possible due to the slow drift times accomplished by the 
introduction of negative ions, such as CS2. Figures 5 illustrates the daerence between an electron track registered in 
GIC, with and without the addition of CS2, respectively. In addition, since diffusion is significantly reduced by 
employing negative ion drift, we can expand the active depth of GIC, thus increasing the effective aperture. 
Electron Track without CS, Electron Track with CS, 
- 
-I+-- 
%- %=z=-_r==== 0- 0 
*T-L.-..-.---.--uw 
-u--, 
0 1000 2000 3000 0 1000 2000 3000 
T~me Samples Time Samples 
Fig. 5. Electron track (sample time versus chathode) in GIC with (left) 
and without (right) the addition of CS2. 
3.3 GIC "Proof of Principle" Imaging 
(number of radiation lengths per 
sample). The best granularity to date is 
obtained with emulsion detectors, 
which have a granularity of typically 
3x10" RLIsample. Another way to 
significantly improve the granularity is 
to decrease the density, in other words, 
use a tracker that has gas as the sample 
medium. A gas medium improves the 
granularity by reducing coulomb 
scattering, minimizing the passive 
volume (such as converter foils 
typically used with pair telescopes) and 
simply increasing the resolution for 
sampling. The raw TD data from an 
electron track produced from a 9 0 ~ r  
source is shown in figure 5. With 400 
ns sampling and an x-y pitch of 400 pm, 
The raw TD data of the electron-positron pair recorded fiom a 6.12 MeV gamma ray ( 1 6 0  excited state) interaction in P- 
10 80%/CS2 20% at a total pressure of 0.55 atm is shown in Figure 6. The drift field was 880 Vlcm and MWD field 
was 4 x 1 0 ~  Vlcm. The apparent truncation of the tracks is due to the particles leaving the active volume. The temporal 
coincidence of the anode and cathode signals allows the raw TD data to be translated into three spatial coordinates and 
energy deposition. The x and y coordinates of each coincidence TD pulse are determined by the anode and cathode 
the granularity of GIC is 8 x 1 0 ~  RLIsample, on par with emulsion detectors. The structure visible in the raw TD data, 
corresponding to the MWD avalanche signals of groups of ionization electrons, illustrates that GIC is capable of 
recording the track with high resolution including the statistics of the ionization process, energy loss mechanism, and 
Moliere scattering. 
(a) Anodes (b) Cathodes 
Fig. 6. Raw TD data of a pair production event in GIC. The 
electronlpositron tracks are seen in the anodelcathode versus sample time. 
- -  
I 
# 
1: 
* 
*- 
e- -- 
1- -- *" 2 
r ' P  
Fig. 7. Reconstructed event (from Fig. 6) of pair production. Red 
arrow indicates direction of incident gamma-ray. 
Fig. 8. GIC prototype in laboratory without 
vacuum chamber. 
strip locations. The z coordinate is derived fiom the sample time 
and the negative-ion drift velocity. The signal amplitude is 
proportional to the energy loss (dE/dx). We refer to the spatial and 
energy deposition as the voxel data, (x, y, z, AE). 
The 3-D reconstruction of the raw pair image data (Fig. 6 )  into voxel 
data is shown in Fig. 7 .  Complete reconstruction of this event is 
not possible because the election and position escaped from the 
limited active volume. Nevertheless, arrows indicate our best 
estimate of the electron-positron directions and incident gamma-ray 
direction. Preliminary analysis, based on a drift velocity of-50 mls, 
indicates an opening angle of -40" for the electron-positron pair, 
consistent with the opening angle of a 6.1 MeV gamma ray 
(Borsellino 1953). 
3.4 1 0 x 1 0 ~ 1 5  cm3 GIC Prototype 
Figure 8 shows our lox lox 15 cm3 prototype GIC in the laboratory 
without the vacuum chamber. The field-shaping grid rests on top of 
the MWD. The active volume of the TPC is bounded by this field- 
shaping grid. The entire camera is encased within a pressure 
chamber. The MWD and grid is located above a motherboard, 
which acts both as a seal for the pressure chamber and connects the 
detector high voltage and signal cables as well as the FEE low power 
to the power supplies and TD outside the chamber. The volume of this prototype is large enough to fully contain the 
majority of gamma-ray interactions, making it possible to fully reconstruct their incident direction and energy. This is 
possible since the pulse height of the signal registered in the MWD is directly proportional to the energy loss, dEIdx, 
along the track. We are currently testing the lox 10x1 5 cm3 prototype in laboratory with radioactive sources. 
4. SIMULATIONS AND ACCELERATOR TESTS 
We have begun simulations using GEANT 4 to study gamma-ray interactions within GIC. We are currently testing the 
response uniformity of the MWDs in the laboratory and investigating the gain characteristics of different gas mixtures 
for the TPC, such as HelCS2 and He/CH3N02. Complete characterization of the energy resolution and angular resolution 
will be obtained with an accelerator run at the Positive Ion Accelerator Facility (PIAF) at the Naval Surface Warfare 
Center in Carderock, MD. The tandem ion accelerator is a Pelletron design, produced by NEC, capable of accelerating 
positive ions with 3MV maximum acceleration voltage. The accelerator produces a monoenergetic beam of gamma rays 
between 0.2-1 5 MeV and neutrons with energies up to 8 MeV. 
5. SUMMARY 
We have successfully demonstrated the capability of imaging high-energy gamma-rays in 3-D with our prototype GIC 
camera. Imaging gamma-rays is essential for identifying signatures gamma-ray lines from the de-excitation of the 1 5 ~ *  
and thus locating HE. Our upcoming accelerator tests will help to further characterize the angular and energy resolution 
in addition to refine our image reconstruction analysis. 
REFERENCES 
[I1 Black, J.K., et al., Proc. SPIE, 4140, 313-323 (2000). 
Borsellino, A., Phys. Rev., 89, 1023-12025 (1953). 
13' Deines-Jones, P. et al., NIM-A, 478, 130 (2002a). 
14] Deines-Jones, P. t al., NIM-A, 477, 55 (2002b). 
Hunter, S.D. et al., IEEE, In Publication (2006). 
[61 Kinsey Bartholomew, & Walker, Phys. Rev. 77, 723 (1950). 
[71 Martoff, S. et al., NIM-A, 440, 355 (2000). 
Mernagh, Harrison, & McNeill, Phys. Med. Biol., 22, 831 (1977). 
19] Piesart, A. & Sauli, F., CERN Technical Report, CERN-84-08 (1984). 


Gamma-Ray Imaging for Explosives Detection

Gamma-Ray Imaging for Explosives Detection

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