Performance Expectations for a Tomography System Performance Expectations for a Tomography System Using Cosmic Ray Muons and Micro Pattern Gas Using Cosmic Ray Muons and Micro Pattern Gas

Detectors for the Detection of Nuclear Contraband Detectors for the Detection of Nuclear Contraband Kondo Gnanvo, Patrick Ford, Jennifer Helsby, Richard Hoch, Marcus Hohlmann, Debasis Mitra

Florida Institute of Technology, Melbourne, FL, USA

Fig 9. Mean scattering angle of U targets located at different positions in the MT station.

Significance analysis

Muon tomography (MT) based on multiple Coulomb scattering of cosmic ray muons appears as a promising way to distinguish high-Z threat materials such as U or Pu from low-Z and medium-Z background with high statistical significance. Spatial resolution of the tracking detectors is a critical parameter for the expected MT performance. High-resolution Micro Pattern Gaseous Detectors, e.g. Gas Electron Multiplier (GEMs), as proposed here would improve overall MT performance. Results of systematic effects due to target dimensions, detector resolution, and MT geometry are also reported. We get positive results when statistically testing a U hypothesis against materials with different Z in a container-like scenario for 10 min exposure. A shorter 1 min exposure time for detecting a 10 cm3 box of threat material appears to be a serious challenge even with detectors with very good position resolution.

We discuss the performance that we can expect from the muon tomography technique in terms of exposure time required for Z-identification and of lower limit on the target size that can be detected. Step 1: We study the significance of the identification of any materials with Z values higher than Iron (back-ground) based on mean scattering angles. (Fig. 11). Step 2: We test the hypothesis that a target material identified as excess over Iron background from step 1 is consistent with presence of Uranium (Figs. 12 - 14).

This material is based upon work supported in part by the U.S. Department of Homeland Security under Grant Award Number 2007-DN-077-ER0006-02. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Department of Homeland Security.

Muon Tomography based on multiple scattering of

cosmic ray muons

Muons are created in the upper atmosphere by cosmic rays. A muon is an elementary particle with a mass of 105.7 MeV/c2. The muon flux at sea level is 104 s-1 m-2 at an average energy of 4 GeV. Muons interact with matter and are deflected at an angle which has a dependence on their atomic number Z. Due to their penetrating nature, muons are good candidates for detecting shielded high-Z materials.

Fig. 1. Deflection of muons due to multiple Coulomb scattering. Scattering angles have an approximately Gaussian distribution with rms width θ0.

Gas Electron Multipliers (GEMs)

A GEM (Gas Electron Multiplier) is a micro pattern gaseous detector for charged particles. It uses a thin sheet of plastic (kapton) coated with metal on both sides and chemically pierced by a regular array of holes a fraction of a millimeter across and apart. A voltage is applied across the GEM foils and the resulting high electric field in the holes makes an avalanche of ions and electrons pour through each hole. The electrons are collected by a suitable device; here a pick up electrode with x- and y-readout.

Fig. 2. GEM detector principle.

Abstract

Standard radiation detection techniques currently employed by portal monitors at international borders and ports are not very sensitive to high-Z radioactive material (U, Pu) if the material is well shielded to absorb the emanating radiation. Muon Tomography (MT) based on the measurement of multiple scattering of atmospheric cosmic ray muons traversing cargo or vehicles is a promising technique for solving this problem. We present results from a detailed GEANT4 simulation of a proposed MT system that employs compact Micro Pattern Gas Detectors (MPGD) with high spatial resolution, e.g. GEM detectors. A basic Point-Of-Closest-Approach algorithm and a more advanced Estimation Maximization algorithm are applied to reconstructed muon tracks for forming 3D tomographic images of interrogated targets. We have devised criteria for discriminating materials by Z and discuss the discrimination power achieved by the technique for various cargo scenarios and integration times. The simulation shows that Muon Tomography can clearly distinguish high-Z material from low-Z and medium-Z material. We discuss how the MT system performs as a function of target location inside the cargo and what performance is expected for scenarios with vertical clutter. We compare the geometric acceptance of a MT station with only top and bottom detectors with a station that uses additional detectors on the sides. Detector resolution is found to be a critical parameter for the performance; i.e. a detector with ~50 μm resolution, e.g. a MPGD, is expected to achieve significantly better Z-discrimination and tomographic imaging capabilities than a detector with ~200 μm resolution, e.g. a drift tube system. The implications of the simulation results for the planned development of a prototype MT station are discussed.

Simulation tools: GEANT4 & CRY

GEANT4 is Monte Carlo tool kit for simulating the passage of particles through matter, developed for high energy physics, nuclear physics, and medical applications. The CRY Monte Carlo generator for cosmic rays was developed by Doug Wright et al. at LLNL and can be used to generate muons with an angular distribution and an energy spectrum corresponding to those of cosmic ray muons, e.g. at sea level.

Fig. 5. POCA accuracy for different Z materials, which is estimated by the fraction of POCA points reconstructed within a given volume around the actual target volume.

A Point-of-Closest-Approach (POCA) algorithm is used to reconstruct two straight muon tracks from detector hits and to determine their closest approach in 3D space. The center point (“POCA point”) of the shortest line between the muon tracks is plotted in 3D. The GEM detectors provide the coordinates of the positions of the incoming and outgoing cosmic muons.

Fig. 4 Normalized distribution of scattering angles for cosmic ray muons crossing different materials for different spatial resolution of GEMs using high-statistics samples

Track reconstruction based on POCA points and mean scattering angle

We made a systematic studies of the parameters affecting the calculated mean value of the scattering angle of a given material, i.e. the effect of target size and thickness (see Fig. 7) on the mean and rms value of the scattering angle as well as the effect of spatial detector resolution (see Figs. 8 & 9). We also report a way to reduce the resolution effect by increasing the gap between the detector layers as shown in Fig. 10.

Systematic effects on the mean angle

Effect of target dimensions

Fig 7. Mean and rms of scattering angles as function of lateral size (top), and thickness (bottom) for various targets.

Effect of spatial resolution

Fig 8. Effect of GEM resolution on the mean and rms values of the scattering angle. Top: 5 mm gap between the GEMs, bottom: 100 mm gap. The effect leads to a shift of the mean angle towards higher values and a widening of the scattering angle distribution (higher rms).

U hypothesis test at 99% confidence

Fig. 12. 40 cm 40 cm 10 cm targets: 10 min. exposure

Al Fe

W U

Pb

Al Fe

W U

Pb

Al Fe

W U

Pb

Al Fe

W U

Pb

Reconstruction using voxels

To analyze the performance of the muon tomography, we divide the detector volume into voxels with 10 cm (or 5 cm) sides. For each voxel the mean value of the scattering angle of all POCA points inside the voxel is displayed, giving a good estimation of the scattering density of the material. Fig. 6 shows the reconstructed mean angles for voxels in a 10 cm slice around z = 0 in the scenario of Fig. 3, for different spatial resolutions (perfect resolution, 50 , 100, 200 micron).

Fig. 3. GEANT4 simulation of a Muon Tomography Station using GEM detectors at both top, bottom and lateral planes with 5 targets (40cm long 40cm wide 10cm tall) made of different Z materials (Al, Fe, Pb, W, U) placed in the center.

Fig. 6. Reconstruction of the mean scattering angles for the scenario with 5 targets from Fig. 3 using the voxel method for detectors with different spatial resolution.

Conclusion

Acknowledgement & Disclaimer

Fig. 14. 10 cm 10 cm 10 cm targets: 10 min. exposure

Significance of any excesses over Iron background at 99% confidence

Effect of the gap between detectors

Fig. 11. 40cm 40 cm 10 cm targets: 10 min. exposure

Fig 10. Effect of the gap between GEM planes on the mean scattering angle for various materials and resolutions. The bigger the gap, the closer the mean scattering angle is to the result obtained with perfect resolution.

This reconstructed mean value and the width of the scattering angle distribution of the muons recorded by the GEM detectors increase with the spatial resolution of the detectors. The gap between the detectors can be increased to reduce the resolution impact as shown in Fig. 8.

Effect of target positions in the MTS

Nucl. Sci. SymposiumOct 19 – 25, 2008

Dresden, GermanyN02 - 451

Fig. 13. 40 cm 40 cm 10 cm targets: 1 min. exposure

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