Chemical composition analysis for X-ray transport container scans. - - PowerPoint PPT Presentation

Chemical composition analysis for X-ray transport container scans.

• A. Zelenaya 1, M. Zelenyi 1,2, A.A.Turinge 1, V.G. Nedorezov 1

1Institute for Nuclear Research RAS 2Moscow Institute of Physics and Technology (SU)

October 10, 2018

Introduction

◮ It is important for national security

to control the movement of dangerous or strategically cargo.

◮ This control can be provided by

scanning transport containers by gamma rays produced by bremsstrahlung.

◮ In this report we consider:

◮ Methodology, existing solution

and our proposing method;

◮ GEANT4 simulation of gamma

rays scanning;

◮ Measurement resolution of

gamma rays detector.

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Methodology: Gamma ray attenuation

T(E0, t, Z) =

E0

• S(E0, E) exp(−µ(E, Z) × t) dE)

E0

• S(E0, E) dE

100 101 Energy, MeV 10

1

, cm2

gr

Attenuation curve

Z = 5 Z = 13 Z = 26 Z = 82

T - transmittance S(E0, E) - response function µ(E, Z) - attenuation t - optical thickness E0 - up-limit energy of bremsstrahlung E - energy of gamma ray Z - charge of nuclei

0.0 2.5 5.0 7.5 10.0 Energy, MeV 10

3

10

2

10

1

100 Probability density

Bremsstrahlung spectrum by 10 MeV electron

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Methodology: Existing solution

Dual energy method

◮ We can’t defined Z if optical thickness t is unknown. ◮ But we can use two electron beams with different energy

which give gamma rays with up-limit energy E (1) and E (2)

0 . ◮ Then we can get Z as a result of minimizing this function:

F(z) = |t(E (1)

0 , z) − t(E (2) 0 , z)|

t(E (1)

0 , z)

→ min

◮ This technique allows to determine scan object as a one from

four possible groups: Zeff ∼ 5, Zeff ∼ 13, Zeff ∼ 26, Zeff ∼ 82.

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Methodology: Disadvantages and proposing method

Disadvantages of dual energy method

◮ It is too difficult to irradiate the target with beams with

different energy.

◮ Low efficiency for target which contains elements with strongly

different charges.

Our method

◮ Use only one electron beam with energy E = 10MeV . ◮ Measure not only the space distribution, but also the energy of

gamma rays.

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Simulation: Preliminary estimates

Steel container Size: 2x2 m Thinknes: 2 mm Detector Pixel size: 1x1 cm W Dangers γ γ γ e - 10 MeV

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Simulation: Preliminary estimates

Very dangerous item

100 50 50 100 x, cm 100 75 50 25 25 50 75 100 y, cm

Uranium cube (6cm) in a lead sphere (thickness – 1 cm)

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Simulation: Preliminary estimates

Search of the explosive Comparison the energy spectrum for aluminium and uranium orb (radius – 1 cm).

0.0 2.5 5.0 7.5 Energy, MeV 1 2 Logarithm ratio of intensity

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Simulation: Preliminary estimates

The energy deposit in detector cells for several materials N – number of detector cells N[Edep < 3 MeV ] N[Edep > 3 MeV ]

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Simulation: Thickness reconstruction for the 1-D case

Simple model

Attenuation of gamma ray flux is defined as: N(E) N0(E) = exp(−

• i

Σmean

i

(E)xi) where xi — thickness of the i-layer, Σmean

i

— mean cross-section for group of materials, N, N0 — the number of gamma.

◮ Disregard secondary

scattering.

◮ Disregard the

annihilation line.

Detector Z ~ 82 Z ~ 13 Z~ 26 Gamma ray source

Reconstruction algorithm

◮ Full thickness is known. ◮ Find thickness using least

squares:

• E

(ln N(E) N0(E)+

• i

Σmean

i

(E)xi))2 → min

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Thickness reconstruction: Example

We consider the object which has 3 layers of aluminium, iron and lead. The figure shows a contribution of every reconstructed material in summary attenuation. The table contains the results of reconstruction.

2 4 6 8 10 Energy, MeV 50 100 150 ln N(E)

N0(E)

Original Al Fe Pb

Material Real, cm Reconstructed, cm Al 20 19.6 Fe 40 41.6 Pb 30 28.7

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Simulation: Thickness reconstruction for the 1-D case

Also we conducted several numeric experiments for different thickness.

◮ Aluminium, Iron and

Lead are used.

◮ Several sets of

thickness with full thickness from 30 cm to 180 cm.

◮ The energy grid

spacing emulates the detector with 10 % resolution.

Z ~ 13 Z ~ 26 Z ~ 82 0 % 5 % 10 % 15 % 20 % 25 % 30 % Relative error

Distribution of reconstruction errors for various numerical experiments

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Simulation: Perspectives

◮ If we use energy-space distribution we can develop the

algorithm for the 3D-tomography

Detector Z ~ 82 Z ~ 13 Z~ 26 Gamma ray source

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Experiment: Energy resolution of the detector

The experiment was conducted by Dr. Guber and Dr. Ivashkin.

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Experiment: Energy resolution of the detector

22Na — 0.511 MeV 22Na — 1.275 MeV 137Cs — 0.662 MeV

The sum of signals from 2 photodiode Noise threshold: 100 KeV Energy, MeV Sigma/Mean 0.511 14.7% 0.662 19% 1.275 13%

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Conclusion

Results

• 1. The measurement of the gamma ray spectrum allows to

identify cargo belonging to the group of materials with certain Zeff .

• 2. Also it allows to define the thickness of layers from different

elements with the accuracy about 25%.

• 3. The energy resolution of the detector based on a BGO

scintillator was studied. For the photodetector with full array

• f pixels the energy resolution is expected about 10%.

Plans and perspectives

With financial support can be developed:

• 1. The program which checks cargo of a transport container for

compliance cargo manifest

• 2. The algorithm for the 3D gamma-tomography.

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Thank you for your attention

100 101 Energy, MeV 10

1

, cm2

gr

Attenuation curve

Z = 5 Z = 13 Z = 26 Z = 82

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