Project DENIDIA: Gamma ray tomography

Computer Engineering Department Technical University of Lodz18/22 Stefanowskiego, 90-924 Lodz, Poland w.mosorow@kis.p.lodz.pl

Volodymyr Mosorovoutgoing researcher at University of Bergen,

Norway

February, 2009

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Action 2: Conception of new tomography systems: Dual ModalityTomography

Partner Institution:Department of Physics and Technology, University of Bergen, Norway

Period of Training: 22.04.2008 - 22.09.2008

Supervisor of training: prof GA Johansen

Details of

staying:

Outline

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1.Physical background of Radiation measurement

2.A MCNP code as a tool for modeling and simulation of a Radioisotope gauges

3.MCNP model of UIB 85 channel gamma ray tomograph

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Radioisotope gauges or Nucleonic control systems (NCS)

Industrial nucleonic measurement system is the combination of one or several radiation sources and one or several radiation detectors

Physical background:Instrumental measurement for control and analysis is based

on the interaction between ionizing radiation and matter.

There are several ways of applying the NCS, among them:• On-line (process),• Off-line (process),• In-situ (well logging),• Used in laboratory (on samples), and• Portable, for site measurements

Relevant target areas of nucleonic control systems:

• oil and gas production,• mining and mineral processing,• paper and plastics industries,• cement and civil engineering industries

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Principles of nucleonic gaugesusing gamma radioisotopes

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Measurement based on the absorption gamma radiation as passes through the process material.

Absorption proportional to changes material density, and measuring path is held constant, this indicates product density.

Principle of measurement

Nucleonic gauges worldwide (statistics of 2000) (source: IAEA)

• USA: more than 100 000 nucleonic gauges• EU: more than 50 000 nucleonic gauges• China: around 50 000 nucleonic gauges• Asia (without China): around 20 000

nucleonic gauges• Latin America: 5000 nucleonic gauges• Africa: 2000-3000 nucleonic gauges• Russia: (not known) around 50 000 nucleonic

gauges

Total : around 250 000 – 300 000 nucleonic gauges worldwide

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Gamma ray radiation•The majority of radioactive disintegrations consist of several steps.

•The daughter nucleus often has some residual energy – the nucleus is in an excited state!

•The excited nucleus emits a particle called a photon with energy equal to the difference between excited state and ground state.

•The gamma-emission usually happens about 10-13 s after primary disintegration.

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•The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element.

Properties of gamma radiation

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Properties of gamma radiation

•Gamma photons have no mass and no electrical charge-they are pure electromagnetic energy.

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Properties of gamma radiation•Their wavelength is short and frequency high showing they are really fast and of high energy. They travel at the speed of light!

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•Highly concentrated gamma-rays can kill living cells

High-energy radiation kills cells by damaging their DNA, thus blocking their ability to grow and increase in number.

Properties of gamma radiation

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•Needs a lead block or a thick concrete block to be stopped. (Lead has a high density and it is not radioactive.)

Properties of gamma radiation

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Measures of Gamma rays

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Photoelectric effect

Compton scattering

Rayleigh scattering

Pair production

Interactions of Gamma-Rays with matter

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• In this case the photon interacts with an atom, completely disappears, and a photoelectron is ejected from on of the atoms shells

• The ejected photoelectron has energy Ee = hν – Eb, where Eb is the binding energy of the photoelectron in whatever atomic state it was leaving the atom in an ionized state

– The atom captures a free electron and/or undergoes a rearrangement of electrons from other atomic states

– In general one or more X-rays are emitted in this process

• The photoelectric effect is the predominant energy interaction mechanism for gamma rays of energies less than a few MeV

• The photoelectron has essentially the entire energy of the gamma ray, the scintillation light emitted

• This interaction provides a good estimate of the energy of the of the gamma ray

Photoelectric absorption

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Compton scattering is the elastic collision of a gamma ray with an electron in the absorbing material

The incoming electron is scattered through an angle θ with respect to its original direction

A fraction of the photon's energy is transferred to the electron which recoils with an energy ranging from 0 to a large fraction of the initial gamma ray energy

Compton Scattering

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Rayleigh scattering (named after Lord Rayleigh) is the elastic scattering process with only a negligible energy transfer.

Rayleigh scattering

*Rayleigh scattering of sunlight in clear atmosphere is the main

reason why sky is blue!

The incident photon with wavelength λ1 interacts with atom and the scattered photon with wavelength λ2 is being emitted with approximately the same wavelength and energy

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in the process of pair production the energy carried by a photon is completely converted into matter, resulting in the creation of an electron-positron pair

since the charge of the system was initially zero, two oppositely charged particles must be produced in order to conserve charge

Pair Production

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The linear attenuation coefficient μ can be defined as:

μ = μ(Rayleigh) + μ(Photoelectric) + τ(Compton) + κ(pair)

which is the total probability of photon interaction per unit length

(unit: cm-1).

•The mean free path is defined as lmfp = 1/μ [cm]

Gamma ray attenuation

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The transmission of mono-energetic photons in a parallel and narrow beam through a thin medium is then:

dI/dx = - μ I

with general solution, I(x) = I0 e- μ x

where I is remaining beam intensity, I0 is the initial intensity, x – is the

thickness of the absorber.

•The mass attenuation coefficient is defined as μ/ρ [cm2/g]

where ρ is the density of the medium.

Lambert-Beer’s exponencial law

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/ρ = (μ(Rayleigh) + μ(Photoelectric) + τ(Compton) + κ(pair))/ ρ

60 keV

Mass attenuation coefficient μ for Soft Tissue (Paper)

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Gamma ray attenuation cdSince Compton scattering processes dominate in many cases, there can be photons that scatter out of the beam being detected, and then later scatter back into the direction of the beam. The fraction of photons that contribute to this additional signal is called ‘Build Up Factor’, B(x,Eγ).

Simplistically, this could be included by the following equation:

I(x) = I0 B(x,Eγ) e- μ x

What is the relationship between cross section and attenuation coefficient?

μ = NAρ σtot /A, where A = atomic weight in g/mol

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Gamma ray attenuation – example problem

What is the transmission of 1 MeV photons through 10 cm of carbon, assuming only Compton Scattering was dominant ?

for σ= 2 x 10-29 m2 per electron,

Carbon has 6 electrons, so σtot= 1.2 x 10-28 m2 per atom

ρ= 2000 kg/m3, NA = 6.022x1023 atom/mol, A=12 g/mol

μ= NAρ σtot /A = 12.7 m-1

I/I0 = exp(-μ x) = 0.28 for x=10 cm

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What is the transmission of 100 keV photons through the same thickness ?

For σ= 5 x 10-29 m2 per electron

I/I0 = exp(-μ x) = 0.0017

Gamma ray attenuation – example problem

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Refs:http://hepweb.rl.ac.uk/ppUKpics/POW/pr_000329.html

Example of particle tracks

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• The Lambert-Beer’s Law is the simplification of real world

• In practice particle transport has a stochastic nature • One of the powerful method for simulations of

particle transport is Monte Carlo simulation However, it is the topic of the following class.....

Conclusion

Dziękuję za uwagę,

Thank you for your attention,

Takk for oppmerksomhet,

Merci pour attention,

Дякую за увагу.