2 radio-physics _ Xrays production and interaction with matter

What are examples of charged particles?

Electrons, positrons, protons and alpha particles.

How do charged particles mainly lose energy when travelling through matter?

Through electromagnetic interactions.

What do charged particles mainly interact with in matter?

Electrons and nuclei of the medium.

What determines the nature of a charged particle's interaction with matter?

Its charge, mass and incident energy.

What is an elastic collision?

A collision involving deflection of the particle's trajectory with negligible exchange of kinetic energy.

What happens to the particle's trajectory in an elastic collision?

It is deflected.

What happens to kinetic energy in an elastic collision?

There is negligible exchange of kinetic energy.

Do elastic collisions cause permanent atomic changes?

no

What is an inelastic collision?

A collision in which energy is transferred to the medium's electrons.

What can energy transfer to electrons cause on inelastic collisions

Excitation and ionization.

What is a radiative collision?

A collision in which a charged particle, when accelerated or decelerated by the nuclear field, emits electromagnetic radiation.

What is the electromagnetic radiation emitted during a radiative collision called?

Bremsstrahlung or braking radiation.

What causes bremsstrahlung?

The acceleration or deceleration of a charged particle by the nuclear field.

Do “impact” and “collision” often refer to the same phenomenon?

yes

What is an impact/collision?

An event in which two or more bodies or particles exert forces on one another over a relatively short time interval.

What can happen during an impact/collision?

There can be exchanges of energy and changes in the particles' trajectories.

What is bremsstrahlung?

Braking radiation produced when a charged particle is accelerated or decelerated by a nuclear field.

Why is bremsstrahlung particularly relevant to X-ray generation?

It produces electromagnetic radiation (photons) when electrons are decelerated.

What happens to a charged particle during bremsstrahlung?

It is deflected by the nuclear field and loses energy as electromagnetic radiation.

How does bremsstrahlung intensity change as particle mass decreases?

It increases.

How does bremsstrahlung intensity change as Z increases?

It increases.

What relationship for bremsstrahlung intensity is given?

I ∝ Z² · z² / m²

What does Z represent in the bremsstrahlung equation? (big Z)

The atomic number of the material.

What does z represent? ( small z)

The charge of the incident particle.

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Bremsstrahlung intensity gets bigger when Z or particle charge gets bigger, and gets much bigger when particle mass gets smaller.

1. Higher Z → more bremsstrahlung

If the electron passes near a material with a higher atomic number, bremsstrahlung production increases.

High Z → ↑ bremsstrahlung

That's why X-ray tubes use a high-Z target such as tungsten (Z = 74).

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2. Smaller particle mass → much more bremsstrahlung

Because mass is underneath the equation, a smaller mass means more bremsstrahlung.

Smaller mass → ↑↑ bremsstrahlung

This is one reason electrons are important for X-ray production.

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3. Higher particle charge → more bremsstrahlung

The particle's charge is also squared:

Higher z → ↑ bremsstrahlung

Bremsstrahlung likes HIGH Z and LOW MASS.

HIGH Z ↑ → more bremsstrahlung

LOW mass ↓ → more bremsstrahlung

HIGH charge ↑ → more bremsstrahlung

And because Z, z and m are squared, their effects are particularly strong.

An X-ray tube fires electrons at a high-Z tungsten target.

So you have:

low-mass electrons + high-Z tungsten

⬇

lots of bremsstrahlung

⬇

X-ray photons are produced

done

What is mass stopping power?

Stopping power used to compare different materials.

What is the symbol for mass stopping power?

S/ρ

Why is mass stopping power used?

To compare stopping power across different materials.

What is the range (R) of a particle?

The total distance travelled until the particle stops.

What happens to the projected range when a particle follows a zig-zag path?

The projected range is smaller than the total distance travelled.

How is range related to average stopping power?

Range is inversely proportional to average stopping power.

What happens in an X-ray tube?

lectrons accelerated through tens to hundreds of kV strike a high-Z anode (+)

What type of material is used for the anode?

high Z material like Tungsten

What happens when electrons strike the high-Z target?

Excitation, ionisation and bremsstrahlung occur.

What can excitation and ionisation in the target produce?

Secondary electrons and characteristic radiation.

When is characteristic radiation produced?

When inner-shell vacancies are filled.

How is bremsstrahlung produced in the anode?

Electrons decelerate in the nuclear field and emit radiation.

What happens to most of the electron-beam energy?

(bremsstrahlung)

It is deposited as heat.

How much of the electron-beam energy is converted into X-rays?

(Bremsstrahlung)

Only a small percentage.

What is a photon?

A particle of electromagnetic radiation, such as an X-ray.

Do photons have mass?

No

Do photons have electric charge?

no

Why don't photons undergo bremsstrahlung

Because photons have no electric charge.

Do photons continuously slow down as they travel through matter?

no

How do photons interact with matter?

Probabilistically through elementary processes

What is the diagnostic X-ray energy range given?

Approximately 20–120 keV.

What are the two dominant photon interactions in the diagnostic range?

Photoelectric effect and Compton scattering.

What happens to pair production in the diagnostic range?

It does not occur.

What are the two levels used to analyse photon interactions?

Macroscopic and microscopic.

What does the macroscopic level examine?

A beam containing millions of photons.

What does the microscopic level examine?

The interaction of a single photon.

What happens to X-ray beam intensity as it passes through matter?

The beam intensity decreases.

Why does X-ray beam intensity decrease in matter?

Because photons interact with the material.

What affects the probability of photon interaction?

The photon's energy.

What happens to photons when they interact with matter?

They undergo elementary interaction processes

What is attenuation?

Reduction in the number of photons as a beam travels through material due to absorption and scattering.

What causes attenuation of a photon beam?

Absorption and scattering.

What happens to the number of photons as a beam passes through material?

The number of photons decreases.

What does HVL stand for?

Half-value layer.

What is the HVL?

The thickness that reduces radiation intensity by half.

What is the equation for HVL?

HVL = ln(2)/μ

What fraction of radiation intensity remains after an HVL?

One half.

What does TVL stand for?

Tenth-value layer.

What is the TVL?

The thickness that reduces radiation intensity by a tenth.

What is the equation for TVL?

TVL = ln(10)/μ

In this section, μ (mu) means the linear attenuation coefficient.

Simply:

μ tells you how quickly the X-ray beam is attenuated as it passes through a material.

read

done

What happens to μ in real polychromatic beams?

varies with spectral energy

mean energy of x-ray when filtered increases because low energy photos are eliminated, so there are less photons but the ones that remain have more energy this means hardness/beam quality increases

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What happens to the beam with filtration?

the beam hardens

Is HVL constant in a filtered polychromatic beam?

no

What happens to beam quality with filtration?

Beam quality increases.

What happens to the mean energy of an X-ray beam when it is filtered?

The mean energy increases.

What happens to HVL and TVL when mean beam energy increases?

increases

What happens to low-energy photons during filtration?

They are eliminated more.

Why does the beam harden during filtration?

The low-energy photons are eliminated, increasing the mean energy.

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In the photoelectric effect, an X-ray photon transfers all of its energy to a bound electron, preferably one in an inner shell.

The electron is then ejected.

Its kinetic energy is:

Ee= E(squiggle) - Eb

So the electron's kinetic energy equals the energy of the incident photon minus the electron's binding energy.

The ejected electron leaves a vacancy. An electron from a more external shell falls into this vacancy. The atom then relaxes by producing either:

• a characteristic photon

The probability of the photoelectric effect:

• decreases rapidly as photon energy increases

• increases strongly with atomic number Z

• is proportional to the density of the medium

• is significant in biological tissues below approximately 30–40 keV

In radio diagnostics, the dependence on Z is approximately Z³.

done

What happens in the photoelectric effect?

The photon transfers all its energy to a bound electron.

Which electrons are preferably involved in the photoelectric effect?

Inner-shell electrons.

What happens to the electron after receiving the photon’s energy?

its ejected

the photon transfers all its energy to a bound electron which is ejected with what

kinetic energy equal to the energy of the incident photon minus the electron binding energy

What is created when the electron is ejected?

A vacancy

What happens to the vacancy?

it is filled by an outer electron

What happens when an outer electron fills the vacancy?

Atomic relaxation occurs

What can atomic relaxation produce?

A characteristic photon

How is characteristic radiation produced in the photoelectric effect?

An outer electron falls into the vacancy and the relaxation energy is emitted as a characteristic photon.

What happens to photoelectric probability as photon energy increases?

It decreases rapidly.

What happens to photoelectric probability as atomic number increases?

It increases.

What happens to photoelectric probability as the density of the medium increases?

It increases proportionally.

🟢 Photoelectric effect = GOOD for image contrast

In the photoelectric effect, the X-ray photon is completely absorbed by the electron.

So:

X-ray photon → completely absorbed → electron ejected

Because the photon is absorbed, it doesn't continue through to the detector.

This contributes to differential absorption, which favours image contrast. Your Text PDF specifically says that a high weight of the photoelectric effect favours contrast.

So remember:

PHOTOELECTRIC = absorption → GOOD contrast ✅

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🔴 Compton scattering = BAD for image quality

With Compton scattering, the photon doesn't disappear completely.

Instead:

X-ray photon → interacts with electron → lower-energy scattered photon + recoil electron

That scattered photon can go in a different direction and contributes to scattered radiation.

Your notes say that scattered photons reduce contrast and add veil to the image.

So:

COMPTON = scattering → BAD for contrast

done

What type of interaction is Compton scattering?

An inelastic interaction of a photon with a quasi-free electron.

What is a quasi-free electron?

An electron with very low binding energy.

Where are quasi-free electrons found in an atom?

In the outer shells.

What happens to the photon in Compton scattering?

It becomes a scattered photon with lower energy.

What happens to the electron in compton scattering

it becomes a recoil electron

What are the two products of Compton scattering?

A lower-energy scattered photon and a recoil electron.

An effective atomic number (Zeff) is used for compounds and mixtures, because materials such as tissues are not made from just one element.

It gives us one overall Z value that represents how the material behaves in terms of its interaction with photons at a given energy.

So, instead of having to consider every element in something like bone or soft tissue separately, we can use its effective atomic number to compare how it interacts with X-rays.

Higher Zeff → interactions increase → material appears more radiopaque (whiter) on the radiograph.

Lower Zeff → relatively more radiolucent → darker.

done

What is effective atomic number

A value defined for compounds and mixtures that reproduces, overall, the Z-dependence of photon interaction processes for a given energy.

Why is effective atomic number useful?

It allows tissues and radiopaque materials to be compared with pure elements.

What happens to photon interactions as Zef increases?

Interactions increase.

What does a higher Zeff generally mean on a radiograph?

The material is more radiopaque and appears whiter