lec 4 radiation interactions

In charged particle collisions, both _________ and _________ are conserved.

Momentum and kinetic energy

In charged particle collisions, the atomic electron is considered what in the equation?

At rest

How much energy (%) can an electron deposit in one collision? What does this tell us about its travel path?

100%, which tells us that electrons can have torturous paths.

How much energy (%) can an alpha particle deposit in one collision? What does this tell us about its travel path?

0.055%, which tells us that alpha particles travel very straight.

What is stopping power and what does it depend on?

The average rate of energy loss of a charged particle in a medium. It depends on the type of particle, its energy, and the medium traversed.

What is the equation for Stopping Power? What is the unit for this equation?

-dE/dx = uQavg

*the u is the attenuation coefficient (the average distance between collisions)

Unit: Measured in units of energy/distance or MeV/cm

What is stopping power often times referred to as? We are familiar with this term already.

Linear Energy Transfer

Since beta particles don't just lose energy in collision, what can they also lose energy as?

Radiative stopping power

What is radiative stopping power?

When a beta particle interacts with the nucleus of an atom and are deflected, giving up some of their energy as a photon (Bremsstrahlung)

A photon is emitted when considering radiative stopping power in beta particles. What can the energy of this photon range from?

It can range from zero to the energy of the affected beta particle.

What is bremsstrahlung yield?

The fraction of energy that is produced when an electron is fully stopped by a material.

*also kinda just know that in the bremsstrahlung yield equation, the Z is the atomic number of the absorber and T is the electron energy.

How do X-ray devices work?

They work by firing electrons at a target. The bremsstrahlung photons created from the interaction of the electrons with a specific target are then directed toward the area of concern.

In X-ray creation, what is the maximum energy that can be achieved?

The energy of the emitted electron, which can be controlled via magnetic pulses.

What is charged particle range?

The distance it travels before coming to rest.

If a charged particle has a high stopping power, what does this mean about its range?

It will have a shorter range

What is the equation for alpha particle range in air?

R (cm) = 0.56E if E is less than 4 MeV

R (cm) = 1.24E - 2.62 if E is greater than 4 MeV

What is the equation for beta particle range in air?

R (g/cm^2) = 0.412T^(1.27-0.0954ln(T) if E is within 0.1 and 2.5 MeV

R(gm/cm^2) = 0.530T - 0.106 if E is greater than 2.5 MeV

Explain positron interaction.

The positron is traveling through the body and loses energy as it interacts with atoms (collision/interactions and radiation.)

The positron eventually runs out of energy.

It meets an electron and they attract each other.

They annihilate each other.

This creates two 511 keV photons, which are emitted at exactly 180 degrees.

What is Bragg Peak?

The point where the rate of energy loss is at a maximum and then quickly falls off.

What is Bragg Peak extremely important for?

External therapy

What is the typical energy level for alpha emission? (MeV)

5 MeV

What is the typical energy level (range) for beta emission?

5 keV to several MeV

Radiation that cannot penetrate the skin is not considered to be an external hazard. What is the thickness of the epidermal layer of the skin?

7 mg/cm^2

Do alpha emitters require much shielding?

No. A piece if paper is enough to shield most emissions, and wood is always sufficient.

What are beta particles generally shielded with?

Thick plastic or plexiglass

What are photons in regard to the electromagnetic spectrum?

Uncharged electromagnetic radiation.

Since photons are uncharged, what can they do?

They do not interact via the coulomb force, so they can travel long distances without interacting. (Which makes them ideal for imaging!)

What are the types of photon interaction and when are they typically dominant?

The photoelectric effect which is dominant for low-energy photons and high Z absorbers.

Compton scattering which is dominant in the middle ranges.

Pair production which is dominant at high energies. (used in PET)

What is the minimum photon energy required for pair production to occur?

1.022 MeV (guess what 1.022 divided by two is? 511 keV!!!!)

What happens in the photoelectric effect?

The complete absorption of a photon and the ejection of an electron.

What happens in Compton scattering?

Photon flies in with energy.

It hits an electron and gives it some of its energy.

The electron gets knocked away. (Compton electron)

The scattered photon changes direction and keeps going with less energy.

*Think of a pool ball hitting another ball

What happens to any extra energy in pair production?

It will be transferred to the kinetic energy of the electron and positron

What happens in Coherent scattering?

Very low energy photons interact. The resulting photon changes direction but does not lose energy. It is basically just deflected. This happens either through Thomson or Raleigh scattering!

What is the difference between Thomson and Raleigh scattering in regards to coherent scattering?

Thomson = photon scatters of a single electron

Raleigh = photon scatters off an entire atom

What is photonuclear absorption?

In very high energy photons, the photon is fully absorbed and a NEUTRON is ejected from the NUCLEUS.

Do photons have a definite range like charged particles?

No. Photons can travel different distances before interacting.

How do we measure photon shielding?

Compare photon intensity before and after passing through a material.

What is attenuation?

The decrease in photon intensity as photons pass through a material.

What is the linear attenuation coefficient?

A measure of how strongly a material reduces photon intensity.

How can we find linear attenuation coefficient if given the mass attenuation coefficient?

Multiply it by the density of the material.

Attenuation is often classified by two different coefficients: the energy transfer coefficient and the energy absorption coefficient. What do each of these measure?

Energy transfer coefficient measures how much energy is transferred to electrons through all forms of interactions.

Energy absorption coefficient measures how much energy is absorbed, mainly through the photoelectric effect.

What is Buildup?

Extra radiation caused by scattered photons in a material

Why does buildup happen?

Radiation is emitted in all directions and must be accounted for, so it is known as buildup.

Half-value layer has to do with _______ shielding.

Photon

What is Half-Value layer? Its equation?

The distance in a material that will reduce the intensity by half. HVL = ln(2)/u

*Tenth-value layer is also used and occasionally even hundredth-value layer can be seen.