Radiation Interactions - Charged Particles

How to quantize number of interactions in a unit mass?

proportional to (C/v)²

C is charge, v is velocity

What is Bragg Peak?

function of Linear Energy Transfer (LET) → as particle slows down, energy deposition rate increases… Bragg Peak then is depth at which max energy (dose) is deposited, ocurs at end of path

What is CSDA?

Continuous Slowing Down Approximation is a shortcut to find particles range. “Average energy loss per unit path length”… CSDA range is when all energy is expended

What does a proton interaction do to an atom? (if interacting with electron and nucleus)

Ionizes it by kicking electron.

Changes element, if high enough energy (Z + 1) → Cyclotrons and Synchrotrons

Where is tumor located for a proton treatment?

At Bragg Peak, CSDA range

Do electrons exhibit Bragg Peak? Why/Why Not?

YES, but it is NOT observed in clinical practice. Because incident electron is scattering on bound electrons, so their random paths blur out the Bragg Peak in the patient

Range & Energy straggling?

different interaction histories, cause by tortuous paths of scattered electrons, gives variations in energies at depths → ENERGY straggling…

and because energies vary, so will penetrative power, so ranges vary as well → RANGE straggling

straggling is the variance in these quantities due to the tortuous paths through material

What interaction is the basis for how we generate x-rays?

Bremsstrahlung radiation

What causes Bremsstrahlung?

Electron decelerated in nucleus’ E-field radiates photon

Energy spectrum and average energy of Bremsstrahlung?

Large spectrum. E_Avg = 1/3 E_Max

Efficiency of Bremsstrahlung production?

1%, mostly heat… Probablity increases with Z of target material → Therapeutic energies are 20-30% efficient

Usefulness of alpha particles? Why?

radiopharmaceuticals. Because they’re so heavy, they pose little external concern → outer layer of dead skin is enough to stop

Charge of alpha particle?

+2

2 categories of neutron interactions?

1. Recoil reactions

   1. Shielding. Why concrete and BPE are good because hydrogenous (m_n=m_p)… little energy lost with higher Z materials

2. Nuclear activations

   1. absorbed neutrons create spontaneous or delayed radioactive decay

Why are neutrons effective at creating nuclear activations?

No charge, so E fields of bound electrons can’t screen neutrons

Will fat or muscle receive higher neutron dose?

Fat (~20% higher neutron dose than muscle)

→ because fat has a higher hydrogen count

What is Indirect Ionization? What sources are indirectly ionizing?

Photons and neutrons → their damage is produced from particles they create… e.g. photons scatter electrons & neutrons create beta, alpha, gamma from nuclear interactions

What is Direct Ionization? What sources?

alphas, betas, and protons → they inflict damage on their own

What atomic material damages DNA?

Free Radicals produced from ionizing radiation