Lecture 16 - Neutrons and Activation

ONCOL 243 - Radiation Safety. University of Alberta

Are neutrons charged or uncharged? How does this affect their penetration ability

Neutrons are uncharged particles, which allows them to penetrate materials more effectively than charged particles like protons or electrons, as they are not repelled by atomic nuclei.

Are neutrons direction or indirectly ionizing?

Neutrons are indirectly ionizing particles, as they do not ionize atoms directly but can cause secondary ionization through interactions with other particles.

What materials attenuate gamma particles?

Materials such as lead, concrete, and steel are effective at attenuating gamma particles due to their dense atomic structure, which absorbs and scatters the high-energy radiation.

what materials attenuate neutrons?

Materials such as hydrogen-rich substances

• such as water, plastics

can lead attenuate neutrons?

no

what type of collision occurs between neutrons and protons?

A neutron-proton collision results in an elastic collision, where kinetic energy and momentum are conserved

• neutron stops completely, proton goes forward

neutron mean lifetime

14 minutes 47 seconds

are neutrons essential for all nuclei?

no

• H-1 can exist without

what are neutron stars

neutron stars are the remnants of massive stars that have undergone supernova explosions, consisting almost entirely of neutrons and exhibiting incredibly high density and strong gravitational fields.

what are two ways a nucleus can be activated to become radioactive?

1. Photoactivation

   • photon hits nucleus and activates it

2. Neutron activation

   • neutrons hit nucleus and activate it

how are photoactivation and neutron activation related?

photoactivation can cause neutrons to be ejected, which can the participate in neutron activation

what is photoneutron production?

The process by which high-energy photons interact with a nucleus to produce neutrons, typically requiring energy above 10 MeV.

what is the energy requirement for photoneutron production?

E > 10 MeV

what are fast energy neutrons called

fast neutrons

what energies of LINAC generate fast neutrons

about 2 MeV

what energy do room scattered fast neutrons have?

100-400 keV.

what are low energy neutrons called?

thermal neutrons

why are they called thermal neutrons

They are called thermal neutrons because they are in thermal equilibrium with their surroundings, having low kinetic energy similar to that of the surrounding atoms.

energies of thermal neutrons

0.04 eV

what two things can (gamma, n) reactions be?

1. photoactivation

2. neutron production

what parts of the LINAC can be made radioactive (activated) by photon irridation?

wedges and compensators

how are wedges and compensators designed to reduce radioactive activation

they are designed with materials with low activation cross sections

after prolonged use of high energy photons (during commissioning), what should be done before entering the treatment room?

wait 10 minutes to let activation products decay before entering room

describe the process of neutron capture (n,y)

1. a fast neutron scatters loosing energy until it becomes a thermal neutron

2. thermal neutron is captured by certain nuclei

3. a gamma ray is emitted as the nucleus transitions to a higher energy state.

   • resulting nuclide is radioactive

Do neutrons scatter more or less than photons?

Neutrons scatter more than photons due to their larger mass and different interaction mechanisms with matter.

what do neutron attenuation and scatter depend on?

the neutron’s energy

what are the three steps of shielding neutrons

1. slow neutrons

2. absorb neutrons

3. absorb gamma rays

how are neutrons slowed

fast neutrons are slowed when scattered off materials which in hydrogen

• water, plastics, concrete

how are neutrons absorbed

with boron and cadmium

why are boron and cadmium used to absorb neutrons

they have high cross section due to resonance energies and other quantum mechanic principles

how are the resulting gamma rays absorbed

the normal way: with lead

what are put into vaults to shield from neutrons

1. long mazes

2. neutron door

what are neutron doors made out of

borated paraffin

• paraffin = plastic = water = slow neutrons

• borated = boron = capture neutrons

describe how photoactivation can occur in the air

are doses to radiation therapists from neutron activation high

no

• especially now with dynamic wedges and MLCs, therapists wont have to touch the activated materials

what can cause an increased buildup of radioactive gases in treatment vaults

improper air ventilation

how many air changes are needed to limit build-up of radioactive gases in the room

5-10 air changes per hour

what type of radiation treatment creates most activation products?

IMRT - intensity modulated radiation therapy

when should machine service personnel be aware of neutron activation

when dismantling or working inside the accelerator head

• exposure may be >1 mCi

what oxygen product may be created in electron mode

Ozone

what is the estimated skin dose from positrons for a therapist entering the room immediatly after a treatment for 5 minutes?

0.15-0.20 uSv

assuming 80 treatment fields per day, what is the annual dose a radiation therapist will recieve?