Interactions with matter last part (1)

Pair Production steps

1. High voltage (1.022 meV) photon is absorbed by
nucleus
2. A pair (negatron and positron) are formed
3. The positron combines with an electron
4. The annihilation reaction takes place, creating
two photons of .511 MeV traveling in opposite
directions from one another.

Pair production

Occurs when an incoming photon has at least 1.022 MeV of energy and interacts with the nucleus of an atom, transferring all its energy to the nucleus. This results in the creation of two particles — a positron and a negatron (electron).

Energy threshold for pair production

1.022 MeV

Negatron behavior

Fills a vacancy in an orbital shell.

Positron behavior

Seeks an electron and interacts with it, resulting in annihilation.

Annihilation interaction

Occurs when a positron and electron collide, destroying each other and producing two photons.

Annihilation interaction energy results

Produces two photons traveling 180° apart, each with 0.511 MeV of energy.

Use of Annihilation Radiation in Positron Emission Tomography (PET)

PET imaging uses annihilation photons created by positron-electron interactions to form diagnostic images.

Source of positrons

PET radionuclides that undergo positron decay.

Process of positron decay

A proton in the nucleus transforms into a neutron, emitting a positron and a neutrino.

Formation of annihilation photons

Result of positron-electron annihilation; two 0.511 MeV photons emitted in opposite directions.

Photodisintegration

Occurs when a very high energy photon (10 MeV or greater) collides with the nucleus and transfers all its energy, making the nucleus unstable.

Photodisintegration result

The nucleus emits a fragment, usually a neutron.

Other particles emitted during photodisintegration

Proton, proton-neutron pair, or alpha particle may be released.