Particles and Antiparticles

Question 1

What happens when a particle and antiparticle meet?

Answer 1

they annihilate and release two high energy photons (gamma)

Question 2

How is this used in PET scanners?

Answer 2

P stands for positron

patients ingests a positron emitting isotope, it meets an electron and emits gamma photons and it is sensed by detectors

Question 3

How are positron emitters formed

Answer 3

stable isotope, in liquid or solid form, placed in front of a beam of protons

some of the nuclei absorb extra protons and become unstable positron emmiters

Question 4

What are the properties of antiparticles?

Answer 4

Dirac’s theory predicted that every particle has an antiparticle

that annihilate when they meet

has the same rest mass

has equal but opposite charge

Question 5

What is pair production?

Answer 5

producing a particle and a antiparticle from photons

Question 6

How is the mass formed?

Answer 6

the energy of the photon is converted into mass

Question 7

What is the minimum energy of photon produced?

Answer 7

rest mass of particle + antiparticle = 2 x rest mass of particle

Question 8

Explain why two photons are formed incstead of a single photon when a helium nucleus annihilates with the anti-helium nucleus?

Answer 8

stationary particle

momentum before annihilation = 0

therefore the momentum after annihilation has to = 0

two photons are released in opposite directions in order to conserve momentum