1.4 Particles and Antiparticles Flashcards
What happens when antimatter and matter particles meet
They destroy each other and radiation is released
What is utilised for a PET scan
The effect of the antimatter and matter particles destroying each other and releasing radiation
What does the P in PET stand for
Positron
Which is the antiparticle of the electron
What is a PET scan used for
Scanning the brain
What happens when a PET scanner is used
A positron-emitting isotope is administered to the patient and some of it reaches the brain via the blood system. Each positron emitted travels no further than a few millimetres before it meets an electron and they annihilate each other. Two gamma photons produced as a result are sensed by detectors linked to computers. An image is built
When does positron emission take place
When a proton changes into a neutron in an unstable nucleus with too many protons
What is the symbol of a positron
0
β
+1
Positron emission equation
A A 0
X –> Y + β + v
Z Z-1 +1
Do positron-emitting isotopes occur naturally
No
They are manufactured by placing a stable isotope in liquid or solid form, in the path of a beam of protons
Some of nuclei in substance absorbs extra protons and become unstable positron-emitters
When was antimatter predicted
1928 by English physicist Paul Dirac
Equation showing the relationship between the energy supplied to the particle to its increase in mass.
E = mc^2
Einstein said that the mass of a particle whe it is stationary (its rest mass m0), corresponds to…
…rest energy m0c^2 locked up as mass
Dirac’s theory of antiparticles predicted that for every type of particle there is a corresponding antiparticle that:
Annihilates the particle and itself if they meet, converting their total mass into photons
Has exactly the same rest mass as the particle
Has exactly opposite charge to the particle if the particle has charge
Who predicted the opposite process of pair production
Dirac
What is the process of pair production
It was predicted that a photon with sufficient energy passing near a nucleus or an electron can suddenly change into a particle-antiparticle pair which would then separate from each other
How is the energy of a particle or antiparticle often expressed
In millions of electron volts (MeV)
Where 1 MeV = 1.60 * 10^-13J
What is 1MeV equal to
1.60 * 10^-13J
When does annihilation occur
When a particle and a corresponding antiparticle meet and their mass is converted into radiation energy
What is produced in the process of annihilation
Two photons (as a single photon cannot ensure a total momentum of zero after the collision)
Minimum energy of each photon produced, hf min
= E0
Where E0 is the rest energy of the particle
In pair production, a photon creates…
… a particle and a corresponding antiparticle, and vanishes in the process
Minimum energy of photon needed =
hf min = 2E0
Eg. electron has rest energy of 0.511MeV Therefore for pair production of an electron and positron from a photon:
Min energy of photon = 2 * 0.511 = 1.022 MeV = 1.64 *10^-13 J
A photon with less energy could not therefore create a positron and electron
When can a positron and electron be made
If a photon has more energy than the minimum energy of photon needed
How can we see the path of alpha and beta particles
Using a cloud chamber
What is a cloud chamber
A small transparent container containing air saturated with vapour and made very cold
The same conditions exist high in the atmosphere
What happens in a cloud chamber
Ionising particles leave a visible trail of liquid droplets when they pass through the air - just like a jet plane does when it passes high overhead on a clear day.
Positive particles in a cloud chamber when there is a magnetic field applied
A positive particle would be deflected by the magnetic field in the opposite direction to a negative particle travelling in the same direction
The slower it went, the more it would bend
