1.4 Particles and Antiparticles

Question 1

What happens when antimatter and matter particles meet

Answer 1

They destroy each other and radiation is released

Question 2

What is utilised for a PET scan

Answer 2

The effect of the antimatter and matter particles destroying each other and releasing radiation

Question 3

What does the P in PET stand for

Answer 3

Positron

Which is the antiparticle of the electron

Question 4

What is a PET scan used for

Answer 4

Scanning the brain

Question 5

What happens when a PET scanner is used

Answer 5

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

Question 6

When does positron emission take place

Answer 6

When a proton changes into a neutron in an unstable nucleus with too many protons

Question 7

What is the symbol of a positron

Answer 7

0
β
+1

Question 8

Positron emission equation

Answer 8

A A 0
X –> Y + β + v
Z Z-1 +1

Question 9

Do positron-emitting isotopes occur naturally

Answer 9

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

Question 10

When was antimatter predicted

Answer 10

1928 by English physicist Paul Dirac

Question 11

Equation showing the relationship between the energy supplied to the particle to its increase in mass.

Answer 11

E = mc^2

Question 12

Einstein said that the mass of a particle whe it is stationary (its rest mass m0), corresponds to…

Answer 12

…rest energy m0c^2 locked up as mass

Question 13

Dirac’s theory of antiparticles predicted that for every type of particle there is a corresponding antiparticle that:

Answer 13

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

Question 14

Who predicted the opposite process of pair production

Answer 14

Dirac

Question 15

What is the process of pair production

Answer 15

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

Question 16

How is the energy of a particle or antiparticle often expressed

Answer 16

In millions of electron volts (MeV)

Where 1 MeV = 1.60 * 10^-13J

Question 17

What is 1MeV equal to

Answer 17

1.60 * 10^-13J

Question 18

When does annihilation occur

Answer 18

When a particle and a corresponding antiparticle meet and their mass is converted into radiation energy

Question 19

What is produced in the process of annihilation

Answer 19

Two photons (as a single photon cannot ensure a total momentum of zero after the collision)

Question 20

Minimum energy of each photon produced, hf min

Answer 20

= E0

Where E0 is the rest energy of the particle

Question 21

In pair production, a photon creates…

Answer 21

… a particle and a corresponding antiparticle, and vanishes in the process

Question 22

Minimum energy of photon needed =

Answer 22

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

Question 23

When can a positron and electron be made

Answer 23

If a photon has more energy than the minimum energy of photon needed

Question 24

How can we see the path of alpha and beta particles

Answer 24

Using a cloud chamber

Question 25

What is a cloud chamber

Answer 25

A small transparent container containing air saturated with vapour and made very cold
The same conditions exist high in the atmosphere

Question 26

What happens in a cloud chamber

Answer 26

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.

Question 27

Positive particles in a cloud chamber when there is a magnetic field applied

Answer 27

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