3.2.1.3 Particles and Radiation: Particles, antiparticles and photons

Question 1

What is c?

Answer 1

The speed of light - 3.00 * 10^8 ms^-1

Question 2

How to calculate the wavelength of light?

Answer 2

lambda = c/f

Question 3

What does an electromagnetic wave consist of?

Answer 3

An electric wave and magnetic wave which travel together and vibrate at right angles to each other in the direction in which they are travelling and in phase with each other.

Question 4

Describe the em spectrum (longest wavelength to shortest)

Answer 4

Radio waves/ microwaves/ infrared/ visible light/ uv light/ x-rays/ gamma rays

Question 5

How are em waves released?

Answer 5

In discrete packets (quanta) - Einstein suggested that EM waves and the energy they carry can only exist in discrete packets, called photons/

Question 6

What is the formula for the energy carried by a photon?

Answer 6

E=hf
(Energy of one photon (J) = Planck’s constant (6.63*10^-34) * frequency of light (Hz))

Question 7

What is the other formula for the energy carried by a photon (in terms of wavelength)?

Answer 7

E= (hc)/lambda

Question 8

What is an antiparticle?

Answer 8

Each particle has a corresponding antiparticle with the same mass and rest energy but with opposite charge.

Question 9

What is the rest energy of the proton/ antiproton?

Answer 9

938.3 MeV

Question 10

What is the rest energy of a neutron/ antineutron?

Answer 10

939.6 MeV

Question 11

What is the rest energy of an electron/ positron?

Answer 11

0.511 MeV

Question 12

What is the rest energy of a neutrino/ antineutrino?

Answer 12

0

Question 13

What is pair production?

Answer 13

Einstein’s famous theory states that energy can turn into mass and vice versa, when energy is converted into mass, you get equal amounts of matter and antimatter (pair production).

Question 14

Only when can pair production happen?

Answer 14

Pair production only happens when there is enough energy to produce the masses of both particles.
- minimum energy needed is the total rest energy of the particles that are being produced.

Question 15

What is produced during pair production?

Answer 15

A particle and its corresponding antiparticle because certain quantities must be conserved.

Question 15

What can be produced during pair production (examples)?

Answer 15

Proton/ antiproton
Electron/positron - tends to happen when a photon passes near a nucleus

Question 16

How do the particles in pair production move after being produced?

Answer 16

They curve away from each other in opposite directions as they are in an applied magnetic field and have opposite charges

Question 17

What is the minimum energy needed for pair production to happen?

Answer 17

Emin = 2E0

Question 18

What is more likely to be produced in pair production?

Answer 18

Electron/positron pair because they have relatively low mass, so low rest energy, so less energy needed for pair production to happen

Question 19

What is annihilation?

Answer 19

When a particle meets its antiparticle. All the mass of the particle and antiparticle gets converted back to energy in the form of two gamma ray photons.

Question 20

Why don’t you get antiparticles in ordinary matter?

Answer 20

They only exist for a fraction of a second before annihilation happens.

Question 21

How can you calculate the minimum energy of a photon produced by annihilation?

Answer 21

Interaction is between particle/antiparticle pair, so two photons would need to have a total energy of at least 2E0 for energy to be conserved in this interaction,
so Emin=E0

Question 22

How do positron emission tomography scanners work?

Answer 22

A positron emitting isotope is put into the bloodstream and the gamma rays produced by the electron-positron annihilation are detected. The gamma rays produced are always produced in pairs moving in opposite directions so they’re easy to distinguish from other gamma rays. Radiation is detected by a scintillator.