Particles and Radiation (AS)

Question 1

Define an Isotope

Answer 1

Isotopes are atoms with the same number of protons, but different number of neutrons.

Question 2

Define specific charge

Answer 2

Know as the charge to mass ratio, and it’s calculated by charge/mass.

Question 3

Briefly describe the strong nuclear force (4)

Answer 3

1. A force which keeps protons and neutrons together and maintains a stable nucleus.
2. It has a range of no more that 3 - 4 femtometres (same as a diameter of a nucleus).
3. Same effect between 2 protons as it does between 2 neutrons or a proton and a neutron
4. It’s an attractive force form 3 - 4 fm down to 0.5 fm. At separations smaller than this, it is a repulsive force that acts to prevent neutrons and protons being pushed into each other.

Question 4

Briefly describe alpha radiation (2)

Answer 4

1. Consists of alpha particles - Mass number is 4 and proton number is 2.
2. As an unstable nucleus emits an alpha particle, the nucleon number decreases by 4, and atomic number decreases by 2.

Question 5

Briefly describe beta radiation (6)

Answer 5

1. Consists of fast moving electrons
2. Charge is equal and opposite to that of a proton, but mass is considerably smaller than that of a proton.
3. This happens as a result of a neutron changing into a proton.
4. Beta particle is created when the change happens and is emitted instantly.
5. In addition, an anti-particle with no charge, called an anti-neutrino is also emitted.
6. Atomic number increases by 1, but nucleon number stays the same (neutron changing into a proton).

Question 6

Briefly describe gamma radiation (4)

Answer 6

1. Electromagnetic radiation emitted by an unstable nucleus
2. It can pass through thick metal plates
3. It has no mass and no charge
4. It is emitted by a nucleus with too much energy, following an alpha or beta emission

Question 7

What did Charles Wilson observe from his cloud chamber? (4)

Answer 7

1. X-rays left wide cloudy tracks inside chamber
2. Alpha particles (highly ionising) left broad, straight, definite length tracks
3. Beta particles left thin, straight or curved tracks
4. Weakly ionising gamma rays left nothing

Question 8

Briefly describe what the spark counter does? (4)

Answer 8

1. Only detects alpha particles, as β and γ radiation do not ionise enough of the air between the metal gauze and the thin wire underneath.
2. When the air particles are ionised by the alpha particles the charged particles produced cause a spark to be formed.
3. The spark jumps across the 500V gap between the gauze and the wire. The spark can be seen, heard and counted by an observer with a microphone.
4. This method shows that α particles have a short range in air.

Question 9

Describe what happens in the geiger-muller counter (4)

Answer 9

1. Ionising radiation enters the tube through a thin mica window, or through the window or sides of the counter.
2. The low pressure inert gas inside the detector is ionised producing a cascade of charged particles that are attracted to oppositely charge electrodes.
3. The small pulse of current produced is detected by the electronic counter, which registers a ‘count’
4. They can detect all 3 types of radiation

Question 10

What are anti-particles?

Answer 10

All particles have a corresponding anti-particle. They have the same mass, rest mass energy, but they have opposite properties such as charge.

Question 11

What is annihilation?

Answer 11

When a particle meets its corresponding antiparticle. This means that the total mass of the particle pair is converted into energy, in the form of 2 gamma ray photons.

Question 12

Why are 2 gamma ray photons always released during particle-antiparticle interaction?

Answer 12

In order to conserve momentum

Question 13

What is pair-production?

Answer 13

When a photon with enough energy can interact with large nucleus and be converted directly into a particle-anitparitcle pair.

Question 14

Describe electromagnetic waves

Answer 14

Consist of an electric wave and a magnetic wave which travel together and vibrate:

1. At right angles to each other and to the direction in which they are travelling
2. In phase with each other. As you can see the 2 waves reach a peak together so they are in step. When waves do this we say they are in phase.

Question 15

What are photons?

Answer 15

Electromagnetic waves are emitted as short bursts of waves, each burst leaving the source in a different direction.

Each burst is a packet of electromagnetic waves and is referred to as a photon.

Question 16

What is the photoelectric effect?

Answer 16

The emission of electrons from a metal surface when light is directed at the surface.

Question 17

What is the electromagnetic force?

Answer 17

The electromagnetic force between 2 charged objects is due to the exchange of virtual photons.
Feynman described them as “virtual” because we can’t detect them directly.

Question 18

What is the weak nuclear force?

Answer 18

The weak nuclear force causes:

1. A neutron to change into a proton in β- decay
2. A proton change into a neutron in β+ decay

In both β+ and β- decay, a new particle and a new antiparticle are created. One is an electron, and positron, and the other is a neutron or an antineutrino.

Question 19

Briefly describe the role of a neutrino in an interaction (2)

Answer 19

1. It can interact with a neutron and make it change into a proton.
2. A β- particle (electron) is created an emitted as a result of the change

Question 20

Briefly describe the role of an anti-neutrino in an interaction (2)

Answer 20

1. It can interact with a proton and make it change into a neutron.
2. A β+ particle (positron) is created and emitted as a result of the change.

Question 21

Give three main features of the exchange particles of W Bosons

Answer 21

Unlike photons these exchange particles:

1. Have a non-zero rest mass
2. Have a very short range of no more than about 0.001 fm
3. Are positively charged (the W+ Boson) or negatively charged (the W- Boson)

Question 22

What happens in β decay to the W Boson?

Answer 22

In each case, the W boson meets a neutrino or antineutrino, changing them into a β- or β+ particle.

Question 23

What happens if no neutron or antineutrino is present for the W Boson? (2)

Answer 23

1. The W+ boson decays into a β- particle and an antineutrino
2. The W- boson decays into a β+ particle and a neutrino

Question 24

What is electron capture?

Answer 24

When at times a proton in a proton-rich nucleus turns into a neutron as a result of interacting through the weak interaction with an inner-shell electron from outside the nucleus (The W+ boson changes the electron into a neutron).