Nuclear Physics

Question 1

Rutherford alpha particle scattering experiment

Answer 1

Rutherford bombarded gold foil with positively charged alpha particles. Most of the alpha particles passed straight through the gold atoms. A few particles were deflected. Very occasionally an alpha particle bounced back in the direction it came from. This lead to the conclusion that an atom had a small, dense positively charged nucleus at its centre and that most of the rest of an atom was empty space.

Question 2

conclusions from rutherford scattering

Answer 2

1. the nuclei must be positive to repel the positively charged alpha particles
2. the nuclei must be small since very few alpha particles are deflected by large angles - most just pass straight through the foil.
3. most of the mass and positive charge must be contained within the nucleus because most alpha particles pass straight through and only the few that pass close to the nucleus are deflected by large angles

Question 3

closest approach of a scattered particle

Answer 3

it does this at the point where its electrical potential energy equals its initial kinetic energy.

Question 4

Alpha (a) decay

Answer 4

An alpha particle is 2 protons and 2 neutrons (i.e. a helium nucleus). When an atom emits an alpha particle its proton number reduces by 2 so it becomes a different element. Alpha particles are positively charged and so can be deflected by electric and magnetic fields.

Question 5

Beta (b–) decay

Answer 5

A b– particle is an electron so it is negatively charged so it can be deflected by electric and magnetic fields. During b– decay a neutron is converted into a proton and an electron and an antineutrino. This increases the proton number by one making the atom a different element.

Question 6

Gamma (g) radiation

Answer 6

Alpha and beta decay can produce a daughter nucleus in an excited state. The daughter nucleus can decay by emitting a gamma ray (i.e. a photon of electromagnetic radiation). Gamma rays have no charge and so are unaffected by electric and magnetic fields.

Question 7

How to identify gamma radiation using simple absorption experiments

Answer 7

gamma ray intensity is halved by 10cm of lead

Question 8

How to identify alpha radiation using simple absorption experiments

Answer 8

alpha particles are stopped by a few cm of air or a thin sheet of paper

Question 9

How to identify beta radiation using simple absorption experiments

Answer 9

beta particles are stopped by a few mm of aluminium

Question 10

Identify gamma using simple electric or magnetic fields

Answer 10

gamma rays are not deflected at all

Question 11

Identify alpha using simple electric or magnetic fields

Answer 11

alpha particles are deflected in the same direction as other positively charged particles

Question 12

Identify beta using simple electric or magnetic fields

Answer 12

beta particles are deflected in the opposite direction to alpha particles

Question 13

Safe handling of radioactive sources

Answer 13

1. always handle with long (30cm) tweezers 2. always return to the container when finished with to limit exposure time 3. point sources away from the body and handle at arms length

Question 14

Relative hazards of exposure to humans

Answer 14

Alpha, beta and gamma radiation can ionise atoms in the body. Alpha particles are the most ionising and gamma rays are the least ionising. Alpha particles are the least penetrating and gamma rays are the most penetrating. alpha particles outside the body can be stopped by cloths / skin and are relatively harmless, alpha particles inside the body are very harmful as they are the most ionising outside the body beta particles and gamma rays are more harmful than alpha particles as the can penetrate into the body.

Question 15

Background radiation

Answer 15

Radiation detectors detect radiation even when no radioactive sources are present. This is because of background radiation which is always present. Background radiation comes from; cosmic rays natural radioactive material in rocks / soil radon gas a small amount comes from medical sources, nuclear power stations and nuclear test Before an experiment the background radiation should be measure so that it can be deducted from the measurements made in the experiment to correct for background radiation.

Question 16

Inverse square law for g radiation

Answer 16

If you double the distance from a gamma ray source then the intensity of the radiation will be one quarter what it was before (this is an inverse square law)

Question 17

Radioactive Decay

Answer 17

If an atom is not stable, it will break down to become more stable. the atom decays by releasing energy, until it reaches a stable form - this is called radioactive decay and it is a random process. You cannot predict when an individual nucleus will decay but with large numbers of nuclei you can use a statistical approach.

Question 18

Activity

Answer 18

The activity of a sample is the average number of disintegrations per second its unit is the becquerel (Bq). One becquerel is one decay per second. DN = change in number of undecayed nuclei Dt = change in time in seconds

Question 19

Decay constant

Answer 19

The decay constant is the probability that a nucleus will decay per second so its unit is s-1. activity = decay constant x the number of undecayed nuclei

Question 20

initial kinetic energy is equal to electrical potential energy

Answer 20

it does this at the point where its electrical potential energy equals its initial kinetic energy. this is just conservation of energy - and if you know the initial kinetic energy of the alpha particle you’ve fired, you can use it to find how close the particle can get to the nucleus.

Question 21

nucleons

Answer 21

the particles that make up the nucleus are called nucleons

Question 22

Can you draw a graph show nuclear radius and nucleon number

Answer 22

Question 23

radius of nucleus (r) is ____________

Answer 23

directly proportional to A1/3

Question 24

the relation of r and A1/3

Answer 24

Question 25

is the volume of proton different from neutron?

Answer 25

they are about the same

Question 26

if the volume of proton is no different from neutron, they have a _______

Answer 26

similar density

Question 27

nuclear density is significantly larger than atomic density - this suggests three things

Answer 27

1. most of an atom’s mass is in its nucleus
2. the nucleus is small compared to the atom
3. an atom must contain a lot of empty space