Radioactivity

Question 1

What did rutherford’s experiment consist of?

Answer 1

Firing a narrow beam of alpha particles at a thin metal foil at same kinetic energy and measuring the angle of deflection.

Question 2

What were the observations of the experiment?

Answer 2

1) Most alpha particles passed through the sheet without being deflected.
2) A small percentage of alpha particles deflected by angles greater than 90 degrees.

Question 3

What did the rutherford observations prove?

Answer 3

1) Most of the atom’s mass is concentrated in the nucleus, a small region at the centre of the atom
2) Nucleus is positively charged as it repels alpha particles.

Question 4

What is the deflection of alpha particle proportional to

Answer 4

1/distance squared. Coulomb’s law.

Question 5

Why does the metal foil have to be very thin in rutherford experiment?

Answer 5

So that alpha particles aren’t scattered more than once.

Question 6

Give magnetic field deflections for radioactive particles

Answer 6

Alpha and beta deflected in opposite directions. Gamma isn’t deflected. Beta deflected more than alpha.

Question 7

Give order of strength of ionisation.

Answer 7

Alpha most ionising, then beta, then gamma, as gamma has no charge.

Question 8

What is the radiation absorbed by?

Answer 8

Alpha - paper and thin metal foil. Beta - 5mm of metal. Gamma - several cm’s of lead.

Question 9

What is the range of air of radiation?

Answer 9

alpha - few cm’s, depends on kinetic energy. Beta - Up to 1m. Gamma - unlimited. Follows the inverse square law.

Question 10

What needs to be subtracted from count rate?

Answer 10

Background count rate.

Question 11

What does gamma radiation follow?

Answer 11

An inverse square law

Question 12

When is gamma radiation emitted?

Answer 12

If nucleus has any excess energy

Question 13

Why is ionising radiation harmful?

Answer 13

Damages living cells by destroying cell membrane which causes cells to die and also by creating free radicals which react with DNA molecules. Damaged dna can cause cells to divide and grow uncontrollably, leading to cancer.

Question 14

What does ionisation level depend on?

Answer 14

Source of ionisation and dose received. Alpha much more ionising, but outside of body isn’t as strong as it would be inside the body. Absorbed by the skin.

Question 15

What is background radiation?

Answer 15

Radiation which occurs naturally due to cosmic radiation and from radioactive materials in rocks, soil and air.

Question 16

Give 4 examples of background radiation.

Answer 16

Air (radon gas), medical, grounds/buildings and food/drink.

Question 17

Give 3 methods of safe handling radioactive sources.

Answer 17

Keep in lead boxes, handle using tongs as far away from body as possible and minimise exposure time of body to the source.

Question 18

What does a decay curve show us?

Answer 18

The mass of a radioactive isotope against time. Mass decreases exponentially.

Question 19

What is the half life of a radioactive isotope?

Answer 19

The average time taken for the mass of the isotope to decrease to half it’s initial mass. Average as decay is a random process.

Question 20

What type of process is radioactive decay?

Answer 20

A random process, as number of nuclei that decays is proportional to the number of nuclei of X remaining.

Question 21

What is the definition of the activity of a source?

Answer 21

The number of nuclei of the isotope that disintegrates per second. The rate of change of number of nuclei of isotope.

Question 22

How does an unstable nucleus become stable?

Answer 22

By emitting an alpha/beta particle. This is unpredictable.

Question 23

What is the decay constant?

Answer 23

The probability of an indiviudal nucleus decaying per second.

Question 24

What is formula for half - life?

Answer 24

ln2/decay constant. Set N/N0 to 0.5 and solve using logs to prove this.

Question 25

What type of graph is used to study nuclear stability?

Answer 25

An N-Z graph.

Question 26

Give properties of the N-Z graph.

Answer 26

straight for light isotopes up to z values of 20. Beyond z=20, line curves up, neutron/proton ratio increases. Extra neutrons help to bind the nucleus together without introducing repulsive electrostatic forces as more protons would do. Beyond z= 60 there are only alpha emitters. Too large to be stable and strong nuclear force unable to overcome force of repulsion between protons.

Question 27

Where are the b minus and b+ emitters lie of n-z graph.

Answer 27

b minus on left as they are neutron rich and become stable by converting a neutron into a proton. B plus on the right hand side as they are proton rich and become stable by converting a proton into a neutron.

Question 28

Why does b + emission not happen often after alpha emission.

Answer 28

Alpha emission is occurring because the nucleus is very neutron rich. beta plus would increase neutron number as a proton is being converted into a neutron, so this would move the nucleus away from the stability belt.

Question 29

What is the main way of measuring nuclear radius?

Answer 29

Firing a beam of high energy electrons accelerated through a large potential difference at a thin solid sample of an element and observing the diffraction pattern produced.

Question 30

Why is a diffraction pattern produced?

Answer 30

The de broglie wavelength of the high energy electrons is of order 10^-15, which is the same as the diameter of the nucleus.

Question 31

What is the wavelength of the the high energy electrons

Answer 31

lamda = hc/E

Question 32

What is nuclear volume proportional to? What can we deduce from this?

Answer 32

mass of the nucleus. Density of nucleus is constant, independent of radius and is the same throughout the whole nucleus.

Question 33

When could a nucleus emit a gamma photon?

Answer 33

If daughter nucleus after emitting a beta or alpha particle still has some excess energy, and is formed in the excited state, it can release a gamma photon to move to it’s ground state either directly or via one or more lower energy excited states.

Question 34

How does carbon dating work?

Answer 34

Living plants take up carbon dioxide through photosynthesis. Small percentage of this is Carbon-14. Once tree dies, proportion of carbon-14 decreases over time as the carbon-14 nuclei decay. Measuring activity of sample, we can calculate the age of the tree.

Question 35

How does argon dating work?

Answer 35

Ancient rocks contain trapped argon as a result of the decay of radioactive isotope of potassium. By measuring activity we can measure the age of the rock sample.

Question 36

What is a radioactive tracer?

Answer 36

They are used to follow the path of a substance through a system. Half - life should be stable enough for necessary measurements to be made and short enough to decay quickly after use. It should emit beta or gamma radiation so it can be detected outside the flow path.