Year 2 Chapters 10 + 11: Radioactivity

Question 1

What was the alpha scattering/ Rutherford scattering/ gold foil experiment?

Answer 1

Alpha source directed at a thin layer of gold foil in a vacuum with a fluorescent screen around them.
Alpha particles fired at the gold foil and were deflected, most passing straight through but some deflected at angles of less than or more than 90 degrees.

Question 2

What conclusions were drawn from the Rutherford scattering experiment?

Answer 2

Atom is mostly empty space- most alpha particles passed straight through with no deflection
Positive nucleus- positive alpha particles were deflected off course which was from repulsive force from positive nucleus
Very small and dense nucleus- very small proportion of alpha particles were deflected so nucleus is much smaller than atom (and its empty space). Alpha particles were deflected and did not move the nucleus, so it is heavier than an alpha particle

Question 3

What were the beliefs about the atom before and after Rutherford’s scattering experiment?

Answer 3

Before- JJ Thomson’s plum pudding model had a cloud of positive charge with negative electrons embedded throughout it
After- Ernest Rutherford’s nuclear model had a small, dense, positively charged nucleus surrounded by negative charge

Question 4

What is alpha decay and what are some properties?

Answer 4

Emission of an alpha particle, helium nucleus from an unstable nucleus to make it more stable. It is strongly ionising due to 2+ charge and is stopped by a few cm of air or paper.

Question 5

What is beta decay and what are some properties?

Answer 5

Emission of a beta particle (fast moving electron) when a proton turns into a neutron or vice versa in an unstable nucleus. They are weakly ionising due to 1- charge and are stopped by ~1m of air or a few mm of aluminium.

Question 6

What is gamma decay and what are some properties?

Answer 6

Emission of gamma rays from an unstable nucleus with too much energy. It is very weakly ionising and has an infinite range in air, obeying inverse square law. Stopped by several m concrete or a few inches lead.
Gamma rays/photons have no charge or mass

Question 7

What are the steps to identifying the type of radiation emitted by a source?

Answer 7

-Use GM tube and counter to find background count rate
-Place source near GM tube and measure count rate
-Place sheet of paper between source and GM tube and see if count rate significantly drops (alpha)
-Repeat with aluminium sheet and lead to check for beta or gamma radiation too

Question 8

How can all three radiation types be used in factories?

Answer 8

Can be used to monitor thickness of certain materials during production
Factories producing paper, Al sheets or steel sheets can place an α, β or γ source and a detector either side of the material being produced
If material is too thick or thin, count rate will change accordingly and so process can be adjusted to produce correct thickness

Question 9

How is gamma radiation used in medicine?

Answer 9

Detector-gamma sources with an appropriate half life can be injected or ingested into a patient and gamma cameras can trace the source to diagnose patients
Sterilisation- can sterilise surgical equipment by killing bacteria
Radiotherapy- gamma radiation can kill cancerous cells in a targeted region but will also kill healthy cells

Question 10

What is an appropriate half-life for using gamma tracers?

Answer 10

Not too short so that it will be able to be picked up my the gamma cameras for long enough
Not too long so that it will not remain in the patient’s body for too long after the procedure

Question 11

What is the inverse square law?

Answer 11

The law that governs the intensity of gamma radiation. Intensity is inversely proportional to the square of the distance from its source because it spreads out in all directions equally.
I = k/x^2

Question 12

How can the inverse square law for gamma radiation be verified?

Answer 12

Using a gamma source and GM tube and counter (required practical), making sure to account for background radiation and taking repeats

Question 13

What are some dangers of radiation?

Answer 13

Alpha and beta radiation are ionising, and can ionise body tissue. Alpha is particularly dangerous is ingested or inhaled, long exposure to gamma can cause mutations and damage to cells

Question 14

What are some safety precautions taken when using a radioactive source?

Answer 14

-Using long handled tongs to handle source
-Keep source in appropriate container
-Keep source as far away from everyone as possible, don’t point at people
-Have shielding, ie lead apron
-If dropped check for leak, wash hands after use, etc.

Question 15

What is background radiation?

Answer 15

Radiation found in small quantities all around us coming from a mixture of natural and man-made sources

Question 16

What are some sources of background radiation?

Answer 16

Radon gas released by rocks
Cosmic rays from space
Nuclear weapons and breakdowns
Medical sources
Radioactive isotopes in rocks

Question 17

How does radioactive decay occur?

Answer 17

Radioactive decay is a random process meaning you cannot predict when the next decay will occur

Question 18

What is λ in radioactive decay?

Answer 18

The decay constant/ constant decay probability which is the probability of a nucleus decaying per unit time

Question 19

How does the number of radioactive nuclei change over time?

Answer 19

It is an exponential decay meaning the time taken for the number of radioactive nuclei in a sample to halve is constant which is the half-life

Question 20

What is half-life?

Answer 20

Half-life (T 1/2) is the average time it takes for the number of radioactive nuclei in a sample to halve

Question 21

How are the half-life and decay constant related?

Answer 21

T 1/2 = ln2/λ

Question 22

What is activity?

Answer 22

The rate of decay of radioactive nuclei in a given isotope

Question 23

How are activity and number of radioactive nuclei related?

Answer 23

A=λN, activity is proportional to number of radioactive nuclei and the decay constant is the constant of proportionality. A measured in Becquerels, Bq

Question 24

How is the decrease in activity linked to half-life?

Answer 24

As activity is proportional to N, the time taken for activity to halve is equal to the half-life

Question 25

When is the decay constant an accurate model?

Answer 25

When there is a large number of nuclei in the sample because it models nuclear decay by statistical means as it is random, so is more accurate with a greater number

Question 26

How does radioactive dating work?

Answer 26

Radioactive isotopes with known half-lives can be used to date objects. eg carbon-14 can date organic objects by measuring the amount of carbon-14 in it and comparing it to the initial amount which is roughly the same in all living things

Question 27

How do activity and half-life affect waste storage?

Answer 27

Radioactive waste has to be stored appropriately, eg waste with a long half-life might have to be stored in steel casks underground to stop the waste from damaging the environment and people in that area far into the future

Question 28

Why can nuclei be unstable?

Answer 28

Nuclei are held together by the strong nuclear force but protons repeal each other due to the electromagnetic forces, so if the forces are out of balance the nucleus will be unstable and so decay

Question 29

What is the graph of N against Z for all nuclei?

Answer 29

A graph showing the stable and unstable isotopes of all elements with the number of neutrons N on the y axis and number of protons Z on the x axis. There is a line of N=Z, a line of stability showing all the stable nuclei, and surrounding it are unstable nuclei

Question 30

What is the shape of the graph for N against Z for nuclei?

Answer 30

Line of stability initially increases with N=Z up to around 20 but then increases more exponentially meaning more neutrons than protons are required for stability in heavier nuclei

Question 31

How does a nucleus decay if it has too many neutrons and how do N and Z change?

Answer 31

Decays through beta-minus decay as a neutron becomes a proton and a β- and antineutrino are released from the nucleus. N - 1, Z + 1

Question 32

How does a nucleus decay if it has too many protons and how do N and Z change?

Answer 32

Decays through beta-plus decay or electron capture. In beta-plus decay, a proton becomes a neutron and a β+ and neutrino are released from the nucleus. N + 1, Z - 1.
In electron capture, an orbiting electron is drawn in by the nucleus, combining with a proton to become a neutron and releasing a neutrino. N + 1, Z - 1

Question 33

How does a nucleus decay if it has too many nucleons?

Answer 33

Decays through alpha decay, emitting a helium nucleus
N - 2, Z - 2

Question 34

How does a nucleus decay if it has too much energy?

Answer 34

Decays through gamma decay, usually after another type of decay leaving the nucleus in an excited state with excess energy, which is emitted as a gamma ray. N and Z not affected

Question 35

Why does the number of neutrons need to be much larger for heavier nuclei?

Answer 35

Electromagnetic force of repulsion between protons becomes larger than strong force, so more neutrons required to increase distance between protons keeping it stable

Question 36

What is special about Technetium-99m?

Answer 36

Te-99m is produced in an excited nuclear state meaning it emits a gamma photon to reach the ground state. It has an unusually long half-life and is ‘metastable’ and is a pure gamma emitter

Question 37

Why is Technetium-99m used in medicine?

Answer 37

It is a pure gamma emitter so it is very weakly ionising and can be picked up by a gamma camera and has a half-life of 6 hours which is short enough for less exposure but long enough for tests to be carried out and can be prepared on site

Question 38

How can nuclear radius be estimated by calculating distance of closest approach?

Answer 38

Alpha particle fired at a nucleus has initial Ek which is known. Electrostatic repulsion causes Ek to be converted into electric potential energy so it slows. Point at which it stops is where all Ek has become electric potential energy. To find electric potential energy, multiple electric potential by the charge of the particle and rearrange for r.

Question 39

How can nuclear radius be estimated using electron diffraction?

Answer 39

Electrons accelerated to high speeds so de Broglie wavelength is around 10^-15m, and are fired at a thin film of material so they diffract between nuclei forming a diffraction pattern on a screen. Has central bright spots with dimmer rings around. You get the equation sinθ = 0.61λ/R

Question 40

How are electron diffraction and closest approach different?

Answer 40

Electron diffraction gives more accurate result as closest approach is an over estimate

Question 41

What is Ro in the equation for nuclear radius with nucleon number?

Answer 41

1.4 fm