Radioactivity

Question 1

Who proposed the first nuclear model of the atom?

Answer 1

Ernest Rutherford

Question 2

Describe Rutherford’s experiment

Answer 2

A thin beam of alpha particles (all with the same kinetic energy) from a source in an evacuated metal container, aimed at a thin metal foil. A microscope with a fluorescent screen at the end rotating round the metal container from 0-almost 180 degrees. He measured the number of alpha particles reaching the detector per minute for different angles

Question 3

Describe the results for Rutherford’s experiment

Answer 3

• Most alpha particles passed straight through the foil with little or no deflection; about 1 in 2000 were deflected
• A small percentage of alpha particles (about 1 in 10000) were deflected through angles greater than 90 degrees

Question 4

Give 3 conditions Rutherford had to have in his experiment and why

Answer 4

1) The alpha particles must have had the same speed (otherwise slow particles would be deflected more than fast particles on the initial path)
2) The container must be evacuated (otherwise alpha particles would be stopped by air molecules)
3) The source of the alpha particles must have a long half life (otherwise later readings would be lower than earlier readings due to radioactive decay of the source)

Question 5

Describe the deflection of alpha, beta and gamma radiation for a magnetic field into the page in Rutherford’s investigation of radioactivity where the beam of radioactive particles enter the field from the left

Answer 5

Alpha particles move up the page (because the current moves in the same way as they do - due to their +ve charge)
Beta particles move down the page (because the current moves in the opposite way as they do - due to their -ve charge)
Gamma radiation remains unchanged (due to it being uncharged)

Question 6

Describe and explain an experiment to investigate the ionising effect of each type of radiation.

Answer 6

A radioactive source is held over an ionisation chamber full of air at atmospheric pressure and a central electrode connected to a picoammeter, a battery supply and a wall electrode. Ions created in the chamber are attracted to the oppositely charged electrode where they are discharged. Electrons move through the picoammeter as a result of ionisation and thus the current is proportional to the number of ions per second created in the chamber.

Question 7

Describe the ionising effect of each type of radiation in an ionisation chamber

Answer 7

• Alpha radiation: causes strong ionisation but ionisation ceases beyond a certain distance if the source is moved away (about 1cm). This is because the particles ionise air molecules.
• Beta radiation: has a weaker ionising effect than alpha radiation but its range in air varies up to a metre or more.
• Gamma radiation: has a much weaker ionising effect than both alpha or beta radiation. This is because the photons carry no charge.

Question 8

Explain how cloud chambers work within radioactivity experiments

Answer 8

The cloud chamber contains air saturated with a vapour at a very low temperature. Because the air is supersaturated, when an ion (caused by ionising particles) passes through the vapour, the ions trigger the formation of droplets in a vapour trail.

Question 9

Describe the 2 different signals that indicate both alpha and beta particles in a cloud chamber.

Answer 9

• Alpha particles - produce straight tracks which radiate from the source and are easily visible. The tracks from the same isotope are all of the same length
• Beta particles - produce wispy tracks that are easily deflected as a result of collision with air molecules. The tracks are less easily seen because they Beta particles are less ionising than alpha particles.

Question 10

Describe the equipment involved in a test to measure the absorption of radiation for different materials.

Answer 10

A source in a sealed container whose radioactive emissions pass through an absorber and into a Geiger tube, which measures the count rate (counts / time)

Question 11

Describe the process of how the radioactive absorption for different materials would be measured.

Answer 11

A source would be set up in front of the material and a Geiger tube. The count rate would be measured without the source or an absorber present to record the background radiation. The background count rate would then be subtracted from the recorded count rate with the source to give the corrected/true count rate from the source.
The count rate would then be measured with the absorber in a fixed position and the count rates would be compared for varying thicknesses of material.

Question 12

Describe a Geiger tube and explain how it works

Answer 12

The Geiger tube is a sealed metal tube that contains gaseous Argon at low pressure. The thin mica window at the end of the tube allows alpha and beta particles to enter (gamma can enter through the wall).
A thin metal rod down through the tube has a positive potential whilst the tube wall is connected to the negative terminal of the power supply and is earthed. This circuit is connected to a pulse counter.
When an ionising particle enters the tube, it ionises gas particles it collides with, which accelerate towards the oppositely charged terminal, ionising more air particles on the way. The discharge of these ions at the terminals creates a pulse of charge which is detected by the pulse counter.

Question 13

Explain what the dead time of a Geiger tube is and explain why the count rate should never exceed about 5000s^(-1)

Answer 13

The dead time is the time taken for the tube to regain its non-conducting state after ionising particles enter it.
It is typically of the order 0.2ms, therefore the count rate should not exceed 5000s^(-1) (1/0.2ms)

Question 14

Describe the charge and makeup of an alpha particle

Answer 14

The nucleus of a Helium atom and is positively charged

Question 15

Describe the set-up and explain how Rutherford devised the structure of alpha particles

Answer 15

He filled a tube ,fitted with two electrodes, with gaseous alpha particles. He applied a voltage, the gas conducted electricity and emitted light.
Using a spectrometer, he proved that the spectrum of light from the tube was the same as that from a tube filled with helium gas.

Question 16

Describe how Beta radiation was proved to be fast moving electrons

Answer 16

A beam of Beta particles were deflected in electric and magnetic fields. The measurements of deflection were used to calculate the specific charge of the beta particles, which was the same as the specific charge of a electron.

Question 17

Give the equation for the intensity (I) of gamma radiation at distance r from the source in terms of k and give the value of k if the source emits n number of photons per second

Answer 17

I = k / r^(2)

k = nhf / 4π

Question 18

For an alpha particle, give:

i) the structure
ii) the range in air
iii) deflection in a magnetic field
iv) absorption
v) ionisation
vi) energy of each particle

Answer 18

i) 2 protons + 2 neutrons
ii) fixed range, depends on energy, can be up to 100mm
iii) easily deflected
iv) stopped by paper
v) about 10^(4) ions per mm in air at STP
vi) constant for a given source

Question 19

For a beta particle, give:

i) the structure
ii) the range in air
iii) deflection in a magnetic field
iv) absorption
v) ionisation
vi) energy of each particle

Answer 19

i) β- = electron (β+ = positron)
ii) range up to about 1m
iii) opposite direction to alpha particles and less easily deflected
iv) stopped by approx 5mm of aluminium
v) about 100 ions per mm in air at STP
vi) varies up to a maximum for a given source

Question 20

For a gamma photon, give:

i) the structure
ii) the range in air
iii) deflection in a magnetic field
iv) absorption
v) ionisation
vi) energy of each photon

Answer 20

i) photon of energy of the order MeV
ii) follows the inverse square law
iii) not deflected
iv) stopped by several centimetres of lead
v) very weak ionising effect
vi) constant for a given source

Question 21

Give the equation for the emission of an alpha particle from element X, with a atomic mass A and Z number of protons

Answer 21

X, with atomic mass X and Z number of protons
——->
an alpha particle + element Y with atomic mass A-4 and Z-2 number of protons

Question 22

Give the equation for the emission of beta- particle from element X, with a atomic mass A and Z number of protons

Answer 22

X, with atomic mass X and Z number of protons
——->
a beta- particle + element Y with atomic mass A and Z+1 number of protons + and an anti-electron nutrino

Question 23

Give the equation for the emission of beta+ particle from element X, with a atomic mass A and Z number of protons

Answer 23

X, with atomic mass X and Z number of protons
——->
a beta+ particle + element Y with atomic mass A and Z-1 number of protons + and an electron nutrino

Question 24

Give the equation for electron capture for the element X, with atomic mass A and Z number of protons

Answer 24

X, with atomic mass A and Z number of protons + an electron
——->
Y, with atomic mass A and Z-1 number of protons + an electron nutrino

Question 25

Give 2 ways in which ionising radiation damages cells

Answer 25

• It can destroy cell membranes, which can lead to cell death
• It can damage vital organelles or molecules such as DNA directly or indirectly by creating free radicals

Question 26

Suggest how people working with equipment that produces ionising radiation could measure their level of exposure

Answer 26

They wear a film badge which contains a strip of photographic film in a light proof wrapper. Different areas of the film are covered by absorbers of different materials and different thickness’s.
When the film is developed, the amount of exposure to each form of ionising radiation can be estimated from the blackening of the film.

Question 27

Give some safety measures which should be taken when storing radioactive materials

Answer 27

• Kept in a lead lined container
• The thickness of the lead in the container must be enough to reduce the gamma radiation from the source to about background level
• The container should be kept under lock and key, and a record of the sources is kept

Question 28

Give some safety measure which should be taken whilst using radioactive materials and why

Answer 28

• Solid sources should be transferred using handling tools such as tongs or using robots so that they are beyond the range of alpha and beta radiation in air.
• Liquid, gas or solid powder should be in sealed containers. This ensures radioactive gas or powder is not inhaled and liquid cannot be spilt onto skin
• Radioactive sources should not be used longer than is necessary because the longer the handler is exposed to radiation, the greater the dose they receive is.

Question 29

Define the half-life of radioactive isotope and give its units

Answer 29

The half-life, T½, of a radioactive isotope is the time take for the mass of the isotope to decrease to half the initial mass

Measured in seconds

Question 30

Define the activity of a radioactive isotope and give its units

Answer 30

The activity, A, of a radioactive isotope is the number of nuclei of the isotope that disintegrate per second.

Measured in becquerel (Bq) where 1Bq = 1 disintegration per second

Question 31

Give the equation to find the energy transefer per second from a radioactive source

Answer 31

Energy transfer per second = AE

A = activity
E = energy of each particle

Question 32

Give the equation for the activity of N number of atoms in terms of λ and state what λ is

Answer 32

A = λN

λ is the decay constant

Question 33

Give the solution for the equation ΔN / Δt = - λN

Answer 33

N = N₀e^(λt)

Question 34

Give the equation for the Activity of a radioactive isotope after time t, with rate constant λ

Answer 34

A = A₀e^(-λt)

Question 35

Give the equation for the half life of a radioactive substance with rate constant λ

Answer 35

T½ = ln2 / λ

Question 36

Define the decay constant and give its units

Answer 36

The decay constant (λ) is the probability of an individual nucleus decaying per second

Units: s⁻¹

Question 37

Give 2 examples of radioactive dating

Answer 37

1) Carbon dating

2) Argon dating

Question 38

Give the formula for the creation of Carbon-14 and state what initiates it

Answer 38

n¹₀ + N¹⁴₇ → C¹⁴₆ + p¹₁

neutron is knocked out of a nuclei by cosmic rays

Question 39

Give the formula for the creation of Argon-40 and states the process by which this happens

Answer 39

K⁴⁰₁₉ + e⁰₋₁ → Ar⁴⁰₁₈ + νₑ

Argon-40 is formed by electron capture

Question 40

Give the equation for the other type of decay of K⁴⁰ than Argon-40, state what type of decay takes place and state how much more probable it is than electron capture

Answer 40

K⁴⁰₁₉ → β⁰₋₁ + Ca⁴⁰₂₀ + ν̅ₑ
Ca⁴⁰ is produced by β⁻ decay
It is 8 times more probable than electron capture

Question 41

Give 4 examples for the application of radioactive tracers, for each giving a brief outline of the method and the tracer used

Answer 41

1) Detecting underground pipe leaks - radioactive tracer injected into flow with a detector on the surface to detect the leak (β-emitter or γ-emitter used depending on depth)
2) Modelling oil reservoirs to improve oil recovery - Water containing tracer injected into reservoirs at high pressure. Detectors at production well monitor breakthrough of isotope (³H₂O used - a γ-emitter with a 12 year half life)
3) Investigating uptake of fertilisers - fertiliser solution containing tracer used on plants and radioactivity in leaves shows uptake (P³² used - a β-emitter with 14 day half life)
4) Monitoring uptake in iodine by thyroid gland - patient given I¹³¹ solution. Activity of thyroid and identical sample compared 24hrs later (I¹³¹ used - contains a β-emitter with 8 day half life)

Question 42

Describe the trend in the band in stable nuclei for a graph of N vs Z

Answer 42

• For light isotopes (0

Question 43

For a graph of N vs Z, where in relation to the band of stable nuclei would you find:

i) α-emitters
ii) β⁺-emitters
iii) β⁻-emitters

Answer 43

i) α-emitters occur above the stability belt above Z = 60. They are too large to be stable becuase the strong nuclear force is unable to overcpome the electrostatic repulsion between the protons
ii) β⁺-emitters occur the right of the stability belt where isotopes are proton-rich compared to stable isotopes.
iii) β⁻-emitters occur to the left of the stability belt where the isotopes are neutron-rich compared to stable isotopes.

Question 44

Explain the change in nuclear energy levels when a γ photon is emitted after an unstable decays

Answer 44

After an unstable nucleus emits an α or a β particle or undergoes electron capture, the daughter nucleus may have been formed in an excited state. The daughter nucleus moves from its metastable state to its ground state emitting a γ photon with energy equal to the change in energy level

Question 45

Explain how the radius of nuclei is determined and give the equation for θmin

Answer 45

Electrons are accelerated through a pd in the order of 100,000,000 volts onto a thin metal sample in a vacuum. A detector is moved from the normal with increasing θ and a graph of detector reading against diffraction angle. There is a general decreasing trend except a minima and maxima. The minima (θmin) depends of the radius R of the nucleus, so the following equation is used:
R sinθmin = 0.61λ
(where λ is the de Broglie wavelength of the electrons)