Radioactivity

Question 1

Describe Rutherford’s experiment

Answer 1

He aimed a narrow beam of alpha particles, all of the same KE, at a thin gold foil and detected where they were scattered. He used a microscope to observe the pinpoints of light emitted when they hit a fluorescent screen and measured the number of alpha particles reaching the detector per minute at different angles. Only 1 in 2000 were deflected and 1 in 10,000 were deflected through angles over 90.

Question 2

What conclusions were drawn from Rutherford’s experiment?

Answer 2

Most of the atom’s mass in concentrated in a small region, the nucleus, at the centre of the atom. The nucleus is positively charged because it repels alpha particles (positive) that approach it too closely.

Question 3

What is count rate?

Answer 3

The number of counts per unit time detected by a Geiger Muller tube. One count is one particle of radiation that enters the tube. Value must be corrected by subtracting the count rate due to background radiation.

Question 4

What completely absorbs each type of radiation?

Answer 4

Alpha- paper and thin metal foil
Beta- about 5mm of metal
Gamma- several centimetres of lead

Question 5

How does a Geiger Muller tube work?

Answer 5

Contains argon at low pressure. A thin mica window allows radiation in. A metal rod down the middle of the tube is at a positive potential and tube wall is connected to the negative terminal of a power supply and is earthed. Gas atoms are ionised along the radiation’s track. The anions attracted to the rod and the cations to the wall. The ions collide with more gas atoms so more ions so, quickly many ions created and discharged at the electrodes. A pulse charge goes around the circuit to a resistor, R, causing a voltage pulse across it which is one count.

Question 6

What is the intensity of radiation?

Answer 6

At a surface is the radiation energy per second per unit area at normal incidence to the surface

Question 7

What is the intensity of gamma radiation related to distance, r, from the source?

Answer 7

Inverse square law. Proportional to 1/r^2

Question 8

Describe electron capture

Answer 8

Some proton rich nuclides can capture an inner shell electron. This causes a proton in the nucleus to change into a neutron with the emission of an electron neutrino at the same time. The inner shell vacancy is filled by an outer shell electron, as a result causing an x-ray photon to be emitted by the atom.

Question 9

How does ionising radiation affect living cells?

Answer 9

It can destroy cell membranes causing cells to die. It can damage vital molecules like DNA directly or indirectly by creating free radical ions which react with the vital molecules. Normal cell division is affected and nuclei become damaged. Damaged DNA can cause cells to divide uncontrollable causing a tumour which may be cancerous.

Question 10

How does a film badge work?

Answer 10

It contains a strip of photographic film in a light proof wrapper. Different areas of the film are covered by absorbers of different materials and thickness. When the film is developed, the amount of exposure to each form of ionising radiation can be estimated from the blackening of the film. If the badge is overexposed, the wearer is not allowed to continue working with the equipment.

Question 11

What is the dose or ionising radiation measured in terms of?

Answer 11

Energy absorbed per unit mass of matter from the radiation.

Question 12

What causes natural background radiation?

Answer 12

Cosmic radiation. Radioactive materials in rocks, soil and air (eg radon gas)

Question 13

What causes man made background radiation?

Answer 13

Medical devices. Nuclear weapons. Air travel. Nuclear power.

Question 14

How should radioactive materials be stored?

Answer 14

In lead-lined containers because most sources produce gamma as well as alpha or beta. Lead must be thick enough to reduce the gamma radiation from the source in the container to about background radiation level. Regulations require the containers are under lock and key and a record of the sources is kept.

Question 15

Regulations for using radioactive materials

Answer 15

Solid sources should be transferred using handling tools (eg tongs). The tools ensure the source is as far away from the user as practicable so beyond range of alpha and beta and lowest intensity gamma. Liquid, gas sources and powders should be in sealed containers so none can be breathed in, spilt or drunk. No source is allowed to touch skin and not be used longer than necessary.

Question 16

Decay curve

Answer 16

An exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.

Question 17

Half life

Answer 17

The time taken for the mass of a radioactive isotope to decrease to half the initial mass.

Question 18

What is the activity of a radioactive isotope?

Answer 18

The number of nuclei of the isotope that disintegrate per second. Unit is Bq where 1Bq is one disintegration per second. It is proportional to the mass of the isotope.

Question 19

What is the power of a radioactive source?

Answer 19

Its activity multiplied by the energy of each particle emitted (provided all the particles have the same energy)

Question 20

Decay constant

Answer 20

The probability of an individual nucleus decaying per second.

Question 21

Which 3 variables have the same formula as the one for number of nuclei?

Answer 21

Mass, activity, corrected count rate

Question 22

How does carbon dating work?

Answer 22

C-14 is formed in the atmosphere as a result of cosmic rays knocking out neutrons from nuclei which collide with nitrogen nuclei to form C-14 and a proton. CO2 taken in from photosynthesis by plants. Once dead, no more taken in so proportion of C-14 decreases as it decays. Can calculate age of sample by measuring it activity if you know the activity of the same mass of living plant.

Question 23

How does argon dating work?

Answer 23

Argon in rocks due to electron capture in K-40. Although beta- decay to calcium is 8x more likely. For every N K-40 atoms now present, if there is 1 argon atom, there must have been N+9 K-40 atoms originally. Therefore can calculate age of rock using half life.

Question 24

What characteristics should a radioactive tracer have?

Answer 24

Half life is stable enough for necessary measurements to be made and short enough to decays quickly after use. Emit beta or gamma radiation so it can be detected outside the flow path.

Question 25

Describe and explain how thickness of paper is monitored?

Answer 25

Beta source on opposite side of paper to detector. If the paper is too thick, the detector reading drops and a signal is fed back to the control system to make the rollers move closer together and make the paper thinner. Beta source with long half life used as alpha would be absorbed completely and gamma would pass straight through without absorption.

Question 26

Describe N-Z graph

Answer 26

Neutron number against proton number. Goes up to N=120 and Z=80. Stability belt follows line N=Z up to (20,20) then N/Z ratio increases so curves up.

Question 27

Where are the emitters of each type of radiation on the N-Z graph?

Answer 27

Alpha above stability belt and occur beyond Z=60 but most have more than 80 protons and 120 neutrons. Nuclei are too large so strong force can’t overcome ES repulsion between protons. Beta- left of stability belt where isotopes neutron rich so convert neutron to proton. Beta+ right of stability belt where isotopes proton rich so convert proton to neutron. Electron capture also right of stability belt.

Question 28

What is an atom in its excited state?

Answer 28

An atom which has more energy than when it is in its ground state.

Question 29

When and why might a nucleus emit a gamma photon?

Answer 29

After emission of alpha or beta particle or electron capture. This is because the daughter nucleus could be formed in an excited state so emits a gamma photon to lose energy to either to a lower excited state or to the ground state.

Question 30

What is the metastable state?

Answer 30

An excited state of the nuclei of an isotope that lasts long enough after previous alpha or beta emission to be separated from the parent isotope.

Question 31

How to measure diameter of nucleus using electron diffraction

Answer 31

Beam of high energy electrons (accelerated by 100mill V) directed at thin solid sample of element. They’re diffracted by the nucleus as their de Broglie wavelength is similar to the diameter of the nucleus. Detect number of electrons per second diffracted through different angles. As angle of detector increases intensity down, then bit up, then down. Angle from first minimum from centre used to find diameter of nucleus using wavelength of electron beam.

Question 32

Graph of ln(nuclear radius) against ln(mass number)

Answer 32

Straight line with gradient 1/3 and y-intercept of ln(r0). r0 is constant 1.05fm.

Question 33

Proving density of nucleus is constant

Answer 33

Volume is 4/3 piR^3. Sub in r0A^1/3 for R where A is mass number. Density is mass/volume. Masses cancel so equation for density is just constants (involves u, atomic mass unit)

Question 34

How to estimate the radius of nuclei using alpha particles

Answer 34

As an alpha particle approaches the nucleus, they repel. The alpha particle momentarily stops at a point, P, so all of its kinetic energy has been converted to electric potential energy. Can use Coulomb’s law and energy is force x distance to equate kinetic energy at start to electric potential energy at P. Use higher energy alpha until number of alpha particles detected at a particular angle changes. This is when the alpha particle just penetrates the nucleus so the separation between the centre of charges will be the nuclear radius.

Question 35

What is the equation linking the angle of the first minimum to the nuclear radius?

Answer 35

Sin(angle of 1st min) = 0.61 x wavelength/nuclear radius

Question 36

Describe how the intensity of gamma radiation varies with distance from its source

Answer 36

Follows inverse square law. Graph of distance from source against one over square root of corrected count rate gives a straight line. There is a negative y-intercept when using GM tube and not fully exposed gamma source. The magnitude of it is the distance between the actual source and the wall of its container add the distance between the actual detector and the wall of the GM tube.

Question 37

What is Technetium-99m and how is it used?

Answer 37

Technetium atoms formed in the metastable state after β- emission of Mo-99. Has a half life of 6 hours and decays to the ground state by λ emission. Can be used to monitor blood flow through the brain and image internal organs.