Radioactivity Flashcards
Carbon dating
Living plants contain a small percentage of radioactive C-14
Negligible decay throughout the lifetime of the plant - half life of 5570 years
Once dead no further carbon is taken in so the proportion of C-14 begins to fall
Calculate age of sample using n = Ne^-lambda x t
Argon dating
Ancient rocks contain trapped argon gas as a result of the decay of K-40 to Ar-40
Half life of K-40 is 1250 million
Age can be calculated by measuring the proportion of Ar-40 to K-40
For every N K-40 atoms if there is 1 Ar-40 then there must of been N+9 K-40 atoms originally
Radioactive tracer requirements
Should have a half life long enough for necessary measurements to be taken but short enough to reduce damage caused
Emit beta or gamma radiation so the radiation is able to be detected outside
Engine wear -> piston ring
Rate of wear of a piston ring can be measured by fitting a radioactive ring
Radioactive atoms transfer from the ring to the oil
By measuring the radioactivity of the oil the mass transferred from the ring can be determined and the rate of wear calculated
Detecting underground pipe leaks
Beta or gamma emitter injected injected into the flow (depending on depth, soil density etc)
A detector on the surface above the pipeline is used to detect leakage
Modelling oil reservoirs mathematically to improve oil recovery
Water containing a tracer is injected into an oil reservoir at higher pressure, forcing some of the oil out
Detection at the production wells monitor the presence of the radioactive isotope to see how much oil is being lost
The tracer is tritaited water, a beta emitter with a half life of 12 years
Investigating the uptake of fertilisers by plants
Plant watered with a solution containing a fertiliser which contains P-32, a beta emitter with a half life of 14 days
By measuring the radioactivity of the leaves, the amount of fertiliser reaching them can be determined
Monitoring the uptake of iodine by the thyroid gland
Patient is given a solution containing sodium iodide which contains a small quantity of radioactive I-133 (beta emitter with a half-life of 8 days)
The activity of the patient’s thyroid and the activity of an identical smaller prepared at the same time is measured 24 hours later
Thickness monitoring
Detector measures the amount of radiation passing through the foil
If the foil is too thick, the detector reading drops and the rollers move closer together to make the foil thinner
and vice versa
The source is a beta emitter with a long half-life
Power sources for remote devices
Satellites, weather sensors etc
Radioactive isotope in a sealed container which absorbs all the radiation emitted by the isotope
Thermocouple attached to container produces electricity through the heat from the absorption of the radiation
Reasons for nuclear instability
Too many neutrons
Too few neutrons
Too many nucleons - too heavy
Too much energy
When does alpha emission occur?
Occurs when nuclei are too massive to be stable - the strong nuclear force doesn’t have the range to hold large nuclei together
E.g. Uranium and radium
When does beta-minus occur?
Beta minus decay occurs in nuclei that are ‘neutron rich’ - have more neutrons than protons
When are gamma rays produced?
After decays the nucleus is often still excited - this energy is released as gamma rays
No change in nuclear constituents but a loss in energy
Also produced when the nucleus captures one of its own orbiting electrons
Energy of emitted radiation and half-life
There is an inverse relationship between half life and energy of emitted particles
So the lower the energy the longer the half life
Conclusions from Rutherford scattering
Most of the atom is empty space - most alpha particles pass straight through
Nucleus has a large positive charge as some alpha particles were deflected by a large angle
Nucleus must be small as few alpha particles were deflected
Most of the mass must be in the nucleus - alpha particles with high momentum are deflected
Experimental conditions for Rutherford scattering
Very thin gold foil - as otherwise the alpha particles would be absorbed
Narrow beam - as otherwise it would be difficult to tell if the alpha particles had been scattered or passed through
Alpha particles must all be travelling at the same speed - otherwise slower alpha particles would be deflected more than faster particles on the same path
Apparatus in an evacuated chamber - otherwise alpha particles would be stopped by air molecules
Source of alpha particles must have a long half life - otherwise later readings would be lower than earlier readings
Ionisation and living cells
Can destroy cell membranes causing cells to die
Can damage vital molecules directly such as DNA or indirectly by creating free radicals
Damages nuclei
Damaged DNA may cause cells to divide and grow uncontrollably - causing a tumour
Damaged DNA may be passed onto future generations as a mutation in the egg or sperm
Using gamma rays for cancer treatment
Rotating beam is used to lessen the damage to the surrounding tissue whilst giving a high dose to the tumour at the centre of rotation
Treatment causes side effects:
Tiredness
Reddening or soreness or skin
Infertility
Safety with radioactive sources
Use long handling tongs
Store in a lead-lined box when not in use
Reduce exposure time by only using the source for as long as is necessary
Wear a film badge - has 3 areas which darken depending on the radiation type
Sources of background radiation
Radon gas
Cosmic rays
Medical practice and industry
Food and drink
Nuclear power
Ground and buildings
Living things
Metastable state
An excited state of the nuclei of an isotope that lasts long enough for the isotope to be separated from the parent isotope
Usually occurs after alpha or beta emission
Metastable technetium formation
Molybdenum isotope 99-Mo has a half life of 67 hours and decays by beta minus emission to form metastable 99-Technetium
Ground state technetium is a beta minus emitter with a half life of 500000 years
Hence a sample of metastable technetium effectively only emits gamma rays
Uses of metastable technetium - monitoring blood flow through the brain
external detectors are used to analyse the activity of the brain’s blood vessels after sodium pertechnate is administered intravenously
Uses of metastable technetium - the gamma camera
A lead shielded detector is used to build up an image of internal organs and structures from emitted gamma rays
For example bone deposits can be located using a phosphate tracer labelled with metastable technetium
Random nature of radioactive decay
Impossible to predict which one of the large number of nuclei in a sample will decay
Impossible to predict when a nucleus will decay
When can background count be ignored in experiments?
When background count is negligible compared to the count rate
When fluctuations in count rate are greater than the background count
Curve of stability
Graph of N against Z
Where N is number of neutrons
Z is number of protons
Starts curving upwards when N = 20
Alpha emitters to the top right of the curve
Beta minus emitters to the middle left of the curve
Beta plus emitters to the middle right of the curve
Limitations of carbon dating
Measures the ratio of carbon 12 to 14 - contamination may result in a variation in sample age
Very old samples can’t be dated as their activity will be negligible or similar to the background count
Typical radius of:
The smallest nucleus
An atom
1 x 10^-15 m (1 fm)
5 x 10^-11 (0.05nm)
What does the significant difference in nuclear density and atomic density suggest?
Density of nucleus - 1.45 x10^17 kgm^-3
Density of an atom - between 10^3 and 10^5 kgm^3
Most of the atom’s mass is in its nucleus
The nucleus is small compared to the atom
An atom is mostly empty space
Two methods of identifying radiation
Test what the radiation is absorbed by using a count rate and paper, aluminium and lead
Use magnetic fields - charged particles (alpha and beta) will be deflected in a circular path, direction and radius depends on charge and mass
Decay constant
Half life
The probability of a give nucleus decaying per unit time
The time it takes for the number of unstable nuclei in a sample of an isotope to halve
OR
The time taken for the activity of a sample of a radioactive isotope to halve
Equation for calculations involving mol
N = nNa
N is number of atoms
n is number of moles
Na is Avagadro’s constant
Calculating intensity of radiation using two points
Using I =k/x^2 for each point we find:
I x^2 = I x^2 where I and x are different for each point
