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A level RadioactivityA level Radioactivity
19/04/23
Graham Perrin
AQA Spec AQA Spec 19/04/23
AQA Spec cont’dAQA Spec cont’d19/04/23
SafetySafety19/04/23
1. 16+ only2. RPO appointed
1. Use logged1. Exposure time
2. Stored and labelled correctly3. Adequate training –
1. Distance
Many simulations available
IonisationIonisation19/04/23
When radiation collides with neutral atoms or molecules it alters their structure by knocking off electrons. This will leave behind IONS – this is called IONISING RADIATION.
particle
Electron
RadioactivityRadioactivity 19/04/23
If a substance is capable of ALWAYS emitting radiation under any conditions we say it is ____________. There are three types of radiation: ALPHA, _____ and GAMMA. These types of radiation are always given off by rocks, _____, building materials, air and cosmic rays around us – this is called BACKGROUND RADIATION. Each type is capable of penetrating different materials:
Sheet of paper
Few mm of _________
Few cm of lead
Words – aluminium, beta, food, radioactive
Types of radiationTypes of radiation 19/04/23
1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a helium atom)
2) Beta () – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle.
3) Gamma – after or decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed.
Unstable nucleus
Unstable nucleus
Unstable nucleus
New nucleus
New nucleus
New nucleus
Alpha particle
Beta particle
Gamma radiation
Inverse Square law for Inverse Square law for 19/04/23
Uses light as a model for radiation. (Both em waves)
The further the better!The further the better!19/04/23
Background RadiationBackground Radiation19/04/23
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
13% are man-made
Uses of radioactivityUses of radioactivity19/04/23
1) Medical uses – gamma rays can be used to destroy cancerous cells or to sterilise medical instruments
2) Tracers – a tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:
Gamma source
Tracers can also be used to develop better plant fertilisers and in medicine to detect tumours:
The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.
Uses of radioactivity 2Uses of radioactivity 219/04/23
Rollers
Beta emitter
Beta detector
Paper
Dangers of radioactivityDangers of radioactivity19/04/23
OUTSIDE the body and are more dangerous as radiation is blocked by the skin.
INSIDE the body an source causes the most damage because it is the most ionising.
Alpha
Beta
Gamma
Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.
Half lifeHalf life19/04/23
The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…
At start there are 16 radioisotope
s
After 1 half life half have
decayed (that’s 8)
After 3 half lives another
2 have decayed (14 altogether)
After 2 half lives another
half have decayed (12 altogether)
= radioisotope = new atom formed
11stst law of radioactive decay law of radioactive decay19/04/23
If there are a lot of unstable nuclei in a sample, then it follows that many of them decay in a period of time.
The fewer the nuclei, the fewer will decay.
NdtdN
N dtdN
A radioactive decay graphA radioactive decay graph19/04/23
Time
(Throw)
Count = Number of dice
1 half life
Exponential decayExponential decay19/04/23
A business is said to go EXPONENTIAL if it’s growth is proportional to the length of time its been open. The longer it’s been running, the faster it grows.
Exponential decay is the inverse relationship.
These types of relationship are governed by a natural ‘irrational’ number. Euler’s number (or Napier’s constant)
e = 2.718…………
22ndnd Law of radioactive decay Law of radioactive decay19/04/23
λt0eNN
33rdrd law of radioactive decay law of radioactive decay19/04/23
λln2
T2
1
Dating materials using half-Dating materials using half-liveslives
19/04/23
Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.
8
8
Answer: The sample was originally completely uranium…
…of the sample
was uranium
4
8
2
8
1
8Now only 4/8 of
the uranium remains – the
other 4/8 is lead
Now only 2/8 of uranium
remains – the other 6/8 is
lead
Now only 1/8 of uranium
remains – the other 7/8 is
leadSo it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
1 half life later…
1 half life later…
1 half life later…
IsotopesIsotopes 19/04/23
An isotope is an atom with a different number of neutrons:
Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.
Notice that the mass number is different. How many neutrons does each isotope have?
A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.
Neutrons and ProtonsNeutrons and Protons19/04/23
Are there always equal numbers of protons and neutrons?
Transforming the nucleusTransforming the nucleus19/04/23
As we have seen both and decay involve changes of the particles within the nucleus. This results in the radioactive isotope changing into a different element.
235 U92
Loses 2 protons and 2 neutrons during decay
235 U92
4
2
231 Th
90
231 ? +
231 Th90
90
Loses a neutron but gains a proton during decay
231 ? +91
0-1
PositronsPositrons19/04/23
decay occurs when the nucleus is unstable due to having too many protons. decay occurs when the nucleus is unstable due to having too many neutrons.More recently a type of unstable nucleus that releases tiny positive particles (a positive version of an electron). This is called + decay (or positron decay).This seems to be the opposite of normal decay (from now on called - decay) i.e. a proton changes into a neutron and a positron.
231 Th
231 Th90
90
Loses a proton but gains a neutron during + decay
231 ? +89
0+1
emissionemission19/04/23
Almost all decays produce ‘over excited nuclei’ which ‘cool down’ by emitting gamma rays. Technetium 99m (meta-stable isotope of technetium) is a source that is widely used in medicine and has a half life of about 6 hours.
Scattering experimentsScattering experiments19/04/23
Rutherford scattering
Electron Diffraction.At high enough energies, electrons can behave as waves.
Rutherford’s size of a Rutherford’s size of a nucleusnucleus
19/04/23
When the is brought momentarily to rest (“having climbed as far as it can up the electrostatic hill”) the work done in bringing it to rest will just equal its initial kinetic energy. When the speed and hence the kinetic energy is zero, all the energy is now electrostatic potential energy.If the momentarily stops when at a distance d from the (centre of) the nucleus of charge Ze, its electrical potential energy is
This equals the initial kinetic energy of the particle. Rutherford used an source given to him by Madame Curie. The energy was ~ 7.7MeV.For gold, Z = 79. Solving gives d ~ 3 ×10-14 m. Compare this with the diameter of gold atoms
When the a is brought momentarily to rest (“having climbed as far as it can up the electrostatic hill”) the work done in bringing it to rest will just equal its initial kinetic energy. When the speed and hence the kinetic energy is zero, all the energy is now electrostatic potential energy.If the a momentarily stops when at a distance d from the (centre of) the nucleus of charge Ze, its electrical potential energy is
For gold, Z = 79. Solving gives
d ~ 3 ×10-14 m
Electron diffraction Electron diffraction (Scattering)(Scattering)
19/04/23
Low energy electrons > crystal informationHigh energy electrons > nuclear information
R0.63
sin
Wavelength of an electron?Wavelength of an electron?19/04/23
h momentum
h mc
hc mc
hf mc E
2
2
De Broglie wavelength shows the duality of energy and matter. Most wavelengths are too small for effects to be noticeable in the macroscopic world.
Nuclear RadiusNuclear Radius19/04/23
This gives a result of the radius being 2.65 × 10-15 m. We need to note the following:•To get an appreciable electron scattering effect, we need to have electrons with a de Broglie wavelength of about the nuclear diameter. This requires very high energies.•The electron diffraction minima are not zero, indicating that the boundary of the nucleus is fuzzy, not sharp.•Since the boundary is not sharp, various methods of determining the radius give rather variable results, from 1.2 fm to 1.5 fm.
R0.63
sin
Nuclear radius / atomic Nuclear radius / atomic numbernumber
19/04/23
By carrying out electron diffraction for different elements, we can investigate nuclear density.
R = r0A1/3
As the nucleon number increases, the volume increases.
[The term A1/3 means the cube root of A, the nucleon number. The term r0 is a constant with the value 1.4 × 10-15 m. R is the nuclear radius.]
Nuclear fissionNuclear fission
19/04/23
Uranium nucleus
Unstable
nucleus New nuclei (e.g. barium
and krypton)
More neutron
s
Neutron
E = mcE = mc2219/04/23
This is possibly the most important equation in the history of Science. What does it mean?The ‘daughter products’ of fission and the neutrons together weigh less than the original neutron and the uranium.The mass difference times the speed of light squared = the energy given off.
Chain reactionsChain reactions 19/04/23
Each fission reaction releases neutrons that are used in further reactions.
Fission reactions Fission reactions summarysummary
19/04/23
Each fission reaction releases energy in the form of _______. In a nuclear power plant this heat is used to boil _______, which is used to drive turbines etc. The energy from each reaction is very ______, but there are ________ of reactions every second. The waste products from these reactions are __________, which is why nuclear power plants are ___________.
Words – radioactive, water, billions, controversial, heat, small
Fuel RodsFuel Rods19/04/23
In a nuclear bomb, the material is packed tightly together above ‘critical mass’.In a reactor the fissionable material is distributed evenly throughout the reactor.The uranium is packed into cylinders called fuel rods and suspended throughout the reactor.
Control RodsControl Rods19/04/23
Control rods, made of materials that absorb neutrons, are held above the reactor.These can be raised and lowered into the reactor to absorb the neutrons released in the chain reaction.If all the neutrons are removed the chain reaction stops.
Cooling the reactorCooling the reactor19/04/23
The important product of a reactor is the heat energy. This is removed to heat water for steam to drive turbines.The heat may be taken out by a gas (like a fridge). These are called AGRs.If the heat is removed by pressurised water, the reactor is called a PWR.
Nuclear power stationsNuclear power stations19/04/23
These work in a similar way to normal power stations:
The main difference is that the nuclear fuel is NOT burnt – it is used to boil water in a “heat
exchanger”
Disposing of radioactive Disposing of radioactive wastewaste
19/04/23
High level waste is immobilised by mixing with ____ making ingredients, melting and pouring the glass into steel containers.
Intermediate waste is set in cement in _____ drums.
The key to dealing with radioactive waste is to IMMOBILISE it. There are a number of ways of doing this depending on how __________ the waste is:
The containers are then kept in stores, often _________.
Words – glass, steel, underground, radioactive
