Radioactivity

Revision

History

• Wilhelm Conrad Roentgen – Discovered x Rays

• Antoine Henri Becquerel– Noticed the effects of Uranium on photographic

plates • Marie Curie– Coined the term Radioactivity

• Pierre Curie– Shared Nobel prize with Marie and Becquerel

Radioactivity

• All substance are made of atoms. These have electrons (e) around the outside, and a nucleus in the middle. • The nucleus consists of

protons (p) and neutrons (n)

Radioactivity

• In some types of atom, the nucleus is unstable, and will decay into a more stable atom. This radioactive decay is completely spontaneous.

• You can heat the substance up, or subject it to high pressure or strong magnetic fields - in fact, do whatever you like to it - and you won't affect the rate of decay in the slightest.

Definition

• Radioactivity is the spontaneous breaking up of certain unstable nuclei, accompanied by the emission of radiation

Types

• When an unstable nucleus decays, there are three ways that it can do so. It may give out:-– an alpha particle (we use the symbol α)– a beta particle (symbol β)– a gamma ray (symbol γ)

• Many radioactive substances emit α particles and β particles as well as γ rays. In fact, you won't find a pure g source; anything that gives off g rays will also give off a and/or b too.

Measuring Radiation

• Instrument: Geiger - Muller tube

α, β or γ radiation Ionises a gas, causes an electric current which is amplified and acts as a detector

Inside a GM Tube

alpha particle (α)• Alpha particles are made of 2

protons and 2 neutrons. (Helium Nuclei)

• This means that they have a charge of +2, and a mass of 4(the mass is measured in "atomic mass units", where each proton & neutron=1)

• α particles are relatively slow and heavy.

• They have a low penetrating power - you can stop them with just a sheet of paper.

• Because they have a large charge, α particles ionise other atoms strongly.

α particle emission

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beta particle (symbol β)• Beta particles have a charge of -1,

and a mass of about 1/2000th of a proton. This means that beta particles are the same as an electron.

• They are fast, and light.

• β particles have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex.

• Beta particles IONISE atoms that they pass, but not as strongly as Alpha particles do.

β particle emission

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gamma ray (symbol γ)• Gamma rays are waves, not particles.

This means that they have no mass and no charge.

• γ rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly.

• γ rays do not directly ionise other atoms, although they may cause atoms to emit other particles which will then cause ionisation.

– We don't find pure gamma sources - γ rays are emitted alongside alpha or beta particles.

– Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.

You need to knowType of Radiation Alpha particle Beta particle Gamma ray

Symbol α βγ

(can look different,depends on the font)

Mass (atomic mass units) 4 1/2000 0

Charge +2 -1 0

Speed slow fast very fast (speed of light)

Ionising ability high medium 0

Penetrating power low medium high

Stopped by: paper aluminium lead

Chemical & Nuclear Reactions

• Alpha decay of Americum 241 and the beta decay of Carbon 14 are Nuclear Reactions– Involve changes in the nucleus

• Chemical reactions involve bond making and breaking ....electron allocation is changed– Formation of compounds– Compounds breaking up– Rearranging aa compound

• Note: Chemical reactions cannot result in an element changing into another element

Uses of Radiation• Smoke Detectors• Smoke alarms contain a weak

source made of Americium-241.

• Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows.

• If smoke enters the alarm, this absorbs the a particles, the current reduces, and the alarm sounds.

• Am-241 has a half-life of 460 years

Uses

• Thickness Control• In paper mills, the

thickness of the paper can be controlled by measuring how much beta radiation passes through the paper to a Geiger counter.

Uses• Sterilising• Even after it has been

packaged, gamma rays can be used to kill bacteria, mould and insects in food.

• This process prolongs the shelf-life of the food, but sometimes changes the taste.

• Gamma rays are also used to sterilise hospital equipment, especially plastic syringes that would be damaged if heated.

Uses• Radioactive Dating • Animals and plants have a

known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues.

• When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a half-life of 5700 years).

• The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.

Uses• Cancer Treatment • Because Gamma rays can

kill living cells, they are used to kill cancer cells without having to resort to difficult surgery. This is called "Radiotherapy", and works because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can.

Radioisotopes

• Isotopes that are radioactive• Most elements of atomic number above 83 do

not have stable isotopes• Many useful isotopes are made artificially. In

nuclear reactors eg. Cobalt-60

Radioactive isotopes

• For a start, just because something is called an isotope doesn't necessarily mean it's radioactive.

•You can think of different isotopes of an atom being different "versions" of that atom.

• Consider a carbon atom. It has 6 protons and 6 neutrons - we call it "carbon-12" because it has an atomic mass of 12 (6 plus 6). If we add a neutron, it's still a carbon atom, but it's a different isotope of carbon.

• One useful isotope of carbon is "carbon-14", which has 6 protons and 8 neutrons. This is the atom we look for when we're carbon dating an object.

Radioisotopes and half-life• Unstable radioactive isotopes are called radioisopoes

• C12 is stable

• C14 is unstable

• Stable Isotopes become less common as the number of Neutrons in the atom increases...all isotopes of elements with atomic numbers >83 are radioactive

Half Life• Point a Geiger counter at a radioactive

substance for a period of time and the reading on the metre decreases as you watch. This is shown on the graph.

• The radioactivity from some substances dies away very fast - perhaps in a few microseconds. Others take thousands of years before you'll notice that the radioactivity had decreased at all.

• In theory, every radioactive substance should stay slightly radioactive for ever - the graph should never actually fall to zero. This means that we can't usefully talk about the "life" of a radioactive source.

Half Life• Instead, we use the

idea of "half-life". This is the time it takes for the radioactivity to fall by half.

8000

7000

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0 1 2 3 4 5 6 7 8 9 10 11 1 2

Half life of 2 hrsReading = 8000 @ time 0

Half life of 2 hrsReading = 4000 @ time 2

Half life of 2 hrsReading = 4000 @ time 6

Background Radiation

• Mainly natural radioactivity, all around us. • Vast majority of our annual dose comes from

radon gas (emits alpha particles) , food & drink, the ground, and cosmic rays (which are gamma rays coming in from space).

• Unless you are having radiotherapy, your dose from medical sources is quite low.

• Many people don't realise that your radiation dose from cosmic rays is increased considerably if you fly a great deal.

Background Radiation

Source %Radon Gas 60Rocks and Soil 10Food and Drink 10Cosmic Rays 10Weapons test fallout 0.2Medical Sources 9Nuclear Waste 0.1