Radioactivity

Radioactivity was discovered by Henry Becquerel and Marie and Pierre Curie.

Henry Becquerel: wrapped crystals of uranium salt in black pepper and left them lying in the dark on a photographic plate, which became partially darkened indicating some form of radiation was being emitted by the salt and penetrating the paper. This emission became known as radioactivity.

Pierre and Marie Curie: By purifying uranium called pitchblende, they discovered two highly radioactive elements, polonium and radium.

Alpha Radiation

Alpha particles consist of two protons and two neutrons (helium nuclei). Therefore, alpha particles are always positive (+2). Alpha particles move relatively slowly, are stopped quite easily and are strongly ionising. Can be hazardous if they get into the body (via the mouth or nose) which could lead to cancer. Alpha particles are deflected by electrical and magnetic fields.

Alpha emission (loss of two protons and two neutrons).

1. The atomic number decreases by 2.
2. The mass number decreases by 4. An example of an a-emitter would be Americium-241 which is used in smoke detectors. Doesn't pose any health hazard due to poor penetrating ability and it has a long half-life.

Beta Radiation

A neutron disintegrates into a proton and emits an electron (a beta particle). B particles are negative (-1), they move quickly, penetrate more deeply than alpha particles and are stopped by 5mm of aluminium. B particles can penetrate deep into the body and can also cause cancer. There is no change in mass number but the atomic number increases by 1. Deflected by electrical and magnetic fields.

Beta emission (neutrons become a proton and an electron)

3. The atomic number increases by 1
4. The mass number remains the same
5. An electron is emitted. An example of a B-emitter would be Carbon 14. When you are alive, the carbon-12 to carbon-14 ratio is constant in your body. When a plant or an animal dies, the carbon 14 starts to decay and the ratio of the two changes. The ratio is evaluated and the age can be determined due to the knowledge that carbon 14 has a half-life of 57000 years.

Gamma Radiation

Y rays are not particles – no mass or charge. Y rays move extremely quickly and have a much greater penetrating ability. Only thick shields of lead or concrete can stop them. Y-rays are very damaging and can lead to cancer. Not deflected by electrical and magnetic fields.

An example of a y-emitter would be Cobalt-60 which is used in cancer treatment.

Radiation Type	What is it?	Examples	Effect of Electrical Field	Penetrating Power	Uses
Alpha Particles +	Helium Nucleus (two protons and two neutrons emitted from the nucleus	Americium-241	Attracted to negative electrode	Stopped by sheet of paper	Smoke detectors
Beta Particles -	Neutron turns into proton and electron. Only electron is emitted	Carbon-14	Attracted to positive electrode	Stopped by 5mm of aluminium	Dating archaeo-logical artefacts
Gamma Ray	High energy electromag- netic radiation (no particles)	Cobalt-60	None	Stopped by thick block of lead	Radiotherapy (kills cancer)

Note: a Geiger-Müller tube may be used to detect radioactivity. A gas is ionised by alpha, beta or gamma radiation resulting in an electric current which is amplified and detected.

Note: in an electric field, beta radiation is attracted to the positive plate, alpha to the negative and gamma is not attracted to either.

Note: transmutation is when an element changes into another element (eg: by the emission of α and β particles).

Widespread Occurrence of Radioactivity

• Background radiation is the low level of ionising radiation that surrounds us at all times. 90% of it comes from natural resources.
• >50% is caused by radon gas which is formed from the decay of radioisotopes found in rocks.
• All isotopes of radon are radioactive (emitting α particles).
• Potential damage to health means that radon barriers are incorporated into new buildings.
• Artificial sources are mostly medical which include cobalt-60 (cancer treatment and x-rays).
• A small amount from the fallout of weapons tests and nuclear waste.

Differences between Nuclear Reactions and Chemical Reactions

Chemical Reactions:

• Involves changes in the electrons of atoms.
• Elements retain their identity (no change in the atomic nucleus).
• Bonds between atoms are broken or formed, but the nuclei remain unchanged.
• Example: Combustion of methane:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Nuclear Reactions:

• Involves changes in the nucleus of atoms.
• New elements may be formed as the nucleus gains or loses particles.
• Example reactions include alpha and beta emissions, where the composition of the nucleus changes.

Examples of Nuclear Reactions

Alpha Emission:

A nucleus emits an alpha particle (2 protons and 2 neutrons), which reduces its atomic number by 2 and mass number by 4.

Beta Emission:

A neutron in the nucleus changes into a proton, emitting a beta particle (an electron) and an antineutrino. The atomic number increases by 1.

Key Differences

Nuclear reactions	Chemical reactions
Changes occur in the nucleus	No change occurs in the nucleus
New element may be formed	New element cannot be formed
No chemical bond formation involved	Transfer or sharing of electrons involved i.e. chemical bond formation

Radioisotopes and their uses

• Radioisotopes are unstable radioactive isotopes. All isotopes of atomic no. Greater than 83 are radioactive.
• Archaeology. Carbon dating. When alive, the $^{12}$C to $^{14}$C ratio is constant, after death, the $^{14}$C begins to decay (β) while the $^{12}$C remains stable, changing the ratio. By measuring the ratio and knowing the half-life of $^{14}$C, the age of the specimen can be estimated.
• Medicine. cobalt-60 (γ) is used in the treatment of cancer – radiotherapy. The gamma rays are directed onto the tumour. Ionising radiation damages the more susceptible cancerous cells.
• Food Preservation. cobalt-60 is used in food irradiation. The gamma rays kill micro-organisms and insects which could cause the food to deteriorate quickly.
• Smoke Alarms. Americium-241 (α). Safe: radiation is not very penetrating. Does not often have to be replaced: long half-life.