P2 Radioactivity

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  • Created on: 05-01-13 10:50

6.1 Observing nuclear radiation

The nuclei of radioactive substances are unstable. They become stable by radioactive decay. In this process they emit radiation and turn into other elements.

The three types of radiation emitted are: alpha, beta and gamma radiation.

Radioactive decay is a random event - we cannot predict or influence when it will happen.

Background radiation is from radioactive substances in the environment, or from space or from devices such as X-ray machines.

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6.2 The Discovery of the nucleus

Once, scientists thought that atoms consisted of spheres of positive charge with electrons stuck into them, like plums in a pudding. This was known as the "plum pudding" model.

Then Rutherford, Geiger and Marsen devised an alpha particle scattering experiment, in which they fired alpha particles at thin gold foil.

Most of the alpha particles passed straight through the foil. This means that most of the atom is just empty space.

Some of the alpha particles were deflected through small angles. This suggests that the nucleus has a positive charge.

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6.2 The Discovery of the nucleus part 2

A few rebound through very large angles. This suggests that the nucleus has a large mass and a very positive charge.

This proved that an atom has a small, positively charged central nucleus where most of the mass of the atom is located.

The "plum pudding" model could not explain why some alpha particles were scattered through large angles.

The nuclear model of the atom correctly explained why the alpha particles are scattered, and why some are scattered through large angles.

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6.3 Nuclear reactions

If an atom loses or gains electrons in becomes charged and is called an ion.

All atoms of a particular element have the same number of protons. Atoms of the same element with different numbers of neutrons are called isotopes.

Therefore isotopes have the same atomic numbers but different mass numbers.

An alpha particle consists of two protons and two neutrons. Its relative mass is 4 and its relative charge is +2. So we represent it by the symbol:

When a nucleus emits an alpha particle the atomic number goes down by two, and the mass number down by four.

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6.3 Nuclear reactions part 2

A beta particle is a high-speed electron from the nucleus, emitted when a neutron in the nucleus changes to a proton and an electron. Its relative mass is 0 and its relative charge is -1. We represent it as:

The proton stays in the nucleus so the atomic number goes up by one and the mass number is unchanged.. The electron is instantly emitted.

When a nucleus emits gamma radiation there is no change in the atomic or mass number. A gamma ray is an electromagnetic wave released by the nucleus. It has no charge and no mass.

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6.4 Radiation

When nuclear radiation travels through a material it will collide with the atoms of the material.

This knocks electrons off them, creating ions - this is called ionisation.

Ionisation in a living cell can kill or damage the cell.

A magnetic or electric field can be used to separate a beam of alpha, beta and gamma radiation.

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Alpha radiation

Alpha particles are relatively large, so have lots of collisions with atoms - they are strongly ionising.

The particles are composed of two protons and two neutrons.

Because of these collisions the alpha particles do not penetrate far into a material. They can be stopped by a thin sheet of paper, human skin and a few centimetres of air.

Alpha particles have a positive charge and are deflected by electric and magnetic fields.

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Beta radiation

Beta particles are much smaller and faster than alpha particles, so they are less ionising and penetrate further.

They are blocked by a few metres of air or a thin sheet of aluminium.

Beta particles consist of fast-moving electrons emitted from the nucleus. The particles have a negative charge and are deflected by electric and magnetic fields in the opposite direction of alpha particles.

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Gamma radiation

Gamma rays are electromagnetic waves so will travel a long way through a material before colliding with an atom.

They are weakly ionising and very penetrating.

Several centimetres of lead or several metres of concrete are needed to absorb most of the radiation.

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6.5 Half Life

We can measure the radioactivity of a sample of a radioactive material by measuring the count rate from it. 

The radioactivity of a sample decreases over time. How quickly the count rate falls to nearly zero depends on the isotope. Some take a few minutes, others take millions of years.

The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve.

The activity of a radioactive source is the number of nuclei that decay per second.

The number of atoms of a radioactive isotope and the activity both decrease by half every half life.

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6.6 Radioactivity at work

The use we can make of a radioactive isotope depends on its half-life and the type of radiation it gives out

For monitoring, the isotope should have a long half-life.

Radioactive tracers should be beta or gamma emitters that last long enough to monitor but not too long.

For radioactive dating of a sample, we need a radioactive isotope that is present in the sample which has a half-life about the same as the age of the sample.

Alpha sources: used in smoke alarms - poorly penetrating (safe) and needs a half-life of several years.

Beta sources: thickness monitoring when making metal foil - it has the right amount of penetration, and a half-life of many years.

Gamma or beta sources: tracers in medicine - HL of a few hours, very penetrating, not dangerously ionising

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Types of radiation

Alpha:  a particle emitted from nucleus, VERY ionising, deflected by ELECTRIC AND MAGNETIC FIELDS

Beta:  B particle emitted from nucleus as electron, NOT VERY ionising, deflected by electric and magnetic fields in the OPPOSITE DIRECTION to alpha particles

Gamma: ray is an electromagnetic wave released from nucleus with no charge or mass, WEAKLY ionising, deflected by NOTHING

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