Revision:Edexcel A Level Physics - Radioactivity

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Radioactivity notes

Have learnt a description of the alpha particle scattering experiment and know how its results led to the nuclear model of an atom.

Alpha particle scattering experiment

• Alpha particles are fired at a thin gold foil
• The coated screen flashes when an alpha particle hits it.
• The majority of alpha particles pass almost straight through the foil.
• Some alpha particles deflect through small angles.
• A very small minority of alpha particles (about 1 in 8000) deflects through more than 90 degrees.

Conclusions made from experiment:

• An atom has a very tiny positively charged centre (the nucleus), which contains most of the atoms mass.
• The nuclei have comparatively large distances between them.

The diameter of an atom is about 10-10 m and that of a nucleus is 10-15 m.

Know the structure of a nucleus and how to find the number of protons and neutrons it contains from its nuclear symbol.

• The structure of an atom was discovered by Rutherford’s scattering experiment by scattering alpha particles from gold foil.
• An atom consists of a very small, central nucleus, containing almost all the atom’s mass, around which electrons orbit
• An atom is neutral, the nucleus is positive, the electrons are negative.
• A nucleus consists of a mixture of particles known as nucleons, symbol *A, where a nucleon is either a proton (positive) or a neutron (neutral).
• A nuclear atom is often represented by its nuclear symbol, from which the number of protons, neutrons and orbiting electrons can be determined.

23 is the 'Mass number, 11 is the Atomic number.

• The Mass number is the total numbers of protons and neutrons in the nucleus of an atom. In this case it is 23 for the sodium atom.
• The Atomic number (also written as proton number) is the number of protons in the nucleus of an atom.
• Subtracting the Mass number from the Atomic number will give you the number of neutrons in the nucleus. *In this case 23-11 = 12 neutrons.
• As atoms are neutral the number of electrons is exactly the same as the number of protons. In this case 11 electrons.

Understand the term isotope.

Isotopes are different atomic forms of the same element which have the same number of protons but a different number of neutrons.

Appreciate that both protons and neutrons have a sub-structure consisting of thrre quarks.

• Both protons and neutrons are known to have their own substructure of particles known as quarks.
• The quark structure of a nucleus can be revealed by scattering experiments using high energy electrons.

Can compare the similarities and differences between the alpha particle scattering experiment and deep inelastic scattering of electrons.

• Evidence for quarks comes from firing electrons at protons.
• Quarks are the basic particles from which protons, neutrons and many other sub-atomic particles are constructed.
• Deep inelastic scattering is a method of probing the structure of subatomic particles in much the same way as Rutherford probed the inside of the atom.
• Deep inelastic scattering of electrons by proton targets revealed that most of the incident electrons interacted very little and pass straight through, with only a small number bouncing back. Much the same as *Rutherford’s scattering experiment.
• This indicates that the charge in the proton is concentrated in small lumps. The charge in the proton is not uniform but split between even smalller charged particles, the quarks
• This is deep inelastic scattering. This scattering is deep as it probes deep into the structure of matter. It is also inelastic because sometimes the electron loses kinetic energy.

Know the nature of the radiations emitted by a radioactive source.

• All emissions from radioactive decay come from the nucleus
• Alpha (positive), beta (usually negative but can be positive) and gamma (no charge) radiations are emitted by a variety of nuclei.

Have learnt the nuclear symbols for an alpha particle, both types of beta particle, gamma radiation, a neutron and a proton.

-symbol for an alpha particle

- symbol for beta plus particle

- symbol for beta minus particle

– symbol for gamma radiation.

- symbol for a neutron

or – symbol for a proton

Can complete and balance nuclear equations.

Balance Nuclear Equations

Rules:

• the total number of protons plus neutrons in the products and in the reactants must be the same.
• the total number of nuclear charges in the products and the the reactants must be the same

Proton: ,

Neutron:

-particle: ,

Electron: ,

Positron: ,

Natural Radioactivity

Beta-particle emission:

Alpha Particle emission:

Electron capture (and positron emission):

• When an atom decays by emitting an alpha particle, it emits two protons and two neutrons.
• When towards the end of its path an alpha particle picks up two electrons and becomes a helium atom.
• In beta minus decay an electron is produced when a neutron in a nucleus splits up into a proton and an ejected beta electron.
• A beta plus particle is a positron (positive electron) produced when a proton in a nucleus changes into a neutron and an ejected beta positron.

Know how to distinguish experimentally between alpha, beta and gamma radiations with reference to their ranges in air and their penetrations through different absorbers.

• Alpha radiation produces a great deal of ionisations (gaining or losing of electrons) i.e. it produces many ions per millimetre along its path. as the alpha particles push their way through material. As a result of this alpha radiation soon runs out of energy and has a very short range.
• Alpha particles are helium nuclei and alpha decay removes two protons and two neutrons from the parent nucleus. (refer back to previous page).
• Beta radiation produces fewer ionisations and so its particles can travel further than alpha particles before running out of energy.
• Gamma radiation produces very few ionisations along its path so therefore has a very large range.
• Gamma radiation is an electromagnetic wave that takes away any surplus energy that a nucleus may have been left with after it has emitted either alpha or beta particles.

Penetration of the radiations emitted by radioactive sources, experiment using Geiger-Muller (GM) tube:

• Use a GM tube with a thin window so that alpha particles can pass into it and so be detected.
• Record a number of count rates with no source present and obtain an average background count
• Background radiation are random emissions from naturally occurring radio-isotopes that must be taken into account whenever performing radioactivity experiments.
• Keep each source a fixed distance from the GM tube (e.g. 1 cm for the alpha source, 3 cm for the beta source, and 6 cm for the gamma source).
• (Place absorbers between the sources and detector).
• Measure the corrected count rate for the alpha source for different thicknesses of paper between it and the G-M tube.
• Repeat for the beta source using thin pieces of aluminium as the absorber.
• Repeat for the gamma source using different thicknesses of lead absorbers.

Ionisation and Penetrating Power

Appreciate the link between a radiation’s ionising ability and its penetrating power or range.

Results of experiment:

• Alpha particles are stopped by 5cm of air, thin paper
• Beta minus 30 cm air, few mm of aluminium, beta plus annihilated on interaction with an electron.
• Gamma can still be detected after passing through several centimetres of lead

These results can be linked with the different radiations ionisations.

{diagram missing showing the results}

Know some sources of background radiation.

• Uranium deposits in the ground, which in turn produce other isotopes that decay.
• Radon gas which spreads throughout the ground and then into the air we breathe.
• Cosmic rays from fast moving nuclei in space.

Half Life

Appreciate that the random and unpredictable nature of an individual decay still leads to an overall predictable decay pattern for the source as a whole and know the meanings and units of ‘activity’, ‘decay constant’ and ‘half life’.

• All radioactive decay is random, the time at which a particular nucleus will decay is unpredictable.
• The activity of a source, is the number of nuclei of that source that decay in one second. It depends on the total number of number of nuclei present at that time.
• The activity of a source decreases with time as the decays taking place reduce the number of nuclei left to decay
• An activity-time graph produces an exponential decay curve: (following page)

• The decay constant is the proportion of the nuclei present that decay in one second.
• Half life is the average time taken for half the nuclei of that radioactive element to decay or the average time for the activity to fall to 50 % of its original value.
• The half life is the same throughout a given decay but varies from source to source.

REMEMBER

• All activity measurements should be adjusted to remove the background activity produced by naturally occurring radio-isotopes and cosmic rays.

When asked to define the decay constant the following word equation is also acceptable:

Decay constant = activity/number of nuclei present

Becquerel (bq): a unit of activity; a count rate of one disintegration per second

= lambda

Have learnt a description of an experiment to measure the half-life of a radioisotope with a half-life of about a minute.

Experiment: Measuring the half-life of protactinium-234.

• Record a number of count rates with no source present and obtain an average background count.
• Shake the ‘protactinium generator’ to transfer the protactinium compound from the lower water-based layer to the upper organic layer.
• When the layers re-establish, place the GM tube alongside the top layer (below).
• Record the count rate at intervals of 10s for 5mins.
• Plot a graph of corrected count rate against time.
• From the graph determine how long it takes for the count rate at any given time to halve its value.

• The GM tube monitors the decay of the protactinium in the top layer.

The half life of this isotope is about one min.

Comments

• Suitable for: A Level physics, especially Edexcel exam board.
• Written by: iceman_jondoe.
• From this post.