Revision

P2bL10 Radiation (Revision)

Key Words

Alpha - a helium nucleus ejected from a radioactive atom.

Atomic number - number of protons in the atom.

Beta - a high speed electron emitted from the nucleus.

Disintegration - when an atom comes apart

Electrons - tiny particles of negative charge. Mass of an electron is 1/1800 th the mass of a proton.

Gamma - an electromagnetic radiation emitted from the nucleus.

Ion - a charged atom.

Ionisation - knocking electrons off atoms.

Isotope - same number of protons, different numbers of neutrons.

Mass number - the total number of protons and neutrons.

Neutrons - particles in the nucleus that carry zero charge.

Nucleus - where the protons and neutrons are.

Proton - particles in the nucleus that carry a positive charge.

Radiation - energy coming from the nucleus in the form of particles or electromagnetic waves.

Radioactive decay - unstable nuclei decay to form other nuclei.

Unstable - the nucleus is likely undergo radioactive decay.

Grade E

Atoms consist of a nucleus, which contains protons and neutrons. There are electrons orbiting the nucleus.

Protons are positively charged;
Neutrons carry no charge;
Electrons are negatively charged.

This atom is neutral. It has the same number of electrons (3) as protons. If an electron is removed, the atom becomes a positive ion. If an electron is added, the atom becomes a negative ion. All chemical reactions between elements involve the electrons.

Here is a picture of carbon 12.

It has 6 protons, and 6 neutrons. Its atomic number is 6 because it has 6 protons. Its mass number is 12, because there are 6 protons and 6 neutrons. Now look at carbon 14:

This atom still has an atomic number of 6, but has a mass number of 14, because there are 6 protons and 8 neutrons. It is an isotope of carbon. An isotope has the same number of protons and a different number of neutrons.

This isotope has too many neutrons and is unstable; it decays radioactively, giving out a beta particle. One of the neutrons turns into a proton and a high speed electron (beta particle) is kicked out of the nucleus.

carbon-14 ® nitrogen-14 + beta particle.

he three kinds of radiation are shown in this table:

*Radiation*	*What it is*	*Range*	*Stopped by*
Alpha (a)	Helium nucleus (NOT atom)	1 cm in air	Skin, sheet of paper
Beta (b)	High speed electron	50 cm in air	3 mm aluminium sheet
Gamma (g)	Electromagnetic radiation	Infinite	Several cm lead

Radiation can be detected by a Geiger-Müller Tube connected to a counter. If you turn one of these on in the lab, it will count even though there is no source out. This is because there is always background radiation, coming from soil, rocks, space, and even living things. Some is man-made, but the rate is so low that it does no harm.

Alpha particles, beta particles, and gamma rays are NOT radioactive themselves.

Grade C

Atoms are often represented like this:

The atomic number is the number of protons

The mass number is the number of protons and neutrons.

Therefore:

Number of neutrons = mass number - atomic number

Remember that if the atomic number stays the same, but the mass number changes, we have an isotope. However, if the atomic number changes, the element changes, even if the mass number stays the same.

In lighter elements the number of neutrons generally balances the number of protons, up to about atomic number 20.

For heavy elements there are many more neutrons than protons:

Element	Mass Number	Atomic Number	Neutrons
Uranium 238	238	92	146
Uranium 235	235	92	143
Plutonium	233	94	139

Nuclei with atomic numbers greater than 82 (lead) are unstable. The nuclei are just too big to hold together permanently. Most decay by alpha decay. Uranium decays this way, but its half life is very long indeed - 4500 million years. Some elements and isotopes have a half life of a tiny fraction of a second.

Many isotopes of elements of atomic number below 82 are unstable, because there is an imbalance of neutrons. Cobalt has a mass number of 59. Cobalt-60 decays giving off gamma rays.

Ionising radiations knock electrons off atoms, making them positive ions.

The ionising radiations above are shown in more detail in the next table:

*Radiation*	*Mass*	*Charge*	*Ionisation*	*Use*
Alpha (a)	4 times mass of a proton	+2e ( 2 × charge of an electron)	Intensely ionising	Smoke detectors
Beta (b)	Mass of electron (1/1800 mass of a proton)	-1e	Moderately ionising	Thickness detectors
Gamma (g)	Zero (it's a wave)	Zero	Weakly ionising	Killing cancer cells

Alpha particles will not penetrate skin, but if placed near living cells, will kill them. That is why it is highly dangerous to ingest an alpha emitter. Gamma emitters are used for medical tracing.

Background radiation is particularly strong where there are a lot of granite rocks. The rocks give off radon, a radioactive gas which can leak through the ground into peoples' houses to cause illness. Houses are now ventilated to reduce the background radiation. Areas at particular risk are South West England and North East Scotland.

Pilots of high-flying aeroplanes are at risk from cosmic rays, as the atmosphere is very thin. Radiographers and other people working with radiation may receive a higher dose of background radiation. They wear film badges which show the amount of radiation to which they are exposed. If they are exposed to too much they are withdrawn from working with radiation for a period of time.

Grade A

The charged radiations (a and b) interact with magnetic fields and electric fields. Gamma rays do not. This can be summed up in the pictures below:

In an electric field:

The alpha particles, being positively charged, are attracted towards the negative plate. Beta particles are negative and are attracted to the positive plate.

For a magnetic field:

The charged particles are deflected by the magnetic field.

Gamma rays, not being charged, are not affected by either an electric or a magnetic field.

When we are doing counts, we need to take the background count into account. This we do by timing the count for a period of time, then doing an average, to give an average count per second.

average count per second = total count in the time period ÷ time period

If we measure over 60 seconds, we would divide by 60.

Each count occurs when a nucleus disintegrates and radioactive decay is an entirely random event. Sometimes there are a lot of counts a second, then only a few. So again we need to time over a period and do an average.

Therefore:

average activity count per second = total average count per second - average background per second

The activity of a source is measured in becquerels (Bq) where 1 Bq = 1 count per second.

A typical alpha decay is that of radium to form radon, a radioactive gas.

Note that:

The mass number goes down by 4;
The atomic number goes down by 2;
The alpha particle is a helium nucleus.

The alpha particle is NOT a helium atom.

Polonium decays by beta decay to form Astatine, one of the halogens.

Note that:

The mass number stays the same;
The atomic number goes up by 1;
The beta particle is a high speed electron;
The curious looking symbol is an electron antineutrino (you are not expected to know this for the exam).

The beta particle (electron) comes from the nucleus, NOT the electron shells.