LOJ Feb 2004

Radioactivity 1

What is radioactive decay?

LOJ Feb 2004

What is an isotope?

• Atoms of the same element which have different numbers of neutronsare called isotopes.

• They have the same chemical properties but different physical properties.

LOJ Feb 2004

What is an isotope?

• There is no such thing as a 'normal' isotope - call it the 'most common' one.

• Some isotopes are radioactive (radioisotopes) others are stable - the most common ones are generally stable but all of the really heavy elements have isotopes that are ALL radioactive.

LOJ Feb 2004

Radioactive substances

Nuclear radiation• comes from the nucleus of a

radioisotope• is a spontaneous process (it is

not affected by pressure temperature etc.)

• Is a random (follows the laws of probability) process.

LOJ Feb 2004

What is radioactive decay?

• When an unstable nucleus splits up (disintegrates):• it emits (alpha) or (beta) radiation;• a different atom, with a different number of protons, is formed.

• Sometimes an energy change within the nucleus also occurs – this results in the emission of a ray

LOJ Feb 2004

What is radioactive decay?

• Radioactivity occurs as a result of changes in the nuclei of atoms (nuclear changes).

• Radioactive isotopes (radioisotopes or radionuclides) are atoms with unstable nuclei.

LOJ Feb 2004

Alpha Decay

• A nucleus that has high mass and too many protons to be stable tends to undergo alpha decay.

• When alpha decay occurs a group of two protons and two neutrons (helium nucleus) comes out of the nucleus. Therefore the proton number decreases by 2 but the nucleon number decreases by 4. The resulting daughter nucleus is of an element 2 positions to the left of the 'parent' in the periodic table.

LOJ Feb 2004

LOJ Feb 2004

• The above equation shows the radioactive decay of Uranium-238 by alpha emission.   – Look at the numbers on the top line (the nucleon

numbers). • 238 = 234 + 4 Therefore the nucleon

numbers balance

– Look at the numbers on the bottom line (the proton numbers).

• 92 = 90 + 2 Therefore the proton numbers balance

LOJ Feb 2004

Beta Decay• When a nucleus has too many neutrons, it

tends to beta decay. • When beta decay occurs a neutron within the

nucleus emits the particle and changes into a proton.

• Therefore the proton number increases but the nucleon number stays the same (only now you have one more proton and one less neutron!).

• The resulting daughter nucleus is of an element one position to the right in the periodic table.

LOJ Feb 2004

LOJ Feb 2004

LOJ Feb 2004

A neutron becomes a proton!

Beta decay is basically the decay of a neutron into the more stable proton

LOJ Feb 2004

• The above equation shows the radioactive decay of Carbon-14 by beta emission. – Look at the numbers on the top line (the nucleon

numbers).

• 14 = 14 + 0 Therefore the nucleon numbers balance

– Look at the numbers on the bottom line (the proton numbers).

• 6 = 7 + (-1) Therefore the proton numbers balance

LOJ Feb 2004

Gamma Emission

• Sometimes, after its emission of an alpha, beta or positron (not on the syllabus!) particle, the nucleus is still in an excited state, called a metastable state.

• In order to get to a lower energy state it emits a quantum of energy in the form of a gamma ray.

LOJ Feb 2004

Gamma Emission

• This is not a highly unstable state otherwise the emission of the gamma ray would immediately accompany the alpha, beta or positron particle.

• Nuclei in the metastable state produce gamma rays at a measurable half-life.

LOJ Feb 2004

• Cobalt 60m decays to give cobalt 60 with a half-life of 5.3 years and technetium 99m decays to give technetium 99 with a half-life of 6 hours.

LOJ Feb 2004

• No matter is emitted from the nucleus therefore the nucleon number and the proton number remain the same.

• Before and after emission of the gamma ray they are the same isotopes of the element but they are different nuclides because the term nuclide incorporates nuclear energy states as well basic structure.