Topic 1 - Basic Radioactivity

Radiation is the process by which an unstable parent nucleus becomes more stable by decay into a daughter nucleus by emitting particles and/or energy.  The basic form can be summed up as:

The decay can consist of several steps.  The unstable nucleus can decay to another nucleus of a different atom by a process called transmutation.  If the new nucleus is unstable it will decay again.  This is known as a decay chain.  There may be several steps, some of which last a very long time indeed, or can be very short.  Some elements have a decay time of thousands of millions of years.  In others the decay time can be microseconds.

Elements have different isotopes.  An element and its isotope have:

·       The same number of protons (and electrons)

·       Different numbers of neutrons.

If the isotope is unstable, it is radioactive and is called a radioisotope.  We must be aware that radioactive decay is NOT the same as nuclear fission.

There are three kinds of radiation:

·       Alpha – a helium nucleus

·       Beta – a high speed electron

·       Gamma – an electromagnetic radiation of wavelength about 10^-14 m.

These kinds of radiation can be emitted individually or in any combination, depending on the type of isotope that is emitting the radiation.  Often when an alpha particle is emitted the nucleus is excited and releases the excess energy in the form of a gamma ray or gamma photon.

When specimens of radioactive isotopes decay they do so entirely randomly.  There is no pattern whatsoever, and the rate of decay is not affected by temperature or other physical constraints, or chemical reactions.

The table helps us to compare the properties of radiation

We will look at the mechanisms of production of alpha and beta radiations later.

We need to be aware that elements with unstable nuclei can be harmful to living organisms.

·       Alpha particles are intensely ionising.  The good news is that they are stopped by a few cm of air or by the skin.  The bad news is that if you ingest an alpha emitter, the radiation quickly will macerate the DNA of living cells, such as the lining of the intestines or lungs.  Then you are in serious trouble.  The main fear from the fall-out of a nuclear catastrophe is from alpha emitters (although you wouldn’t want to take a gamma source to bed with you).

·       Beta particles can penetrate the body, but are stopped by a few mm of Aluminium.  They are less damaging than gamma rays or alpha particles.  They are weakly ionising.  Medical tracers are radioisotopes that are beta emitters

·       Gamma rays are considered the most dangerous form of radiation, as they are very penetrating.  They are attenuated by several centimetres of lead, but not stopped completely.  So they can pass easily through our bodies.  Surprisingly, they cause very little ionisation, which causes genetic damage, and are not absorbed very efficiently by DNA, so quite a long exposure to gamma rays is needed to destroy DNA completely.  However random damage can be done by smaller doses.  It can be repaired by the cell’s repair mechanisms, but mis-repair can cause mutations, which can lead to cancer.  Intense radiation can mess up DNA sufficiently to cause radiation sickness.  This can of course apply to other radiations as well.

In the early days of radiation research, people had little clue as to how dangerous the stuff was. In those days lumps of uranium were used as ice-breakers at parties (“Darling, do come and feel my magic metal.”); the metal felt warm, and gave the person feeling it a massive dose of radiation!  Today the nuclear industry takes safety very seriously indeed, and workers are rigorously monitored.  If it appears that personnel are being exposed to higher levels of radiation than they should be, they are withdrawn from that work.  Safety must the primary consideration in every function of the nuclear industry.  However, things can go wrong as in any human activity, e.g. falsification of records, or unauthorised experiments, such as those that led to the Chernobyl disaster, when 7 tonnes of caesium-137 was scattered over Europe.