The radiation can be detected and measured in several ways:

• By use of a Geiger-Muller (GM) tube and counter. This electronically amplifies the ionising effect of the radiation and is used for very accurate measurements of radioactivity and it can detect a single radioactive event.
• A Geiger-Muller (GM) tube and counter set up in the laboratory may record a background radiation of 25 counts per second.
  • That means 25 individual, mainly gamma rays, and some beta particles (probably no alpha particles) are 'hitting' the approximately 1cm² detector area every second.
  • So, think how many must hit your body!, but don't worry, we seem to have survived millions of years of evolution so far, and the body's repair system can deal with a few hits!
  • Just out of curiosity, look up how many neutrino's we survive from passing through our body from the Sun every second! its scarry!!!!!
• Photographic film reacts to radiation in the same way as it does to light. It is used in film badges by workers in the nuclear industry and hospitals to monitor how much radiation people are exposed to in their potentially harmful environment. The film is developed after specified time interval, and the amount of 'exposure' or darkening of the film is a measure of how much radiation has 'hit' the person.
• The activity of a radioactive source is measured in ...
  • Becquerel units (Bq, s^-1), 1 becquerel = 1 disintegration of an unstable nucleus per second.
  • or in curie (Ci, 3.7 x 10^-10 s^-1), 1 curie = 3.7 x 10¹⁰ disintegrations per second.
  • A disintegration means the decay or breakdown of an individual unstable nucleus,
  • so 1 curie = 3.7 x 10¹⁰ becquerel of unstable nuclei decaying per second.
• Doses of radiation are measured in gray, sievert or roentgen.
  • Gray units (Gy, J kg^-1) are based on the absorbed dose of ionising radiation energy in joules per kilogram of absorbing material.
    • 1 Rad = 10^-2 Gy
  • Sievert units (Sv, J kg^-1) are based on the dose equivalent of ionising radiation and these units seem to the most important when dealing with health and safety issues.
    • 1 Rem = 10^-2 Sv
  • Röentgen units are based on the ionising effect of the radiation.
    • 1 röentgen = 2.58 x 10^-4 C kg^-1 (charge in coulombs per kilogram of material)
• Radioactive contamination in a material e.g. its activity in food, might be measured in Bq/Kg or Bq/litre.
• Biologically significant levels of radiation:
  • Maximum dose allowed for general public: 5 mSv/year (mSv = millisievert = Sv/1000, 1 mSv = 100 mRem)
  • Maximum dose allowed for radiation workers (medical, industrial, nuclear power): 50 mSv/year
  • Natural background dose rate: 1.25 mSv/year
  • Maximum dose due to atmospheric atomic weapon testing 1954-61: 12µSv/year (µ = micro = 10^-6)
  • Maximum dose due to medical and industrial use: 120µSv/year
  • Average dose due to nuclear reactors: 2µSv/year
  • Threshold for nausea ('radiation sickness'): 1 Sv in a few hours
  • Threshold for death: 1.5-2.0 Sv in a few hours (not 100%, but fatalities start to occur in the days or weeks after exposure to the radiation)

• If a Geiger counter is set up anywhere in the world it will register (hopefully!) a low level of radioactivity.
• This is called the background radiation and there are two sets of sources for it.
• When doing accurate experiments this background radiation must be taken into account.
• The background radiation is measured and subtracted from any experimental results using radioisotopes.

• Radiation from outer space eg cosmic rays from the Sun.
• Radioactivity from naturally occurring radioisotopes in rocks at the surface eg there are traces of radioisotopes of uranium in granite rocks.
• The radioactive gas Radon is formed in the process, and can build up to harmful levels in cellars.
• Radioactivity from naturally occurring radioisotopes deep in the Earth's core, the energy released keeps the core very hot and heats the magma in the Earth's mantle

• Emissions from nuclear power stations (governed by health and safety legislation, they are allowed to emit tiny amounts of radioactive material into the environment).
• Safe storage of nuclear waste from power stations is a current problem that is yet to be solved for the long-term future. It is very contentious issue for obvious health, safety and environmental reasons and no satisfactory solution has been found to the problem of safe waste disposal.
  • The used radioisotopes and nuclear fuel most be processed into a safer form eg a glass solid. This solid waste is stored in long-term and leak-proof containers which could be buried in a deep and well shielded storage area underground.
  • BUT even before this long-term process, nuclear reactor/weapon waste is particularly and exceptionally dangerously radioactive due to radioisotopes with short half-lives. So initially it is stored in containers under water until it has 'cooled off' and safer to handle.
  • Some idea of the dangers and problems in handling radioactive materials are mentioned below and the long-term considerations in the notes on half-life data.
• Radioisotope tracers are used in industry and hospitals (see later) and so their use and disposal must be carefully controlled.
• Nuclear accidents, the worst being at Chernobyl power station in Russia. Parts of the Lake District in England are still contaminated from the 'fallout' in the rain.
• Atomic weapons testing in the 40's, 50's and 60's. The 'super powers' were testing their latest nuclear bombs in the air or on the surface, producing contaminated dust in the atmosphere. Some of the radioisotopes formed in the explosions, like strontium-90, are still around.

7. The Dangers of Radioactive Emissions - beware of radio-isotopes!

The penetration trends and the effects of Ionisation

All radioactive emissions are extremely dangerous to living organisms. When alpha, beta or gamma radioactive emissions hit living cells they cause ionisation (ionization) effects, they can kill cells directly or cause genetic damage eg to the DNA molecules. High radiation doses cause burn effects and can kill cells. However, low doses don't kill the cells, but if they are genetically damaged and can still replicate, these mutations can lead to the formation of cancerous cells and tumor development later. When alpha, beta and gamma radiation collide with neutral atoms or molecules they knock off electrons and convert them into charged or ionised particles (ions). Positive ions are formed on electron loss and negative ions are formed by electron gain. The positive ions maybe unstable and very reactive and cause other chemical changes in the cell molecules. The 3 radiations have different capacities to cause cell damage.

• If the radioactive source gets inside the body the 'danger' order is alpha > beta > gamma.  The bigger the mass or charge of the particle, the bigger its ionising impact on atoms or molecule. BECAUSE the order of mass is 4 > ¹/₁₈₅₀ > 0, and for electric charge the order is 2+ > -1 > 0.  If the radioisotope is in the body the radiation impacts directly on cells with the consequences described above.
• However, if the radioactive source is outside the body, the order danger is reversed to gamma > beta > alpha because the danger order follows the pattern of penetrating power. The smaller the mass and charge the more penetrating the radiation (reverse the order of above). Gamma and beta are the most penetrating and will reach vital organs in the body and be absorbed.  Most gamma passes through soft tissue but some is inevitably absorbed by cells.  Alpha radiation would not penetrate clothing and is highly unlikely to reach living cells.
• Because of the dangers of this ionising or atomic radiation, all workers and medical staff who are likely to be near radioactive or ionising sources must wear lapel radiation badges containing photographic film to monitor their exposure to radiation. The film is regularly developed and the darker the film the more radiation would have impacted on the person. 
• Examples of precautions that can be taken include:
  • Radiographers wear lead lined aprons and anyone else involved in radiotherapy cancer treatment must take particular precautions and radiation monitored.
  • In nuclear fuel preparation and reprocessing, as much work is done using robotic control systems in behind steel, concrete, lead or thick lead glass panels for visual monitoring of the situation.
  • In research laboratories, experiments are conducted in sealed fume cupboards at the laboratory side and technicians work through sealed whole arm gloves through a thick lead glass front. You can also reduce the pressure in the fume cupboard so there is no chance of pressure leakage out into the laboratory area.

The uses of radioactive isotopes depends on their penetrating power and the value of their half-life (see later).

• Because alpha particles are easily stopped, an alpha source is used in some smoke detectors. A sealed alpha source of Americium-241 (half-life 458 years, producing constant signal) sends a stream of alpha particles to a sensor across an air gap. Any smoke present will block the alpha particles and change the sensor signal, this change in signal triggers the alarm. Beta and gamma radiation would be of no use because the smoke particles would not stop them, no change in signal, no alarm triggered!

• Most Beta particles are stopped by a few mm or cm of solid materials. The thicker the layer the more beta radiation is absorbed. A beta source is placed on one side of a sheet of material. A detector (e.g. a Geiger counter) is put on the other side and can monitor how much radiation gets through. The signal size depends on thickness of the sheet and it gets smaller as the sheet gets thicker. Therefore the signal can be used to monitor the sheet thickness. The half-life must be quite long so that change in the signal does not result from rapid decay.
• This idea is used to control production lines of paper, plastic or steel sheeting. Before the sheet material passes through 'flattening' rollers, it passes between a beta source and detector. The detector signal is checked against that for a preset thickness. If the signal is too big the sheet is too thin and the rollers are moved apart to thicken the sheet. If the signal is too small the sheet is too thick and the rollers are moved closer together.

• Gamma radiation is highly penetrating and so gamma sources are used where the radiation must be detected after passing through an appreciable thickness of material. This is used in various tracer situations and usually the half-life should be relatively short to avoid any health hazards.
• A gamma emitting tracer can be added to the flow of water in a pipe and the outside of the pipes monitored with a Geiger counter. Any leaks would be detected by an increase in radiation reading. The flow of water in underground streams can be followed in a similar way.
• Radiotherapy: It seems ironic that the very radiation which causes cancer, can also be used to treat it. A beam of gamma radiation is directed onto the tumor site to kill the cancer cells. Unfortunately the radiation passes through the 'good' tissue too and kills or damages 'good' cells. Modern techniques use multiple rotating gamma sources that are focused on to the tumor. This means the surrounding 'good cells' are less frequently hit and minimises potential harmful side-effects on the rest of the body (e.g. sickness or other mutations). Radiotherapy also avoids the need for intrusive surgery which has its own risk factors. The gamma emitters used have relatively long half-lives to give the instrument a good working life.
• Gamma radiation can be used in a non-destructive way to test the structure of a material.
  • In a sense it is an alternative to X-ray photography for more dense materials e.g.
  • It is used test the structure and quality of pipe welds.
    • A gamma source is placed inside the pipe and photographic paper wrapped around the weld.
    • If there is any gap or flaw in the weld, more gamma radiation gets through and shows up as increased exposure on the 'gamma-ray picture'.
    • Its better to find out the fault now, rather than later when it fractures, and has to be 'dug up' or retrieved from the bottom of the sea!
• Because gamma radiation is so deadly and penetrating it can be used to sterilise surgical equipment or packaged food:
  • The radiation is deadly for bacteria even in the most microscopic pockets of apparently smooth and shiny stainless steel of surgical instruments.
  • It is very convenient for 'convenience' food!. After cooking and sealing in a plastic packet, you don't need to reopen to complete the sterilization to give it a long shelf-life!
  Technium-99 is a gamma emitter (half-life 6 hours) and is used as a medical tracer.
  •  In a suitable chemical form, it is injected into the body and its 'movement' can be followed. Time is allowed for the radioactive tracer to spread and its progress tracked with a detector outside the body.
  • The patient can be placed next to a 'detection screen' that shows where the radioactive tracer is.
  • The effective function of organs like the liver and digestion system can be checked.
  • Similarly, a patient can breathe in air with a gaseous gamma emitter in it, and the effectiveness and structure of the lungs can be checked.
  • The half-life should be long enough to allow good detection BUT NOT too long to be dangerous to the body over a period of time.
  • Beta sources can be used, though not as penetrating as gamma. 
  • Alpha sources are too readily absorbed to show up with a Geiger counter and so are not suitable for these 'tracer' applications.

• Some atomic nuclei are very unstable and only exist for a few seconds or minutes. Others are very stable and take millions of years to decay away to form another atom. A measure of the stability of a radioisotope is given by its half-life.
• The half-life of a radioisotope is the average time it takes for half of the remaining radioactive atoms to decay to a different atom. It means in one half-life of time, on average, half of the undecayed unstable nuclei of a particular isotope disintegrate.
• It can vary from a fraction of a second to millions of years!
• The radioactivity of any sample will decrease with time as the unstable atoms decay to more stable atoms, though sometimes by complex decay series routes e.g. ₉₂U isotopes eventually decay to ₈₂Pb isotopes.
• An example of what this means is shown in an Excel file. (after viewing the file, use <== or BACK to return here)

Four Uses of decay data and half-life

For Higher students only. The older a sample of a radioactive material, the less radioactive it is. The decrease in radioactivity follows a characteristic pattern shown in the graph or decay curve. After every half-life, (in this case 5 days, working out from the graph), the % radioisotope (or radioactivity) is halved, producing the initially steeply declining curve which then levels out towards zero at infinite time!

(1) Determination of the half-life of a Radioisotope

• The radioactivity from a radioisotope is measured over a period of time. Graphical or mathematical analysis is performed to calculate the time it takes for the radioactivity of the isotope to halve. The radioactivity is likely to be measured in terms of the count rate so the half-life will be the time it takes for the count rate to halve.
• An example of what this means is shown in an Excel file. (after viewing the file, use <== or BACK to return here) or shown in the diagram above.
• You need to practice these sort of calculations of half-life determination, radioactive residue left, and dating calculations (see below) using the multiple choice QUIZ (higher GCSE)

• From the half-life you can calculate how much of the radio-active atoms are left e.g. after one half-life, ¹/₂ is left, after two half-lives, ¹/₄ is left, after three half-lives, ¹/₈ is left in other words its a 'halving pattern' etc.
  • Example Q: The half-life of a radioisotope is 10 hours. Starting with 2.5g, how much is left after 30 hours?
    • 2.5g =10h=> 1.25g =10h=> 0.625g =10h=> 0.3125g (after total time of 30h)
• The half-life of a radioisotope has implications about its use and storage and disposal. If the half-life is known then the radioactivity of a source can be predicted in the future (see (1) above). Plutonium-244 produced in the nuclear power industry has a half-life of 40 000 years! Storage of waste containing these harmful substances must be stable for hundreds of thousands of years! So we have quite a storage problem for the 'geological time' future! see also dangers and background radiation.
• Radioisotopes used as tracers must have short half-lives, particularly those used in medicine to avoid the patient being dangerously over exposed to the harmful radiation, but a long enough half-life to enable accurate measurement and monitoring of the tracer.

(3) Archaeological dating with the isotope carbon-14

* Most carbon atoms are of the stable isotope carbon-12. A very small % of them are radioactive due to carbon-14 with a half-life of 5700 years. It decays by beta emission to stable nitrogen-14. Archaeologists can use any material containing carbon of 'organic living' origin to determine its age. This can be bone, wood, leather etc. and the tecnique is sometimes called radiocarbon-14 dating.

• When the 'carbon containing' material is in a living organism there is a constant interchange of carbon with the environment as food or carbon dioxide. This means the carbon-14 % remains constant. When the organism is dead the exchange stops and the carbon-14 content of the material begins to fall as it radioactively decays.
• Compared to when it was 'alive', if an object has ¹/₂ of the expected carbon-14 it must be 5700 years old. If it only has ¹/₄ (¹/₂ of a ¹/₂) of the expected carbon-14 left, the object it must be 11400 years old (5 700 + 5 700). If only ¹/₈ (¹/₂ of ¹/₄) left it is 17100 years old (11 400 + 5700) etc. etc. The decay curve for carbon-14 is shown in an Excel file. (use BACK button to return here)

• Certain elements with very long half-lives can be used to date the geological age of igneous rocks and even the age of the Earth. has a half-life of 1.3 x 10⁹ years. It decays to form .
• If the argon gas is trapped in the rock, the ratio of potassium-40 to argon-40 decreases over time and the ratio can be used to date the age of rock formation i.e. from the time the argon gas first became trapped in the rock. The method is more reliable for igneous rocks, rather than sedimentary rocks because the argon will tend to diffuse out of porous sedimentary rocks but would be well trapped in harder and denser igneous rocks.
  • If the ⁴⁰Ar/⁴⁰K ratio is 1.0 (50% of ⁴⁰K decayed, 50% left )  the rocks are 1.3 x 10⁹ years old
  • If the ⁴⁰Ar/⁴⁰K ratio is 3.0 (75% of ⁴⁰K decayed, 25% left)  the rocks are 2.6 x 10⁹ years old
  • If the ⁴⁰Ar/⁴⁰K ratio is 7.0 (87.5% of ⁴⁰K decayed, 12.5% left)  the rocks are 3.9 x 10⁹ years old
    • These are worked out on the basis of 100% =half-life=> 50% =half-life=> 25% =half-life=> 12.5% etc. etc.
• Long lived isotopes of uranium (element 92) decay via a complicated series of relatively short-lived radioisotopes to produce stable isotopes of lead (element 82). The uranium isotope/lead isotope ratio decreases with time and so the ratio can be used to calculate the age of igneous  rocks containing uranium compounds.

10. What happens overall in Alpha and Beta Radioactive Decay?

• Alpha Decay e.g. the nuclear equation  

  • A helium nucleus, the alpha particle, of 2 protons and 2 neutrons is emitted at high speed/kinetic energy from the nucleus.

  • The residual atom (sometimes referred to as the politically incorrect 'daughter nuclide'!*) has a mass number of 4 less, and an atomic number of 2 less, than the 'parent' or original atom.

  • Most atoms with an atomic number of over 82 (Pb) usually undergo alpha decay.

  • * apart from Marie Curie, in the late 19thC/early 20thC, nuclear physics was dominated by male scientists! 

• ^- Beta^- Decay e.g. the nuclear equation   

  • A neutron in the nucleus changes spontaneously into a proton and a high kinetic energy electron forms the emitted beta particle.

  • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number rises by 1.

  • This tends to happen with isotopes with too many neutrons to be stable (too high an n/p ratio) and lies above the stability curve shown in a previous graph. By changing a neutron to a proton the n/p ratio is reduced to an isotope lying in the stability band.

• Balancing: The changes can be represented as nuclear equations and they must balance in mass number and nuclear or emitted particle charge (protons in alpha decay, protons and electrons in beta decay). In (1) mass = 235 = 231 + 4 and protons = 92 = 90 + 2. For (2) mass = 14 = 14 + 0 and for protons/beta charge = 6 = 7 + (-1).  In either case a new element is formed ie the 'transmutation' of one element to another has happened. It also means that there can never be a 'pure' Radioisotope.

• Gamma emission: The emission of gamma radiation from a nucleus does not involve any change in the atomic (proton) number or mass number.

  • When a 'new' nucleus is formed it tends to have excess energy making it potentially unstable.

  • To become more 'nuclear stable' the nucleus loses some energy as a burst of gamma radiation but the proton and neutron numbers do not change.

• ⁺ Positron emission (beta⁺ decay): e.g.

  • A proton changes to neutron  and  a 'positive electron' called a positron is expelled with very high kinetic energy. A positron has the same mass as an electron but carries a positive charge (it is the 'anti-matter' particle of the electron!).

  • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number falls by 1.

  • This tends to happen with isotopes with too few neutrons to be stable (too low an n/p ratio) and lies below the stability curve shown in a previous graph. By changing a proton to a neutron the n/p ratio is increased to an isotope lying in the stability band.

11. The production of Radioisotopes - artificial sources

To meet the industrial and medical demand for Radioisotopes (as described earlier) many are made by allowing stable isotopes to be hit by neutrons in a nuclear reactor. Note again, the balancing of nuclear equations eg

to make carbon-13, used as a chemical tracer carbon in studying the mechanisms or organic chemistry reactions

to make sodium-24, which can be used in tracer studies of animal blood circulation

to make cobalt-60, used as the gamma source for cancer radiotherapy.

• At the extremely high temperatures in the 'heart' of stars the atomic nuclei have such enormous speeds and kinetic energies that on collision they can fuse together.

• The extremely high energy is needed to overcome the natural and massive repulsion of the two positive nuclei involved.

• The process by which a heavier atomic nucleus is made from two smaller atomic nuclei is called fusion and these changes also release enormous amounts of energy.

• The smallest atom is hydrogen, this is converted to helium and gradually all the other elements up to uranium must have been formed in stars like the Sun.

• Examples of fusion nuclear equations (get the balancing?) ....

• (a) (initially a heavier isotope of hydrogen is formed and a positron)

• (b) +

• (c)

• (d)

• (e)

• (f) (the most abundant helium isotope found today)

• (g)

• (h) helium nuclei fuse to form lithium, beryllium etc.

• (i) then from carbon to oxygen etc.

• (j) and lots of alternative fusions like ... etc.

• (k) gradually building up elements with increasing atomic and mass numbers, and finally the massive isotope of uranium, the biggest 'naturally' occurring atom!

• (a), (b) and (f) are believed to be the main initial energy releasing fusion nuclear reactions in the Sun, they happen quite nicely at 15000000^oC!

• On 'Earth' super-heavy' elements are being made in nuclear reactors by bombarding elements like uranium (atomic number 92) with lighter particles (described below).

• Heavy atomic nuclei tend to be naturally unstable and for example, many long lived isotopes of uranium (U₉₂) finally decay via a series of relatively short-lived radioisotopes to produce stable isotopes of lead (₈₂Pb).

• No element higher than uranium (₉₂U) is found in nature except for traces of neptunium (₉₃Np) and plutonium (₉₄Pu) isotopes. These are found in uranium ores but are produced by neutron-uranium collisions rather than from the Earth's origin. The neutrons come from the spontaneous fission of the unstable uranium isotope ²³⁵U and gives rise to heavy element 'synthesis' sequence.

  • eg ²³⁸U == + n ==> ²³⁹U == beta decay ==> ²³⁹Np == beta decay ==> ²³⁹Pu

• Even heavier or 'trans-uranium' elements can be made by bombarding a heavy atomic nucleus with a smaller ionised atom particle, in an ion particle accelerator.

  • However many of the heaviest are only produced in minute quantities as little as a few hundred atoms in accelerator collisions.

  • In an accelerator the two atoms are ionised and accelerated in powerful electromagnetic fields to very high speeds eg close to speed of light, but in opposite directions and are then allowed to collide. The high kinetic energies are needed to overcome the repulsion of the two positive nuclei.

  • See examples 1. to 3. below.

• The heavier elements are also made by neutron bombardment in a nuclear reactor.

  • Although most neutrons partake in nuclear fission reactions (see next section), in some cases this will create a bigger nucleus.

    •  e.g. Np and Pu 'natural' examples above and example 4. below.

• So, from these two methods, a whole series of man-made or 'artificial' elements from atomic number 93 to 112 have been synthesised.

• Where they are formed in nuclear reactors from neutron collision (e.g. plutonium), they can be chemically separated in quantities ranging from micrograms to kg in order study their physical and chemical properties.

• Note again, the balancing of nuclear equations e.g.

1. formation of einsteinium from uranium and nitrogen nuclei

2. formation of californium from uranium and carbon nuclei

3. formation of lawrencium from californium and boron nuclei

4. formation of americium from plutonium and neutrons

You can find out about the heaviest elements made so far from various Periodic Table sites and six have been picked out for more detailed information - elements in alphabetical order BUT look for atomic numbers over 100 and click on 1 - 6 etc.!!