For some reason, not fully understood, the atoms of certain elements are not stable; that is, they do not keep the same structure. They spontaneously emit one or more parts of their structure. These emissions, which are always from the nucleus of the atom, make the atom smaller. It will also possess less energy or be changed in some other way. Whatever happens, by losing something the atom appears to decay. The emissions are called radioactivity and so the whole process is known as RADIOACTIVE DECAY.
The majority of naturally occurring elements have stable atoms; just a few of the elements with large atoms are unstable. However, stable atoms can be made to be unstable, or radioactive, by bombarding them with sub-atomic particles such as neutrons or protons. Unstable atoms of this type are usually referred to as RADIO-ISOTOPES.
Radioactivity is both spontaneous and random; spontaneous because no external energy needs to be supplied to the decaying nucleus - all the energy for the emission comes from the atom itself; and random because the radiations are emitted at irregular intervals and are not predictable.
After an emission has occurred the resulting atom may be stable or unstable. If it is stable it remains as it is, but if it is unstable then further emissions can be expected, with the production of other atoms. No matter whether the change is to a stable or an unstable nucleus, the change is not reversible. It is not possible to control the changes, which are unaffected by temperature, pressure or any chemical reaction.
Types of emission:
Unstable atoms will emit one of three types of radioactivity, often called radiations. The three types are: ALPHA, BETA and GAMMA, named after the first three letters of the Greek alphabet.
These three radiations are called IONISING RADIATIONS because they have the ability to upset the electrical balance of atoms, thus producing ions. You can find out more about the three types of radiation later in this section.
Discovery:
The discovery of radioactivity was not the work of one person, nor was it the type of discovery that can be given a specific date. It was more of an on-going series of discoveries that linked together. Indeed, discoveries are still being made, so the full story of radioactivity is still not known. It is possible to record the important steps in mankind's discovery of radioactivity and to link the observational and theoretical work of a large number of scientists that gives us our current understanding of the subject. Some early key discoveries are listed below.
1895: Wilhelm Rontgen, who discovered X-rays, noticed that certain substances glowed when struck by X-rays. These substances were radioactive, although radioactivity had not been given a name at this time.
1896: Henri Bequerel attempted to reverse Rontgen's discovery by using radioactive materials to produce X-rays. He discovered the radioactivity of uranium.
1897: Joseph Thomson discovered electrons as particles within the atomic structure.
1898: Marie and Pierre Curie discovered highly radioactive elements such as radium.
1899: Ernest Rutherford discovered the radioactive gas, radon.
1900: Pierre Curie named two different types of radiation, alpha and beta.
1900: Paul-Ulrich Villard discovered and named gamma rays.
1900: William Crookes discovered radioactive thorium-234.
1901: Max Planck proposed the quantum theory, that is, that radiations travel in small 'packets' which he called quanta.
1902: Ernest Rutherford and Frederick Soddy understood that the nuclei of atoms change after radioactive emissions have occurred.
1903: William Ramsay and Frederick Soddy discovered that helium is produced by the decay of radium.
1905: Albert Einstein proposed his theory of relativity.
1905: Egon von Schweidler formed the statistical laws that govern radioactive decay.
1910: Frederick Soddy introduced the concept of isotopes.
1911: Ernest Rutherford suggested that most of the mass of an atom is concentrated at its centre and is positive.
After the First World War research became intense, leading to the unleashing of energy from the nucleus of the atom.
Detection and measurement (The Geiger Counter):
Radioactivity can be detected in a variety of ways. It gets its name from the fact that radiations disrupt radio waves, causing interference on a radio receiver.
One of the earliest ways of detecting the presence of radioactivity was with photographic plates and films, that is, glass or plastic coated with a chemical such as silver bromide. The silver bromide behaves in a similar way to when exposed to light. A piece of photographic material exposed to radioactivity will appear dark after development; the greater the exposure the darker the film will be.
The most usual form of detection and measurement of radiations of this type is by the use of a Geiger-Muller tube (G-M tube), named after two scientists who did much of the early work related to radioactivity.
Hans Geiger (1882 -1945) was a German physicist who introduced the first successful instrument for the detection and counting of alpha radiation. Erwin Muller (1911 - 1977) was a German physicist who, in 1951, moved to the USA where he became professor of physics at Pennsylvania State University. Muller was an expert on ionisation, the principle upon which the G-M operates. The ideas of these two scientists were combined to form the modern day Geiger-Muller tube.
The G-M tube is an aluminium tube with a wire through its centre, placed in line with the sides of the tube. The tube is given a strong negative charge while the wire is made strongly positive. Both of the electrical charges on the electrodes are the result of being connected to an extra high tension supply (EHT) of at least 400 volts. A thin mica window seals the front of the tube while an insulator seals the back. External connections from the EHT are made through the insulator. Air has been removed from the tube and replaced with argon at low pressure and a trace of bromine.
The thin mica window allows the entry of active particles and gamma radiation. If any of these enter the tube then ionisation of the gas inside occurs. Ionisation means that the electrical balance of one or more atoms is upset and the atoms of the gas concerned are now charged particles called IONS.
If the ions are positively charged then they will be attracted to the negative electrode (in the case of the G-M tube). If the ions are negatively charged then they are attracted to the positive electrode. A G-M tube is an example of a device known as an ionisation chamber. As the ions arrive at the electrodes small electrical pulses are produced. These pulses are very small so they must be amplified (made bigger) before they can be detected and read easily. The amplifier is connected to a ratemeter or countmeter which displays the number of pulses received.
The ratemeter display gives the number of pulses received in a short time interval, usually 2 seconds. The countmeter display shows the running total of pulses received and continues to rise until the display is reset or until the counter is switched off. The meter is also usually the EHT supply for the G-M tube.
Instead of being sent to a ratemeter or countmeter the amplified pulses could be sent to an audio amplifier connected to a speaker, in which case the pulses would be heard as a series of clicks. The higher the radiation level the faster the clicks would be produced. This provides a quick means of detecting or monitoring radiactivity.
The G-M tube is very sensitive and quite fragile and so it is usually mounted in a holder which also supports the amplifier. The use of a suitable stand also assists with the measurement of the distance between the G-M tube and the source being investigated.
The amplifier is connected to the meter by a co-axial cable along which the pulses pass. Co-axial cable is used because this has a central copper core to carry the pulses, surrounded by a fine mesh of wire. The surrounding mesh prevents external electrical interference affecting the pulses from the G-M tube. This type of cable is sometimes referred to as screened cable.
