| | |
|---|---|
| ACTIVATION ANALYSIS |
By changing a known fraction of an element into a radioactive form, the products of that radioactivity, whether alpha particles, beta particles or gamma rays, can be measured giving an estimate of the total amount of the element in question. |
| ALPHA PARTICLE |
Inside the nucleus of an atom there is a melange of mobile positive protons and neutral neutrons. 2 protons and 2 neutrons sometimes stick together as a unit called an alpha particle which can be emitted at high speed from an atom's nucleus as a positively charged unit, which is one form of radioactivity. The nucleus of a helium atom, composed of two protons and 2 neutrons, is in fact an alpha particle. |
| ANODE |
See electrode. |
| ARTIFICIAL RADIOACTIVITY |
Particles such as protons, alpha particles or neutrons can enter the nucleus of another atom when made to travel very fast. As the particle enters the nucleus of the atom it might stay there and add to its number of protons and neutrons or cause it to break up, in either case creating an atom of another element, which would usually be radioactive. For example, an isotope of sodium is made by bombarding magnesium-24 with deuterium ions. The disturbed nucleus emits an alpha particle leaving radioactive sodium-22. |
| ATOM |
Atoms are the building blocks of all matter. A group of atoms bound together makes a molecule. For example, water molecules (H2O) consist of two atoms of hydrogen linked to one atom of oxygen. An atom consists of a nucleus surrounded by negatively charged electrons. The nucleus is made up of positively charged protons and usually some neutrons. An atom has the same number of protons as electrons but for each element (eg carbon, gold, oxygen) the number is different and the element is defined by that number. The number of neutrons can vary even between different atoms of the same element. The electrons are minuscule in comparison to the protons and neutrons. See also isotope. |
| BECQUEREL |
A rate of decay, (not a unit of risk) in the measurement of radioactivity. 1 Becquerel represents 1 radioactive disintegration per second. Its effects depend on which radioactive material is involved. |
| BETA PARTICLES |
A beta particle is an electron that has been formed when a neutron in the nucleus of an atom breaks down into an electron and a proton, and the electron is ejected at high speed. An atom which gives off beta particles is displaying one sort of radioactivity. |
| CATHODE |
See electrode. |
| CURIE |
A unit for measuring the rate of radioactive decay. 1 Curie (Ci) = 3.7 x 1010 disintegrations per second. Not used much nowadays. |
| CYCLOTRON |
A machine for accelerating ions to very high speeds using magnetic and electric fields. See chapter 11 for a full description. |
| DEUTERIUM |
Deuterium is an isotope of hydrogen. An atom of common hydrogen has a nucleus consisting of 1 proton only, but deuterium, which used to be known as heavy hydrogen, has a nucleus made up of 1 proton and 1 neutron. |
| DEUTERON |
A deuterium nucleus, a positively charged particle composed of 1 proton and 1 neutron. |
| DIODE |
The simplest sort of valve consisting of heated electrodes in an evacuated tube; the heated cathode gives off electrons that are attracted to the anode. It is used for its ability to allow current to flow in one direction only. |
| ELECTRODE |
A conductor that can lead an electric current into or out of a liquid, a gas or a vacuum.
A cathode is an electrode responsible for emitting or transmitting electrons into the space between two electrodes and an anode is an electrode that collects them. The movement of the electrons is the basis of an electric current, although the current is said to flow in the opposite direction to the electron flow. |
| ELECTROMAGNETIC WAVES |
A magnetic field produces waves in one direction, and an electric field produces waves perpendicular to them. The two move together as electromagnetic waves, at the speed of light. In increasing order of wavelength and decreasing frequency, the electromagnetic spectrum includes gamma-rays, X-rays, ultraviolet, visible light, infrared, microwaves and short medium and long radio waves. |
| ELECTRON |
See atom. |
| ELEMENT |
See atom. |
| FLUORIDE |
Fluorine atoms combine in pairs to make molecules of the toxic gas fluorine or F2. However, if a salt of fluorine such as sodium fluoride, NaF, is dissolved in water, it divides into sodium ions, Na+ and fluoride ions F-. |
| GAMMA RAYS |
During radioactive disintegration of a nucleus, gamma rays may be emitted. They are a form of wave energy, like light or X-rays, but more energetic. When absorbed by matter, gamma rays cause fast moving electrons to be given off and so induce radioactivity in the materials they hit. |
| GEIGER COUNTER |
An instrument for detecting and counting the particles given off by radioactive materials. |
| GOLD LEAF ELECTROSCOPE |
Gold leaf is gold beaten out into very thin sheets. Two pieces hanging from an electrode can make a device for demonstrating the presence of an electric charge by moving apart under mutual repulsion. Once in this state, the gold leaves can be used to detect or measure radioactivity, which will ionise the gas between them, allowing them to fall back to their original position. |
| HALF-LIFE |
The time it takes for a given mass of a radioactive element to use up half its activity. |
| HEAVY HYDROGEN |
See deuterium. |
| HORMESIS |
A material harmful or lethal at high doses, which is useful at low doses. An example is vitamin D, of which small and regular doses are essential. However, an excess leads to hypervitaminosis D, which is unpleasant and disabling. |
| IN VIVO |
Inside the living body. |
| ION |
An ion is the result if a normally neutral atom should gain an electron and become negatively charged, or lose an electron and become positively charged. In either case an ion will move under the influence of an electric field as described under 'electrode'.
Gases can be ionised: in a neon tube electrons are removed from the neon atoms by the electric current forced to pass through the tube and as the electrons return to complete the neon atoms again a red light is given off. |
| ISOTOPE |
Isotopes are different forms of the same element having different numbers of neutrons in the nucleus. For example, chlorine atoms always have 17 protons, but they may also have 18 or 20 neutrons, giving a total number of nuclear particles of either 35 or 37. Carbon-12 (or C12) and Carbon-14 (C14) both have 6 protons, but one has 6 neutrons and the other 8. Carbon-12 is stable but carbon-14 is radioactive as its nucleus is unstable. |
| MAGNETRON |
Used as an oscillator in radar transmitters and microwave cookers. A valve in which electric and magnetic fields cause electrons from a central cathode to gyrate on their way to a slotted anode. The cavity magnetron is an improved version with cavities cut in the solid, cylindrical anode. |
| MeV |
A measure of energy. One mega-electron-volt is one million electron-volts of particle energy. |
| MICROCURIE |
A millionth of one Curie. |
| MICROMETRE |
One millionth of a metre, written μm. Similarly, a microgram is one millionth of a gram, written μg. |
| MICROWAVES |
Electromagnetic waves with short wavelengths between 1 mm and 30 cm, less than normal radio waves. |
| MOLECULE |
See atom. |
| NANOGRAM |
One thousand millionth of a gram. |
| NEUTRON |
See atom. |
| NUCLEUS |
The nucleus is the positively charged central part of an atom containing protons and neutrons. The sum of the number of protons and neutrons gives an element's weight, protons and neutrons having almost the same mass. A particular element always has the same number of protons, but it can have various numbers of neutrons giving different isotopes of that element. See also atom. |
| PENTODE VALVE |
An electronic valve with 5 electrodes. 2 of the electrodes are the cathode and anode of simpler valves and the other three are grids placed between them to modify the valve as a high frequency amplifier. |
| POLARISATION OF LIGHT |
The electromagnetic waves which form a beam of light normally vibrate in all possible side-to-side directions. Light is said to be polarised if most of the vibrations are all in one direction. |
| PROTON |
See atom. |
| RADAR |
RAdio Detection And Ranging. Microwave radiation from specialised valves is emitted to explore the surrounding area. Any of the radiation which returns is assumed to have bounced off a hard object and so the whereabouts of unseen objects can be detected. |
| RADIO |
Electromagnetic waves, generated by high frequency alternating currents, provide a method of signalling without connecting wires. |
| RADIOACTIVITY |
A process by which atoms of one element break up to form atoms of another element. Some atoms have so much energy stored in the nucleus that they spontaneously disintegrate, shooting off bits of themselves. For example, the nuclei of some isotopes of uranium or radium shoot out alpha particles. Other atoms release beta particles or gamma rays, or a combination of gamma rays and alpha or beta particles. When an alpha or beta particle is emitted from a nucleus, the nucleus left behind is then a different element as it has a changed number of protons. The new element might be stable, or it might itself be radioactive and decay (break down) further. Some radioactive atoms can last a long time before breaking up. In the case of uranium-238 it takes 4500 million years for half the atoms in any sample to disintegrate into another element. Others break up more quickly: it takes only half a second for half the atoms in a sample of poloniumm-214 to decay, making it highly radioactive, but for a short time only. |
| RADON |
A radioactive gas formed by the disintegration of radium. Radium, a radioactive metal, is found in minuscule amounts in most soil and rocks and in slightly greater quantities in granite or rocks containing uranium ores. Its decay product, radon, seeps upwards through the soil, and may be concentrated within a well draught-proofed house built above radium-containing rocks. |
| REM |
One rem is the dose received when any kind of radiation producing the same biological effect as 0.01 joules of X-rays is absorbed by each kilogram of a person or organ. Average annual dose from natural sources in Britain is about 100 millirems. |
| SIEVERT |
A unit of dose of radioactivity equal to 100 rems. |
| SYNCHROTRON |
A specialised form of cyclotron designed to accelerate particles to even greater speeds. |
| THERMO-LUMINESCENCE |
Light emitted by previously irradiated minerals on heating. |
| TRITIUM |
Tritium is an isotope of hydrogen, each atom having a nucleus of 1 proton and 2 neutrons. |
| VALVE |
An electronic device that allows electric current to pass in one direction only. From this basic function, valves can be designed to amplify electric currents and perform a variety of electronic tasks. |
| VELOCITY MODULATION |
A method of producing the outgoing microwave radiation of radar by the so-called 'bunching' action of a high frequency electric field on a stream of electrons. |