Radioactivity: Glossary of Terms

(in alphabetical order)

Activity
The activity of a radioactive sample is the number of emissions of nuclear radiation from that sample in one second. It is measured in becquerel (Bq)
Alpha particle
 
 
 
 
 
 
 
 

 

An alpha particle  is: 
  • A particle of nuclear radiation that is made up of two protons and two neutrons. It is therefore a helium nucleus with a nucleon number of 4 and a proton number of 2. Animated gif: click here
  • It has a mass of 4.0 u (see atomic mass unit)
  • Being massive (on a nuclear scale!) and having a double positive charge its ionising power is high.
  • 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. For an animated gif click here.

  • As its ionising power is so high it does not penetrate very deeply into matter before its energy has been used up. Its penetrating power is therefore very low (absorbed by 10 cm of air, 0.01 mm lead or a sheet of paper).
  • The following symbols are commonly in use:

(Greek letter 'a' - alpha)

(Greek letter 'a' - alpha)

(Chemical symbol for helium)

Anion
A negative ion.
Antimatter
When an antimatter particle meets with a matter particle they annihilate each other! Both particles disappear and two gamma rays are produced instead. Antimatter does not get very far through matter before it reaches its matter counterpart but the gamma rays produced can travel a long way through matter.

An example of antimatter is the positron.

Artificial Nuclear Transmutation
The human involvement of changing one element into another by firing neutrons at a nucleus. On absorbing the neutrons the nucleus changes into a different isotope.  Animated gif: click here This is often a radioactive nucleus that then decays to produce a nucleus of different element. See transmutation.

e.g.

(See transuranic elements)

Atom
An atom is the smallest chemically indivisible part of an element. An atom has a central nucleus composed of protons and neutrons. The number of protons in the nucleus of an atom determines which element the atom is. The elements are listed in the periodic table.
Click here to view the image of the atom
Atomic Mass Unit
Atomic Mass Unit (u): This is a convenient mass unit to use when dealing with the masses of atoms and nuclear particles. Were the mass expressed in kilograms the values would be so tiny they would be difficult for students to deal with.

The unit is defined by taking the mass of one atom of carbon12 as being 12u. This makes the mass of a proton and a neutron to be equal to 1u (when measured to the degree of accuracy needed for GCSE level)

1u = 1.661 x 10-27 kg

1kg = 6.020 x 1026 u

(Note - Some text books use the abbreviation amu)

Atomic Number Z
(Also called the proton number, but at GCSE call it the atomic number in exams - using the physicist's term may lose you marks) - The atomic number of an atom is the number of protons in the nucleus of the atom. Each element has its own atomic number. 
Background Radiation

This comes from five main sources:

Food and drink

Human activity

Cosmic rays

Rocks and Soil

Radon

Background Rate
The activity from background radiation. If a Geiger-counter is switched on in a room it will register ionising radiation. This will vary according to location (see background radiation).

Before an experiment with a radioactive source is carried out the background rate must be established. This is done by switching on the Geiger-counter when the source is not present and recording the average activity for twenty minutes or so.

This value must then be deducted from any readings taken with the source present so that the true activity of the sample can be found.

Becquerel
This is the unit of activity of a radioisotope. It is the number of spontaneous nuclear transformations in one second.

The abbreviation is Bq.

(The old unit is the curie)

Beta Particle
A beta particle  is: 
  • a particle of nuclear radiation that is a fast electron. Animated gif: click here
  • It has a mass of 0.000055 u (see atomic mass unit)
  • Being of tiny mass (even on a nuclear scale) and having only a single negative charge its ionising power is low compared to the alpha particle.
  • As its ionising power is low it can penetrate quite deeply into matter before its energy has been used up. Its penetrating power is therefore moderate (absorbed by 1m air, 0.1 mm lead or 3mm aluminium sheet).
  • When a nucleus has too many neutrons, it tends to beta decay.

  • The following symbols are commonly in use:

(Greek letter 'b' - beta)

(Greek letter 'b' - beta)

('e' stands for electron)

Binding Energy
The mass of the nucleus is less than the total mass its nucleons would have if they were separated. The mass defect is the difference in mass between the total mass of the separated nucleons and the mass of the nucleus. The mass defect is equivalent to the energy released when the nucleons are bound.

Binding energy is the energy that must be supplied to the nucleus in order to separate it into its nucleons.

The binding energy is the energy equivalent of the mass defect.

Carbon Dating
Methods of finding out the age of artefacts that contain materials that were once alive. When it dies a living organism takes in no more carbon from the atmosphere and the percentage of C-14 will decrease. The ratio of C-14 to total carbon is found and from this the age can be calculated.
Cation
A positive ion.
Cobalt
Chemical symbol (Co), 

Cobalt was discovered in 1735 by Georg Brandt. 

Description: a silver-white, lustrous, hard metal that can be magnetised. Naturally occurring (with other metals) as the ores cobaltite and smaltite Cobalt alloys are used in very hard cutting tools, high-strength permanent magnets, and jet engines. 

A radioactive form of cobalt, cobalt-60, is prepared by exposing naturally occurring cobalt-59 to the radiations of an atomic pile. 

The nucleus absorbs a neutron and becomes cobalt-60. This is a gamma and beta emitter. The beta rays are of low energy (said to be 'soft') and are easily absorbed by a metal coating on top of a cobalt source. The cobalt is then effectively only giving gamma rays to the surroundings.

It has a half life of 5.3 years and decays to produce a daughter nuclide of nickel-60 (Stable)

It is used:

  • in industry in the inspection of materials to reveal internal structure, flaws, or foreign objects or as a radioactive tracer and
  • in medical science in cancer therapy
Collimator
A device that produces a parallel beam of radiation. This is either done by refraction (as with light), bending the rays until they are parallel using a system of lenses or (as with gamma rays) by absorbing (using a lead grid system) all of those rays that are not travelling parallel to one another.
Compound
Two or more different atoms that are chemically bonded together form molecules of a compound. The compound often has very different properties from the elements it is made up of.
Cosmic Rays
High-energy particles of ionising radiation from space.

(See Appendix B)

Curie
The Curie (Ci) is an old unit of activity. One curie is a very large amount of radioactivity. It is more common to find activities given in millicurie or microcurie
 
 

1 Ci = 37 GBq = 37,000,000,000 Bq

Daughter Nucleus
The name given to a decay product of the parent nucleus.

Decay product

A nuclide or radionuclide produced by radioactive decay. It may be formed directly from a radionuclide or as a result of a series of successive decays through several radionuclides.
DNA
Deoxyribonucleic Acid - double helix molecule that forms the genetic material of all living cells. It controls the structure and function of cells and is the material of inheritance; therefore damage to the coding can cause mutation.

Ionising radiation can cause damage to the structure of DNA so that on division the new cell is altered (mutated). Repair to damage to DNA is carried out by 'repair enzymes'. If mutants that lack such a facility are more susceptible to radiation damage and to express the damage so produced.

Nuclear radiation therefore can have harmful effects on living matter, resulting in mutations and cancer (tumours). Very high dose can cause cell death.

Electromagnetic radiation
All electromagnetic radiation is pure energy (zero mass) and part of the electromagnetic spectrum. Light energy is the most familiar part of this.

All electromagnetic radiation travels at a speed of 3.0 x 108 m/s in a vacuum.

Electromagnetic Spectrum
This is a continuous band of electromagnetic radiation arranged in the order of decreasing photon energy, decreasing frequency or increasing wavelength. Each part is named according to its origin and frequency/wavelength range. Light energy is the most familiar part of the spectrum and it is often referred to as the 'family of light'. Some parts of the e.m. spectrum can be directly detected by humans, others cannot. 

See the table below:

indicates that the rays are harmful because they are of high enough energy to be ionising radiation.

Name
Typical wavelength (m)
Sources
Detectors
Gamma Rays

g-rays

10-12
Nucleus of radioactive atom
Geiger-Muller Tube
X-rays

10-10
X-ray tube
Photographic film
Ultraviolet (UV)

10-8
Very hot objects, the Sun, sparks, mercury lamps
Photographic film, causes a sun tan, makes fluorescent substances glow
visible
10-7
Hot objects, the Sun, fluorescent substances, lasers
Human eye, photographic film, LDR
infra-red
10-5
Warm or hot objects, the Sun
Skin, blackened thermometer, thermistor
microwave
10-2
Magnetron - found in microwave ovens, mobile phone transmitters, satellite communications
Aerial with a microwave receiver set
TV and radio
10+2
Radio transmitters, radar transmitters
Aerial with a radio/TV receiver set

Click on graphic to enlarge

Electron
Sub-atomic particle that orbits the nucleus of the atom. It has a mass of 0.00055u (atomic mass units) and unit negative charge (-1). 
Electron volt
Energy can be calculated in terms of electron volts (eV) 

The eV is a very small unit of energy. It is used a lot in nuclear calculations. 

From GCSE electricity we know that 

E

(joule)

=
Q

(coulomb)

V

(volt)

So, a joule could be thought of as a 'coulomb volt'

Similarly, if we don't use a full coulomb of charge, but only the charge on an electron 'e' 

E

(electron volt)

=
Q

(change on one electron)

V

(volt)

The charge on an electron is only 1.6 x 10-19 C

Therefore the eV is only 1.6 x 10-19 J

= 0.000 000 000 000 000 000 19 J (very tiny!)

Element
An element is a substance that contains only one type of atom (all of its atoms have the same proton number). It cannot be chemically broken down into any simpler substance. Elements are listed in the periodic table.
Fundamental Forces of Nature
Name Domain Exchange Particle
Relative strength
I






 

Strong Nuclear Force Within the nucleus - acts between nucleons over a very short range Pions (mesons)
1
II
Electromagnetic Force Between charged particles - binds atoms and molecules together photons 
10-2
III
Weak Nuclear Force Within the nucleus - governs radioactive beta decay involves leptons w bosons and z-particles
10-6
IV
Gravitational Force Acts between all masses - very important for large masses in space such as planets and stars gravitons (not yet detected!)
10-38
Gamma Camera
An imaging device that formulates pictures based upon spatial distribution of gamma ray intensities. For more details and diagrams see medical applications section of human activities
Gamma Ray

 

The gamma ray is: 
  • As its ionising power is so low it penetrates very deeply into matter before its energy has been used up. Its penetrating power is therefore very high (about 99.9% is absorbed by 1 km of air or 10 cm lead). Very few of the gamma rays emitted from the Sun reach the Earth's surface because the atmosphere is thick enough to absorb virtually all of them.
  • For gamma ray emission to occur the nucleus must be in an excited state after emitting an alpha, beta or positron particle. Sometimes it stays like that for quite a while before the gamma ray is emitted (see metastable state), sometimes it is instantaneous. When gamma emission occurs there is no emission of matter particles therefore the nucleon number and the proton number remain the same. The remaining nucleus is of the same isotope but at a lower energy state.
  • The following symbol is commonly in use:
(Greek letter 'g' - gamma)
Geiger-Müller Tube
Often called a Geiger-counter the GM Tube is used to count the activity of a radioactive sample. It is a special type of ionisation chamber that can be used to detect alpha, beta or gamma radiation. The output of the tube is a voltage that can either be connected to a scaler that gives an accumulated total of counts or a rate meter that gives an output in becquerel. Sometimes the output is a speaker that gives a loud 'click' for each particle it detects. The output can also be connected via an interface to a computer.
Gray
The gray (Gy) is the unit of absorbed dose
Half Life
The half-life of a radioisotope is the time taken for half of the atoms in a sample of that radionuclide to decay (emit nuclear radiation). It can also be expressed as the time taken for the activity of the sample to halve.

It has the following accepted symbols:

Ion
An ion is an atom that is being orbited by a different number of electrons than the number of protons in its nucleus. If it has too many electrons to be neutral it will have a negative charge and be called an anion. If it has too few electrons to be neutral it will have a positive charge and be called a cation.
Ionization
The process by which a neutral atom gains or loses an electron thereby becoming an ion.
Ionizing power
The ionizing power of ionising radiation measures how many ions are formed in a given area when the radiation passes through it. Alpha particles have a double charge and are very massive compared to beta and gamma they are therefore very strongly ionizing, interacting with many electrons in the orbitals of atoms before they lose their energy. Beta particles with their much smaller mass and single charge have less ionising power. The least ionizing radiation is composed of gamma photons because it is neutral and has no mass. 

Ionizing power is often quoted as ion pairs per centimetre of air. 

(~10/cm)

low power

<

(~103/cm)

medium power

<

 

(~105/cm)

high power

It indicates how densely packed incidents of ionisation will be. In human tissue ionisation occurring in a limited location means concentrated tissue damage and will increase the probability of cell death or mutations due to DNA damage.

NB - the ionising power comparison is for beams of comparable intensity.

Ionizing radiation
Radiation of high enough energy to cause ionization. The energy of a photon of electromagnetic radiation or nuclear radiation is given to an orbital electron. If it is sufficiently high enough to promote the electron out of the influence of its nucleus, it is ionizing radiation and an ion will be formed.
Isotope
An element can exist in several different isotopes. Each isotope of an element has the same proton number but a different number of neutrons, making the nucleon number different. All isotopes of an element have the same chemical properties (the way they react with other substances) but different physical properties (boiling point, melting point, density etc.). Some isotopes are more common than others but there is no such thing as a 'normal' isotope. The nucleus of some isotopes is unstable, such isotopes are termed radioisotopes.
LINAC - linear accelerator
A linear accelerator produces X-rays of such high energy that they are in the same energy range as gamma rays. They can then be used fro radiotherapy. The advantage of the LINAC over a fixed radioactive source is that it can be switched off when not in use, whereas the cobalt 60 source has to be sealed in lead and treated with care when not in use. They can also produce rays just of a required energy and intensity.

However they are very expensive to buy and not all hospitals can afford one.

Mass defect The mass of the nucleus is less than the total mass its nucleons would have if they were separated. The mass defect is the difference in mass between the total mass of the separated nucleons and the mass of the nucleus. The mass defect is equivalent to the energy released when the nucleons are bound.

Binding energy is the energy that must be supplied to the nucleus in order to separate it into its nucleons.

The binding energy is the energy equivalent of the mass defect.

Mass Number A
Mass number is the average number of nucleons found in a large sample of an element. Naturally occurring chlorine exists in the form of two isotopes Cl37 (25%) and Cl35 (75%). Therefore the 'average' mass of a chlorine atom is 35.5. Chemists use mass number because they deal with large quantities of atoms. Physicists prefer the term nucleon number and deal with individual isotopes rather than the naturally occurring mixture of them, but the GCSE syllabuses like you to use the term mass number - so do that at that level!
Matter
Matter is anything which occupies space. There are three states of matter: solid, liquid and gas. It is made up of atoms.
Metastable State
Sometimes, after its emission of an alpha, beta or positron 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.

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 isotope of the element but they are different nuclide because the term nuclide incorporates nuclear energy states as well basic structure.

Neutron
Nuclear particle that has zero charge and a mass of 1u (atomic mass unit)
Nuclear radiation
Nuclear radiation emanates from the nucleus of an atom. It is ionising radiation. There are four types of nuclear radiation, three are made of matter: alpha, beta and gamma and the fourth is an antimatter particle: the positron.
Nucleon
The name given to a particle within the nucleus (a proton OR an neutron)
Nucleon Number A
(See mass number) - The nucleon number of an atom is the number of neutrons in the nucleus of an atom. Each element can have several different nucleon numbers. Each one corresponds to a different isotope of the element.
Nuclei
Plural form of nucleus (Latin nouns ending in 'us' usually have plural forms that end in 'i')
Nucleus
The nucleus of an atom contains all of its protons and neutrons. It is a tiny dense centre to an atom that is orbited by its electrons.
  • Nuclear sizes are expressed in femtometres (10-15 m), where as atomic sizes are expressed in 10-10 m. This means that an atom's diameter is about 100.000 times bigger than its nucleus! 
If you made a model of an atom (to scale) and had the diameter of the nucleus to be 1mm (a peppercorn) the diameter of the atom itself would have to be 100m!
  • Nuclear mass is virtually the same as the atomic mass because the electrons have such tiny mass compared to the mass of electrons and protons.
Nuclide
An atom defined by its proton number, nucleon number and energy state. Thus it is not only clearly identified as a particular isotope of that atom but also its energy state is clearly defined. 
Particle Accelerator

A particle accelerator is a device used to project charged particles at high speed into matter. Electric and magnetic fields are used to provide the forces to accelerate and control these charged particles at speeds approaching the speed of light. When these charged particles strike the nucleus of an atom, they alter its stability and new particles may be produced.Various devices used to detect subatomic particles are the Geiger counter, ionization chamber, bubble chamber, cloud chamber, spark chamber, scintillation counter, and photographic film.

Penetrating Power
The penetrating power of nuclear radiation depends upon the ionising power of the radiation. The radiation continues to penetrate matter until it has lost all of its energy. The more localised the ionisation the less penetrating power it will possess.

Alpha particles are the least penetrating as they are the most densely ionising; beta particles are more penetrating than alpha because they are less ionising and gamma have the most penetrating power.
Click here to view this as an animation

Periodic Table
The Periodic table is a chart used primarily by chemists. It summarises the elements by arranging them in the order of their atomic number in such a way that similarities and trends can be appreciated. 

Each row (or PERIOD of the table) exhibits a range of properties that can be interpreted by how the electron orbitals of the atom are filled. Each column or GROUP of elements exhibits similar properties (like a family show similar characteristics). 

PET
Positron emission tomography. (More details)
Photoelectric Effect

The emission of electrons from matter by photons of electromagnetic radiation

Photomultiplier
A photomultiplier is a device in which incident photons create measurable electrical pulses. The device is based on the photoelectric effect. It uses large electric fields to accelerate electrons and, through a cascade sequence, amplify the signal. When it is large enough it can be sent to electronic circuitry for analysis and display. 
Photon
A photon is a name given to a quantum of electromagnetic energy. It is usually used to refer to photons of visible light if the part of the electromagnetic spectrum is not otherwise specified.
Positron
A positron  is an antimatterbeta particle
  • a particle of nuclear radiation that is a fast anti-electron. 
  • It has a mass of 0.000055 u (see atomic mass unit)
  • It has a charge of +1 (opposite to an electron!)
  • When it meets with an electron it annihilates it! Both particles disappear and two gamma rays are produced instead. It does not get very far through matter before it reaches another electron therefore its ionising power is great.
  • As its ionising power is great it cannot penetrate quite deeply into matter before it is annihilated. Its penetrating power is therefore very low indeed but it produces gamma rays and these have great penetration power.
  • When a nucleus has too many protons, it tends to positron decay.
  • When positron decay occurs a proton within the nucleus emits the particle and changes into a neutron. Animated gif: click here .Therefore the proton number decreases but the nucleon number stays the same (only now you have one more neutron and one less proton!). The resulting daughter nucleus is of an element 1 position to the left of the 'parent' in the periodic table. For an animated gif click here.
  • The following symbols are commonly in use:

(Greek letter 'b' - beta)

(Greek letter 'b' - beta)

('e' stands for electron)

Proton
Nuclear particle that has unit positive charge (+1) and a mass of 1u (atomic mass unit). The number of protons (proton number) in a nucleus determines which element the atom belongs to.
Proton Number Z
(Also called the atomic number) - The proton number of an atom is the number of protons in the nucleus of an atom. Each element has its own proton number.
Quantum of Energy
A quantum of energy is a little packet of energy. The quantum theory developed at the beginning of the last century tells us that energy changes in atoms happens when these little packets of energy are taken in or given out. The value of the quantum of energy can be calculated using the equation:

E = hf

where:

E is the energy in joules

h is the Planck constant

and f is the frequency in hertz

Rad
The old unit for absorbed dose was the 'rad'

1rad = 0.01 J/kg = 0.01Gy

Radiation Dose
  • Absorbed dose (D) is measured in gray (Gy). This is a measure of how much energy is absorbed by unit mass from the ionising radiation it is exposed to.
1Gy = 1 J/kg

The old unit for absorbed dose was the 'rad'

1rad = 0.01 J/kg = 0.01Gy


  • Dose equivalent (H) is measured in sievert (Sv). It takes into consideration not only the intensity of the radiation source and the time of exposure but also the different ionising powers (and therefore potential hazard) to the recipient.
The old unit for dose equivalent is the rem

1 rem = 0.01 Sv

Radioactive
A word used to describe substances that contain atoms that emit nuclear radiation. How radioactive a substance is depends on its activity.
Radioactive Decay
It is process of spontaneous transformation of a radionuclide by the emission of nuclear radiation

Radioactive nuclear decay occurs whenever a nucleus is in an energy-state that is not the lowest possible for its nucleon number. This state may occur naturally (which essentially means that it was created in that state when formed within a star) or by artificial means (neutron or photon irradiation).

  • The emission of the nuclear radiation is a purely random event. It cannot be predicted exactly when an atom will decay, only that a certain number will decay in a given time. The mathematics of probability is used for this requires a large number of atoms to be considered. {See half life and radioactive decay series).
  • The rate of decay depends on the number of undecayed nuclei present, so with each decay event there is a decrease in the activity of a radioactive sample.
Radioactive Decay Series
When ever a decay occurs the nucleon number either stays the same (gamma or beta emission) or reduces by four (alpha emission). Therefore if you divide the nucleon number by four you will either have no remainder or 1, 2 or 3. Mathematically we could say that the nucleon number could be described by 4n, 4n+1, 4n+2 or 4n+3. 

See here.

Radioactive Label
See radioactive tracer.
Radioactive Tracer
A small amount of radioisotope is made to replace a non-radioactive isotope of an element in a compound. The path of that radioisotope or one of its daughter nuclei (decomposition product) is then monitored by detection of emitted nuclear radiation. It is also called a radioactive label.
Radioactivity
The property of a radionuclide to spontaneously emit ionising radiation. It arises from the breakdown of an unstable nucleus
Radioisotope
A radioisotope is an isotope that has an unstable nucleus. The nucleus emits a nuclear radiation to attain stability by a process called radioactive decay
Radionuclide
An atom of a radioisotope. A radioactive atom defined by its proton number, nucleon number and energy state. Thus it is not only clearly identified as a particular radioisotope of that atom but also its energy state is clearly defined giving an indication of whether it will emit a gamma ray to attain a more stable lower energy state.
Radiosensitive
Some biological cells divide more rapidly than others. these are the ones that are more likely to suffer damage from ionising radiation. They do not have sufficient time to 'repair damage' before they are required to divide again. Cancer cells fall into this category.
Radiotherapy
The destruction of malignant tumours with a high dose of gamma radiation that will result in cell death

See radiation therapy for more details

Rate of Decay
This is the number of particles of nuclear radiation emitted in one second. It is measured in becquerel.
Rem
The old unit for dose equivalent.

1 rem = 0.01 Sv

Sievert
The unit for the dose equivalent of ionising radiation received by a person.

The abbreviation is Sv.

A dose of 1 Sv is very high. The maximum annual permitted dose level (PDL) for the average person (i.e. one not working with ionising radiation) is just 5mSv - the average dose is about 2mSv.

Specific Charge The specific charge of a nucleus is the total charge of the nucleons (in coulombs) divided by the total mass of the nucleons (in kilograms). Specific charge is measured in C-1kg
Strong Nuclear Force
This is the strongest of the four fundamental forces of nature. It only has a very short range - to the order of femtometres (10-15m). It is the force that holds the nucleus together.
Tc 99m
Technetium 99m is a very useful radioisotope. It is the most commonly used one in hospitals because it is ideal to use with a gamma camera and only emits gamma rays. (See here)
Transmutation
Transmutation is the process by which the nucleus of a radioactive atom undergoes decay into an atom with a different number of protons, until such time as a stable nucleus is produced. See artificial transmutation. (See the simulation)
Transuranic Elements
These are elements with proton numbers greater than 92. The transuranic elements are not naturally occurring, they are artificially created.

They were made by firing neutrons at the nuclei of heavy atoms such as uranium. These are then neutron rich and decay by beta emission to produce atoms with bigger proton numbers. This is called artificial transmutation. The naturally occurring isotopes decay by alpha emission thereby reducing the proton number. It is only since the dawn of the nuclear age that elements with proton numbers bigger than 92 have existed on our planet.

X-ray
The X-ray is: 
  • a photon of high energy, short wavelength electomagnetic radiation that is not made up matter at all.. it is pure energy. 
  • the result of an atomic electron transition for a very heavy element.
  • It is produced in an X-ray tube. High energy electrons are fired at a metal (usually tungsten) target within an evacuated tube. The interaction of these high energy electrons with the electrons orbiting the metal atom produces X-rays - they are NOT nuclear radiation (cf gamma ray production). Some very powerful X-ray machines can produce X-rays that are of higher energy than most gamma rays. (see LINAC)
  • Having no mass and no charge its ionising power is very low.
  • As its ionising power is so low it penetrates very deeply into matter before its energy has been used up. Its penetrating power is therefore very high (about 99.9% is absorbed by 100 m of air or 1cm lead).
Zone of nuclear stability

The ratio of neutrons to protons in the nucleus increases from 1:1 as the nucleon number of stable isotopes increases. Those lying outside the zone are generally radioactive and those within it generally stable.