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Radiation is a phenomenon in which energetic particles or energy waves travel through a medium or space. Radiation is a form of energy, which, in turn, is the ability or capacity to induce an action, or cause or make something happen. Ideas about radiation arose to fit experimental evidence. Therefore, it is sometimes helpful to run through the histories of the various discoveries that lead to current ideas on radiation.

Historically, mankind has had direct knowledge only of visible light. From 1670 to 1672, Isaac Newton, the English physicist, made detailed studies of color and optics, which lead to the publication of his “Opticks,” in 1704. Throughout the eighteenth and nineteenth centuries, a number of famed scientists (such as Gauss, Faraday, Hertz, and others) studied the properties of electrical and magnetic fields, developing the science of electrodynamics. During the 1860s, James Clerk Maxwell proposed that visible light is a phenomenon of electromagnetic waves, travelling through space. According to this idea of electromagnetic radiation, the possibility arose of a spectrum of radiation of varying wavelengths and energy. This turned out to be true, with the discovery of radio waves, less energetic than visible light, and, with the discovery of x-rays, more energetic than visible light. In 1895, Wilhelm Röntgen discovered and named x-rays. When experimenting with a vacuum tube, he noticed a fluorescence on a nearby plate of coated glass. In one month, he discovered the main properties of x-rays that we understand to this day.

There are other kinds of radiation besides electromagnetic radiation. Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. He noticed that when uranium salts were placed near a photographic plate wrapped in black paper, there was a deep blackening of the plate. It soon became apparent that the uranium salts were emitting a form of radiation that could pass through paper. At first, it seemed that the new radiation was similar to the, then recently-discovered, x-rays. Further research by Becquerel, Ernest Rutherford, Marie Curie, and others showed that this form of radiation was significantly more complicated.

In 1898, Marie Curie coined the word “radioactivity” to describe the spontaneous emission of radiation from matter. Rutherford was the first to understand that the radiation consisted of three different types, corresponding to a positive charge, a negative charge, and a neutral charge, which he named after the first three letters of the Greek alphabet: alpha, beta, and gamma. The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes.



Ionizing radiation

Even though there are several kinds of radiation, all radiation is classified, according to its harmfulness, into two categories: ionizing and non-ionizing radiation. Radiation with sufficiently high energy can ionize atoms, causing electrons to be stripped (or 'knocked out') from an electron shell, which leaves the atom with a net positive charge. Ionization disrupts chemical bonds, which can damage cells and cellular DNA, as well as generating toxic chemicals, which interfere with cellular metabolism.

Ionizing radiation is another way of saying, harmful, or dangerous radiation. Alpha particles, beta particles, gamma rays, x-ray radiation, and neutrons are all energetic enough to ionize atoms. Lower spectrum electromagnetic radiation, like radio waves, micro-wave, or visible light cannot ionize atoms, and are therefore called non-ionizing radiation. The word radiation commonly refers to ionizing radiation only, but it may also refer to non-ionizing radiation like radio waves or visible light.

All ionizing radiation causes similar damage at a cellular level. But, because alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage, such as radiation burns to the skin. Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body like radiation sickness, and increasing the incidence of cancer rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances, which, depending on the substance's chemical nature, can produce both diffuse and localized internal damage.



Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) consist of two protons and two neutrons bound together into a particle, identical to a helium nucleus. The particle is therefore positively charged, and can be deflected by a magnetic field. Alpha particles are a highly ionizing form of particle radiation, and have low penetration depth. They can be stopped by a few centimeters of air, a sheet of paper, or by the skin. However, when isotopes emitting alpha particle are ingested, they are far more dangerous than their half-life or decay rate would suggest.

Alpha particles are fragments of an unstable nucleus which are emitted from the nucleus as the nucleus becomes more stable. When an atom emits an alpha particle, the mass number of the atom decreases by four, due to the loss of the neutrons and protons from each alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons, and the atom becomes a new element. The process of emitting an alpha particle sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy.


Beta particles (named after and denoted by the second letter in the Greek alphabet, β)

are high-energy, high-speed electrons emitted by certain types of radioactive nuclei such as (potassium) K-40. Beta particles are negatively charged, and can be deflected by a magnetic field. Beta particles are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. An unstable atomic nucleus with an excess of neutrons may undergo beta decay, where a neutron is converted into a proton and an electron.

Although the beta particles given off by different radioactive materials may vary in energy, most beta particles can be stopped by a few millimeters of aluminum. Being composed of charged particles, beta radiation is more strongly ionizing than gamma radiation. When passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off bremsstrahlung x-rays.


Gamma and X-Ray

Gamma radiation (named after and denoted by the third letter in the Greek alphabet, γ) is a form of high frequency electromagnetic radiation, released from an unstable nucleus, as it becomes more stable, upon emitting either alpha or beta radiation. X-rays are the same kind of radiation as gamma radiation, but generated by a different process and discovered and named under different circumstances. The two types of radiation are now usually distinguished by their origin: x-rays being electronically produced when electrons undergo energy level transitions as they orbit the atomic nucleus, while gamma rays are emitted by the nucleus as the result of radioactivity.

This type of radiation is predicted from the electromagnetic theory of radiation.

However, such radiation is also characterized as high energy photon particles, which are said to have neither mass nor electric charge. These highly energetic photons are deeply penetrating and are difficult to stop. They can be stopped by thick layers of material, where stopping power of the material depends mostly on its total mass.



In 1931, it was discovered that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. This new kind of radiation was identified as neutron radiation.

Neutron radiation is a kind of ionizing radiation which consists of free neutrons. Neutrons may be emitted from any number of different nuclear reactions. The combination of an alpha particle emitter and certain light-weight isotopes is still a common neutron source.

Neutrons readily pass through most material, but interact enough to cause biological damage. Neutron radiation is considered to be the most severe and dangerous radiation available. Neutrons can travel great distances, but can be shielded by hydrogen rich material. The most effective shielding materials are water, polyethylene, paraffin wax, or concrete, where a considerable amount of water molecules are chemically bound to the cement.





Concepts of measuring radiation have gradually changed over the years. Exposure, dose, and dose equivalent are 3 different measuring concepts that have corresponding units. For each concept, there is usually a traditional unit, and an SI unit. SI stands for the French phrase, systeme internationale, which means, international system of weights and measures, also known as the metric system.



Exposure is a measurement of ionization in air, of x-ray radiation and low to medium energy gamma radiation. Exposure cannot be measured for other kinds of radiation. The concept of exposure is considered scientifically obsolete; however, it is still often used. The term exposure is sometimes used imprecisely as a synonym for dose.

The röntgen (R) is the traditional unit of exposure, which represents the amount of radiation required to create 1 esu of charge in 1 cubic centimeter of dry air.

The coulomb per kilogram is the SI unit of ionizing radiation exposure, and is the amount of radiation required to create 1 coulomb of charge in 1 kilogram of matter.

1 röntgen = 0.0003 coulombs per kilogram.



Since the effects of gamma and other ionizing radiation on living tissue is more closely related to the amount of energy deposited than ionizing capacity, exposure is a scientifically obsolete concept and has been superseded by the concept of absorbed dose. Radiation energy deposited in matter is called “dose.”

Dosimetry is the measurement, method of measurement, or instrument of measurement of dose. An ionization chamber that is calibrated to give a read-out in dose, can measure dose for lower energy gamma and x-ray radiation. Geiger detectors and scintillation detectors can be calibrated to give measurements in dose-rate, when working with specific, known isotopes. There are also a number of dose measuring systems which are used in personnel dosimetry monitors or badges. Dose can also be mathematically calculated for any kind of radiation if the identity of the irradiating nuclide is known, and some idea of the amount of activity can be measured. The rad is the traditional unit of dose.

1 rad = 100 ergs per gram or 0.01 Joules per kilo-gram.

The gray (Gy) is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of matter.

1 gray = 100 rads.

1 rad = 10 milli-grays.


Dose Equivalent

Dose equivalent is a refinement of dose, accounting for the biological effect of radiation on human tissue. For gamma rays it is equal to the absorbed dose. For other kinds of radiation at various energies, and for various parts of the body, there can be weighting factors applied to the absorbed dose, to achieve the dose equivalent. These weighting factors are empirically derived. The rem is the traditional unit of dose equivalent.

1 rem = 100 ergs per gram or 0.01 Joules per killo-gram.

The sievert (Sv) is the SI unit of dose equivalent, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of matter.

1 sievert = 100 rems.

1 rem = 10 milli-sieverts


Understanding Measurements

(Stated in SI units):

Below 0.5 sieverts, there are no measurable effects. From 1 to 2 sieverts, symptoms of radiation sickness first appear. Radiation dose of 2 to 4 sieverts is often fatal. Dose in excess of 4 sieverts is fatal.

(Stated in traditional units):

(Below 50 rems, there are no measurable effects. From 100 to 200 rems, symptoms of radiation sickness first appear. Radiation dose of 200 to 400 rems is usually fatal. Dose in excess of 400 rems is fatal).

NRC and agreement state regulations limit the whole body occupational dose limit for radiation workers to 5 rems in a year. The limit for members of the public is 100 milli-rem in a year.