Rough Guide to Radiation

Types of Radiation
Units of Measurements

This is just a very rough guide, to explain some of the jargon bandied about in this report, and put the figures quoted in some context. You will have to look elsewhere, for a more scientific treatment of the subject of radiation.



Isotopes are variants of the same element which have the same chemical properties (ie. same number of protons and electrons), but different numbers of neutrons in their nucleii (and thus different nuclear/radiological properties).
Terms like U238 or uranium-238 simply mean a uranium isotope with an atomic weight of 238. This consists of 92 protons (like all the Uranium isotopes) and 146 neutrons.

Another term bandied about is biological half life, which refers to how long toxic/radiological substances remain in the body.

Transuranics is the term for elements heavier than uranium (eg. plutonium). All are man-made, and in existence only since the dawn of the atomic age.

See also the LLRC's Jargon Buster page

Types of Radiation

Ionizing radiation is any form of radiation that has enough energy to knock electrons (or nuclear particles) out of atoms, thus creating ions (and hence damaging living cells, for instance). The most familiar forms of radiation are Alpha, Beta, Gamma-Ray, X-Ray and Neutron.

Alpha particles are doubly charged Helium (chemical symbol, He) ions - basically Helium nuclei, with 2 neutrons and 2 protons. They are low-energy, and cannot even penetrate human skin, but they can be very damaging if released by radiation sources within the body.

Beta particles are simply high-energy electrons, and although less ionising than alpha particles, they travel much further, eg. they can penetrate human skin.

X-Rays are a form of EM (electro-magnetic) radiation, and so are Gamma-Rays, which have a much higher frequency (and hence, energy) than X-Rays. Gamma rays are highly dangerous photons emitted by the nucleus (whereas X-Rays are emitted by electrons), but EM radiation is not a significant factor in DU.
Note that EM frequencies below X-Rays (eg. microwave, infra-red, ultra-violet, radio waves and visible light) constitute non-ionising radiation, due to their lower energies.

Neutron radiation consists simply of neutron streams. More penetrative than Alpha (because these uncharged particles are more "stealthy"), but with similar ionising power.


Units of Measurements

The main units of measurement are:
Bq - Becquerels (SI replacement for Curies)
Ci - Curie
Gy - Gray
Sv - Sievert

Becquerels measure the activity of a radiation source, by measuring the number of atoms that disintegrate in 1 second (each such disintegration resulting in a radioactive emission). 1 Becquerel represents 1 disintegration per second.
The older unit of measurement is the Curie, which represents 37,000,000,000 disintegrations per second (the rate of disintegration occurring in 1 gramme of radium), ie. 1 Ci equals 37 Billion Bq.

Grays measure the absorbed dose, ie. the amount of energy absorbed by 1 gramme of organic tissue. I Gray is equal to 1 J/Kg (1 Joule of energy per kilogramme of tissue).
The older unit of measurement is the Rad, and 1 Gray equals 100 Rads.

Sieverts measure the equivalent dose, which represents the biological effects of radiation absorption, because for a specific absorbed dose, the ill effects will vary according to the type of the radiation.
The biological effect is computed by multiplying Grays by the Quality Factor assigned to that type of radiation. The quality factor of photons (Gamma) and electrons (Beta) is arbitrarily defined as 1, hence it ranges from 5-20 for neutrons, and is 20 for Alpha particles. Having thus adjusted for "quality", 1 Sievert of X-Rays would have the same effect as 1 Sievert of neutrons (absorbed by the same part of the body).
The older unit of measurement is the Rem (Roentgen Equivalent Man/Mammal), and 1 Sievert equals 100 Rem.

Finally, there is something called an effective dose, which is obtained by weighting the equivalent dose with the sensitivity of the type of tissue being irradiated. This is also measured in Sieverts.

The lethal effective dose is about 10 Sv, ie. death occurs within a few days after radiation exposure. At doses of 4 to 6 Sv, the survival rate probability is 50% within the first 4 weeks after exposure. At about 0.5 Sv, so-called "non-stochastic" radiation damage occurs, ie. direct effects on the human body (eg. cataracts, changed blood count). Up to about 100 mSv no direct radiation damage occurs but there may be random stochastic effects.
The annual radiation dose to the German public is estimated at about 20 mSv (ie. 0.02 Sv) per year, and the limit for professional exposure is set at 2.4 mSv. The annual dose in the US is estimated at 100 millirads per year.
[from the German Maritime and Hydrographic Agency]

This Yugoslavian environmental site tabulates the severity (in Grays) of various radiation doses