There are a several different units of measurement for radiation emitted and absorbed from radioactive materials. Exposure is measured in terms of a particular amount of radiation per a particular duration of time.
A curie is a unit used to measure radioactivity. It is named for Madam Curie, an early pioneer in research into radioactivity. It is a measurement of the number of atoms in a group of radioactive atoms that are giving off radiation at a particular moment. One curie is equivalent to thirty seven billion radium atoms emitting radiation. One curie is about ten million times the usual amount of radiation in a human body. A picocurie is one trillionth of a curie. Curies are often measured in the number of curies per gram of a solid substance. Curies per liter measures the number of curies in a liquid substance.
A becquerel is a unit of radioactivity named for Henri Becquerel who shared a 1903 Nobel Prize with Madam Curie for their research on radioactivity. It is defined as the quantity of radioactive material in which one nucleus decays each second. In a given amount of radioactivity the number of becquerels changes over time. When working with short live isotopes, it is necessary to note the exact date and time in the past or future for a particular becquerel count. There are 37 billion becquerels in one Curie.
When using some sort of detector such as a Geiger counter to measure radioactivity, counts per second or counts per minute are often used. These measurements can be converted to becquerels by taking into account the radiation background, the efficiency of the detector, the sample size and the way that the sample may absorb its own radiation.
A rem is a unit used to measure radiation exposure. It indicates how much radiation has been absorbed by someone or something. A calorie is a general measurement of energy. It takes one calorie to raise the temperature of one gram of water one degree centigrade at sea level. One rem is equivalent to two-millionths of a calorie. A rem is much more that people usually absorb from the natural environment each day. Rems are also broken down into millirems which are one-thousandth of a rem.
A unit called a gray is also used to measure absorbed ionizing radiation. It is defined as one joule of ionizing radiation absorbed by one kilogram of matter. A joule of energy can produce one watt of energy for one second. We measure electric usage in our homes in terms of kilowatt hours or one thousand watts per hour. One gray is equivalent to one hundred rem.
The most important unit for this blog is the sievert. It is a unit of dose equivalent radiation. It was developed in order to evaluate the biological effects of absorbed radiation instead of just the amount of radiation absorbed by the body. It is named for Rolf Maximilian Sievert, a Swedish scientist who worked on the biological effects of radiation. One sievert is equivalent to the effect of absorbing one gray of gamma radiation. Sieverts are measured by multiplying the grays of radiation by a weighting factor. Electrons and photons of all energies (beta and gamma particles) have a weighting factor of one. Protons have a weighting factor of two. Heavy ions and fission fragments (such as alpha particles) have a weighting factor of twenty. Neutrons have a weighting factor that is dependent on their linear transfer energy which can vary from less than ten for a weighting factor of one, ten to one hundred with a weighting factor between one and thirty, and greater than one hundred for a weighting factor greater than thirty. Sieverts are also measured in millisieverts or one-thousandth of a sievert and microsieversts or one-millionth of a sievert. One sievert is equivalent to 100 rem.
The effective dose of radiation absorbed by a person is found by taking the averages of all irradiated tissues whose weighting factors add up to one. Bone marrow, colon, lung, stomach, breast and other tissues each have a factor of twelve hundredths which combine to yield a factor of seventy two hundredths. Gonads weight as eight hundredths. Bladder, esophagus, liver and thyroid each have weights of four hundredths and collectively contribute sixteen hundredths. And finally, bone, brain, salivary glands and skin each have weighting factors of one hundredth which adds up to four hundredths. All together, these add up to one for the whole body.