Radiation units

I've been confused about radiation units forever and finally decided to settle the question. I'm sharing this so you can become just as confused as I was and then, hopefully, enlightened.

We describe radiation with rads, roentgens, rutherfords, sieverts, rems, curies, becquerels, and grays - why so many units?. Most of these are were abandoned/deprecated as we learned more about what radiation is and how it works. Last years fashion of roentgens, rems, rutherfords, and rads are old news girl, the new hotness this season are becquerels, grays, and sieverts.

These units are used to measure both the energy and rate of ionizing radiation (radiation strong enough to knock electrons off of atoms) and to the immediate and eventual harm this causes to human tissue. To measure the amount of radioactive decay in a substance you should use Becquerels but it took us a while to get there:

• Curies (Ci) (1910) - defined to measure the quantity of radium emanating in equilibrium with 1 gram of radium.
• Roentgen (R) (1928) - Used to describe the electric charge freed in a volume of air.
• Rutherfords (Rd) (1946) - defined as the activity of a quantity of radioactive material is which one million nuculei decay per second.
• Becquerels (Bq) (1975) - defined as 1 nuclear decay per second.

There are 36996 rutherfords and 37 billion becquerels in a curie.

To measure the probability of developing a fatal cancer after exposure to radiation use sieverts. They replaced roentgens which were established in 1976. These units are often described as stochastic units because they represent a probablity you will get sick. Again we had a different unit for a while:

• Roentgen equivalent man (rem) (1976)
• Sievert (Sv) (19??)

One sievert represents a 5.5% chance of developing a fatal cancer. My little radiation meter measures everything in sieverts.

To calculate the odds of getting sick from radiation exposure you need to quantify the amount absorbed by your tissue. This is currently measured in grays:

• Rad (rad) (1953) - the dose causing 100 ergs of energy to be absorbed in one gram
• Gray - the absorption of one joule of radiation energy per kilogram

There are 100 gray in one rad.

Grays are also used as units describing kerma - the sum of the initial kinetic energy of all charged particles liberated by ionizing radiation.

There's a bunch of terms used to describe how well radiation is absorbed by tissue including:

• Absorbed dose (measured in grays) - the energy absorbed in matter
• Equivalent dose (measured in sieverts) - the stochastic health effect of radiation taking into consideration how much a particular biological tissue absorbs the radiation
• Effective dose (measured in sieverts) - same as equivalent dose but the sum total of all biological tissues
• Committed dose - The risk if an effective dose was applied uniformly to the whole body(?)

I'm still unclear about the difference between the terms for different doses and it seems orthagonal to the intent of this post so I'm dropping it.

One interesting note I came across while wading through the wikipedia.

The typical human body contains roughly 0.1 μCi (14 mg) of naturally occurring potassium-40. A human body containing 16 kg (35 lb) of carbon would also have about 24 nanograms or 0.1 μCi of carbon-14. Together, these would result in a total of approximately 0.2 μCi or 7400 decays per second inside the person's body (mostly from beta decay but some from gamma decay).

posted by na on December 19, 2023 09:25