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FAQ - Radiation units - oh brother....

Posted: Mon Jun 03, 2019 10:53 pm
by Richard Hull
What is a "Rutherford", "a sunshine unit", a "REP" a "Rel"? All were defacto standards in nuclear radiation units and all are now gone.

Sometimes it seems that all measurement standards are not standards at all, but a rolling and roiling hodge-poge what what folks think are pretty units in any any given culture, county or choice du jour. All standards are standards for a little while until what is loosely termed a "standards committee" decides...."Out with the old and in with the new"....."long live the new!"

You would think that physics is one of the last bastions of invariability and stability. Wrongo! The sciences tend to love to change units across the board on a 50 to 100 year cycle. So, two to three generations, at most, live in a sweet spot where colleges use a standard that remains somewhat unchanged. Still, the poor students reading old texts were sent to conversion tables to make sense in their age of the units being used in old texts. Likewise, those poor graduates who are unfortunate enough to suffer a complete shift in units occur in their post grad years or worse, late in their careers, must again scramble to interpret their learned and long accepted working units into, the "new units".

I was one of those poor bastards. In college I was forced in one class to deal with antiquated English units and in Physics, CGS metric units, belched out into an MKS unit world and late in life stumbled kicking and screaming into the "new" SI unit world.

As this is a radiation forum we can speak in both the old unit system, Curies, rads, rems etc. and be able to recognize and work with the new SI units Becquerels, Grays, Sieverts. Fortunately, the conversion is a matter of a tortured power of ten path as you will note in the URL supplied below.

There are three categories in most radioactivity units discussion.
1. Activity
2. Dose rate
3. total absorbed dose

Activity is related to a radioactive source material. Regardless of the amount of material in a source, it is referred to as a "source". Activity is usually referred to as the number of radioactive decays or "disintegrations" occurring per unit time. (normally in one second) ...A rate of decay
old system = Curie symbol Ci
new system = Becquerel symbol Bq

Dose Rate. This is the amount of absorbed radiation energy emitted or absorbed per unit time in a target or flesh per unit time. Such units are typically smallish in nature and are a fraction of a much larger total dose. Most counters give a dose field rate that if you remain in the measured field for one hour this is the dose you will receive. example.... a counter reads 130 millirem....Stay in it for 5 minutes and you receive a total dose of 1/12 X130 or 10.8 mR
old system - Rem, milliRem symbol mR or mR/hr
new system - Sievert, micro Sievert symbol Sv, uSv

Absorbed dose . This is a total energy emitted or aborbed by a target regardless of the start or stop time of the of the exposure.
old system - RAD or Rem
new system - Gray

Read up on the units and conversions found at ... quivalence

Richard Hull