What Dose!?

Most common radiation “dose” that we, in radiation protection, are concerned of is either the “rem” or “sievert” of “mili-“ or “micro” levels, respectively. Both units “rem” and “Sv,” define the same kind of the, overall, biological implications of a damage to a living tissue (probability of cancer occurrence … ) induced by absorption of certain amounts of the various sorts of the ionizing radiation of different energies. This is an abbreviated introduction of what “dose” could be. Are you still with me?

The common, international, basis of the term “dose” is the Absorbed Dose of 1 “gray” (Gy) which measures quantity of ionizing energy equal to 1 “J(oule)” of energy absorbed by one kilogram of anything “massive.” How large is 1 joule? If, for example, a drop of water, ~ 50 mg, absorbs 1 J of energy, it would cause rise in its temperature by ~100F (compute how much the temperature in 0C of 1 ml of water will rise if it absorbs 4.18 J of energy.)That is physics - still with me?

One J(oule) energy of ionizing radiation could, also, be coarsely “measured” by about 6 trillion (6 × 1012) gammas of 60Co (~1.25 MeV each.) When absorbed by 1 kg of living tissue it become proportional to ~1 Sv of the “biological” or Equivalent Dose (compute how many Ci of the 60Co generates this many gammas/0.17 hrs.) Thus, the Absorbed Dose is “physical,” while the Equivalent Dose is “biological” dose. Dose of 1Sv is a huge one! Approximately 0.4 Sv dose induces nausea while ~4 Sv may be deadly.

The “rem” (roentgen equivalent in man) is, like the “sievert,” the measure of “biological” dose. But now, like Sv, it is based on the physical unit of energy “rad” (radiation absorbed dose) that is 1/100th of 1 gray and thus 1/100th of 1 J/kg. In the end, the “rem” will be one 100th of “sievert,” but it will be explained in better terms in the .pdf PPT module offered below (how much, approximately, is the “rad” in MeV?) Though only 1/100 of Sv, 1 rem is still a large dose when accumulated in a very short period of time.

The most important fact we need to glean from all of this is that all radiation dose units are based on, and derived from, a very precisely defined physical measures of absorbed ionizing radiation energy, i.e., 1 J(oule)/kg = 1 Gy and rad  = 1/100th of either one. The “roentgen” R ~ 7/8 rad - in air -, that is still so useful concept, is not basic to SI dose units. Although so important in forming our understanding of radiation protection, it is not used as a “dose” measure anymore (what is the numerical correspondence between 1 J/kg air and 1 roentgen?)  However, all of it, I mean the entire nuclear field and radiation protection in particular, in a way, was initiated by the brilliant work of that “rascal” Wilhelm, thus it is still desirable and necessary to keep him in the game (how many Gy/min - in air - corresponds to the ion chamber instrument reading of 3.7 × 10-1 R/hr ? Is it a high or a low dose rate?)

The Absorbed Dose .pdf file offered below is one of the PPT modules of the series Common Radiation Protection Equations. In it I am trying to describe the relations between “roentgen,” “joule,” “gray,” and “rad.” I hope that the offered presentation will help our RP technicians in somewhat better understanding of the basic radiation protection concepts. Please, review the .pdf PPT file and let me know what could be done to make it better.


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