RADIOLOGICAL UNITS

I have received several letters asking to explain radiological units, old and new, even though I have done that often, but long ago.

There are four quantities that describe different aspects of radioactivity and its effects. Radioactivity manifests itself by the alpha, beta or gamma rays that are shot out when the nuclei of radioisotopes disintegrate, one after the other at random intervals, the nucleus splitting into fragments which are themselves new nuclei of either stable or radioactive elements. The alpha particles (=helium nuclei), beta particles (=electrons) and gamma rays (electromagnetic radiation) ionize the air and are partially or wholly absorbed by matter exposed to them.

The activity describes only the source, but none of its effects. The old unit is the Curie (Ci), equal to the activity of 1 gram of radium. The new (metric or standard international, SI) unit is the becquerel (Bq), which simply gives the number of disintegrations per second. Since 1 g of radium has 37 billion disintegrations per second, 1 Ci = 3.7 x 10^10 Bq.

The intensity of, or exposure to, the radiation is measured by the number of ion pairs (electron and positive ion) produced by gamma or X rays in 1 cc of air. If it produces 1 electrostatic unit of charge, the exposure is 1 roentgen ®. It has no specially named SI unit (i.e., it is expressed in coulombs per cubic meter).

The absorbed dose is the energy absorbed by a material, e.g., human tissue, and is proportional to the exposure times its dura-tion. The old unit, defined by an energy of 100 ergs absorbed in 1 cc of the material is the rad ®; the SI unit is the gray (Gy). 1 gray = 100 rads.

The dose as defined above is purely physical and takes no ac-count either of the type of radiation (for the same energy, alpha particles will do more harm than gamma rays) or the biology of the exposed tissue (the same physical dose will do more damage to, say, the gastro-intestinal tract than to a hand or foot). The effective dose taking account of this by multiplying the physical dose by various pertinent factors is measured in rems, 1 rem being the dose absorbed by "average" human tissue when exposed to gamma rays. The dose absorbed, say, by the lung from alpha particles is then upgraded in rems accordingly. The metric or SI unit is the sievert;

1 sievert = 100 rem.

It has become customary in the US to express doses and effec-tive doses in centigrays and centisieverts (1/100 of a sievert), so that you are in a way using the new and old units at the same time:

1 cGy = 1cS, and 1 rem = 1 cGy.

However, Mr. H.E.B. of Nashua, N.H., writes that all of this is too complicated and could I please translate this into pounds and ounces since (he correctly writes) 1 Ci is the activity of 1 g of radium = 0.00222 lb.

Well, starting from that, yes, one can get an idea, at least for a point source. If you calculate the dose delivered by 1 g of radium at a distance r, you will find that the coefficient is just about one when you use rems/hr and yards:

where / means "over" or division. Now you have a weight and can play around with the formula (which I saw in Health Physics and Radiological Handbook, Nucleon Lectern Assts., 1984, p. 41). Divide both sides of the equation by 1,000, and you can use it with millirems and milligrams. For distance in feet multiply the right side by 3^2=9. If the radioisotope is not radium, but something else, you need to look up tables of specific activity (activity per unit weight, usually curies per gram) and take the ratio how much gamma radia-tion the isotope puts out compared with radium, e.g., cesium-134, 1300; cesium-137, 87; iodine-131, 124,000; uranium-238, 0.0000003333, etc.

Which reminds me about a typical news item a month or two ago, when a truck carrying nuclear fuel collided with some other vehicle in New England and "spilled" its cargo, i.e., the container. That, of course, immediately made all the news bulletins. "For-tunately," said the boob toob dopes, "the container did not split open." Of course it didn't, it is designed to stay intact when dropped from 100 ft onto spike set in concrete. But what if it had? The fuel is uranium, so anybody approaching a 1 kg splinter to within 3 feet and gaping at it for 20 minutes would have got a dose of 0.1111 microrems, significantly less than newstwister Ted Turner got from Jane Fonda during their wedding night.