Is there a conversion factor between the radiation units Curie and Roentgen?
Curies (Ci) and Roentgens (R) are both related to radiation, but they describe different properties.
A Curie is a unit associated with the number of radioactive disintegrations per second in a particular sample of radioactive material. The Curie describes the activity of a radioactive source. One Ci of radioactive material produces 37 billion disintegrations per second. A disintegration per second is also known as a Becquerel (Bq).
A Roentgen, on the other hand, is a measure of the amount of charge produced in a particular sample of air from ionizing radiation (i.e. - a type of radiation that has enough energy to remove an electron from an atom, producing ions). The technical definition is the amount of X or gamma radiation that produces one electrostatic unit of ionic charge in one cubic centimeter of dry air at standard temperature and pressure. The Roentgen describes the exposure of air from a radioactive source.
Radioactive decay produces various types of radiation in the form of particles (alpha, beta, neutron) and photons (x-rays, gamma rays). A radioactive source will emit these radiations at various frequencies, depending on its activity and its decay mode. Each type of radiation, depending on its energy, produces a different amount of ionization of air, and hence a different exposure. Alpha particles, for example, will produce substantially different amounts of ionization than highly penetrating gamma photons. The total exposure produced from a radioactive source is therefore related to the total number and type of radiation emissions from that source; the total number of emissions is related to the activity of that source.
Hence, there is no general equivalence between Curies and Roentgens, but a certain number of Curies of a particular radioactive material with a known size and shape will produce a certain number of Roentgens at a specified distance.
Aside: Since most people are interested in the effects of radiation on tissue, the Roentgen is not a practical measurement because it describes the ionization of air. The rem, or Roentgen equivalent man, is a more familiar measure of ionizing radiation. The rem describes the effective dose to tissue - a unit more meaningful for people. The rem is basically a measure of the amount of energy absorbed per unit mass of tissue. The Roentgen (charge produced in air) is indirectly linked to the rem (energy absorbed in tissue); the standard comparison of the effect of ionizing radiation in tissue to that in air is given that 1 rem is approximately equal to one Roentgen of 200-kV X-radiation. The rem has recently been replaced with the Sievert (1 Sv = 100 rem).
Scott Wilber, President, ComScire - Quantum World Corporation, and
Erin Niven, M.Sc., Ph.D. Candidate, Medical Physics, Ontario, Canada
'Physicists like to think that all you have to do is say, these are the conditions, now what happens next?'