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