RSH Data & Documents

"Health Effects of Low-level Radiation"

by Dr. Sohei Kondo

Title Page: Front matter

Table of Contents

Terminology of radiation dose

Terminology of
radiation dose

Health effects of ionizing radiation result from microscopic events of energy deposition in the cells of the human body. The energy deposition arises from the passage of charged particles (electrons, protons, helium ions, etc.) with high energy through the cells, giving rise to clusters of molecules, ionized or excited, along the particle's track. Ionization of molecules is the major cause of biological effects of ionizing radiation (Lea, D.E.: Actions of Radiations on Living Cells, Cambridge University Press, 1955). However, since we cannot measure directly the ionization events in the cell, we conventionally use the term dose defined as the average of the total energy absorbed per unit mass of the tissue. For fast neutrons and X or gamma photons, we often use the kerma defined as the sum of the initial kinetic energies of all the charged particles liberated per unit mass of the tissue by such indirectly ionizing particles.

The unit for the dose or kerma was formerly called the rad and has been recently called the gray with symbol Gy. One Gy corresponds to a joule per kilogram of the tissue. Submultiples of 1 Gy are often used, such as the centigray cGy (1 cGy = 1 rad). Throughout the present book, unless otherwise stated, I use the rad unit for it is close to levels of radiation frequently referred to in the book. Consequently, I use the formerly used term rem (rad equivalent man) to take into account the fact that one rad of alpha particles or fast neutrons is Q times more harmful than one rad of beta particles, by saying that one rad of alpha particles is equivalent to Q (=20 for radioprotection) rem. The new unit is named the sievert, symbol Sv, and submultiples of 1 Sv are often used such as the millisievert (mSv; 1 mSv = 0.1 rem).

The new unit for the measure of radioactivity is the becquerel with symbol Bq; 1 Bq is the rate per second of a radioactive nuclide being disintegrated. The formerly used unit Curie (Ci) is related to 1 g of ²²⁶Ra. That is, 1 Ci = 3.7x10¹⁰ Bq. Both units are used in the book.

Table 1.  Relationship between old and new radiation units

Reference
Lea, D.E. (1955) Actions of Radiations on Living Cells, Cambridge, Cambridge University Press