Atomic Radiation and Life

The radiation dose given off by an X-ray machine or by isotopes is usually measured by determining the number of ions produced in a volume of gas. Since these carry an electric charge there are a number of extremely delicate methods by which they can be detected. The widely used Geiger counter consists essentially of a wire stretched inside a cylindrical tube, so arranged that an electric current can pass between the wire and the tube only when there are ions in the gas. Consequently, when an ionizing particle passes through the tube, an electric signal is given out. In this way the number of ionizing particles given off by a radio-active source can be accurately counted. This is called the activity of the material. It is measured in a unit called the curie after the discoverer of radium. The activity of one gram of radium together with its decay products is equal to one curie. Every time an atom disintegrates a beta- or alpha-ray is given off, together with a certain amount of gamma radiation.

The activity in curies can tell us nothing about the dose of radiation given off by the radio-active material, since the curie measures only the number of ionizing particles emitted, independent of their range or energy. If, for example, we put next to the skin one curie of radio-active cobalt, which gives off energetic gamma-rays, the dose received on the surface will be one five-thousandth part of the dose received from one curie of polonium which gives off alpha-particles. On the other hand the gamma-rays from the curie of cobalt will penetrate deeply, while the alpha-rays will not affect anything which lies more than two one-thousandth of an inch below the surface of the skin.

The best way of defining the dose of radiation which an irradiated material has received is in terms of energy. We have seen that on exposure to ionizing radiation electrons, or other sub-atomic particles moving at great speed, lose energy to the surrounding molecules. The amount of' energy gained by the irradiated substance is clearly the important factor, and will determine the biological changes produced. The most widely used unit for measuring X-ray and gamma-ray dosage is the roentgen - named after the discoverer of X-rays. The remarkable property of ionizing radiation is that the small amount of energy represented by a few hundred roentgens can kill a man.

The primitive embryonic cell known as the zygote, which is formed after the entry of the sperm into the ovum, is very sensitive so radiation. For example, 80 per cent of mice, exposed to 200 rads of X-rays within the first five days after conception, fail to give birth. Smaller doses give rise to a lower incidence of pre-natal death, but an appreciable reduction in the average litter size has been observed with 50 rads.

At first the embryo grows by cell division without differentiation and becomes firmly implanted in the wall of the uterus. This requires about eight days in human beings and five days in mice. Then differentiation begins, and the individual organs and limbs are formed; the embryo takes shape. During this period it is in the greatest danger. Now radiation no longer kills-the damaged embryo is not re-absorbed or aborted, but proceeds to a live birth which is abnormal. These malformations can be very great, so as to give horrible and distressing monsters, which are, however, quite capable of living for a time. The incidence is particularly high in the early stages of the active development of the embryo.

The period of major organ production is over after about three months in human beings, and the foetus then develops its finer aspects and generally grows and develops. Exposure to doses insufficient to produce severe radiation sickness in the mother no longer produces gross deformities which can be recognized in small experimental animals. But the absence of striking changes in the newborn does not mean that the irradiation has been without harm. The general effect is less obvious, but none the less serious, and irradiation at the later stages of pregnancy results in very marked growth reduction, giving small babies which develop into smaller adults. Their life span is reduced and their reproductive organs are often affected so that they grow up sterile. Damage to the brain and eyes was found a few weeks after birth in all cases which had been irradiated in the foetal stage with zoo rads, and there is a significant incidence after 200 rads. Since only gross disorders of the brain can be detected in experimental animals, it seems likely that much smaller doses will give effects which are serious in man.

The detailed picture of the influence of radiation on prenatal development has been obtained from studies with animals. Unhappily, sufficient human cases are known to make it certain that the same pattern also occurs in man; and we can confidently superimpose a human time-scale on the mouse data. Some of our information is derived from the survivors of the atom bombs in Japan. The children of the women who were pregnant and exposed to irradiation at Nagasaki and Hiroshima are, on average, shorter and lighter and have smaller heads, indicating an under-developed brain. Some show severe mental deficiencies, while others were unable to speak normally at five years old.

Most of our knowledge comes from expectant mothers who were irradiated for therapeutic or diagnostic reasons. Many cases are described in the medical literature of abnormalities following exposure of the embryo. Most of these arose twenty or thirty years ago at a time when radiologists did not know of the great radio-sensitivity of the foetus. A detailed survey showed that, where a mother received several hundred roentgen within the first two months after the implantation of the embryo, severe mal-development was observed in every child, a high proportion of whom lived for many years.

(From Atomic Radiation and Life by Peter Alexander.)