Radioactive fission products, whether they are biochemically inert or biochemically active, can do biological damage when either outside the body or within.
        X-rays and gamma rays are photons, i.e. high-energy light-waves. When emitted by a source, for example, radium or cobalt, located outside the body, they easily pass through the body, hence they are usually called penetrating radiation. The familiar lead apron provided for patients in some medical procedures stops X-rays from reaching reproductive organs. A thick lead barrier or wall is used to protect the X-ray technician. Because X-rays are penetrating, they can be used in diagnostic medicine to image human bones or human organs made opaque by a dye. These internal body parts are differentially penetrable. Where bones absorb the energy, no X-rays hit the sensitive X-ray film, giving a contrast to form the picture of the bones on the radiation-sensitive X-ray plate. High-energy gamma rays, which easily penetrate bone, would be unsuitable for such medical usage because the film would be uniformly exposed. In photography jargon, the picture would be a `white out' with no contrasts. No radiation remains in the body after an X-ray picture is taken. It is like light passing through a window. The damage it may have caused on the way through, however, remains.
        Some radioactive substances give off beta particles, or electrons, as they release energy and seek a stable atomic state. These are small negatively charged particles which can penetrate skin but cannot penetrate through the whole body as do X-rays and gamma rays.
         Microscopic nuclear explosions of some radioactive chemicals release high-energy alpha particles. An alpha particle, the nucleus of a helium atom, is a positively charged particle. It is larger in size than a beta particle, like a cannon-ball relative to a bullet, having correspondingly less penetrating power but more impact. Alpha particles can be stopped by human skin, but they may damage the skin in the process. Both alpha and beta particles penetrate cell membranes more easily than they penetrate skin. Hence ingesting, inhaling or absorbing radioactive chemicals capable of emitting alpha or beta particles and thereby placing them inside delicate body parts such as the lungs, heart, brain or kidneys, always poses serious threats to human health.[3] Plutonium is an alpha emitter, and no quantity inhaled has been found to be too small to induce lung cancer in animals.
        The skin, of course, can stop alpha or beta radiation inside the body tissue from escaping outwards and damaging, for example, a baby one is holding or another person sitting nearby. Also, it is impossible to detect these particles with most whole body `counters' such as are used in hospitals and nuclear installations. These counters can only detect X-rays and gamma rays emitted from within the body.
        Splitting a uranium atom also releases neutrons, which act like microscopically small bullets. Neutrons are about one-fourth the size of alpha particles and have almost 2,000 times the mass of an electron. If there are other fissionable atoms nearby (uranium 235 or plutonium 239, for example) these neutron projectiles may strike them, causing them to split and to release more neutrons. This is the familiar chain reaction. It takes place spontaneously when fissionable material is sufficiently concentrated, i.e. forms a critical mass. In a typical atomic bomb the fissioning is very rapid. In a nuclear reactor, water, gas or the control rods function to slow down or to absorb neutrons and control the chain reaction.
        Neutrons escaping from the fission reaction can penetrate the human body. They are among the most biologically destructive ot the fission products. They have a short range, however, and in the absence of fissionable material they will quickly be absorbed by non-radioactive materials. Some of these latter become radioactive in the process, as was noted earlier, and are called activation products.