NEUTRON RAYS

Evening Post, Volume CXXI, Issue 141, 16 June 1936, Page 14

NEUTRON RAYS

AND LIVING THINGS

SOME STRIKING RESULTS

AMERICAN EXPERI-

MENTS

In the February issue of the proceedings of the U.S. National Academy of Science there appear two papers by members of the staff of the University of California reporting the results of experiments carried out by them to ascertain the effects produced in the tissues of living plants and animals by irradiation with the recently discovered variety of corpuscular rays termed neutrons, says a writer in the Melbourne "Age."

As the name neutron implies, these rays, are, in distinction to cathode rays —which are high-speed electrons—or the alpha rays from radium—which carry a charge of positive electricity— electrically uncharged or neutraL Except in this one respect they appear to be practically identical with the nucleus of the hydrogen atom, or proton. But this one difference is sufficient to account for their extraordinary power of penetrating matter, for carrying no electric charge they pass near or even through atoms unperturbed by the intense forces which the electrical fields within these exert upon other charged particles. Thus it is that, whereas a plate of lead half an inch thick will almost completely obstruct a beam of the fastest electrons which can be artificially produced—or even those of higher speed which naturally emerge from radio-active substances —a beam of neutrons can penetrate a plate ten times this thickness with only a fractional reduction of their number and speed.

BEAM OF "DEATII-KAYS." It is clear that such rays might be expected to travel a mile or more thrbugh air before being completely dispersed or absorbed, and, as we shall see, could such a beam be -produced in sufficient intensity, it might well justify the title of a beam of "deathrays."

• Only in the rare cases when the neutron!—itself a , particle of extremely minute ; dimensions—encounters the equally minute central core or nucleus of an "atom does it suffer a sudden check arid loss of energy, and that, as the law of collision requires, the greater the smaller the mass of the atom encountered, so that, paradoxically, paraffin is more opaque to a beam of neutrons that is lead. The neutron may even enter the' nucleus of an atom and be retained there, forming thus an atom of a new kind, eften radio-active.

But, even when capture does not result from nuclear collision, the atom which is struck receives an impetus which toakes it, for a brief instant, a formidable agent of destruction to other atoms which lie in its path of recoil. Like an alpha-ray, this recoiling atom tears off one or more outer electrons from every atom it encounters, converting them thus temporarily into electrically-charged atoms or tons with a vastly enhanced chemical activity.

. Now since this chemical activity resulting from ionisation by other radiations—whether short waves of light (ultra-violet light or X-rays), fast-mov-ing electrons, or "the more massive alpha-rays—is, in every case, associated with intense biological action, often of a kind most destructive to the living cell, it was obviously important to ascertain whether or not a similar potency inheres in the neutron.

Had the only method of producing neutron rays been the original one of bombardment of a metal by alpha-rays, this might have been a difficult matter to test, for such a source, depending as it does on the minute quantities of radium available, must ever remain almost incomparably weak when compared, for example, with the intensity of X-rays from a powerful X-ray bulb.

But Professor Lawrence, of California, discovered more than a. year ago that the ions of "heavy" hydrogen—or deuterium, as it is now termed—when impelled to enormous speed by an electric force of several million volts in a wonderful apparatus called a cyclotron, designed and constructed by him, are even more effective agents in causing the metal beryllium and other elements to emit neutrons than are alpha-rays.

Neutron beams comparable in intensity with X-ray beams of moderate strength can be obtained with Lawrence's, machine. To test their effect on living matter, two different series of experiments have been carried out, one by Lawrence himself in collaboration with his brother, Dr. John Lawrence, of trie Yale University School of Medicine, in which rats were chosen as the subjects of experimentation; and the other by Dr. Raymond Zirkle, of the Johnson Foundation for Medical Physics, University of Pennsylvania, and a collaborator from the staff of the X-ray department of the University of California. These experimenters caused the rays to fall upon germinating wheat grains and studied the subsequent growth of the sprouts. Comparison of the biological effect of the neutrons was made with that produced by X-ray beams, from the 900,000-volt plant of California University. In each case the strength of the beams was estimated from their ionising effect when passed through air—a method which has now become the standard one for measurement of X-ray dosage.

MORE POTENT. The results of the neutron irradiation, whether on the living plant or the living animal, have proved these rays to be vastly more potent for the destruction of life than are X-rays. Whereas the rats were found capable of enduring a dose of 500 rontgen units of X-rays without permanent injury to their health —for, although a considerable decrease in the count of white and red corpuscles in the blood followed this irradiation, it was found to be only temporary—a rat which received only 72 units of the neutron rays sickened and finally died. Lawrence estimates that the biological effect of neutrons is approximately 10 times as great as that of X-rays.

Substantially the same deduction is darwn from the experiments on the sprouting grains of wheat. A decided retardation in growth of the sprouts was observed to follow both from the treatment with X-rays and from that by neutrons, but in the latter case one rontgen unit -produced the same retardation as 20 X-ray units.

Zirkle and his colleague, Aebersold, discuss the question, important in regard to possible future therapeutic applications of neutrons, whether some tissues are more sensitive to the action of neutron rays than others. The greater effect on their wheat grains as compared with the animal subjects of Lawrence's experiments would certainly appear to indicate a very marked difference in their destructive action on plants and animals respectively; but whether such a differential activity will be found to exist in regard to healthy and diseased human tissues can, of course, be finally settled only by experiments.