COSMIC RAYS-EXPERIMENT and CONJECTURE ALEXANDER W. STERN 36 Crooke Ave., Brooklyn, New York
The addition of experimental knowledge concerning ultra-penetrating radiation has tended to raise rather than to settle problems concerning its nature and origin, until it has now become one of the most fierplexing, albeit interesting, subjects of modern physics. It i s suggested that a complete solution of the nature and origin of cosmic rays may have to wait for a n adequate theory of the interaction of radiation and matter based on a relativistic quantum mechanics.
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LTHOUGH we know now a great deal more about this penetrating radiation, nevertheless the nature and origin of the cosmic rays remain as much an open question as thirty years ago when Elster and Wilson first detected them. These investigators found that their electroscopes would discharge, no matter how much precaution was taken to prevent any leakage of the electric charge from their instrument. Essentially, an electroscope consists of a glass jar in which is suspended a pair of very thin gold leaves or strips. The gold leaves are supported by a metal rod which passes through the top of the jar, so that an electric charge may be imparted to the gold leaves, "charging" the electroscope. The two leaves then having the same kind of charge, will repel each other and spread apart. Once charged, the leaves will remain in this divergent position, unless there is something inside the electroscope to carry the charge away, causing the leaves to come together and discharging the instrument.
Elster and Wilson found that the leakage of the charge from their electroscopes could be reduced by shielding their instruments with a metal box a few inches thick, demonstrating that it was some kind of penetrating radiation coming from the outside that was responsible for the discharge of their instrument, and not a fault of the insulating material. This radiation could penetrate a metal box a few inches thick, get through to the instrument, and ionize or electrify the surrounding air, which would then conduct the charge away from the electroscope. The problem immediately arose: "What was the nature of this radiation?" At that time radioactivity (which is the spontaneous emission of rays of enormous energy by certain substances) was the most discussed phenomenon in the scientific world, and the common opinion was that the radioactive material in the earth was responsible for the penetrating radiation observed by Elster and Wilson. There was nothing to disprove this view until 1910, when Gockel, a Swiss scientist, set out to test this hypothesis by going up in a balloon to a height of a little less than three miles. If the radioactive material in the earth were solely responsible for the penetrating radiation, then a t a height of three miles this radiation should be entirely absorbed. Gockel found that, although the rate of discharge of his electroscope decreased for a time as he ascended, it nevertheless began to increase as soon as the balloon was above the influence of the radioactive materials of the earth. This, I believe, is the first direct evidence of penetrating radiation coming from above. From 1910 to 1914 Hess
in Austria and Kolhorster in Germany checked and extended these important observations of Gockel. Hess had taken balloon readings to 5200 meters, while Kolhorster's observations reached a height of 0000 meters. The careful measurements of these investigations put Gockel's results on a quantitative basis. In fact, Hess and Kolhorster were the first to measure the cosmic radiation. The war put a stop to further work, and nothing was done until 1922, when Millikan and Bowen built recording electroscopes and sent them up in sounding balloons to a height of nearly ten miles. They, too, had found an increasing rate of discharge of their electroscopes with increasing height above the earth, but their rate of discharge was only onequarter of the rate derived from the data of Hess and Kolhorster. Quantitative, and not qualitative, discrepancies worry scientists. Immediately doubts arose about the cosmic origin of the penetrating radiation, and a number of other possibilities were suggested. The two which attracted the most attention were C. T. R. Wilson's suggestion that the ionization or electrical conductivity of the air caused by thunderstorms was responsible for the observed effects, and the other, the presence of radioactive material in the upper atmosphere. In order to test or eliminate these possibilities, direct measurements were made of the penetrating power of the rays with substances other than the atmosphere (such as water or lead) free from radioactive contamination. L shall pass over the early experiments along these lines, as they did not materially advance the subject, and come to the work of Millikan and Cameron. A new era in cosmic ray research began with the admirable work of Millikan and Cameron during the years 1925 to 1931. With new, air-tight electroscopes, which they sank a t the bottom of the deep, high, snowfed mountain lakes in California, which are practically free from radioactive contamination, they obtained reliable data concerning the penetrating power and the variation in intensity of the radiation with geographical location. An extensive series of measurements made in two lakes three hundred miles apart revealed that the cosmic rays were exactly alike in the two different localities. It was found that the main component constituting about 90% of the radiation has a penetrating power of about sixty feet of water, which upon calculation gives an equivalent energy of approximately 25,000,000 volts.* Their measurements also gave evidence of the existence of two weaker components of the cosmic radiation with energies of the order of 100,000,000 and 200,000,000 volts, respectively. For -the purpose of extending and checking their
* The energy of a cosmic ray beam is derived from iti; absorption coefficient or penetrating power by a mathematical formula which gives the relation between the penetrating power of a beam and its frequency. The frequency multiplied by Planck's constant "h" gives the energy in ergs, which, when multiplied by 6.28 X 10". is converted to energy expressed in electron-volts. A 25 million-volt photon is one having an energy equivalent to that acquired by an electron which has fallen through a drop of potential of 25 million volts.
California work, Millikan and Cameron, in the fall of 1926, went to South America. They chose two lakes in the Bolivian Andes, Lake Miguilla (altitude 15,000 ft.) and Lake Titicaca (altitude 12,500 ft.), in which to cany out another group of experiments similar to those they made in California. Their South American results revealed that the cosmic rays enter the earth with equal intensity in the southern and northern hemispheres, thus showing that the earth's magnetic field has no effect on the radiation. Because electrons are affected by a magnetic field, and should therefore give less intense rays near the equator than near the pole to which they will be attracted, this finding constitutes one of the strongest arguments for the hypothesis that the cosmic rays consist of high-frequency radiation rather than high-speed electrons. Also, the suggestion of C. T. R. Wilson that the cosmic radiation is the result of the collisions of high-speed electrons, which have acquired enormous energies in thunderstorms, with our atmosphere does not receive support from these South American experiments of Millikan and Cameron, since the lakes where the measurements were made are free from thugderstorms, being situated in valleys which are screened from such effects by the surrounding mountains. Tests were also made for any directional effects in the radiation with the result that no such effects were found, the intensity of the radiation remaining the same whether the Milky Way, the sun, or any other portion of the sky was overhead or not. To get conclusive evidence that the radiation is not affected by a magnetic field, Millikan went to Churchill, Manitoba, the nearest settlement on earth to the north magnetic pole, and established the fact that the cosmic rays are not influenced by the earth's magnetic field. The measnrements of the intensity of the radiation a t Churchill agreed with the measurements made in California and South America. Finally, in 1931, as a result of careful measurements with their new and extremely sensitive electroscope, Millikan and Cameron verified the existence of a weak component constituting a very small percentage of the radiation, but with an energy of the order of 1000 million volts! Millikan estimates this component to comprise about i/soo of the whole beam, but very recent work points to the fact that Millikan underestimated the strength of this most penetrating part of the radiation. To summarize, what are the direct conclusions from Millikan's experiments? First, that there is an extremely penetrating radiation coming from above which is not affected by any heavenly body, the intensity of the radiation remaining the same whether coming from the direction of the Milky Way, the sun, or any other celestial body. This is a remarkable property, because one readily sees that, since all directions in the heavens are equally effective, the radiation must come from beyond our galactic system. The second is that the cosmic rays are not in any way influenced by the earth's magnetic field, and the third, that the radiation is independent of geographical location, a
fact difficult to reconcile with the hypothesis which makes the earth's atmosphere the origin of the radiation. The observed penetrating power of the radiation is given by Millikan and Cameron as ranging from 60 feet to 600 feet of water with equivalent energies of approximately 25 million to 250 million volts, but recently these investigators have verified the existence of a cosmic ray, first discovered by Regener, with a penetrating power of about 2400 feet of water, representing an energy of the order of 1000 million volts! Radiation resulting from the complete annihilation of a hydrogen atom would have this same energy. Taking these experimental facts into account, let us consider what is the basis for Millikan's assertion that the "observed cosmic rays are the birth cries of the common elements out of positive and negative electrons." The evidence for atom-building going on in interstellar space rests chiefly on the agreement between the observed values of the penetrating power of the cosmic radiation and the theoretical values computed on the assumption that the radiation is produced by the act of creation of the common elements out of their elementary constituents. By using the Dirac formula Millikan found that the agreement between the observed values and the theoretical values corresponding to the creation of helium, oxygen, and silicon out of hydrogen was so good as to leave no doubt in his mind that the observed cosmic rays aie in fact the birth-cries of the infant atoms of helium, oxygen, and silicon. But in 1928 a more accurate formula based on the relativistic wave mechanics was introduced by Klein and Nishia. The calculated absorption coefficients, or the penetrating powers, from the Klein-Nishina equation differ materially from those given by the Dirac equation. With the new equation the computed coefficients agree very well with the observed co&cients in the hydrogen to helium transformation, but with the heavier elements (oxygen, silicon, and iron) the agreement is at best only qualitative. And in 1932 Anderson, working in Milikan's laboratory, obtained proof showing that it is the nucleus, the core of the atom, that is the principal factor in the absorption of the cosmic radiation, not the outer electrons of the atom, as was heretofore supposed. Since both the Dirac and the KleinNishina equations give only the absorption due to the outer electrons, the formulas are inaccurate, and we are left without a valid formula to test the atombuilding hypothesis. Anderson's work revealed that, in a considerable number of cases, the cosmic radiation causes the disintegration of the atom. Another very important result of Anderson's beautiful experiments in photographing cosmic ray tracks is that as high as one-tenth of the photographed tracks show energies of the order of 1000 million volts, which is what is demanded for atom-annihilation, not atom-building. Consequently, Millikan is now compelled to add atomannihilation (Science, May 13, 1932) as a possible hypothesis to explain the origin of the cosmic rays, despite the fact that he has brought forward his previous suggestion of atom-building as evidence that the Creator is
still on the job! One sees therefore that the atombuilding explanation holds only for the softest component of the cosmic radiation, the formation of helium out of hydrogen. When we come to the more energetic components, the evidence for atom-building processes is insubstantial. Now, as to the nature of the radiation. Chiefly, there are two reasons for regarding it as composed of photons instead of electrons. First, no process of nature is known which could endow electrons with so much energy, and second, the complete absence of any effect on the radiation by the earth's magnetic field. But nothing is known concerning the properties of high-energy electrons moving with the velocity of light, nor have we a complete theory of the interaction of radiation and matter which would tell us what happens when a photon collides with an electron. Only an adequately developed relativistic quantum mechanics could give us this knowledge, but there are tremendous intellectual difficulties which stand in the way of its formulation. For all these reasons we can only draw tentative conclusions about the origins and nature of cosmic radiation. What other facts have been discovered concerning ultra-penetrating radiation? In 1929, Dr. Bathe and Professor Kolhorster created some excitement in the scientific world by presenting evidence for the corpuscular nature of the penetrating radiation. The experiment was performed by simply placing two Geiger tube counters one above the other at a distance of five centimeters apart, the space between the counters being screened by lead, and noting the number of coincident deflections in the counters. A Geiger counter consists of a tube evacuated to a few millimeters pressure, in the center of which is a wire. A battery maintains a field between the wire and the walls of the tube, so that a current just refuses to pass. At this critical stage the passage of a single electron of sufficient energy through the tube will produce enough ionization to allow a momentary current to pass between the wire and the tube, which is made detectable by connecting an electrometer to the tube, the deflection of which is shown on a moving film. Simultaneous photographic registration by the two counters would show that an ionizing particle must have passed through both counters. When the experiment was performed it was found that 20% of the total number of deflections of one counter were coincident with deflections in the other. This percentage is so high that it can only be explained on the basis that the coincidences were due to the same corpuscular ray entering both counters. It is possible, as suggested by Lord Rutherford, for a photon of radiation to produce a coincidence by liberating an electron in one counter, passing through and liberating another electron in the second counter. Coincidences of this kind can occur from the interaction of the radiation with the nucleus, liberating a fast beta-ray electron from the inner core of the atom. The difficultyhere is that, whiie a recoil electron is in the forward direction, the radiation excited in the nucleus is likely to be distributed in all directions, and there-
27 fore unlikely to pass through the second counter. To prove that these fast electrons are not due to scattering associated with such penetrating radiation, the penetrating power of the electrons was measured by placing a block of gold four centimeters thick between the counters, the diminution in the number of coincidences thereby giving a measure of the absorption of the corpuscular rays. One naturally expects that if these corpuscular rays consist of secondary electrons they will be more easily absorbable than the penetrating radiation which caused them. Bothe and Kolhorster performing the experiment found the same absorption coefficient as was obtained from the electroscopic experiments of Millikan working with unfiltered cosmic Lays, thus showing that the penetrating power of these electrons is as hieh as the radiation itself. The surprising result of Bothe and Kolhiister's work led Mott-Smith and Locher, last summer, to plan and perform an important experiment designed to remove the discrepancy between the findings of MottSmith, Millikan, and Rossi on the one hand, and Bothe and Kolhorster on the other. Mott-Smith had already performed an experiment giving no evidence that the penetrating radiation could be deflected by a strong magnetic field, although in his work, electrons with as high an energy as 2 X lo9volts would have been measurably deflected, afid Rossi, in Italy, obtained the same results with his magnetic deviation experiments. Most of the evidence was, therefore, on the side of Millikan when Mott-Smith and Locher began their work. Their idea was to combine a Wilson cloud chamber apparatus and two Geiger counters in such a way that every corpuscle that operates the counters must pass through the cloud chamber, giving a definite correlation between an ion track appearing in the chamber and the discharge of the two counters. (A W'ison cloud chamber is an instrument in which a corpuscle is made to give evidence of its presence by producing a track of water droplets.) Electrons and protons, like historical personages, are known by the traces they leave behind them. The Wilson cloud chamber is based on the fact that water vapor in air, even in a supersaturated condition, will not condense into droplets unless there is something for it to condense on. Either dust particles or ions will sdice. A corpuscle shot through a Wilson cloud chamber containing dust-free air of just sufficient saturation, will, by knocking the electrons out of the atoms in its path, cause a train of ions to form. The water vapor then immediately condenses, and there is formed a white streak of tiny droplets sitting on the ions, marking the path of the corpuscle. This line of droplets may be photographed and studied. If no ion tracks are found in the chamber in association with the discharges of the counters, we have good evidence in favor of photons as comprising the penetrating radiation, since photons do not produce ion tracks. One could then, by postulating a few new, albeit unusual, properties for photons (such as discharging a counter), reconcile the results of Millikan,
Mott-Smith, and Rossi with those of Bothe and Kolhorster. The other alternative is, of course, the production of ion tracks in conjunction with the coincident discharges of the counters. This would be decided evidence in favor of the electronic nature of the radiation. The result of the experiment was quite unexpected. Mott-Smith found ion tracks closely resembling those of fast beta rays . from a radioactive source, showing that a cosmic ray particle went through the apparatus. But if the cosmic radiation consists of high-speed electrons, how can one explain the findings of Millikan showing that the radiation is not influenced by the earth's magnetic field, and the magnetic deviation experiments of Mott-Smith and Rossi which did not record any magnetic deflection of the cosmic radiation. The results of Millikan, MottSmith, and Rossi would require the electrons to have the prodigious amount of energy represented by 1O"e volts in order to escape deflection. On the other hand, if the cosmic radiation does consist of these very highenergy electrons, they would have greater penetrating power than was recorded by cosmic-ray measurements. Besides, nobody knows how electrons can acquire such enormous energy. However, if one were to hold to the photonic nature of the radiation, how could one explain the observable tracks in the cloud chamber? Can a cosmic ray photon produce by collision an electron with energy greater than itself? Others have suggested that the cosmic radiation may consist of a new type of particle-the neutron. The neutron, as its name implies, carries no charge and therefore cannot be deflected by a magnetic field. It is assumed to be a proton and an electron in such close combination as t o neutralize each other's charges and behave as a single particle. But Millikan and Anderson have shown the neutron hypothesis to be untenable, since a calculation reveals that neutrons in collision with atoms could not give rise to such high-energy electrons as those observed when the cosmic rays encounter atoms. P. A. M. Dirac, one of the greatest exponents of the new quantum theory, bas shown recently in a highly technical paper that the quantum physics allows the existence of isolated magnetic poles-units of magnetism-which are similar to electrons, the unit of electricity. I have written a popular account of his paper for Science of February 12, 1932, and presented therein Professor 0. W. Richardson's suggestion that there may be enough objects in the universe with the intrinsic energy of these magnetic poles to account for the otherwise unaccountable ultra-penetrating radiation. Finally, there is the fact brought out by Professor F. A. Lindemann. Because of their enormous energy, the constituents of the ultra-penetrating radiation have masses comparable to that of the hydrogen atom, and it becomes difficult to detect any magnetic effect. At 109 volts, for example, which is the voltage of the cosmic radiation, electrons, protons, and photons all have comparable masses, and the three fundamental constituents of the universe become indistinguishable by their behavior.
Each type of experiment suggests a new interpretation of the cosmic rays-waves, electrons, protons, neutrons, and radiations from a new kind of substance made up of magnetic atoms. All the foregoing interpretations, explanations, and conjectures show the urgent need of a deeper study of the cosmic rays. Aided by a grant from the Camegie Foundation, there has already been organized by Professor A. H. Compton, of the University of Chicago, a world-wide expedition with the purpose of furnishing the most adequate tests yet devised to distinguish whether the cosmic rays are photons or electrons. According to a news dispatch in the N m York Times of January 3, 1932, Professor Compton will start in March on a six months' expedition to Peru, New Zealand, Australia, Hawaii, and Alaska. Three other cooperating expeditions will be organized, one in South America, another in South Africa, and a third in the Himalaya Mountains. Their work will supplement the old measurements of Millikan, with observations taken in standard ionization chambers a t eighteen stations distributed all over the world, and a t heights ranging from 1000 to 20,000 feet. But the addition of more experimental facts is not all that is necessary. Whenever there has been a great advance in physics, theory and experiment have gone hand in hand. The experimental facts need to be correlated oefore we can see their significance,and an adequate theory of the cosmic radiation developed before we can have a clear understanding of one of the most important questions of modem physics. A knowledge of the nature and origin of cosmic radiation may profoundly influence our ideas concerning the origin and destiny of the universe, and give us a deeper insight into our study of the ultimate constituents of the world -matter and energy, which. after all, lie a t the bottom of that manifestation of nature we humans like to clothe in mystery-life. The unknown in science is not the "unknowahle." In that most characteristic pursuit of modem man, the adventure that is science, man achieves his dignity and nobility. ADDENDUM, NOVEMBER 28, 1932 In the July 1, 1932, and September 1, 1932, issues of the Physical Revim and also in a signed article for the N m York Times, Professor A. H. Compton reports some of the results of his cosmic ray survey. Measurements made by himself and other members of the Cosmic Ray Expedition in the American Rockies, Hawaii, New Zealand, Peru, the Equatorial Pacific, Southeastern Australia, Panama, Mexico, and Northeastern Canada show that the intensity of the cosmic radiation varies with latitude. A slight variation in the intensity of the rays with the time of day was also detected. The results of the measurements reveal that the cosmic radiation is about 20% stronger near the earth's magnetic pole than near the equator, and is slightly stronger during the daytime than during night-time. The slight variation between daytime and night-time radiation becomes more pronounced at high altitudes. At 4930 meters above sea level the
average intensity of the radiation coming to the earth between 10 and 4 in the daytime is ll/& stronger than the radiation coming between 10 and 4 in the night-time. These findings are in flat contradiction to Professor Millikan's work, and show that a t least a good part of the penetrating radiation entering our atmosphere consists of charged particles-either electrons or protons. The evidence so far favors electrons. Professor Compton has also gathered new evidence confirming that the cosmic ray carries a stupendous amount of energy, of the order of lo9 volts. Yet the origin of this radiation is more of a mystery than ever. If the cosmic radiation were waves, one could account for their incomparable energy by postulating that these waves were emitted by the destruction of atomic nuclei, and for the softer components of the radiation one could assume that they were emitted by the formation of atomic nuclei. But now that we have evidence for the electronic nature of the cosmic rays, we are at a total loss to explain how an electron could acquire so much energy! If the cosmic rays are electrons, then this is the first evidence we have of a very singular and significant processthe disintegration of atomic nuclei by capture or collision with high-energy electrons. Such an interaction would be a very complicated affair, and is little understood a t the present time. If the nuclei of atoms are composed of protons and neutrons, as some physicists are now inclined to believe, then one must assume, in order to explain Anderson's photographs, that the cosmic ray, when interacting with the nuclei of atoms, breaks up the neutron and causes the emission of either one or both of its constituents. (It is remembered that Anderson has succeeded in photographing the tracks of nuclear electrons and protons liberated by the cosmic radiation.) That is to say that when high-energy radiation (either photons or electrons) interacts with neutrons, the neutron no longer behaves as a statistical particle-as a unithut that its constituent proton and electron interact independently.
