GUEST AUTHOR Willard F. Allen Department of Chemistry University of Alberta Edmonton, Alberta, Canada
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Textbook Errors,
CIS.^ Rays
A
very common misapprehension among chemists is that cosmic rays are high-frequency electromagnetic radiations from interstellar space. Although y-rays are produced in cosmic ray showers, it was shown as long ago as 1929 (1) that most cosmic rays are fast-moving charged particles. Despite this, several modern chemistry texts1 still classify them as electromagnetic radiation, with no mention of particles. The following is an outline of the current picture (2). Investigations of radioactivity in the period 190025, showed that there was a background of ionizing radiation present everywhere on the earth. While some of this could be accounted for by radioactive materials in or close to the detecting devices, a small but measurable intensity appeared to exist everywhere. When it was found that this portion of the background increased with altitude, it was assumed to be of extraterrestrial origin, and was given the name "cosmic rays." Cosmic rays have been shown to have the following characteristics: Their intensity increases with altitude to a maximum at about 65,000 ft, after which it decreases t.o ahout half its maxi~numvalue, and remains constant. Their intensity is highest at latitudes above 45'N and 45'8, and drops to a minimum at the geomagnetic equator. At sea level the minimum is about 14% below the maximum. Near the equator there is a preferred direction of approach. More cosmic rays arrive from the west than from the east. Cosmic rays, except a t very high altitudes, come in showers, each of which appears to originate at a point. If shielding is used, e.g., lead plates in a cloud track chamber, showers appear to originate within the shielding. This results in an increase in intensity (counts per minute) as shielding thickness is increased, until a maximum is reached, after which intensity falls off to a steady value. Tracks of cosmic ray showers in cloud chambers and photographic emulsion show the presence of nucleons with Z from 1 to 26, electrons, positrons, and mesons of several types, and imply the existence of photons, neutrons, and neutrinos. I n fact positrons, p- and rrSuggestions of material suitable for tbk column and guest columns suit,ahlefor ~ubliestiondirectlv are eaeerlv .. . solicited. Thev 4~o11111 he seut wirh w many detnilsns pwiihlc, and pwrtirdnrly with r r f e n w e s to mudern trxthwkr. 1 0 \\'. I!. Elrcrhxrrlt, Sehool uf ('hemwry, Gcor~inIn~titutsof Trrhnolug-y, . l t l m r ~G , twr~iu ~~~
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1 Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at l a s t two independent standard hooks
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mesons were all discovered first in cosmic ray tracks, and neutrons from cosmic rays are responsible for the formation of carbon-14 in the upper atmosphere (3). Cosmic rays have been classified as hard or soft, depending upon their penetrating power. The hard component contains high-energy nucleons and mesons, while the soft component includes electrons, photons, and low-energy nucleons and mesons. Cosmic ray energies range from 2 X lo8 to l0l8 ev (0.2 Bev-lOlo Bev), although most fall within the range from 2 X lo8 to 10" ev. There is only a small time variation in intensity of cosmic rays. This is most pronounced with the lowenergy components, and is related to solar flare activity. These observations are currently explained as follows. Primary cosmic rays are high-energy nucleons (about 66% protons, 25% helium nuclei) which interact with atoms and molecules in the earth's atmosphere to produce a variety of particles and radiations classed as secondary cosmic rays. Because of its very high energy a primary cosmic ray produces a large number of secondary rays, some of which produce third and fourth generation particles. This accounts for cosmic ray showers, and for the increase in intensity with thickness of shielding (the atmosphere is a special type of shielding) up to a maximum. For greater thicknesses of shielding the absorption of rays more than compensates for any casrade effects. Few primary cosmic rays penetrate to sea level. Because of their positive charge, primary cosmic rays may be deflected by the magnetic field of the earth. Particles approachiug the earth along magnetic lines of force are unaffected, while those whose paths cut across magnetic lines of force are subjected to forces according to the "right hand rule." Particles traveling toward the earth's magnetic poles, or in a north-south direction, are little affected; but particles approaching the equator in an east-west direction undergo maximum deflection, those approaching from the east being deflected down, those from the west, up. This results in a decrease in the radiation obsewed at sea level from the east compared to that from the west, and a decrease in the total obsewed intensity at the equator compared to that a t high latitudes. Both these effects are most pronounced with low-energy particles, as might be expected. There are several possible origins of the primary cosmic rays. Some low-energy particles apparently come from the sun; others presumably come from more distant stars. High-energy particles may come from violent cosmic events such as exploding stars, or may start as low-energy particles which are accelerated
by passage through magnetic fields in space. Cosmic rays are complex, and of perhaps limited chemical applications. Their detailed study is the concern of physicists, but they may logically be included in discussions of nuclear composition and reactions such as are found in many general chemistry courses. If they are included, a brief hut reasonably complete treatment such as that given here seems warranted.
Literature Cited 1. BOTEE, W.,AND KOLHORSTER, W., Z.Phys., 56, 751 (1929). 2. (a) HOOPER, J. E., AND SCHARFP, M., "The Cosmic Rl~disr tion," Methuen, London, 1958. ( b ) Rossr, B. B., "Cosmic Rays," McGraw Hill, New York, 1964.
3. L ~ B YW. , F., "Radiocarbon Dating," University of Chicago Press, Chicago, 1952.
Volume 43, Number 1 1 , November 1966
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