The Birth-Time of the World and Other Scientific Essays Part 22

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Before leaving our review of the general laws governing the pa.s.sage of alpha rays through matter, another point of interest must be referred to. We have hitherto spoken in general terms of the fact that ionisation attends the pa.s.sage of the ray. We have said nothing as to the nature of the ionisation so produced. But in point of fact the ionisation due to an alpha ray is sui generis. A glance at one of Wilson's photographs (Fig. 14.) ill.u.s.trates this. The white streak of water particles marks the path of the ray. The ions produced are evidently closely crowded along the track of the ray. They have been called into existence in a very minute instant of time. Now we know that ions of opposite sign if left to themselves recombine. The rate of recombination depends upon the product of the number of each sign present in unit volume. Here the numbers are very great and the volume very small. The ionic density is therefore high, and recombination very rapidly removes the ions after they are formed. We see here a peculiarity of the ionisation effected by alpha rays. It is linear in distribution and very local. Much of the ionisation in gases is again undone by recombination before diffusion leads to the separation of the ions. This "initial recombination" is greatest towards the end of the path of the ray where the ionisation is a maximum. Here it may be so effective that the form of the curve is completely lost unless a very large electromotive force is used to separate the ions when the ionisation is being investigated.

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We have now reviewed recent work at sufficient length to understand something of the nature of the most important advance ever made in our knowledge of the atom. Let us glance briefly at what we have learned. The radioactive atom in sinking to a lower atomic weight casts out with enormous velocity an atom of helium.

It thus loses a definite portion of its ma.s.s and of its energy.

Helium which is chemically one of the most inert of the elements, is, when possessed of such great kinetic energy, able to penetrate and ionise the atoms which it meets in its path. It spends its energy in the act of ionising them, coming to rest, when it moves in air, in a few centimetres. Its initial velocity depends upon the particular radioactive element which has given rise to it. The length of its path is therefore different according to the radioactive element from which it proceeds. The r.e.t.a.r.dation which it experiences in its path depends entirely upon the atomic weight of the atoms which it traverses. As it advances in its path its effectiveness in ionising the atom rapidly increases and attains a very marked maximum. In a gas the ions produced being much crowded together recombine rapidly; so rapidly that the actual ionisation may be quite concealed unless a sufficiently strong electric force is applied to separate them.

Such is a brief summary of the climax of radioactive discovery:--the birth, life and death of the alpha ray. Its advent into Science has altered fundamentally our conception of

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matter. It is fraught with momentous bearings upon Geological Science. How the work of the alpha ray is sometimes recorded visibly in the rocks and what we may learn from that record, I propose now to bring before you.

In certain minerals, notably the brown variety of mica known as biot.i.te, the microscope reveals minute circular marks occurring here and there, quite irregularly. The most usual appearance is that of a circular area darker in colour than the surrounding mineral. The radii of these little disc-shaped marks when well defined are found to be remarkably uniform, in some cases four hundredths of a millimetre and in others three hundredths, about.

These are the measurements in biot.i.te. In other minerals the measurements are not quite the same as in biot.i.te. Such minute objects are quite invisible to the naked eye. In some rocks they are very abundant, indeed they may be crowded together in such numbers as to darken the colour of the mineral containing them.

They have long been a mystery to petrologists.

Close examination shows that there is always a small speck of a foreign body at the centre of the circle, and it is often possible to identify the nature of this central substance, small though it be. Most generally it is found to be the mineral zircon. Now this mineral was shown by Strutt to contain radium in quant.i.ties much exceeding those found in ordinary rock substances.

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Some other mineral may occasionally form the nucleus, but we never find any which is not known to be specially likely to contain a radioactive substance. Another circ.u.mstance we notice.

The smaller this central nucleus the more perfect in form is the darkened circular area surrounding it. When the circle is very perfect and the central mineral clearly defined at its centre we find by measurement that the radius of the darkened area is generally 0.033 mm. It may sometimes be 0.040 mm. These are always the measurements in biot.i.te. In other minerals the radii are a little different.

We see in the photograph (Pl. XXIII, lower figure), much magnified, a halo contained in biot.i.te. We are looking at a region in a rock-section, the rock being ground down to such a thickness that light freely pa.s.ses through it. The biot.i.te is in the centre of the field. Quartz and felspar surround it. The rock is a granite. The biot.i.te is not all one crystal. Two crystals, mutually inclined, are cut across. The halo extends across both crystals, but owing to the fact that polarised light is used in taking the photograph it appears darker in one crystal than in the other. We see the zircon which composes the nucleus. The fine striated appearance of the biot.i.te is due to the cleavage of that mineral, which is cut across in the section.

The question arises whether the darkened area surrounding the zircon may not be due to the influence of the radioactive substances contained in the zircon. The

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extraordinary uniformity of the radial measurements of perfectly formed haloes (to use the name by which they have long been known) suggests that they may be the result of alpha radiation.

For in that case, as we have seen, we can at once account for the definite radius as simply representing the range of the ray in biot.i.te. The furthest-reaching ray will define the radius of the halo. In the case of the uranium family this will be radium C, and in the case of thorium it will be thorium C. Now here we possess a means of at once confirming or rejecting the view that the halo is a radioactive phenomenon and occasioned by alpha radiation; for we can calculate what the range of these rays will be in biot.i.te, availing ourselves of Bragg's additive law, already referred to. When we make this calculation we find that radium C just penetrates 0.033 mm. and thorium C 0.040 mm. The proof is complete that we are dealing with the effects of alpha rays. Observe now that not only is the coincidence of measurement and calculation a proof of the view that alpha radiation has occasioned the halo, but it is a very complete verification of the important fact stated by Bragg, that the stopping power depends solely on the atomic weight of the atoms traversed by the ray.

We have seen that our examination of the rocks reveals only the two sorts of halo: the radium halo and the thorium halo. This is not without teaching. For why not find an actinium halo? Now Rutherford long ago suggested that this element and its derivatives were

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probably an offspring of the uranium family; a side branch, as it were, in the formation of which relatively few transforming atoms took part. On Rutherford's theory then, actinium should always accompany uranium and radium, but in very subordinate amount. The absence of actinium haloes clearly supports this view. For if actinium was an independent element we would be sure to find actinium haloes. The difference in radius should be noticeable.

If, on the other hand, actinium

was always a.s.sociated with uranium and radium, then its effects would be submerged in those of the much more potent effects of the uranium series of elements.

It will have occurred to you already that if the radioactive origin of the halo is a.s.sured the shape of a halo is not really circular, but spherical. This is so. There is no such thing as a disc-shaped halo. The halo is a spherical volume containing the radioactive nucleus at its centre. The true radius of the halo may, therefore, only be measured on sections pa.s.sing through the nucleus.

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In order to understand the mode of formation of a halo we may profitably study on a diagram the events which go on within the halo-sphere. Such a diagram is seen in Fig. 15. It shows to relatively correct scale the limiting range of all the alpha-ray producing members of the uranium and thorium families. We know that each member of a family will exist in equilibrium amount within the nucleus possessing the parent element. Each alpha ray leaving the nucleus will just attain its range and then cease to affect the mica. Within the halosphere, there must be, therefore, the acc.u.mulated effects of the influences of all the rays. Each has its own sphere of influence, and the spheres are all concentric.

The radii in biot.i.te of the several spheres are given in the following table

URANIUM FAMILY.

Radium C - 0.0330 mm.

Radium A - 0.0224 mm.

Ra Emanation - 0.0196 mm.

Radium F - 0.0177 mm.

Radium - 0.0156 mm.

Ionium - 0.0141 mm.

Uranium 1 - 0.0137 mm.

Uranium 2 - 0.0118 mm.

THORIUM FAMILY.

Thorium CE - 0.040 mm.

Thorium A - 0.026 mm.

Th Emanation - 0.023 mm.

Thorium Ci - 0.022 mm.

Thorium X - 0.020 mm.

Radiothorium - 0.119 mm.

Thorium - 0.013 mm.

In the photograph (Pl. XXIV, lower figure), we see a uranium and a thorium halo in the same crystal of mica. The mica is contained in a rock-section and is cut across the cleavage. The effects of thorium Ca are clearly shown

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as a lighter border surrounding the acc.u.mulated inner darkening due to the other thorium rays. The uranium halo (to the right) similarly shows the effects of radium C, but less distinctly.

Haloes which are uniformly dark all over as described above are, in point of fact, "over-exposed"; to borrow a familiar photographic term. Haloes are found which show much beautiful internal detail. Too vigorous action obscures this detail just as detail is lost in an over-exposed photograph. We may again have "under-exposed" haloes in which the action of the several rays is incomplete or in which the action of certain of the rays has left little if any trace. Beginning at the most under-exposed haloes we find circular dark marks having the radius 0.012 or 0.013 mm.

These haloes are due to uranium, although their inner darkening is doubtless aided by the pa.s.sage of rays which were too few to extend the darkening beyond the vigorous effects of the two uranium rays. Then we find haloes carried out to the radii 0.016, 0.018 and 0.019 mm. The last sometimes show very beautiful outer rings having radial dimensions such as would be produced by radium A and radium C. Finally we may have haloes in which interior detail is lost so far out as the radius due to emanation or radium A, while outside this floats the ring due to radium C.

Certain variations of these effects may occur, marking, apparently, different stages of exposure. Plates XXIII and XXIV (upper figure) ill.u.s.trate some of these stages;

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the latter photograph being greatly enlarged to show clearly the halo-sphere of radium A.

In most of the cases mentioned above the structure evidently shows the existence of concentric spherical sh.e.l.ls of darkened biot.i.te. This is a very interesting fact. For it proves that in the mineral the alpha ray gives rise to the same increased ionisation towards the end of its range, as Bragg determined in the case of gases. And we must conclude that the halo in every case grows in this manner. A spherical sh.e.l.l of darkened biot.i.te is first produced and the inner colouration is only effected as the more feeble ionisation along the track of the ray in course of ages gives rise to sufficient alteration of the mineral. This more feeble ionisation is, near the nucleus, enhanced in its effects by the fact that there all the rays combine to increase the ionisation and, moreover, the several tracks are there crowded by the convergency to the centre. Hence the most elementary haloes seldom show definite rings due to uranium, etc., but appear as embryonic disc-like markings. The photographs ill.u.s.trate many of the phases of halo development.

Rutherford succeeded in making a halo artificially by compressing into a capillary gla.s.s tube a quant.i.ty of the emanation of radium. As the emanation decayed the various derived products came into existence and all the several alpha rays penetrated the gla.s.s, darkening the walls of the capillary out to the limit of the range of radium C in gla.s.s. Plate XXV shows a magnified section of the

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tube. The dark central part is the capillary. The tubular halo surrounds it. This experiment has, however, been antic.i.p.ated by some scores of millions of years, for here is the same effect in a biot.i.te crystal (Pl. XXV). Along what are apparently tubular pa.s.sages or cracks in the mica, a solution, rich in radioactive substances, has moved; probably during the final consolidation of the granite in which the mica occurs. A continuous and very regular halo has developed along these conduits. A string of halo-spheres may lie along such pa.s.sages. We must infer that solutions or gases able to establish the radioactive nuclei moved along these conduits, and we are ent.i.tled to ask if all the haloes in this biot.i.te are not, in this sense, of secondary origin. There is, I may add, much to support such a conclusion.

The widespread distribution of radioactive substances is most readily appreciated by examination of sections of rocks cut thin enough for microscopic investigation. It is, indeed, difficult to find, in the older rocks of granitic type, mica which does not show haloes, or traces of haloes. Often we find that every one of the inclusions in the mica--that is, every one of the earlier formed substances--contain radioactive elements, as indicated by the presence of darkened borders. As will be seen presently the quant.i.ties involved are generally vanis.h.i.+ngly small. For example it was found by direct determination that in one gram of the halo-rich mica of Co. Carlow there was rather less than twelve billionths of a gram of radium, We are

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ent.i.tled to infer that other rare elements are similarly widely distributed but remain undetectable because of their more stable properties.

It must not be thought that the under-exposed halo is a recent creation. By no means. All are old, appallingly old; and in the same rock all are, probably, of the same, or neatly the same, age. The under-exposure is simply due to a lesser quant.i.ty of the radioactive elements in the nucleus. They are under-exposed, in short, not because of lesser duration of exposure, but because of insufficient action; as when in taking a photograph the stop is not open enough for the time of the exposure.