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Atomic Projectiles and their Collisions with

Light Atoms: SIR ERNEST RUTHERFORD.... 467 Second Award of the Elliot Medal ......... 473 Proposed Constitution and By-laws of the

American Association for the Advancement of Science .......

............. 474 Scientific Events:

The Southwestern Geological Society; The American Physical Society; The History of Science and the American Historical So ciety; The Section of Zoology of the American Association; The Deflection of Light by Gravitation and the Theory of Relativity... 477

Scientific Notes and News ................ 479

University and Educational News .......... 485

Discussion and Correspondence:

A Helium Series in the Extreme Ultra-violet:
of the Term Acceleration: PROFESSOR C.
M. SPARROW .....

..... 481


ATOMS The discovery of radio-activity has not only thrown a flood of light on the processes of transformation of radio-active atoms; it has at the same time provided us with the most powerful natural agencies for probing the inner structure of the atoms of all the elements. The swift a-particles and the high-speed electrons or B-rays ejected from radio-active bodies are by far the most concentrated sources of energy known to science. The enormous energy of the flying a-particle or helium atom is illustrated by the bright flash of light it produces when it impacts on a crystal of zinc sulphide, and by the dense distribution of ions along its trail through a gas. This great store of energy is due to the rapidity of its motion, which in the case of the a-particle from radium C (range 7 cm. in air) amounts to 19,000 km. per second, or about 20,000 times the speed of a rifle-bullet. It is easily calculated that the energy of motion of an ounce of helium moving with the speed of the a-particle from radium C is equivalent to 10,000 tons of solid shot projected with a velocity of 1 km. per second. · In consequence of its great energy of motion the charged particle is able to penetrate deeply into the structure of all atoms before it is deflected or turned back, and from a study of the deflection of the path of the a-particle we are able to obtain important evidence on the strength and distribution of the electric fields near the center or nucleus of the atom.

Since it is believed that the atom of matter is, in general, complex, consisting of positively and negatively charged parts, it is to be anticipated that a narrow pencil of 2-particles, after passing through a thin plate of matter, should | 1 An address before the Royal Institution of Great Britain, June 6, 1919.

Notes on Meteorology and Climatology:

Aerological Work-Winds; Airplanes and the Weather: DR. CHARLES F. BROOKS .... 483

Special Articles :

A Preliminary Note on Foot-rot of Cereals

in the Northwest: B. F. DANA .......... 484 The New Haven Meeting of the National

Academy of Sciences ................... 486 The American Mathematical Society: PRO

FESSOR F. N. COLE ........ ............. 487 Meeting of the Committee on Policy of the

American Association for the Advancement of Science : DR. L. 0. HOWARD .......... 487

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be scattered into a comparatively broad beam. Geiger and Marsden showed not only that much small scattering occurred, but also that in passing through the atoms of a heavy element some of the a-particles were actually turned back in their path. Considering the great energy of motion of the a-particle, this is an arresting fact, showing that the a-particle must encounter very intense forces in penetrating the structure of the atom. In order to explain such results, the idea of the nucleus atom was developed in which the main mass of the atom is concentrated in a positively charged nucleus of very small dimensions compared with the space occupied by the electrons which surround it. The scattering of a-particles through large angles was shown to be the result of a single collision where the a-particle passed close to this charged nucleus. From a study of the distribution of the particles scattered at different angles, results of first importance emerged. It was found that the results could be explained only if the electric forces between the a-particle and charged nucleus followed the law of inverse squares for distances apart of the order of 10-11 cm. Darwin pointed out that the varia tion of scattering with velocity was explicable only on the same law. This is an important step, for it affords an experimental proof that, at any rate to a first approximation, the ordinary law of force holds for electrified bodies at such exceedingly minute distances. It was also found that a resultant charge on the nucleus measured in fundamental units was about equal to the atomic number of the ele ment. In the case of gold this number is believed from the work of Moseley to be 79.

Knowing the mass of the impinging a-particle and of the atom with which it collides, we can determine from direct mechanical principles the distribution of velocities after the collision, assuming that there is no loss of energy due to radiation or other causes. It is important to notice that in such a calculation we need make no assumption as to the nature of the atoms or of the forces involved in the approach and separation of the atoms. For example, if an a-particle collides with

another helium atom, we should expect the
a-particle to give its energy to the helium atom,
which could thus travel on with the speed
of the a-particle. If an a-particle collides
directly with a heavy atom-e. 9., of gold of
atomic weight 197—the a-particle should re-
trace its path with only slightly diminished
velocity, while the gold atom moves onward in
the original direction of the a-particle, but
with about one fiftieth of its velocity. Next,
consider the important case where the a-par-
ticle of mass 4 makes a direct collision with a
hydrogen atom of mass 1. From the laws of
impact, the hydrogen atom is shot forward
with a velocity 1.6 times that of the direc-
tion, but with only 0.6 of its initial speed.
Marsden showed that swift hydrogen atoms set
in motion by impact with a-particles can be de-
tected like a-particles by the scintillations
produced in a zinc sulphide crystal. Recently
I have been able to measure the speed of such
H atoms and found it to be in good accord
with the calculated value, so that we may con-
clude that the ordinary laws of impact may be
applied with confidence in such cases. The
relative velocities of the a-particles and recoil
atom after collision can thus be simply illus-
trated by impact of two perfectly elastic balls
of masses proportional to the masses of the

While the velocities of the recoil atoms can be easily calculated, the distance which they travel before being brought to rest depends on both the mass and the charge carried by the recoil atom. Experiment shows that the range. of H atoms, like the range of a particles, varies nearly as the cube of their initial velocity. If the H atom carries a single charge, Darwin showed that its range should be about four times the range of the a-particle. This has been confirmed by experiment. Generally, it can be shown that the range of a charged atom carrying a single charge is mu’R, where m is the atomic weight, and u the ratio of the velocity of the recoil atom to that of the a-particle, and R the range of the a-particle before collision. In comparison of theory with experiment, the results agree better if the index is taken as 2.9 instead of 3. If, however, the

recoil atom carries a double charge after a the number of a-particles scattered through collision, it is to be expected that its range 5° was observed to be about 200,000 times would only be about one quarter of the corre- greater than the number through 150°. The sponding range if it carried a single charge. variation with angle was in close accord with It follows that we can not expect to detect the the theory, showing that the law of inverse presence of any recoil atom carrying two squares holds for distances between 3.6 X 10-12 charges beyond the range of the a-particle, cm. and 4.3 X 10-11 cm. in the case of the but we can calculate that any recoil atom, of gold atom. The experiments of Crowther in mass not greater than oxygen and carrying a 1910 on the variation of scattering of B-rays single charge, should be detected beyond the with velocity indicate that a similar law range of the a-particle. For example, for a holds also in that case, and for even greater single charge the recoil atoms of hydrogen distances from the nucleus. and helium should travel 4 R, lithium 2.8 R, We have seen that Marsden was able by carbon 1.6 R, nitrogen 1.3 R, and oxygen 1.1 the scintillation method to detect hydrogen R, where R is the range of the incident a-par- atoms set in ewift motion by a-particles up ticles. We thus see that it should be possible to distances about four times the range of the to detect the presence of such singly charged incident a-particle. In Marsden's experiatoms, if they exist, after completely stopping ments a thin-walled glass tube filled with the a-particles by a suitable thickness of ab- radium emanation served as an intense source sorbing material. This is a great advantage of rays. Since the lack of homogeneity of for the number of such swift recoil atoms is the a-radiation and the absorption in the minute in comparison with the number of a glass are great drawbacks in making an acparticles, and we could not hope to detect them curate study of the laws controlling the proin the presence of the much more numerous duction of swift atoms by impact, I have a-particles.

found it best to use for the purpose a homoIn order to calculate the number of recoil geneous source of radium by exposing a atoms scattered through any given angle from disc in a strong source of emanation. Fifteen the direction of flight of the a-particles, it is minutes after removal from the emanation the necessary, in addition, to make assumptions as a-rays from the disc are practically homoto the constitution of the atoms and as to the geneous, with a range in air of 7 cm. By nature and magnitude of the forces involved special arrangements very intense sources of in the collision. Consider, for example, the a-radiation can be produced in this way, and case of a collision of an a-particle with an in the various experiments discs have been atom of gold of nuclear charge 79. Assuming used the x-ray activity of which has varied that the nucleus of the a-particle and that of between 5 to 80 milligrams of radium. Allowthe gold atom behave like point charges, re- ance can easily be made for the decay of the pelling according to the inverse square law, it radiation with time. can readily be calculated that, for direct col. In the experiments with hydrogen the lision, the a-particles from radium C, which is source was placed in a metal box about 3 cm. turned through an angle of 180°, approaches away from an opening in the end covered by within a distance D=3.6 X 10-12 cm, of the a thin sheet of metal of sufficient thickness center of the gold nucleus. This is the closest to absorb the a-rays completely. A zinc sulpossible distance of approach of the a-particle, phide screen was mounted outside about 1 and the distance increases for oblique collis- mm. away from the opening, so as to allow for ions. For example, when the a-particle is the insertion of absorbing screens of alumiscattered through an angle of 150°, 90°, 30°, nium or mica. The apparatus was filled with 10°, 5o, the closest distances of approach are dry hydrogen at atmospheric pressure. The 1.01, 1.2, 2.4, 6.2, 12 D respectively.

H atoms striking the zinc sulphide screen In the experiments of Geiger and Marsden, were counted by means of a microscope in the usual way. The strong luminosity due to the and magnitude of the forces involved in the B-rays from radium was largely reduced collision when the nuclei approach closer than by placing the apparatus in a powerful mag- a certain distance. netic field which bent them away from the In addition to these peculiarities, the numscreen.

ber of H atoms is greatly in excess of the If we suppose, for the distances involved number to be expected on the simple theory. in a collision, that the a-particle and hydrogen For example, for the swiftest a-rays the numnucleus may be regarded as point charges, it is ber which is able to travel a distance equivo easy to see that oblique impacts should occur alent to 10 cm. of air is more than thirty much oftener than head-on collisions, and times greater than the calculated value. The consequently that the stream of H atoms set variation in number of H atoms with velocity in motion by collisions should contain atoms of the incident a-particle is also entirely the velocities of which vary from zero to the different from that to be expected on the maximum produced in a direct collision. The theory of point charges. The number diminslow-velocity atoms should greatly preponder- ishes rapidly with velocity, and is very small ate, and the number of scintillations observed for a-particles of range 2.5 cm. should fall off rapidly when absorbing screens It must be borne in mind that the proare placed in the path of the rays close to the duction of a high-speed H atom by an zinc sulphide screen.

a-particle is an exceedingly rare occurrence. A surprising effect was, however, observed. Under the conditions of the experiment the Using a-rays of range 7 cm., the number of H number of H atoms is seldom more than atoms remained unchanged when the absorp- 1/30,000 of the number of a-particles Probtion in their path was increased from 9 cm. ably each a-particle passes through the structo 19 cm. of air equivalent. After 19 cm. the ture of 10,000 hydrogen molecules in traversnumber fell off steadily, and no scintillations ing one centimeter of hydrogen at atmoscould be observed beyond 28 cm. air absorp- pheric pressure, and only one a-particle in tion. In fact, the stream of H atoms re 100,000 of these produces a high-speed H sembled closely a homogeneous beam of a-rays atom; so that in 109 collisions with the moleof range 28 cm., for it is well known that cules of hydrogen the a-particle, on the averowing to scattering, the number of a-particles age, approaches only once close enough to the from a homogeneous source begin to fall off center of the nucleus to give rise to a swift some distance from the end of their range. hydrogen atom. The results showed that the H atoms are pro- We should anticipate that for such collisjected forward mainly in the direction of the ions the a-particle is unable to distinguish a-particles and over a narrow range of veloc- between the hydrogen atom and the hydrogen ity, and that few, if any, lower velocity atoms molecule, and that H atoms should be liberare present in the stream.

ated from matter containing free or comIf we reduce the velocity of the a-particle bined hydrogen. This is fully borne out by by placing a metal screen over the source, experiment. it is found that the distribution of H atoms From the number of H atoms observed it with velocity changes, and that the rays are can be easily calculated that the a-particle no longer nearly homogeneous. When the must be fired within a perpendicular distance range of the a-rays is reduced to 3.5 cm., of 2.4 x 10-18 cm. of the center of the H the absorption of the H atoms is in close nucleus in order to set it in swift motion. accord with the value to be expected from the This is a distance less than the diameter of theory of point charges. It is clear, there the electron, viz. 3.6 X 10-13 cm. The genfore, that the distribution of velocity among eral results obtained with a-rays of range 7 the H atoms varies with the speed of the cm. are similar to those to be expected if the incident a-particles, and this indicates that a a-particle behaves like a charged disc, of marked change takes place in the distribution radius of about the diameter of an electron,

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travelling with its plane perpendicular to the direction of motion.

It is clear from the experiments with hydrogen that, for distances of the order of the diameter of the electron, the a-particle no longer behaves like a point charge, but that the a-particles must have dimensions of the order of that of the electron. The closest distance of approach in these collisions in hydrogen is about one tenth the corresponding distances in the case of a collision of an a-particle with an atom of gold.

The results obtained with hydrogen in no way invalidate the nucleus theory as used to explain the scattering of a-rays by heavy atoms, but show, as we should expect, that the theory breaks down when we approach very close to the nucleus structure. In our ignorance of the constitution of the nucleus of the a-particle, we can only speculate as to its structure and the distribution of forces very close to it. If we take the a-particles of mass 4 to consist of four positively charged H nuclei and two negative electrons, we should expect it to have dimensions of the order of the diameter of the electron, supposing, as seems probable, that the H nucleus is of much smaller dimensions than the electron itself. When we consider the enormous magnitude of the forces between the a-particle and the H nucleus in a close collisionamounting to 6 kg. of weight-it is to be expected that the structure of the a-particle should be much deformed, and that the law of force may undergo very marked changes in direction and magnitude for small changes in the closeness of approach of the two colliding nuclei. Such considerations offer a reasonable explanation of the anomalies shown in the number and distribution with velocity of the H atoms exhibited for different velocities of the a-particles.

When we consider the enormous forces between the nuclei, it is not so much a matter of surprise that the nuclei should be deformed as that the structure of the a-particle or helium nucleus escapes disruption into its constituent parts. Such an effect has been carefully looked for, but so far no definite evidence of such a disintegration has been

observed. If this be the case, the helium nucleus must be a very stable structure to stand the strain of the gigantic forces involved in a close collision.

W e have seen that the recoil atoms of all elements of atomic mass less than 18 should travel beyond the range of the a-particle, provided they carry a single charge. Preliminary experiments, in which the a-particles passed through pure helium, showed that no longrange recoil atoms were present, indicating that after recoil the helium atom carries a double charge. In a similar way no certain evidence has been obtained of long-range recoil atoms from lithium, boron, or beryllium. It is difficult in experiments with solids or solid compounds to be sure of the absence of hydrogen or water-vapor, which results in the production of numerous swift H atoms. These difficulties are not present in the case of nitrogen and oxygen, and a special examination has been made of recoil atoms in these gases. Bright scintillations were observed in both these gases about 2 cm. beyond the range of the a-particle. These scintillations are, presumably, due to swift N and O atoms carrying a single charge, for the ranges observed are about those to be expected for such atoms. The scintillations due to recoil atoms of N and O are much brighter than H scintillations, although the actual energy of the flying atom is greater in the later case. This difference in brightness is probably connected with the much weaker ionization per unit of path due to the swifter H atom.

The corresponding range of the recoil atoms was about the same in oxygen, nitrogen and carbon dioxide. Theoretically, it is to be anticipated that the N recoil atom should give a somewhat greater range than the O atom. The recoil atoms observed in carbon dioxide are apparently due to oxygen, for if the carbon atoms carried a single charge they should be detected beyond the range of O atoms.

The number of recoil atoms in nitrogen and oxygen and their absorption indicate that these atoms, like H atoms, are shot forward mainly in the direction of the a-particles. It

atoms. it in and other much brighter

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