Elements and Atoms: Chapter 19
What is the Alpha Particle? Rutherford

No one contributed more than Ernest Rutherford (1871-1937; see photo in Chemical Achievers at Chemical Heritage Foundation) to an understanding of radioactivity and its domain, the atomic nucleus, in the early years of the 20th century. Working in J. J. Thomson's laboratory, Rutherford distinguished between two different forms of radioactivity, alpha (α) and beta (β) [Rutherford 1899]. He worked out the time-dependence of radioactive decay and introduced the term half-life [Rutherford 1900]. With Frederick Soddy, he realized that radioactive decay actually tranforms an atom of one element into an atom of a different element [Rutherford & Soddy 1902].[1]

This work was sufficient to earn Rutherford a Nobel Prize in 1908.[2] But some of his most important work was still ahead. Large-angle scattering of α particles was first reported in his laboratory [Geiger & Marsden 1909]. His correct interpretation of that scattering led to the realization that most of the mass of an atom is concentrated in a tiny core or nucleus [Rutherford 1911]; thus it is to Rutherford that we owe the nuclear atom and nuclear physics. The experiments which found a physically measurable quantity associated with atomic number were also carried out in his laboratory [Moseley 1913, 1914]. During the World War, Rutherford discovered that some atoms could be induced to fall apart in a process of artificial transmutation [Rutherford 1919; view photo of apparatus at Science Museum, London].

Rutherford characterized the α particle in work extending over several years with a variety of co-workers. The selection reproduced below represents the final step in the identification of the α particle as a positively-charged helium atom. (I would say "helium nucleus," but Rutherford had not yet discovered the nucleus.) In importance to the development of understanding about the atom, this paper does not rank with those cited above. It does, however, illustrate the simple and careful experimental methodology of Rutherford.


The Nature of the α Particle from Radioactive Substances[3]

Ernest Rutherford & Thomas Royds, Philosophical Magazine 17, 281-286 (1909)

The experimental evidence collected during the last few years has strongly supported the view that the α particle[4] is a charged helium atom, but it has been found exceedingly difficult to give a decisive proof of the relation. In recent papers, Rutherford and Geiger[5] have supplied still further evidence of the correctness of this point of view. The number of α particles from one gram of radium have been counted, and the charge carried by each determined. The values of several radioactive quantities, calculated on the assumption that the α particle is a helium atom carrying two unit charges, have been shown to be in good agreement with the experimental numbers. In particular, the good agreement between the calculated rate of production of helium by radium and the rate experimentally determined by Sir James Dewar[6], is strong evidence in favour of the identity of the α particle with the helium atom.[7]

The methods of attack on this problem have been largely indirect, involving considerations of the charge carried by the helium atom and the value of e/m of the α particle. The proof of the identity of the α particle with the helium atom is incomplete until it can be shown that the α particles, accumulated quite independently of the matter from which they are expelled, consist of helium. For example, it might be argued that the appearance of helium in the radium emanation was a result of the expulsion of the α particle, in the same way that the appearance of radium A is a consequence of the expulsion of an α particle from the emanation. If one atom of helium appeared for each α particle expelled, calculation and experiment might still agree, and yet the α particle itself might be an atom of hydrogen or of some other substance.[8]

We have recently made experiments to test whether helium appears in a vessel into which the α particles have been fired, the active matter itself being enclosed in a vessel sufficiently thin to allow the α particles to escape, but impervious to the passage of helium or other radioactive products.[9]

The experimental arrangement is clearly seen in the figure. The equilibrium quantity of emanation from about 140 milligrams of radium was purified and compressed by means of a mercury-column into a fine glass tube A about 1.5 cms. long. This fine tube, which was sealed on a larger capillary tube B, was sufficiently thin to allow the α particles from the emanation and its products to escape, but sufficiently strong to withstand atmospheric pressure. After some trials, Mr. Baumbach succeeded in blowing such fine tubes very uniform in thickness. The thickness of the wall of the tube employed in most of the experiments was less than 1/100 mm., and was equivalent in stopping power of the α particle to about 2 cms. of air. Since the ranges of the α particles from the emanation and its products radium A and radium C[10] are 4.3, 4.8, and 7 cms. respectively, it is seen that the great majority[11] of the α particles expelled by the active matter escape through the walls of the tube. The ranges of the α particles after passing through the glass were determined with the aid of a zinc-sulphide screen.[12] Immediately after the introduction of the emanation the phosphorescence showed brilliantly when the screen was close to the tube, but practically disappeared at a distance of 5 cms. Such a result is to be expected. The phosphorescence initially observed was due mainly to the α particles of the emanation and its product radium A (period 3 mins.). In the course of time the amount of radium C, initially zero, gradually increased, and the α radiations from it of range 7 cms. were able to cause phosphorescence at a greater distance.

The glass tube A was surrounded by a cylindrical glass tube T, 7.5 cms. long and 1.5 cms. diameter, by means of a ground-glass joint C. A small vacuum-tube V was attached to the upper end of T. The outer glass tube T was exhausted by a pump through the stopcock D, and the exhaustion completed with the aid of the charcoal tube F cooled by liquid air. By means of a mercury column H attached to a reservoir, mercury was forced into the tube T until it reached the bottom of the tube A.[13]

Part of the α particles which escaped through the walls of the fine tube were stopped by the outer glass tube and part by the mercury surface. If the α particle is a helium atom, helium should gradually diffuse from the glass and mercury into the exhausted space, and its presence could then be detected spectroscopically by raising the mercury and compressing the gases into the vacuum-tube.

In order to avoid any possible contamination of the apparatus with helium, freshly distilled mercury and entirely new glass apparatus were used. Before introducing the emanation into A, the absence of helium was confirmed experimentally. At intervals after the introduction of the emanation the mercury was raised, and the gases in the outer tube spectroscopically examined. After 24 hours no trace of the helium yellow line was seen; after 2 days the helium yellow was faintly visible; after 4 days the helium yellow and green lines were bright; and after 6 days all the stronger lines of the helium spectrum were observed. The absence of the neon spectrum shows that the helium present was not due to a leakage of air into the apparatus.[14]

There is, however, one possible source of error in this experiment. The helium may not be due to the α particles themselves, but may have diffused from the emanation through the thin walls of the glass tube. In order to test this point the emanation was completely pumped out of A, and after some hours a quantity of helium, about 10 times the previous volume of the emanation, was compressed into the same tube A.

The outer tube T and the vacuum-tube were removed and a fresh apparatus substituted. Observations to detect helium in the tube T were made at intervals, in the same way as before, but no trace of the helium spectrum was observed over a period of eight days.[15]

The helium in the tube A was then pumped out and a fresh supply of emanation substituted. Results similar to the first experiment were observed. The helium yellow and green lines showed brightly after four days.[16]

These experiments thus show conclusively that the helium could not have diffused through the glass walls, but must have been derived from the α particles which were fired through them. In other words, the experiments give a decisive proof that the α particle after losing its charge is an atom of helium.

Other Experiments.

We have seen that in the experiments above described helium was not observed in the outer tube in sufficient quantity to show the characteristic yellow line until two days had elapsed. Now the equilibrium amount of emanation from 100 milligrams of radium should produce helium at the rate of about .03 c.mm. per day. The amount produced in one day, if present in the outer tube, should produce a bright spectrum of helium under the experimental conditions. It thus appeared probable that the helium fired into the glass must escape very slowly into the exhausted space, for if the helium escaped at once, the presence of helium should have detected a few hours after the introduction of the emanation.[17]

In order to examine this point more closely the experiments were repeated, with the addition that a cylinder of thin sheet lead of sufficient thickness to stop the α particles was placed over the fine emanation tube. Preliminary experiments, in the manner described later, showed that the lead-foil did not initially contain a detectable amount of helium. Twenty-four hours after the introduction into the tube A of about the same amount of emanation as before, the yellow and green lines of helium in this case after one day was of about the same intensity as that after the fourth day in the experiments without the lead screen. It was thus clear that the lead-foil gave up the helium fired into it far more readily than the glass.

In order to form an idea of the rapidity of escape of the helium from the lead some further experiments were made.[18] The outer cylinder T was removed and a small cylinder of lead-foil placed round the thin emanation-tube surrounded the air at atmospheric pressure. After exposure for a definite time to the emanation, the lead screen was removed and tested for helium as follows. The lead-foil was placed in a glass tube between two stopcocks. In order to avoid a possible release of the helium present in the lead by pumping out the air, the air was displaced by a current of pure electrolytic oxygen[19]. The stopcocks were closed and the tube attached to a subsidiary apparatus similar to that employed for testing for the presence of neon and helium in the gases produced by the action of the radium emanation on water (Phil. Mag. Nov. 1908). The oxygen was absorbed by charcoal and the tube then heated beyond the melting-point of lead to allow the helium to escape. The presence of helium was then spectroscopically looked for in the usual way. Using this method, it was found possible to detect the presence of helium in the lead which had been exposed for only four hours to the α rays from the emanation. After an exposure of 24 hours the helium yellow and green lines came out brightly. These experiments were repeated several times with similar results.

A number of blank experiments were made, using samples of the lead-foil which had not been exposed to the α rays, but in no case was any helium detected. In a similar way, the presence of helium was detected in a cylinder of tinfoil exposed for a few hours over the emanation-tube.[20]

These experiments show that the helium does not escape at once from the lead, but there is on the average a period of retardation of several hours and possible longer.

The detection of helium in the lead and tin foil, as well as in the glass, removes a possible objection that the helium might have been in some way present in the glass initially, and was liberated as a consequence of its bombardment by the α particles.

The use of such thin glass tubes containing emanation affords a simple and convenient method of examining the effect on substances of an intense α radiation quite independently of the radioactive material contained in the tube.

We can conclude with certainty from these experiments that the α particle after losing its charge is a helium atom. Other evidence indicates that the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water.[21]

University of Manchester,
Nov. 13, 1908

Notes

[1]The idea of one element changing into another was foreign to the chemistry and physics of the time; it was associated with fantasies of a pre-scientific era. Rutherford is supposed to have urged his colleague, "Don't call it transmutation. They'll have our heads off as alchemists." [Pais 1986]

[2]It is ironic that a man who quipped, "All science is either physics or stamp collecting," [Blackett 1963] won a Nobel Prize in chemistry. Because his work touched on the nature of the elements, chemistry was the field in which it was recognized with a Nobel.

[3]Communicated by the Authors. [original note --CJG]

[4]Fairly early in the study of radioactivity, three different types of radioactive phenomena were identified. Rutherford first distinguished between two kinds which he unimaginatively labeled α and β [Rutherford 1899]. (Following that precedent, the third kind of radioactivity was called γ.) Rutherford found that one kind of radiation (β) was more penetrating than the other (α), and thus distinguished them on the basis of their range. But what were these rays? Were they particles like cathode rays? Were they energy like light or X-rays? β rays were identified as electrons by the early 20th century. γ rays are a high-energy form of electromagnetic radiation like visible light or X-rays but with a much shorter wavelength.

[5]Proc. Roy. Soc. A. lxxxi, pp. 141-173 (1908). [original note --CJG]

[6]Proc. Roy. Soc. A. lxxxi. p. 280 (1908). [original note --CJG]

[7]How might one go about characterizing the α particle? The first step was to distinguish it from other radioactive phenomena, which Rutherford first did by noting that it was not as penetrating as β particles. In fact, Rutherford went on to measure how far α particles from various radioactive materials could travel through air. (See note 12 below.) Then measuring the charge to mass ratio (e/m) and, if possible, the charge separately, would be helpful. After all, the e/m ratio proved to be crucial to characterizing cathode rays (chapter 16), and that line of inquiry developed several techniques for measuring e/m and the charge. With this information in hand, Rutherford could tell that the particle in question was not something as light as an electron or as heavy as an atom of uranium, but was more like a helium atom. Was it a helium atom? The next step would be to try to collect α particles and see if the collected material is in fact helium. Indeed, William Ramsay and Soddy observed production of helium in the presence of an α-emitting substance, namely radium, as early as 1903 [Ramsay & Soddy 1903].

[8]What Rutherford calls radium emanation is now known to be a particular isotope of radon gas, namely 222Rn. And radium A is actually an isotope of polonium, 218Po. (See next chapter for more on isotopes.)

Rutherford notes that just because helium is produced at the same rate as α particles does not mean that α particles are helium. He gives two examples of radioactive decay processes which produce α particles:

radium --> "radium emanation" + α and "radium emanation" --> radium A + α.
In the first case, radium emanation is produced at the same rate as the α particle; it is not an α particle, however, but another fragment (by far the larger, in fact) of the decaying radium atom.

[9]Wait a minute! How can the vessel be thin enough to let α particles through but not helium atoms if α particles are charged helium atoms? This is not as contradictory an apparatus as it may appear. The α particles, whatever they are, are ejected from the decaying atom with great speed, enabling them to penetrate thin walls that are impervious to ordinary gases including helium. The idea here is to make a barrier that will let through only α particles, and to examine those accumulated α particles once they have slowed down. And we will see that Rutherford & Royds make sure that α particles can get through (note 12) and that ordinary helium cannot (note 15).

[10]Radium C is actually an isotope of polonium, 214Po.

[11]The α particles fired at a very oblique angle to the tube would be stopped in the glass. The fraction stopped in this way would be small under the experimental conditions. [original note --CJG]

[12]How might one measure the range of α particles? All one needed was a way to detect the particles. Then simply vary the distance between the source of the particles (i.e., the radioactive substance) and the detector. There were already several ways of detecting particles emitted by radioactive materials. We have already seen the use of photographic plates (by Becquerel, chapter 17) and a device sensitive to electric charge (by Curie, chapter 18). Rutherford used a technique based on the fact that α particles could make certain phosphorescent materials, such as zinc sulfide, visibly sparkle. One could observe and manually count the number of sparkles (or scintillations) one saw (in a dark room, of course). About this time, Hans Geiger and Rutherford invented an electrical device to detect and amplify the charge α particles carried [Rutherford & Geiger 1908a].

At any rate, scintillations on the zinc sulfide screen outside the thin tube show that α particles can get out of that tube.

[13]The apparatus surrounds the thin tube containing the emanation with empty space, pumped free of any impurity. This was the space in which the α particles were to be collected.

[14]Most important, the space surrounding the thin tube was to be free of any extraneous helium before the start of the experiment. So Rutherford & Royds look for helium in that space by looking for the lines characteristic of the spectrum of helium. The spectrum is a particularly sensitive method of identifying helium; that is, helium can be detected in very small quantities. At first they see no helium, but after the experiment has been running for a few days, they see some. Conclusion: helium enters the collection space during the course of the experiment. While they see helium, they do not see neon. Conclusion: the helium they see did not leak in from the atmosphere, for if it did, neon would leak in with it.

[15]How do Rutherford & Royds make sure that helium cannot get through the thin-walled tube? They put helium in the tube, then look for it in the outer collection tube and find none.

[16]For good measure, they repeat the first experiment. Thus they know that helium is collected reproducibly in the outer tube and only under conditions in which α particles leave the thin-walled tube.

[17]At this point, the main question of the experiment has been answered conclusively, as stated at the end of the previous paragraph. The experimenters are now pursuing a loose end, a slightly unexpected observation: helium does not accumulate in the outer tube as rapidly as α particles are produced and leave the inner tube. Why not? Maybe it takes a long time for them to wiggle free from the glass walls of the outer tube where many of them are stopped. How might one test this explanation? Try stopping the α particles with a different material, and see if the accumulation of helium is faster or slower.

[18]At this point, the attention of the investigators appears to have shifted to the ability of lead to retain captured α particles. In fact they are investigating one more possible explanation of helium in their earlier experiments. "How do we know," they ask, "that the helium was not originally in the glass wall of the outer tube and was only released by exposure to α particles?"

[19]That the air was completely displaced was shown by the absence of neon in the final spectrum. [original note --CJG]

[20]They eliminated the possibility that α-particle bombardment released helium already present in the glass in two ways. First, they changed the outer glass tube for one made of lead foil and later tinfoil, and still got helium. Second, they showed that they obtained helium only after bombardment by α particles. In effect, they could collect α particles within metal foils, remove the source of the particles, and then release helium from the foils: that helium came from the already-collected α particles.

[21]None of the experiments described in this paper demonstrates that the α particle is a charged particle. Other experiments carried out previously showed that it carries two units of positive charge [Rutherford & Geiger 1908b].

References


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