Rutherford’s Reluctant Role in Nuclear Transmutation

May 18, 2019 — By Steven B. Krivit —

Second in a Series of Articles on the Rutherford Nitrogen-to-Oxygen Transmutation Myth

Physicist Sir Ernest Rutherford (1871-1937) is a legendary figure in science history. Some people consider Rutherford to be among the 10 greatest physicists in history. Some call him the father of modern physics. The world’s first confirmed transmutation of one element into another has been described by many people as among Rutherford’s three greatest accomplishments. The discovery, however, belonged instead to a research fellow named Patrick Blackett, who worked in Rutherford’s lab at Cambridge University. This article discusses in detail Rutherford’s role in the research that preceded Blackett’s discovery.

Although a few historians recorded the discovery correctly, the myth that the discovery belonged to Rutherford ran strong for 70 years. Blackett’s discovery is described in “The World’s First Successful Alchemist (It Wasn’t Rutherford).”

The Dawn of Atomic Science
The dawn of atomic science took place as the late 1800s moved into the early 1900s. Eventually, old ideas yielded to new ones. In 1897, Joseph John Thomson discovered the electron while experimenting with cathode rays. Thomson’s discovery challenged the prevailing view that the atom was the smallest component in nature and thus indivisible.

In October 1901, chemist Frederick Soddy (1877-1956) and Rutherford made the first observation of natural nuclear disintegration of an unstable element. They observed that a radioactive source naturally decayed by itself, losing its radioactive power. For example, they reported the change of radioactive thorium to what they called thorium-X. They knew that thorium-X was some kind of gas in the argon family, but they did not make a transmutation claim, nor did they specifically claim that they had created a new element. Instead, they simply called thorium-X an “emanation.” They did suspect that they had created a new element, but they didn’t have experimental evidence to explicitly identify that new element, let alone make a claim of transmutation.

Although Rutherford suspected that he and Soddy had accomplished an elemental transmutation, he was reluctant to make such a bold claim. His letter to Sir William Crookes, a prominent chemist who was editor and publisher of Chemical News, reveals this fact. Rutherford had proactively asked Crookes for his help to get his and Soddy’s paper accepted for publication by the Chemical Society in the event that opposing points of view were to develop.

“Although of course it is not advisable to put the case too bluntly to a chemical society,” Rutherford wrote, “I believe that in the radioactive elements we have a process of disintegration or transmutation steadily going on.” (Badash, 1966, 89)

The emanation was later identified as radon. Rutherford also revealed his reluctance when he admonished Soddy in the moment of their discovery. Elemental transmutation was viewed as the work of charlatans, far outside the realm of legitimate science, isolated to the disgraced practitioners of alchemy. It made perfect sense that Rutherford would not want to taint his reputation with a claim of transmutation. But Soddy was less worried about what other people would think. Years later, Soddy recounted the event to his biographer Muriel Howorth:

My mind was always occupied with transmutation. That is natural; I was a chemist. You will remember my paper on “Alchemy and Chemistry”; I made that goal quite clear. Also at that time, I had been working on the lectures on gas analysis which I had been asked to give in the university.

That is why, perhaps, when Rutherford showed me the emanation which was not thorium, nor alpha nor beta particles, but [something] which could be blown about, I drew his attention to the fact that it would be a gas. I do want to show, as fairly as I can, that it was not very strange that I should be the first to discover and announce quite confidently the natural transmutation of radioactive elements. After all, I had tried the emanation out with every reagent that should have absorbed any known gas, and it had passed right through each one. It was natural to infer that it must be one of the newly found argon gases, though not one that [William] Ramsay had already discovered.

I was, of course, tremendously elated to have discovered transmutation — the goal of every chemist of every age, but looking back through the years it was, perhaps, the courage to express my conviction which was the laudable feature about it. I had written in my paper, “Alchemy and Chemistry,” that transmutation had not yet been found; yet when the time came to investigate the phenomenon, the whole thing seemed too devastatingly simple. The fact that this was, in reality, transmutation, flashed through my brain, and I could hardly believe what I knew to be true.

I remember quite well standing there transfixed as though stunned by the colossal import of the thing and blurting out — or so it seemed at the time: “Rutherford, this is transmutation: the thorium is disintegrating and transmuting itself into an argon gas.” The words seemed to flash through me as if from some outside source.

Rutherford shouted to me, in his breezy manner, “For Mike’s sake, Soddy, don’t call it transmutation. They’ll have our heads off as alchemists. You know what they are.” After which, he went waltzing round the laboratory, his huge voice booming “Onward Christian so-ho-hojers.”

Rutherford and Soddy’s collaboration soon ended. Soddy went to work with Sir William Ramsay (1852-1916), and in July 1903, they reported the results of their experiments. They had obtained evidence that, indeed, one element was created from another. They observed and reported that, through the natural radioactive decay process, radium had changed to helium. The two chemists — not Rutherford — earned the right to claim discovery of the first natural elemental transmutation. Yet nature was doing all the work. Ramsay and Soddy were merely the observers. The power to deliberately cause the transmutation of one element to another was not yet within man’s control.

In those formative decades in the early 20th century, scientists realized that the structure of the atom was a key mystery waiting to be solved. Rutherford hypothesized that atoms had a central core, a nucleus. By 1911, scientists understood that naturally radioactive substances emitted alpha particles (helium nuclei). Using these powerful natural projectiles, Rutherford directed a beam of alpha particles at a thin gold foil and observed a scattering anomaly. On the basis of this observation, he proposed the nuclear model for atomic structure. In July 1913, Niels Bohr introduced his own model of the atom. In December of that year, Rutherford confirmed the existence of the atomic nucleus that established his concept as the accepted model of the atom; atomic science gave way to nuclear science.

Soon, researchers began to suspect that a third type of particle — neither an electron (beta particle) nor a helium nucleus (alpha particle) — was ejected by radioactive atoms. They called it by various names: H particle, hydrogen atom, and positive electron. By 1914, Ernest Marsden, five years out of college, working as a research assistant in Rutherford’s laboratory, observed and published results that suggested, but did not prove, that, in addition to alpha particles, hydrogen atoms were also emitted from radioactive sources. Marsden was unable to finish his experiments because he accepted a new appointment at Victoria College in New Zealand, but Rutherford continued the work. He tested whether the particles detected by Marsden were, in fact, hydrogen atoms and, if so, what was the source of those hydrogen atoms. Historian Thaddeus Trenn’s quotes from Rutherford reveal his thinking at the time. [1]

“The exceedingly small dimensions found for the hydrogen nucleus add weight to the suggestion that the hydrogen nucleus is the positive electron, and that its mass is entirely electromagnetic in origin,” Rutherford wrote.

The idea of what he would later call the proton was becoming clearer to Rutherford. And, as Trenn wrote, “in 1914, Rutherford had already stated his own conviction, that the alpha particle was only a secondary constituent, the primary one being hydrogen.” Again, Trenn quoted Rutherford.

“The helium nucleus has a mass nearly four times that of hydrogen. If one supposes that the positive electron, i.e. the hydrogen atom, is a unit of which all atoms are composed, it is to be anticipated that the helium atom contains four positive electrons and two negative,” Rutherford wrote.

This is a crucial point to understand: Rutherford anticipated that the hydrogen atom, as the proton was then called, was a constituent of that which all atoms are composed. The proton, as Rutherford correctly anticipated, was not another element but a fundamental building block of all elements, a sub-atomic particle. By shattering the atom, he hoped and expected that he would obtain the proof of the nucleus, as he wrote to Bohr in 1917:

I … have got, I think, results that will ultimately prove of great importance. … I am detecting and counting the lighter atoms set in motion by alpha particles, and the results, I think, throw a good deal of light on the character and distribution of forces near the nucleus. I am also trying to break up the atom by this method. (Stuewer, 1986, 322)

In 1919, Rutherford published his observations and conclusions in a series of four papers titled “Collisions of Alpha Particles with Light Atoms” and, with them, just as he had sought, proof of this new particle. He began by explaining how he intended to follow Marsden’s work:

On the nucleus theory of atomic structure, it is to be anticipated that the nuclei of light atoms should be set in swift motion by intimate collisions with alpha particles. From consideration of impact, it can be simply shown that, as a result of a head-on collision, an atom of hydrogen should acquire a velocity 1.6 times that of the alpha particle before impact. Such high-speed “H” atoms should be readily detected by the scintillation method. This is shown to be the case by Marsden [in 1914], who found that the passage of alpha particles through hydrogen gave rise to numerous faint scintillations on a zinc sulfide screen placed far from the range of the alpha particles. (Collisions I, p 537, ¶ 1)

In a second paper, Marsden [in 1915] showed that the alpha-ray tube itself gave rise to a number of scintillations like those from hydrogen. Similar results were observed within alpha-ray tubes made from quartz instead of glass, and also with a nickel plate coated with radium C. The number of H scintillations observed in all cases appears to be too large to be accounted for by the possible presence of hydrogen in the material, and Marsden concluded that there was strong evidence that hydrogen arose from the radioactive matter itself. (Collisions I, p 538, ¶ 2)

We have seen that Marsden in his second paper had some indications that the radioactive matter itself gave rise to swift H atoms. This, if correct, was a very important result, for previously the presence of no light elements except helium had been observed in radioactive transformations. (Collisions I, p 538, ¶ 3)

And thus, Rutherford observed evidence of emitted particles that, because of their distance travelled, could not possibly be alpha particles. By all appearances, they were hydrogen atoms. They had a positive charge, were deflected by a magnetic field, and had the same range and energy as hydrogen atoms. But, as Rutherford explained, he did not know whether the detected hydrogen atoms originated from the radioactive source or were the result of alpha particle bombardment on hydrogen occluded somewhere in the system:

It has been shown in paper I that a metal source, coated with a deposit of radium C, always gives rise to a number of scintillations on a zinc sulphide screen far beyond the range of the alpha particles. The swift atoms causing these scintillations carry a positive charge and are deflected by a magnetic field, and have about the same range and energy as the swift H atoms produced by the passage of alpha particles through hydrogen. These “natural” scintillations are believed to be due mainly to swift H atoms from the radioactive source, but it is difficult to decide whether they are expelled from the radioactive source itself or are due to the action of alpha particles on occluded hydrogen. (Collisions IV, p. 581, ¶ 1)

In the fourth paper in the series, Rutherford revealed that he found the evidence he was looking for:

From the results so far obtained, it is difficult to avoid the conclusion that the long-range atoms arising from the collision of alpha particles with nitrogen are not nitrogen atoms but probably charged atoms of hydrogen or atoms of mass 2. If this be the case, we must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus. (Collisions IV, p. 586, ¶ 1)

Rutherford had answered the uncertainty left by Marsden, with data revealing that the emission of the hydrogen atom was indeed the result of the alpha bombardment reaction. Rutherford did not, however, have data that would shed light on the next logical steps in the exploration; instead, he was limited to formulating a working hypothesis. Rutherford hypothesized that the energy of the impinging alpha particle caused the nitrogen atom to partially disintegrate, knocking a “hydrogen atom” from its nucleus. He also assumed that the alpha particle remained intact after striking and recoiling from the target nitrogen atom:

Considering the enormous intensity of the forces brought into play, it is not so much a matter of surprise that the nitrogen atom should suffer disintegration as that the alpha particle itself escapes disruption into its constituents. (Collisions IV, p. 587, ¶ 1)

His four Collisions papers concerned only the experimental evidence showing that hydrogen atoms were indeed a reaction product of the alpha bombardment of nitrogen rather than a direct emission of the radioactive source. Nothing in his Collisions papers concerned the identity of the residual nucleus after alpha bombardment, let alone any determination that it was an atom of oxygen, or for that matter, any discussion of transmutation of elements. Nothing in Rutherford’s 1919 published work described:

• the pursuit of evidence of elemental transmutation
• the identification of any new element from transmutation
• the correct interpretation of the process that resulted in the emission of the proton and the transmutation of nitrogen to oxygen

A year later, in 1920, at a meeting of the British Association for the Advancement of Science, Rutherford suggested that the particle in the nucleus of the hydrogen atom should be called a proton, and he rightly earned the honor of naming it.

The pursuit of elemental transmutation had fascinated scientists for ages. But this was never Rutherford’s fascination. His focus was on particles, particle kinetics, and nuclear structure.

After completing his 1919 work, Rutherford knew that the identity of the residual nucleus was an unanswered question. He assigned Patrick Blackett (1897-1974), a research fellow in his laboratory, to resolve the question. In 1921, Blackett had just completed his studies at Cambridge and had earned his bachelor’s degree. For the next four years, Blackett performed the experiments that allowed him to identify the transmuted element — oxygen — and identify the correct process that resulted in the world’s first confirmed artificial elemental transmutation. Blackett published his results in 1925, and Rutherford was not a co-author. [2]

More than a few historians have been confused about Rutherford’s role in this history.

Next Article: The Nobel Foundation’s Retraction of the Rutherford Transmutation Claim 

References
1. Trenn, Thaddeus (March 1974) “The Justification of Transmutation: Speculations of Ramsay and Experiments of Rutherford,” Ambix, 21(1), p. 53-77
2. Blackett, Patrick Maynard Stewart (Feb. 2, 1925) “The Ejection of Protons From Nitrogen Nuclei, Photographed by the Wilson Method,” Journal of the Chemical Society Transactions. Series A, 107(742), p. 349-60