Millennials Cockcroft and Walton crack open the atom (87 years ago!)

On April 14, 1932, Dr. Ernest Walton noticed the telltale signature of alpha particles after bombarding a lithium target with protons: the lithium broke into two helium nuclei.  John Cockcroft, his colleague, and Prof. Rutherford confirmed this was the case, independently doing the experiment two additional times.  The three men penned a letter to Nature that same night announcing the first controlled nuclear transmutation of one element (lithium) into “alpha particles” (later confirmed as Helium nuclei.)  They had split the atom. Wow!  Controlled disintegration of Lithium.

Screen Shot 2019-04-18 at 10.30.18 AMThe “Atom Splitter” of an Earlier Generation.

I remember first hearing about Ernest Walton from a fellow physicist at Trinity College, Dublin during a whirlwind tour of his Physics Department before high tea at the Faculty Club.  “Have you heard of this Irish physicist?  How about that Irish physicist?”  Sadly, I had never heard of ANY of them, yet they should be remembered.  Like Dr. Ernest Walton.  Hell, he won the Nobel Prize in Physics in 1951, after WWII which ended by splitting the atom with far more energy by using neutrons.

IMG_0092“Artificial Production of Fast Protons”, Letter to The Editor, Nature 129, 3250, pp242 (old numbering system in original wrappers) February 13, 1932


Dr. Ernest Thomas Sinton Walton was born in 1903, and died in 1995 Belfast, Northern Ireland. In 1922 Walton won scholarships to Trinity College, Dublin for the study of mathematics and science.  He was awarded bachelor’s and master’s degrees from Trinity in 1926 and 1927, respectively.   He was accepted as a research student at Trinity College, Cambridge under the supervision of Rutherford.  At the time there were four Nobel Prize laureates on the staff at the Cavendish lab and a further five were to emerge, including Walton and Cockcroft. Walton was awarded his PhD in 1931 and remained at Cambridge as a researcher until 1934.  It turns out, he had a strong commitment to the Christian Faith.

Sir John Douglas Cockcroft, OM, KCB, CBE, FRS was born in 1897 and served on the Western Front with the Royal Field Artillery during World War I, suffering gas attacks and trench warfare.  Afterward, he studied electrical engineering at Manchester Municipal College (never underestimate the training in city colleges!).  He then won a scholarship to St. John’s College, Cambridge, where he sat the tripos exam in June 1924, becoming a wrangler.  There were no “veterans benefits”, nor any of that.

Ernest Rutherford accepted Cockcroft as a research student at the Cavendish Laboratory, and Cockcroft completed his doctorate under Rutherford’s supervision in 1928 [NOT 1925, as posted in APS News ].

Screen Shot 2019-04-18 at 10.52.00 AM

A schematic an early two-stage Walton-Cockcroft voltage multiplier


Cockcroft and Walton used this to perform the first controlled artificial disintegration of an atomic nucleus, first through a fusion-like and then a fission process, as in…

ZXA  :    3Li7 + 1H1 → 2He4 + 2He4

where the lower subscripted number “Z” is the positive-charge of the nucleus.  From their early 1930-2 view, the Li(7) nucleus [meaning with a nuclear mass of 7 protons + 4 electrons] first became a Li(8) [8 protons + 4 electrons] by absorption of just one of those “swift” protons, then split into two “alpha” particles.

Rutherford had earlier discovered “protons,” from the decay of nitrogen in 1917 by bombarding it with “alpha particles”, the truly first alchemy or transmutation of an element, although completely uncontrolled.

It is truly mind-blowing that the neutron was not discovered and published by the time of Walton and Cockcroft’s experiment.  Not until later, on June 1, 1932, did Sir James Chadwick, the English physicist, publish his results using scattering data to calculate the mass of this neutral particle.  Since the time of the Cavendish Lab, it had been known that the atomic mass number “A” of nuclei is a bit more than twice the atomic number “Z” for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus.

As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of number “A” protons.  How was that possible?  And Why?

But by this time it was known from the uncertainty principle and from “particle-in-a-box” type confinement calculations that there just wasn’t enough energy available to contain electrons in the nucleus.

An experimental breakthrough came in 1930 with the observation by Bothe and Becker that bombardment of Beryllium with “alpha particles” from a radioactive source produced neutral radiation which was penetrating but non-ionizing. They presumed it was gamma rays, but Curie and Joliot showed that when you bombarded a paraffin target with this radiation, it ejected protons with energy about 5.3 MeV.  This proved to be inconsistent with gamma rays, as can be shown from momentum and energy analysis.

Taken as a whole, not a very efficient method for probing the interior of the atom nor the processes of dissecting it, but to the point of the tasks at hand.  Because their preferred method of detection was an early cloud-expansion chamber, they were also able to determine the kinetic energy of the “alpha particles” and verify Einstein’s conclusion – “ENERGY IS EQUIVALENT TO MASS (within error bars if one assumes the “alpha particle” is 4 protons + 2 electrons).”

A feat popularly known as splitting the atom, the device could combine and split many more things in the study of nuclear processes.  And did, in later years.  With a maximum achievable accelerating potential of ~800kV, this was rarely needed.   The quantum tunneling effect to disintegrate the lithium nucleus, based on the theoretical work of George Gamow sufficed.  As a result, their results both confirmed the fission and the effect known as tunneling.

Georgiy Antonovich Gamov was born in 1904 outside Odessa in the Russian Empire.  His education at the local gymnasium was often interrupted by shelling during the First World War, so he was to a large extent self-taught.  His education continued at the Novorossiya University in Odessa  to 1923, and then at the University of Leningrad from 1923.  He studied optics and, later, cosmology.  At Leningrad, Gamow became friends with two other students of theoretical physics, the brilliant Landau and Ivanenko.  The three became known as , “The Three Musketeers”, meeting regularly to discuss and analyze the ground-breaking papers in quantum mechanics.

On gaining his PhD from the University of Leningrad in 1928, he worked on quantum theory at the University of Göttingen in Germany, where his research into the atomic nucleus provided the basis for his habilitation.  He was invited to the Institute for Theoretical Physics at the University of Copenhagen by Bohr from 1928 to 1931, with a fateful(?) break in 1929 to work at the Cavendish Laboratory.

It was during this time, Gamow described the theory of the “alpha decay” of a nucleus via quantum tunneling, the first successful explanation of the behavior of radioactive elements using quantum theory.  Classically, it takes an enormous amount of energy  for a particle to enter or leave the high-energy potential of the nucleus of an atom.  In quantum mechanics, however, there is a finite-probability that the particle can “tunnel” through the potential and escape (Helium alpha decay), as Gamow illustrated a priori through a relationship between the half-life of the particle and the energy of the emission.

So, three (3) amazing things were accomplished in these Walton-Cockcroft experiments of 1932:

(1) The Energy-Mass equivalence equation was quantitatively demonstrated using the fixed “alpha particle”, validating Einstein,

(2) Quantum Tunneling was demonstrated practical, validating Gamow’s work, and

(3) Transmutation of elements through various processes is valid, releasing stored nuclear energy in vast quantities if done rapidly.

Bombardment using neutrons instead of protons and the rapid  release of energy envisioned in 1932 through as yet undiscovered process of “chain reactions” using neutrons were years off.

After the war, Cockcroft became the director of the Atomic Energy Research Establishment (AERE) at Harwell, where the low-powered, graphite moderated GLEEP became the first nuclear reactor to operate in western Europe when it was started on 15 August 1947.  This was followed by BEPO in 1948.  Cockcroft was involved in the design of the reactors and the chemical separation plant at Windscale.

Significantly, under his direction it took part in frontier fusion research, including the ZETA program.  His insistence that the chimney stacks of the Windscale reactors be fitted with filters was mocked as Cockcroft’s Folly – until the core of one of the reactors ignited and released radionuclides during the Windscale fire of 1957.  No fatalities at Windscale occurred, because of the filters.  He, perhaps single-handedly, prevented a catastrophe like that which would follow the incineration of Chernobyl’s graphite-moderated reactor.  

One might ask why? What gave him the insight that others lacked?

Perhaps it came while busting open atoms in the Cavendish Lab decades before, through processes not yet clear and unleashing sub-atomic particles unknown.  How many times did he feel “lucky” then?  Or did he feel, if not humble, a little more cautious when not only taking his life into consideration, but that of a nation?

For his service to humanity, Cockcroft was awarded The Presidential Medal of Freedom  (USA) and the Chevalier de la Legion d’Honneur  (Republic of France).


POSTSCRIPT

Why do I write these blips, and never seem to finish?  I think it helps me explore different aspects of physics.  Like Fission.  Like Fusion.  Like how the HELL did anyone research this mine-field?  Like the immutability of the “alpha particle” being something different?  We always said when I was a punk kid student, “Why did they call it an alpha particle for so long?  It’s a Helium nucleus, right?  They didn’t know it was 2 protons and 2 neutrons.  They thought it was still 4 protons and 2 electrons – or something! 

The APS NEWS had a Physics History on this subject, but got a couple facts wrong and missed a few, so I decided to look into it.  I mean they started that article with the famous E =mc2 like everyone in 1932 knew that or used that form.  Not.


<=   THE OTHER VIEW  =>

 

This is Rutherford, looking his maniacal self.  Look at those eyes.  In the event, just an opinion.  I wonder how much he planned out as a campaign; how much came from his own maniac-drive upon his students and researchers.  If planned, he developed from 1907 until his death in 1937, the near-perfect “ATOMIC ARMY OF CONQUEST.” 


Start with Prof. Dr. Hans Geiger who was awarded a doctorate in 1906.  He received a fellowship to the University of Manchester and worked as an assistant to Sir Franz Arthur Friedrich Schuster FRS FRSE , a British-German physicist.  In 1907, after Schuster’s sudden retirement, Geiger began to work with his successor, Ernest Rutherford, and in 1908, along with Ernest Marsden, an undergraduate student under him, conducted the famous “gold foil experiment.”  This process allowed them to count alpha particles and led to Rutherford’s award of the 1908 Nobel Prize in Chemistry.  

In 1912, after a series of exciting experiments, Geiger was named Head of Radiation Research at the German National Institute of Science and Technology in Berlin, and left the UK.  Rutherford had just sent another of his students as a researcher to Geiger a year later in 1913, an English theoretician – James Chadwick – when war broke out.  Chadwick arrived just in time to be interned for the war (WWI) as an “enemy alien” counting beta particles for Geiger. 

Geiger served in the German Army as an officer in the field artillery, somewhat like Cockcroft was at that very moment in the Royal Field Artillery on the opposite side of no man’s land.  After WWI, Chadwick rejoined Rutherford, who was now Director-Head of the Cavendish Lab, and theorized the neutron “just after” Walton and Cockcroft’s experiments.    I mean, it seemed they were all part of the same Rutherford-Geiger Scientific Empire. But I digress and slightly jest 😉


 

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