Discovery of D-D fusion

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EUROfusion was established in 2014 to succeed the European Fusion Development Agreement (EFDA). This article stems from EFDA times and may be outdated.

 

 

“The most interesting and important reaction which we have observed is that of heavy hydrogen on heavy hydrogen itself. Experiment has shown that diplogen is not appreciably affected by bombardment with alpha-particles from polonium, and we have been unable to detect any specific action of protons on diplogen for energies up to 300,000 e-volts. We were therefore surprised to find that on bombarding heavy hydrogen with diplons an enormous effect was produced. (…) Subsequent observation at much lower bombarding potentials showed that we were dealing in reality with a very large emission of protons.”"The simplest reaction which we can assume is

1D2 + 1D2 —> 2He4 —> 1H1 + 1H3

(…) If we neglect the energy of the bombarding particle (…) the mass of this helium atom must be 4.0272, and it therefore possesses an excess energy over the normal helium atom, of mass 4.0022.” ”It seems clear that the production of the isotope of hydrogen of mass 3 in these reactions is established beyond doubt. (…) The possible existence of this isotope has been discussed by several writers and although a careful search has been made no evidence of its presence has been found. It seems probable, however, that it could be formed by the process we have considered in sufficient quantity to be detected ultimately by spectroscopic or positive-ray methods. ”

 

“It was (also) at once evident that there was present a very intense radiation capable of producing an undiminished effect on the

counter through 20 cm of lead. As a check on this a search was made for recoil nuclei with the linear counter, and it was found that neutrons are emitted in numbers comparable with the number of protons.”

 

“In order to account for the production of neutrons of the observed energy and number we have been led to assume the transformation

1D2 + 1D2 —> 2He4 —> 2He3 + 0n1

in which the unstable 2He4 nucleus first formed breaks up into a helium isotope of mass 3 and a neutron”

 

“No evidence of the existence of an 2He3 isotope has been obtained by ordinary methods, although the possibility of its existence has been suggested at various times. It is not unlikely that while the new isotope may prove to be unstable over long periods it may yet have a sufficiently long life to be detected by counting methods and in the expansion chamber.”

 

M.L.E. Oliphant, P. Harteck and Lord Rutherford in “Transmutation effects Observed with Heavy Hydrogen”, published in Proceedings of the Royal Society, A, vol. 144 (1934), p. 692 (full version)

 

 

 

       Note

 

       In the original article the authors use the term “diplogen” for deuterium (i.e. heavy hydrogen), and “diplons” for deuterium nuclei.

 

 

The authors recognised neutrons, although the discovery of the neutron was

  • announced only two years before, by James Chadwick (see his article and the Nobel Prize in 1935). It must have helped that James Chadwick worked in the same laboratory!
  • The two possible D-D fusion reactions were correctly identified, and a third option was discussed – a gamma decay of the interim 2He4 nucleus. The article stated that “it was impossible to decide whether a gamma ray of high energy is present”. Today we know that the fusion reaction is instantaneous, and that the interim nucleus is not properly formed. Therefore, the gamma decay option hasn’t enough time to occur and so it is extremely rare.
  • There was a real master-stroke: products of both D-D reactions were named even though neither tritium nor helium 3 were known at that time. Consequently, this very article is often considered to mark the discovery (although indirectly) of tritium 1H3 (current preferred notation is 1T3)
  • The observations also supported the previous indirect discovery of helium 3 (2He3) made by M. Oliphant, B.B.Kinsey and Lord Rutherford in their studies of the lithium disintegration by proton in 1932. This reaction had first been observed  by J.D. Cockcroft and E.T.S. Walton as the first nuclear reaction ever – only a few months before, and again in the same laboratory! (Nature article, Nobel prize in 1951).
  • However, it was only in 1939 in the US Berkeley National Laboratory that Luis W. Alvarez (Nobel prize in 1968) and Robert Cornog succeeded in directly observing tritium and helium 3 isotopes (“Helium and Hydrogen of Mass 3″, Phys. Rev. 56 (1939), page 613). In their measurements, a cyclotron accelerator was first used as a mass spectrometer and tritium was indeed produced by D-D fusion.
  • The authors speculated that helium 3 could be quite an unstable element. It is also known that Lord Rutherford thought tritium would be stable and tried to separate it from water. Therefore it came as a surprise that L.W. Alvarez and R. Cornog found  helium 3 to be a stable isotope, while tritium was unstable!  Although helium 3 is a stable isotope, it is very rare on Earth: There is only one helium 3 atom in one million helium 4 atoms (helium 4 being a product of natural ”alpha” radioactivity). Tritium is a beta-source with a half-life of 12.3 years.
  • The article on the discovery of fusion reactions gives us a fascinating insight into an intense period of early research into nuclear physics. The experimental work was undertaken in the Cavendish laboratory by Marcus L.E. Oliphant (1901-2000), Paul Harteck (1902-1985) and world famous Ernest Rutherford (1871-1937). Lord Rutherford had already won the 1908 Nobel Prize in Chemistry for the discovery of alpha and beta radioactivity, but the best was yet to come – in 1911 he published hisdiscovery of atomic nucleus. In this article on D-D fusion, published in 1934, a few details deserve special attention:

All the incredible developments of the 1930s seem very remote today, when D-D fusion reactions are well understood and can be observed in most fusion experiments. At JET, the D-D fusion neutrons provide a valuable tool to measure plasma properties, and therefore neutron diagnostics at JET are being enhanced with new instruments.

 

Picture of the month

 

picture of Sudbury Neutrino Observatory

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