Who discovered fusion—Blackett or Oliphant?

Most historians credit a 1934 experiment by Mark Oliphant in Ernest Rutherford’s laboratory at Cambridge as the first demonstration of nuclear fusion. Oliphant used an ion-acceleration apparatus to shoot a beam of deuterium nuclei into targets that also contained this “heavy” form of hydrogen. 

Deuterium (²H₁) nuclei have both a proton and neutron, with twice the mass of ordinary hydrogen nuclei with only a proton (¹H₁).

Oliphant’s experiment is described in "Transmutation Effects Observed with Heavy Hydrogen.” Its summary says:

“An account is given of the effects observed when dipoles are used to bombard targets of compounds containing heavy hydrogen. It is found that a group of protons... is emitted in very large numbers… A discussion of the reaction which gives rise to them is given, and reasons are advanced for supposing that the short-range group consists of nuclei of a new isotope of hydrogen of mass 3...

Neutrons have been observed in large numbers as a result of the same bombardments… it is suggested that they arise from an alternative mode of breaking up of the unstable form of helium nucleus formed initially by the union of two diplons.  This other mode results in the explosion of a neutron and a helium isotope of mass 3…

Both fusion and fission occurred in these two reactions. First, two hydrogen-2 nuclei (Oliphant called them "diplons") fused and formed an unstable helium-4 (⁴He₂) nucleus. It immediately disintegrated. Sometimes it split off a proton to form radioactive hydrogen-3, “tritium” (³H₁). Other times it released a neutron to form the rare stable helium-3 isotope (³He₂). Both of these products are more massive than deuterium (²H₁), so either ot the two reactions could be called “fusion.”

However, in 1925, Patrick Blackett, as a research student in Rutherford’s laboratory at Cambridge, had already conducted a cloud chamber experiment demonstrating that the helium-4 (⁴He₂) nucleus--an alpha particle-- can fuse with nitrogen (¹⁴N₇) to form the rare (stable) oxygen-17 isotope (¹⁷O₈), while releasing a proton (¹H₁).

The image below is from Blackett's 1948 Nobel Lecture, "Cloud chamber researches in nuclear physics and cosmic radiation."

Why is Blackett’s experiment not interpreted as the first demonstration of fusion? More importantly, has the failure to acknowledge his work skewed our understanding of fusion? 

Most scientists are convinced that fusion requires highly specialized equipment such as that used by Oliphant. But Blackett had shown that rocks containing thorium ores emit alpha particles that rarely, but predictably, cause nitrogen to transmute to oxygen.

Nuclear weapons developers would argue that Oliphant’s results were more important than Blackett's. Oliphant had demonstrated that tritium can be produced from deuterium. Both of these forms of "heavy" hydrogen are key ingredients in modern nuclear weapons, along with plutonium. First plutonium splits, then its fission energy induces deuterium and tritium to fuse, greatly boosting the explosive power of the bomb.

Blackett and Oliphant were both members of the top-secret British MAUD Committee, formed in 1940 to decide if an atom bomb was feasible. Their report helped launch the Manhattan Project, led by Robert Oppenheimer. Oliphant felt so strongly that the Americans needed to act on the report and build a bomb that he traveled to Washington DC to lobby the U.S. government. He worked on the Manhattan Project at the Berkeley Radiation Laboratory under Ernest Lawrence, designing the electromagnetic calutron that enriched the uranium used in the Hiroshima bomb, and traveled to Oak Ridge, Tennessee to supervise work on the Y-12 National Security Complex. He later expressed regret about civilian killings and joined the Pugwash movement, advocating for the abolition of nuclear weapons

Blackett tutored Oppenheimer while both were at Cambridge. They clashed, because Oppenheimer did not want to spend time in the laboratory, preferring to focus on theory. There is even a story that Oppenheimer was so upset with Blackett that he tried to kill him with a poisoned apple. 

After World War II Blackett published a book,The Military and Political Consequences of Atomic Energy, warning about military exploitation of science and the development of nuclear weapons. 

For his work in improving the cloud chamber, and for using it to study cosmic radiation, Blackett was awarded the Nobel Prize in Physics in 1948. He also invented the sensitive magnetometer that provided data proving the reality of seafloor spreading, continental drift, and plate tectonics.