Who Discovered Gamma Rays? The Dated Record

Key takeaways

• Paul Villard found gamma radiation in Paris in April 1900. His notes of 9 and 30 April report a radium beam that a magnet could not bend and that crossed 0.2 mm of lead.
• Villard gave the rays no new name. Rutherford’s paper of February 1903 is the earliest place we found the letter gamma, and he defined it as non-deviable by a magnetic field and very penetrating.
• Whether gamma rays were particles or waves stayed open for 14 years. Paschen’s 1904 magnet test set a heavy lower bound on any particle, and crystals gave the answer.
• In May 1914 Rutherford and Andrade diffracted the soft gamma rays of radium B from rocksalt. Their lines gave wavelengths of 0.079 to 0.137 nm, photon energies of about 9 to 16 keV.
• The record has loose ends. Becquerel doubted Villard’s result in April 1900, and the first use of the word gamma may predate 1903.

Who discovered gamma rays? Paul Villard did, in Paris in April 1900, while studying radium. Ernest Rutherford named them in 1903. In May 1914 Rutherford and E. N. da C. Andrade showed that gamma rays are waves like X rays, by diffracting them from a crystal.

This piece follows the format of our dated series, which began with what Nikola Tesla invented. Each date below points to a journal volume, an archive scan or an agency record. Where the record is silent or disputed, we say so. For a lighter read on the same rays, see our 12 facts about gamma rays.

Gamma ray timeline: who showed what, and when

Dates are session dates of the Paris Académie des sciences or issue dates of the journal. A paper’s date is when it was read or printed, which can trail the experiment. Becquerel’s 1896 find opens the story told in our piece on the path to nuclear fission.

What each person actually showed

A discovery is rarely one act. This table separates what each person measured from what the measurement left open.

Villard’s two notes, read in the original

Villard worked in the chemistry laboratory of the École Normale Supérieure. He was studying how the bendable rays of radium reflect and refract, as cathode rays do. His first note, read on 9 April 1900, described an odd result. A straight beam crossed an inclined aluminium plate without refracting.

A magnet showed why. The rays divided into two separate groups, one bent and one straight. The straight beam blackened a plate 25 cm away behind several sheets of black paper and an aluminium sheet. It also crossed a lead plate 0.2 mm thick.

The second note, read on 30 April, compared the two groups directly. About 1 cm of glass stopped the bent rays and barely weakened the straight ones. A strip of lead 0.3 mm thick wiped out the track of the bent beam and only dimmed the other. Charged rays lose energy steadily along their path, a process covered in our explainer on the Landau distribution and energy loss. Villard called the straight beam the X rays of radium, the counterpart of what happens in a Crookes tube.

The Curies had already described a component that no magnet bends. Becquerel described it as non-deviable and little penetrating. Villard said his rays were less absorbable than those, and his second note gives the plate evidence.

Becquerel did not accept the result at once. On 30 April he told the Académie that Villard’s refraction experiment was inconclusive. He added that if Villard’s observations were exact, the cause of the disagreement might lie in the radium product Villard used. Another possibility was weak, very penetrating rays like uranium’s, which show only after a long exposure. Rutherford’s 1905 book later credits Villard with first drawing attention to the rays and says Becquerel confirmed the result.

Who named them gamma, and when

Rutherford had named the first two radiations in January 1899. Studying uranium, he wrote that the radiation is complex. One part is very readily absorbed and “will be termed for convenience” the alpha radiation. The more penetrative part is the beta radiation.

The letter gamma appears in his paper of February 1903 on alpha rays from radium. Its first paragraph lists three types of radiation, and the third reads “The γ rays, which are non-deviable by a magnetic field, and which are of a very penetrating character.” The AIP History catalog also dates the naming to 1903. In the 1905 edition of Radio-activity he says the three types “for brevity and convenience have been termed by the writer” alpha, beta and gamma.

We searched text scans of the 1901 and 1902 volumes of the Philosophical Magazine and found no earlier use. His 1905 book also cites a 1902 paper in the Physikalische Zeitschrift on gamma rays from uranium and thorium, which we could not open. So 1903 is the earliest date our sources support, and an earlier first use cannot be ruled out.

Light or particles? The 14-year argument

After 1900 the rays had a test and a name, but no identity. Two results shaped the debate. Early gas experiments seemed to separate gamma rays from X rays, yet Rutherford noted that the X rays used were much softer than the gamma rays. In 1904 Paschen looked for deflection in a very intense magnetic field and found none. Rutherford reports that charged particles near light speed would then need an apparent mass at least 45 times that of a hydrogen atom.

Crystals settled it. W. H. and W. L. Bragg and Moseley had shown that a crystal sorts X rays by wavelength. Rutherford and Andrade applied the method to radium. Their source was a thin-walled tube of radium emanation; the paper gives about 100 millicuries as an example of its strength. A 24 hour exposure gave a marked image from the reflected gamma rays, where X rays need about 10 minutes. A strong magnet bent away the beta particles that would otherwise fog the plate.

The plates showed sharp lines at fixed angles. Two strong lines fell at almost exactly 10° and 12° on rocksalt. A second crystal, heavy spar, gave the same lines after scaling, which the authors say puts it “beyond doubt” that they are true diffraction lines. They then excited lead with beta rays from the same apparatus and measured its lines at 10° 2′ and 12° 0′. These agree with the strong lines of radium B. The authors conclude that radium B has the same nucleus charge as lead, although its atomic weight is 214 against 207.

A crystal sorts waves by wavelength. Lines at fixed angles, matching the X-ray lines of lead, put these gamma rays in the same family as X rays. That is our reading of the result. The paper reports wavelengths and line spectra and does not use the word electromagnetic.

From angle to wavelength to energy

The Bragg relation for first order is wavelength = 2 × d × sin θ. Rutherford and Andrade took the grating space of rocksalt as d = 2.814 × 10⁻⁸ cm from Moseley. Photon energy follows from the NIST relation that 1 eV corresponds to 8.065544 × 10⁵ reciprocal metres. So the energy in eV is 1 divided by the product of the wavelength in metres and 8.065544 × 10⁵.

The three soft lines give 9.08 to 15.6 keV, which is X-ray territory. The last row shows why the harder rays were difficult. At about 1° the diffraction line sits little more than 3 mm from the centre of the direct band on the plate, according to the authors.

After 1914: photons and the sky

In May 1923 Arthur Compton published a quantum theory of X-ray scattering, in which a radiation quantum carries momentum as well as energy. By the space age, gamma-ray astronomers counted single photons. Explorer XI launched on 27 April 1961 to detect the sources of high-energy gamma rays. Kraushaar and Clark wrote in 1962 that fewer than 100 energetic photons had given the first glimpse, and that their discussed sample numbered 22.

Gamma-ray bursts came from a different programme. The Vela satellites began in October 1963 to check compliance with the nuclear test ban treaty. Vela 4a and 4b recorded an event on 2 July 1967 that Klebesadel counts as the first observed burst. His team published 16 bursts in 1973. The same programme is the subject of our piece on the Vela incident and bhangmeter physics.

Where the record is uncertain

The first use of the name gamma is the loosest date. The earliest print use we could open is February 1903, but Rutherford’s book points to a 1902 paper we could not read.

Becquerel’s role is also unclear. Rutherford cites Becquerel’s note on page 1154 as a confirmation of Villard. The note on that page reads as a challenge, and we did not check Becquerel’s later papers.

Dates of discovery also vary by source. Physics Today says Villard presented his results in April and May 1900. The Comptes rendus volume shows his two notes at the sessions of 9 and 30 April. Physics Today adds that the work drew little attention at first. Gerward’s 1999 history of the discovery is listed under the sources as further reading.

Last, “discovered” needs care. Villard saw a component of radium’s radiation and likened it to X rays. He did not claim a new species of ray or give it a name. Rutherford supplied the name, and the 1914 crystal spectra supplied the physics.

Sources

• Comptes rendus de l’Académie des sciences, vol. 122 (January to June 1896), Becquerel, pp. 501 to 503, Internet Archive scan
• Rutherford, Uranium radiation and the electrical conduction produced by it, Philosophical Magazine 47, January 1899, pp. 109 to 163, Internet Archive scan
• Comptes rendus de l’Académie des sciences, vol. 130 (January to June 1900): Villard pp. 1010 to 1012 and 1178 to 1179, Becquerel pp. 979 and 1154 to 1157, Internet Archive scan
• Rutherford, The magnetic and electric deviation of the easily absorbed rays from radium, Philosophical Magazine (6) 5, February 1903, p. 177, Internet Archive scan
• Rutherford, Radio-activity, second edition, Cambridge University Press, 1905, Project Gutenberg
• AIP History of Physics catalog, Villard, Identification of gamma radiation
• Rutherford and Andrade, The wave-length of the soft γ rays from radium B, Philosophical Magazine 27, May 1914, pp. 854 to 868, Internet Archive scan
• Rutherford and Andrade, The spectrum of the penetrating γ rays from radium B and radium C, Philosophical Magazine 28, August 1914, pp. 263 to 273, Internet Archive scan
• Compton, A quantum theory of the scattering of X-rays by light elements, Physical Review 21, 483 (1923)
• NASA NSSDCA, S 15 (Explorer 11), 1961-013A
• Scientific American, Kraushaar and Clark, 1962, reprinted in 50, 100, 150 years ago
• NASA, A brief history of the discovery of cosmic gamma-ray bursts
• NIST CODATA, electron volt to inverse metre relationship
• Physics Today, Paul Villard
• Gerward, Paul Villard and his discovery of gamma rays, Physics in Perspective 1, 367 to 383 (1999), further reading