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November 5, 2014

Radioactivity was discovered a little more than a century ago by French physicist, Henri Becquerel, who was awarded the 1903 Nobel Prize in Physics for its discovery. Becquerel, for whom the unit of radioactivity (decays per second) is named, found that salts of uranium would expose photographic plates protected by black paper. The source of this radiant energy was first thought to be X-rays, but further work by Pierre and Marie Curie, and by Ernest Rutherford and others, revealed these rays to be different.

Henri Becquerel (1852-1908)

Portrait of Henri Becquerel (1852-1908), circa 1905, from the Smithsonian Institution Libraries.

The unit of radioactivity was once the curie (Ci), but it became the becquerel (Bq) when SI units were standardized.

One Ci = 37 GBq.

(Photograph by Paul Nadar (1856–1939), via Wikimedia Commons.)

It was Rutherford who discovered that radioactive decay is an exponential process. After Rutherford, all radioactive elements were characterized by their half-life, a contraction of Rutherford's 1907 term, "half-life period," that describes when half the original nuclei in a sample have decayed. This was a strange law, indeed, since physicists of that period could see no reason why atoms created at the same time should decompose at different times. Rutherford and his contemporaries could not explain this statistical phenomenon, which is now known to arise when quantum vacuum fluctuations exceed an activation energy for the decay process.

Exponential decay law in the 1903 Rutherford-Soddy paper

Exponential decay law for radium in a 1903 paper by Rutherford and Frederick Soddy.[1] The "Emanation" is radon.

(Via arXiv.)[2)]

By 1930, Rutherford and his colleagues were quite confident that this exponential law was true; and, that the half life was a constant for each particular radionuclide.[3] Of course, the key to good statistics is having a large sample size, so any small variation in measured half life in a given short interval would have just been considered to be sampling noise. As I wrote in two previous articles (Radioactive Decay Surprise, September 4, 2008 and Radioactive Decay, August 30, 2010), some experiments seemed to indicate a periodic variation in the radioactive decay of some radionuclides.

In 2008, physicists at Purdue University analyzed data on the radioactive decay rates of silicon-32 (32Si) and radium-226 (226Ra) obtained during extended measurements in the 1980s at Brookhaven National Laboratory and the Physikalisch-Technische Bundesanstalt. They found a small (0.1%) variation with a period of one year (see figure).[4-5] They further reported that a solar flare on December 13, 2006, significantly decreased the decay of manganese-54 (54Mn) in their laboratory.[4]

Deviation of <sup>226</sup>Ra radioactive decay from an exponential law

Deviation of 226Ra radioactive decay from an exponential law (blue), along with the 1/R2 variation of the Earth-Sun distance (red). (Fig. 3 of ref. 5, via arXiv, modified for clarity.)[5)]

The Purdue team proposed that the observed effect might be caused by an influence of the Sun's neutrino flux on the radioactive decay process, but one problem with any solar interpretation is that the maxima and minima of the data did not always coincide with the times of closest and farthest approach to the Sun. Also, the Earth's distance from the Sun varies by a few percent, but the effect is seen at the 0.1% level.

There is evidence against such an effect. Deep space probes are generally powered by radioisotope thermoelectric generators in which the heat of radioactive decay is harvested to make electricity. The generator on the Cassini spacecraft uses plutonium-238 as its source of energy, and the spacecraft's trajectory took it through much of the Solar System, from a flyby of Venus for a gravity assist, to an orbit of Saturn. An analysis of the spacecraft's power output showed no correlation as a function of distance from the Sun.[6-7] At a 90% confidence level, the supposed variation with (1/R2) is less than 0.84 x 10-4, and with (1/R) is less than 0.99 x 10-4.[7]

Scientists from Purdue University teamed with colleagues from Ohio State University and elsewhere to monitor the radioactivity of chlorine-36 (36Cl) for seven successive years, from July, 2005 to June, 2011, and they found the same variation.[8] The measurements were made with a Geiger-Müller counter, and they were careful to guard against environmental affects known to exist for such a detector.[8] A team of German scientists from the Physikalisch-Technische Bundesanstalt have performed the same measurement, and they have published a null result (see figure).[9-10] Chlorine-36 normalized activity.

Chlorine-36 normalized activity, as measured at Ohio State University (red) and Physikalisch-Technische Bundesanstalt (black). The blue line represents the reciprocal square of the distance between the Sun and Earth in astronomical units. (Physikalisch-Technische Bundesanstalt image, modified for clarity.)

The German team notes that many radiation detectors are sensitive to environmental parameters, and that the data supporting the seasonal change in decay rate for silicon-32, chlorine-36 and radium-226 were based on gas detectors.[10] The German experiment didn't use a Geiger-Müller counter. Instead, a liquid scintillation counting method was used in which a small amount of radioactive material is mixed with an organic liquid scintillator. The apparatus used three photomultiplier tubes for light detection from the scintillator. The method eliminates problems of self-absorption by the source material.[9-10]

The German team's chlorine-36 measurements were started in 2009, and their data overlap with the Purdue/Ohio State data for about two years.[9-10] The data show fluctuations that are more than one order of magnitude lower than the Ohio data, and the data show no seasonal variation.[9] The German team also undertook experiments with strontium-90 and yttrium-90, and these don't show any seasonal variation after a half year's data collection. Those experiments are continuing.[10]


  1. Ernest Rutherford and Frederick Soddy, “Radioactive change,” Philosophical Magazine vol. 5 (1903), pp. 576-591.
  2. Helge Kragh, "Rutherford, Radioactivity, and the Atomic Nucleus," arXiv, February 5, 2012.
  3. S. E. Rutherford, J. Chadwick, and C. Ellis, "Radiations from Radioactive Substances" (Cambridge University Press, 1930).
  4. Do nuclear decay rates depend on our distance from the sun? (arXiv Blog, August 29, 2008)
  5. Jere H. Jenkins, Ephraim Fischbach, John B. Buncher, John T. Gruenwald, Dennis E. Krause and Joshua J. Mattes, "Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance," arXiv, August 25, 2008.
  6. Ian O'Neill, "Using Cassini to Test Radioactive Decay Rate Variation," Astroengine, September 26, 2008
  7. Peter S. Cooper, "Searching for modifications to the exponential radioactive decay law with the Cassini spacecraft," arXiv, September 24, 2008).
  8. Jere H. Jenkins, Kevin R. Herminghuysen, Thomas E. Blue, Ephraim Fischbach, Daniel Javorsek II, Andrew C. Kauffman, Daniel W. Mundy, Peter A. Sturrockf, and Joseph W. Talnagic, "Additional experimental evidence for a solar influence on nuclear decay rates," Astroparticle Physics, vol. 37 (September 2012), pp. 81-88.
  9. K. Kossert and O.J. Nähle, "Long-term measurements of 36Cl to investigate potential solar influence on the decay rate," Astroparticle Physics, vol. 55 (March, 2014), pp. 33-26.
  10. Scientific news from Physikalisch-Technische Bundesanstalt, Division 6, "Precise activity measurements on 36Cl samples refute a dependence of the decay rate on the distance between the Earth and the Sun.

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