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Radioactivity
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.
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 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 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/R
2) 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, 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]
References:
- Ernest Rutherford and Frederick Soddy, “Radioactive change,” Philosophical Magazine vol. 5 (1903), pp. 576-591.
- Helge Kragh, "Rutherford, Radioactivity, and the Atomic Nucleus," arXiv, February 5, 2012.
- S. E. Rutherford, J. Chadwick, and C. Ellis, "Radiations from Radioactive Substances" (Cambridge University Press, 1930).
- Do nuclear decay rates depend on our distance from the sun? (arXiv Blog, August 29, 2008)
- 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.
- Ian O'Neill, "Using Cassini to Test Radioactive Decay Rate Variation," Astroengine, September 26, 2008
- Peter S. Cooper, "Searching for modifications to the exponential radioactive decay law with the Cassini spacecraft," arXiv, September 24, 2008).
- 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.
- 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.
- 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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Linked Keywords: Radioactivity; century; France; French; physicist; Henri Becquerel; Nobel Prize in Physics; unit of radioactivity; salt; uranium; photographic plate; paper; radiant energy; X-ray; Pierre Curie; Marie Curie; Ernest Rutherford; Henri Becquerel (1852-1908); Smithsonian Institution Libraries; curie (Ci); becquerel (Bq); International System of Units; SI units; Paul Nadar (1856–1939); Wikimedia Commons; exponential decay; exponential process; half-life; atomic nucleus; nuclei; physical law; atom; statistical mechanics; quantum vacuum fluctuation; activation energy; radium; Frederick Soddy; radon; radionuclide; statistics; noise; experiment; periodic function; Purdue University; data; silicon; 1980s; Brookhaven National Laboratory; Physikalisch-Technische Bundesanstalt; solar flare; manganese; laboratory; Earth-Sun distance; arXiv; Sun; neutrino; energy flux; percent; deep space exploration; space probe; radioisotope thermoelectric generator; heat; energy harvesting; electricity; Cassini-Huygens spacecraft; plutonium-238; energy; spacecraft; trajectory; Solar System; Venus; gravity assist; orbit; Saturn; correlation and dependence; confidence interval; confidence level; colleague; Ohio State University; chlorine-36; Geiger-Müller counter; environmental; sensor; detector; Germany; German; Physikalisch-Technische Bundesanstalt; null result; astronomical unit; liquid scintillation counting; organic compound; organic liquid; scintillator; laboratory equipment; apparatus; photomultiplier tube; order of magnitude; strontium-90; yttrium-90; Ernest Rutherford and Frederick Soddy, “Radioactive change,” Philosophical Magazine vol. 5 (1903), pp. 576-591.