Sunday, September 7, 2008

Radioactive decay rates depend on Earth-Sun distance?

Reports are floating around the scientific blogs (such as at arXiv Blog) about a new paper recently published on the Cornell Preprint server. Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance, by Jenkins, Fischbach, et al., has created a stir in the physics community. It may eventually rank up there with the Pioneer Anomaly in the 1990s and reports of a Fifth Force in the 1980s.

The results are intriguing as they involve measurements conducted at two different sites, Brookhaven National Laboratory (BNL) and Physikalisch-Technische-Bundesandstalt (PTB) in Germany, as well as two different isotopes, 32Si (a beta emitter) and 226 Ra (an alpha emitter), respectively. 36Cl (a beta emitter) was used as a calibration source in the BNL experiment. Two independent sites rule out a local variation at the experimental site. The BNL experiment also exhibits this annual variation in the ratio of 32Si to 36Cl, suggesting there must be an isotopic difference in the response of the nuclei to this effect. The authors suggest a solar effect, reported in another paper, Perturbation of Nuclear Decay Rates During the Solar Flare of 13 December 2006.

I've not yet heard of any Creation “Scientists” claiming that their theories predict this type of annual effect, but I suspect it is only a matter of time before they do. Reports of even the most miniscule changes in radioactive decay rates excite creationists a great deal since much of radioisotope dating depends on relatively constant rates of radioactive decay. I've examined a number of creationists claims of variation in radioactive decay rates (see Claims of Accelerated Radioactive Decay but have found none that specifically predict variation based on the Earth-Sun distance. Like psychics, such successful predictions by creationists will be ex post facto. Lest the creationists get too excited, it should be noted that both experiments exhibit this variation at the 0.2% level (above and below a constant value), nowhere near large enough to turn a 4.5 billion year old Earth into a 6000 year old Earth.

There are a number of interesting aspects of the Jenkins et al. analysis that hint this is a measuring systematic. One is that the amplitude of the variations seems to vary significantly from one year to the next. It is not perfectly correlated with the Earth-Sun distance. Another interesting issue is that the oscillation appears to cover the same range (0.2%) when examining the 226Ra decay as well as the ratio of the 32Si/36Cl decay. Is the 36Cl calibration source unaffected by this phenomenon? Perhaps 32Si is unaffected and 36Cl has an inverse correlation? If 32Si and 36Cl were identically affected, we would expect them to cancel out. As noted by the authors, there also appears to be a significant phase shift between the Earth-Sun distance and decay variation. This suggests an effect driven by another variable which may be coupled to the annual variation. Finally, the data are from the 1980s so it may be difficult to re-examine some of the techniques used to test for other systematics. Of course, the fact that this process seems to occur over a relatively short annual time scale will make it easier to verify today.

On a similar historical note, it was realized in the 1970s that no one had examined if the Newtonian inverse-square law operated at laboratory distances as well as it worked on solar system scales and larger. Previous researchers had determined the value of Newton's 'G' under the assumption that the inverse-square law was valid. When the historical data were finally examined with that assumption removed, the data suggested that at short ranges, gravity was not an inverse-square law (see Why do we believe Newtonian gravitation at laboratory dimensions? by Daniel R. Long). This launched a search in the 1980s and 1990s for a possible 'fifth force', in addition to the four fundamental forces known to physics.

It is interesting that the second author in the Jenkins paper, Ephraim Fischbach, is one of the researchers involved in the Fifth Force search (see Ten Years of the Fifth Force by Ephraim Fischbach). Interest in a fifth force died down as improvements in measurement technology seem to verify inverse-square at very short ranges (see Laboratory tests of gravity).

I suspect there are similar systematic measuring errors in the current Jenkins work as well. It is nonetheless an intriguing observation that definitely requires verification from other sources.


W.T."Tom" Bridgman said...

More discussion of this topic, including some issues related to creationism, on the physics arXiv blog.

W.T."Tom" Bridgman said...

P.S. Cooper [1] reports on the comparison of power output of the RTG (radioisotope thermal generator) aboard Cassini spacecraft which rules out variation with spacecraft-Sun distance for 238Pu to a level 1/350 the size reported by Jenkins et al.

Semkow et al.[2] reports that temperature variations of the air between the radioactive source and the detector can explain the annual variation reported by Jenkins et al.

[1] P. S. Cooper. Searching for modifications to the exponential radioactive decay law with the Cassini spacecraft.
ArXiv e-prints, September 2008.
[2] T. M. Semkow, D. K. Haines, S. E. Beach, B. J. Kilpatrick, A. J. Khan, and K. O’Brien. Oscillations in radioactive exponential decay. Physics Letters B, 675:415–419, May 2009. doi:10.1016/j.physletb.2009.04.051.

JREF UnrepentantSinner said...

Look like Creationists finally got around to the ex post facto claim of credit.

"The suggestion that decay rates may be affected by neutrinos is nothing new. The TalkOrigins Web site cites a reference to Henry Morris mentioning the possibility as early as 1974 and Davis Young discussing it in 1988 (“Claim CD004,” 2004)."