No seasoning: Fluctuating flux flummoxed

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  • Published: Oct 15, 2014
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: No seasoning: Fluctuating flux flummoxed

In flux

The decay rate of certain radioactive nuclides does not fluctuate with the terrestrial seasons because of changes in the flux of solar neutrinos passing through our planet, according to new research that debunks the hypothesis.

The decay rate of certain radioactive nuclides does not fluctuate with the terrestrial seasons because of changes in the flux of solar neutrinos passing through our planet, according to new research that debunks the hypothesis.

In 2012, researchers Ephraim Fischbach and Jere Jenkins in the US and their colleagues published data concerning the radioactive isotope chlorine-36. Their findings suggested that Earth's seasons were causing fluctuations in the rate of decay of this unstable nuclide. Perhaps, they posited, the fluctuations were due to the changing distance between the Sun and the Earth as our planet follows its elliptical orbit around our star. The changing difference is not the factor that gives rise to the summers and winters of the temperate zones, that is due to our tilt relative to the ecliptic plane in which we orbit the Sun. However, there is a fairly substantial difference between Earth's closest approach to the sun, the perihelion, which occurs in early January (146 million kilometres) and the farthest we orbit, known as aphelion, which occurs in early July (152 million km).

Peri to ap

Is this almost 4 percent difference sufficient to somehow affect nuclide decay rates?The researchers attributed the apparent fluctuations they observed in chlorine-36 and other radioactive isotopes to changes in the flux of solar neutrinos reaching the surface of the Earth. Somehow a 4 percent distance difference would change the radioactive decay rate. It is a controversial idea, especially given that neutrinos are such minute subatomic particles that pass through most matter at incredible high rates continuously with very, very few interactions.

Measurements of the decay rates of long-lived radionuclides have been carried out over the course of several years. The resulting data, for example, are used to determine the half-life of the respective radionuclides, or to test the stability of the detector system used. Long-lived reference sources are even used to compensate for changes in the efficiency of detectors. In the long-term measurements, considerable fluctuations occur, depending on the type of detector used. In some cases, these fluctuations seem to be correlated with the respective season. However, the data fluctuations claimed by Fischbach and Jenkins are not in phase with perihelion and aphelion. So, they suggested that the rate change correlates with phenomena occurring within the Sun's interior that cause changes in the neutrino flux.

Triple confidence

Researchers, Karsten Kossert and Ole J. Nähle, at Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute in Braunschweig have sharpened and wielded Occam's razor to show that the seasonal fluctuations and their cumbersome explanation are merely an artefact of fluctuating sensor sensitivities. They too looked at chlorine-36, they began their measurements in 2009, and used a liquid scintillation counter. In their experiment, a small quantity of the radioactive material is put directly in the organic liquid scintillator, which precludes interference from self-absorption of the radiation. Additionally, they used the TDCR (Triple to Double Coincidence Ratio) procedure to determine the radioactivity without reference to an external source once more removing a possible source of error. The TDCR liquid scintillation measurements thus have clear advantages over simple decay counters.

The PTB results show none of the seasonal fluctuation published by the US team, in fact the fluctuations overall are far less prominent. If there were some solar neutrino influence on radioactive decay rates here on Earth, the PTB measurements should have revealed it. They do not. Instead, the team suggests, it is simple environmental factors such as changes in temperature, humidity and atmospheric pressure that have affected instruments in past experiments to give rise to fluctuations not in the actual decay of atomic nuclei but simply in the sensitivity of the detectors. The team has carried out additional long-term measurements on strontium-90 and ttrium-90 and these too show no seasonal fluctuations. Moreover, there is overlap in the experiments with the US team, but the Germans do not even see the phase and amplitudes reported by the former.

Related Links

Astroparticle Phys, 2014, 55, 33-36: "Long-term measurements of 36 Cl to investigate potential solar influence on the decay rate"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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