R.J. de Meijer

No evidence for antineutrinos significantly influencing exponential β+ decay

High-precision measurements were conducted on the time evolution of gamma-ray count rates during reactor-on and reactor-off periods to investigate the possible influence of antineutrinos on nuclear decay. This experiment was triggered by a recent analysis (Jenkins et al., 2009) of long-term measurements suggesting a possible link to variations in nuclear decay rate and solar neutrino flux. The antineutrino flux during reactor-off periods is mainly due to geoneutrinos and four orders of magnitude lower than during reactor-on periods. No effects have been observed for the two branches in the decay of (152)Eu and the decay of (137)Cs, (54)Mn and (22)Na. The upper limit determined of the ratio Δλ/λ for (22)Na is (-1±2)×10(-4), and (54)Mn is (-1±4)×10(-4). In comparison to the interpretation of Jenkins et al. our measurements do not show any such effect to at least two orders of magnitude less. Hence either the hypothesis of Jenkins et al. is not true or else one of two rather unlikely possibilities must also be true: either the effect of neutrinos on β(-) decay differs considerably from the effect of antineutrinos on β(+) decay, or the effect of antineutrinos on β(+) decay must be identical to their effect on β(-) and electron-capture decay.

Determination of soil, sand and ore primordial radionuclide concentrations by full-spectrum analyses of high-purity germanium detector spectra

The full-spectrum analysis (FSA) method was used to determine primordial activity concentrations (ACs) in soil, sand and ore samples, in conjunction with a HPGe detector. FSA involves the least-squares fitting of sample spectra by linear combinations of (238)U, (232)Th and (40)K standard spectra. The differences between the FSA results and those from traditional windows analyses (using regions-of-interest around selected photopeaks) are less than 10% for all samples except zircon ore, where FSA yielded an unphysical (40)K AC.

γ-Ray spectrometry of radon in water and the role of radon to representatively sample aquifers

Measurement of radon in water by gamma-ray spectrometry using a HPGe detector has been investigated to determine aquifer characteristics. The radon activity concentration is determined by taking the weighted average of the concentrations derived from gamma-ray lines associated with (214)Pb and (214)Bi decay. The role of accurate radon data to representatively sample aquifers was also investigated by studying a semi-cased borehole. A simplified physical model describing the change of radon concentration with the pumping time, reproduces the data and predicts the time for representative sampling of the aquifer.

Monitoring the radon flux from gold-mine dumps by γ-ray mapping

The exhalation of radon from the large mine dumps at the gold mines in South Africa is a potential health hazard. Determination of radon fluxes from these dumpsites is problematic due to the scatter in the data in time and place and the cost involved in getting a representative sample. gamma-ray spectroscopic analysis of soil samples from a dumpsite indicates that as much as 30% of the formed radon may escape, resulting in a disturbance to the secular equilibrium of the U-238 decay series. A method is proposed to quantitatively assess the radon flux from such dumpsites by using a mobile gamma-ray detector system

The Case for an Underground Neutrino Facility in South Africa

Experiments in physics, Astro-particle physics and cosmology that require careful shielding against cosmic rays include dark matter searches, studies of radioactive decays, and neutrino detection experiments. The need for such shielding has motivated the construction of laboratory caverns in mines and adjacent to tunnels under mountains. There are currently about a dozen such laboratories, in existence or under construction, all in the Northern Hemisphere. A motivation has been made for the establishment of a Southern Hemisphere facility. In this paper a feasibility study of measurements of radon in air (using electret ion chambers and alpha spectroscopy), background gamma ray measurements (inside/outside) the tunnel using scintillator (inorganic) detectors, cosmic ray measurements using organic scintillators and radiometric analyses of representative rock samples for the establishment of such a facility in the South Africa is presented.

The Moon’s formation revisited

T he falsification of a hypothesis and its replacement by a new testable hypothesis are part of progress in science. With respect to the formation of the Moon the classic Giant Impact Hypothesis (GIH) as described in the recent article by Geiss, Huber and Rossi, Europhysics News 45/4, 25-30, was developed when other hypotheses had to be discarded after the first analyses of the composition of lunar rocks, returned to Earth in the Apollo missions. The GIH appeared to explain the first-order physical and chemical features of the Earth-Moon system, including its angular momentum and the depletion of iron in the Moon compared to the Earth. The GIH was not seriously challenged for over thirty years since its inception in the mid-1970s [1]. However, over the past decade, increasingly large cracks have appeared in the armour of the GIH. More precise analytical techniques have revealed an astonishing similarity in both the elemental and isotopic composition between lunar rocks and the Earth’s silicate (rocky) crust and mantle. Similarities encompass major elements including silicon and titanium [2] as well as trace elements including neodymium and tungsten [3]. Such similarity is irreconcilable with smooth-particle hydrodynamic computer simulations of the classic giant impact of a Mars-sized planet into the young Earth, because those all predict that the Moon should consist predominantly of impactor material rather than of terrestrial material [4]. Several attempts have been made to fix this fundamental problem with the GIH. One suite of models has investigated whether lunar and terrestrial mantle material can be completely homogenised elementally and isotopically after the giant impact. The answer appears to be: not for all elements that show the uncanny Earth-Moon resemblance, and not for all of the silicate Earth and the Moon [5]. Collisional parameter space for giant impact models has also been stretched to try and fit the compositional similarities [6]. Impacts in which the impactor is either significantly smaller than Mars or as large as the Earth itself lead to predicted lunar compositions which are closer to that of the silicate Earth, more consistent with observations. But such impacts only work if they are accompanied by Earth-Moon system angular momenta that are significantly larger (by about a factor of 3) than today’s values. Such momenta are actually very close to the limit of rotational stability for the Earth. The main advantage of impacts accompanied by very high angular momenta is that they would release material predominantly from the proto-Earth rather than the impactor. But the disadvantage is that the high angular momentum and energy have to be syphoned off after the giant impact by a resonance involving Earth, Moon and Sun. At present it is unclear whether this mechanism can be invoked to remove the large amount of excess angular momentum that accompanies these alternative giant impact models. Both measurements of lunar rock compositions and hydrodynamic models agree that the classic GIH, involving a Mars-sized impactor and a constant angular momentum, must be rejected. As summarised above, new impact-based hypotheses have been developed, but these require additional assumptions and a process to remove large amounts of angular momentum. Some alternative hypotheses that do not start with the premise that a giant impact caused the formation of the Moon have also been proposed. We developed a hypothesis in which the Moon is formed of terrestrial material at an angular momentum close to the present value. Our hypothesis [7] is based on the concentration of fissile material concentrated in the Core-Mantle Boundary (CMB) of the Earth by a mineral called calcium silicate perovskite. By natural concentration the fissile material gets concentrated and spontaneously leads to georeactors [8]. Triggered by a small impact or by natural concentration processes, concentration of fissile material in the georeactor causes the reactor to become supercritical leading to a nuclear explosion. This explosion produces a shock wave propagating towards the surface where it ejects ironpoor silicate material into space, from which the Moon eventually forms. The shock wave emission does not disturb the isotopic and elemental composition of terrestrial silicate rock material. The presence of georeactors has been shown to be feasible and simulations indicate that such a shock wave emission is realistic [8]. At present our hypothesis is at least as consistent with observations as the latest impact-based hypotheses. At the moment, instead of being a done and dusted deal, the formation of our Moon remains shrouded in mystery. One reason for this may be that our present knowledge of the composition of the Moon is mainly based on the analysis of some of the 380 kg of samples collected at the Moon’s surface from a small area on its near side. Future lunar missions that could bring more material