The soft recoiling of a nucleus after it has been struck by a low-energy neutrino is a mechanism that physicists have long been keen to get a grip on. Low energy neutrinos probe entire nuclei at once, and as a result there is a strong likelihood of these interactions occurring compared to other neutrino scattering processes, potentially enabling detectors to become very compact. But the catch, which has prevented the construction of such detectors until now, is that a successful detector would have to be acutely sensitive at low energies.

Juan Collar and colleagues from the University of Chicago and Canberra Industries in the US, however, have now built a prototype detector that has just that. Like existing germanium detectors, the new detector spots softly-recoiling nuclei as they ionize surrounding atoms in the semiconductor. But unlike existing germanium detectors, impurities distributed within the p-type semiconductor help the ionized charges to move, effectively "pulling" them towards the detector's electrodes. The physicists also reduced electronic noise by using much smaller electrodes that minimize unwanted capacitance.

The most significant application presents itself in nuclear reactors, where there is increasing concern that fuel rods could be removed illicitly and used to make nuclear weapons. Although in principle a reactor's neutrino signature would change following illicit behaviour, detectors proposed hitherto have been so large (over 30 cubic metres) and sensitive to noise that they cannot get within the 20 metre radius required to monitor with any reliability. Collar's device, at over 1,000 times smaller (and roughly half a kilogram), is unobtrusive enough for it to monitor a reactor at close range.

But the applications don't stop there. Because the detector responds in the same way to all known types of neutrino (electron, muon and tau), it could possibly provide the first concrete evidence for the much-debated fourth type: the so-called "sterile" neutrino.

Collar says that his team are now looking to lower the energy threshold of their prototype further still, so that they detect even more low-energy neutrinos using the same mass of germanium.