Physicists in Europe have resolved a long-standing puzzle regarding the energy released when an isotope of the rare-earth element holmium decays via electron capture. They say that their extremely precise measurement of the mass difference between mother and daughter nuclides will be crucial in helping to pin down the unknown and very small mass of the neutrino. Other experts, however, insist that this measurement will not help to significantly reduce the upper limit on neutrino mass.
Experiments in the late 1990s and early 2000s showed that neutrinos oscillate from one flavour to another as they travel through space, and that therefore they have mass – in contradiction with the Standard Model of particle physics. However, such experiments can only establish the difference in mass between the three flavours, and not their absolute masses. But establishing the absolute value could point scientists to new physics beyond the Standard Model and identify the role of neutrinos in galaxy formation. A number of experiments including the Electron Capture 163Holmium experiment (ECHo) at the University of Heidelberg in Germany have therefore been developed to try and pin down that mass.
ECHo will begin in 2016 with the aim of measuring neutrino mass using the phenomenon of electron capture in holmium-163. The neutron-deficient isotope absorbs an electron from its inner shell, so converting one of its protons into a neutron and thereby changing into stable dysprosium-163. This results in the emission of an electron neutrino, as well as X-ray photons and Auger electrons – as an outer electron drops to fill the hole left by the absorption – and sometimes a gamma ray from the excited nucleus.
The experiment involves surrounding several milligrams of holmium-163 with gold and then measuring the temperature rise of the gold as it absorbs the particles and radiation from each decay event. The number of times that a specific energy is absorbed is then plotted as a function of that energy. This generates an energy spectrum with a shape that depends subtly on the mass of the neutrino.
Klaus Blaum, director of the Max Planck Institute for Nuclear Physics in Heidelberg, says that this analysis of energy conservation depends crucially on knowing the total energy released by the nuclear decay – in other words the maximum energy value at the end point of the spectrum. That quantity has been measured many times over the last 35 years using a variety of techniques. All such techniques to date, however, have been indirect, and have resulted in significant systematic errors and a consequent wide distribution of results. The current officially recommended value is 2550 eV, for example, but energies as high as 2800 eV have also been measured.
The latest work, in contrast, provides a direct measurement of the decay energy by measuring the mass difference between holmium-163 and dysprosium-163. Neutrons generated at the Institut Laue-Langevin in Grenoble, France, were used to convert erbium-162 into holmium-163, which, after being purified and processed, was ionized and placed in the SHIPTRAP Penning-trap mass spectrometer at the GSI laboratory in Darmstadt. The frequency – or, more precisely, the phase – of oscillation of the ions within the trap was measured and compared with that of a sample of dysprosium, with the frequency difference then providing a direct measure of the mass difference of the isotope.
The result was 2833 eV, and the uncertainty just a few tens of eV. “With our measurement, this problem is solved,” says Blaum, who points out that his group had done a large amount of test measurements beforehand.
Blaum and colleagues conclude that the new measurement should allow the upper limit on the electron neutrino mass to be reduced from its current value of about 225 eV to some 10 eV, once ECHo starts taking data around the end of 2016. Use of higher charge states for the mass spectrometry, to increase oscillation frequency and so reduce statistical uncertainty, should then help to push neutrino-mass sensitivity further down to about 1 eV over the next three to five years, says Sergey Eliseev, also at the Max Planck Institute for Nuclear Physics in Heidelberg.
However, Angelo Nucciotti of the University of Milan-Bicocca, who works on the rival HOLMES electron-capture project, points out it will now be harder than previously thought to reduce the upper limit on neutrino masses – because a neutrino of a given mass will have a smaller effect on the electron-capture spectrum when the decay energy is larger. He also disputes the idea that an accurate mass-spectrometry-based measurement is key to increasing to neutrino-mass sensitivity. “It is known,” he says, “that solid-state or chemical effects can shift the spectrum’s end-point enough to make an independent measurement using single-charged ions in a vacuum of little use.”
Are improvements futile?
Flavio Gatti, a nuclear physicist at the University of Genoa, agrees. Further improvements to the Penning-trap measurements, he argues, could be “useless or difficult to use without making large systematic errors”.
The current best upper limit on neutrino mass is about 2 eV for the electron antineutrino, and this comes from experiments that study beta decay. This limit should be further reduced to about 0.2 eV by the €60m KATRIN facility, which will then study the decay of tritium using a 10 m-diameter electrostatic spectrometer, when it switches on in 2016 at the Karlsruhe Institute for Technology in Germany. However, Nucciotti says that electron-capture experiments might eventually surpass the sensitivity of KATRIN. “This will be very challenging,” he cautions. “But not impossible. It will require a technological (and financial) effort at least comparable to that of KATRIN.”
KATRIN co-spokesperson Guido Drexlin argues that both beta-decay and electron-capture experiments are needed, given, he says, “the different systematic effects in both physics processes and detection methods”. He points out that of the more than 100 nuclei that undergo these kinds of decay, only tritium and holmium have the decay energy and other characteristics suitable for sub-eV neutrino-mass experiments. “It is very important to exploit both nuclei to the fullest extent,” he says.
The research is described in Physical Review Letters.