Beta Decay and the Cosmic Neutrino Background

AbstraktAbstract

In 1964 Penzias and Wilson detected the Cosmic Microwave Background (CMB). Its spectrum follows Planck's black body radiation formula and shows a remarkable constant temperature of T-0 gamma approximate to 2.7 K independent of the direction. The present photon density is about 370 photons per cm(3). The size of the hot spots, which deviates only in the fifth decimal of the temperature from the average value, tells us, that the universe is flat. About 380 000 years after the Big Bang at a temperature of T-0 gamma = 3000 K already in the matter dominated era the electrons combine with the protons and the He-4 and the photons move freely in the neutral universe. So the temperature and distribution of the photons give us information of the universe 380 000 years after the Big Bang. Information about earlier times can, in principle, be derived from the Cosmic Neutrino Background (CvB). The neutrinos decouple already 1 second after the Big Bang at a temperature of about 10(10) K. Today their temperature is similar to 1.95 K and the average density is 56 electron-neutrinos per cm(3). Registration of these neutrinos is an extremely challenging experimental problem which can hardly be solved with the present technologies. On the other hand it represents a tempting opportunity to check one of the key element of the Big Bang cosmology and to probe the early stages of the universe evolution. The search for the CvB with the induced beta decay nu(e) +(3) H -> 3 He + e(-) is the topic of this contribution. The signal would show up by a peak in the electron spectrum with an energy of the neutrino mass above the Q value. We discuss the prospects of this approach and argue that it is able to set limits on the CvB density in our vicinity. We also discuss critically ways to increase with modifications of the present KATRIN spectrometer the source intensity by a factor 100, which would yield about 170 counts of relic neutrino captures per year.

In 1964 Penzias and Wilson detected the Cosmic Microwave Background (CMB). Its spectrum follows Planck's black body radiation formula and shows a remarkable constant temperature of T-0 gamma approximate to 2.7 K independent of the direction. The present photon density is about 370 photons per cm(3). The size of the hot spots, which deviates only in the fifth decimal of the temperature from the average value, tells us, that the universe is flat. About 380 000 years after the Big Bang at a temperature of T-0 gamma = 3000 K already in the matter dominated era the electrons combine with the protons and the He-4 and the photons move freely in the neutral universe. So the temperature and distribution of the photons give us information of the universe 380 000 years after the Big Bang. Information about earlier times can, in principle, be derived from the Cosmic Neutrino Background (CvB). The neutrinos decouple already 1 second after the Big Bang at a temperature of about 10(10) K. Today their temperature is similar to 1.95 K and the average density is 56 electron-neutrinos per cm(3). Registration of these neutrinos is an extremely challenging experimental problem which can hardly be solved with the present technologies. On the other hand it represents a tempting opportunity to check one of the key element of the Big Bang cosmology and to probe the early stages of the universe evolution. The search for the CvB with the induced beta decay nu(e) +(3) H -> 3 He + e(-) is the topic of this contribution. The signal would show up by a peak in the electron spectrum with an energy of the neutrino mass above the Q value. We discuss the prospects of this approach and argue that it is able to set limits on the CvB density in our vicinity. We also discuss critically ways to increase with modifications of the present KATRIN spectrometer the source intensity by a factor 100, which would yield about 170 counts of relic neutrino captures per year.