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[en] Direct neutrino mass experiments are complementary to searches for neutrinoless double β-decay and to analyses of cosmological data. Up to recently all direct neutrino mass experiments have been performed with tritium. Starting with the pioneering experiments by Curran, Angus and Cockroft as well as by Hanna and Pontecorvo the last tritium beta decay experiments at Mainz and at Troitsk have achieved upper limits on the neutrino mass of about 2 eV/c2. The KATRIN experiment under construction will improve the neutrino mass sensitivity down to 200 meV/c2 by increasing strongly the statistics and – at the same time – reducing the systematic uncertainties. Commissioning measurements with half of the KATRIN experiment, the huge main spectrometer and the detector, have been performed just recently. As an alternative to tritium β-decay experiments cryobolometers investigating the endpoint region of 187Re β-decay or of the electron capture of 163Ho are being developed.
[en] The KArlsruhe TRItium Neutrino experiment will measure the endpoint of the tritium-β-spectrum by means of an electrostatic retarding spectrometer (MAC-E-Filter), which will allow determination of the mass of the anti νe with 200 meV/c2 sensitivity (90% C.L.). To achieve this, it is necessary to monitor the retarding voltage of the spectrometer with 3ppm precision, which is realized using a combination of a high voltage divider and a monoenergetic source of conversion electrons (83mKr). For this purpose, a condensed Krypton source (CKrS) will be installed at the Cryogenic Pumping Section (CPS) of the KATRIN experiment. The CKrS will be mounted on an UHV manipulator to be able to scan the whole flux tube of KATRIN. To control the thickness of the condensed gas in the source, a specialized ellipsometry setup with analyser and detector in the vacuum has been developed. Another upgrade to the CKrS is the installation of an ablation laser, to remove residual gas contaminations from the substrate.
[en] The absolute scale of neutrino masses is very important for understanding the evolution and the structure formation of the universe as well as for nuclear and particle physics beyond the present Standard Model. Complementary to deducing statements on the neutrino mass from cosmological observations, two different methods to determine the neutrino mass scale in the laboratory are pursued: the search for neutrinoless double β decay and the direct neutrino mass search. For both methods currently experiments with a sensitivity of O(100) meV are being set up or commissioned.
[en] Windowless silicon photodiodes of type Hamamatsu S172308 special and S3590-06 with dimensions 10 x 10 x 0.5 mm3 have been found to be suitable as detectors for 18.6 keV electrons from the endpoint region of the tritium β-spectrum at the Mainz neutrino rest mass experiment. To reduce the leakage current of the photodiode and the various temperature dependent noise contributions, the diode and the first stage of the charge sensitive preamplifier have been cooled with liquid nitrogen. At a temperature T = -97deg C an energy resolution of 1.1 keV (FWHM) has been achieved for 60 keV γ-rays and 1.6 keV (FWHM) for 22 keV electrons. The density thickness product px of the dead layer of the entrance window has been determined by energy loss measurements of monoenergetic electrons to be 45 μg/cm2. (orig.)