The Karlsruhe Tritium Neutrino experiment (KATRIN) aims to measure the electron neutrino mass from the β-decay of tritium with an unprecedented sensitivity of 0.2 eV/c2. A central component is the electro-static spectrometer section, where the energy of the decay electrons will be measured. It comprises two large ultra-high vacuum vessels, the pre-spectrometer with a volume of 8.5 m3 and the main spectrometer with a volume of 1250 m3. Both spectrometers are already on site at Forschungszentrum Karlsruhe where the experiment will take place. Extensive tests of the vacuum performance and electro-magnetic properties are currently conducted.
The electron energy will be analysed by putting a variable electrostatic retarding potential of about -18.6 kV on the vacuum vessel. An inner electrode system with a slightly higher voltage helps to shape the potential inside the vacuum. Only electrons with energies above the retarding potential can pass the spectrometer section and are counted with a silicon detector. The count-rate close to the endpoint of the β-spectrum is very low (a few mHz). Therefore the measurement strongly depends on low background rates. While materials have been carefully selected with regard to low intrinsic radioactivity, another possible sources of background depend on the quality of the vacuum. For example cosmic ray induced electrons from the walls or electrons captured in penning traps can interact with residual gas molecules and can be reflected on the detector. This leads to very stringent requirements on the vacuum, aiming at an ultimate total pressure of below 10-9 Pa and a wall outgassing rate below 10-13 Pa m3/s cm2.
This paper describes the design, manufacturing and first result of vacuum measurements with the large main spectrometer. Outgassing results of the 650 m2 stainless steel surface will be presented. The start of the neutrino measurements is expected for 2010.
|