| Troitsk Neutrino Mass Experiment - Spectrometer. The schematic diagram of the Troitsk setup is given in Fig.1. An essential part of the spectrometer and of the tritium source is a set of superconducting coils, which produce a continuous longitudinal magnetic field through the whole setup so that configuration of the spectrometer magnetic field is a magnetic bottle with enormous bottleneck ratio. The source magnetic field intensity is shown in the Fig.1. The cylindrical electrode in the central part of the spectrometer is an integral electrostatic analyzer. All the magnetic and electrostatic fields are adjusted in such a way so as to ensure the adiabaticity of the electron motion through the source and through the spectrometer along the magnetic field lines. Therefore, the electron detector (a Li-drifted silicon counter 20 mm in diameter) detects only those electrons which have been produced in the magnetic field flux tube crossing the detector surface. This flux tube nowhere touches the walls except rear part of the source. The background rate proved to be independent on the spectrometer voltage. The latter enabled one to make measurements of the tritium spectrum by scanning only the potential of the spectrometer. The spectrometer resolution function is presented by the step function with almost linear slope from 0 to 1. In most of the measurements the width of the slope or energy resolution was adjusted to be 3.7 to 4.7eV (FW) at 18.6 keV by proper choice of the magnetic fields in the spectrometer. The spectrometer luminosity was 0.27 sq.cm in the first runs of measurements and 0.22 sq.cm in the last runs. The gaseous tritium is injected at the center of a 3 meter-long tube inside the solenoids and is being pumped out by mercury diffusion pumps installed at both ends of the tube. After some additional compression in the buster mercury pump and after purification in a system of the hot getter tritium returns to the injection point thus providing continuous circulation. The temperature of the tritium tube is maintained at 26 to 28 K. The efficiency of the detector was almost 100% within the window from 6 to 20 keV. The high efficiency of the detector is due to the electrons scattered backwards by the detector surface with partial loss of their energy being returned back by the field of the electrostatic analyzer, and bounce in this manner until they are fully absorbed. The superconducting system was cooled by a helium refrigerator of about 60 W cooling power (at 4.6K) working on-line in the two-phase circulation regime. The main vacuum pumping in the spectrometer was done by the cold surfaces of the cryostats. Fig.1 Experimental set-up. 1, 2 vacuum tank; 3, 4 electrostatic analyzer; 5 grounded electrode;
6, 7, 8, 9 superconducting coils; 10 warm coil; 11 N2 jacket; 12 Si(Li)
detector; 13 fast shutter; 14 Ti-pump ; 15 cold valve; 16 Hg diffusion
pump; 17 T2 purification system; 18 electron gun; 19 argon pump.
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