Experiment at Mainz
The Mainz Neutrino Mass Experiment
- Introduction -
The electrons, which are starting from the tritium source (light blue rectangular) in the left solenoid into the forward hemisphere, are guided magnetically on a cyclotron motion around the magnetic field lines (blue lines) into the spectrometer, thus resulting in an accepted solid angle of nearly 2 .
On their way into the middle of the spectrometer the magnetic field drops by nearly 4 orders of magnetitude. Therefore the magnetic gradient force transforms most the cyclotron energy into longitudinal motion. This is illustrated in the lower part of the figure by a momentum vector. Due to the slowly varying magnetic field the momentum transforms adiabatically, therefore the magnetic moment keeps constant:
Summarizing this transformation:
the electrons, isotropically emitted at the source, are transformed into a broad beam of electrons flying almost parallel to the magnetic field lines.
This parallel beam of electrons is running against an electrostatic potential made up by a system of cylindrical electrodes (green in figure 1). All those electrons, which have enough energy to pass the electrostatic barrier are reaccelerated and collimated onto a detector, all the other electrons are reflected. Therefore the spectrometer acts as an integrating high-energy pass filter. The relative sharpness of this filter is only given by the ratio of the minimum magnetic field in the middle plane and the maximum magnetic field between electron source and spectrometer:
By scanning the electrostatic retarding potential the spectrum can be measured.
As tritium source the Mainz Neutrino Mass Experiment uses a film of molecular tritium quench-condensed onto graphite subtrate (HOPG). The film has a diameter of 17 mm and a typical thickness of 40 nm, which is measured by laser ellipsometry.
In the years 1995-1997 the Mainz setup was upgraded. A new doublett of superconducting solenoides was installed between tritium source and spectrometer (see figure 2). By its inner LHe cooled cryotrap it decouples the vacua of both parts of the setup. electrons from the source are guided magnetically around the corner without losses. To the contrary tritium molecules, which are evaporating from the source and which were producing the major part of the background before, now are frozen out at the cryotrap.
As second substantial improvement a new cryostat now provides temperatures of the tritium film below 2 K to avoid a roughening transition of the film, which was a problem of earlier Mainz measurements. The roughening process is a temperature activated surface diffusion process, therefore low temperatures are necessary to get time constants much longer than the measurement duration. The film is kept at a temperature of 1.86 K within a few hundreds of a Kelvin over several month.
The full automation of the apparatus and remote control allows to perform long term measuremnts of several months per year.
- Results -
which is compatible with a neutrino mass of zero. Considering its uncertainties this value corresponds to an upper limit on the electron neutrino mass of:
These values and an alternative analysis
of the data were presented at the international conference NEUTRINO 2000. They represent the world's best sensitivity on a neutrino mass in a direct neutrino mass experiment.
It should be mentioned that in the very likely case of neutrino mixing our measured value of the electron neutrino mass corresponds to an average over all neutrino mass eigenstates contributing to the electron neutrino according to its mixing |Uei2| (more correctly: if the mass eigenstates are not resolved by the experiment):
Our very precise data on the tritium spectrum can also be used to test the "Troitsk-anomaly", a small excess of counts near the endpoint, as reported by the Troitsk neutrino mass experiment. The Mainz data support the Troitsk hypothesis only partly, a final answer cannot be given yet. However, the postulated half year period of the "Troitsk-anomaly" is contradicted by our data.
- Outlook on a large tritium spectrometer with sub-eV sensitivity -
To reach a sub-eV sensitivity a larger spectrometer providing higher signal rate and better energy resolution is needed. Our simulations have shown that a spectrometer with 7 m in diameter (see figure 4) providing an about 100 times larger effective cross section of the analysing plane compared to the Mainz spectrometer would reach a sensitivity on the neutrino mass of below 0.4 eV. Such an experiment would make profit not only from a gaseous molecular tritium source as the present Troitsk experiment but also from a quench-condensed molecular tritium source as the Mainz experiment. The advantage of having both sources is that they have complementary systematic uncertainties and would allow to check each other. By an additional time-of-flight analysis the spectrometer can transform from an integrating high pass filter into a narrow band filter (MAC-E-TOF mode) which enables to improve the investigations of systematics and to check the tritium decay spectrum for local anomalies with unprecedented precision. In a first proof of principle experiment this new method was proven to work with the present Mainz spectrometer (J. Bonn et al, Nucl. Inst. and Meth. A421 (1999) 256).
Currently the feasibility and the physical prospects of such a large tritium spectrometer with 7 m diameter is being discussed by the neutrino groups of Karlsruhe, Mainz and Troitsk. An ideal place for such an experiment would be the Forschungszentrum Karlsruhe/Germany. The prospects and the scientific case of this future experiment is going to be discussed at an international workshop at Bad Liebenzell/Germany in January 2001.
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