The Mainz Neutrino Mass Experiment          

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The Mainz Neutrino Mass Experiment

- Introduction -

To become sensitive to the mass of the electron antineutrino by measuring the endpoint region of the tritium decay spectrum a spectrometer of both a high energy resolution and a high luminosity is essential. At Mainz a new type of spectrometer was developed especially for this purpose. It is called MAC-E-Filter from Magnetic Adiabatic Collimation followed by an Electrostatic Filter. The main features are illustrated in figure 1: Two superconducting solenoids (marked as rectangulars with crosses inside) are producing a magnetic guiding field.

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 -

By various investigations we improved at the same time the systematic uncertainties significantly. The main systematic uncertainties are originating from the physics and the properties of our quench-condensed tritium film:
  • Roughening transition of the tritium film:
    Together with the group of P. Leiderer/Konstanz we investigated the roughening transition of quench-condensed molecular hydrogen films, its speed dependence on the temperature and on the hydrogen isotope (L. Fleischmann et al., J. Low Temp. Phys. 119 (2000) 615, L. Fleischmann et al., Eur. Phys. J. B16 (2000) 521). These measurements showed, that this process does not play any role anymore at such low temperature as 1.86 K, at which the tritium films are prepared and kept since the upgrade of the Mainz setup. Our recent tritium data proof the validness of this statement.

  • Inelastic scattering of electrons within the tritium film:
    We improved our film thickness measurement by laser ellipsometry down to a relative uncertainty of a few percent. Together with the group from the neutrino mass experiment at Troitsk/Russia we measured the energy loss function of electrons in the tritium film by dedicated measurements with conversion electrons from a 83mKr source (Aseev et al., Eur. Phys. D10 (2000) 39). We found some descrepancies between solid and gaseous tritium, which are in good agreement with quantum chemical calculations made by A. Saenz (also: Aseev et al., Eur. Phys. D10 (2000) 39).

  • Neighbour excitation:
    In the case of a quench-condensed tritium film in a few percent of the decays the electron shell of neighbour molecules is excited due to the fast nuclear charge change during the decay. The small differences observed between the energy loss functions of solid and gaseous tritium have to be applied for the quench-condensed tritium film.

  • Self-charging of the tritium film:
    We observed a self-charging of the tritium film resulting in a difference of the electric potential of about 3 V between top and bottom layer of the tritium film. This effect is due to the fact that about 1 billion electrons are leaving the tritium film per second, the positive ions are remaining. By dedicated studies (H. Barth et al., Prog. Part. Nucl. Phys. 40 (1998) 353) with conversion electrons we characterised this effect, its size and time dependence.

The data of the last four runs of 1998 and 1999 does not show any residual problem (see the region close to the endpoint figure 3). The most sensitive analysis on the neutrino mass, in which the last 70 eV of the spectrum below the endpoint is used only, gives

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 -

By collecting more data by the Mainz Neutrino Mass Experiment a sensitivity on a neutrino mass of about 2 eV/c2 can be reached. This is not enough to clarify the urgent open questions for particle physics (determing the neutrino mass scale and making a distinction between different neutrino mass scenarios) and for cosmologically (clarification the significance of neutrinos for dark matter and their role in structure formation) mentioned in the introduction. For this goal a sub-eV sensitivity on the neutrino mass is needed. At present the only direct way to probe neutrino masses with this sensitivity is still the investigation of the endpoint region of tritium decay. And still the best experimental approach is following the concept of a MAC-E-Filter, which was developed and run by the neutrino mass experiments at Mainz and Troitsk with great success.

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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