KATRIN Proposal _ 320 KATRIN – A next generation tritium beta by sofiaie


									KATRIN                                         Proposal # 320
 KATRIN – A next generation tritium beta-decay experiment with sub-eV
              sensitivity for the electron neutrino mass

 M. Charlton1*, A.J. Davies1, R.J. Lewis1, H.H. Telle1, D.L. Wark2, J. Tennyson3, N. Doss3,
P.T. Greenland3, P.J. Storey3, R.J. Reid4, J.D. Herbert4, O.B. Malyshev4 and K.J. Middleman4
           Department of Physics, University of Wales Swansea, Singleton Park, Swansea SA2 8PP
                 CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX
     Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
                   ASTeC, CCLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD
                                          UK principal investigator

Overview of KATRIN
KATRIN [1] will attempt a determination of the rest mass of the electron (anti-) neutrino by
precision measurement of the shape of the tritium β-decay spectrum in the vicinity of its
endpoint. There has been a long history of tritium endpoint experiments. Most recently, the
experiments at Mainz [2] and Troitsk [3] have used similar technologies to those proposed for
KATRIN, such that most of the important systematic effects can be anticipated and precluded
by appropriate engineering design and physics input. In essence the experiment will measure,
by electrostatic means, the energy dependence of the tritium β-decay electrons with an energy
resolution of the order of 1 eV. Overall sensitivity to the neutrino rest mass will be around
0.2 eV.

Figure 1. Schematic illustration of KATRIN showing the main components of the proposed experiment. The
present Mainz experiment is shown for comparison of scale.

The schematic of KATRIN, shown in figure 1, illustrates the main features of the
experiment. The apparatus will be around 65 m in length (c.f. the current Mainz experiment,
which is around 5 m long), and is grossly divided into source, spectrometer and detector
regions. Energy analysis of the β-spectrum will be performed by the spectrometers. A
preliminary cut will be provided by the pre-spectrometer to restrict the number of charged
particles continuing into the main spectrometer, where the detailed spectrum will be
measured. A purpose-built segmented detector constructed from an array of silicon diode
detectors will be used to count the β-particles. Significant experimental challenges exist in all
these areas and the UK is contributing to several, as detailed below. 2003 was the first full
year that the UK team had involvement in KATRIN, and we are now an integrated part of the
collaboration. The experiment will be constructed at Forschungzentrum Karlsruhe (FZK),
Germany. Key components have been ordered and some have already been delivered and are
under test. Full funding for the main hardware has been approved, including
21.5 M€ allocated by the German Ministry for Education and Research for the period 2003-8.
Gaseous Tritium Source - Monitoring
This source will consist of a 10 m long, 70 mm diameter cylindrical tube fed centrally by a T2
gas supply of high isotopic purity. It is crucial that the gas purity is continuously monitored.
This will be achieved using a laser Raman technique under development at Swansea. A
dedicated Ph.D. student has been allocated to this task. Progress during the last year is
summarised below.

An assessment was made of preliminary Raman data from FZK and improvements necessary
to achieve the required T2 monitoring sensitivity identified.

Test measurements were carried out, using a custom-built Raman light collection assembly,
including an edge-filter (for laser excitation line suppression) and fibre-coupling, in
conjunction with a spectrometer similar to that at FZK. Both a D2-filled gas cell, on loan
from FZK, and a mock-up cell to allow for modifications in excitation / light collection
arrangements were used. Reduction in scattered light for a baffled set-up geometry of the
latter was confirmed. Repeat measurements to clarify the issue of stray-light, encountered in
the original FZK measurements, are planned with the full FZK set-up at Karlsruhe, and will
take place in the first quarter of 2004.

Tests were conducted using a single-mode (SLM) fibre in conjunction with a small 10 mW
CW laser and a multi-mode (MLM) fibre in conjunction with a 10-100 mJ pulsed laser (both
532 nm Nd:YAG lasers). Both could be coupled efficiently, achieving up to 50%
transmission of radiation. However, the beam quality suffered severely using the MLM fibre
such that proper beam collimation is difficult. Hence, the originally proposed set-up with a
pulsed laser might not be viable. However, next-generation CW lasers and improved SLM
fibre technology promise to provide the required Raman excitation photon flux using fibre-
coupled light delivery, thought to be vital for long-term reliability. Test measurements and
discussions with equipment manufacturers are on-going. Furthermore, spectrometer and
detector specifications are now finalised.
Gaseous Tritium Source - Molecular Physics and Modelling
The windowless gas tritium source will be the mainstay of the experiment, and it is crucial to
understand the properties and underlying physics of the source and how these affect the
energetics of the β-particles. Significant work in this area is being undertaken in the
molecular physics group at UCL, and a dedicated Ph.D. student has been appointed.
Following consultation within the wider KATRIN collaboration, major topics requiring
urgent attention were identified, as below.

Deleterious β-spectrum endpoint effects can occur if species other than T2 are present in the
source. An initial study suggests that decays from T3+ (or T2D+ / D2 T+) could influence the β-
distribution in the critical energy range. However, present models suggest that little T3+ will
be formed in the source.

It is necessary to undertake a sensitivity analysis of the final state β-distribution as a function
of source parameters, in particular: isotopic composition, temperature, and the T2 ortho/para
ratio. Work has been started to extend previous, high accuracy, studies to up to 30 eV below
the endpoint to probe the influence of these parameters. It is also necessary to estimate the
likely density of dimers (and possibly higher clusters) in the source. To date, only H2 and D2
have been considered and the results suggest a non-negligible proportion of dimers. This
work will be used to inform a calculation on T2 dimers in due course. Since T2 is heavier, and
therefore supports more dimer states, the concentration of dimers in T2 gas at the source
temperature of 30 K will be higher than in both H2 and D2. It should be possible to obtain
some information on the ortho/para ratio and dimer content from dedicated Raman

Vacuum System
From the gaseous tritium source at 10-3 mbar, to the large vacuum chamber/main
spectrometer at a pressure below 10-11 mbar, KATRIN has exacting vacuum standards and
challenges. In 2003, the CCLRC Accelerator Science and Technology Centre (ASTeC)
through its Vacuum Science Group based at Daresbury Laboratory joined KATRIN on the
strength of its expertise in designing, conditioning and operating large scale UHV systems.
Subsequently, the Group has contributed significantly to two areas of the vacuum aspects of
the project. The first is advising on the vacuum conditioning, test and measurement
programme for the pre-spectrometer vessel recently delivered to FZK. The second is a joint
programme between Prof. Felix Sharipov of Universidade Federal do Parana, Brazil, Dr.
Xueli of FZK and Dr. Oleg Malyshev of ASTeC to model the transmission of tritium through
the differential pumping lines between the source and the spectrometer. This involves
calculations in the difficult transition flow regime. Initial results are encouraging.

Some training of personnel from the University of Karlsruhe in pressure measurement
techniques at XHV has also been given in the Group's Laboratory. The Group will contribute
to the specification for the main spectrometer vessel over the next few months.

The KATRIN Spectrometers – Modelling
The performance of the KATRIN spectrometers is crucial to the success of the experiment. In
particular, the main spectrometer is now envisaged to have a diameter of 10 m and be over
20 m in length. Modelling the fields and particle trajectories in this device, and the pre-
spectrometer, is of vital importance, and work is underway at Swansea.

Initial studies concentrated on upgrading in-house axially-symmetric 2D Boundary Element
codes to work under the Windows operating system on the specially purchased 3 GHz (with 2
GB RAM and 2x120 GB discs) work station. In addition to computing electric fields and
potentials in multi-electrode, multi-dielectric configurations, these codes have been modified
so that they can now trace particle trajectories in relativistic situations and take into account
an arbitrary magnetic field imposed on the volume of space under consideration.

Current work is aimed at finding an efficient way to import the electrode and dielectric data
for the KATRIN pre-spectrometer. This may be carried out by either digitizing hard-copy
printouts of the apparatus or by devising software to extract electrode coordinates from the
Auto-Cad files used to make the drawings. In parallel, evaluation of suitable commercial
software packages for computing electric and magnetic fields and potentials in a full 3D
situation, and for tracing resulting particle trajectories, is being undertaken.

1. See the KATRIN website at www-ik1.fzk.de/tritium
2. Ch. Weinheimer et al., High precision measurement of the tritium β spectrum near its endpoint and upper limit on the neutrino mass.
Phys. Letts B 460 (1999) 219
3. A.I. Belsev et al., Results of the Troitsk experiment on the search for the electron antineutrino rest mass in tritium beta decay. Phys. Letts.
B 350 (1995) 263 and V.M. Lobashev et al., Direct search for mass of neutrino and anomaly in the tritium beta-spectrum. Phys. Letts B 460
(1999) 227

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