Accurate Calibration of Raman Systems: At the Karlsruhe Tritium Neutrino Experiment

226 Pages·2014·6.25 MB·English

by  Magnus Schlösser (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Magnus Schlösser
Accurate
Calibration of
Raman Systems
At the Karlsruhe Tritium
Neutrino Experiment
Springer Theses
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Magnus Schlösser
Accurate Calibration
of Raman Systems
At the Karlsruhe Tritium Neutrino
Experiment
Doctoral Thesis accepted by
Karlsruhe Institute of Technology, Germany
123
Author Supervisor
Dr. MagnusSchlösser Prof.GuidoDrexlin
Tritium LaboratoryKarlsruhe InstituteforExperimental Nuclear Physics
InstituteforTechnical Physics Karlsruhe Instituteof Technology
Karlsruhe Instituteof Technology Eggenstein-Leopoldshafen
Eggenstein-Leopoldshafen Germany
Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-06220-4 ISBN 978-3-319-06221-1 (eBook)
DOI 10.1007/978-3-319-06221-1
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Supervisor’s Foreword
Inourquestforunderstandingtheuniverseanditsmattercontentneutrinosplaya
key role: they are, arguably, the most fascinating elementary particles known as
manyoftheirintrinsicpropertiesareunknown.Theopenquestionoftheabsolute
neutrino mass scale will be addressed with unprecedented accuracy by the
Karlsruhe Tritium Neutrino (KATRIN) experiment, currently under construction.
This thesis focusses on solving one of the grand challenges of high precision
tritium b-decay spectroscopy: to continuously measure the isotopic content of an
ultra-luminouswindowlessmoleculargaseoustritiumsource.Theextensiveworks
of this thesis beautifully demonstrate that laser-based Raman spectroscopy meets
thestringentbenchmarksofachievingaprecisionof0.1 %withsamplingtimesas
short as 60 s over the entire KATRIN data taking period of five calendar years.
Core among the many achievements is an accurate calibration of the Raman
system by two independent methods over a broad range of wavelengths covering
allsixmolecularhydrogenisotopologues.Therigorouscombinationofexperiment
and theory of this thesis allows the KATRIN experiment to explore sub-eV
neutrino masses with minimized systematics, while also opening novel applica-
tions for Raman spectroscopy.
Eggenstein-Leopoldshafen, February 2014 Prof. Guido Drexlin
v
Abstract
NeutrinosarebyfarthelightestfermionsintheStandardModelofparticlephysics
and also the most numerous fermionic particles in the universe. Originally, they
were believed to be massless. However, later neutrino oscillation experiments
indicated that neutrinos actually carry (some very small) mass, making them the
lightest fermions in the Standard Model of particle physics. Their absolute mass
scale is highly relevant both in particle physics and cosmology.
Several methods for measuring the neutrino mass scale exist of which high-
precisionelectronspectroscopyofthetritiumb-decayisthemostsensitive,model-
independent method today. Within the context of the said method, the Karlsruhe
Tritium Neutrino experiment, KATRIN, is the next-generation direct neutrino
mass experiment. It is targeted at improving the current experimental sensitivity
realized in the Mainz and Troitsk experiments of the late 1990s, from 2 eV/c2
down to 200 MeV/c2 (90 % C.L.).
This can only be achieved if systematic uncertainties are minimized; a key
parameteristheisotopiccompositionoftritiumgasinthewindowlesssource.This
composition needs to be monitored inline and in near-time, and Raman
spectroscopy was selected as the method of choice, being non-destructive and
non-contact. For the KATRIN experiment to achieve the aforementioned
sensitivity, the actual source gas composition needs to be determined on short
sampling time scales ofthe order ofoneminutewith atruenessbetter than 10 %,
and a precision of 0.1 %. This implies that the Raman source monitoring
measurements need to mimic or better these boundary conditions; and it is
essential that they are met for the full range of hydrogen isotopologues (H ,
2
HD, D , HT, DT, and T ) encountered in the source. Consequently, accurate
2 2
calibration procedures are paramount, and it is this aspect which is central to this
thesis.
Within the frameworkof thisthesis,several(independent)calibrationmethods
have been studied. Two were identified as being the most promising methods,
yielding excellent, complementary results: (i) a gas sampling technique, which
encompasseshighaccuracy,butwhichisdifficulttoapplytotritiatedspecies;and
(ii) an approach via theoretical Raman signals (theoretical intensities from
quantum theory plus spectral sensitivity of the Raman system), which covers all
six isotopologues. Both methods exhibited their individual merits and difficulties;
however, in cross-calibration test they proved to be very successful.
vii
viii Abstract
For the first approach, a custom-made mixing device for H , HD, D was
2 2
employedandthestatisticalandsystematicuncertaintiesofthecalibrationmethod
were thoroughly investigated.
For the latter method, ab initio transition matrix elements adapted from the
literature were incorporated. These are associated with the molecular polariz-
ability, and the theoretical predictions have been verified by accurate Raman
depolarization measurements. For this a (two-step) correction model has been
developed, which was applied successfully and which was capable to account for
all aberrations related to the experimental setup. In addition, the spectral
sensitivity of the Raman system needed to be determined. For this a NIST-
traceable luminescence standard (with very small calibration uncertainty) was
adapted to almost perfectly replicate the Raman light excitation geometry.
Comparative studies of the two above methodologies were carried out for the
non-radioactive hydrogen isotopologues (H , HD, D ); the results yielded an
2 2
agreement of better than 2 % for the relative Raman response functions. This is
less than the estimated (dominant) uncertainty of the theoretical Raman signal
approach of about 3 %. These results suggest that the trueness requirement of
10 % for the species with high relevance for KATRIN (T , DT, D and HT) will,
2 2
in all likelihood, be exceeded.
Thecalibrationapproachesdevelopedwithintheframeworkofthisthesisoffer
great potential to be applicable to other applications with conditions and
requirements similar to KATRIN. This includes nuclear fusion power plants
where tritium and deuterium are used as fusion fuel; or the real-time monitoring
and control of environmental gases and combustion processes.
In summary, the results obtained in the research work underlying this thesis
have contributed in a significant way to implement a Raman analysis system,
which exceeds the precision and trueness requirements for KATRIN. This means
that the properties of the gaseous source of KATRIN experiment are well-
understood which is an essential prerequisite in the envisaged high-precision
analysis of the neutrino mass data. This therefore will help to unravel one of the
most fascinating open issues of astroparticle physics—the intrinsic nature of
neutrinos.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Discovery of the Neutrino . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Neutrino Oscillations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Role of Massive Neutrinos. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1 Particle Physics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.2 Cosmology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4 Measurement of the Neutrino Mass. . . . . . . . . . . . . . . . . . . . . 14
1.4.1 Indirect Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4.2 Direct Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2 The KATRIN Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.1 Tritium (cid:2)-decay Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.1.1 Tritium (cid:2)-decay and the Neutrino Mass. . . . . . . . . . . . . 31
2.1.2 The MAC-E-Filter Measurement Principle. . . . . . . . . . . 34
2.1.3 Results of Previous Neutrino Mass Experiments
at Mainz and Troitsk. . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2 The Karlsruhe Tritium Neutrino Experiment. . . . . . . . . . . . . . . 36
2.2.1 Projected Sensitivity on Neutrino Mass . . . . . . . . . . . . . 37
2.2.2 Experimental Overview. . . . . . . . . . . . . . . . . . . . . . . . 38
2.3 Properties of the WGTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.3.1 Column Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.3.2 Isotopologue Composition . . . . . . . . . . . . . . . . . . . . . . 42
2.4 Accuracy Requirements for the Monitoring
of the Source Gas Composition. . . . . . . . . . . . . . . . . . . . . . . . 43
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3 Theory of Quantitative Raman Spectroscopy . . . . . . . . . . . . . . . . 53
3.1 Introduction to Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . 54
3.1.1 Rotational and Vibrational States
in Diatomic Molecules. . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1.2 Basic Principles of the Raman Effect . . . . . . . . . . . . . . 58
3.1.3 Description of Raman Intensities . . . . . . . . . . . . . . . . . 61
ix
x Contents
3.2 Raman Spectroscopy on Hydrogen Isotopologues . . . . . . . . . . . 65
3.2.1 Pre-Requisites of the Experimental Systems. . . . . . . . . . 66
3.2.2 Theoretical Values for the Polarizability . . . . . . . . . . . . 67
3.2.3 Previous Raman Studies on All Hydrogen
Isotopologues Including Tritium. . . . . . . . . . . . . . . . . . 68
3.3 Calibration for Quantitative Analysis. . . . . . . . . . . . . . . . . . . . 69
3.3.1 Possible Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.3.2 Calibration Strategy for the KATRIN LARA System . . . 71
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.1 Raman System Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.1.1 Overview of the Scheme of Raman Measurements . . . . . 75
4.1.2 Raman Systems at the TLK . . . . . . . . . . . . . . . . . . . . . 76
4.1.3 Components of the Raman Systems . . . . . . . . . . . . . . . 78
4.2 Spectrum Acquisition, Processing and Analysis. . . . . . . . . . . . . 85
4.2.1 Data Acquisition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.2.2 Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.3 Analysis of Actual System Performance. . . . . . . . . . . . . . . . . . 95
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5 Calibration Based on Theoretical Intensities
and Spectral Sensitivity (Method I). . . . . . . . . . . . . . . . . . . . . . . . 101
5.1 Motivation and Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.2 Calculation of Theoretical Intensities. . . . . . . . . . . . . . . . . . . . 102
5.3 Verification of Theoretical Intensities
via Depolarization Measurements . . . . . . . . . . . . . . . . . . . . . . 106
5.3.1 Definition of the Depolarization Ratio. . . . . . . . . . . . . . 107
5.3.2 Model of Depolarization Ratios as Observed
in the Experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.3.3 Routine for Correcting Observed
Depolarization Ratios . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.3.4 Measurement of Depolarization Ratios
of All Six Hydrogen Isotopologues. . . . . . . . . . . . . . . . 119
5.4 Measurement of the System’s Spectral Sensitivity. . . . . . . . . . . 132
5.4.1 Requirements for the Determination
of the Spectral Sensitivity . . . . . . . . . . . . . . . . . . . . . . 132
5.4.2 Possible Calibration Sources for the Measurement
of the Spectral Sensitivity . . . . . . . . . . . . . . . . . . . . . . 133
5.4.3 NIST-traceable Luminescence Standard SRM 2242. . . . . 135
5.4.4 Resulting Spectral Sensitivity and Discussion. . . . . . . . . 143
5.5 Discussion of the Calibration Results. . . . . . . . . . . . . . . . . . . . 144
5.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147