Cet ouvrage et des milliers d'autres font partie de la bibliothèque YouScribe
Obtenez un accès à la bibliothèque pour les lire en ligne
En savoir plus

Partagez cette publication

Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
Put forward by
Dipl.-Phys. Werner Maneschg
Born in: Brunico, Italy
Oral examination: May 11, 2011Low-energy solar neutrino spectroscopy with Borexino:
Towards the detection of the
solar pep and CNO neutrino ux
Referees:
Prof. Dr. Wolfgang Hampel
Prof. Dr. W Kr atschmerLow-energy solar neutrino spectroscopy with Borexino:
Towards the detection of the solar pep and CNO neutrino ux
Borexino is a large-volume organic liquid scintillator detector of unprecedented high radiopurity
which has been designed for low-energy neutrino spectroscopy in real time. Besides the main ob-
7jective of the experiment, the measurement of the solar Be neutrino ux, Borexino also aims at
detecting solar neutrinos from the pep fusion process and from the CNO cycle. The detectability
of these neutrinos is strictly connected to a successful rejection of all relevant background compo-
nents. The identi cation and reduction of these background signals is the central subject of this
dissertation. In the rst part, contaminants induced by cosmic-ray muons and muon showers were
11analyzed. The dominant background is the cosmogenic radioisotope C. Its rate is10 times
higher than the expected combined pep and CNO neutrino rate in the preferred energy window of
11 11observation at [0.8,1.3] MeV. Since C is mostly produced under the release of a free neutron, C
can be tagged with a threefold coincidence (TFC) consisting of the muon signal, the neutron cap-
11ture and the subsequent C decay. By optimizing the TFC method and other rejection techniques,
11a C rejection e ciency of 80% was achieved. This led to a neutrino-to-background ratio of 1:1.7,
whereby 61% of statistics is lost. The second part of the work concerns the study of the external
208background. Especially long-range 2.6 MeV gamma rays from Tl decays in the outer detector
parts can reach the scintillator in the innermost region of the detector. For the determination of
228the resultant spectral shape, a custom-made5 MBq Th source was produced and an external
11calibration was carried out for the rst time. The obtained calibration data and the achieved C
rejection e ciency will allow for the direct detection of solar pep and possibly also CNO neutrinos
with Borexino.
Spektroskopie niederenergetischer Sonnenneutrinos in Borexino:
Analysen zum Nachweis des solaren pep- und CNO-Neutrino usses
Borexino ist ein gro volumiger Detektor, der mit organischem Flussigszin tillator von einer bisher
noch nie erreichten geringen Eigenradioaktivit at gefullt ist und fur die Echtzeitspektroskopie
niederenergetischer Neutrinos konzipiert wurde. Neben dem Hauptziel des Experiments, der Mes-
7sung des solaren Be Neutrino usses, wird auch der Nachweis von Sonnenneutrinos aus dem pep-
Fusionsprozess und dem CNO-Zyklus angestrebt. Die Nachweisbarkeit dieser Neutrinos h angt von
der erfolgreichen Unterdruc kung aller relevanten Untergrundkomponenten ab. Die Identi zierung
und Reduktion verschiedener Untergrundsignale ist das Hauptthema dieser Dissertation. Im er-
sten Teil der Arbeit werden myon-induzierte Untergrund e analysiert. Der dominierende Unter-
11grund ist das kosmogene Radioisotop C, dessen Rate10 mal h oher ist als die erwartete pep-
11und CNO-Neutrinorate im bevorzugten Beobachtungsfenster von [0.8,1.3] MeV. Da C meistens
11unter Emission eines Neutrons entsteht, kann C ub er eine Dreifachkoinzidenz (DFK), bestehend
11aus dem Myon-Signal, dem Neutroneinfang und dem C-Zerfall identi ziert werden. Die DFK-
11Methode und weitere Techniken zur Unterdruc kung von C wurden optimiert, dadurch wurde
11eine C-Unterdruc kungse zienz von 80% und ein Neutrino-zu-Untergrund-Verh altnis von 1:1.7
erreicht. Dabei geht 61% der Statistik verloren. Der zweite Teil der Arbeit besch aftigt sich mit
der Untersuchung des externen Untergrundes. Vorwiegend langreichweitige 2.6 MeV Photonen, die
208durch Tl Zerf alle in den au eren Detektorkomponenten emittiert werden, k onnen den Szintilla-
tor im inneren Bereich des Detektors erreichen. Um die spektrale Form des externen Untergrundes
228zu bestimmen, wurde eine5 MBq Th-Quelle eigens angefertigt und damit erstmals eine ex-
terne Kalibration durchgefuhrt. Die gewonnenen Kalibrationsdaten werden zusammen mit den
11optimierten C-Unterdruc kungsmethoden den direkten Nachweis solarer pep- und wom oglich auch
CNO-Neutrinos in Borexino erm oglichen.vi CONTENTS
Contents
1 Introduction 1
2 Neutrinos and solar physics 3
2.1 Neutrinos: a door to physics beyond the Standard Model . . . . . . . . . . . . . . . 3
2.1.1 Neutrino oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.2 Open questions in the neutrino sector . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Own analysis: potential of near-future neutrino mass experiments . . . . . . 9
2.2 Solar physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 The Standard Solar Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Solar fusion reactions and solar neutrinos . . . . . . . . . . . . . . . . . . . . 13
2.2.3 Helioseismology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Borexino: physics goals and rst results . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Challenge for the pep and CNO neutrino analysis with Borexino . . . . . . . . . . . 23
3 The Borexino experiment 27
3.1 Description of the detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.1.1 Conceptual design of the Borexino detector . . . . . . . . . . . . . . . . . . . 27
3.1.2 Detection principle and optical properties of the scintillator . . . . . . . . . . 30
3.1.3 Signal processing and event generation . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Data analysis tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.1 Data reconstruction codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.2 Simulation tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3 Achieved background levels in Borexino . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 Detector stability and operations on the detector . . . . . . . . . . . . . . . . . . . . 40
3.4.1 Operations on the scintillator . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.4.2 Duty cycle calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4 Analysis of muon-induced backgrounds 44CONTENTS vii
4.1 Prompt muon signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.1.2 Muon detection and muon rate estimation . . . . . . . . . . . . . . . . . . . . 45
4.1.3 Muon track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1.4 Analysis of the radial distribution of cosmic-ray muons . . . . . . . . . . . . . 53
4.1.5 Analysis of the angular distribution of cosmic-ray muons . . . . . . . . . . . . 55
4.2 Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.2 Detection of muon-induced neutrons in Borexino . . . . . . . . . . . . . . . . 57
4.2.3 Analysis of the reconstructed energy of muon-induced neutron clusters . . . . 60
4.2.4 Neutron rate and neutron multiplicities . . . . . . . . . . . . . . . . . . . . . 63
4.2.5 Position reconstruction and lateral distribution of neutrons . . . . . . . . . . 67
4.2.6 Propagation of neutrons in the Borexino scintillator . . . . . . . . . . . . . . 70
241 94.2.7 Analysis: Propagation of neutrons from an Am Be source . . . . . . . . . 74
4.2.8 Analysis: of muon-induced neutrons . . . . . . . . . . . . . . . . 80
4.3 Cosmic-ray induced radioisotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3.1 Production ofes through cosmic-ray muons and showers . . . . . 86
114.3.2 The threefold coincidence method and classi cation of C coincidences . . . 88
114.3.3 Procedure for the rate estimation of C decays via the TFC method . . . . . 90
114.3.4 Determination and analysis of pure C data sets . . . . . . . . . . . . . . . . 93
114.3.5 C rejection techniques and optimization strategies . . . . . . . . . . . . . . 98
114.3.6 Applying the C subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
114.3.7 Other background components besides C . . . . . . . . . . . . . . . . . . . 109
5 Analysis of the external background 113
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.1.1 Origin of the external background . . . . . . . . . . . . . . . . . . . . . . . . 113
5.1.2 Motivation for an external calibration . . . . . . . . . . . . . . . . . . . . . . 116
5.1.3 The external calibration system . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.2 Production and characterisation of an external source . . . . . . . . . . . . . . . . 118viii CONTENTS
5.2.1 Requirements to the source . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
2285.2.2 Production of a custom-made 5.4 MBq Th source . . . . . . . . . . . . . . 120
5.2.3 Encapsulation and sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
2285.2.4 Characterisation of the Borexino Th source . . . . . . . . . . . . . . . . . . 124
2285.2.5 Full characterisation of the custom-made Th . . . . . . . . . . . . . . . . . 128
5.3 First external calibration campaign in Borexino . . . . . . . . . . . . . . . . . . . . . 129
5.4 Analysis of energy and position reconstruction . . . . . . . . . . . . . . . . . . . . . 130
5.4.1 Data selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.4.2 Analysis of the energy spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5.4.3 Analysis of the radial distribution . . . . . . . . . . . . . . . . . . . . . . . . 144
5.5 Comparison of the calibration data with Monte Carlo simulations . . . . . . . . . . . 150
2285.5.1 Simulation of the point-like Th source . . . . . . . . . . . . . . . . . . . . . 150
5.5.2 Spectral shape information from the calibration data . . . . . . . . . . . . . . 153
6 Summary 156
A Appendix 158
A.1 Muontrack reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
A.2 Borexino: background and results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
A.3 Radioactive decay chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
A.4 Reference tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
A.5 plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173o
g¨n.
sel nhn

M€

,
Â
¡li
n
jo
fhsÈ
t˜n
n
màn
Šndrec

dikast
n
aÐ,
n
âpeÈ
nai,


1 Introduction
No, by Zeus, judges, since he says
that the sun is a stone and the moon is earth.
Plato, Apol. D 26
Our Sun has been often referred to as the Rosetta Stone of astrophysics. Due to its proximity to
our home planet, astronomers have the opportunity to study in detail dynamic processes occurring
in the visible outer stellar layers and to record vast spectroscopic data from the solar photosphere.
Over the last decades, the detection of solar neutrinos and helioseismogical measurements has pro-
vided information about the interior of the Sun. Several theoretical models of the solar structure
were developed, they make use of some of this information (as boundary conditions) and predict
subsequent observables.
The so-called Standard Solar Model (SSM) which is based on the thermal and mechanical equi-
librium, the nuclear energy production and the observed chemical abundances in the photosphere,
also makes predictions about the solar neutrino uxes. Since the early beginnings, solar neutrino
experiments have faced several discrepancies between the SSM predicted and measured solar neu-
trino uxes, known as the \solar neutrino problems" [1, 2, 3, 4]. The puzzle was nally solved
by the non-astrophysical explanation of the existence of neutrino oscillations, which provided the
rst hint of physics beyond the Standard Model of elementary particle physics. This discovery
has led to a rapidly evolving eld of neutrino physics which includes elementary particle physics,
astrophysics, cosmology and geology.
The work presented in this thesis has been carried out in the framework of the solar neutrino
experiment Borexino. It is located deep underground at the Laboratori Nazionali del Gran Sasso
in Middle Italy and has collected data since May 2007. The detector is lled with 300 tons of
organic liquid scintillator of unprecedented radiopurity, which has the unique possibility to probe
solar neutrinos in real time and in the sub-MeV region, but also to detect geo-, reactor- and
supernova-antineutrinos.
7 8Besides the already measured solar Be and B neutrino uxes, Borexino also has the potential in
principle to measure neutrinos from the pep fusion reactions and from the CNO cycle taking place
in the solar core. Both neutrino uxes can be observed in Borexino in the same energy window and
they are of special interest. On the one hand, the pep neutrino rate is almost model-independently
related to the rate of the pp neutrinos, which constitute90% of the total solar neutrino ux.
On the other hand, the CNO cycle is expected to contribute only1.5% to the total solar energy
production, but to become dominant in heavy and late-stage stars. Thus both measurements are
of paramount importance for solar physics and the development of a theory of the structure and
evolution of stars.
The objective of this thesis was the identi cation, characterisation and rejection of backgrounds in
Borexino, aiming at detecting the solar pep and CNO neutrinos.
A primary background is induced by the residual muon ux at the experimental site. Muons and
muon showers crossing the scintillator can produce radioisotopes in large quantities which mimic
the neutrino signals. As shown in this work, a detailed data analysis was carried out in order to
describe the muons and muon-induced signals. Based on this a sophisticated strategy,
including a special coincidence method, has been applied and optimized to suppress the muon-

Un pour Un
Permettre à tous d'accéder à la lecture
Pour chaque accès à la bibliothèque, YouScribe donne un accès à une personne dans le besoin