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Measurement of the electron antineutrino angular correlation coefficient a with the neutron decay spectrometer aSPECT [Elektronische Ressource] / Martin Simson

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Technische Universitat MunchenLehrstuhl fur Experimentalphysik E18Measurement of the electron antineutrino angularcorrelation coe cient a with the neutron decayspectrometer aSPECTMartin SimsonVollst andiger Abdruck der von der Fakult at fur Physik der Technischen Universit atMunc hen zur Erlangung des akademischen Grades eines Doktors der Naturwissen-schaften genehmigten Dissertation.Vorsitzender: Univ.-Prof. Dr. A. J. BurasPrufer der Dissertation: 1. Dr. O. Zimmer2. Univ.-Prof. Dr. K. SchreckenbachDie Dissertation wurde am 31.08.2010 bei der Technischen Universit at Munc heneingereicht und durch die Fakult at fur Physik am 21.09.2010 angenommen.aContents1 Introduction 72 Theoretical background 112.1 Theory of -decay . . . . . . . . . . . . . . . . . . . . . . . . 112.1.1 Fermi’s theory . . . . . . . . . . . . . . . . . . . . . . 122.1.2 V A . . . . . . . . . . . . . . . . . . . . . . 142.1.3 Standard model . . . . . . . . . . . . . . . . . . . . . 152.2 Measurable parameters in neutron decay . . . . . . . . . . . 172.3 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.1 Lepton spectra . . . . . . . . . . . . . . . . . . . . . 212.3.2 Proton spectrum . . . . . . . . . . . . . . . . . . . . 233 Description of the spectrometer 253.1 The concept of adiabatic invariance and the magnetic mirrore ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2 Transmission function . . . . . . . . . . . . . . . . .

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Technische Universitat Munchen
Lehrstuhl fur Experimentalphysik E18
Measurement of the electron antineutrino angular
correlation coe cient a with the neutron decay
spectrometer aSPECT
Martin Simson
Vollst andiger Abdruck der von der Fakult at fur Physik der Technischen Universit at
Munc hen zur Erlangung des akademischen Grades eines Doktors der Naturwissen-
schaften genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. J. Buras
Prufer der Dissertation: 1. Dr. O. Zimmer
2. Univ.-Prof. Dr. K. Schreckenbach
Die Dissertation wurde am 31.08.2010 bei der Technischen Universit at Munc hen
eingereicht und durch die Fakult at fur Physik am 21.09.2010 angenommen.aContents
1 Introduction 7
2 Theoretical background 11
2.1 Theory of -decay . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 Fermi’s theory . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2 V A . . . . . . . . . . . . . . . . . . . . . . 14
2.1.3 Standard model . . . . . . . . . . . . . . . . . . . . . 15
2.2 Measurable parameters in neutron decay . . . . . . . . . . . 17
2.3 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1 Lepton spectra . . . . . . . . . . . . . . . . . . . . . 21
2.3.2 Proton spectrum . . . . . . . . . . . . . . . . . . . . 23
3 Description of the spectrometer 25
3.1 The concept of adiabatic invariance and the magnetic mirror
e ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2 Transmission function . . . . . . . . . . . . . . . . . . . . . 29
3.3 The retardation spectrometer . . . . . . . . . . . . . . . . . 32
3.3.1 Design of the electric elds . . . . . . . . . . . . . . . 33
3.3.2 of the magnetic eld . . . . . . . . . . . . . . 36
3.4 Systematic e ects . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.1 Adiabatic transmission function . . . . . . . . . . . . 39
3.4.2 Non-adiabatic proton motion . . . . . . . . . . . . . 41
3.4.3 Residual gas . . . . . . . . . . . . . . . . . . . . . . . 42
3.4.4 Background . . . . . . . . . . . . . . . . . . . . . . . 44
3.4.5 Doppler e ect due to neutron motion . . . . . . . . . 47
3.4.6 Edge e ect . . . . . . . . . . . . . . . . . . . . . . . . 47
3.4.7 Detection e ciency . . . . . . . . . . . . . . . . . . . 49
3Contents
4 The detection system 51
4.1 A short introduction to semiconductors and semiconductor
detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 The principle of a silicon drift detector . . . . . . . . . . . . 53
4.3 The aSPECT detector . . . . . . . . . . . . . . . . . . . . . 54
4.3.1 The chip . . . . . . . . . . . . . . . . . . . . 54
4.3.2 Mechanical setup . . . . . . . . . . . . . . . . . . . . 56
4.4 Signal processing electronics . . . . . . . . . . . . . . . . . . 60
4.4.1 The ampli cation boards . . . . . . . . . . . . . . . . 60
4.4.2 Digital electronics and the trigger algorithm . . . . . 61
4.4.3 Data structure . . . . . . . . . . . . . . . . . . . . . 64
4.4.4 Readout software . . . . . . . . . . . . . . . . . . . . 66
4.4.5 Powering of the electronics . . . . . . . . . . . . . . . 67
4.5 Simulations of detector properties . . . . . . . . . . . . . . . 68
4.5.1 The partial event model . . . . . . . . . . . . . . . . 68
4.5.2 Results of the simulations . . . . . . . . . . . . . . . 69
4.5.3 Electron simulations . . . . . . . . . . . . . . . . . . 74
4.6 Investigations of detector properties . . . . . . . . . . . . . . 76
4.6.1 Energy calibration . . . . . . . . . . . . . . . . . . . 76
4.6.2 The pa accelerator . . . . . . . . . . . . . . . . . . . 78
4.6.3 Detector test setup . . . . . . . . . . . . . . . . . . . 79
4.6.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5 Beamtime 85
5.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . 85
5.1.1 Beam tailoring . . . . . . . . . . . . . . . . . . . . . 86
5.1.2 Alignment of the spectrometer . . . . . . . . . . . . . 90
5.1.3 Neutron beam pro les . . . . . . . . . . . . . . . . . 92
5.1.4 Magnetic eld measurements . . . . . . . . . . . . . . 93
5.2 Data taking . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.2.1 Measurement process . . . . . . . . . . . . . . . . . . 96
5.2.2 The instrument control setup . . . . . . . . . . . . . 98
5.2.3 Neutron counter performance . . . . . . . . . . . . . 99
5.3 Overview of the beamtime . . . . . . . . . . . . . . . . . . . 101
6 Data analysis 105
6.1 Fitting of the pulses . . . . . . . . . . . . . . . . . . . . . . 105
4Contents
6.1.1 The pulse function . . . . . . . . . . . . . . . . . . . 105
6.1.2 Event types . . . . . . . . . . . . . . . . . . . . . . . 107
6.1.3 Incorrectly sorted events . . . . . . . . . . . . . . . . 108
6.2 Extraction of the correlation coe cient . . . . . . . . . . . . 115
6.2.1 Dead time correction . . . . . . . . . . . . . . . . . . 117
6.2.2 Background subtraction . . . . . . . . . . . . . . . . 117
6.2.3 Integration of the count rate . . . . . . . . . . . . . . 118
6.2.4 Dependence on the integration limits . . . . . . . . . 120
6.3 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.3.1 Automated shutter movements . . . . . . . . . . . . 122
6.3.2 Closed shutter . . . . . . . . . . . . . . . . . . . . . . 124
6.4 Correlated events . . . . . . . . . . . . . . . . . . . . . . . . 129
6.5 Baseline shifts . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.5.1 Trigger e ciency . . . . . . . . . . . . . . . . . . . . 131
6.5.2 Pulse-height e ects . . . . . . . . . . . . . . . . . . . 133
6.5.3 Electronics tests . . . . . . . . . . . . . . . . . . . . . 136
6.5.4 Correction of the saturation e ect . . . . . . . . . . . 144
6.6 Electronic noise . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.7 Trigger settings . . . . . . . . . . . . . . . . . . . . . . . . . 148
7 Conclusion and outlook 151
A Appendix 155
A.1 Detector voltages . . . . . . . . . . . . . . . . . . . . . . . . 155
A.2 Instrument control programs . . . . . . . . . . . . . . . . . . 157
A.2.1 DAQ PC . . . . . . . . . . . . . . . . . . . . . . . . . 157
A.2.2 Decode PC . . . . . . . . . . . . . . . . . . . . . . . 157
A.2.3 Magnet control PC . . . . . . . . . . . . . . . . . . . 158
A.2.4 Control PC . . . . . . . . . . . . . . . . . . . . . . . 159
A.3 ROOT data structure . . . . . . . . . . . . . . . . . . . . . . 159
A.3.1 Decode tree . . . . . . . . . . . . . . . . . . . . . . . 160
A.3.2 Fit tree . . . . . . . . . . . . . . . . . . . . . . . . . 161
5Contents
61 Introduction
The current understanding of elementary particles and their interactions
is described by the standard model (SM) of particle physics. This theory,
developed since the 1960’s, classi es the known elementary particles into
two major groups, Quarks and Leptons. Each group is divided in three
generations containing two particles each. Every particle has an antiparticle
with the same mass but opposite charge.
Despite the huge success of the SM, it cannot answer all the questions that
arise in modern particle physics.Among other shortcomings, the SM only
describes three of the four known fundamental interactions: The strong,
the weak, and the electromagnetic force. The SM does not contain gra-
vity, furthermore it cannot explain the baryon asymmetry in the universe.
Therefore the standard model, with its many parameters, might be a \low-
energy" limit of a more general description of the physical world. One of
the goals of modern experimental particle physics is to probe the SM in va-
rious di erent systems. The results of such experiments provide new input
for theorists, but also allow to test the predictions of yet unproven theories
beyond the SM, like for example super symmetry.
One such system is the neutron, it allows precision measurements which
may indicate failures of the SM. In part, the information can be compared
to results gained in high-energy, accelerator based experiments, but the
neutron o ers complementary results as well. For example, although the is electrically neutral, it consists of charged constituents which
might lead to a non zero electric dipole moment. Up to date all results of
these studies are compatible with zero, still they already provided strong
constraints on di erent theories beyond the SM [1].
Although the neutron is stable as long as it is bound in a nucleus, free
7Chapter 1 Introduction
neutrons are unstable due to the slightly higher mass of the neutron n
compared to the sum of the masses of the proton p and the electron e . It
decays with a life time of about 15 minutes into a proton, an electron, and
an electron antineutrino :e
(1.1) n!p +e + + 782:3 keV:e
The released energy is given by the di erences of the masses of the neutron,
proton, and electron [2].
In general, the decay of a nucleus into an electron, an antineutrino, and
the daughter nucleus is called beta-minus ( ) decay. The minus refers to
the negative charge of the outgoing electron. The decay of the free neutron
is the simplest form of a