Development of a Liquid Scintillator and of Data ... - Borexino - Infn

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Summary and Outlook possible to distinguish clearly between the different currently allowed oscillation scenarios. The LENS experiment aims at a real-time charged current detection of the Be and, eventually the pp neutrinos. By measuring the CC and the neutrino-electron scattering (CC + NC) rate of the Be neutrinos, their total flux can be determined and compared with solar model predictions. The comparison of the results from BOREXINO and LENS can then give an unambiguous proof for oscillations of the Be neutrinos. The identification of the neutrino signal in BOREXINO relies on the distinction of a Comptonlike edge in the electron energy spectrum, due to the monoenergetic (862 keV) Be neutrinos scattered off electrons. Since the neutrino signal does not have any event-by-event signature, reduction and rejection of the background signals from radioactive decays are essential. As the expected signal rate in BOREXINO is less than ÓÙÒØ× ØÓÒ ¡ Ý in the energy range from 0.25 - 0.8 MeV, the requirements on the radiopurity of the detector components are extremely challenging: for the liquid scintillator, a maximum concentration of U and Th of g/g and of Kof g/g is allowed in order to achieve a signal/background ratio even in the case of total conversion of into . The necessary background suppression can be achieved only if a large fraction of the background events can be rejected by their pulse shape characteristics (alpha decays) and by spatial cuts (discrimination of external background by fiducial volume cut). It is therefore crucial to provide several independent analysis methods which can be used for cross checks of the efficiency and systematic uncertainty of the applied cuts. In this work, a new method for the spatial reconstruction of scintillation events for BOREXINO and the CTF was developed and successfully tested with Monte Carlo simulations and CTF2 data. From laboratory measurements and source tests in the CTF1, the parameters for an optical model of the light propagation in the liquid scintillator have been evaluated and applied in a tracking program developed for BOREXINO and the CTF. The reconstruction compares the ADC and TDC data from a real event with values from simulations on a spatial grid. The resolution of this reconstruction method therefore depends strongly on the understanding of the light propagation mechanisms. Calibration measurements with localized sources in the CTF2 were performed in order to study the systematic behaviour of the reconstruction code. A new liquid scintillator based on phenyl-o-xylylethane (PXE) was tested as an alternative choice for BOREXINO. Its physical and chemical properties make it superior to pseudocumene (PC) in some respects, e.g. the higher density (target mass) and the higher flash point (safety), but there are also some limitations which need further investigation. In particular, the C concentration was about 6 times higher than in the PC measured in the CTF1 ( C/ C ¡ ). The C level may vary, though, depending on the petroleum source, and also for the PC a new batch of scintillator from another source could have a higher C/ C ratio. The very low U concentration that was measured in PXE by NAA ( ¡ g/g) could not be confirmed by CTF measurements, as in the CTF1 the sensitivity of the detector was substantially reduced due to the failure of a large fraction of the PMTs, and in the CTF2 a Rn background of the order of several g/g from an unidentified source was present. The Th concentration measured in the CTF2 ( ¦ ¡ g/g) is compatible with the limit of ¡ g/g that was measured with NAA, confirming the high intrinsic radiopurity 114
Summary and Outlook of this scintillator. The «¬ discrimination efficiency in PXE was found to be within the requirements for BOREXINO. The calibration measurements performed in the CTF2 showed that the optical properties of the PXE are generally comparable to those of PC, but more detailed studies of the exact timing behaviour and absorption/reemission probabilities of PXE would be needed. The development of large volume low background scintillation detectors, that was successfully demonstrated with the CTF, opens new perspectives for low energy neutrino detection: with future solar neutrino experiments using the liquid scintillation technique (BOREXINO, LENS ...) it will be possible to determine the flux of the solar Be neutrinos in both charged and neutral current, which is needed in order to settle the neutrino mass and mixing parameters. If a liquid scintillator with an extremely low C content could be found, even the ultimate challenge of a real-time detection of the low energetic pp neutrinos could be met by BOREXINO. 115
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Fakultät für Physik der Technisch
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Abstract The first chapter contains
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Übersicht an Radionukliden in BORE
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Contents 3 The Counting Test Facili
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Contents viii
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1 Solar Neutrinos Figure 1.1: The p
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1 Solar Neutrinos 1.2 Detection of
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1 Solar Neutrinos With this detecti
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1 Solar Neutrinos 1.2.3 Future Expe
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1 Solar Neutrinos 1.3.2 Astrophysic
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1 Solar Neutrinos Using the unitari
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1 Solar Neutrinos km in the earth.
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1 Solar Neutrinos 16 Solution ¡Ñ
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2 The Borexino Experiment The relat
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2 The Borexino Experiment Figure 2.
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2 The Borexino Experiment 2.2 The D
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2 The Borexino Experiment 2.3 Detec
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2 The Borexino Experiment 2.3.2 Anc
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2 The Borexino Experiment which per
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2 The Borexino Experiment 2.4 Backg
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2 The Borexino Experiment ns; Rn-
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2 The Borexino Experiment volume wo
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3 The Counting Test Facility (CTF)
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4 Position Reconstruction of Scinti
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5 Particle Identification with a Ne
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Page 86 and 87: 6 Test of a PXE based scintillator
Page 108 and 109: 7 The Source Runs in CTF2 transpare
Page 110 and 111: 7 The Source Runs in CTF2 7.3 The S
Page 112 and 113: 7 The Source Runs in CTF2 102 of th
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Page 116 and 117: 7 The Source Runs in CTF2 7.4.2 Rec
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Page 120 and 121: 7 The Source Runs in CTF2 45 40 35
Page 122 and 123: 7 The Source Runs in CTF2 112 reemi
Page 126 and 127: Bibliography [Ade98] E. G. Adelberg
Page 128 and 129: Bibliography [Bra00] L. De Braeckel
Page 130 and 131: Bibliography [Lom97] P. Lombardi, D
Page 132 and 133: Bibliography [Vog96] R. B. Vogelaar
Page 134 and 135: Danksagung An dieser Stelle möchte
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Development of a Liquid Scintillator and of Data ... - Borexino - Infn

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