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

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6 Test of a PXE based scintillator for Borexino 6.1 Motivation for an alternative scintillator Initially, BOREXINO was going to use a PC based liquid scintillator ( Ñ) and a surrounding water buffer ( Ñ) [Bel91]. It turned out that the solution adopted for the scintillator containment vessel, namely a thin nylon membrane with 0.1 mm thickness, cannot hold a buoyancy force bigger than a few tons. Thus it became necessary to match the density of the scintillator and the buffer liquid within 1 %. There are two possible solutions to this problem: - find a buffer medium whose density matches the density of PC and which is optical transparent, non-fluorescing and chemically inert (not attacking the nylon, steel, the concentrators or the PMTs); - find a liquid scintillator with optical properties similar to PC and a density close to 1.0 g/cm, so that water can be used as buffer medium. Considering the second strategy, a new scintillator based on PXE ( Ñ )was suggested by R. Raghavan [Rag95] and subsequently tested in laboratory measurements as well as in the CTF. The main advantage of PXE compared to PC is its higher density. Taking a fixed detector geometry, a higher density means more target mass in the same volume, and, as the shielding against the external background is more efficient in a medium with higher density, the fiducial volume can be further increased, which additionally increases the target mass. Assuming the fixed volume of 321 m for the scintillator, the mass of the PXE would be 316 t compared to 281 t for PC. Taking into account the electron densities, the signal rate would increase from 55 events per day to about 61 events per day in the same volume. For the shielding against the external background the higher density of the water buffer is also advantageous. For comparison, a simulation of the external gamma ray background induced by the radioactivity of the PMTs and concentrators was performed for the two scenarios: PC scintillator/PC buffer and PXE scintillator/water buffer. The contamination of the PMTs is: Th 43,6 g/piece, U 127,9 g/piece, K 6,1 g/piece. The contamination of the concentrators is: Th 0.25 g/g, 76
6.2 Laboratory Measurements with PXE U and K negligible. Table 6.1 shows the results of the simulation. Even without any radial cuts, the background is a factor of 3 - 4 lower for PXE/water than for PC/PC. Increasing the radius of the fiducial volume from 300 to 320 cm, which would give for PXE/water about the same background contribution than for PC/PC, would yield a 20 % higher target volume, or a 35 % higher target mass, which means that the Be neutrino signal rate would increase from 55 events per day to about 75 events per day. 250 - 800 keV 800 - 1400 keV Radius PC/PC PXE/Water PC/PC PXE/Water (cm) counts/day counts/day counts/day counts/day 300 0.22 0.05 1.1 0.13 325 1.15 0.30 4.9 0.71 350 8.5 1.8 23 4.8 375 66 14 103 25 400 434 109 386 105 425 1248 345 844 243 all 1921 455 1023 298 Table 6.1: Contribution of the PMTs and concentrators to the background in BOREXINO, placing the fiducial volume cut at different radii. From a Monte Carlo simulation for the two different scenarios: PC scintillator/ PC buffer, and PXE scintillator/ water buffer [Gri01]. 6.2 Laboratory Measurements with PXE 6.2.1 Basic properties of PXE For the use in a large scale low background experiment not only the optical properties and the radiopurity (which will be discussed in more detail later), but also other physical properties have an impact on the choice of the material. These are e.g. density (target mass, background shielding), flammability (safety) and viscosity (liquid handling). Also the cost, availability and legal classification of the material play an important role. PXE and PC are both aromatic hydrocarbons, made up of similar molecular structures and with generally similar chemical properties. The molecular structure of the solvent and the fluor molecules of the scintillator mixtures under discussion are shown in fig. 6.1. The basic physical properties of PC and PXE are compared in table 6.2. PXE has a 10 % higher density than PC, whose advantages were already explained. PC is legally a hazardous material because of its flammability code (flammability class III). The transportation requirements are those of UN code 1993. PXE is not hazardous, and has no special UN hazard code or related transportation 77
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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
Page 36 and 37: 2 The Borexino Experiment 2.3.2 Anc
Page 38 and 39: 2 The Borexino Experiment which per
Page 40 and 41: 2 The Borexino Experiment 2.4 Backg
Page 42 and 43: 2 The Borexino Experiment ns; Rn-
Page 44 and 45: 2 The Borexino Experiment volume wo
Page 46 and 47: 3 The Counting Test Facility (CTF)
Page 56 and 57: 4 Position Reconstruction of Scinti
Page 72 and 73: 5 Particle Identification with a Ne
Page 88 and 89: 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
Page 114 and 115: 7 The Source Runs in CTF2 7.4 Analy
Page 116 and 117: 7 The Source Runs in CTF2 7.4.2 Rec
Page 118 and 119: 7 The Source Runs in CTF2 7.4.3 Ene
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 124 and 125: Summary and Outlook possible to dis
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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