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

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6 Test of a PXE based scintillator for Borexino - The light yield of PXE/TP is 0.68, slightly less than PC/PPO (0.73). - The quenching factors of alpha particles were measured in small samples with Rn spiked scintillator. These are listed in table 5.1 for PXE/TP and PC/PPO. The effect of alpha quenching is about 20 % weaker in PXE/TP than in PC/PPO, which means that the alpha peaks appear at somewhat higher energies. - The «¬ discrimination efficiencies of PXE/TP and PC/PPO under laboratory conditions are comparable. With the beta identification efficiency fixed at 98 %, the alpha identification efficiency is 99.7 % for PC/PPO and 98.4 % for PXE/TP/bisMSB in the energy range from 80 - 135 pe [Lom97]. 6.2.3 Radiopurity As PXE is a petroleum derivative like PC, similar radiopurity levels may be expected. PC is obtained directly from the fractional distillation of petroleum, while PXE requires further processing: the raw materials styrene and xylene are obtained from the fractional distillation of petroleum, and then react with each other. The product is washed with water and distilled a second time. After the distillation a solid column unit with clay is utilized at the plant, mainly to remove residual water dissolved in the PXE. These further process steps introduce considerable optical and inorganic impurity levels. Therefore, R. Raghavan suggested the implementation of the solid column chromatography (or gel filtration) within BOREXINO, as opposed to using the one at the plant, so that we could control this step by ourselves. In laboratory experiments he found that silica gel would suit this purpose well. Column chromatography with silica gel is a commonly used technique for the cleaning of liquids. Silica gel is a powder with highly porous grains (grain size several 100 m, pore size few nm), offering a large contact surface (several Ñ ). The surface consists of reactive silanol groups (Si-OH). Ionized and polarizable impurities in the liquid are removed by surface adsorption, while the nonpolar scintillator molecules are unaffected. The fine powder structure of the silica gel also has a filter action on particulates. Laboratory tests of the column purification with PXE/TP together with neutron activation [Gol97] showed a clear reduction of the concentration of almost all impurities in the liquid scintillator, which are of concern for BOREXINO (see table 6.3). These results made us confident to build a large column (volume 80 l) for the purification of several tons of scintillator, to test this cleaning procedure in the CTF. Commercially available silica gel has a rather high intrinsic U concentration ( g/g), so that the Rn emanation from the silica gel itself becomes a problem, as this leads to high concentrations of the long-lived Rn daughters in the scintillator. Thus, we were looking for a supplier of high purity silica gel or the possibility to fabricate high purity silica gel by ourselves starting with cleaner raw materials [Ang97]. The Ra activity and Rn emanation 80
6.2 Laboratory Measurements with PXE PXE + TP, concentration in g/g Element before purification after purification purification factor Cd ¡ Cr ¡ Fe ¡ In ¡ K ¡ La ¡ Lu ¡ Na ¡ Rb ¡ U ¡ Th ¡ W ¡ Zn ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ ¡ - ¡ ¡ ¡ ¡ Table 6.3: Purification factors for solid column chromatography on PXE/TP, obtained from a laboratory test with a 20 ml column. From [Gol97]. rate of several samples of commercially available silica gel are shown in table 6.4. The Rn emanation of the high purity silica gel from Eka Nobel was below the detection limit, and hence it was chosen for the use within BOREXINO. To avoid the contamination of the scintillator with long-lived Rn daughters, the scintillator is purged with nitrogen gas directly after the column to remove the Rn before it decays in the scintillator (see fig. 6.4). silica gel sample Ra activity (Bq/kg) Rn emanation (Bq/kg) Merck, 0.2 - 0.5 mm, 60 A ¦ ¦ Roth, 2-4mm ¦ ¦ Eka Nobel, Kromasil, m Table 6.4: Ra activity and Rn emanation of different samples of commercially available silica gel [Rau99]. The silica gel from Merck was used in the CTF1. The silica gel from Eka Nobel was used in the CTF2 and will be used for BOREXINO. ¡ 81
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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
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 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
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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