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

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6 Test of a PXE based scintillator for Borexino Limit on U In the CTF U is assayed via the characteristic delayed coincidence between the decays of its daughters Bi and Po(half life of Po: 164 s). Assuming secular equilibrium of the whole decay chain, the measured decay rate of these daughters can be expressed as uraniumequivalent, ÓÙÒØ Ý ¡ ØÓÒ Í ¡ Å ¡ Æ ¡ where A is the measured activity, ¡ a is the decay constant of U, M = 238 g/mol the molar mass, and Æ Avogadro’s number. Data taking with 4 tons of PXE lasted 52 days. The initial Rn contamination was small (90 counts per day), so that the Rn introduced during the filling should have decayed away (to a level of 1 count per day) within 4 weeks. We observed, however, after an initial decrease a constant Rn activity of about 1 count per hour (see fig. 6.6). Moreover, this activity was not uniformly distributed, but peaked around the vertical axis (see fig. 6.7). Along the vertical axis, the distribution was not homogeneous in time, but seemed to be slowly moving downwards, while twice some new activity was introduced from the top (fig. 6.8). One possible explanation could be, that at the end of the volumetric filling, we reintroduced some water at the top of the Inner Vessel. As the densities of PXE and water are very close, the water droplets would move downwards only very slowly. Some droplets could stay attached to the tube walls and detach only occasionally, e.g. because of vibrations. As the activity in this ‘column’ around the z-axis was almost 1 count per hour, the specific activity of a l water droplet accounting for the whole Rn activity would have to be ÕÑ . However, the Rn and Ra activity in the CTF water were measured to be 30 mBq/m and 3 mBq/m respectively [Sim00], so that this theory can be ruled out. The nature of the Rn contamination is still unknown. A systematic effect of the position reconstruction could be excluded by the source Bi-Po counts / 12 hours 88 60 50 40 30 20 10 fit function: P1. P2 e + P3 0 750 800 850 900 950 1000 1050 1100 1150 1200 x10 4 Figure 6.6: Rn activity versus time for the runs 758 - 789. time in s
6.3 The PXE Tests in the CTF measurements, as will be shown in chapter 7. Applying a radial cut on the Bi- Po event distribution, we could derive an upper limit for the intrinsic U contamination. For the analysis, I took two periods at the end of the 4 ton runs: run 773 - 780 (7.32 days, August 21 - 29), and run 783 - 789 (6.98 days, August 31 - September 8). The following cuts were applied to select the Bi- Po events: - energy of the group 1 event: more than 100 pe (efficiency 95 %); - energy of the group 2 event: between 200 pe and 400 pe (efficiency 99 %); - coincidence time between 5 s and 800 s (efficiency 94%). Two further cuts served to reject events originating from cosmic ray muons: - the muon veto must not have triggered (no muonflag); - the meantime (i.e. the average time of all hit PMTs) must be below 25 ns. With these two cuts, muon events can be discriminated with a high efficiency while scintillation events are not affected. A varying radial cut around the z-axis was applied to reject the events inside the column. The efficiency of this cut together with the results obtained are listed in table 6.6. The best value obtained is Í ¦ ¡ for a cylindrical cut around the z-axis Ê Ñ. run773 - 780 run 783 - 789 radius (m) mass (t) no. of U equiv. no. of U equiv. events ( g/g) events ( g/g) 3.94 121 ¦ 101 ¦ 3.64 79 ¦ 59 ¦ 3.22 55 ¦ 41 ¦ 2.72 42 ¦ 32 ¦ 2.14 31 ¦ 24 ¦ 1.53 26 ¦ 17 ¦ Table 6.6: Result of the U analysis after the application of different cylindrical cuts around the z-axis. The errors given are statistical errors only. 89
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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)
Page 48 and 49: 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
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Page 114 and 115: 7 The Source Runs in CTF2 7.4 Analy
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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 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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