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

6.3 The PXE Tests in the CTF
in equilibrium with their progenitors Ra (t a) and Th (t a), while
the NAA directly measures the parent isotopes U and Th. Only in the case of secular
equilibrium both methods would yield the same results. The uranium measurement in the CTF
could be affected by a significant background coming from Rn in the water, that permeates
through the nylon barrier (a very precise evaluation of this background is not possible; it might
have as well reached a level of ¡ g U/g). The thorium measurement is not affected by
such a kind of background. As can be seen from table 6.5, the values measured in the CTF1
and with NAA are compatible with secular equilibrium in both decay chains.
After the scintillator had been unloaded from the CTF, we continued the purification tests in
the Module-0 with a water extraction of the 5 tons of scintillator with 2 tons of deionized water,
and subsequently a circulation over a 40 l column (about 6 times the total volume). After each
purification step a sample was taken for the NAA. In the end, a new record limit for the U
and Th concentration in PXE was measured [vH99]:
Í ¢
Ì ¢
After these operations the scintillator was stored in one of the EP tanks, while the CTF and
Module-0 were dismounted and upgraded during the years 1998 and 1999.
6.3.3 The PXE Test in the CTF2 2000
The PXE test in the CTF2 was carried out from May to September 2000. For the filling operations
the upgraded Module-0 was used which also allowed to test this system for its performance
and leak tightness.
The PXE scintillator, that had been used for the test in CTF1 in 1996, was stored underground
in one 7-ton tank of Module-0 during the last years and was used again for the new test in
CTF2. The scintillator composition was È ÌÈ Ð ×ÅË ÑÐ . As the exact
status of the scintillator was unknown (the tank had been opened several times during the last
years to change flanges and sealings, and to connect the new pipes), we decided to clean the
whole scintillator by running it in batches over a 40 l Silica gel column. The Silica gel was
preconditioned by heating it up to about 100 Æ C and flushing it continuously with nitrogen
for about 15 h to remove adsorbed water. The column operation needed about 1 day per ton
scintillator. The scintillator was continuously running over the column with a flow rate of
1.5 - 2.5 l/min and from there into one of the two 1-ton-buffer-tanks, where it was flushed with
nitrogen (RN ) to remove the Rn introduced by Rn-emanation from the silica gel (see fig.
6.4). Once one of the buffer tanks was completely filled, the nitrogen flushing was continued
for several hours with high purity nitrogen ( ÕÑ ). For the filling the remaining gas
volume at the top of the buffer tank was filled up with deionized water, then the tank was
connected to the CTF Inner Vessel and the scintillator was pumped into the Inner Vessel by
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