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

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