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

2.4 Background
- Potassium is ubiquitous with a concentration of 2.4 % in the earth’s crust. Naturally
occurring K has an isotopic abundance of ¢ with a half life of ¢ years;
89 % of its decays proceed by beta emission with an endpoint of 1.3 MeV and 11 %
decay by electron capture with the emission of a 1.46 MeV gamma. The tolerable limit
for BOREXINO is 10 g/g (KÒØ) in the scintillator. The potassium content in the fluor
PPO as delivered was measured by NAA to be ¡ g/g [Gol97]. By water extraction
of a concentrated solution (100 g/l PPO in PC) this could be reduced several orders
of magnitude, so that the final potassium concentration in the CTF scintillator (1.5 g/l
PPO in PC) was below the sensitivity limit of both the CTF ( ¡ g/g) and NAA
( ¡ g/g). The purity requirement is thus not directly confirmed, but it is expected
to be met because of the high efficiency of the water extraction.
- Be can be formed by proton and neutron reactions on C from cosmic radiation. It
decays by electron capture with a half life of 53 days, with a 10 %-branch emitting a
478 keV gamma. In equilibrium at the earth’s surface Be gammas are expected to give
a count rate of 2700 per day and ton of scintillator [Vog96]. This rate can be reduced by
bringing the scintillator underground directly after distillation, before the equilibrium
rate of Be is reached (after one day of surface exposure the expected gamma rate is
per day and ton), and storing the scintillator underground for a sufficient period of
time to allow the Be to decay, or by removing the Be through purification. At a small
rate, Be will also be produced on site by cosmic ray muons (see paragraph 2.4.3).
Though there is no clear signature for a single neutrino event, there are several methods to
discriminate certain classes of background events:
- correlated events: the method consists of tagging delayed coincidences in the U and
Th decay chains ( Bi- Po, ¬-« with t s; Bi- Po, ¬-« with t
Isotope Abundance T [a] Concentration [g/g]
Cd 12.2 % ¡
In 95.8 % ¡
K 0.01 % ¡
¡
¡
¡
La 0.09 % ¡ ¡
Lu 2.59 % ¡ ¡
Rb 27.8 % ¡ ¡
Th 100 % ¡ ¡
U 99.3 % ¡
Table 2.2: Trace element concentrations at an equal amount of 1 event/(100t day) to the background
rate in BOREXINO in the energy range from 250 - 800 keV, after application of all cuts
(correlated events, «/¬ discrimination, statistical subtraction).
¡
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