a design study for a cobra upgrade to - Institut für Kern- und ...

14 3 The COBRA experiment
1420:480:340:13:1 [18].
In contrast to the other secondary cosmic rays, muons cannot be
stopped by a few meters of shielding material. The muon flux can
only be reduced by going deep underground, where it is for instance
reduced by 4 orders of magnitude in the 3500 meters of water
equivalent (mwe) deep LNGS. There the atmospheric muons
with energies of about 10 GeV up to a few TeV can interact either
directly with the detector material or can produce tertiary neutrons
near the detector. Their contribution to the background rate
can be further reduced with an active muon veto. A more serious
impact on low-level experiments have cosmogenic isotopes.
These can be produced in materials, which are exposed to cosmic
radiation. (n, γ) interactions or spallation processes with the constituent
isotopes can produce various radionuclides. Radionuclides
with half lives of month or years, being still present in the
components, long after these have been installed underground,
can be a major problem.
Primordial radioisotopes are isotopes with half-lives of more than 10 9
years, being produced before the formation of the earth. Isotopes
from the 238 U, 235 U and 232 Th decay chains and 40 K are
most likely to find. They are in the laboratory surrounding rock
and in smaller or larger concentrations in all detector components.
This requires shielding of the detector from the surrounding
background decays, where primary the produced neutrons
and gamma rays must be shielded. Also, a thoughtful selection of
the detector and shielding material and a subsequent careful handling
of these, in order to prevent a subsequent contamination of
the surfaces, is necessary. With the highest prominent gamma line
of 2.614 keV from the 232 Th decay chain, these background components
are beyond the most interesting Q-values of double beta
decays, but can disturb the experiments nevertheless. The noble
gas 222 Rn is part of the 238 U decay chain and can leak out of the
surrounding rock, can accumulate in the laboratory air and can
diffuse into the experimental set-up. It decays under the emission
of a 5.5 MeV alpha particle and is therefore a problem for double
beta decay experiments. Additionally, its decay products are also
radioactive, can attach themselves to surfaces near the detector
and can increase the background level further on [18].
Anthropogenic radioisotopes are mainly fallout from nuclear
weapons testing and nuclear accidents, although they can
also exist naturally in lower concentrations. Materials, being
produced before nuclear weapons testing and especially the