Shielding studies for COBRA

A shielding is an essential part of any low background experiment. Its task is to minimise the amount of external particles and their secondaries reaching the detector. There are three types of particles a shielding is needed for: gamma rays, muons and neutrons. Most of the gamma rays originate from natural radioactivity, which leads to energies up to 3 MeV with the vast majority of the flux below the 2.6 MeV gamma ray peak of 208Tl. Already 15 cm of lead or an equivalent passive shielding reduces the natural gamma ray flux sufficiently. Gamma rays induced by neutrons and cosmic muons can have much higher energies, but also have a considerable lower flux. The muons are usually of cosmic origin and thus can have very high energies. They are however highly ionising particles, which allows the use of a veto and of tracking methods to prevent them from generating background.
As neutrons are more challenging to shield and potentially a very dangerous background source for the COBRA experiment, special emphasise is placed on them. (See the next section for more details on neutron shielding.) In addition the radioactive purity and the cosmic activation of the shielding materials have to be considered.

Neutron shielding

Simulated spectrum of the neutron induced energy deposition in a COBRA-detector made up of 64000 crystals for different shieldings with
the same total thickness. The black full line indicates a pure lead shielding, the blue dotted line a standard multilayer shielding, the full blue line
a composite like shielding and the dashed grey line an optimised multi layer shielding. The original energy spectrum of the incoming neutrons
was calculated using a parametrisation for the muon induced neutronflux by [1] with values appropriate for the LNGS.

As neutrons do not carry an electrical charge, they are difficult to detect. They can however produce nuclear recoils, which could mimic the signal of low background experiments. Furthermore 113Cd, which has a natural abundance of 12.22 %, has a very high cross section for the capture of thermal neutrons. This leads to additional background in the CdZnTe-crystals of the COBRA-experiment. In an underground environment like the LNGS there are two main neutron sources to consider. Neutrons from local radioactivity and neutrons induced by cosmic muons. Below 10 MeV the main contribution is due to spontaneous fission and (α,n)-processes due to interactions of α’s from natural emitters with light target nuclei in the rock. Neutrons with energies above 10 MeV are produced by nuclear reactions induced by cosmic ray muons. However, the flux of these neutrons is reduced considerably by beeing in an underground laboratory (see figure 1). At the LNGS neutrons from radioactive processes have a total flux which is about three orders of magnitude larger then the muon induced flux. However, due to their low energy they are easily shielded. Therefore, our main focus is on finding shieldings which are effective against the high energy muon induced neutrons.
To study the shielding, Geant4 with the low energy expansion for dark matter experiments and MCNPX are used. An array of 64000 COBRA crystals with a total mass of about 400kg is simulated, surrounded by various shieldings. Types of shieldings considered are: shieldings consisting of a single material layer, multi layer shieldings, large water or liquid scintillator tanks and composite like shieldings with many thin layers of different materials. First results of this study are shown in figure 2.

 

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