Radon induced surface contaminations in low background experiments

In low background experiments the reduction of all possible radioactive contaminants is a crucial point for detector construction. This is also true for the surface contaminants, either those introduced during the production of detector components or those introduced during handling, treatment or storage. One of the most critical issue in this field is the control of the contamination induced by 222Rn and its progenies in the environment where the detectors are assembled and stored. Radioactive atoms can stick on detector components and create a net increase of the contaminants present on their surfaces, introducing an additional—often not negligible—source of background. The reduction of this kind of contaminations can become of primary importance in the case of fully sensitive devices, like cryogenic particle detectors. In this paper the analysis on the Rn sticking factor for copper and tellurium dioxide—the two main materials used for the construction of the CUORE detector—is discussed. The diffusion of radioactive atoms inside the detector components is considered in order to evaluate the effective contribution of Rn exposure to the background counting rate of an experiment.


Low background experiments
DBD2ν & DBD0ν DM interactions with RM rare α/β decays < 10 -2 -10 -4 c/keV/kg/y < 10 -3 -10 -4 c/kg/d < 10 -2 -10 -xx c/kg/d (8) Pb ( 210 Pb and 210 Po are at the equilibrium). We refer to the 210 Pb activity because we assume that after a long period of time (t τ Rn ), all the 222 Rn daughters have decayed and have populated the 210 Pb level of the Rn decay-chain. We consider the 210 Pb as an "integrator" of all the nuclei that have stuck on the surface ( 218 Pb and 214 Po).
A 0 Pb is derived from the equation: A Po is the activity of the sample measured after its exposure to Rn (t τ Rn , in our case t ∼ 1 y), and λ X is the mean half-life of the X element.
Finally the sticking factor is computed using the following formula: where S is the slab surface and t exp the time exposure.
After a thorough analysis we can state that the Radon A 0 Pb is derived from the equation: A Po is the activity of the sample measured after its exposure to Rn (t τ Rn , in our case t ∼ 1 y), and λ X is the mean half-life of the X element.
Finally the sticking factor is computed using the following formula: where S is the slab surface and t exp the time exposure. After a thorough analysis we can state that the Radon  Pb evaluated from 210 Po contamination. "prompt" (t ~ h) and "delayed" (t ~ 1.5 y). However the events that are in the tail are a small fraction of the overall activity (about 15%), assuming the continuum contribution negligible.

Pb production
We compare the plots in Fig. 4.19, which show the di↵erential activity measurements (that are just the di↵erence of two acquired spectra of the same sample),  fact the more the samples are kept in the Rn-box the longer is the time at 21 disposal for di↵using in the inside. Obviously the higher is the concentr of radon in the atmosphere the faster and deeper will be the di↵usion, sin just depends on the concentration gradient between the environment and th cavity (or canyons) on the surface of the sample created following the produ and cleaning procedure which make the copper crystal amorphous and no regular, especially on the surface (see Fig. 4  We suppose to have a surface (S) exposed to a high radon concentration atmosphere (n, particles per unit volume). If we wish to compute the number of radon nuclei which hit our surface per unit time and unit surface (it is just a flux), Γ, we have to consider the number of nuclei in the volume v · t, where v is the particles velocity and t the unit time. If we integrate the solid angle with which all the particles in the v · t see the surface S: The bolometric technique Energy deposited in the absorber produces a measurable temperature rise.