TECHNOLOGIES FOR OBTAINING RADIO-PURE MATERIALS; METHODS OF LOW RADIOACTIVITY DETECTION ∗

This paper discusses selected problems arising from the presence of radioactive background sources in experiments searching for extremely rare processes. Physical properties of 42 K ions, present in argon as a progeny of 42 Ar, were investigated. A model of 42 K ions behavior in liquieﬁed argon is presented. Also, construction and operation of an electrostatic 222 Rn monitor of gaseous nitrogen is outlined.


Introduction
Liquefied noble gases are extensively used in ultra-low background experiments searching for rare processes. The cryogenic liquids serve as a radiopure material for passive shielding and as a cooling medium (liquefied argon in the GERDA experiment [1]) or both as a target and detector (two phase argon TPC in the DarkSide experiment [2]). The ultra-low background experiments are aiming at maximization of their sensitivity to registering rare events by mitigating any possible background sources. Careful material selection and on-line monitoring for background sources is, therefore, crucial. E.g. the detection limit for half life-time of 76 Ge regarding neutrinoless double beta decay (0ν2β) is inversely proportional to the square root of the background index B E where A is the 76 Ge isotope abundance, is the 0ν2β decay registration efficiency, m mol is the molar mass of Ge, m is the detector mass, T is the duration of measurement, B E is the background index in region of interest (at expected 0ν2β decay energy), δ E is the energy resolution, and N A is the Avogadro number. Therefore, the most effort is put forth in lowering the experimental background.

42 K in liquid argon
42 K is produced in beta decays of 42 Ar, naturally present in argon ( 42 Ar/ nat Ar < 3 × 10 −21 g/g). High energy released in beta decay of 42 K (Q = 3525 keV) exceeds the 76 Ge 0ν2β decay energy (Q = 2039 keV), therefore, 42 K is a potential source of background for e.g. the GERDA experiment.
Potassium may form positive ions as a result of 42 Ar beta decays occurring in the cryogenic liquid. Electric potential present in the active volume of a detector (biased bare germanium diodes in the GERDA experiment or TPC in the DarkSide experiment) is responsible for transport of the ions. In consequence, initial concentration of the impurities homogeneously distributed in the volume of the detector may be altered. Figure 1 presents the described processes. To fully understand the radioactive contribution of 42 K beta decays to the background, one has to study the germinate recombination processes and transport properties of the cations. The germinate recombination is responsible for immediate neutralization of the cations following the beta decay. This process determines, depending on the external electric field strength, the amount of cations retaining their charge. Spatial range of the fraction of ions surviving the germinate neutralization varies with mobility and ionic half life-time of the ions (neutralization of the ions by electronegative impurities like oxygen). The properties of 42 K were extensively studied in dedicated experiments (Liquid Argon Germanium facility in the framework of the GERDA experiment -LArGe).

222 Rn and its progenies
The DarkSide experiment is searching for interactions of Cold Dark Matter particles (Weakly Interacting Massive Particles -WIMPs) with atoms of liquefied argon. The signal is detected by a two-phase Time Projection Chamber. Alpha decays of 222 Rn progenies may mimic the signal from WIMPs, serving as a potential source of the experimental background.
222 Rn belongs to the decay chain of 226 Ra, present in all materials used for construction of the experiments. Radon is diffusing and emanating from surfaces of materials, being an inert noble gas. 222 Rn may also be ejected from thin surface layers of a material as a recoil atom of 226 Ra alpha decay. 222 Rn then enters active volumes of the detectors by dissolving in other gases easily. Figure 2 shows the 222 Rn decay chain.  The electrostatic 222 Rn monitor (400 l volume) was recently constructed and deployed to operation for the DarkSide experiment. 222 Rn content is instantly controlled in various gases used in the experiment: air exhausted from clean-rooms, nitrogen and argon. 222 Rn concentration detection limit achieved by the device is better than 10 mBq/m 3 .

Conclusions
Ultra-low background experiments demand supreme purities of the construction materials. Also, during their operation on-line monitoring of the background index is essential. Technologies for obtaining highly radiopure materials rely on careful material selection and understanding of the physical properties of the radio-impurities. Dedicated experiments focused only on the impurities need to be designed to study their nature.