Study on cosmogenic activation above ground for the DarkSide-20k project

The activation of materials due to the exposure to cosmic rays may become an important background source for experiments investigating rare event phenomena. DarkSide-20k is a direct detection experiment for galactic dark matter particles, using a two-phase liquid argon time projection chamber filled with 49.7 tonnes (active mass) of Underground Argon (UAr) depleted in 39Ar. Here, the cosmogenic activity of relevant long-lived radioisotopes induced in the argon and other massive components of the set-up has been estimated; production of 120 t of radiopure UAr is foreseen. The expected exposure above ground and production rates, either measured or calculated, have been considered. From the simulated counting rates in the detector due to cosmogenic isotopes, it is concluded that activation in copper and stainless steel is not problematic. Activation of titanium, considered in early designs but not used in the final design, is discussed. The activity of 39Ar induced during extraction, purification and transport on surface, in baseline conditions, is evaluated to be 2.8% of the activity measured in UAr from the same source, and thus considered acceptable. Other products in the UAr such as 37Ar and 3H are shown to not be relevant due to short half-life and assumed purification methods

Messung organischer Spurengase in der arktischen Troposphäre : Hinweise auf einen regionalen halogeninduzierten Ozonabbau

Tropospheric ozone depletion events have been observed for more than ten years in the arctie during spring. These events are characterized by a rapid loss of ozone from normal levels of 30-40 ppb to values below the detection limit (< 1 ppb). These events last for a couple of hours to a few days and are accompanied by elevated concentrations of filterable bromine and low mixing ratios of many hydrocarbons. Therefore, reactions with halogen atoms are thought to cause the phenomenon. Although there have been several intensive field studies in the past the chemical mechanism of the halogen induced ozone depletion is still not fully understood. Further, the source ofthe halogen atoms is still unknown. The purpose of this study was to verify and extend the database of arctic tropospheric organic trace gas rnixing ratios in order to provide additional evidence for the suggested halogen atom induced ozone depletions. Owing to their low concentration direct measurements of halogen atoms are presently not possible. However, an indirect determination of these concentrations can be deduced from changes in hydrocarbon patterns. In-situ measurements of C$_{2}$-C$_{8}$ hydrocarbons and C$_{1}$-C$_{2}$ halocarbons, canister sampling, and in-situ determinations of carbon monoxide were carried out between Apr. 18 - June 12, 1995 and Mar. 29 - May 15, 1996 during the ARCTOC (ARCtie Tropospherie Ozone Chemistry) field campaigns at Ny Älesund, Svalbard (78°56'N, 11°55'E). For these measurements the present gas chromatographic systems were modified. The chromatographie resolution of the peaks, the accuracy of the measurement, the detection limits, and the sampling frequency were improved considerably. Tropospheric ozone depletions were observed in both field campaigns. In each ozone depletion event concurrent depletion of alkanes and ethyne, but no significant change in benzene, chloromethane, or CO mixing ratios were observed. The change in the organic trace gas pattern gives evidence for halogen radieals being involved in the chemical cyc1es during ozone depletion. For major ozone depletion periods time integrated chlorine and bromine atom concentrations were ca1culated from the depletion of the mixing ratio of light alkanes and ethyne, respectively. The time integrated Cl and Br radical concentrations were in the order of 10$^{10}$ s/cm$^{3}$ and 5 $\cdot$ 10$^{12}$ s/cm$^{3}$, respectively. The observed ozone los ses can be explained quantitatively with these data. The results show that free bromine atoms are the major cause for ozone depletion (more than 95%). The observed mixing ratios of halocarbons lead to the conc1usion that halogenated organic compounds are no significant source of halogen atoms