COSINE-100 is a direct detection dark matter search experiment that uses a 106 kg array of eight NaI ( Tl ) crystals that are kept underground at the Yangyang Underground Laboratory to avoid cosmogenic activation of radioisotopes by cosmic rays .
Even though the cosmogenic activity is declining with time , there are still significant background rates from the remnant nuclides .
In this paper , we report measurements of cosmogenic isotope contaminations with less than one year half-lives that are based on extrapolations of the time dependent activities of their characteristic energy peaks to activity rates at the time the crystals were deployed underground .
For longer-lived ^ { 109 } Cd ( T _ { 1 / 2 } = 1.27 y ) and ^ { 22 } Na ( T _ { 1 / 2 } = 2.6 y ) , we investigate time correlations and coincidence events due to several emissions .
The inferred sea-level production rates are compared with calculations based on the ACTIVIA and MENDL-2 model calculations and experimental data .
The results from different approaches are in reasonable agreement with each other .
For ^ { 3 } H , which has a long , 12.3 year half-life , we evaluated the activity levels and the exposure times that are in reasonable agreement with the time period estimated for each crystal ’ s exposure .