Correlated background induced by radioimpurities in Double Chooz

The Double Chooz reactor antineutrino experiment, located at the Chooz nuclear power plant in France, provides a precise measurement of the θ₁₃ parameter. After data taking with the far detector since April 2011, the near detector was finished in October 2014 and data taking started within this year. Although this enables a significant reduction of both reactor and detector related systematics uncertainties, several sources of background have to be taken into account to allow a measurement of the neutrino mixing angle θ₁₃ with the desired precision. The talk will focus on the background induced by radioactivity in the near detector. Events coming from the decay of ²¹⁴Bi and ²¹²Bi followed by the α-decay of ²¹⁴Po and ²¹²Po, referred to as BiPo coincidences, provide a fast coincidence signal between β^- and α signal, which is also spatially correlated. It can be easily distinguished from other background. Therefore it provides tagging of the number of decays within the U decay chain and the Th decay chain. This provides a tool to prove the radiopurity of the Double Chooz Detector. It shows that the radiopurity in Double Chooz is well within the specifications of less than 10^-13 g/g of U and Th in all parts of the inner detector. However, these events cannot be neglected for a measurement of θ₁₃ and have to be addressed in further ongoing studies.