Currently, EDDA uses a polypropylene fiber target of about 5 micrometers diameter. Therefore the interaction region between beam and target has a neclegible size. Using a polarized atomic beam target the situation will change. Due to the increased size of the interaction region (diameter about 10 mm) a second, independent detector with high spatial resolution is needed to allow the reconstruction of the reaction vertex.
The example below shows, that the inner layer also provides an effective suppression of background reactions.
So, even using the fibre target, the inner layer will be helpful to suppress background reactions and to gain higher redundancy in EDDA's data.