Benchmarking the PreSPEC@GSI experiment for Coulex-multipolarimetry on the $$\pi (p_{3/2})\rightarrow \pi (p_{1/2})$$ spin-flip transition in $$^{85}\hbox {Br}$$

A first performance test of the Coulomb excitation multipolarimetry (Coulex-multipolarimetry) method is presented. It is based on a $$^{85}\hbox {Br}\,\pi p_{3/2}\rightarrow \pi p_{1/2}$$ spin-flip experiment performed as part of the PreSPEC-AGATA campaign at the GSI Helmholtzzentrum fur Schwerionenforschung (GSI). Via determination of background levels around the expected $$^{85}\hbox {Br}$$ excitations as well as measured $$^{197}\hbox {Au}$$ excitations, an upper limit for the M1 transition strength of the $$1/2_1^-\rightarrow 3/2_\text {g.s.}^-$$ transition in $$^{85}\hbox {Br}$$ and a lower beam time limit for upcoming experimental campaigns utilizing Coulex-multipolarimetry have been inferred. The impact of the use of AGATA in its anticipated $$1\pi $$ configuration on these estimates is deduced via Geant4 simulations.