Unfolding the response of a zero-degree magnetic spectrometer from measurements of the Δ resonance

The magnetic spectrometer FRagment Separator at GSI has been used to investigate the in-medium Delta-resonance excitation in peripheral heavy-ion reactions. The resolving power of this spectrometer makes it possible to disentangle the longitudinal-momentum loss induced by the excitation of the Delta resonance in the projectile residues produced in isobaric charge-exchange collisions. However, beam emittance, electromagnetic interactions of projectile and residual nuclei in the target, and the accuracy of the tracking detectors limit the final resolution. The characterization of the Delta resonance requires then to unfold the measured longitudinal-momentum distribution from the response of the spectrometer. In this work, we use an unfolding procedure based on the Richardson-Lucy method with a regularization technique to optimize the stability of the solution against statistical fluctuations. The method is validated using measurements of isobaric charge-changing collisions with a Xe-136 beam at 500 A MeV. (C) 2012 Elsevier B.V. All rights reserved.