Eliminating the effect of SRC beam-energy spread on the BigRIPS focal plane

At RIKEN RIBF, precise and systematic studies on the 1s binding energies and widths of pionic-Sn atom usig the Sn(d,$^3$He) pion-transfer reaction are planned, but a relatively large energy spread of the RIKEN Superconducting Ring Cyclotron (SRC) poses a difficulty in achieving the required resolution. ``Dispersion matching'' is a way of adjusting the optical settings of the beam line to overcome this problem. It makes positions of particles on a focal plane independent of the beam-momentum spread, while ensuring the particles with different Q values focused at the different positions on the focal plane. We recently made an experiment for testing the dispersion matching at RIKEN. In this experiment, a $^{14}$N beam of 250 MeV/nucleon was used; the value of energy per nucleon being the same as that of the deuteron which will be used in the experiment on pionic-Sn atom. We tried some ion-optical settings and verified that particles with a finite momentum spread could be focused to one point. In the Hawaii meeting, we will present the detailed analysis of this test experiment.