Backward spin polarizability $��_��$ of the proton

Using the Mainz $48 cm ��\times 64 cm$ NaI(Tl) detector and the segmented G��ttingen recoil detector SENECA in coincidence, Compton scattering by the proton at $��^{lab}_��= 136^\circ$ has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities $��_��= - 38.7 \pm 1.8$ and $-27.2 \pm 3.1$ $(\times 10^{-4}fm^4)$, respectively.