A list of PMT directions sorted by PMT channel, on DU-1, floor-1

def pmt_angles(self):
        """A list of PMT directions sorted by PMT channel, on DU-1, floor-1"""
        if self._pmt_angles == []:
            mask = (self.pmts.du == 1) & (self.pmts.floor == 1)
            self._pmt_angles = self.pmts.dir[mask]
        return self._pmt_angles