A new avalanche photodiode arrangement has been installed in cell 4 to receive X-rays scattered from a beam absorber. This is similar to the one currently used operationally in cell 19 to check the purity of the beam delivery in single and 16 bunch modes. This new device, however, accepts a much higher count rate with a gated electronic detection system allowing purity measurements to be made with a higher dynamic range (parasitic bunches at a few x 10-10 in single bunch can now be detected.).