  A novel system to monitor the intensity and the stability of a bunched beam
of $\simeq 1.2\times 10^{7}$ antiprotons ($\bar{p}$s) with a length of $\simeq$
250 ns (FWHM) and to measure its trapping efficiency in a Penning trap is
described. This system operates parasitically detecting the pions from the
annihilation of part of the beam in a degrader. Six plastic scintillators have
been coupled from one side to six proximity focused HPDs without preamplifiers
and operating in current mode. This device works in the stray field of the
ATHENA magnet with no loss of efficiency; the gain can be varied from zero to a
few thousands with a precision better than 0.1% and the dynamic range is larger
than 8 orders of magnitude. Linearity and stability have been measured up to
charge responses of 100 nC, corresponding to the beam completely dumped. The
beam counter has been calibrated in two different and independent ways giving
consistent results.
