During the year 2000 the HERA positron beam was unintentionally lost several times in the vicinity of the HERMES interaction point. As a result the leakage current of the prototype module of the Lambda Wheels increased dramatically. Such events will reduce the life time of the silicon detector and the read-out chips. For that reason it was decided to develop a Beam-Loss Monitor (BLM), which creates a trigger signal if the beam starts to diverge from its usual orbit. The trigger signal activates a kicker magnet that is installed in the HERA positron ring. The kicker deflects the beam such that it is dumped in a controlled fashion.

The BLM consists of two sets of three polystyrene ionisation chambers and one dummy detector of the same size. The figure show the BLM as it was installed at HERMES in 2002. A trigger is generated if two of the three chambers generate a signal above threshold, while the dummy gives no signal. After construction the BLM was calibrated at an intense X-ray source in Mons (Belgium). As a result the output of the BLM can now be specified in units of Gy/s.

Sideview of the Beam Loss Monitor. The dummy chamber is shown on the far right of the figure.
The gas flow, the support structure and a cross sectional view are shown as well.

Since the spring of 2002 the BLM is fully operational. Using the BLM three types of beam loss events have been observed: (i) fast losses (within one HERA revolution) causing very little radiation; (ii) losses spread out over many hundreds of revolutions, which cause a fair amount of radiation; and (iii) slow losses, causing a significant amount of excess radiation. The BLM is able to dump the beam for the last two event classes within a few milliseconds, and has done so in a few cases. The BLM turns out to be the fastest beam-dump system of all HERA experiments.