Requirements and Environment

The measurement of CP violation requires an interaction rate of around 40 MHz, i.e. 5 interactions per bunch crossing. One has to compare this with the natural loss rate of the proton beam. With a typical current of 80 mA (i.e. protons) and a lifetime of 100 hours the HERA proton beam just loses 30 MHz of protons. This demonstrates that the target has to collect very efficiently the protons before they get lost, and that the target has to scrape away protons from the tails of the beam in case the initial lifetime is too high. The target efficiency is defined as the ratio between the interaction rate in the HERA-B target and the total HERA proton loss rate, which is given by the current and the lifetime. A target efficiency above 50% is aspired not to reduce the proton lifetime below 50 hours. At this accepted level the target don't cut severely into the efficiency of the other HERA experiment because the HERA luminosity lifetime is usually less than 10 h, mainly determined by the electron lifetime and the emittance growth. The interactions produced on the target follows the Poisson statistics:

where describes the probability to observe n interactions in a bunch crossing (bx) if the mean number of interactions per bx is . The variance of the Poisson distribution is equal to the mean value , i.e. one gets a broad distribution. The capability of the HERA-B detector, optimized for a mean of five overlaid events, is limited by high occupancies and high radiation doses. The following lists summarizes the basic operation conditions to the target by means of the three most important efficiency requirements:

Rate and Target Efficiency:

To achieve the aspired rate of 40 MHz the impact on HERA and the other HERA experiments has to be small. This requires a very high target efficiency of at least 50% and an effective reduction of background produced in the target.

Running Efficiency:

A nearly continuous operation of the target is necessary to obtain  sec measurement time within one year. The target steering has to be therefore very secure and has to avoid any harm or even the loss of the proton beam; which would then cost at least several hours to refill HERA. In addition it has to be very reliable, fast and easy to be operated. A proper online monitoring is necessary to recognize problems, e.g. in the rate stability or the background very early. And last but not least the coordination with HERA and the other HERA experiments is essential to obtain an effective use of filled proton beams.

Reconstruction Efficiency:

Due to the limitations of the HERA-B detector capabilities in resolving events with very much interactions a constant rate without spikes but with equal distribution from all wires for all filled proton bunches is needed. In addition the interactions should come out of a small time window (  nsec) within the 96 nsec bunch distance.