Comments on the possibility of replacing RICH1 with threshold Cherenkovs

The replacement of RICH1 with two threshold Cherenkov counters is a relatively simple and "cheap" option. Many counters of this type exist and have been operated successfully over many years. For these reasons this option was put forward in the CHEOPS Letter of Intent, despite the well-known drawbacks of this solution in comparison with the RICH option.

The main advantages of the RICH solution are:

1. the inherent very high granularity, which allows to disentangle the Cherenkov rings from close tracks. Quite complicated patterns are expected for COMPASS events on the basis of the present Monte Carlo simulations;
2. the higher number of photons ( one radiator rather than two in the same space), which gives a higher efficiency;
3. the upper momentum limit for particle separation is much higher, since it does not depend on the threshold.

The disadvantages of the RICH solution are its cost and its complexity, which could affect the time of construction. Since anyway one RICH will be built, the technical problems will be solved, and there remains the less complex task of duplicating the construction efforts within the given cost and time frames.

On the other hand, photon detection with MWPC's with CsI photo-cathodes allows to obtain an excellent granularity at reasonable costs: after several years of R&D [23], the proposed technique has proven to be feasible, and guarantees performances which are just unthinkable by using PM's. The presently remaining uncertainties on the efficiencies of mass-produced photo-cathodes do not affect us much. Our estimate of 34 photo-electrons per particle is based on CERN test results on the five large-area photo-cathodes which have been constructed in 1995. Some groups (HERA-B, HADES) do not reproduce the CERN results and claim QE values a factor of about 1.5 smaller. This would still leave us with more than twenty photo-electrons per ring, which is known to give excellent results (for comparison, the Omega-RICH had about 15 photo-electrons per ring).

The main parameters of the two options are given in Table 3.16.

  
Table 3.16: Comparison of Cherenkov parameters for the two options.

The two threshold Cherenkovs have the same total length as the RICH, and their design is that given in the CHEOPS Letter of Intent.