Proposal for the BELLE muon system upgrade on the basis of scintillator technique

The BELLE KL and muon detector (KLM) is based on resistive plate chambers (RPC). Already at the present background rate of about 0.1 Hz/cm2 in the end cap region RPC efficiency is about 95% only. The background rate at Supper KEKB is expected to be an order of magnitude higher. This will result in unacceptably low RPC efficiency (below 50%). It is believed that the background is dominated by neutrons. It might be possible to reduce the background somewhat by putting light material moderator after the lead shields at the detector sides facing the tunnel. However it is not easy to estimate the background rate. It will be known reliably only after commissioning of the Super KEKB and the upgrade of the BELLE detector. Therefore it looks more prudent to substitute RPC with a faster detector at least in the end cap region. KLM based on scintillator counters with wave length shifting (WLS) fibers can cope with the background rate at least two orders of magnitude higher than the present one. The conceptual level properties of KLM based on scintillator technique and feasibility of its construction are discussed in this paper. The proposed system consists of two independent scintillator planes forming a superlayer in every gap in the magnet yoke. KLM based on scintillator counters with WLS fiber light collection can work efficiently at the background rates 100 times larger than the present one. This is a well established experimental technique in case of multichannel PM (Photo-Multiplier) readout. It is even more attractive and elegant with the SiPM readout. However in this case a lot of R and D is still required. (M. Suetake)