HKS Water Cerenkov counter

Figure 45 shows the refraction index dependence of the number of photoelectrons from the Cerenkov counter. In the momentum region of interest (1.05 $\sim$ 1.35 GeV/$c$), protons in Lucite (n=1.48) emit over 50% Cerenkov photons of it for K$^+$ in Lucite; the on-line separation of protons was difficult with the Lucite Cerenkov counters that have been used for the E89-009 experiment. Cerenkov light emission in water by protons is much less and water Cerenkov is more suitable for K$^+$, p separation. Therefore, water Cerenkov counters were chosen instead of Lucite ones in the HKS spectrometer. Two layers of counters (HWC1 and HWC2) are prepared to achieve an on-line rejection efficiency of $5\times10^{-4}$ for protons, which will result in a proton contamination rate in the kaon trigger below 100 Hz. These counters will be installed behind the second TOF wall. A wavelength shifter (amino-G-salt) will be doped in pure water to make the light response isotropic and to increase the number of Cerenkov photons which can be detected by PMT. Figure 46 shows a schematic structure of the water Cerenkov counter. The effective volume of one segment is 150 $\times$ 350 $\times$ 75 mm$^3$. The box is made of white acrylic plates with two UVT transparent windows viewed by Hamamatsu 2''$\phi$ PMTs (H7135).

Diffusion reflectance of various materials (Millipore membrane, Goretex sheet, titanium dioxide paint and acrylic plate $etc.$) were measured with spectrophotometers (Shimadzu UV-2400 and ISR-240A). Figure 47 shows the result of the reflectance measurements and PMT's quantum efficiency. Taking chemical stability and easiness of fabrication into account, we decided to make the water Cerenkov box of white acryl without any reflection material inside.

With 1.2 GeV/$c$ unseparated beam at KEK-PS, the prototype version was beam-tested. Figure 48 shows number of photoelectrons detected by the prototype water Cerenkov counters filled with de-ionized water ($> 18$M$\Omega\cdot$cm) and a wavelength shifter, 10 mg/$l$ amino-G-salt (2-amino-6,8-naphthalene-disulfonic acid). The amino-G-salt solution gave much larger number of photoelectrons than pure water, and thus it is easier to separate K$^+$s from protons.

The responses of the water Cerenkov counter to 1.05$\sim$1.35 GeV/$c$ K$^+$ and protons were measured at KEK and a realistic beam profile at the water Cerenkov counter position in the HKS setup was estimated by GEANT Monte Carlo simulation. Combining both results, the kaon detection probability and proton contamination to the kaon trigger were calculated (figure 49). The result is that one layer of WC counters detect 97.3% of kaons and 2.3% of protons remain in the kaon trigger. Therefore, two layers of WC reduce the proton rate to 5 $\times 10^{-4}$ and it satisfies our requirement. Mass production of the water Cerenkov box is now in progress and they will be ready for shipping to Jlab by mid. June.