High resolution Kaon spectrometer (HKS)

General specifications of the HKS spectrometer are given in Table. 2. The HKS is designed to achieve simultaneously 2 $\times$ 10$^{-4}$ momentum resolution and 20 msr solid angle acceptance with the splitter. Figure 16 shows the angular and momentum acceptance of the HKS spectrometer with the splitter calculated with a GEANT simulation code. The HKS is placed rotated horizontally by 7 degrees with respect to the beam to avoid zero degree positive particles, mostly positrons. The solid angle is more than 20 msr over the momentum region of 1.2 GeV/$c$ $\pm$ 10%, as designed. Figure 17 shows the momentum resolution obtained with the code as a function of momentum for three different positions of the tracking chambers with respect to the focal plane. In the simulation, realistic matter distributions such as a vacuum window and chamber windows, and drift chamber position resolutions were taken into account. Even with a modest chamber resolution of 200 $\mu$m, momentum resolution of $2 \times 10^{-4}$ will be achieved.

The detector system for the HKS and Enge is summarized in Table 3. The HKS detector system has similar configuration to the one in SOS. However, as seen in Table 4, HKS singles rate is dominated by pions, whose rate will be up to a few MHz. In order to achieve efficient pion rejection rate as high as 10$^{-4}$, two layers of aerogel Cerenkov counter with refractive index of 1.055 will be installed. For the proton rejection, Lucite Cerenkov counter with wavelength shifter in it will be employed, so that the Cerenkov counter has good efficiency for the wide range of incident angles. Time resolution of as good as 80 ps is a goal for the time-of-flight scintillators. By having the good time resolution, we plan to minimize the distance between the two time-of-flight wall and achieve large solid angle. A Gas Cerenkov counter will also be installed as a trigger counter so that positrons can be tagged for calibration purposes. The tracking chambers of HKS should have high rate capability and accept the rate up to a few MHz.