FM-PMTs

Hamamatsu (HPK) type R6680 fine-mesh photomultipliers, with a 2-inch diameter and 24 stages of 2000 mesh/inch dynodes, have been selected for the TOF counter. The 24 dynode stages provide a gain of $3 \times 10^6$ at a high voltage below 2800 V in a magnetic field of 1.5 T. The bialkali photocathode with an effective diameter of 39 mm covers 50 % of the end area of each TOF counter. The transit time spread is 320 ps (rms), the rise and fall times are 3.5 and 4.5 ns, respectively, and the pulse width is about 6 ns at FWHM. Figs. (a) and (b) show gains and time resolutions of eight typical FM-PMTs as a function of magnetic field strength. The measurements were performed with the field parallel to the tube axis, using an N2-dye laser of 420 nm wavelength. Although the gain was reduced by a factor of about 200 at 1.5 T, the time resolution was not significantly affected. Degradation of the time resolution of about 10 to 15 % was observed when the magnetic field was raised from 0 to 1.5 T. All FM-PMTs were tested for their gains and time resolutions before the final module assembly.

Figure: 1.5 T field test of FM-PMTs. (a) Gains and (b) time resolutions as a function of magnetic field.

FM-PMTs were attached to the TOF counter ends with an air gap of $\sim$0.1 mm. In the case of the TSC counters the tubes were glued to the light guides at the backward ends. The air gap for the TOF counter selectively passes earlier arrival photons and reduces a gain saturation effect of FM-PMT due to large pulses at a very high rate. As the time resolution is determined by the rising edge of the time profile of arrival photons at PMT, the air gap hardly affects the time resolution.