Belle

The Belle spectrometer is built around an Oxford Instruments superconducting magnet provided by Physikon1 dedicated to studies with high transverse fields and high frequencies. To this end, its counters all have short light guides with high-field mesh dynode phototubes: the muon counter and active collimator each have four phototubes, and are mounted in the beamline vacuum; the four positron counters and the muon veto are from a cryostat insert and sample holder. The magnet is a superconducting split solenoid delivering 7.5 T (75 kG) from 100A (750 G/A) and which runs in persistent mode. The Oxford Instruments cryostat is a hybrid between a gas flow and cold finger cryostat as the helium flow is through a ring around the sample area - the sample is cooled by helium exchange gas surrounding it. (This system has the benefit of not having the gas become very dense at low temperatures.) There is a phase separator on the inlet and counterflow cooling in the transfer line, allowing the cryostat to reach 1.7K. Although the counters are optimized for high transverse fields (with spin-rotated muons), Belle can be used for longitudinal field experiments by using the muon veto counter as the forward counter and the active collimator as the backward.