Gated-Dynode photomultiplier tube assembly for the boronated scintillator detector of the ISS-CREAM experiment

We present the design of a four-photomultiplier tube (PMT) assembly for measuring light generation in the Boronated Scintillator Detector (BSD) of the Cosmic Ray Energetics And Mass experiment for the International Space Station (ISS-CREAM). The light produced in the scintillator includes a contribution from the capture of thermalized neutrons produced in cosmic ray interactions, as well as delayed fluorescence, in the range of several microseconds following the incident cosmic ray impact. The BSD is held in an insensitive state until a cosmic-ray trigger causes a rapid turn on of the PMTs to detect late light signals, several microseconds after the trigger. The design includes two pulsed high-voltage supplies, which serve to gate the first two dynodes of the Hamamatsu R1924A PMTs used in the assembly. When the dynode gates are held in the "off" state, this scheme is shown to suppress >10⁵ photons from injection into the tube's multiplier stage, while being capable of rapid switching into a stable operating mode in less than 1 $\mu$s. This mitigates any backgrounds from afterpulsing that might otherwise be present in the PMT response following the cosmic ray incidence.