Photon detector is one of the main types of detectors in high-energy and nuclear physics. Photon counting is a fundamental method for the acquisition of maximum information from the light signal. This method is widely used for the detection of very weak light signals. PMTs with discrete dynodes, MCP-PMTs, avalanche diodes, hybrid PMTs are usually used as photon counting detectors. As compared to the other types of detectors MCP-PMT have the advantage of high gain and fast response, larger active area, low power consumption, stable operation under exposure to magnetic fields. Semiconductor optical detectors have smaller active area, lower time resolution and worse noise performance as compared to MCP-PMTs.
Low noise level is the necessary condition for photon counting. Detection efficiency is strongly dependent on the threshold level set for the photon-counting mode (discrimination). Pulse height distribution is the critical feature for operation in photon-counting mode. Lower pulse-height resolution and higher peak-to-valley ratio provide better photon counting. The lower threshold of the pulse height is usually set in the valley position.
A photon counting photomultiplier tube (PMT) Sapphire-2A based on MCP for operation in the UV spectral range has been designed exhibiting a unique value of peak-to-valley ratio. The device parameters are listed in the information sheet. Counting characteristic and pulse height distribution are presented in the figures.
In terms of construction MCP-PMT Sapphire-2A is a glass-to-metal vacuum unit with a cesium-telluride photocathode deposited on a magnesium fluoride substrate, a chevron stack of two microchannel plates and a metal anode. Electrons from the photocathode are focused on the MCP input by electrostatic lens field. Sapphire-2A peak-to-valley ratio is typically 20 whereas dynode PMTs for similar applications have typical value of about 2. Such a high peak-to-valley ratio is not documented in literature for any MCP-PMT.
High gain level of electron flows generated by chevron stacks of MCPs, low dark currents, low natural noise in the UV spectral range allow for the detection of superweak UV signals and single photon counting at the device input.
It must be stressed that the use of PMTs with secondary electron emission provided by a chevron stack of MCPs instead of conventional dynodes allows for improving time resolution, applying gating for noise level reduction and increasing resistance to magnetic fields. MCP-PMT designed with a solar-blind photocathode makes it possible to improve threshold sensitivity due to the leveling of ambient light background. PMT design with electrostatic focusing of photoelectrons at the first MCP input along with the efficient ways of outgassing increases PMT resistance to overloads and improves real operating life as compared to biplanar PMTs.