How to count PMT pulses Victor Kornilov Sternberg astronomical institute I count photons 25 years.

Classical scheme of a photon counting Pulses from PMT A~3mV, I<10  A Pulses from amplifier to counter A~3V, I~60mA

Problems Both signals have the same frequency characteristics And output pulses influent at input pulses Difficulties are enlarged for pulses < 30ns Ground problem, External noise Evident solution: To count pulses near amplifier, in this case a system of data transmission into host computer needed. (Hamamatsu and ETL produce a photometric modules with RS232 interface)

Devices control and Data acquisition Physical layer 1 – RS485 (Recommended Standard) interface Physical layer 2 – Inexpensive components (RISC  controllers) Logical layer 1 - Exchange by bytes united in addressed packets, terminated by special signals, CRC Logical layer 2 – Nonsymmetrical structure, exchange via host only, passive, active, induced transaction Logical layer 3 – compact commands, data packets, direct access to module parameters, containers Interface boards, converters, drivers, software. We use this unnamed system 5 years (described in MASS documentation).

Transaction modes 1 - Command (reply signal only) 2 – Data request 3 – Active data transmission

Properties of the RS485 Communicate multiple nodes bi-directionally over a single twisted pair. Excellent noise rejection (differential balanced) High data rate with long cable General robustness.

Considerations of RS485 Configuration Termination and stubs Data rate vs cable length Cabling Fail-safe biasing Grounding and shielding Useful papers: Application noteswww.national.com RS-422 and RS-485 Application Note

Amplifier-discriminator Preamplifier - transimpendance SA5205, band 600MHz, K=9 (Philips) Pulse discrimination – Ultra fast comparator AD8611, 4ns Digital control of discrimination level Noise <200  V Deadtime with Hamamatsu R647 – 17ns, with R7400 ~10ns Permits to measure fluxes up to 10 Mp/s (if counter permits too…)

Counter, control and exchange Pre-counter 74HC(T)4520, fmax=60MHz Kernel -  controller ATMega channels with 16 bits, may be 24 bits Hardware generation of integration strobe In system reprogramming of  -programm PMT current control Exchange rate up to 2Mbit/s