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<10A

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: www.national.com, Application notes

www.bb-europe.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

~10nsPermits 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 ATMega8-16

• 2 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