Modeling a Photomultiplier Tube - Page 1
Forum section: electronics / projects

author: Randall W. Lott
author_rank: Regular Contributor
timestamp: June 03, 2015, 01:12:53 am
content: Hey guys,I've come across a very involved project of developing a low-noise transimpedance amplifier (TIA) for a current-output PMT module.There seems to be a whole lot of information on photodiode amplifiers, but not for PMTs.It's relatively hard to prototype a circuit for these modules, because they are being used for a wide range of light intensities and they are currently used in a light-controlled environment.I've tried to measure the output impedance of the module, but I obviously failed to get any reading. Also, I don't have access to a pico-ammeter capable of capturing these very small current signals.The dark-current of the PMT is around 20pA and the maximum output current specified by the datasheet is 10uA, which is not true. I was able to measure up to 800uA for some pulses (4Vp on the TIA output across a 5k feedback resistor). I've tried to contact Hamamatsu, but they haven't helped.Thank you!

author: moffy
author_rank: Super Contributor
timestamp: June 03, 2015, 02:48:04 am
content: I am sorry, but what exactly is your question? To model one I assume they look like a very high impedance current source with small leakage. To measure leakage current use a fA input op amp (LMCXXXX - CMOS) as an integrator and measure the output after a suitable interval. Make sure you null out your offsets, and leakage currents on the board can be a killer.

author: LukeW
author_rank: Frequent Contributor
timestamp: June 03, 2015, 03:00:36 am
content: What's the application?Single photon counting? Gamma-ray spectroscopy etc?Do you need to preserve timing information? Do you need to preserve energy information, eg. for spectroscopy?http://www.becker-hickl.com/pdf/ampmt.pdfhttp://www.cremat.com/PMTs.htmwww.ortec-online.com/download/Preamplifier-Introduction.pdfMight be helpful.

author: Randall W. Lott
author_rank: Regular Contributor
timestamp: June 03, 2015, 04:58:59 am
content: It's for flow cytometry. I need to maintain the Gaussian pulse shape, but also cater for a wide range of light intensities.Thank you!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2015, 09:03:51 am
content: If you know the intensity and gain ranges, I should think a current source and capacitor is a good enough model.There's not really much about the V-I characteristics there, it's a high impedance, and delivers current (or charge, if you're only getting blips and pulses, as in some astronomy and physics applications).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2015, 07:21:48 pm
content: Usually the PMT has a large a mount of internal gain, and thus there is no need for a really low noise amplifier after that.At a fixed gain the PMT does not offer a large dynamic range (high S/N ration), so the amplifier should not be so difficult. This is especially true for low light signal where a PMT might be useful.The PMT gain depends very much on it's supply voltage and often can be adjusted. So the dark signal also depends very much on it. This may be essentially 0, a small very noisy current or at high gains sizable individually resolvable pulses (e.g. 10 to 1000s per second).The output is rather close to a perfect current source, with little (few pF) of capacity.My feeling is that flow cytometry might not really need a PMT and might be better of with photodiodes (possibly avalanche type and maybe CCD chips). A PMT is great at very low levels of blue or UV light and if a large area or very fast (ns range) detector is needed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2015, 08:05:52 pm
content: CCDs would be interesting as you could track multiple shadows across the sensor; it would take a huge amount more processing power of course, since you'd need the image analysis stuff. (That said, probably nothing an RPi can't do.) Might be useful for high flow and mixed cell identification. (Go nuts, add stains and identify by color? Add multispectral sensors to the mix?)Tim