Passive integrator for PRA / PMT

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peterpion
Posts: 46
Joined: Mon Mar 21, 2005 5:08 pm
Real name: PeteRowse

Passive integrator for PRA / PMT

Post by peterpion »

Hi, thought I would post this as a different post rather than include in previous post as its quite a different topic. Please move is wrong place.

Regarding using PRA I've been searching the forums and finding many references that say the information I am asking about is already there in many places so I am sorry to post about it. If someone could point me towards the right posts or search strings that would be much appreciated.

My query is about (the frequently asked topic of) getting PRA working (well). I have a scintillation counter (geo bicron I think they are called, someone else posted a picture of one). But its heavily modified with a new dynode divider and a second socket on the probe for taking off the signal. I've used a variable PSU I built which seems fine, but I am having problems integrating the pulses so they are suitable for PRA. Ive read several posts referring to Richards FAQs describing how to extract a signal but I can't find those posts, and also I have read that a (quote) simple passive resistor capacitor integrator as described on these forums is known to work well, but I can't find those posts either.

In PRA I don't get any kind of spectrum, but on a scope I can see the pulses are good. So I think it must be the passive integrator I need, my pulses are probably not stretched enough. Unless its an impedance problem with the soundcard. But this has probably been covered already in the previous posts on this topic.

Also I saw some posts by Steven S who is apparently selling an interface device, I wonder if this is required for good results, and if not, well there must be some point to having it, could anyone say what advantages it provides?

Can anyone help me find the posts that discuss this passive integrator?

TIA, Pete
peterpion
Posts: 46
Joined: Mon Mar 21, 2005 5:08 pm
Real name: PeteRowse

Re: Passive integrator for PRA / PMT

Post by peterpion »

Thought I better answer this question since I have found the answer myself (or rather 'an answer'), and leaving it dangling here without a solution seemed the wrong thing to do.

I am sure there are others here who already know of it, but for people like me who do not, and find this question looking for the same thing, I found a project called theremino (http://theremino.com/en/blog/gamma-spectrometry/), which tackles the topic I asked about (although its not passive but just has an active buffer after the pulse stretching section). It also provides software although I don't know if its got any advantage over PRA, but as for the hardware, they have put together a circuit which both generates the HV for a PMT and also stretches and buffers the output pulses. They claim to have shaved a couple of percent off the best yet PRA FWHM with software tweaks too, but I can't vouch for that yet.

Personally, already having a HV psu, all I needed was the pulse stretcher and buffer (shaded red in the schematic), but I also decided to use their modified USB sound card idea (not on schematic), which gave me 192kHz sampling, as well as adding a high pass filter right at the sound card to remove noise introduced by handling the cable from the PMT to the sound card. Whether this helps I don't know, but the USB sound sticks are only about 2 dollars each so its no real loss. I also did not use C8 as I already had a DC blocking capacitor in my PMT assembly (this capacitor is only required if your input to this circuit is high voltage direct from the PMT, I connected the PMT output directly to the C8-C9 junction).

The creators of the project have stated that its completely freeware, so, in case the project disappears, I will attach the schematic , I hope this is OK by the forum rules, I have not seen anything to the contrary. It might be useful for others to link to in the future too. The circuit certainly does produce nice slow pulses which with 192kHz sampling, look very smoothly sampled in the oscilloscope window in the accompanying software. The designers power it from the USB 5V supply, but I am currently using 6V of batteries, which makes managing noise that bit easier to start off with. I have also attached their PCB layout since even if you are building this dead bug style or on matrix board, its useful to have a layout to start with and adapt IMO. Also, the designers say that the mosfet used in the PSU past of the circuit has no replacement as its required to work at a very low voltage, so if you are building the PSU but want to use a different mosfet you might need to think about how to do it. Also, shorting SV1 results in higher voltage at the price of higher ripple apparently. I recommend going to the original project home if you want to build it though, if its still available, as there is much more info available at the project home page.

I guess the answer to my original question (passive integrator) would be the components R8, C9, R9, C10, R10 and C11, but after passing through that network, the signal will be very weak, and trying to put it into a 50 ohm bit of co-ax or even directly into a medium impedance sound card will probably not work well, but the buffer stage after seems to work well.

Hope this is useful to someone sometime!
PmtAdapter_V3.3_SCH_NoText_small.jpg
PmtAdapter_V3.3_3D_Bottom_800_small.jpg
PmtAdapter_V3.3_3D_Top_NoComp_800_small.jpg
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Richard Hull
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Re: Passive integrator for PRA / PMT

Post by Richard Hull »

Thanks for this post. This is an interesting and useful little PMT supply and amp buffer. I would want to use battery power too. Really nice.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
peterpion
Posts: 46
Joined: Mon Mar 21, 2005 5:08 pm
Real name: PeteRowse

Re: Passive integrator for PRA / PMT

Post by peterpion »

I thought it was pretty clever squeezing the psu plus buffer into so few components - the only drawback I suspect is getting that mosfet, but if you would use batteries, which seems better considering the low current drain, then I imagine the circuit could be run at a higher voltage, maybe 12v, and then probably the mosfet choice would be much easier. Or possibly a dc-dc converter if you really want USB power. The designers do say that noise / ripple has to be very carefully managed with short wires etc, so additionally I might be tempted to split the board into 2 parts and screen them from each other a bit better to make construction less exact perhaps.
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