Charge sensitive amplifier for photomultiplier - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 12, 2020, 05:00:16 pm
content: Hey all!So I've recently scored a vintage NIM bin full of modules - 0-5KV PSU, shaping amplifiers etc. See attached.Either way I want to try connecting some different detectors to it, namely PMT based detectors and maybe also some standard GM-tubes and such.Problem is with the photomultipliers atleast, is that they need a preamp, and they're usually fairly pricey on ebay. I haven't been able to find many preamp designs for scintillation counters on the internet, and the manuals for the proffessional ones only has fairly vague schematics. I have no real idea how to design something like this, but I've whipped something up based on the sparse info in the litterature. This brochure has a bit of theory:https://www.ortec-online.com/-/media/ametekortec/manuals/113-mnl.pdfSee attached.I'm not sure what kind of amplifier is used, but every block diagram seems to have the preamp at unity gain.Does this look reasonable, or have I missed something? Does anyone have a schematic for an actual scintillation counter preamp?Thanks in advance!

author: jmelson
author_rank: Super Contributor
timestamp: January 12, 2020, 06:39:20 pm
content: Quote from: ChristofferB on January 12, 2020, 05:00:16 pmWell, you may NOT actually need a preamp, and you often don't need a CHARGE sensitive preamp. Given a decent amount of lightfrom whatever you are doing, and enough voltage on the PMT, you can get a Volt or more of signal. Due to space charge effects,the tubes start to get non-linear with much over a Volt of signal.The charge sensitive preamp is just one where the feedback capacitor dominates the response, and the DC gain is set highby a large-value feedback resistor.Back in the day (consistent with that NIM gear you got) a preamp was a whole module full of discrete transistors. Nowadays, even thecheapest op-amp is more sophisticated that that.Jon

author: jmelson
author_rank: Super Contributor
timestamp: January 12, 2020, 06:43:19 pm
content: Your schematic appears that it is for a Geiger-Mueller tube, not a PMT. Generally, the safest way to go is with a negative supply, and then the tube anode is at ground. The other option is a positive supply, where the anode is at the power supply voltage, and the signal is capacitively coupled to whatever isreceiving it. That puts the full power supply voltage across the coupling capacitor, if it ever breaks down your electronics are SHOT!Jon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 12, 2020, 06:57:00 pm
content: Well this is for a scintillation counter - so a crystal that's kept in complete darkness, and then gives a light when a particle passes through it. Maybe I can try without, but adding an opamp buffer couldn't hurt could it?The circuit is very close to what is seen in the link I provided, and it should be usable with both positive or negative HV bias on the tube. The original, that is.Alternatively, since I have plenty of NIM amplifiers, maybe I could just have the shunt RC circuit and then ommit the amplifier?Thanks for the interest!--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2020, 09:04:13 pm
content: With a PMT one can set the PMT gain relatively high and than do not need a very special amplifier. The main first decision is wether the kathode or anode of the PMT is near ground. With the anode = output at ground the amplifier is not that complicated unless very fast response is needed. A relatively normal OP should be sufficient. The type of amplifier depends on how the peak measurement is done:The classical way is to have the amplifier do some peak shaping and than have a circuit to measure the peak high or area. The slower the amplifier the wider the peaks.Beside analog peak capture a fast ADC could sample the whole peak to calculate the area.An alternative way would be to have the amplifier more charge amplifier like, this is with a very long time constant. The current peaks than correspond to steps in the charge that could be measured more directly with relatively slow sampling and a discharge from time to time. If the charge gets to large.I have done such a circuit some years ago, with no really special parts except the PMT and Scintillator. AFAIR the amplifier was something like LF356 - so nothing special. The peak measurement was with 2 OPs peak hold circuit and an AVR µC to digitize the peak hight. The slightly tricky part is the high voltage supply that needs to be quite stable, as the PMT gain depends on the voltage.With not to high a count rate the peaks can be wide enough to get away with relatively slow amplifiers.

author: sdouble
author_rank: Frequent Contributor
timestamp: January 12, 2020, 10:54:04 pm
content: I must partly disagree with what was written above.Typical spectroscopic chain inclide a charge sensitive preamp attached to your PMT output, a shaping amplifier to increase the signal amplitude and to reduce the analog bandwidth to improve the signal/noise and finally and ADC to get the total charge produce by your PMT.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 12, 2020, 11:50:16 pm
content: Yeah, that's what I was planning to do as well. The pulse shape is of big importance, as I've understood it, and all the amplifier inputs are 93 ohm. That's pretty low impedance to feed a current pulse into. But hey, Isn't a simple op amp buffer enough to do the trick? I've never really dealt with charge sensitive circuits like this before.Thanks,--Chris

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 13, 2020, 12:23:35 am
content: But omitting an amplifier is not going to ruin my PMT as i see it, so I guess the solution is to make a HV tee/preamp box with ROOM for a little buffer, and then compare preformance.Thanks for the insight!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 13, 2020, 11:55:42 am
content: I found a design from physicsopenlab that's exactly as simple as I was hoping for, while still giving decent peak shapes:I am going to attempt to copy this, but with a few more features, like selectable capacitance, and I will also give it a split rail +-6VPSU so I can get the full pos/neg output swing.Maybe the HV bias could be built in to this box as well, to save myself an additional MHV cable. Maybe It should all just be built into the PMT base.ref:http://physicsopenlab.org/2016/04/21/pmt-pulse-processing/Thanks for the insight!--Chris

author: jmelson
author_rank: Super Contributor
timestamp: January 13, 2020, 09:02:12 pm
content: Quote from: ChristofferB on January 12, 2020, 06:57:00 pmYou certainly do not need a charge-sensitive amp to count pulses. Just a simple linear amp, or maybe no amp at all if your signal levels are big enough.Generally, all PMT tube bases have the signal on a separate cable than the HV. It just all works much better that way, and only a few unitsI've seen do it the one-cable way. We have a Victoreen meter that can use EITHER a GM tube or a scintillator/PMT that is like that.Do you already have a PMT with tube base?Jon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 13, 2020, 10:05:06 pm
content: Yeah making a base with two connectors might be the nicest solution. I don't have a PMT yet, but I'm eyeing one of this guy's kits on the Ebay:eBay auction: #https://www.ebay.com/itm/3-Scintillation-Detector-BASIC-KIT-Includes-PMT-Scintillator-Electronics-BNC/263250251252It's pretty cheap, it's a nice, large area PMT and I get a Scintillation crystal in the same diameter. Oh and that specific PMT and scintillator are the EXACT dimensions to fit inside a standard danish PVC drain pipeso making a light proof enclosure should be a breeze. Other people has just used humongous diameters of heat shrink but I think that looks a bit dodgy.Either way, The last preamp circuit is like, a 5 minute build, so nothing is lost trying with and without and see the difference.thanks again!

author: richnormand
author_rank: Unknown
timestamp: January 13, 2020, 10:51:03 pm
content: I would recommend you go through this classic before doing your project:https://psec.uchicago.edu/links/Photomultiplier_Handbook.pdfIt is a reprint of the old RCA photomultiplier handbook. In particular the tube divider base wiring, coupling and preamp requirements for different applications.I mount my scintillator/tube in black pvc pipes. with matching end caps for gamma work. Works quite well.I also used four big coffee cans for two 5" plastic scintillators.The two units are mounted a meter apart and feed nim preamps, delays, coincidence gates and correlators for a directional muon telescope.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 12:43:15 am
content: Oh that really is cool! I'll have to read up on that! - And I see the charge sensitive detector is still just one op amp. These new silicon photomultipliers are really like magic. Maybe I should try adding one of those to my setup as well.I was just messing around with a blinded PIN diode with a charge sensitive amplifier as a quick test, it didn't really display any pulses that could be attributed to radiation but it DID put out a very sharp pulse about 1/sec. To test out my new NIM modules I put it through a shaping amplifier and look at that beautiful gaussian peak! Works as well as it did in 1981!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 08:56:03 am
content: A photodiode with sufficient amplification can also work as a radiation detector. Especially alpha particles can produce nice peaks, rather similar (just weaker) to what one gets from a scintillator and PMT. The DIY solution I had build was actually first made for an open diode (lense removed from an BPW24).The peak forming amplifier is nothing that fancy - just a kind of band-pass filter that transforms a short and possibly asymmetric pulse to a more symmetric one. So it's just a simple amplifier with a few suitable selected RC values to give a nice pulse and not too much ringing to the negative side. There is no absolute need to get a nice symmetric pulse - the pulse shape may just make the job of the pulse detector a little easier and could improve the SNR a little, but not dramatic.For tests I used a LED and square wave generator to produce similar light pulses (something like LED with some 100 Ohms in parallel and 50 pf to square wave generator).The silicon PMTs are easy to use, but the real thing is still more sensitive and usually larger area.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 11:49:27 am
content: Yeah, I have looked at the naked photodiode option as well, I think I might build one of those as well. The nice, gold-plated silicon surface-barrier detectors used for alpha spectroscopy are really nice but really, REALLY costly. A versatile charge sensitive preamp that would work with both PMT's and photodiodes would be really elegant, but I doubt it's more convenient than just having two little preamp boxes.I can't find a good source for the OPA354 op amps used (apparantly CMOS opamps are really good for these charge sensitive amp stages) but I think the ltc6240 is very similar.I guess J-fet op amps could be used as well, but bipolars probably aren't suited.thanks again!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 01:02:28 pm
content: The PMT signal is not at all small. So the amplifier is thus not critical unless one wants very short pulses for high count rates. So rather normal OPs like LF356, TL072 , TLC272 or similar should be good enough. If high speed is really needed, there are plenty of fast FET based OPs, like AD8065.With a relatively high level from the PMT (enough gain there) and thus not to small a capacitor even a BJT based OP could work. There is no need to have the amplifier much faster than the pulse former and peak analyzer. Chances are the peak analyzer will set the speed that can be used. The natural source are usually weak and thus no need for short pulses.The photo-diode needs more amplification. With an extra preamplifier directly at the diode the signal from a photo-diode can be compatible with the PMT signal. So the same analyzer part could be used from there. In the dark the affordable BPW24 with the lens removed works, though a little fragile without the window. It's at least an affordable DIY solution.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 01:34:58 pm
content: Great, I'll try that then!I have a very nice Shimadzu photodiode from an UV-VIS spectrometer I tore down. I think I'll try removing the lens from that and use that.I'll start with the photodiode preamp, I think, since I'm still scrounging SHV connectors and cables for the HV of the PMT.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 02:35:20 pm
content: The Shimadzu photodiode sounds like an expensive model. The BPW24 is no longer available at Reichelt for cheap (it was at some 2.50 EUR AFAIR), but still not that expensive.The suitable preamplifier is just the normal TIA with some voltage applied to the detector. AFAIR I did not even use a special OP type (e.g. TL071 like). Today my choice would be something like OPA172. The amplifier should be close to the detector as capacitance matters.One would still need a alpha source, as the diode is essentially only good for alphas. It can also detect some beta radiation, but here some energy can be lost as the diode may not be thick enough. Also the energy is usually smaller and not that characteristic. So the PIN detector only makes sense with a suitable source - I had build the detector for use in school, where they have some sources.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 02:53:11 pm
content: That sounds reasonable. I tried making one a few weeks ago (generating the pulse seen in prev. picture) but it just oscillated slightly, giving a pulse/sec or so. All signals vanished when I put a finger on the ground plane of the board too. I think it's prett imperative that the CSA is actually in a shielded can, and not just on copper clad board.I'll start over with a much better understanding!I use a victorian uranium glass stopper that registers something like twice the background radiation with a beta/gamma GM probe. I think that's the ideal source, since it's practically harmless, and completely encapsulated (in glass matrix). The stopper is also symmetric, so the geometry is pretty simple.--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 04:22:40 pm
content: The uranium glass is OK as a gamma and beta source. However there would not be much alpha, as only the very surface is active here. So alpha sources are usually a very thin film with high content of the active isotope. Anything deeper than even a fraction of 1 µm would already see reduced energy. So the photodiode type sensor is kind of limited. With a good alpha source the energy loss in air is clearly visible. So with distance it is not only less intensity, but also lower energy.The amplifier needs some shielding, but I don't expect / remember it to be so bad. It is important to find the right amount of capacitance in feedback and avoid other capacitance to the surrounding. A point to watch for is avoid capacitive loading to the OP by a (shielded) cable directly at the output. The TIA usually has no phase reserve to spare.With the scintillator and PMT the dark count rate from the natural background is at something like in the 100 counts per second, depending on the lower cut off. There are plenty of weak pulses, still well above the PMT's own background. So the scintillator + PMT is a lot more sensitive than a GMT.For the test I used some potassium salt. It was still weak (so it took something like 10-60 minutes for a spectrum) but worked to get a dominant line to get an idea of the energy scale. I would expect a lot of lines from uranium, so not that ideal for a first test and calibration.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 05:02:29 pm
content: The multiple lines might be a problem, true. It might still be possible to figure out where in the spectrum you are, we'll see.I have some large potassium chloride crystals as well, I might try those as well. I think the photodiode should be sensitive to beta particles as well.In any event, alpha spectrometers are always evacuated to reduce the amount of loss from air. Usually they are little aluminium boxes with a vacuum tight door, a three way valve to let in vacuum and vent the chamber, and then a gas tight coaxial feedthrough to mount the detector (which is most often a surface barrier detector). Could probably be built into a Hammond box with a vacuum seal in the lid!They make for pretty interesting NIM bin modules:eBay auction: #https://www.ebay.com/itm/Tennelec-TC-256-Alpha-Spectrometer-Nim-Bin-MODULE/273344483103a little pricey though.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 05:03:25 pm
content: If i use a dual op amp, wouldn't using the 2nd amplifier as a buffer solve the cable capacitance issue?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 05:33:46 pm
content: An extra buffer can solve the cable capacity issue. In most cases just a 100 Ohms resistors is enough too. Just an OP directly to a BNC output is not a good idea - though I have found that the hard way in an expensive commercial instrument: with a short cable everything is OK, but more than 1.5 m coax and there was oscillation where it should not be.The photo-diode would also detect beta radiation, but beta has no nice sharp lines and lower energy. One would not see the noise in the energy very much as the spectrum is smooth an boring anyway.There is not much electronics to an alpha spectrometer. So much of the spectrometer could be the vacuum chamber.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 14, 2020, 05:40:17 pm
content: Good to know! I'm going to buffer it and add a 93 ohm resistor in series - the impedance required by the spectroscopy amplifier.Here's the schematic i'm going for. I have no idea how i messed up the resolution so bad, it looks like it was drawn in a ZX-Spectrum CAD program!You're right about beta spectroscopy. There must be some easy, natural alpha sources available. Rain water? Has a lot of fast decaying atmospheric radionuclides from cosmic ray spallation in the upper atmosphere as far as I remember.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2020, 09:15:39 pm
content: I would skip in the test input, just to keep the input capacitance small. If a test signal is need, create it optically with a LED.Similar I would avoid the extra connectors / longer cable to the detector - here every pF counts. The FB capacitor would be more like a gimmick wire close to the input. AFAIR I even skiped the coupling cap and had the detector directly at the TIA. For a final version I would consider a low noise OP here (e.g. OPA172, OPA141).AFAIK there may be one Alpha source sometimes still available: some old ionizing type smoke detectors. They are phased out some time ago so it could become difficult, possibly even illegal to get. A tiny piece of uranium mineral could be an option too. No need to have much as only the upper µm counts. Radon gas is also a Alpha source, but hard to handle in vacuum. Thorium in some form (e.g. welding electrode, gas lamp) may work too.

author: StillTrying
author_rank: Super Contributor
timestamp: January 14, 2020, 09:56:51 pm
content: To preserve the shape of 200ns wide 20uA to 1mA current pulses a single transistor is very hard to beat, even the physical layout isn't very critical, a small piece of stripboard is fine if you're sensible with the strips.

author: jmelson
author_rank: Super Contributor
timestamp: January 14, 2020, 10:01:14 pm
content: Quote from: ChristofferB on January 13, 2020, 10:05:06 pmNote that those plastic scintillators give a VERY short pulse, just 10-15 ns, compared to crystalline scintillators like NaI.So, any circuitry after the PMT needs to be able to properly handle those extremely short pulses. Unless your preamphas a 5ns rise time or so, you will end up with no signal on the output.Jon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 15, 2020, 12:31:19 am
content: Quote from: Kleinstein on January 14, 2020, 09:15:39 pmThat sounds reasonable, I'll try putting the diode directly on the bias circuit and ommit the longer lead.I was under the impression that gimmick caps were only useful at fairly high frequencies. I put in a 0.5-5 pf trimmer cap for now.I'll figure the source thing out, even without any source I should still see some background pulses.Quote from: jmelson on January 14, 2020, 10:01:14 pmThat's good to know. Yeah it looks as if there's a reason why the NaI(Tl) and CsI(Tl) with/without thallium are very popular. They're also less robust, and more expensive. Getting a 3" one would likely cost an arm and a leg!But it should be possible, right? Using a high speed op amp of course. Actually the datasheet for Bicron BC412 lists pulse width as around 5 ns. Still, they seem generally applicable, even on field scintillation counters.

author: sdouble
author_rank: Frequent Contributor
timestamp: January 15, 2020, 02:27:21 am
content: If all what you want is counting, you do not need an amplifier I guess. The pmts have a typical max gain of 10^6 to 10^7 which is usually enough to get pulses of 100s of mV at least.for the detector you were mentioning, you will get pulses of 10s to 100s of ns duration which is long enough for triggering a scope for instance.Typically people use a discriminator to deliver a logic pulse out of a pmt signal. the good ol' NIM logic is still in use. negative (-800mV/50ohms) high.wheb people are after highly accurate timing they do use a constant fraction discriminator to limit the so-called walk effect.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 15, 2020, 07:06:09 am
content: What I really want to do is pulse height analysis and stuff like coincidence/anticoincidence experiments. So after going through a shaping amplifier to widen the pulse, and a gate to generate a logic indication of a detection, I want to feed an ADC and then a multichannel analyzer.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 15, 2020, 07:15:33 am
content: Actually, the OPA354 cmos opamp they used in the scintillator schematic I posted seems to be plenty fast enough for a plastic scintillator preamp. Maybe I should just use that then.The reason I included the 'test' input is because every single preamp I've seen for setups like this, Ortec, Canberra etc. brand has it, and it is used to put a marker peak in your alpha/gamma energy histogram to calibrate with.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2020, 04:27:32 pm
content: For coincidence experiments at high count rate one may want to preserve a high speed. For just getting the pulse amplitude, there is no need for a super fast amplifier. The pulse shaping is essentially some well behaved band-pass filter. So the is not real need to have the input amplifier faster than the pulse shaper and amplitude analyzer. If the pulse is much shorter than the response time of the amplifier, one would still see a pulse, but a wider one and with reduced peak amplitude, but the same area under the pulse. So a fast amplifier is not absolutely needed. A slow amplifier could be seen as part of the pulse shaping.For the amplitude analysis there are several options and this also effects the preferred pulse shape. The 3 main methods I know of are:a) analog peak detector circuit and "slow" ADC (probably the easiest method - I have build this type).b) fast ADC and numerical amplitude calculation (usually more from area than peak)c) analog integration with some baseline restoring and slower but higher resolution ADC and looking for "steps" instead of peaks.The last method does not need an extra peak shaper.The amplifier should normally be stable enough, so that one does not need an extra marker. The weak point is more the stability of the high voltage that sets the PMT gain. So if at all a test pulse would need to be optical. The alternative is known isotope from time to time.

author: r6502
author_rank: Regular Contributor
timestamp: February 28, 2021, 09:01:27 pm
content: Hello all,this thread is a little older, but I'd like to show a reverse engineered schematic of a preamp for an older Harshaw PM with an NaI (TI) crystal installed. Shematic is very simple 2 transistor version and they used older germanium types. Mine is the type 6S6/E detector.I have attached some photos of the detector and its preamp and the schematic of the preamp.it is still working fin. I use also some NIM modules for pulse sharpening and using a MCA card located in an older PC made by EG&G.Guido

author: coppercone2
author_rank: Super Contributor
timestamp: February 28, 2021, 11:52:50 pm
content: I have one of those scillnators with a crystal but I took it apart since it was not working, it had a broken amplifier tube circuit of some kind. I have been meaning to do something with it but the whole gamma thing is a little unsettling, the only project I made was a cast containment jar (like a heavy lip balm tin) to house the radioactive sample that it came with, very weak but I decided to keep it in a lead box, it was bugging me every time I thought about it, but the rating on the test sample is safe by goverment levels.

author: r6502
author_rank: Regular Contributor
timestamp: March 01, 2021, 05:55:54 am
content: Hello all,I had too look a bit, but I found a catalogue for Harshaw radiation detectors - it is hard to find on the web, so decided to share it here. I'm not sure about Harshaw, but when you search fot the company you can find the website of Saint-Gobain. In ther history of they point on Harshaw.https://www.crystals.saint-gobain.com/about-usA few years ago, when I got my 2 Detectors from Harshaw, I contacted Saint-Gobain, but they did not have any literature in the archive. For the 2nd detector, it is a 12SW12/3EX that has a larger crystal, I'm not sure, what photomultiplier Harshaw used. from the outside you are not able to see the mfg of the photomultiplier tube. If someone has an idea, what's inside - please let me knew.Guido

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 01, 2021, 07:10:56 am
content: Thank You r6502 !

author: penfold
author_rank: Frequent Contributor
timestamp: March 01, 2021, 07:15:46 am
content: I'm speculating a bit, but it may be an ADIT brand tube... especially if Harshaw had any relationship with Bicron (I'm a bit unsure whether the dates check out there, not knowing the age of your tube and any history of ADIT before the early 80's)Are there any clues about the diameter of the PMT itself? It looks like a 2" base but 1.5" crystal. I would guess its maybe something similar to an ADIT B51B03, I couldn't tell you if that was the historically correct model number used when the detector built, but I'd be pretty confident its the same design of tube

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 03, 2021, 09:25:52 pm
content: Thanks for the info! It's good to have a reference design!I've actually had so much success not using a preamp at all - just an R to bias and a C to a spectroscopy amplifier - with perhaps the highest resolution I've so far seen with my small 1.15x1.15x3 square scintillation probe. Attached below is a couple of hours of 232Th mantle counting time, background subtracted.Obviously gain varies hugely with cable lenght this way!