Question on a Pre-amplifier for a 3He

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Dennis P Brown
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Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

I noticed that in the excellent youtube vid that Joe Gayo referenced, the 3He detector tube had a pre-amplifier before it was feed into a amplifier then the discriminator (the NIM box system). If I use a commercial system that can handle a scintillation detector, (so it has an amp of some type) should I use a pre-amplifier then? Is a pre-amp normally required and if so, why?

Should this also be done for a BF3 detector?

Aside: I did run my fusor at 1500 watts (the cathode glowed red; blew my 15 map fse in the process.) My BF3 tube with the commercil scintillation detector system (adj. to 2300 volts) read nothing - hence by question on that tube as well.
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Re: Question on a Pre-amplifier for a 3He

Post by Alex Aitken »

Maybe I can help a bit. Yes you need a preamp. A photomultiplier in an scintillator setup can easily add a gain of 1 million to what the photocathode produces. On top of the gas gain, a proportional counter is limited to maybe 1000 or 10'000, so the signal is a lot lower. The He3 tube produces the smallest signal and needs the best shielding and the lowest noise preamp.

There is a lot of lore about these, and how to make them work, set the discriminator, checking the setup for gamma immunity etc. Do it badly and like Pons and Fleischmann, you can be easily mislead. Aside from picking up gamma is set wrong, they can trigger from EMF and even just if they get hot supposedly.

I have some tubes I've not played with yet, but they are corona tubes and work a bit differently. I'd be copying Doug Coulter's stuff for those. Aside from searching for proportional counter preamps and info, I'm not sure where I would look.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

In general the signal out of the tube is in the single millivolt range. There is a lot of noise floating around at that level! You don't want to try and ship this to a preamp via a cable to a remote NIM bin. This is why preamps are all self contained single boxed items....

Ideally, you want near zero, if not actually, zero distance between the signal out of the tube and the input to the preamp. Once you get a large fraction of a volt out of the preamp, you can, with some confidence, cable this signal to a more distant NIM amplifier. With good cabling and termination it will get there with a widely separated signal to noise factor that is easily further amplified. Any noise can now be discriminated out to just produce a real clean pulse signal. Oh, yes! You will need a preamp for a BF3 tube as well

Once you have a clean pulse signal that represents only a true neutron count, you can then do all manner of things with it in other NIM modules. You count it, shape it, make it narrower or wider, set off an alarm signal or blink a light indicating a neutron is detected or make a beep or click at the same time.

The key is to get a real signal to the NIM bin for further messaging. The preamp does this.

Attached photo of intimate zero length preamp to 3He connection. HN male to BNC female then male to male BNC adapters. As there are a total of three metal to metal electrical connections in this adapter string, it is critical to hold mechanical rigidity between the tube and preamp as any movement here would or could induce noise.

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3He to Preamp.jpg
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

Thank you both for the reply's - just I'll add a pre-amp to the 3He tube. Any suggestions from e-bay? Not sure what I would look for in a pre-amp.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

All suitable preamps for use should be NIM compatible in that they are smallish self contained metal boxes with the key NIM standard 9 pin power plug found on NIM modules. the input end typically has a single HVBNC connector and the other end a SHV BNC for the bias and two or more common BNC connectors. All of the common NIM manufacturers have made them. Ortec, Tennelec, etc.

The only other method to preamp is to build your own using one of the little Cremat modules made for this purpose such that all you need is power and a few components at the bias input end of the preamp. Carl Willis and others have used this $100 small all in one preamp module to effect a very nice preamp for neutron proportional tubes. I attach the CR-110 preamp spec sheet. The simple circuit shown on the spec sheet is all you need. Remember to make sure all the bias and input caps are 3000 volt caps at the value noted. If not, you will have the bias voltage appear at the little preamp module and instantly destroy your $55.00 spent. The .01 uf input cap can be replaced by a .002-.005uf 3kv cap.

Order this $55.00 easy to use Cremat module off amazon at

https://www.amazon.com/s?me=ANMGJLZHRBP ... PDKIKX0DER

For used NIM preamps found on E-bay search.... Charge sensitive preamp, Nuclear preamp, proportional tube preamp, NIM preamp. Tennelec preamp, Ortec preamp , etc.
Note: From my quick search on eBay, I fear a great deal of insanity prevails in the pricing, (over pricing) of those preamps that I found.

Richard Hull
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CR-110-R2.1 (1).pdf
(249.02 KiB) Downloaded 206 times
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
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Re: Question on a Pre-amplifier for a 3He

Post by Rex Allers »

I have no direct thoughts on a preamp but this current ebay listing looks to be the same model shown in Richard's pic.

https://www.ebay.com/itm/124943588769?h ... Sw9dJhYcxW

(PRINCETON GAMMA TECH RG-11B/C)
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

Note: the PGT amp I use and the one in this image is from a HPGe system. If the user has removed it from same, he must have also removed the critical FET front end buried in the head on the "cold finger" and moved it into the preamp box supplying a HV BNC as an input. The seller has carefully avoided showing us the open hole on the opposite, (input), end of the preamp box face.

I fear this preamp is incomplete and just a pull off the cold finger with the critical input circuit FET left in the head. I see no input connector!! Do not buy this without full info on how the seller has properly moved the FET and components into the preamp box and what type of input connector he has on the other end of the box!!!!!!

I know, because I had to do this on my PGT which I ripped off of the cold finger found on a surplus HPGe system. Buyer be very aware.

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
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Re: Question on a Pre-amplifier for a 3He

Post by Joe Gayo »

TI has just introduced two new JFETs that look promising for CSAs - https://www.ti.com/amplifier-circuit/sp ... ducts.html
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Re: Question on a Pre-amplifier for a 3He

Post by Rex Allers »

Richard, thanks for the warning on the ebay listing I shared. I just looked at your pic and saw the same model listed. Your description and warnings do make it sound like this ebay item is probably not useful.
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

Thank you Richard for the direct link for the require CSpA* - I certainly had no idea where such items are obtained nor what would be the best units. That is extremely helpful, to say the least.

Learning the "in's and out's" of neutron detection details is not as fun as other aspects for those of us more attuned to vacuum systems and brute power supplies (I have been rather unpleasantly surprised - and also alarmed - by my HV unit: arc overs are now a plague of late. Mounted all secondary components on a glass plate. Wood is just too conductive.)

Delicate electronics and such for pre-amps is a bit of an esoteric topic for me - yet I guess it is part of the learning curve for better signal detection - at least cleaner. Frankly, more of the person that exploits essentially finished components rather than the type of that design and builds such systems from scratch. Those here that do are certainly a few steps ahead of most of us.

I do appreciate the pointers and knowledge you have provided - this, I realize has been gained via hard work and study of this subject matter. Many here, I'd think - myself very much included - consider these as 'black boxes'. Even through I have used schematics and repaired some black box systems, the design of these systems is a mystery that eludes my interest and quickly suppresses my attention span. ;)

Speaking of the 'black boxes', any reason the Cremat pre-amp's come in different flavors - like the CR-110, 113 or 200-100ns? After reading a bit, rather deep topic ... my attention span started to fade rather quickly with the required cross reference readings in order to follow the technical definitions in the spec sheets.

*The company provides a lot of useful info, as well
Last edited by Dennis P Brown on Wed Mar 23, 2022 4:20 pm, edited 1 time in total.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

The 110 is the one you want as it is the most sensitive of the bunch. Unfortunately, with no real electronic construction skills and shielding skills, most would have finishing issues in such a build, which by my view and history would be a dawdle of simplicity.

Proportional tubes and ion chambers demand critical preamps, critical shielded housing and critical cabling and termination, if taken to a remote location for further amplification, signal conditioning and readout. Sorry 'bout that but it is what it is.

The all-in-one box, (detector-thermalizer-preamp-amp-conditioner-readout), is called a portable neutron counter and is traditionally outrageously and killer expensive. Nice to have roaming about a reactor facility sniffing out pesky leaks, but terrible if you don't want to be within arms length of a working fusion reactor for a protracted period of time, especially if you are really kicking out a ton of fast neuts!

For the under 10e6 neut/sec systems the portable is fine as you can be about 3 feet away from the portable and read it without being exposed to any degree. For the electronically adroit, you can tap into the pulse line just before readout where the pulse signal is "amplifier output-hot" and conditioned, bore a hole in your nice portable counter and put a BNC connector there, then ship the signal via cable to a remote readout location.

In the end of all of this....

Those with the verve and who really are serious are told from the beginning that this entire endeavor is a skill building adventure where you will be dragged over the coals and through the mud of many sciences and technologies. In the end, if totally successful, and if you stay with it after you win it, you will have done much more than just do fusion. You will be belched out of the adventure, wiser and more capable in more disciplines than most anyone you may ever meet in your life.

Think about it!

This philosophizing is not a joke. It is real, a fact that will amaze not only yourself but others around you.
Look!....
You built a lab area. (Carpentry)....You supplied wired electrical outlets suitable to the lab's energy requirements (electrician)...You planned the entire scenario (mental engineering and developmental planning along many lines)..... You searched and scrounged needed materials. (engineering procurement & finance).....You assembled a working vacuum system with instrumentation related to same and suitable to the task. (Vacuum technologist)....You fabricated and, or, oversaw the building of a nuclear reactor out of metal (Material Scientist, Machinist, Welder).... You added a nuclear fuel source and gas transport system to the reactor vessel (combination material scientist, engineering procurement and vacuum technologist)....You may have designed, assembled and brought together the electronics and electrical systems needed to acquire data from the fusion process as well as power it. ( Electronics engineering, electronic technician, electrician and electrical worker).....You are careful to select and oversee the proper data gathering from the nuclear process to supply meaningful data (nuclear metrology specialist) From beginning to end you have been totally involved in the effort (Large engineering systems integrator and ramrod of a large number of systems workers of varying skills)

You are a sort of renaissance man in the world of engineering and the sciences assuming you have done all of the above with your own hands. Yes, you have had help from others who have done it through the written word (fusor.net). You know that the written word will only go so far, so you have dug in deep on your own research into the effort and you have experienced a constant "laying upon of the hands", loss of treasure, loss of time in your life doing all of this. Specific challenges during the adventure are to be surmounted and the tedium endured. Self-enrichment is costly in time and treasure.

You can now bask in the glow of a major accomplishment few dare to attempt and even fewer succeed at. You are self-enriched, and justifiably, self confident about any future tasking of yourself. You can intelligently speak from experience across many disciplines and technologies and sciences.

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
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

Putting it that way, I almost feel like I might, with more effort, be worthy of creating a star in a 'jar'! Thank you.

I ordered the CR-110 (I did read the spec's and after cross referencing a few terms - hoped - that was the correct choice. Very glad it is.) When it arrives I'll assemble it into a working pre-amp. I'll post the results now that I appear to have addressed the arc over in my diode bridge and ballast resisters.

I really want to explore and get better at understanding, building and testing neutron detector equipment - through NIM system is a bit too far, I feel, to attempt at this stage. It is true that black boxes, and turn-key systems are nice (and, of course, expensive) but doing it oneself is, as you so eloquently said, makes one a better and more knowledgeable person (paraphrasing a bit.)

I am a bit worried that too many people today do not understand even the simple things they depend on - like an electrical switch for their room lights - much less the skills needed to build a small research building or repair their own car (yes, even the advanced cars - really, a good bit easier now with youtube demo's and the computer interface to tell you the very problem but fewer people appear to even try!)

Thank you Richard, for all your essential help and frankly, your knowledge and breath of skills leaves me in awe. A number of people here are extremely skilled and knowledgeable and this is what certainly makes this forum so unique and amazing.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

Might I recommend their circuit in the spec sheet. It will work and give you an output that is good. the .01uf input cap can be a .005 @ 3kv.
I might also recommend two nine volt alkaline batteries as the supply to the 110 as in the diagram below.

Click on image to enlarge.

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Re: Question on a Pre-amplifier for a 3He

Post by Joe Gayo »

Richard, did you intend to snip out the 0V ground reference for the CR-110 module? My guess is they are using a discrete JFET with a standard opamp, and the non-inverting input of the opamp is probably referenced to 0V ground.

Quality 9V batteries are key. The impedance of the batteries can actually create enough of an offset compared with external grounded equipment (e.g. counter) that there is measurement degradation. I use 9V with 0.5 ohm impedance.

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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

I intended to use two 9 volt transistor batteries to power the unit - certainly don't want an external supply adding noise; through, I think I'll use a single on/off switch for breaking the common ground connection on the batteries rather then a dual switch on the hot side. I think this arrangement would be simpler ... or is that not acceptable for some electronic reason?

So this preamp's output signal produces a pulse that is riding on a 28 volts bias?

Certainly, unlike my current detector systems that use a single HV cable (600 - 2500 V) to both power and detect the signal, I would guess that any such system (like my Eberline or similar unit) would then need its input filtering cap to be bypassed? This gets back, I think, to your previous post about tapping into the std handheld field unit. Also, does the port I use matter - i.e. Geiger or scintillation?

Sorry for all the questions for a topic that is obvious to the electronic knowable here; however, adding a preamp to my detector circuit/system is getting a bit more complex ...err, fun then I had naïvely first considered.

I've been w/o my big box oscilloscope for some years (sweep failed) so I'm SOL relative to signal diagnostics - I do have a hand held unit; through, that device isn't very user friendly - too few control knobs, far too many auto's.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

OOPS! Ground reference replaced correctly in the original above. All is OK right now. The above circuit is fully correct now.

Batteries should be good for neutron counting. I would not trust them on gamma spec or energy resolving efforts. One could include/add, +/- 6v TO-92 LDO regulators to lower the impedance using the 9v batteries. LIPO batteries or NimH batteries might be better.

I would not switch the ground as the full unreferenced voltage would be permanently applied all the time. It is never a good idea to switch on only the ground reference in electronics.

Why would there be a 28 v bias on the output? Everything is pretty much done for you in this little marvel. +/- 3V unsaturated output swing in the spec sheet @ 50 ohm output impedance. Where did the 28v come from??

For energy resolution stability, all supplies need to be very quiet. This is more for PMT and Gamma spec work, rather than raw counting. One can pile on the gain after the preamp in shaping and discrimination. Part of this is the value of NIM in the long run, unless you build it all in electronics from scratch. Any critical work in this area demands the use of a DSO scope if one is really nit picky about the waveforms sent to energy resolving circuitry.

Richard Hull
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

To quote the Master - oops! I mis-typed the 28 volts but would not the system have 18 volts on the signal/pulse line from the batteries?

Could my Eberline or similar device read that low a voltage through its HV capacitor? Might it be advisable to remove that capacitor from its circuit?

I'll certainly use a double throw switch on the hot side of the batteries.

Richard, you really want me to look down into that rabbit hole - now I'm cross referencing LDO-regulators and how to use them. Yes, looks easy enough but again, this is certainly getting ... well, interesting as a topic and project.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

Again, there is a myriad of preassembled circuit in this little gem. All op amp circuitry in precision gear tends to like positive and negative supplies to retain waveform shape above and below the input reference. It is only at the output that the signal is put into a nice finished form with reduced voltage about the reference ground. Nuclear circuitry might even shape the waveform a bit for a special purpose. For simple counting, you are just looking for a good solid pulse per detection. The waveform is not hyper critical as long as it is fast and of a suitable width to be counted by your counter.

Much more attention needs to be given to such matters if you are looking for energy resolving value in the waveform with low noise. Voltage drift from batteries as they are expended, noise from chemical action, (RARE), and high impedance in their sourcing the power can all foul energy resolving stability.

Batteries that can typically supply huge currents at voltage are low impedance types. LIPO and NimH are such batteries. For counting top of the line alkaline are acceptable. Re-chargeable NimH or LIPOs would be nice if you wish.

Regardless, even for simple counting I use a DSO to see what I have at most all points in nuclear circuit chaining. It is just good practice.

Richard Hull
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

OK, I see why a single component 9 volt regulator will not work. The drop out voltage. So a 9 volt regulator isn't gonna work - its min. voltage is well above the 9 volt regulator. Ok. Trying to learn a bit on this subject. So a LDO looks to be the best fit.
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Re: Question on a Pre-amplifier for a 3He

Post by Richard Hull »

Again, are you looking to use this for other than counting neutrons and use it for energy resolution? If for energy resolution you will need a 6 volt positive and a 6v negative Low Drop Out regulator in a TO-92 case. You might try two 9 volt Lithium batteries and avoid the LDOs. In the spec sheet they like the +/- 6v supply regime. It is not demanded.

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
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Re: Question on a Pre-amplifier for a 3He

Post by Peter Schmelcher »

A good chapter on neutron detection.
https://www.lanl.gov/org/ddste/aldgs/ss ... 79-406.pdf
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Re: Question on a Pre-amplifier for a 3He

Post by Alex Aitken »

Joe Gayo wrote: Wed Mar 23, 2022 2:21 am TI has just introduced two new JFETs that look promising for CSAs - https://www.ti.com/amplifier-circuit/sp ... ducts.html
These look interesting. On paper the jfe2140 doesn't beat the nxp discontinued bf862 and the input capacitance is a bit larger. Having said that noise is often conservative, and input capacitance is often wildly wrong. I do note that the jfe2140 is a matched dual transistor device, and the jfe150 is a single device with about twice the input capacitance, about twice the transconductance at twice the current and it makes me think a jfe150 is just a jfe2140 die with the devices in parallel.
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Re: Question on a Pre-amplifier for a 3He

Post by Joe Gayo »

The other way around. 2140 is two 150s.
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

Thank you, Richard. Nice to get more clarity on this topic. I don't intend it for energy resolution just simple counting.

Wish I understood even a single electronic related word of Alex's post - guess google is needed again.
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Re: Question on a Pre-amplifier for a 3He

Post by Dennis P Brown »

The CSA chip and a circuit board arrived (via USPS - was super fast.) I intend to use this mostly for the 3He detector systems so these will operate under 750 volts (power.) As such, I will use the 'not so' high voltage connectors that are rated for under 1.5 KV - but in the future, I will likely add the 3 KV connectors/cables to the box for the BF3 tube. I've ordered said HV connectors & cable from a Chinese company (the prices are just ridiculously low) but the 'slow boat from China' is starting to be the norm - ironically, all the delay is in China, not American ports/transport.
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