E-bay PMTs ... Such a Deal!

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E-bay PMTs ... Such a Deal!

Post by guest » Mon Aug 20, 2001 3:17 pm

I believe it was Tom Dressel who posted the heads up on the PMTs offerd on E-bay. I took his advice a couple of weeks ago and checked it out. There was an offer for a quantity of four 3" PMTs. I immediately recognized in the photo, the rare spider web signature of the short and fast 4879 RCA PMT. I did the "buy it now" thing and have received the items.

They are really hard to get now and are bi-alkali tubes with the round bucket detector which is a bit faster and more sensitive that the venetian blind type tubes.

Data on this tube is just not to be found. It uses a standard basing as with many 10 stage tubes which I discovered by removeing the black out tape and following the elements with my eye into the base and comparing it with some EMI tubes I have.

These came without socket or dynode strings. (bummer)

Fortunately, I have several sockets with properly wired strings on hands. (old hamfest buys.)

I assembled one with a common plastic scintillator this past weekend and BOY are they slick! AND sensitive.....

Thanks for the heads up on this. I hope others got some of these puppies as the guy has been selling lots of four like this for about a month. Note* they are "pulls", but the photons haven't worn them out yet. (They are as good as the day they were made... 6-79 date code)

Richard Hull

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Re: E-bay PMTs ... Such a Deal!

Post by Tom Dressel » Mon Aug 20, 2001 4:46 pm

Actually it was Richard Hester who posted the info about PMT's on E-bay.

I just checked E-Bay, and someone in Richmond, VA has a EMI 9856 PMT for sale with the dynode string. Does anyone know, will this tube work in a neutron detection system?

Tom Dressel

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Re: E-bay PMTs ... Such a Deal!

Post by guest » Mon Aug 20, 2001 6:11 pm

Well what do you know about that!! That PMT with base etc is one of the ones I sold at my Teslathons over 4 years ago. I am unfamiliar with the name of the e-bay seller as presented, but its is absolutely my old tube. I trimmed the leads exactly as show for sale.

Yes, That tube is actually the exact tube/socket & string/ pre-amp used in my 2" BC-720 detector shown being assembled in the second fusor video.

I found about 20 of these complete systems in a local junk yard in the mid 90's and sold them over the next 4-5 years to buddys and tech-heads.

Nice to see my stuff makin' th' rounds, although it appears this guy bought it from me and never used it.

If you get this item, I think I may still have some tech data, which I gleaned from it way back then.

Richard Hull

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Re: E-bay PMTs ... Such a Deal!

Post by Tom Dressel » Mon Aug 20, 2001 9:44 pm

Well I bought the PMT.
Richard, did I understand you to say that the thing has the pre amp on it?

You wouldn't happen to have the shielded enclosure you built for that PMT laying around collecting dust would you?

I have all the NIM components that I need including the SCA, counter, rate meter,gated integrator and 0-10 KV power supply. I also have a proportional counter preamp which I assume would work with a BF3 tube but not a PMT.

The deuterium delivery system module is finished, connected to the chamber, and leak tested. It is all VCR fittings and will hold a hard vacuum for weeks (I have not yet connected the D2 bottle). I have set up the system with a series of valves that allow me to bleed in air to get every thing set up. I can then close off the air bleed, bypass the capillary leak to evacuate all the tubing and the reservoir, before filling it with deuterium.

I have a really good, tight vacuum system, with a diff pump, and cold trap.

So all I need to do is get the PMT neutron detector built, figure out the cabling for all the NIM components, and possibly get a few more KV out of my power supply and I am ready to do fusion.

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Re: E-bay PMTs ... Such a Deal!

Post by Tom Dressel » Tue Aug 21, 2001 2:52 am

Well, I heard from the E-Bay seller of the EMI 9856 PMT, It will soon have my germs on it.

The seller bought it at a hamfest 3 years ago, probably from the guy that bought it from Richard Hull. It was never used by the seller.

Richard, I would appreciate any info about the tube you might have.

Thomas Dressel

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Re: E-bay PMTs ... Such a Deal!

Post by Richard Hull » Tue Aug 21, 2001 3:23 am

Tom,

That's right, there is a full preamp on a board attached to the rear of that socket. Look closely again at the photo on the E-bay ad.

I have the original housing I made up for my PMT in use. It was too tough to make to leave moldering in a corner. Sorry.

If the guy forwards all the paper work he quoted from in the E-bay ad, you will get a diagram for hookup. I never sold a PMT without a decent hookup diagram. If he doesn't send the paper work, then I can forward it to you.

I am glad to hear that you are almost ready to crank up for fusion and look forward to the reports in future.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: E-bay PMTs ... Such a Deal!

Post by guest » Tue Aug 21, 2001 5:26 am

When I first started considering a PMT/plastic scintillator based system for neutron detection, I was concerned about getting my hands on a really fast PMT to match the rise and fall times of the typical plastic scintillator, which are about 1nsec and 2-4 nsec, respectively. I have since moderated my stance somewhat. The difference between fast and "slow" PMTs is mostly in the dynode structure. Fast PMTs use mesh dynodes, while slower PMTs use a box/grid or venetian blind dynode structure. The mesh dynodes are good at preserving the temporal structure of the input pulse, but are not all that efficient. Box and grid and venetian blind dynodes are very efficient, but mush out the width of the input pulse.
Consider the priorities of the fusor experimenter - the count rate is pretty low, and one couldn't care less about preserving the temporal structure of the input pulse. What is really needed is a a decent amplitude output pulse, period. Having the pulse be a fair representation of the input amplitude would be nice in case one wants to do some computer analysis to estimate the input neutron energy, but this is merely gravy, and most folks will simply apply an SCA or some other sort of amplitude discriminator to the output pulse and not bother any further.
In summary, what you really need to get started is a PMT, either fast or "slow" (even the slow ones have a rise time of ~10-15 nsec, which is not by any means slouchy),a plastic scintillator, a fast preamp, probably some pulse stretching since most NIM modules (and a lot of counters) want input pulse widths of >100 nsec, followed by a good counter with an input level control or an SCA/counter combination.
Because the slower PMTs smear out the time signature of the input pulse, you can get by with a slower preamp than would seem to be indicated by the rise and decay times of the plastic scintillator. I am taking advantage of this to run a blisteringly fast preamp at higher gain to get a good output signal. This will be follwed by a fast charge sensitive amplifier (an integrator with a short time constant) to stretch out and amplify the preamp output, then to a discriminator and counter. I plan to run the PMT and preamp without any scintillator at first to get an idea of the noise due to the PMT. The PMT is a 3" Burle with rise time of 12nsec. I'll post more about the preamp design when it is proven in action...

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Re: E-bay PMTs ... Such a Deal!

Post by Tom Dressel » Tue Aug 21, 2001 1:08 pm

Richard Hester:
How about posting a pix and some discussion about the NIM layout too, for the not so smart guys like me.

Tom Dressel

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Re: E-bay PMTs ... Such a Deal!

Post by guest » Tue Aug 21, 2001 2:22 pm

Richard Hester has posted wisely and beat me to the punch. He learned the old fashion way that most all nuclear researchers destroy the pulse fidelity and it is rare to find anyone interested in unit nanosecond data.

Most good PMT systems and gamma spectros using them actually avoid fast times as the electronics gets easier and simple cabling is the order of the day.

I personally stretch most of my pulses whether geiger or PMT out to 10 usec for processing!! (it is hard to lose such a pulse as this to capacitive loading or sloppy circutry) Ideally, dead times should be in the 10% range and this would still allow for over 10k counts per second!! It is a rare geiger tube that can count this fast, quench, and recombine for the next pulse. Anyone working with radiation in that range needs to reconsider their life's work. (that's over a half million (CPM!).

Most Nim stuff likes 1-5 usec widths with about 100nsec rise times. Unless you are working with ADC circuitry, the max frequency encountered in NIM work is about 1 mhz. This means simple 50 ohm BNC cabling with some attention to termination required.

There is always the issue of what is being counted. It is important to remember that PMTs traditionally see only gamma with the NaI (Tl) and platic scintillators. Geigers usually only count beta with windowed units adding alpha to the mix. Geiger tubes are terribly inefficient with gamma above x-ray energies. BF3 and He3 tubes are for thermal neutrons only. Pin diodes can be turned to detect gamma as well in amateur systems, albeit poorly. There are many more detection systems than these, but they are often so specialized, complicated and expensive that the amateur would have to dedicate himself to the effort.

The above detectors will, if skillfully applied, detect most anything we will need to be looking at.

I really hate suggesting this, and it is not a must have, but a good digital storage scope operated by the informed neophyte will speak volumes and help debug a lot of odd situational problems in pulse handling. There are few as good for the dollar spent than the Tektronix TDS-210. I call this series, the "lunch box" scopes. I bought mine years ago when they were introduced at $995 new with 3 year warranty. Armed with this scope for digital and pulse work (scientific) and the Old Tektronix 460 for analog (service) work, there is little that can escape the trained eye of an engineer or technician.

If all you have is an analog scope (hamfest buy) then you absolutely must obtain an ortec, tennelec or canberra NIM pulser to diagnose pulse issues. (regular pulses of variable amplitude available)

With no scope you can still do good work provided you have some idea about the pulse amplitudes and widths expected and delivered by your detector, system or instrument.

Richard Hull

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Re: E-bay PMTs ... Such a Deal!

Post by Tom Dressel » Wed Aug 22, 2001 1:57 am

I have an analog scope. It is a BK precision 10 KHz duel trace scope, probably used in a TV repair shop. I bought it in a Norfolk, VA pawn shop 18 years ago. I used It to make a photoplethysmograph, a device to monitor blood flow to skin and muscle flaps. At the time I knew NOTHING about electronics I built it all by reading Archer's Engineer's Notebook by Forrest M Mims. The analog scope is probably not going to help me much.

Al Dieken has many digital storage scopes. He sells them at hamfests. I know he will sell me one cheap, or let me use one for a week or so to get things set up and debugged.

Tom Dressel

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