Hornyak detector build

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Hornyak detector build

Postby Doug Coulter » Sun Sep 04, 2011 2:00 pm

Quite awhile back, we bought some things from Eljen, among them was a "professional" hornyak button for detecting fast neutrons. I had put it on a dual cathode super sensitive phototube from a PET scanner and tried it with marginal results, primarily having trouble with EMI in the setup I had then, along with (then) a poorly performing fusor as a test source. Well, the EMI issues have been fixed at their source, and the fusor is kicking butt and taking names, so I resurrected that project to see how it does now. Being zinc sulfide based, now that I've exposed it to light for these pix, I can't use it for about a day, but that's fine - glue needs to dry anyway. The phototube is an interesting super-linear one I got out of a super research grade mass spectrometer BillF scrounged somewhere that I don't plan to bring back up in its current form anyway. Not a photon counter, this tube, but a decent one I believe. I plan to put it in an old CD ion-chamber detector I tore all the original parts out of, since those are "as deaf as a post" and I don't have any need for it in the original form. But, nice box, nice meter.
Here are the parts:
Hornyakparts.jpg
Parts before assembly

That's a steel tube I welded a thin (.010") steel end onto to let the neutrons in. the tube wrapped with black tape with the Hornyak button on the end, coupled with silicone optical grease. I made a PVC plug for the back end to let the wires out without letting light in. It's glued with Permatex high temperature silicone goop, which I've found is nicely light tight to the wavelengths these tubes are sensitive to. The big coil you see is for degaussing the whole affair -- phototubes don't like even weak magnetic fields. And here it is assembled. I'll get in a test next fusor run.
I plan to make a logarithmic converter to drive the meter, and to add a speaker for any pulses this produces, and have a nice portable neutron detector - probably won't be real super sensitive, but we'll see when I test it.
Hornyak.jpg
Assembled and glued, now we wait for the glue, and the long persistence of the ZnS to wear off.

9106B.pdf
Datasheet for the phototube used here.
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Re: Hornyak detector build

Postby Starfire » Sun Sep 04, 2011 2:44 pm

Doug - what are using for the moderator? you will need to thermalise the fast Neutrons for it to work, also have you a Mu-metal shield as steel will not provide magnetic shielding?
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Re: Hornyak detector build

Postby Doug Coulter » Sun Sep 04, 2011 3:13 pm

No moderator is needed for this. A Hornyak button doesn't use one, and won't detect slow neutrons at all (and in fact, I may take advantage of this by using a moderator with a hole in it to make it directional).

It depends on a fast neutron knocking a proton out of the plastic encapsulant and into the phosphor, which is chosen to be as numb as possible to things other than charged particles - ZnS stinks at lighting up on X rays for example. So, the detector is complete as shown.

Most detectors do need a moderator, because they use a neutron capture reaction that has a cross section that rises as the energy goes down, and the capture reaction provides enough energy to detect -- that's how the boron-10 and and He-3 and Li-6 detectors work. This uses the energy of the fast neutron itself. In theory, this could also give you better time resolution (no time-smear in the moderator), but in practice the ZnS phosphor is slow, so no big win there. Other phosphors light up too easily with X rays, it's one of those trade-offs. However, my super expensive neutron physics books show a fast one that uses a 30 mil thick plastic "radiator" which neutrons kick protons out of, which then hit a scintillator crystal for detection -- you can even get a clue about how energetic the neutrons were with a fancy one like that -- nothing like as precise as a gamma spectrometer because the neutrons don't always knock a proton straight on, nor do they always give the proton their full energy. The authorities say it's worth about 10 bins in a spectra covering about a 2::1 energy range. So, that's better than no information, but not that super great. In our case, we think we already know the neutron energy from the DD or DT reactions, so no big deal (and those should be quite easy to tell apart).

Here's a pic of the phototube facing side of a Hornyak button made I think by Jon Rosenstiel. This isn't the one I used, but mine looks about the same. I'm shining a bit of UV on it to light it up.
HornyakButton.jpg
Homebrew Hornyak


The reason for the rings is ZnS has a terrible index of refraction that isn't matched by anything I know of - so it's white and not very transparent, it tends to trap its own light. So it's cast into the plastic in this pattern so you can get some more of the light out of it and into the phototube. The plastic provides the protons to be knocked out by fast neutrons. I've made these myself. They're not super sensitive, but once a fusor is cranking, you don't need super sensitivity (or fragility, or emi sensitivity) so I thought I'd try this, since I paid pretty high for the button (but nothing for everything else!).

I think the steel will give me *some* shielding, as long as it isn't magnetized itself. We'll see...I've been using this stuff to hold photo tubes a lot around here and they seem to work fine if I degauss the tube first. I don't plan to use this near big fields. If I do, I'll cut some mumetal for it. But that's a last resort. Just bending mumetal means it loses its goodness and needs to be re-annealed in hydrogen again...not so great.
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Re: Hornyak detector build

Postby Doug Coulter » Mon Sep 05, 2011 12:55 pm

And, tada, here are some neutrons from my static-master/Be sources. As usual, click the pic. Dunno why .gif gets those silly scrollbars.
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Neutron pulse from Hornyak/phototube
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Big one
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I'm pretty sure all these with the fast initial risetime are real events. It did take "heroic" work to cut radiated noise from the CCFL down to where I can see these this clean (it already had a two stage RC filter on the DC output of .47uf an 100k times two). I put the CCFL/diodes/filter in its own box, grounded. Phototube load is 47k with a x1 scope probe on it. Now and then I see a pulse without that fast initial risetime, and am wondering what happened there. But since the neutron scattering is more or less random, I suppose it could have knocked out a few low energy protons (or they were low energy by the time they got to a ZnS molecule) that then scattered all around in there to make the rest of the pulse. I'd further suppose that the much more rare big guy was perhaps a cosmic event, not a neutron from my little source (which, btw, the other detectors I have can't see above background, hmmm). The jury is still out till I can expose this to some mono-energetic neutrons from a fusor where there's no question about which is the source and which is "random junk from the sky or elsewhere". That's going to take a bit more than the setup I have clip-leaded into existence on the bench just now, though.
HornyakLashup.jpg
Bench setup, power supply (double RC filter with large C) in the bud box, running off bench supply.


I note that a 10mv pulse on 47k implies ~213 nanoamps. This is going to need some very excellent shielding to be EMI resistant! And the opamp and logger element are going to have to be top rate to show this level on the meter in the old CDV-715 box. Fun! The HV supply (CCFL+doubler) is drawing right around 1 watt, so it's not going to be real easy on batteries, I may do rev 1 with a wall wart as this needs regulated voltages throughout. While I can make the HV a quadruplier and lose some of the power drain (some is just the bias current for the Royer oscillator in the CCFL), the regulator would add it back in spades...Phototubes aren't great on batteries unless one uses a many step multiplier and drives all the dynodes off taps from that, avoiding the drain from the divider resistors which aren't needed in that case. Someday -- that's another project for another day.
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Re: Hornyak -- preamp

Postby Doug Coulter » Sat Sep 24, 2011 2:47 pm

I've been fooling with preamp designs lately, and so I made one just for this, as it was desirable to have more gain, along with the ability to drive longer cables. A little more speed is nice, but in this case I actually sort of slowed the preamp down a little to better match the long decay signal from the hornyak phosphor. Here is the schematic.
HornAmp.gif
Phototube preamp for slow phosphors, gain 150


I'm really getting nice results with that LED bias scheme, so I'm using it again. You may need to hunt a bit to find a low enough forward voltage led for this, I found the yellow one we used on the standard counter was just right, so I'm using that (1.641v with the 100k bias resistor, or about 100ua). Note I didn't need a coupling cap on the input - I connected straight to the phototube anode and used the tube with a negative supply. I only needed one protection diode (if I even needed any) as it can only pull negative of ground.

I used our regular CCFL, and no doubling for this, so it would need 9 volts to get to the 8xx volt output voltage, leaving me a nice supply to also run the preamp off. I wanted fairly high preamp volts to be sure to be able to make full 5v CMOS levels (it makes about 8v peak). Due to burning some more current in the input xistor, as well as the cable driver, this isn't a real good one for batteries as is, though it does perform well over wide power supply voltages due to the led trick. Probably 5-6 ma.

I wanted more gain as this isn't a high output thing, and the goal was to direct drive a cmos counter input on the standard counter (which is a schmitt-trigger input, tightly defined levels). Threshold such as it is, is set by the gain. This almost completely ignores some pretty fierce X rays, and also misses marginal neutron collision events, making it kind of numb, but on my fusor at roughly 1 m neuts/second, it counts at right about 1k cpm -- easy conversion to do in your head, so I like it as is, and it's way out of the cosmic background also.

I built this, the detector head, and a regulated 9v supple and 880v HV supply into an old yellow CD-715 box. Now I'll have to come up with a trick to drive the meter -- the plan is to take this setup up to HEAS and put it on Richard's fusor while he runs and kinda get a demo done, and a cross calibration with his lab out of it, if he's willing to let us do that.

I'll get some more pix up of the "finished" thing soon. I applied what I call optimal effort to this. I didn't bother with "pretty" or even "rugged" or "slick" - I didn't know it was going to work for sure, so I did the minimal kludge to prove that first. If I make more (and that's looking likely now) they'll be nicer on the eyes and better other ways. But until you know it's going to be worth the effort, a good engineer won't waste man hours on it.

While not as sensitive as the Russian tubes, this is completely free from spurious counts -- they aren't -- has a super low background - they don't, doesn't temperature drift (they do), and by far the most important - I can get phototubes (ones better than the one I used are cheap) and make hornyak buttons in any quantity at any time, forever, where we are dependent on luck to get those Russian tubes...

Reliable neutron detectors, cheap, for all -- it's finally in reach! This setup is even portable, though I am running it off a wall wart at present.
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Re: Hornyak detector build - guts

Postby Doug Coulter » Sat Sep 24, 2011 4:30 pm

This is using the preamp shown above, our usual CCFL and lm317 HV power supply, this time not doubled, but filtered with a series 10k and a .47 uf cap (lots of filter), and the Hornyak head. Here's what the guts look like. I'll see if I can figure a trick to make the meter read in the bargain -- it's a nice meter they used (but I haven't found out the full scale current, I'd bet it's pretty low). This is running off a 12v DC wall wart for now. That phototube really eats some power in the resistors -- a 100ma one wasn't quite enough to hold up at full DC HV output.

HornyGuts.jpg
Guts


As I said above, this wasn't a high-effort build. It was more like "can this work?". I knew that with the sensitivity of the preamp (a few mv to get a full scale output) I'd have to at least take care of EMI issues -- the fusor makes some, and the CCFL does too. So I folded up some Cu flashing to shield the power supply (and heatsink the regulator) inside the old CD box. I made a clamp for the sensor with another piece of the same steel tubing. As you can see, the preamp is just held in there with one screw for now -- I'll be fooling with that more to get a meter output and so on, so no point in super-mounting it up yet. Obviously, a quality build would use terminals, some more heat shrink tubing etc - but this is just a prototype, not even an "alpha" much less a beta. A real one such as we might make for sale later would of course be a lot nicer (see standard counter for example). By making our own hornyak head, using very thin plexiglass sheets laminated with ZnS:Ag and epoxy, we should be able to about double or triple the sensitivity per square of this, and by making it bigger, get another double or so sensitivity. That would be in a real comfortable range for fusor type things. In a low output, pretty stable run on my fusor at around 1 million neuts per second, this counted about 16.7 cps, or about 1000 cpm -- way way above its cosmic background. But that was with a working fusor (even if not running at its peak), and right up against the tank wall almost. A beginner might want just a little more sensitivity, dunno. This will be just fine for fusor tuning with the standard counter as the data aquisition and logging for me, and that frees up our more sensitive, but also bulkier and more delicate (not to mention expensive) other neutron detectors for other uses -- this is fine for what I need here. Since I'm going for ever higher fusor output, the numbness of this is actually an advantage, along with the log plot feature of the std counter software - I want to see movements on that, not piddly few-percent improvements!

Of course, I kept the goodies out of the original CD box, especially the electrometer tube, high value resistors, and ion chamber. The rest might wind up as junk my wife recycles. Nice box though.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Hornyak detector build - final details

Postby Doug Coulter » Sun Sep 25, 2011 4:45 pm

Well, the thing works in tests just fine. I did a fusor run and plotted the results using the standard counter which you can see here. One more thing was nagging me. The original box has a very nice 50ua meter in it. It would look dumb to remove it, and felt pretty dumb not to be using such a fine piece (go check what they cost, if you can find one these days). Hmmm. Well, a peak detector (pulse stretcher) followed by a log converter sounded about right if I could do it cheap and easy, so that's what I did. Here's the additions.
HornyMtr.gif
Additional details, pseudo log converter for meter

One thing I forgot to mention (sometimes it's hard to be expert and a good teacher too - you forget everyone doesn't already know stuff that's second nature) is that I used the preamp power rail as the return for the anode coax. This way, and noise on the rail is "common mode" since the pnp input and signal ground reference is actually the supply rail in this circuit. Otherwise, I'd have given up a ton of power supply rejection, which I probably needed here. That short piece of coax doesn't have the shield wired at all at the tube end inside the grounded steel pipe.

Now on to the fun stuff. The output of the preamp is about a volt off the rail, and it's pretty stable due to the nice LED biasing scheme. I needed to get rid of that volt, and to do a peak holder, as well as get something like a log function so the meter would have some usable range. I added two 2n3906's as emitter followers to start. The first one acts as a diode, a diode drop, and a current amplifier to help charge my peak holding capacitor. The second one isolates that cap and adds another Vbe of drop to get me real near zero volts to the rail. A resistor to a diode gives me the log function (sort of, purists will hurl, but it does work). Due to the low current on the transistors, I didn't get rid of quite enough of the DC, so I had to add a 200 ohm resistor load to the final one to get the Vbe to be a little bigger. Since it's a 50 ua meter, and we might get .7v on that diode, we wanted 14k or so for a range resistor. 15k was close enough. This has enough range that put up near a running fusor in the 1e6 neuts/second range reads just half scale, but the sub-threshold phototube and preamp noise keep the meter off the zero, so if nothing else, you know it's working immediately. So, we have a reliable "it's working" indicator, and a reading of half scale when it's time to get away from a too loud radiation source, with room to spare for better output when that happens. Just right!

Now, since I did all this "optimal effort" meaning it's a sloppy kludge, it's not real robust. That really doesn't matter here, it's a prototype to the Nth degree, and can easily be cleaned up and made more physically solid anytime. And we will, unless we think of a better way to detect neutrons from fusors that has all the attractiveness this has, mainly being we can make these all day long and not have to depend on luck in finding some surplus crucial piece -- all the parts here are readily available and cost far less than even a BF3 tube retail. This isn't the only neutron detector project underway at the moment - I'm also looking into H recoil large area counters (also needs no moderator) and possibly manufacturing our own B10 tubes (not BF3 if we can make them work with just boron lining, since BF3 is truly nasty stuff to work with - and is a lousy counter gas, as is 3He).

Looking at the commercial prices, the fact that I paid almost $150 for the hornyak button isn't a show stopper. The fact that I can (and have) make them makes it even better! Another $150 was spent on 5 grams of ZnS:Ag -- that's enough to make quite a few of these, as you only need a few milligrams per. We'll have to see if the other approaches can beat this one on all counts. Stay tuned!
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X-Ray Shiled your Hornyak Button?? Re: Hornyak detector bui

Postby George Dowell » Wed Sep 28, 2011 1:10 pm

See the patent .pdf.

This German company states "It has long been known".......... that a layer of ZnS(Ag applied to and around a scintillation
crystal will boost the low energy gamma detection capability, down to <30 keV to about 15 keV.

This was a recent topic on the Spectrometry board where we discussed the possible reasons for this statement, is it true, and what's going on.

Conventional wisdom on the US side of the pond is that ZnS(Ag) while being the penultimate alpha detector, rejects gamma rays period.

Yet the German information persists.

To put an end to it one way or the other, a listmember did an X-Ray analysis of one of our GEOelectronics ZnS(Ag) Alpha Scintillator screens.

The results were astonishing and totally unexpected. Under a weak X-Ray beam ( only 10 microAmps), the ZNS(Ag) lit up with a blue glow, easily seen by the naked eyes. Form 15 to 50 keV,(instrument limit) it just go brighter and brighter. See .jpg's. The black background one is as taken with a not too sensitive Nikon CCD camera, no intensifier at all. The Blue background was digitally enhanced. To the naked eye, the blue glow was very evident.

A microfocus X-Ray beam is used, the actual spot is abiut 3/8".

Test Everything.


Geo
Attachments
50 keV FULL.jpg
ZnS(Ag) @ 50 keV X-ray digitally contrast enhanced
50 keV.jpg
ZnS(Ag) @ 50 keV X-ray as taken
ZnS as Gamma Booster.pdf
ZnS as Low Energy Gamma Booster Patent
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Re: Hornyak detector build

Postby George Dowell » Wed Sep 28, 2011 1:29 pm

Got Mu Metal around that PMT??? Always a good idea.

Ferromagnetic domains will change in that steel tube when it is moved in relation to the earth's magnetic field. Inside the PMT, the electron beam, especially those from the photocathode and first dynode will shift. This is of course not good for metrology because over time the shift will change making repeat measurements change accordingly. OK for prototype- get a non mag tube and use some Mu Metal.....


I have some if you need.


For adding speaker to counter projects:

http://www.qsl.net/k/k0ff//Speak2Me%20Module/




Geo
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Re: Hornyak detector build

Postby Doug Coulter » Wed Sep 28, 2011 7:33 pm

I check you on all that, Geo.

I explicitly tested for X ray sensitivity, and yes, there is some. Most of my fusor is covered with 3/32" lead, overlapped and soldered, but I left a couple holes for:

1. Seeing X and gamma rays (not all are produced at power supply volts, a spectrum is interesting and I plan to post one soon -- seems fusion generates some ~ 2mev photons -- I 'll get a calibrated number though - that one is a guess from comparing pulse heights on a scope between Cs-137 and the fusor, not calibrated otherwise). Obviously, I'm not going to take my gallon jug Nai down there and expose it to the fusor and activate the I in it - I'll use a crummier one that should still be good enough.

2. Having a place to put my neutron activation oven as close in as possible.

Most people don't shield their fusors, saying sq law is enough. I call bull on that - better to be safe, and I've found that even a hole in the shielding in the back results in plenty of X rays in the front, where I am, even on a geiger counter which is pretty numb to them. They seem to scatter off the air itself, and of course, all the heavy metal machines. I try really hard to keep my exposure well below 1 mrem per run and don't run that often usually, only when there's something new to test. The system is really reliable and repeatable now, so I don't need to run it as much.

Knowing that ZnS has at least some sensitivity to X rays (How could it not? Even UV makes it go!) I explicitly tested this near a hole in my own fusor shielding, as it's going up to HEAS with Bill to look at Richard's unshielded fusor and show off our new Standard Counter project. Yup, problems there, though the X ray pulses weren't as tall as the better neutron ones. I experimentally determined the thinnest lead that stopped the noise while looking at the analog output, which was about .040" thick and made a conformal cap of lead sheet for the whole tube. It still reads about the same on neutrons, but now doesn't change when I move it from behind the thick shield to where there's none. ZnS isn't perfect, but it does respond a lot better to charged particles than to X rays, which most other phosphors I know of don't share. I would otherwise have used a faster phosphor, or scint plastic, but those are optimized the other way - they do X rays better than charged particles from neutron knock-ons. I did mess with that one awhile back, but by the time you stop the X rays well enough to shut up the plastic, you've pretty much either stopped or slowed the neutrons so much you don't have enough energy left to knock protons out of the plastic either.

I could use some mu metal fer sure. I heard if you bend it up too much it has to be re-annealed (in ugh, hydrogen) (Arnold magnetics books) with a very long heating/cooling cycle. I did degauss the 4130 chrome-moly tubing before putting the tube in it, but yes, the spec on that phototube says keep it below 3 gauss or it won't work right. And yes, I've seen things "self magnetize".
There's not a lot of room in there, it would have to be thick to go inside - the ID of the pipe is only about 20 mils over that of the tube/detector and the tape holding the detector on.
I did degauss the tubing before putting in the phototube, of course. Couldn't use Al tubing as it has a capture resonance for fast neutrons right where we're working here -- didn't seem wise.
I don't have data on every material there, but some books mention the Al issue, and also He having a big cross section such that some labs use He filled balloons to filter out fast neutrons(!).

In this case, I decided to use a rather sloppy (but very stable) threshold of the schmitt trigger input of the PIC uP, and just vary the preamp gain to get it about right - threshold is about 4v, and the peak preamp output is about 9v and I set gain to just clip those. This results in missing a good number of counts -- the energy transferred to the phosphor from any one collision is fairly random, producing pulses all the way down to almost zero. I chose gain to make the threshold about 4x the X ray pulse height and various noise in the preamp and phototube. That means I miss some pulses of course, but don't get false hits from anyting other than cosmic rays, and that rate is pretty low ~7-8 cpm. A small gain shift wouldn't make much difference to this, but a big one would. I used tight regulation on the phototube supply and tested that since that's another gain-shift issue, and is a real big effect on multiplier tubes.

Another effect that's worrying is the ZnS:Ag long phosphorescence. This might cause a DC shift that would affect things, as well as take some of it "out of the game" if it's busy doing that, it probably doesn't do the normal fluorescence thing the same. We might be seeing just that in the curves I've already posted up here - the output seems to trend down slightly with time on all runs. I will therefore do one with a count from this in time sync with one of the other neutron detectors and see if it's the sensor - I suspect it is.

My take on neutron metrology (which seems to be generally agreed with) is that none of these real-time things -- boron, 3He, or this thing, can really be counted on to be better than perhaps 10% in otherwise *perfect* conditions. I've noted changes in my other tubes simply due to walking near -- the bag o' water & fat that is "me" adds enough more moderator to the system to change their output! Richard H is fanatic about never moving his, but that's not the only source of errors I've seen, to say the least. Millivolt signals in the presence of serious EMI are fraught with possible problems even in things that seem "tight". The Russian 3he tubes sometimes take off like a relaxation oscillator for a few seconds (the pulses look different on analog, and sound different which is why I use audio monitoring on all these). This is why my own religion is to use silver or Indium activation and count that with a beta sensitive counter when I need to be really sure. For me, the realtime things are mainly useful for real time tuning of the neutron source -- for example does a little more gas in the tank make more? I know before my hand is off the valve. Does more current make more neutrons in scale with the increased power? (answer, no) - for that sort of thing they are great. For computing the actual numerical neutron output from the fusor? No way. Too many ways to fool them.

There are zero ways to fool silver, and heck, it's cheap and easy to do, it just doesn't give you real time feedback for tuning.

In short, there are "issues" it seems with every neutron detector technology there is. The Hornyak has its blessings though - I can and have made them myself - so I am not dependent on a lucky score to have them to sell. And you can always get phototubes. Joe and I may make some boron tubes - we'll be having a F/F meeting after HEAS to discuss things like that. Another approach for fast neutrons (no moderator seems to be a feature here, they're big and heavy) is to look at recoils off hydrogen or helium in a big proportional tube. Those are fairly X ray numb too, but the signal is tiny. Again, the advantage is that they're cheap and we can just make them for other hobbyists.

Geo -- I made you a subforum to list anything you want to sell in...it's all yours, go for it! The more the merrier, and we all benefit, I believe. Look for "Geo's" under Vendors.

Edit, nice little speaker driver circuit you did. The one shot should make it a lot easier to hear. I worry a bit about the peak current in the output transistor, but if you have pulse width control you can control that nicely (speakers have inductance). I am planning to use an lm386 chip here for other reasons. I can put a peak in the response that matches speaker resonance if I like, but I've found that on several things I've got here -- being able to hear the pulse width and height give the user additional information. And in fact, the best "sound improver" is Joe Sousa's pill bottle "tick to tock" converter -- it really works nicely and is passive. The 386 draws very little power, works down to 4v and it's less parts overall. I have also built small stereo amps into rack panels with speakers at the sides as part of every gear rack I have here, with switched inputs and BNC's on the panels, with a "tape out" to go to computer soundcards for recording runs.

I've found with my big expensive 3He tube that during neutron bursts, not only do some of the pulses double or more in height, but also get wider. This is easy to hear since without the fixed pulse stretcher, they get more "bassy" or "thumpy". I've been working with pulsed fusion, so this is nice to have for me, dunno if it would help anyone else. Having done a lot of time as an audiophool, a musician etc, I have pretty good ears and like not having to look to get the info while I'm trying other things. Of course, if your counter is a geiger, there's no point, all the pulses look alike except ones marginally inside the dead time - and you might as well just make them all uniform in that case.
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