Neutron Scintillation Tube/Counter

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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown »

My ST-360 (power supply/amp/counter instrument) has been repaired yet again (third time); two of the three pads on the diode clamp location (less than 1.1 mm 0.5 mm for the area that the pads are in; the pads are just 0.2 mm square) which are utterly gone thanks to my soldering skills. Working under a rather high power microscope and with limits of less than 1/3 mm (the ultra-fine soldering tip looks like a blunt rod!) is very difficult. I had to cut a fine 0.2 mm wide copper foil to create a new pad and 'wire' led to one of the contentions to replaced one of the pads I destroyed along with its connection lead that goes to another connector point. As to the other damaged pad I simply created a small pile of solder to 'act' as a new pad for the terminals to the diode (three terminals.)

The good news is that the ST-360 unit is working fine for both its Geiger counter connector and relative to its scintillation connector with each respective detector in place (these detector tubes work well with the high voltage the ST-360 produces.) I used the room's background (fairly well know from past experience) and then a uranium sample to confirm the proper counting with each detector (the scintillation circuit has an extra amp.)

Considering how difficult replacing that three pin diode is, I am very lucky it is working again.

The bad news is my high voltage supply causes the ST-360 to 'see' all the line noise (for the capacitor's output in the interface box) and treat these'pulses' as events resulting in a few thousand counts a minute - I have tried not using a ground between the ST-360 and the interface box - no help. My guess is something might be wrong with the comparator voltage (or chip) and I may look into that

I am lost why this problem is still occurring since my first supply and detector worked so well with the ST-360. That is, before I caused the first diode clamp chip failure. While this new system isn't battery powered, the voltage output looks very steady (with a digital volt meter.) Until I can replace my oscilloscope, I can't look at the output of the supply's wave form. Unfortunately, the ST-360 has a max HV of 1200 v. Too low to operate the russian boron or He-3 tubes (Though I did try.) So, I am stuck trying to address this issue - I have a working, proven fusor but now can't get my detectors to work again.

On the bright side, forcing me to delve into the design and operation more deeply (really would have preferd not too, however.)
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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Since yesterday, I had a few ideas on what might be causing issues with my detector system. So, I did some more work this morning on the interface system while I still had some time before I leave.

First, I connected the power supply directly to the ST-360 (by-pass the interface box) and the ST-360 was rock steady - the triggering was not caused by the supply. Did a little more testing and discovered the issue is rather complex and I don't yet fully understand all the causes. Grounding certainly matters but isn't the answer alone. I removed the cap/bleed resister from the interface box to avoid noise/false triggering events.

Next, I have a secondary voltmeter (using a voltage divider) reading the actual voltage on the detector. Turns out, the 100 M-ohm load (between the supply and detectors/ST-360) causes the voltage at the detector to fall to 1/2 to 1/3 what is displayed on the digital power supply: really not too surprising (hind-sight) but I have little experience using 100 M-ohm resistors in circuits and didn't think this out - that the detector tubes, both my scintillation and Geiger, draw a finite but not insignificant current causing the voltage drop. I have to almost triple the applied voltage to get the desired voltage at the scintillation detector.

The really encouraging results are that both my Geiger and scintillation tubes work great with my digital supply, interface box, and ST-360 when I set the correct applied voltage to the detector tube. For instance, the scintillation tube requires 850 - 900 volts and that means I set the digital supply to about 3000 volts. Then I get excellent results from the scintillation tube using my know test sources. Similar for the Geiger tube; it works at 720 volts and then that requires that I set 1550 volts on the digital supply (both these digital settings result in my other voltage meter at the detector tubes to read their correct operational voltages.) When I do this, then the count readings on the ST-360, fed thru the interface box, match my standards. Again, these voltage issues are caused by the 100 M-ohm resistor. Of course, I could lower the resistors inline with the supply but the objective is to use my He-3 tube and the paper posted here suggests 100 m-ohm for the russian detectors.

This is a relief since I now can rule out power supply issues. Of course, if I was just going to use the scintillation and Geiger tubes, none of this would be required. The ST-360 provides more than enough voltage to run these tubes. However, the issue is the He-3 and boron tubes. These require voltages that significantly exceed my ST-360's capabilities; worse, those voltages will fry my diode clamp chip. Hence, I need to buffer between the ST-360 and my interface box.

One issue I need to address is my neutron scintillation tube; while I believe there are issues with it, that tube has the potential to work just with my ST-360. I will look at that if I have time this weekend.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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Realized that since a voltage drop occurs in the interface box's circuit when I turn on the ST-360, this means that this device has an innate impedance; exploiting that, I should be able to drop the voltage the ST-360 "see's" simply by using a resistor in series with the ST-360 input line (signal.) This will not affect the voltage that the detector tube see's, at all.

So, in-line with this idea, I installed the 15 k-ohm resistor I had on hand and it did drop the "820 v DC" that the scintillation detector "see's" (though that remained at the detector tube) to essentially zero at the ST-360. Critically, the ST-360 still saw and responded to the gamma induced signal for the detector. So the ST-360 worked fine with the scintillation detector using my uranium source (through maybe a bit fewer counts.)

Guess I need a series of resistors to determine the best signal x-fer to the ST-360, which will keep the highest voltage for the detector tubes safely below the max. rated value the ST-360 can handle. This should solve my issues for both the He-3 and boron gas tubes. That will remain to be done this weekend.
Last edited by Dennis P Brown on Fri Dec 01, 2017 1:12 pm, edited 1 time in total.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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The new lower valve resistor worked; the best (lowest) resistor I had on hand was a 27 ohm; this allows the ST-360 to "see" a 1:10 ratio in the voltage reduction or when the power supply is at 2000 volts (and the detector) the ST-360 experiences just 200 volts. This is close enough for now that I am not concerned with further maximizing the signal to the unit (maybe double the voltage at some point via a still lower resistor.)

The unit works well now with the ST-360 at this lower resistance when I use the scintillation detector (for gamma's.)

I have decided to start up the fusor and see if the RF noise from the unit creates any problems with the (repaired) ST-360 and the interface/new power supply.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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I fired up the fusor so it had a steady plasma and was up to its normal voltage/current values: 30 kV at 30 milli-amps; D2 was at 14 microns. Everything looked good and was operating in a fully stable manner identical to how it has in the past. I had the ST-360 in stdby mode (power for that unit on but not set to the count mode.) The neutron detector tube had no power, however. So I then set the ST-360 to count mode (neutron detector tube still off) and the unit remained steady (no counts, even with the fusor running all out.) At least the ST-360 was not seeing any RF noise even with all wires and interface box connected to the ST-360 (including the not as yet powered He-3 tube)!

I decide to put the correct power to the He-3 tube and allow the ST-360 to go on line. Immediately, the ST-360 display started counting (the fusor did have deuterium and was running at the correct voltage/current for fusion); Watching the ST-360's display it appeared the detector tube was recording about 120 counts/min (didn't really time the run.) Not sure if this reading was real, so I removed the moderator and the counts lowered to just 2-3/min (again, guess on time.) Apparently (maybe) the He-3 tube is working after well over a year fighting this issue.

However, before I could do another run (I put the Moderator back in to prepare for a more measured repeat run), the fusor power supply's main power suddenly failed with no warning. This has never happened before and I was, at first, at a total lost what happened. After some quick checking, I determined the main fusor supply's fuse had blown (but not why.)

I turned off the ST-360 to protect it while I did checks of the main fusor/supply and determined the fuse failed protecting the unit (why was strange since everything had, until then, been stable.)

I replaced the fusor's fuse. Then as I slowly turned up the fusor's power supply, and around 20-22 kV, I'd get a runaway of current but I saw no plasma form or 'light up'. A few more attempts with identical results convinced me something in the fusor is most likely wrong; so I shut the main fusor down.

After the DP cools, I will further investigate what is going on and I hope nothing major has gone wrong.

I am, to say the least, both confused and rather surprised that just as I got my new He-3 neutron detector up and possibly working, my reliable fusor just failed in a major manner. This is really strange and makes me wonder if I have bad Karma or something - ;).

Regardless of this aggravating outcome, I am extremely happy, none-the-less. First, I again managed to successfully repair my ST-360 counter box; in addition, finally,after much effort and a number of failures, my new interface box works exactly as designed/intended (a lot of work went in to designing, figuring out failure modes, as well as required rebuilds.) Also, my new digital power supply both works and appears noise free (luckily, that digital supply covers the voltage range I require for my detectors); further, and very importantly, the new He-3 tube/counter/interface system also appears to be all working together properly and the neutron detector system exhibits no noise issues with the fusor; even when the detector is powered up with the fusor running (my EMF shielding for the tube appears very successful.)

Better still, and the whole point of this long effort (again, over a year) my new He-3 neutron detector system appears to be working as intended; that is, the He-3 detector and counter might have been detecting neutrons! :)

Well, I know what I have to do this weekend. Hope it isn't serious now that the neutron detector system and its previously large array of problems might very well be solved. I guess frustration and fusors are synonymous words, maybe?

Aside: just realized we had a very small Earth quake here yesterday and my new, reduced volume anode screen might have been moved (it isn't fixed - maybe do something about that.) I am getting an indication that my cover screen has moved off the DP throat opening - plasma was, after the incident, reaching my TC gauge. Didn't think about this so, maybe something was moved by the quake. Will see.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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Checked the fusor and the Earth quake did move my RF shielding screen over the DP throat; this is serious since it exposes all my gauge sensors to the high voltage plasma. Also, and maybe the cause of the current runaway my fusor experienced, the main viewing port/HV feed-thru was far too coated with metal (eroded, no doubt, from the electrode.) This might very well have created the conduction path leading to current run away by the fusor's power supply.

I disassembled the fusor's top and inspected the system. I removed and cleaned the main view port window. I re-assembled the fusor and placed it under vacuum. I have the DP heating up now.

I just tested the fusor at 50 microns and a stable plasma was easy to ignite and maintain a steady current (through very low voltage.) So far, what I am seeing makes sense for my theory of the main cause. Of course, until I test the fusor in full mode, can't know for sure.

I am including a picture of my new neutron detector's main system (less the detector tube, which is mounted on the fusor.)

The interface box is very simple but allows the high voltage from the digital supply to be put through a 100 M-ohm resistor, then this output has a voltage divider to monitor the net voltage that develops with the draw down by the detector tube (and even the ST-360); that is, this reduced output supplies the detector tube via another co-axial connector; then this detector line is fed into a simple resistor (to drop its voltage a good bit more (1/10)) allowing the detector's output signal (detector pulse) to reach the ST-360 to then be amplified and counted/displayed. All that interface box contains are a number of resistors, wires, and connections to various terminals located around the panels to be connected to the other devices. In some ways, that interface box is far too simple considering the issues its solves.

Neutron detector system's digital power supply (bottom, center left, large and mostly black panel), new interface box/electronics (top blue/black box), ST-360 amp/pre-amp and counter display (right, white box), and the 120 volt line filter (small light tan box in background and to the right). Cables connected; the voltmeter (yellow and to far left) gives real voltage on the detector tube. The digital supply displays voltage it is set to and currently providing/reading internally
Neutron detector system's digital power supply (bottom, center left, large and mostly black panel), new interface box/electronics (top blue/black box), ST-360 amp/pre-amp and counter display (right, white box), and the 120 volt line filter (small light tan box in background and to the right). Cables connected; the voltmeter (yellow and to far left) gives real voltage on the detector tube. The digital supply displays voltage it is set to and currently providing/reading internally
Last edited by Dennis P Brown on Fri Dec 01, 2017 8:22 pm, edited 7 times in total.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

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After the aforementioned repairs, I ran the fusor; and as previously with deuterium and using the new He-3 detector operating (at its correct voltage); the ST-360 obtained 130 counts/min with a moderator this time (ran two one minute runs - nearly identical numbers), and obtained 6/min with no moderator. And again, I obtained all of one full run (i.e. one with a mod and without; then one with the mod again. Didn't manage to get the second no mod run.)

These runs were done only after I finally, and with effort, got the fusor stable.

Should have realized that there were more extensive issues with the fusor than I first suspected. The previous issues were rooted deeper than my initial inspection indicated - turns out the cathode's ceramic cylinder had a hairline fracture all the way around its circumference and this was severely leaking current causing random surges in the fusor's current performance and leading to runaway conditions, at times. Also, sudden burning of some of the ceramic material from the post. Before this became very noticeable, the system had settled down and ran smoothly for the first three test runs but rapidly, thereafter spiraled into trouble. The system caused such a sudden power spike that the breaker for the power strip jumped and I forgot that the fusor was at full power. So, when I turned the breaker on, the fusor went all out and my ammeter for the fusor was burned out.

So, before I can use the system again I need to disassemble the cathode and cut a glass cylinder to cover/shield this problem with the ceramic support post - that will take a few days at the least. Also, before I can operate the fusor at all again, I will need to locate a new milli-ampmeter to replace the one I just lost (hopefully, that is all that was burned out/damaged - I did test the x-former and it appeared to have no voltage issues.) As a result of these problems, just as I was finally making real progress with the new neutron detector system, yet another set of not insignificant issues arise and need to be addressed - worse, rather time intensive (considering the nearly year effort to create a new neutron detector system (barely), not all that long, maybe.) Bottom-line, the fusor will be down some weeks before I end up finding and ordering/get delivery of another meter that covers the range of the old one. I guess I should go digital like the voltmeter display for the fusor PS but that might be a good bit of an effort and maybe more costly than I want (I do have a old surplus digital meter (somewhere ... ) but my first effort to get it to work as an amp meter was a failure - not sure I want to revisit that problem again.)

Almost a bit comical how these problems occur - fix one support issue and another one appears (I really do need to put a check list together for start up and shut down.) But then, these devices are being pushed hard, are somewhat underrated for their loads, and for me, these devices/parts are often surplus and unless one really over builds and/or uses new stuff ($$$), the old saying of fix and repair daily is starting to sound all too familiar of late. Not sure what I am achieving patching up the fusor after I develop new support detectors (which are not finished, for that matter.)

Aside: So, today I did manage a great deal of success with the new He-3 detector system but this effort also resulted in the creation of some new problems with the fusor - I guess one way to burn up a day off ... ;) .
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Richard Hull
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Re: Neutron Scintillation Tube/Counter

Post by Richard Hull »

Digital ammeters are easy to make with the totally free Harbor Freight DVMs. I now have 83 of them in boxes here at the lab. They give 'em away and I bring 'em home, take out the batteries and throw them in a large box. No digital ammeter for a fusor should cost more than $0.00.

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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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown »

Some rather good news on the broken meter problem - I got up early this morning and I traced the circuit for my meter in the fusor (like much of what I build, a rather simple circuit since the x-former has a tap for current measurement and to ground the x-former) and discovered that the problem was just a failure in the ground path; the LED light I use for the "High Voltage Active" warning is part of the meter's ground path (and hence, the x-former; ok, not the 'brightest' design idea I have had using an LED ... lol.) The LED light had been fried thus removing my grounding path for both the meter & x-former. Easy problem to by-pass for a simple test.

Placed the fusor under vacuum and ran the x-former; had a nice plasma at 30 microns and a few kilo-volts. The meter with the by-pass worked well displaying a nice match to voltage increase with the current following - so, will re-wire that ground path and ignore the LED (more trouble to remove that unit from the panel.)

Yes, a digital meter is useful (and cheap!) but analog meters do have their charms ... like taking massive abuse and keep working - the LED, not so much able, it appears ...lesson for the digital age, maybe?

As for building the new glass cylinder for my electrode column, my diamond coated band-saw blade is now junk so I will need to get a new one to cut the glass section. Well, I prefer this problem over the meter problem, any way (simple mechanical - electrical can be more sinister than they appear and far more serious than one expects - glad that is not the case.) Besides, the blade was very old, anyway.

The cracked ceramic cylinder for the electrode is far more serious a problem than I had suspected - the internal metal column is the source of sputtering material and its rate is serious. This will, soon, cause another 'short' if allowed to continue. So the fusor is definitely down until that repair is complete. Dodged a bullet with that problem - that shorting problem could damage or burn out my x-former. So the discovery was fortuitous, in a fashion.

I will, when I have time, post the simple circuit I used to both reduce (for my ST-360) the tube's applied voltage and its current - including the voltage divider; that is a useful feature to read the actual detector voltage, rather then just the applied voltage by the supply - these have rather large differences. While the interface case is over kill in size for what it does, since it was salvaged from a previous project long ago (a furnace controller), not too concerned; like the digital power supply (bought over ten years ago for a rather low price considering its 'current' usefulness) hind-sight, sometimes works out nicely.
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