Neutron Scintillation Tube/Counter

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 8:19 pm

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.

MVC-018L.JPG
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 Sat Dec 02, 2017 1:22 am, edited 7 times in total.

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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 11:00 pm

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 » Sat Dec 02, 2017 4:48 am

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

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Dennis P Brown
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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Sat Dec 02, 2017 12:25 pm

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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