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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Richard Hull
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Re: Neutron Scintillation Tube/Counter

Post by Richard Hull » Sat Nov 18, 2017 12:10 am

I was the slowest of all here. Starting the effort in 1997 with no internet content related to amateur fusion, I built a demo fusor in about 4 months, but I had all the fixin's on hand. I didn't have to purchase a thing, but I soon would start what has become about a $10,000+ spending spree over the last 19 years in the fusion quest. I will also note that it took 7 long years to get a plus-ultra- neutron counter and 4 years to get, (make), a suitable mega-mark, fusor power supply.

I worked the demo fusor II for 2 years, reading cover-to-cover over 4 volumes on gaseous conduction. Then came the Farnsworth TV site on "songs.com" put up by the perfesser. When Paul found out, during his research, that Farnsworth had pursued fusion, he added a path on his songs/Farnsowrth site to see if there was an interest in Farnsworth's fusion effort. This effort on Paul's part was an immediate success! My first fusion was in fusor III which never exceeded 150,000 n/s. What I consider my real full blown fusor IV came in 2004. All of this was after the demise of "songs.com" and the passing of our "intranets" fusor site and the founding of this site in 2002. I have threatened over the last 10 years to make fusor V, but hey....slow is slow, and slow continues.

I love history. The history of this amateur effort is long and interesting. If you would like to see where it all began, check out the archived Songs postings to see how little we knew and see some names still around today, way back when.

http://www.fusor.net/old-boards/songs.com/

Note: the first post at the top is actually the last post in 2000 scrolling down will take you to the first postings.

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.

Silviu Tamasdan
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Re: Neutron Scintillation Tube/Counter

Post by Silviu Tamasdan » Sat Nov 18, 2017 8:45 pm

We're getting nostalgic now, but that old board (which I wasn't a part of) is an interesting view in the past. Thank you for posting that.
There _is_ madness to my method.

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Re: Neutron Scintillation Tube/Counter

Post by Richard Hull » Sun Nov 19, 2017 5:05 am

Yes we had stars in our eyes then and thought all fusion came from hard deuteron collisions in the poissor within the inner grid. Innocence has been lost over the many years. Without quantification but with a lot of good qualification we now know much more about when, where and how fusion is done in the simple fusor. The verve and willingness to pursue amateur fusion remains unabated.

I had a blast re-reading a large fraction of those early postings over the last few days. I am glad to have resupplied the URL so others could take a trip down memory lane when this entire subject was new and vital.

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 » Wed Nov 29, 2017 12:21 am

An update and notes for my effort to start up my neutron detector systems again under this thread: the previous weekend, my home built scintillation tube/power supply caused the ST-360's counter's diode voltage clamp to fail yet again. Oh well, microscope level soldering again. I realized my current system just isn't good enough - both the voltage control and interface system.

So I started deciding how to improve the system. That is when I realized that I have in storage an EC-650 "Gel-cell" power supply; this has a digital read out and complex feed-back control to provide any voltage from 0-6000 VDC (+/-) output (up to 100 ma.) While the unit is very stable (unit weights a good 40 lbs, however) I noticed that it did vary a few volts every now and again - apparently line variation in my home; so I added a line conditioner. The unit is now rock steady. So, I will use this both for my Russian neutron tubes and my scintillation tubes as a voltage source.

To enable this system to operate these tubes and my counter safely, I built a new and better interface box. This box has three co-axial connector outputs - one for the high voltage (HV) input, a second connector for the detector line and the third for the signal output from the detector to the counter/pre-amp (ST-360.)

Internally for this interface box I have a removable resistor (currently about 10K-ohms) for the voltage draw down on the signal output side of the cap that is used to both prevent HV from the counter and provides the data input to this counter. This cap/resistor feeds the output connector to the ST-360. The connector for the detector tubes has a 100 M-ohm resistor in-line and that resistor then connects to the other side of the capacitor (the HV side); and these, in turn, are connected to the high voltage input coaxial connector from my EC-650. Finally, I have a 1000:1 resistor bridge hookup inside to enable me to monitor voltage independent of what the EC-650 displays (these have 'banana type plug' connectors on the box.)

This system appears to provide a very stable voltage and enables both a proper resistance for the detector input voltage along with a safe signal connector for my ST-360 (the cap should block the HV but pass the signal.) I guess I will need an oscilloscope to determine the optimum resistor for the cap (however, I am using the value suggested in the paper for russian tubes.)

Until I repair the ST-360, I will experiment this weekend with a pre-amp system I have and a digital counter in order to test if I can get this power supply/interface system to work with a simple Geiger Tube. Once I am confident of its operation, I can repair the ST-360 and maybe try that and this new system with my He-3 tube.

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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Wed Nov 29, 2017 1:45 pm

After re-reading Rex Allens' excellent and detailed comment on Silvio Tamasdan thread "Criticize my Neutron Detector Setup" I realize that I am making a few fundamental mistakes in my interface box - the primary issue being the capacitance of my co-axial cable between the signal cap and the detector tube. I am referencing the thread and in Rex's post his second diagram showing the co-axial cable connection to the detector tube: viewtopic.php?f=13&t=11954&start=10

It would appear from his post that I should enclose my neutron detector tube within the interface box and directly connect the 'buffer' or signal capacitor directly to the tube so that part of my circuit has no significant capacitance of its own (between this buffer cap and the tube); then, run the HV to this via a short co-axial cable thru my 100 M-ohm resistor in the box and then use this to power the detector (see the first drawing of that post by Rex.)

I do have room in the box to add a pre-amp but that depends on powering such a device. I will consider that in a later mod of the system. For now, I will most likely continue to use a long coaxial cable to output the signal from the 'buffer' capacitor to my counter system.

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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Thu Nov 30, 2017 7:48 pm

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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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 11:52 am

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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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 2:45 pm

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 6:12 pm, edited 1 time in total.

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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 6:11 pm

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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Re: Neutron Scintillation Tube/Counter

Post by Dennis P Brown » Fri Dec 01, 2017 6:56 pm

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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