Neutron test of a BF3, He3, and Scintillation (fast) tube

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Dennis P Brown
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Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

I finally fixed the leak in the fusor: it went from 2 microns to 100 microns over 24+ hours (likely simple outgassing.) In a test, it bottomed out to 1 * 10^-5 torr after ten minutes of pumping. I have 7 inches of teflon and 2 inches of paraffin wax as a moderator. The distance from the center of the chamber to the center of the neutron tubes is 16 inches. The power supply is rated to 32 kV and 30 ma continuous (though it can handle up to 100 ma for short times.) The detector system is based on the Ludlum 2000. All cables are coaxial and the BF3 is fully shielded as is the scintillation tube in metal cases. The He3 tube is not encased in an extra metal tube.

The BF3 is about 1.5 inches in diameter and about 16 inches long. The He3 tube is about 3/4 inch in diameter and 16 inches long as well. The scintillation medium volume is 3 inches in diameter by two inches thick.

The fusor was ultra stable - once set it didn't have any hiccups or variation in current once I set all parameters. As I barely adjusted the D2 inlet leak valve, the fusor current was dramatically but smoothly changed - both down (small reduction in D2) and up as I increased the D2 flow.

I ran the Ludlum 2000 with the BF3 tube (@ 1600 v; and Disc 1/2 full turn). The fusor was @32 kV and 28-35 ma. The tube responded at about 60 counts/min. and maybe 1 count per min. without the moderator. It didn't seem affected by the fusor when set near it w/o the moderator but fusor still at full power. The performance of the detector did follow the fusor power (60 counts/min at full fusor power and near zero with no measurable current but still @ 32 kV.) I was surprised that sometimes the detector 'ran away'. Whether that occurred because I was too close to its max operating voltage and a breakdown occurred or just something else, I have no real idea.

The He3 tube really puzzles me for two reasons. I ran it at 500 volts (rather low (!?) but it didn't like higher voltages at all - that is, it would quickly suffer a breakdown and run away if I used 700 volts). The DISC adjustment was set at about 1/8 turn. In the moderator with the fusor at full power the Ludlum display was running wild - consistently it showed 3000 counts/min. Again: fusor at 32 kV and 30 ma. As a sanity check, I set the DISC to Zero turns and the He3 had no counts. When the moderator was removed the system would stop all counts and the detector would just sit happy (no counts) till I replaced the moderator. Then it would run like crazy. Ditto it followed the fusor power as I adjusted the gas flow (so the fusor always saw 32 kV but current was taken from 30ma down to 0 ma). The counts slowed and then stopped as I did this. It didn't 'see' any noise at all when I exposed the naked tube (no extra shielding) to the fusor body as the fusor was under full current/voltage with a fully lite plasma. So, the detector tube didn't seem to care at all about fusor noise (as I'd expect it should.)

Yet the number of counts is huge - far above what I'd expect for a He3 system. Especially with that voltage - a voltage that seems so low but it certainly doesn't like higher voltages.

The scintillation tube ran at about 1000 v (Wagged) and provided 40 - 50 counts/min and followed the fusor power as well; however, there is no way to test for noise issues directly but it is well electrically shielded and behind a thick shield of ceramic and lead. I did move it further away (still shielded) and the count rate quickly dropped essentially to zero at 3 feet from the fusor center. I did get a runaway with it once. Not sure how or why can could occur unless the detector can have a runaway due to voltage(?)

With so little experience with detectors, wondering if these results make sense and indicate these devices were likely responding only to neutrons.

Overall, except for the occasional runaway with the BF3, and the far too high count rate of the He3 tube, these devices acted as if they were responding to the fusor (and its neutrons.) Neither tube cared if I handled them at voltage.
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Detail of the BF3 tube (inside metal shield), moderators, and Chamber - top
Detail of the BF3 tube (inside metal shield), moderators, and Chamber - top
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

I still have another BF3 tube I'd like to test but just am not certain that my setup is acting as it should. Hence the long post above. Rather paranoid but of late, that seems appropriate (failure of the TC-246, and the previous leak, and the new failure of a new pre-amp (not the one I showed a few days ago. Just having bad luck or maybe, just incompetency.) Also, I am curious about activation (of silver). One requires a moderator to do this? That is, slow neutrons are necessary, rather than fast? If so, then a thick moderator is better? This, of course, assumes a sufficient neutron rate and a decedent Geiger counter tube.
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Richard Hull
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

Wow! what a mishmash of reporting.

I was amazed you had a nice stable fusor that ran at the insane 32kv @ 28-32 ma. Pressure, pressure, pressure??? Grossly incomplete data given on the fusor at a totally stable operating point. Thus, I can say nothing.

With a good conditioned fusor V at 32kv @7 microns of D2 and 10ma. My 4 ATM Reuter and Stokes 3He tube, in moderator 8-inches from the fusor, is clicking away at about 30,000 cpm. The other day in the final part of my 5 day run up of fusor V I ran the system to 42kv @ 12ma at 9microns of D2 and counted 78,000 CPM.

Do you have a precise pressure on your 3He tube?
The BF3 should have been going nuts as well!

Again what the hell pressure were you running on D2 to need the unheard of 28-32ma at that high voltage! My grid typically runs white hot and near melt at 15ma when doing fusion. I never like or achieve pressures over 16 microns and currents over 14ma at 43kv when I log 128,000 cpm and am just past the mega mark TIER.

Your results are confusing to me in the extreme. What was your running pressure of D2?

Richard Hull
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Oops: see, just not paying attention on my part. The pressure (D2) was 10 -15 micron operating range but for counts, set at 12 microns D2 for runs. My grid was red to white hot (the later more so). The wire grid is a 4 mm thick steel alloy shaped into a std grid configuration. The plasma was a light blue and I ran for ten minutes 'on' for each detector, and then off for ten minutes off to allow the grid to cool.

So my D2 pressure appears to match your operating conditions.

I also forgot to include pics of the He3 and Scintillation systems. So, I'll include those. I might be running the BF3 tube low on voltage but at 1800 volts it runs like the He3 tube. Maybe correct, maybe not. The DISC setting certainly is an issue depending on the applied voltage.
Attachments
Fusor Cathode/Plasma
Fusor Cathode/Plasma
D2 Pressure
D2 Pressure
He3 in Moderator
He3 in Moderator
He3 tube
He3 tube
Fast Neutron Scintillation tube
Fast Neutron Scintillation tube
Last edited by Dennis P Brown on Thu May 11, 2023 7:58 am, edited 3 times in total.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

From past posts, I see that the operating voltage for the CHM-32 He3 is 700 volts. So my 500 volts was low. I've tried 700 volts and that looks similar (but then I adjusted the DISC setting for stability/sensitivity.) They achieve 280 counts/10 min. with a Po/Be source. So guess my fusor isn't too unreasonable for my high count rate.

see: viewtopic.php?p=77866&hilit=CHM+32#p77866
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Here I remove and reinstall the He3 Detector twice into the moderator. I use the std Microsoft player for the vid.

The first vid is just changing the current (not voltage - that remains at 32 kV) by decreasing the D2 gas flow. There is lag in both the TC gauge and the system removing the D2. So the track isn't one-to-one, of course. The counts follow the gauge with the innate lag (but the counts do closely follow the current very well - that ma gauge is too far away to include in any vid.)

The second vid of removing the detector is obvious on the results. These are each about 46 Megs in size.
Attachments
D2VSCurrentshort.mp4
D2 vs. Current of He3 Detector Performance
(28.37 MiB) Downloaded 223 times
VideoDectector.mp4
Video of He3 Detector Operation
(46.31 MiB) Downloaded 252 times
Last edited by Dennis P Brown on Thu May 11, 2023 8:45 am, edited 6 times in total.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Jim Kovalchick »

Hi Dennis,
It's good to see you active in your lab.

A couple of questions/suggestions:

1. For your thermal neutron sensing tubes there is such a thing as too much moderator but there is not such a thing as too much reflector. I recommend parking your tube as close to your fusor as you can only limited by the thickness of moderator needed to slow neutrons to thermal. Too much moderator give neutrons a chance to be absorbed or wander before hitting your detector. Being too far either because of extra moderator or geometry obviously reduces your flux.
2. Reflect. In a couple pictures it looks like you have nothing behind the tube. You would see lots more neutrons if you stack some reflector behind it. Use more paraffin, water, HDPE, spare blocks of Be(lol) etc.
3. What's that between the fusor and your fast scintillator? I assume its shielding, but I see some cardboard and plastic that will tend to slow your neutrons before they get to your fast scintillator.


I'm probably giving you suggestions you already well understand, but I thought I'd share my experiences.

Good luck,

Jim K
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Thanks - struggling (again). So I certainly do appreciate your post. I'll add a reflector for the next run. The the main shield for the fast detector is a 2.5 inch thick ceramic block (protected with a plastic mess) and a 3/16 inch lead sheet panel in a wood frame. I guess I can experiment with amount of moderators and switch the blocks around.

Balancing all the parameters for proper neutron operation is certainly an issue. That is why I decided to post these and try different detectors for comparison. I am nervous on voltage and using the Ludlum - don't want to damage the unit.

Sanity check using some numbers for a He-3 detector
I guess I will calculate a possible rate for an arbitrary fusor that some have posted here - a value of 250 k neutrons/sec isn't too unreasonable for this calculation.

So for my detector at a distance of 16 inches from the fusor center this represents a 'spherical area' of 3220 sq. for all the fusor neutrons to pass through. My detector 'area' is roughly 12 sq. inches at this distance. So the cross sectional area of my detector to this imaginary spherical surface is roughly 0.004% of this total area. Then for 250 k neutrons/sec one gets 931 neutrons/sec crossing the detector 'volume' per second. Now, assuming a 15% capture rate of thermal neutrons (I've seen values here at the forum that claim 30% is typical*), one would than expect 140 n/sec using this rate.

So to get 3000 n/min means I'm recording 3000/60 or 50 neutron/sec. This implies (ignoring moderator absorption and other issues like detector volume variation over the 12 in^2 'surface') my fusor would be producing roughly 90 k neutrons/sec. Not exactly a high rate compared to others here but I think believable for the size, voltage, and D2 supply. Assuming my math isn't in error.

So my 3000 counts/min. by the detector tube isn't all that high of a value for the setup using these assumptions.

From some spec sheets I've crossed checked, the CHM-32 tube has an operating voltage of 750 v (yes, it can go far higher but then there are issues, apparently.) The tube likely has a pressure of 2 atm He-3. However, not certain of these values but they are a lower bound from reading the various sources and could be higher.

* viewtopic.php?p=2944&hilit=He3+neutron+efficiency#p2944
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

I agree with Jim move the 3He in closer for higher count numbers. Try your best to smoothly and evenly surround the 3He tube with moderator. I had a 1" diameter 22 inch long 4 ATM SS body 3He tube in the center of a 6" PVC pipe filled with water from 2004-2019 with the pipe moderator up near the fusor IV. Worked fabulously well all those years!

Warning never do this with any 3He tube with an aluminum body tube!!! Jon Rosenstiel did this with his aluminum tube and electrolysis via some process ate a hole into his 3He tube ruining it.

With fusor V, in 2019, I moved to a block (pile) of HDPE and see no real difference over my water moderator.

Now as to your fast scintillator, use only the lead sheet over your scintillator. Ditch all that other crap.

Put your detectors near to the fusor.

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
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Will certainly do and thank you for the advice - I was rather in the dark on this issue to say the least - more is not better. I agree and I should experiment on trying to fix a better (more optimum) moderator thickness. Maybe a FAQ on moderators would be nice since that is an area that is both necessary but not covered in any depth.

The art of neutron detection is certainly something that is more involved than I appreciated, previously - not just electronics alone.

Agree with the 'fast neutron' detector issue - bad arrangement and I'll only add some lead (have to get some more.) Learning curve still after all these years ;) But that is what makes this forum so valuable.

I really want to attack the neutron detection issue as many ways as possible to learn more about these detectors and their in's and out's.

At some point I'd really like to try activation but seriously doubt my fusor can generate enough neutrons currently. Certainly the geometry of the moderator and detectors needs work. Maybe, if I have the time, try a smaller fusor body. Certainly, try and build the 80 kV x-former and see what that can do with the current fusor. Why I upgraded the shielding. Always more to try. But that is where the fun occurs.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

I took Jim's and Richard's advice and I reduced the amount of modulator (removed 40%) and used that removed section to act as a reflector behind the He-3 tube. The fusor power was 32 kV, 30 ma with a D2 pressure of 10 microns (same as yesterday). This removal of some moderator also meant that the detector was moved closer to the fusor center (about 3.5 inches closer.)

The net result was my count increased to a little over 7000/min compared to the previous 3000/min. A rather large increase for so little effort - thanks again for that excellent advice!

I experimented with the fusor total power (reduced it to 25 kV; pressure remained at 10 microns) and the count went down to 3700/min.

After 15 mins, I notice a general reduction in counts till I was getting about 3000/min @ full power. Apparently the chamber was getting hot (as was the cathode -very.) Gave it 30 minutes to cool and then fired it up again. With the power at my previous max settings, the count rate returned to 7200/min.

So while my fusor neutron rate/production hasn't changed at all, of course, the detector change has resulted in an increase in the counts detected. More thermal neutrons are reaching the detector. This puts the device at what I think is a more reasonable rate of 180 k neutrons/sec. Still rather low end compared to many here but I'm just trying to get a handle on detectors and electronics again. I should experiment more and try to optimize the moderator even more but have other issues for today. Should have time this weekend with the forecast rain.

Overall, I am very pleased with the stability of the fusor. Keeping it clean, performing a plasma ashing (using trace air @ 5 microns and 10 - 12 kV), and running it down before starting to below 10^-4 torr all appear to enable this stability. After that process it always fires up easily and holds all power parameters rock steady. The plasma behaves just as steady, as well. No loss of plasma or any glitches or hiccups when running. I can dial down either the voltage or D2 flow and the plasma responses in a steady fashion till plasma is extinguished. Then it smoothly reignites as either parameter is increased again.

The steady loss in counts simply confirms what many here already know and that water cooling would enable better long-term operation. For those wanting to cool the cathode I would think that too would improve long-term count rate. Again, old hat for many here but just interesting to see the effect so clearly in my fusor.

These results make me consider that I should maybe build a smaller one. And if I do, certainly will add water cooling.

For my next effort, I want to replace the primary amp in the NIM box (maybe next weekend and assuming that one works ...) and then I should be able to get the NIM electronics back online again for neutron detector interface. Also, I'll add some Pb shielding to the fast scintillation detector and then try that device again (through my neutron generation rate had better be a good bit higher if that device is going to have any chance of making believable detection.)

As for activation, I am building an electronic Geiger counter that interfaces with a computer for recording counts/time. However, I seriously doubt I have enough neutrons with this fusor to be successful. Another reason to build a smaller unit.
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Why I need a small building for the Fusor System
Why I need a small building for the Fusor System
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

Glad to here you note that while you have read much for years, it is nice seeing it all unfold before you in your own working system. Nothing teaches like the doing and observing with your own eyes the work proceeding from the effort of your own hands and expenditure of treasure. Sinks in real good don't it? You own it in a special way now.

Don't over lead up the fast system. Scintillators are traditionally wonderful counters of x-rays and gamma. You just want to use the lead to filter the x-rays from the fusor. You should not have to use a thick piece. The piece you had seems OK.

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
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Thanks for the shield advice on the scintillation detector. Again, my ignorance on that subject is vast.

So very true about learning by 'seeing' the actual issue by doing the experiment is very enlighten. In any case, these results have convinced me to try a smaller fusor chamber - bet I do find some surprises (not all good) if I manage to assemble such a unit (bought a three way 100 mm connector - now need a window for it. Once more into the machine shop.) This hobby has certainly been addictive in many ways - the hardware is an interesting part as well (through that can be frustrating for obvious reasons.)

I really need to study neutron interactions with matter (i.e . moderators) and figure out what is optimum thickness for some materials (via calculations rather than blind experiment - but that will have to do for now till I have time to read up on that subject.) Knowing the neutron initial energy certainly will make that task far easier. However, I've never read about the function of a neutron reflector (through in hindsight that makes so much sense.) So posters here are such a great deal of help. Never would have thought of that aspect of moderators and detectors.

I really want to look into neutron detection far more and see what and how these detectors differ. That a He-3 tube works 'better' at far lower voltage than often sighted in spec's is a surprise that I'd like to explore more via reading now I have a 'on-hands' experience. Also, capture rate and total counts caught me by surprise for said units. Wasn't expecting those results at all.

I must also add that NIM amp units are becoming very hard to find via ebay. Guess most available units have been either surplused or junked (through inoperative units are what is often being sold of late - not on purpose; testing is not something most sellers can do.)

I found an Ortec 575 that looks like it has many features of the Tennelec 246. Hope this one works (though not holding my breath) - I don't want to give up on the NIM box/units since these offer other advantages compared to a one-in-all unit like the Ludlum 2000.

Testing via a pulse generator of these amp electronic units is something I know little about (and will likely ask questions later. Like what max voltage is acceptable or are attenuators required?) Certainly I know even less on basic repair unlike some experts here, like you. Did fix a Ludlum hand unit and a black box 'all-in-one detector driver/amp via circuit diagrams and more easy electric parts (i.e no dreaded multi-function chips). Those chips are, and remain for me, complete black boxes (and to most people except those knowledgeable in the mysterious ways of that black art ;) )

I'm certainly looking forward to this up coming HEAS meeting. Amazed that you can tolerate so many of us for that amount of time. Both you and your wife are certainly generous to host this meeting, much less arrange and operate it.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

Thanks for the kind words on the HEAS meetings. October 7th will be this 34th year bash. As always, I will post the opening for invitation RSVPs in June or July.

You have a Russian 3he tube of unknown fill or boost gas ratio. Seems to work at the ultra low 700 volt level. No U.S. made 4 atm tube 22" long will not even begin to work below 1300 volts. Most work around 1600 volts.

U.S. made BF3 tubes work from about 1500 volts for the tiny Nancy Woods tubes to 3000 volts for large 2" diameter 20" long monsters.

Richard Hull
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Just a side issue that, once again, I noticed during my latest series of runs. I always measured no significant x-rays from my fusor (same as back ground (BG)) either at the metal side or at the glass window while the fusor was 'running'. Anze operates an identical chamber at the same voltage and he got some x-ray generation signal at his fusor window location (as reported by his school's safety officer.) This puzzled me so I decided to mount my Geiger-Muller tube for permanent monitoring near the window (see pic.)

I get, just like before, essentially BG counts from/for x-ray detection from my Geiger-Muller Detector when operating at this location during a typical fusor run: i.e. D2 pressure from 9 - 12 microns, at 32 kV and at any current but generally from 20 - 30 ma.

The x-ray detector location is positioned at the window port. My Ludlum hand held detector operates with a Geiger-Muller detector tube that is 0.5 inch dia. by about 8 inches long. No steel casing/shell cover. The detector tube is about 4.5 inches from the thin glass plate (quartz) and exactly 12 inches from the center of the fusor/cathode.

So, as I dial down just the D2 flow rate (and the pressure drops from 10 microns to under 5 microns and current goes to zero) the voltage remains at 32 KV and, of course, the neutron production then falls off to 'zero', measurable. However, as the pressure falls, my x-ray production soars! That is, the Geiger-Muller counter increases by at least an order of magnitude.

I would guess that the electrons which are normally colliding with deuterium ions (and losing energy) are now more free (?) to collide with the fusor shell at full energy. Yet the pressure change doesn't appear all that large a drop (relative to free mean path increase) so I'm surprised by an order of magnitude increase in x-ray production. This large increase really caught me off guard and certainly could be an issue that I didn't suspect under these conditions (happens rarely.) Previously, I never bothered to measure x-ray production under those conditions since I didn't consider that such a change would impact the x-ray hazard level.

So, while this confirms that my fusor while operating produces too few x-rays at the window to distinguish from back-ground counts it does indicate an x-ray issue does occur under these specific conditions. Never read anyone else posting this type of occurrence, previously.

What really surprised me was that with such a small decrease in overall gas pressure in the chamber (10 microns to just under 5 microns) this resulted in a radically higher x-ray count - over a full magnitude increase over BG counts. This is not a trivial increase. I'd guess for higher voltages (systems 40 kV and up) this effect would be even more pronounced under similar circumstances.

Apparently, a minor change in pressure can lead to a major increase in x-ray production - something that is obvious in hindsight - but something I didn't know or even suspect. The pressure in a fusor matters a great deal more than I ever thought relative for this hazard.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Rich Gorski »

Dennis,

Interesting results on the X-ray transmission through your quartz viewport but hard to understand why you're seeing only background through it at 32kV. In my experience with 5/8" thick Borosilicate glass on my ring accelerator device I see plenty of X-rays getting through that even at 15kV (I forget the rate numbers but the x-rays were easily detected with my Ludlam M3 with pancake GM detector). Borosilicate glass being about 80% SiO2 should be less transparent to X-rays than your quartz viewport which is likely fused silica with a > 80% SiO2 content. The Borosilicate will have more metals in it and thus less transparent to X-rays and deep UV. Are you sure your GM tube is not in the shadow of something possibly inside your chamber? From the photo it looks like the GM tube is not aligned with the chamber axis but maybe that's just a camera angle thing.

Not sure what to think about the X-ray increase with lower pressure. Interesting though.

Rich G.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

I will echo Rich's comments. At 35kv with a Victoreen ion chamber instrument right at the steel shell of the fusor I read 3mr/h at the window 60mr/h. Thus, I use heavy lead at the window. (Actually behind the camera and around the window as there is a lot of scattering. The ion chamber is a must have for sniffing nasty leaks. at my operators position 6 feet from the fusor I get 1mr/h at 43kv. A GM counter approaching the window screams an uncountable amount at 42kv. At 42kv, 12ma, 10 microns, right at the window, the GM counter has zero counts as the tube is locked into a completely saturated condition! No counts, the GM is dead to the world. Under these conditions 3 feet from the unshielded window, the GM is screaming and indicates about 500X background. 1/4-lead tames it at 3 feet to background.

Do you have a portable GM counter? Put it next to the window.

Richard Hull
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Rather concerned by the GM's lack of response during the normal run. The Geiger counter may have issues considering what you two indicate. Thanks for this input!

The Dosimeter (Pen type) I placed close to the view port says 18 Roentgen* (after 10 - 15 minutes.) This Pen was also exposed two times (about a minute) to the fusor having full power but with reduced D2 flow. This was done to recreate the times the GM counter increased by 10x.

The dosimeter reading (average of two units) appears to be more in line with what you both are saying. My calibrate GM unit died some years ago so maybe this GM unit is lying.

By the way, the view port faces 90 degrees from my position - the mirror allows me to see into the fusor w/o exposure. I do have Pb shielding between me and the fusor (besides its own steel wall.)

I have an old, surplus 'ion chamber' detector (Same make as yours, Richard.) Not sure it works anymore but guess I could try it and see if it responds to the window flux and if it does, then see if it offers similar readings to the Pen Dosimeter. That would take care of two issues at once.

I will add the current GM unit responds nicely to a small uranium ore sample. So I assumed it was operating well enough to detect x-rays. That doesn't appear to be the case except when the flux is rather high. Strange it would operate that way. Again, a rather unpleasant surprise.


Update to this post:

Went out and pulled the old surplus unit. It is a Victoreen CDV-715 ion chamber unit.

The CDV-715 acts as if it is ok. Passed the 'self check' and I was able to set the unit to its scale's zero point. Fired up the fusor and exposed both the 715 ion chamber detector to the same position to the fusor's glass plate as the GM tube. The Ion chamber unit was set on the 0.1x scale (unit: Roentgen/hour). The 715 ion chamber scale read, after twenty minutes, 1.1 R/hr or correcting for the scaling factor: 0.11 REM/hr or 110 mRem/hr.

Does this appear inline with what you would expect?

The dosimeter appears rather high as I understand the units but am I miss reading that? The scale is 0-200 R. I remeasured using the same pen and it read 5 R (though this time I didn't cycle the D2 gas so the x-ray emission would be lower.) This unit was calibrate twenty years ago so may be inaccurate - besides its simple age (1960's!) Also, while I can't find a source, I assume this is R/hr as well.

I guess a new GM counter is something that needs to be moved to my priority list.

* As I understand the units - this is also about equal to a REM (for Roentgen equivalent man).
Last edited by Dennis P Brown on Sun May 14, 2023 6:57 pm, edited 2 times in total.
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Rich Gorski
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Rich Gorski »

Oh my! It never occurred to me that a saturated GM counter could read zero... but yeah that makes sense that it would if the count rate is higher than the dead time. No pulses. So if your GM meter reads zero its probably telling you something bad. Either its dead (which is bad) or your in a high radiation field which is even worse.

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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Richard Hull »

110 mrem/h is about right for the level at your incredibly giant glass window.
I have 200mrem dosimeters and at the 3/4" diameter glass on fusor V, over a 30 minute run, unshielded, it reads about 18 mrem.

18 R! My God Man! That will change your blood chemistry a bit! Are you sure about your dosimeter??? I hope it is busted or is actually a 200mrem unit

Finally, dosimeters are "too late for you" devices. If they are calibrated and accurate, whatever they read that's the dose you got. Past tense! Too late for you! You have to live with it or maybe get sick for a while or die with it!

Ion chambers read in mrem/hr. If the ion chamber says you are in a 50mrem/hr gamma or x-ray field, you gotta' hang around in that field for a full hour to get the 50mrem absorbed dose.

Dosimeters have NO time component!! Just total "already received dose" in mrem or rem depending on the scaling.
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Re: Neutron test of a BF3, He3, and Scintillation (fast) tube

Post by Dennis P Brown »

Thank you, Richard and that certainly helps me to clarify what the values that I'm reading and what one to believe. (Also, about time I reread the about the various radiation units again.) That flag by Rich was a good catch.

The 715 is far more likely correct since it is not conceivable that the dosimeter pen could record a number that high. I am only at 32 kV max. From your readings it does appear that the very old 715 might be operating in a reasonable range.

That all said, guess a new GM counter is essential. Might as well get a 'pancake' probe as well since I would like to try activation.

Until I can confirm the 715 number, I'll assume worse case (not the dosimeter, though), and just keep the shielding I use in place and treat the window as a dangerous source (I always have previously.) The existing shielding is more than sufficient even if the level is 110 mRem.

Also, decided to do a simple test. I placed a sheet of Pb shielding between the He-3 detector system (still with its primary moderator) and the fusor to confirm that the He-3 wasn't responding to the x-rays (it shouldn't). The Pb shielding made no difference, which is a big "of course". But nice to do a sanity check.

Looking ahead now that this radiation issue is better understood by me and what I still need to do there.

Next, I need to get more involved with analyzing the neutron generated pulse signals and using the scope with the detector.

Also, I really need to do a makeover of the fast neutron scintillation counter system since it does not appear to be working. I have another PMT much larger) and will install that tube onto the plastic detector material. See if it makes a difference.

Hopefully, the new NIM Amplifier works, and I can get that detector system up and running to aid in the study of the various detector output signals.

The BF3 does not appear to provide a proper signal either but that could be due to voltage and/or DISC issues. Again, doing some real diagnostic appear critical.

Finally, I need to break my chamber down and see what is involved with adding a smaller chamber onto my pumping system. That could be a significant machine shop build and I need to determine what couplings that new 'T' will require.

Always something new and/or an issue to address. The fusor hobby does keep one working due to there always being a next idea/project to address/attempt.
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