Neutron Detection
Of course the vast majority of folks who embark on building a fusor, do so because they ultimately want to do fusion and prove that they have done so. That proof is that they can detect the neutrons which are produced by half of the D-D fusion reactions.
I was no different, and so last spring, soon after I got my deuterium supply system properly connected to the fusor in a vacuum tight way, I tried to make neutrons.
I failed.
Or at least I failed to successfully detect any neutrons that I might possibly have made.
In my experience, getting properly setup to detect neutrons, and getting to a state where you KNOW that your setup will detect neutrons is the hardest part of building a fusor and proving that it does fusion. Why? Because unless you just go fork over $4000 - $6000 to Ludlum to buy a brand new Model 15, or Model 12-4, or Model 2363, you can’t be confident that your neutron detectors will actually work properly. Without doing a lot of work.
I purchased a few used complete neutron detectors off Ebay. Given the overall cost of those used Ebay detectors, I would have been better off just buying a single new one from Ludlum. Especially since none of the used neutron detectors I bought were able to successfully detect the low levels of neutrons I was initially producing with my fusor. In the end it was a 1x24 inch Texlium He3 tube that I bought a few years back from Richard Hull at HEAS in the HDPE moderator I described in an earlier post, that I coupled to a Ludlum 2200 scaler for neutron detection. That 24 inch long He3 tube was by far the most sensitive neutron detection device of all the different tubes and meters that I had.
Last spring (March 2022), I tried using a couple of Ludlum 2363 counters with the “Thors hammer” probe. I never got any counts that were statistically different from background. I tried using a couple different Ludlum Model 15s that I bought used off Ebay as well. Ditto on the failure to detect neutrons at statistically significant levels. (I think there may be issues with the tubes in both of those Model 15s. One of them has a tiny He3 tube, and the other has a larger BF3 tube.) Of course when I bought them, I had no way of really testing them, so I couldn’t know for sure if they worked before the window for returning them closed.
Bottom line is that you don’t know if the neutron detection equipment you are buying off Ebay actually works. The only way you can know for sure, is if you have a neutron source. If you don’t KNOW that your fusor makes neutrons, and you don’t have access to a neutron source, then you really are flying blind. If you don’t get any statistically significant counts, you don’t know if it is because your fusor is actually NOT making neutrons (certainly a very distinct possibility), or if your neutron detection equipment just doesn’t work, or isn’t calibrated properly, or the bias voltage level on the tube is too low, or the threshold on the meter is too high. Now I suppose I could have forked over some dough to Ludlum to calibrate one or more of the meters I bought, but I decided to go a different route. The do everything yourself route. It costs more, takes the longest, and is harder, but that is how you maximize what you learn.
I wanted to be ABSOLUTELY SURE that I could detect neutrons. The only way to do that, was to either purchase, build, or borrow a neutron source. I didn’t know anyone in my area with access to a source, so borrowing was not an option. Purchasing a source, I assumed would be hard. Now, I could be wrong about this, but I suspect that most entities with neutron sources, especially if they are reasonably strong sources, are probably required to have an RSO (Radiation Safety Officer), or a Health Physicist, or both. Certainly you can’t really find neutron sources on Ebay. At least I could not. I must confess I did try. Neutron sources are part of some kinds of moisture meters, and some kinds of density meters, but the kinds with neutron sources do not show up on Ebay. I’m pretty sure they are not supposed to ever show up on Ebay. In my experience, they don’t.
That leaves building a source. So, I thought I would just follow Carl Willis example, go the easy route, and buy an Antistatic Nuclespot 5mCi Polonium source for $289, and use it with a $50 Beryllium disk bought from Ebay to make a simple source. The Nuclespot would only last a year or so, but that would be sufficient time for me to test and calibrate my neutron tubes and meters. So, I tried. I ordered one back towards the end of August 2022. They told me it would be a 12 week wait. Well, it turned out that was wrong, because I finally called them towards the end of January 2023 and cancelled my order and asked for a refund. Nuclespots are in my experience, at least right now, impossible to get. Evidently the manufacturer made some changes to the manufacturing process, and due to that is now dealing with getting NRC approval for the modifications. At least that is the story they told me.
Siemens Pyrotronics F3/5A to the rescue… these are a LOT more expensive now on Ebay than they were just a couple of years ago. I bought a couple from a fellow member of this forum post HEAS, had another one I had purchased off Ebay a couple of years ago, and so was able to make a temporary, very weak ~200uCi Americium 241 Beryllium (AmBe) neutron source. This is only 1/25th as strong as the Nuclespot source would have been, but the advantage is that the half life of Americium is 1143 times longer than the half life of the Polonium in the Nuclespot. The Americium won’t be mostly decayed in a year. (In fact it still retains more than 90% of its original activity levels, even though it is on the order of 60 years old! Yes, it has been a LONG TIME since they put 80uCi of Am241 in a smoke detector… Modern ones have only 0.8uCi or thereabouts.) If I need a neutron source in a year, I can just put the Beryllium foil back in close proximity to the sealed Americium foil, and BINGO!, neutrons. Not very many neutrons mind you, but neutrons. Americium 241 is an alpha source, and Beryllium has the property that if you hit it with an energetic alpha particle, between 30 and 60 times out of a million hits, it will spit out a neutron. If we go with the low end of that estimate, 200uCi is 7,400,000 Becquerel (decays per second), which means we should be getting 7.4*30 = 222 n/sec. In reality the most counts I got with the source next to a neutron tube inside a moderator, was 100-200 cpm depending on the tube I was testing. But the point is, I could get counts! So I could be sure which neutron detection setups were working, and which were not.
None of the Ebay acquired all in one neutron detectors (the model 15’s and the model 2363’s) would register statistically significant count rates with my very weak source. This of course was quite disappointing.
Many of the folks on fusor.net who use stand alone neutron tubes, do so with specialized NIMBIN equipment. They use either homegrown, or specialized commercial preamps, coupled to shaping amplifiers, discriminators, and MCAs etc. I didn’t want to deal with any of that. I wanted to see if I could get some of these stand alone tubes to work directly with stock Ludlum meters – ala Mark Rowley with his Ludlum 3 and Russian CHM11 tube combination. (Although I guess in his case, he did put a bias/ballast resistor in series with the CHM11 tube.)
Well, it turns out that you CAN get standard He3, BF3 and B10 tubes to work directly with Ludlum 2200 and Ludlum 1000 scalers. No bias resistors, no preamps, no shaping amps, no discriminators required. BUT, not using a preamp, means you have to run at least the He3 tubes with a higher bias voltage, to get the tube gain up to a point where the stock meters will register the counts with their threshold adjusted very low.
In order to be sure that I knew exactly what the bias voltages and thresholds of my meters were, I bought a used Ludlum Model 500 pulser from a vendor in the calibration business, and used that to calibrate both the HV meter as well as the threshold potentiometer on the Ludlum 2200. (To double check the HV meters, I also bought a couple Sensitive Research electrostatic voltmeters off of Ebay. Which BTW are very cool pieces of hardware that require no batteries or external power source to function and to which you can directly connect the high impedance HV sources on radiation meters because the electrostatic voltmeter input impedance is on the order of 1e15 ohms.) The Ludlum documentation on the 2200 says that it will work with proportional tubes, but the threshold needs to be set to about 2mV for them. So, I increased the amplifier gain (screwdriver adjusting screw with a lock nut that is labeled DISC on the front of the 2200) on the 2200 by turning the set screw clockwise, so that 1.0 on the threshold pot was a 3.3mV threshold as measured on the Model 500, and 10.0 on the pot was a 33mV threshold. This allowed me to set thresholds anywhere from 2mV up to 30mV or so with better resolution on the pot than the initial lower gain amp setting allowed. After locking in the DISC amplifier setting with the lock nut, I adjusted the threshold pot down to 0.6 which corresponds to a ~2mV threshold.
The Texlium He3 tube has an HN RF connector on the end (many of the Reuter Stokes neutron tubes also have an HN connector on them), so I bought an HN to C adapter (UG-702/U) and screwed it onto the HN Texlium connector. Then used a stock Ludlum C to C cable to connect the Texlium tube directly to the Ludlum 2200. No bias resistor or preamp involved at all. (I intentionally did NOT use BNC connectors anywhere in the setup because I wanted to be able to push the bias voltage up to the 2500V limit of the 2200 if needed, without having to worry about significantly exceeding the HV capacity of the BNC connectors.) Put the tube in the moderator with the weak AmBe source and slowly started raising the bias voltage. Nothing at 1000V, nothing at 1500V, nothing at 1600V, nothing at 1650V. Finally when I hit a little above 1700V I got a single count. At 1800V I got a couple more counts. When I got up to about 1850V the count rate started to go up. Turns out, that if you run my Texlium He3 tube at 1900V with a 2mV threshold, it will work perfectly with a Ludlum 2200 no additional hardware required. Which makes it really easy to use.
Now granted, with a preamp, I could probably run the tube at a much “cooler” voltage of 1500V or so. So for the purpose of extending tube life as long as possible, using a preamp and a lower bias voltage is better. But this tube is not going to be running anything close to 24x7. More like a couple hours a week on average, absolute max. So for now, I am perfectly content to run it the simple way, directly connected to my Ludlum 2200.
I had a number of other tubes that I tested to determine what kind of bias and threshold settings were needed to run them directly connected to a Ludlum 2200. This is what I found along with typical counts per minute inside an HDPE moderator with my weak AmBe source, as well as typical background cpm:
Tube/Meter:.....................................Bias and Threshold:.........AmBe............Bkgnd:
Texlium stainless He3 tube 1x24.............1900V bias 2mV threshold..95cpm...........8cpm
Reuter Stokes Aluminum He3 tube 1x22....1900V bias 2mV threshold..101cpm.........12cpm
Russian SI19 1.125x8 He3 Corona............2050V bias 35mV threshold.70cpm..........18cpm
GE B10 1.5x17..................................715V bias 10mV threshold...40cpm..........1cpm
Nancy Woods BF3 1x6.........................1975V bias 2mV threshold...12cpm ..........5cpm
Ludlum Model 15 BF3.........................1750V bias ?? threshold.......2cpm............0cpm
Ludlum Model 2363 hammer.................?? bias ?? threshold............6cpm............6cpm
For the Ludlum model 2363 I just ran it as a unit as it was configured when I purchased it. I left the probe attached to the meter, and put the probe a few inches away from the unmoderated AmBe source.
Here is a picture of the tubes listed above: from bottom to top Texlium, RS, S19, GE B10, Nancy Woods, and the Model 15 BF3
Bottom Line:
1) Large He3 tubes are the most sensitive. But they are hard to come by and cost $$$.
2) You can directly connect many tubes to Ludlum 2200 Scalers and they will work perfectly – with an appropriately low threshold and a hotter bias than would be required with a preamp.
3) Ludlum 2200 scalers are great instruments that can be used to drive many different radiation measuring devices.
4) To be SURE you can measure neutrons with your instrumentation, you need a neutron source.
5) Ludlum 500 Pulsers are nice for making sure your instruments are properly calibrated and for accurately determining bias voltage and threshold voltage levels.
6) If you don’t buy a ready to go, calibrated, guaranteed functional, reasonably sensitive neutron measuring system, getting one setup and being sure it really works, is non trivial.
7) Ebay purchased neutron detection instruments like the Model 15, Model 12-4 and Model 2363 are most certainly not guaranteed to work in all cases. Sometimes (maybe more often than not) you will get a dud. Make sure you can figure that out before the return window closes. (ie: that neutron source I keep talking about, is kind of important…)
Joe.
