Detector noise problem

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Dan Knapp
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Re: Detector noise problem

Post by Dan Knapp » Wed May 30, 2018 12:58 am

I had indicated I would post the solution when this problem was solved. It is now at least partially solved, so here it is.

Richard suggested operating the He-3 tube at higher voltage to increase the size of the neutron peaks versus the noise peaks. It turns out there is a published paper on this issue from the cold fusion era (perhaps the only useful product of cold fusion work?): T. Aoyama, et al., Radioisotopes 40:188-192, 1991 (copy attached). Cold fusion people were apparently claiming detection of spark noise peaks as detected neutrons. In this paper they generated sparks with an induction coil and showed the peaks heights did not change with He-3 tube bias whereas the neutron peaks increased, and that neutron peaks could be raised above the spark noise with higher bias voltage, as Richard suggested.

Rich Feldman suggested a low pass filter. We built a filter (circuit attached) designed to pass the neutron peaks and block the ringing noise peaks shown in the original post. It blocks the big noise peaks, but doesn't completely eliminate noise.

Peter Schmelcher suggested looking at the quality of the connectors being used. I replaced the signal input BNC on the preamp box with a silver plated panel mount HN connector that attaches directly to the He-3 tube eliminating two BNC connections. I also replaced the BNC bias voltage connector with an SHV connector. I replaced the Cremat supplied banana jacks for the preamp power connections with a DB-9 connector and made a shielded power cable to connect to the power jack on the back of the NIM Amp module.

Not directly related to the noise problem, but as another change from the original post, I made a new 200K / 40W ballast resistor by connecting eight 100K / 5W ceramic film resistors in a series parallel configuration mounted on a sheet of Lexan inside a piece of 1-1/2 inch PVC pipe.

Using all of these changes, I am now able to count neutron peaks that go away when the tube is removed from the moderator. I found that one problem I was having in trying to observe neutron peaks disappearing upon removal of the tube from the moderator cylinder is that I was seeing thermalized neutrons being reflected from my stack of plastic containers filled with boric acid solution (some may recall my earlier post on this shielding arrangement). Since it is obvious that I'm not producing enough neutrons to need shielding, I rolled the shield stack to the other side of the room, and this problem went away.

Operating the fusor at 25 kV, 8 ma, and 3 mTorr of deuterium, with the He-3 tube at 1500 V and 30 cm. from the center of the fusor, I see a count of 10.5 neutrons per second that goes to less than one per second upon removal of the tube from the moderator. There are still some noise peaks even with the low pass filter, which I block by raising the lower limit control on the SCA to eliminate all counts with the tube out of the moderator. It is apparent from the scope traces that this setting is also blocking smaller neutron peaks, so I still don't have an optimal solution. However, by counting peaks that go away upon removal of the tube from the moderator, I can be confident that the peaks I am counting are neutrons. Based upon a previous calibration of the He-3 tube located 30 cm from a Californium-252 neutron source, this count corresponds to an isotropic neutron production rate of 8500 per second. This calibration was done with a lower bias (mfr. recommendd 1100 volts) and a much lower limit on the SCA (just enough to block gammas). We need to do another calibration in situ with the same settings. It is expected that the calibration source will give fewer counts with the higher SCA lower limit, and thus we were actually producing more than 8500 n/s.

This neutron production rate is nothing to write home about, but at least we now have a positive control for neutron detection. With a power supply that goes to 60 kV / 60 ma, we should be able to generate a much larger neutron flux; but the system is very difficult to stabilize. The x-ray power supply is being controlled by the vendor's LabView interface, which is written to allow voltage adjustment only in 1 kV increments (OK for an X-ray supply but not for a fusor!). We only have the standalone LabView software interface without the source code, so I can't modify it for finer control. There is clearly a need for fine analog control of fusor voltage. With this crude control of voltage, the system has a tendency to go into runaway mode with grid overheating and current spiking to overload shutdown of the supply. At present we’ve not been able to get above 25 kV without the system going into runaway and overheating the cathode.

This experience of trying to quickly cobble together a positive control for neutron detection has given me much appreciation of the fine art of tuning a fusor, and the need for fine control of voltage. I have a couple of Spellman -50 kV / 4 ma supplies that I can parallel to get 8 ma with fine analog control of voltage, but this current is at the edge of adequate. With some modifications, I should be able to near double this current output, so it looks like this will be the next step.

Hopefully, this description will be of use to others using He-3 detectors. Clearly, accurate counting of neutrons with He-3 tubes is not a trivial undertaking (which may have been the root of cold fusion fiasco). Thanks to all who offered suggestions.
Attachments
NeutronDetectionPaper.pdf
Paper on neutron counting in the presence of spark noise.
(604.72 KiB) Downloaded 129 times
lowpassfilter.pdf
Schematic of low pass filter.
(56.34 KiB) Downloaded 136 times
Last edited by Dan Knapp on Wed May 30, 2018 11:25 am, edited 2 times in total.

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Richard Hull
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Re: Detector noise problem

Post by Richard Hull » Wed May 30, 2018 2:12 am

Dan,

I am glad you found a workable solution and that the many comments helped. I printed out the nice paper on the 3He detector setup and noise elimination. I have harped for some time the need to set the discriminator level high enough to take care of local noise. The fusor kicks out a very strong and consistent neutron that is obvious on a scope to be rivaled only by cosmics which can't be removed. In general operating a 3He tube can go a good bit beyond its ideal voltage in spec sheets which is often a broad range, anyway, provided you have the original spec sheet.

You also begin to see, as so many other folks who stuck with fusion in a fusor, that amazing results require amazing control skills. Manual skills in control that lead to better and better results with a "feel" of operation of each fusor are often a learned function by a stable hand at the "wheel" with better than average feedback to the human by multiple measuring devices. These devices guide control.

A computer controlled system for a fusor might best be implemented by the person who has first mastered his fusor's "fine control" hysteresis characteristics, manually.

Great work and thanks again for the paper...

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: Detector noise problem

Post by Dennis P Brown » Wed May 30, 2018 10:23 am

In your low band pass filter circuit, what are the values of those resistors: Ohms, kilo-ohms or mega-ohms?

Dan Knapp
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Joined: Wed Aug 06, 2008 12:34 pm
Real name: Dan Knapp

Re: Detector noise problem

Post by Dan Knapp » Wed May 30, 2018 11:11 am

Resistors are 100 ohms. Purpose is to make input and output impedances 50 ohms to match preamp output impedance and cables (amp module has high input impedance).

In rereading my post, I realized I failed to thank all who had made suggestions. Thanks for your input on this.

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