Detector noise problem

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Dan Knapp
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Detector noise problem

Post by Dan Knapp » Fri Apr 20, 2018 1:31 am

We are having a problem with discriminating neutron peaks using a He-3 detector tube and seek ideas on how we might solve the problem. The detector is a one inch 4 atm. Saint-Gobain “ruggedized” He-3 detector tube operated at the recommended 1150 volts. The tube is inserted into a central bore in a 6.5” diameter cylinder of HDPE as the moderator. We are using a Cremat CR-110 charge sensitive preamplifier feeding an Ortec 490 Amplifier/Single Channel Analyzer and then an Ortec 871 Counter Module. We are monitoring the preamp output and the SCA output with a dual channel oscilloscope. This counting arrangement works fine with a Californium neutron source where we can discriminate the neutron peaks from the gamma peaks with the SCA, but in the presence of a fusor plasma, there are electrical noise peaks which are higher than the neutron peaks, and thus cannot be discriminated by the SCA. The photos below show scope traces for a noise peak and a neutron peak. Evidence that the second type of peak is a neutron peak is that these peaks disappear when the He-3 tube is removed from the moderator.
First a little background on the experiment. Dan Barnes and I have been working for some time now on extending the Los Alamos work on the Penning trap as a small fusion reactor (https://nucleus.iaea.org/sites/fusionpo ... /Knapp.pdf). We have recently begun attempts to actually do D-D fusion in the Penning trap. We decided as a positive control to implement a spherical fusor in our vacuum system. The fusor has an 8 inch diameter grounded spherical anode (8 longitudes; 5 latitudes) made from 1/16 inch 316SS welding rod. The cathode is a 2 inch sphere of 0.022 inch 304SS wire (6 longitudes; 1 latitude). The cathode is connected to a Universal Voltronics -60kV/60mA X-ray power supply via a 120K ballast resistor. Pressure control is accomplished by closing the gate valve to the turbo pumped chamber and pumping through a one inch flexible metal hose and bellows valve. Deuterium is admitted from a one liter buffer volume via a Granville-Phillips 203 Variable Leak. A stable plasma without excessive cathode heating is obtained by adjusting the deuterium pressure to about five millitorr at a cathode potential of -25 kV, with about 6 mA cathode current.
Under these conditions, a large number of peaks are observed consisting of spikes about 200 nsec wide and lower amplitude broad peaks about 200 usec wide. Detail of the two types of peaks are shown in the attached photos. The broad peaks disappear when the detector tube is removed from the moderator, and on this basis are assumed to be neutron peaks. The noise spikes (as well as the neutron peaks) are only present when the plasma is lit. The detector tube is attached to the preamplifier via a 3 inch coax line with BNC connectors. The spikes disappear when the coax and tube are detached from the preamp, but reappear when the coax without the tube attached is connected to the preamp input. Thus it appears that the noise is entering the system via the preamp input. Both types of peaks disappear when the plasma extinguishes.
We have searched for possible ground loops by trying many different ground connection schemes (normally the entire system is grounded in a star pattern to a large copper buss connected to earth ground). We have also tried a large number of different arrangements of additional shielding. The only thing that made any difference was that adding a grounded foil shield around the preamp power cable attenuated the noise spikes a bit. We tried a new Cremat preamp module, but it made no difference. Putting an isolated grounded foil shield around the outside of the moderator and extending over the coax to the preamp made no difference as did putting an insulated shield on the tube inside the moderator. Moving the detector/moderator/preamp off the main system chassis reduced the number of neutron peaks (moved the detector farther away from the source) but made no difference in the noise.
We are seeking advice on how else we might attack this noise peak problem. Hopefully, the shape of the noise peaks with the ringing tail might be a clue to the source. Any suggestions would be appreciated.
Attachments
noise peak.jpg
noise peak
neutron peak.jpg
neutron peak

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Richard Hull
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Re: Detector noise problem

Post by Richard Hull » Fri Apr 20, 2018 4:43 am

If you look at the fusor IV's neutron detector you will see there is no cable between the preamp and the Rueter & Stokes 4 atm 3He tube. This is the ideal. I do run about 6 feet of coax from the preamp to the pulse shaper and SCA in my NIM bin.

The first scope trace is a damped wave and might be indicative of an arcing within the system. A common wave form from an arc discharge in a line transmitted or picked up elsewhere.

I assume the second scope image is not a simultaneous capture on the DSO but a noise pulse on one channel overlayed with the neutron pulse as a time ordered example for us to see.

Wherever there is coax, I clamp on a ferrite block externally. I also attach ferrite blocks on the HV line right as it enters the fusor. I have about 8 of these blocks on my coax and 1 on the HV line. These blocks help prevent coax ground bounce bumps and squelch resonant "antenna" line noise on the shield. One can never establish a "good enough ground" around a fusor. Ferrite blocks are usually contained in a plastic clampable housing designed to be snapped together around a shielded cable near the cable's connector ends. Long cables in a noisy environment can have several along its length.

It is great that you have a scope for analysis. I use my DSO for such noise hunts regularly. Noise introduced after a preamp can usually be easily dealt with as it can be attenuated and the clean premap neutron pulses can be amplified over the noise and the noise discriminated out. However if noise gets into the preamp and is greater than the neutron pulses, your are sunk! I run my 23 inch long, 1 inch diameter, 3He tube at about 1600 volts. You might try boosting your voltage a bit to increase the neutron pulse amplitude over that of the noise pulses. Hopefully, doing this will increase the pulse amplitude without increasing the noise pulses. First try a few ferrite blocks on that short BNC cable between the tube and preamp.

This sounds line a ground issue, arcing or shield bounce noise. Use a little portable AM radio to search for noise sources. The fusor, itself, is typically noisy.

Richard Hull
Attachments
IV const 04 (32).JPG
Princeton Gamma Tech preamp directly connected to 3He tube
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Dan Knapp
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Re: Detector noise problem

Post by Dan Knapp » Fri Apr 20, 2018 11:02 am

Sorry for not explaining the two traces. The blue trace is the preamp output, and the yellow trace is the count output from the SCA. On the noise trace, there is no count pulse because the threshold was set to reject the noise peak (which also blocked the neutron peaks). The neutron peak trace was captured with the SCA threshold set lower and hence had an accompanying SCA output peak on the yellow trace.

Peter Schmelcher
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Re: Detector noise problem

Post by Peter Schmelcher » Fri Apr 20, 2018 6:41 pm

Looking for EMI is a witch hunt and your coupling could be M or E field or simply conducted.

If it were me I would float the preamp with a 9V battery and only connect the scope to begin the hunt.
If I could float the scope on batteries I would and also position it on a plastic shelf.
Then start introducing various shielding and grounding ideas and observe the results.

Don't overlook that all BNC RF connectors are not created equal. Nickle plating has bit me in the past. I always use silver or gold for both inner and outer contacts when I do low signal work.

Best of Luck
-Peter

Bruce Meagher
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Re: Detector noise problem

Post by Bruce Meagher » Fri Apr 20, 2018 8:13 pm

A few years ago I had noise issues with a large 2” x 36” He3 tube connected to Cermat 110 preamp (mounted on a Cermat 150 board and in their box). I had a 6’ coax cable between the preamp and the detector that was a significant factor. I switched to an HP 5554A preamp and a shorter cable and got much better results. I didn’t really fully investigate the issue because I was more interested in the neutrons.

A couple things to potentially consider:

Try directly connecting the preamp to the detector with just a high quality connector. Switch out the input connector on the Cermat 150 box to a better quality one (assuming you are using their preamp board and box). Trying a different preamp brand. Run the NIM bin, HV power supply, preamp, and scope on a UPS power supply disconnected from the mains. Also have you tried capping (not terminated) the end of the BNC cable to see if the noise is still present?

Good luck!

Bruce

Dan Knapp
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Re: Detector noise problem

Post by Dan Knapp » Sat Apr 21, 2018 2:29 am

We’ve tried running the preamp with battery power, attaching the preamp to the tube with a double male bnc (quality of connector unknown, however), and capping the preamp input with a terminator (which eliminates the noise as does leaving the preamp input BNC open.
We also tried ferrites on the preamp to tube short coax and on the preamp output cable per Richard’s suggestions. We already had ferrites on the HV line.
We have another preamp on the way, and also plan to build one per an extensive discussion here several years ago. I’m wondering if we should try some different R values on the ballast to try to reduce the noise at its source.
A possible solution is to select the “fatter” neutron peaks with a pulse shape analyzer/single channel analyzer/time to amplitude converter/single channel analyzer, but we haven’t found any TAC NIM modules on eBay.
If and when we cure the problem, I’ll post the solution. Thanks again for the input, and we welcome any other ideas.

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Rich Feldman
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Re: Detector noise problem

Post by Rich Feldman » Sat Apr 21, 2018 5:34 am

Looks like there's a huge difference between the frequency content of the bad pulses and the good ones.
The bad ones ring at about 10 MHz, and the big spike is of comparable frequency.
The good ones would not change much if viewed with a probe and 'scope bandwidth of 100 kHz (3.5 us risetime).
Does your 'scope vertical channel menu have any bandwidth limit (digitally filtered) options?

Can you put a real lowpass filter in the path?
Not just some ferrite. Those are usually specified for loss at 100 MHz. Often chosen by designers in a trial and error process, until functional problems stop, and EMI regulatory compliance test sweeps pass with reasonable margins.

Details would depend on where the signal path is accessible, and the associated voltages and impedances. My personal experience with radiation detection instrumentation is rudimentary.

Here's one homebrew passive LPF: http://www.k5td.com/500kHz_Low_Pass_Filter.html
Here are some you can buy: https://kiwa-electronics.com/100-khz-lo ... ilter.html https://www.thorlabs.com/newgrouppage9. ... up_id=8613
I bet they're all much fancier than you need, in terms of flat passband and sharp cutoff. DC voltage ratings might be an issue.
Richard Feldman

Peter Schmelcher
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Re: Detector noise problem

Post by Peter Schmelcher » Sat Apr 21, 2018 9:11 am

Try disconnecting the HV tube bias supply and cap or short the connection.

Dan Knapp
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Re: Detector noise problem

Post by Dan Knapp » Sat Apr 21, 2018 10:53 am

The noise is still there with the detector HV bias turned off.
The low pass filter suggestion sounds like a possible solution. The neutron peaks have a fast rise time, so I’m not sure how they will be affected; but it’s an easy thing to try.
Thanks again for the comments.

John Futter
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Re: Detector noise problem

Post by John Futter » Sat Apr 21, 2018 9:11 pm

dont forget that input capacitance is directly proportional to noise
each input pf adds noise and limits bandwidth in a charge amplifier.
this seen in some ORTEC preamps where they try to match input capacity to the feedback capacitor ie they change the feedback cap with a switch.

the best match is when input cap = feedback cap

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