Joe Gayo's lab tour

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Joe Gayo
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Re: Joe Gayo's lab tour

Post by Joe Gayo » Mon Jul 01, 2019 7:23 pm

@Lukas - If you are talking about the small oscillations that follow a trendline, then this is a result of the statistical nature of neutron measurement. The counts are averaged over 500 msec but I only report rates that have been averaged over 5 seconds or more and not a single measurement spike.

@everyone - Above 90kV I'm having a saturation (maybe?) issue with my detector where the counts increase asymptotically and the numbers become very high. Is this because of a lack of deadtime? I would assume it would stop counting if the BF3 tube just constantly discharged because the signal would be constantly high. Or is it saturated but there is enough of an AC component to the current that it's registered as erroneously high counts? I need to set up an additional detector at a greater distance...

I have tried reducing shaping time (which seemed to help) on the amplifier to allow higher count rates. With a shaping time of 4usec, I would think it would support many kHz of rates. The detector seems fine to 3 kHz count rate.
Last edited by Joe Gayo on Tue Jul 02, 2019 12:07 am, edited 1 time in total.

Lukas Springer
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Re: Joe Gayo's lab tour

Post by Lukas Springer » Mon Jul 01, 2019 11:21 pm

Damn statistics, ruin everything! :D

That sounds like signal pileup in your detector, have you tried just using a higher distance to your reactor?
faster shaping can do only so much and most gas proportional detectors are not made for high counting rate.

Joe Gayo
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Re: Joe Gayo's lab tour

Post by Joe Gayo » Tue Jul 02, 2019 12:22 am

Yes, I’m going to set up an additional detector at a greater distance (The present detector location is calibrated and gives nice resolution for low current/voltage operating conditions), but I was curious about typical resolving time for a BF3 detector system. I could setup a signal generator to test. I would think dead time issues would reduce the count rate, not increase (I found a paper citing dead-time under 10usec).

https://www.tandfonline.com/doi/pdf/10. ... 96.9732021

Another question: I understand how pile-up can distort spectroscopy measurements but in pure counting situations how does the count rate multiply?

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Re: Joe Gayo's lab tour

Post by Robert Dwyer » Tue Jul 02, 2019 12:55 am

I think there should be decent dead time in your system that can cause pulse-pileup, but I am unsure why that would make you have multiplying count rates. I think there are three places that will give you dead time, and measurements of these with an o-scope could help diagnose this issue:

1. Tube dead time: This stems from the recovery period due to the voltage drop across the tube when a current flows from a pulse. The current needs to drop far enough so there is sufficient voltage to cause a large enough pulse to be formed for the amps and shaping electronics to detect the event.

2. Electronics Dead time: This will come from the any NIM or other electronics used for pulse shaping and analysis suhc as amps, scas, etc.. there is a time were the electronics are busy with a single event and cannot resolve any other inputs that come into the electronics. On many NIM modules there is an output on the back labled 'DT' which sends a logic signal while the module is busy with an even allowing you to better track deadtime on the electronics front.

3. Scaler Dead time: I am putting this in a different category than the 'electronics' category, as it has less to do with pulse shaping, but more the counting end of thing. This could include hardware such as physical counters or scalers, but also could extend to digitizers, mca's or even software, that adds deadtime to the system were, at high count rates, pulses may be lost.

As your tube is a gas tube, I think Lukas is right in suggesting most of your issues are stemming from the tube itself (if it is a pileup related event). The deadtime, if not found on manufacturer's sheet, can be measured using an oscilliscope. Setting the oscilliscope to trigger on a pulse from the detector each event, but not clear the screen. After about 50-100 pulses, you should see the time between the pulses (dead time). There are many mathematical models you can find that have been done on correcting for dead time, but the easiest way is to move the detector farther back. These models, and more in depth explanations of dead time can be found online. I suggest Glenn Knoll's book which is great investment for both the hobbiest and professional interested in radiation counting, as it explains things far more in-depth and better than I ever could.

Below is an image showing both the dead-time and recovery time in a geiger tube, which will be similar to what is probably happening in your tube.
Dead_time_of_geiger_muller_tube.png
The dead time should keep you from seeing too high count rates. If your tube is not properly quenched, then perhaps there may be an issue there. If there is nothing erroneous in your dead time measurments or pulse traces, then I suspect an issue with counting electronics such as your scaler. At 90kv sufficient noise may be entering your system as well. Both the dead time measurements and a farther detector will give you a good start as to figuring out this issue.
If we throw more money at it, it will have to work... right?

Joe Gayo
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Re: Joe Gayo's lab tour

Post by Joe Gayo » Tue Jul 02, 2019 1:07 am

I have a bubble dosimeter reading of 4.8E+7 which seems ridiculous...

@Robert
I have Knoll’s book and agree it’s a must. There are several things I’m going to investigate with the counting electronics (plus add the greater distance detector)
Last edited by Joe Gayo on Tue Jul 02, 2019 1:18 am, edited 1 time in total.

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Re: Joe Gayo's lab tour

Post by Robert Dwyer » Tue Jul 02, 2019 1:18 am

Does this reading corellate with the high count rate from the BF3 tube? If so then you may be seeing real counts. I think setting up both a bubble dosimeter along with another tube at a greater distance at the same parameters of voltage, current, and pressure and see if you get the same yield. If the dosimeter is reading these results I think the likelihood of real events is true. At those neutron numbers pileup events would still be visible, but a trend should still be seen with the BTI with regards to a count rate.

If you do see evidence of these numbers being real, my next suggestion is activation of some kind. Silver, Rhodium, Silvers, etc... to verify these yields.
If we throw more money at it, it will have to work... right?

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Re: Joe Gayo's lab tour

Post by Joe Gayo » Tue Jul 02, 2019 1:23 am

Yes, but I want a lot more data. I’ve had some runs where the rates were much higher but unfortunately I didn’t have a bubble dosimeter present.

I’ve activated silver, indium, and manganese quite easily to many times background. The device can run at 5E+6 for extended periods (60 minutes).

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Re: Joe Gayo's lab tour

Post by Richard Hull » Tue Jul 02, 2019 10:26 am

I would take three routes here with large numbers you are apparently getting.

BTI at some increased range. (BTI dosimeter needs to be fresh and trusted, obviously.) If in excellent shape, different ranges should jibe based on inverse square law plus or minus a bit, but not to an extreme.

Silver activation - Always a true indicator of Neutrons. Should hold to some semblance of the inverse square law as well.

Definitely move the BF3 tube out farther. The suggestion about an o'scope picture of the preamp output is a must.... To eliminate the issue of noise. due to proximity. If I were there, I would be all over the preamp output with a good DSO like ugly on a gorilla. Electronic counting is tough and noise loves a home in long cables that are terminated poorly or not at all. Even with good technique, the energy and RF at 90kv in a fusor might be an issue. This is especially true if the moderator and tube are in close proximity to the fusor. If all is fine with the electronics its count at various ranges should obey the inverse square law just like the dosimeter. Neutrons, while they can scatter somewhat in air, have decent straight line ranges much like gamma rays over laboratory distances.

Sometimes I find I just can't do it all to a degree that make some of my efforts payoff like I want. I have knowledable friend in the HEAS here that I can call upon in critical endeavors, mostly in data gathering.

Much of the above that I have suggested should rely on a good lab assistant as you will have your hands full with controlling the fusor in the experiment. This is especially true for the multiple capture and saving of screen images on the DSO during the experiment. The lab assistant is almost a must have and should have some familiarity with the process and equipment. A third person could be a "step-and-fetch-it". Such a person need not have any experience, just a willingness to help the two researchers in whatever small intermediate tasks that are menial yet immediately helpful. (log book entries called out to him.)

I have even used a large cadre of very experienced fusor people at the HEAS gathering in October for a lot of tasks I fret over when alone.

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.

Joe Gayo
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Re: Joe Gayo's lab tour

Post by Joe Gayo » Mon Jul 22, 2019 4:16 pm

I'll post more details in the near future, but I was having issues of gamma/x-ray pile-up above 90kV. These normally low amplitudes pulses pilled up to such an extreme level that they crossed the discriminator threshold. This was resolved in the typical fashion with increased distance, shielding, and discriminator level.

I have re-validated the results below 90kV and my previous numbers stand. I have some new record results that I will be posting ...
Last edited by Joe Gayo on Mon Jul 22, 2019 9:32 pm, edited 1 time in total.

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Richard Hull
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Re: Joe Gayo's lab tour

Post by Richard Hull » Mon Jul 22, 2019 5:29 pm

Indeed, X-rays get vastly more intense as the voltage advances. There is a trade off which Joe is helping to quantify here. His chambers are of low Z aluminum which typically reduce X-ray production, but at high voltages, what x-rays are produced make the chamber absolutely transparent to the higher energy x-rays that are produced. Interesting....

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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