Neutrons? - Please Critique

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Garrett Young
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Real name: Garrett Young

Neutrons? - Please Critique

Post by Garrett Young »

My counts seem way too high and I need some critical feedback.

Detector Tube: http://www.lndinc.com/products/417/

Distance from Chamber: 14 cm

Moderator: 7.6 cm of HDPE

Pressure: 13 mTorr

https://youtu.be/7FOfLvmOcyA

Edit: Video of entire setup

https://youtu.be/er0bQVbHN_0

Edit 2: My chamber doesn't have a viewport so I'm unable to provide images of the plasma
- Garrett
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Richard Hull
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Re: Neutrons? - Please Critique

Post by Richard Hull »

No real data again.

What is the voltage you are applying to the tube? I would think no more than 1200-1400 volts.

What kind of preamp are you using?

How did you window the preamp signal to the readouts.

Take the naked tube and put it up near the fusor. Pulling it out of the moderator is pulling it away from the fusor.

I am suspecting RF noise is your issue. If you were getting the counts you claim, you would be dead now.

in a moderator you need to get a background count in the range of 5-15cpm.

I see you will have a bubble detector on the way. Believe it first and electronics last. I will figure you might see a bubble a minute, if you are doing fusion at all. Nearly 200,000 counts per second, if real, would turn the dosimeter into a pile of foaming bubbles in seconds.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Garrett Young
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Re: Neutrons? - Please Critique

Post by Garrett Young »

I agree that there is something seriously wrong with the counts an suspect noise as well. I've dramatically decreased the distance between the pre-amp (TC 175) and detector tube to 10 cm. The moderator/tube assembly is now electrically shielded from the power supply and other sources of interference. The tube is being biased (as shown in the video) at 1150V in accordance with the manufacturers data sheet. The amplified signal is being asymmetrically window discriminated with an Ortec 550A with a low value established with a radium gamma source. To be honest, I don't understand your comment of "no real data". I presented what I thought was relevant and asked for feedback.
- Garrett
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Richard Hull
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Re: Neutrons? - Please Critique

Post by Richard Hull »

I needed to know the tube voltage. Some folks want to push the voltage up too high. This can cause a detection of x-rays.
You did not give type of pre-amp used or that you had used a radium source to set the window. We cannot assume any of this.

Sound like you did things correctly could you take an ohmic reading between your fusor's SS shell and the preamp's shielded cable? Should be virtually zero ohms give or take.
I just suspect noise of some type.

Try your bubble detector immediately, once it arrives. Remember to shield it from the fusor chamber heat. Place it down under the chamber, about 3 inches or more away from the metal to avoid rising heat thermals. The detector is meant to be no more than room temperature or false reading will abound.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Garrett Young
Posts: 123
Joined: Fri May 26, 2006 9:51 pm
Real name: Garrett Young

Re: Neutrons? - Please Critique

Post by Garrett Young »

The resistance between pre-amp cable shield and vacuum chamber reads essentially zero (0.2), but the high voltage line from the power supply is un-shielded and is probably radiating massive amounts of noise. As mentioned in my last response, I've moved the detector/moderator so that there is a grounded surface (the stainless steel table top) that separates both the chamber and high voltage.

Here is my NIM configuration:

3106D
+1150 Volts

TC175

TC243
coarse gain 50
fine gain 1
peaking time 8 microseconds
shaping time 4 microseconds
BLR auto, asymmetric
input negative
output unipolar

550A
upper level 10 Volts
lower level 3 Volts
integral mode

In addition, I have an oscilloscope monitoring the amplifier output into the SCA which has helped immensely in the debug process. I have the settings dialed in and the background rate is 4.3 (30 counts over 7 minutes). I'm going to change to a timing SCA because occasionally there's a small leading peak which is counted and would also improve interference robustness.
IMG_1457.JPG
More information about my system:
Dual Stage Rotary Pump (only)
Manual shutoff valve between the pump and chamber
4 mTorr measured base operating pressure
0.3 mTorr/min leak rate.
10L D2 Lecture Bottle Tank
Line Regulator 30"Hg - 15 psi
0.0005 cv needle valve
Spellman DXM30N600X4215 30kV 20mA
- Garrett
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Richard Hull
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Re: Neutrons? - Please Critique

Post by Richard Hull »

Thanks for the extra data. The scope is the way to go. 4 cpm background, in moderator, for that small tube is just about perfect! I get between 6 and 8 cpm with my much larger Reuter and Stokes 3He tube. This tells me you have a noise issue alone; which the grounded table top might take care of. If you are doing fusion, a well adjusted 3He tube like yours, in moderator, should get between 100 and 3000 cpm. It all depends on a number of operational and placement factors.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Garrett Young
Posts: 123
Joined: Fri May 26, 2006 9:51 pm
Real name: Garrett Young

Re: Neutrons? - Please Critique

Post by Garrett Young »

One other note, I shortened the cable between the pre-amp and detector from 120cm to 10cm which obviously helped. I had to change the input connector on the pre-amp to a MHV to work with the parts I have on hand.
- Garrett
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