Latest project

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Jon Rosenstiel
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Latest project

Post by Jon Rosenstiel »

See the Images du Jour section for a photo of my recently completed neutron detector. (Maybe for now I should call it my cosmic ray muon detector)!

Jon Rosenstiel
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Richard Hull
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Re: Latest project

Post by Richard Hull »

Nice looking detector Jon!

I was a bit stunned at the count being as high as it was. My BF3 units mihgt give 0.5cpm. The PMT unit with BC-720 also is in that range as well. Perhaps you will have to dial the discrimination up a bit. It would take a known neutron source to tell what that level would be though.

If you could get a really hot Radium source such as an old wwII aircraft dial, remove the glass and place it hard against the face of the PMT housing you should discrimenate until it can't be detected over background.

Nice work....

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
Jon Rosenstiel
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Re: Latest project

Post by Jon Rosenstiel »

To get 20cpm, (cosmic ray muons), the discrimination was set to 30mV. I realize that this is probably kinda low, but I'm mainly trying to get my feet wet here. When it comes time to actually count neutrons I hope to have the real thing, (a BC-720 scintillator), and much more knowledge.
The largest cosmic ray pulse I've seen in the past few days was 212mV. I did a 8 minute run earlier tonight with the trigger level set at 70mV. I recorded 50 counts which gives 6.25cpm. The average pulse height for those 50 counts was 108mV. (PMT Anode potential at 900Vdc) Does what I've done here make any sense?

What kind of pulse heights are you getting with the BC-720?
(I should note here that I am using a Hamamatsu R1306-02 pmt).

My next project is to make a BC-720 replica to see how it compares to my present scintillator. Rough calculations I've done indicate that the "target" construction of the BC-720 increases the surface area by about 3 times.
I plan to use a ZnS(Ag) to casting resin ratio of 10%. (Similar to Hornyak buttons). I'll machine the "target" on a lathe, then place it back in the mold and pour clear casting resin over it. When finished it will be a polished "hockey puck" 2" in diameter and 3/4" thick.

Any and all comments welcome.
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Richard Hull
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Re: Latest project

Post by Richard Hull »

Sounds like you are doin' just fine, Jon.

The great thing about the BC-720 was that the output for a large radium source gammas was only about 10-20mv, while cosmic ray events could be anywhere, but usually under 75 mv. Neutrons were always 200 mv or more. When serious, I just set the discriminator for 150mv and left all but the most potent cosmics and all terrestrial stuff in th' scud way below. I could record as many as 2cpm cosmic/background but normally it was under 1 cpm.

Your attempt at making a nice neutron scintillator is laudable and will be watched closely by us all. Good luck.

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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Re: Latest project

Post by guest »

One thing that would help correlation of results, discriminators, etc., would be to know what the gain in each setup, including PMT gain (varies as function of voltage) and preamp. I'm interested myself, as it looks like I'm a week or three from having a 5" BC-720 -like system.
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Re: Latest project

Post by jim-frank »

Jon- What type of casting resin works for these? ( A brand name would be nice) I bought some ZnS-Ag from Richard Hull and I plan to try making a scintillator for my 2" PMT, just to play with.
Jim Frank
Jon Rosenstiel
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Re: Latest project

Post by Jon Rosenstiel »

Richard, I've been working on stabilizing the various settings the past couple of nights. I did four 15 minute runs tonight with the scope trigger level set at 100mV. I'm not yet completely satisfied with the counts/minute that I'm getting. (about 1.6 cmp) The cpm dropped off slightly with each successive run. That may be due to: (1)Insufficient warm up time of my test equipment. (2)Cosmic ray muon variation due to time of day. (3)PMT hv supply is not warmed up adequately. I've noticed that when it is first turned on the hv is around 5 or 6 volts high. It will slowly drop to 900V after about 30 minutes. I need to more thoroughly monitor the preceding items to get a handle on what is happening.
(Just how sensitive is the pmt to hv fluctuations)?
Jim, the casting resin I used is called "Castin'Craft Clear Casting Resin. Made by ETI. Web site: www.eti-usa.com In case you haven't used this stuff before, (like me), their website has some good tips. I ended up machining a mold from HDPE. (as they recommend)
Worked great! The resin takes plenty of time to harden, (compared to epoxy), so don't stress if nothing seems to be happening!

Jon Rosenstiel
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Re: Latest project

Post by guest »

If you've had the PMT out in the light for a while, the base noise level will be up quite a bit . Operation in the dark for a few hours will straighten this out.
My HV supply also drifts a few volts downward after turn-on. I think there are a couple of things going on: 1) The reference voltage on the TL431 is probably drifting down a bit as it heats up. Since the reference is only 2.5V and is multiplied all the way up to 900V, it doesn't take much of a reference drift to realize an appreciable shift in output voltage. A 16.7mV downward shift in the reference will result in a 6V output shift. 2) The top resistor in the voltage divider might also be drifting down a bit due to both heating and voltage effects. Using a couple of resistors instead of one would help out this problem. Using a better compensated reference will help fix 1). Still, 6V out of 900 isn't too shabby.
Which preamp are you using on your PMT?
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Re: Latest project

Post by guest »

The reason I am badgering both Richard Hull and Jon Rosensteil for data on their preamps is that it gives valuable information on the actual PMT output signal that can be actively used for preamp and discriminator design. For example, if the Hamamatsu preamp design were used for both efforts, we know that the transimpedance of this amplifier is 4V/ma. Richard Hull says that neutron signals in his setup are 200mV or more. Simple manipulation puts the peak PMT output current for this signal level at 50uA. It would also be useful to know the max output current to avoid preamp saturation.
Jon Rosenstiel
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Re: Latest project

Post by Jon Rosenstiel »

The pmt has been in it's light tight housing for a good week now, so that shouldn't be the problem.
Later tonight I'll monitor the hv supply. After thinking about what's happening I'm pretty sure I'll get consistant cosmic ray counts once I let the hv supply stabilize.
I'm using the Hamamatsu preamp.

Jon Rosenstiel
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Re: Latest project

Post by guest »

I'll check, but you may have to actually operate the PMT under voltage (in the dark) for a few hours to exorcise the effects of previous light exposure.
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Richard Hull
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Re: Latest project

Post by Richard Hull »

My system with the 2" BC-720 also used the Hamamatsu tube and preamp. I sold those items freely to a number of members of this fusion site about 2 years ago. It was the 2" hamamatsu PMT, mating socket and pre-amp ready to rock and roll with just a +/- 15vdc and 900v supply needed to use. I had about 35 sets of those when I started, but have only a few for my personal use remaining.

I have just posted to image du jour another image of the large BC-720 I have. It seems there was a desire to see it again. It will be up for a week or two. If you like it and want to reference it in the future, download it and save it on your computer's drive.

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
Jon Rosenstiel
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Re: Latest project

Post by Jon Rosenstiel »

Jon Rosenstiel wrote:
> Richard, I've been working on stabilizing the various settings the past couple of nights. I did four 15 minute runs tonight with the scope trigger level set at 100mV. I'm not yet completely satisfied with the counts/minute that I'm getting. (about 1.6 cmp) The cpm dropped off slightly with each successive run. That may be due to: (1)Insufficient warm up time of my test equipment. (2)Cosmic ray muon variation due to time of day. (3)PMT hv supply is not warmed up adequately. I've noticed that when it is first turned on the hv is around 5 or 6 volts high. It will slowly drop to 900V after about 30 minutes. I need to more thoroughly monitor the preceding items to get a handle on what is happening.

I did a hv output voltage vs. time test on my, (Richard Hester's design), pmt power supply today. For a starting point, the temp in my lab, (garage), was 65F.
After 5 min............906V
After 45 min..........901V
After 1.5 hr............900V
Once the 1.5 hr point was reached the supply was dead stable the for remainder of the day.

I also ran a test of the cosmic ray count vs. hv supply voltage, (900V and 905V). For this test the trigger voltage of the counter was held at 72mV. I did five, alternating, 15 minute runs. (15 minutes at 900V, 15 minutes at 905V, repeated five times). After averaging the 900V setting gave 4.04cpm and the 905V setting gave 4.82cpm, about 0.8cpm higher. Not really a large difference, but nonetheless a difference. I consider it a gentle reminder to leave no stone unturned when dealing with sensitive electronic equipment.

Jon Rosenstiel
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Re: Latest project

Post by Jon Rosenstiel »

Richard Hull wrote:
> Nice looking detector Jon!
>
> I was a bit stunned at the count being as high as it was. My BF3 units mihgt give 0.5cpm. The PMT unit with BC-720 also is in that range as well. Perhaps you will have to dial the discrimination up a bit. It would take a known neutron source to tell what that level would be though.
>
Richard, is that correct, 0.5cpm? By any chance you didn't mean 0.5 counts/second did you? I'm asking because I see Scott Little was getting 12cpm background, (0.2cps), with his BC-720 detector.

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Richard Hull
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Re: Latest project

Post by Richard Hull »

That's right 1 (one) single count every two (2) minutes. This is just about what I get with the BF3 systems too. I found that if the discriminator was set too low I got about 8-10 cpm background. A test with a super hot Co60 source doubled that count and that should not happen! Raising the discrim. level to a point where the Co60 source was not even noticed led to at most, 1 cpm, but that varied diurnally. (1.5cpm to .4cpm) over a weeks testing. I ultimately had to run a background test just before a fusor run and right after data collection and average the results to determine a local time background.

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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