Modern neutron detection - Ludlum's new scintillator

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.
Matt_Gibson
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Matt_Gibson »

Hi Joe,

I was in Neutron only mode and the actual detector was well away from the chamber at the time (probe was up next to chamber). I probably need to remove the lead and do a short run to see if the high readings can be repeated again.

-Matt
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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

There is always a learning curve on a new detection system. 1/4-inch of lead in front of your probe, wherever it is, would eliminate 100% of the x-rays, yet would not diminish your neutron counts at all.

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Chris Seyfert
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Chris Seyfert »

Richard,

If I may, that's mostly accurate, though fast neutron scattering is often forgotten about. At 2.5 MeV, the total neutron cross section for the stable lead isotopes is about 7.5 barns, nearly all due to scattering. If I've run the numbers correctly, about 16% (!) of any 2.5 MeV neutron flux passing through a 1/4" sheet of lead will scatter in some way.

One must be careful, here! This does not necessarily mean that the detected flux would change by 16%, as the majority of the interactions will be small-angle scatter. If one's detector is very close to the lead, the scattered neutrons will not move much from their original trajectory and the flux will be essentially unchanged. However, if one's detector is a reasonable distance from the lead sheet, then the situation is more complex.

Probably not a major consideration for most amateurs, but another wrinkle to be aware of in the world of neutron detection.
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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

I always speak of jamming the lead fast against the moderator containing the detector related to keeping x-rays out of the counting regime. I use sheet lead pressed hard against my He 3 moderator. For measurement purposes I want all such protective, exclusion measures be taken hard at the detector.

For health and safety purposes they are taken at the source.

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: Modern neutron detection - Ludlum's new scintillator

Post by Matt_Gibson »

Anyone made any more progress with these as far as calibration goes? I know there was some work being done with bubble detectors…
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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

Mine finally arrived, looks in really very good condition.

Using the standalone application I got 3.1 CPM over 48 min on the first test (6pm uk) and 5.2 CPM over 32 minutes (7am). I'm going to leave it running for 24 hours and see what it averages out as.

Silly experiment idea / question, If I was to take a short drive to my local nuclear power plant and setup the kit on a public footpath, which happens to bizarrely close to the main containment dome, do you think a tiny number of stray neutrons would be detectable over background?
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Liam David
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Liam David »

The count rate seems a little low; I got around 9.5 cpm over 18 hrs, although I am at some 700 m in elevation. One would have to look up the neutron flux dependence on altitude.

You won't register anything above background near the reactor. If you did, consider the dose rate to workers in the facility per the inverse square law + lots of concrete.
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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

I thought it was a bit low to, however we are less than 10m above sea level, in a shallow valley.
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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

I am at 166 feet above sea level and my average count is on the order of 8 CPM, by varies from a low of 6 cpm to as high as 11 CPM depending on solar activity, it seems. My largest ever 60 min count average was 16cpm during one of the early 2000s CME that brought the aurora to Richmond skies at night

It is very rare to drift off the 8CPM mark by more than 1 or 2 counts, ever.

24"X6"X6" moderator with a 18" sensitive volume in a 1" diameter 3He, 4ATM tube.

Establishing two things are critical

1. Making sure you are counting only neutrons. (Use a very intense source of gamma rays to discriminate out 100% of detections.)
2. Find and constantly check your local background (mostly 100% cosmogenic in origin)

There is no such thing as a 100CPM background level in a well constructed and discriminated neutron counter. With a background under 20 cpm in a BF3 or 3He based system, fusion is easily detected as CPM rates are easily and obviously above background at very low fusion levels.

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6"X6"X24" moderator 3He, 4ATM tube
6"X6"X24" moderator 3He, 4ATM tube
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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Jim Kovalchick
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Jim Kovalchick »

Reading through this thread, I saw a post saying that the moderator trick won't work with this detector because the energy response is flat. I agree with this logic but propose a different version of the moderator trick. Putting a thin neutron absorber like cadmium foil between a fast neutron source like a fusor and the detector should not change the reading much. Now put a moderator like hdpe between the foil and the fusor and the reading should drop dramatically. Pulling the foil in the same arrangement with moderator in place will increase the reading. Certainly more complicated than a moderator test with a typical thermal neutron detector, but manageable.
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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

After a longer run I'm getting 5.6 CPM over a few hours, which is lower that what others have posted but not orders of out.

Clearly need to figure out how to calibrate these things against a known source.

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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

As the counter seems to be in order around background, you might assume it to be counting neutrons only. Now a hot source of gammas like a significantly hot piece of U ore would be nice or a 10uCi Cs137 source would warrant no real increase in counts. A pound or more of a thorium chemical like thorium nitrate would also help to be sure that no gammas are counted.

Beyond the above, you ought to assume you have a good neutron counter. Calibration is not necessary as it is the numbered counts per minute (digital counts), that you are most interested in.

Calibration is important for health science reasons. This is something you will not have to worry about as your first fusion efforts will not produce a significant fluence level or a long time of exposure.

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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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

Thanks Richard, in that case sounds like it's going to be good to go.

Uranium ore is somewhat expensive to get hold of over here from my searching so far, however tried it with a big radium gauge (WW2 aircraft vintage, 32k cpm via a gmc-320) and it made no difference to the recorded neutron count. The log files on these detectors show the neutrons and gamma as separate columns though. While bits for the fusor slowly come together, shame I don't have a very weak neutron source to use to experiment with the detector.
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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

Never fear. You will have a somewhat weak neutron source the first time you do fusion in the 20kv range. As voltage goes up with some good current behind it, your fusion/neutron source will grow very strong.

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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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

To kick off some data sharing and following Liams tip of moving the hammer 5x-10x its width away from the fusor, I did a quick test.

10 minute run with 5 minutes at 5cm distance and 5 minutes at 50cm, 36kv 6ma. The fusor was left running and the probe carefully moved to the pre-measured distance.

5cm average 35.2 mrem/h
50cm average 1.16 mrem/h
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Matt_Gibson »

I looked back over my best run, here it is:

Got ahold of a bubble detector (23b/mR) thanks to Anze. Did a few runs today, the best one:

Using my new transformer with my 6 stage CW full wave multiplier:

66kV and 10mA at 11.66microns. Bubble detector is 35cm from the chamber center. I got 18 bubbles in 60 seconds. My Ludlum PRESCILA probe was averaging 105mR/hr at 7.6cm from chamber center.

Plugging the bubble detector data into https://www.gammaspectacular.com/fusion_calculator.html
gave me 5.6E6 n/s.

Looks like the probe isn’t very accurate when it’s right up against the chamber. Most runs with the bubble detector seemed to suggest that my probe was off by a factor of 7.5 when right up against the chamber. Sadly, my bubble detector is defunct now so I can’t try to place the probe at the same distance as the bubble detector.

-Matt
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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

Somebody may correct my maths here but I make 105 mrem/h 594,400 n/s at 7.6cm.

Moved mine even further away and getting around 1mrem/h at 71cm vs 28mrem/h at 5cm. I make this 494,100 vs 68,600 very close to 7 times different!
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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

I would love to see a lead plate of 1/8-inch thickness placed between a closeup hammer and the fusor. Make sure the lead shields the entire probe.
Just to see if any of the gammas are getting counted with and without the plate in place.

While all in the probe is undoubtedly RF shielded, remember that the fusor up close is a fearsome RF generator.

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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Liam David
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Liam David »

I don't have any specific numbers, but when I ran without lead between the hammer and fusor (hammer against chamber), the gamma counts, even in neutron-only mode, swamped the neutrons by an order of magnitude.
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Emma Black
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Emma Black »

I thought the gamma counts were coming from the 2363's internal detector? I get pages of zeros on mine no matter how close, albeit at a lower voltage.

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Richard Hull
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Richard Hull »

From this point on, hammer owners will have to hammer this out among themselves. I do not own a hammer and don't feel competent to offer any thing more than suggestions base on general neutron counting experience. The hammer is a specific device that is interestingly constructed with seeming many modalities related to internal software.

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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Liam David
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Re: Modern neutron detection - Ludlum's new scintillator

Post by Liam David »

I've found that the hammer itself is sensitive to x-rays from the fusor. I haven't used the internal tube at all, and at your voltage, the internal tube won't detect anything. I'll take some data within the next month or so to quantify the hammer's sensitivity to fusor x-rays.
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