Modern neutron detection - Ludlum's new scintillator

[Andrew Seltzman]

The moderator trick will not work with this type of detector (proton recoil) though since it's energy response is almost flat from thermal to 100MeV. Unlike a He3 of BF3 detector which can be removed from it's moderator, in the PRESCILA probe, the detecting element and moderator is the same element.

[Matt_Gibson]

I’ll update my thread for my next run

I just wanted to add some raw/crude results to this one since this detector is “new” and I have little idea how to interpret the data other than seeing a large jump in background count.

-Matt

[Dennis P Brown]

Couldn't one still could add an additional moderator and see the change in counts? That would be rather straight forward method to confirm a real neutron signal.

[Andrew Seltzman]

An additional moderator shouldn't make much difference on a PRESCILA probe since it is sensitive to both fast and thermal neutrons. A moderator with a neutron absorber should make a difference though by reducing neutron rate, however it would physically be a huge moderator compared to that used in a HE3 or BF3 tube since the detector head is much larger in diameter. Running with deuterium and hydrogen separately should produce similar electrical noise output and is likely a simpler test.

[Finn Hammer]

All,

Bruce was right, after all (never doubted that, of course, just too befuddled to respond to it at the time ) there are other ways to communicate with the 2363, than via mathcad.
Of course, getting the first numbers across for initial assessment of the functionality of the items, the matcad code was essential, and this rush was just another aspect of the Ludlum 2363 frenzy, which some of us experienced first hand.

But for the average Joe, like myself, to whom mathcad language looks unreasonably gibberishly, communicating with something like C+ would be preferable.
I have just tested the RS232 interface with a terminal, and it works well, (of course it does, it is a Ludlum!) this means that it will indeed be trivial to build a small microprocessor based querying adapter, with a readout displaying whatever parameter is desired, probably to present neutron rate and averaged neutron rate, even save these data to a SDcard or something.
Here is a little snip of the first attempts, soon crowned by luck:

The learning curve was not that steep this time. RTFM. Case sensitive for the most.

To arrive at an accumulated, or averaged count number can also be aquired by the Ludlum interface. It has, apart from the 1/2 secont dump facility, also the option to set an arbitrary count interval, with averaging over the interval, as seen here, where I set the interval to 10sec. But it could be 600Sec. It even sports a countdown to the next count!, right handy in the case of long count intervals.

Ludlum interface showing the averaged arbitrary count interval.

My fondness of this instrument is increasing by the day, it's versatility is steadily increasing!

Cheers, Finn hammer

[JoeBallantyne]

I bought this instrument as well, and the shipment finally arrived this week after getting stuck in the bowels of FedEx for a couple of weeks.

I don't yet have a neutron producing fusor to test it with, but I did a background run, and most of the time it spits out about 6 counts per minute. Sometimes up to 8, sometimes as low as 3 or 4, but typically 6.

I didn't yet try running Andrew's script, or dumping out anything over the RS232 port, simply used a timer on my phone to countdown a minute and manually counted the higher toned neutron output clicks from the speaker. Did that about 10 times in a row, and the most frequent result was 6 cpm.

Joe.

[Matt_Gibson]

Joe,

How’s the condition of the Ludlum? Finn and myself got ours in good/great shape while others got some in rough shape.

6cpm is around what I get.

-Matt

[JoeBallantyne]

I would say the original condition when shipped was decent, but certainly not new. There was significant damage incurred during shipment due mostly to failure of the seller to appropriately package and mark the shipment. Needless to say, I was not happy when I saw the box, or when I opened it up. I am of course following up with the seller.

Joe.

[Matt_Gibson]

Any new progress with this? I’m curious as to how these are comparing to known detectors.

I had a confusing experience with mine:

As I was pushing my fusor voltage up close to 60kV (pulls around 7-8mA at 13 microns or so) I had my readings skyrocket to over 1600mR/hr. I was able to repeat this over and over again. Once I realized that X rays are now a problem again (I was previously limited to 40kV) I stopped and added lead sheeting. After adding the lead, I was never able to get anywhere near the results.

To date, my best results after adding the lead, is 60mR/hr. This was done at 60kV 7mA 12microns, hammer about 5cm from chamber center.

I’m stumped as to what is going on, but guess that the X rays were interfering.

[JoeBallantyne]

There are two independent detectors. One for neutrons, one for gamma. I believe the bottom end of the gamma response is about 60keV.

I expect you were simply measuring xrays at 60keV. Which you would have been getting once you pushed the voltage up to 60KV. Clearly you were getting a lot of them.

You should have been able to switch counting the xrays on the meter on and off by simply toggling from N+G to N. N+G will count the xray exposure, and N only should not.

Joe.

[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

[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.

Richard Hull

[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.

[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

[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…

[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?

[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.

[Emma Black]

I thought it was a bit low to, however we are less than 10m above sea level, in a shallow valley.

[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.

Richard Hull

[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.

[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.

[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

[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.

[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

[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