Neutron detector on eBay

[henryhallam]

The same seller who sold me my rather odd Albatross neutron counter also has another on eBay at the moment: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 3816275910

He's a good seller, I haven't had my counter arrive yet so I can't tell you how they perform but he says that it powers up and shows background levels.
From the manufacturer's website, it looks like the counters work by activating silver foil and comparing the reading from a geiger next to that and one without any activated material.

Henry

P.S. Feedthrough arrived, woohoo! Now I only need the hemispheres...

[john_h]

Wondering what "pulse neutron detector" means. Does it imply that the detector is slow to recover?

Also wondering what is tunneled into that block of moderator. Is is a GM tube? A pair of them?

Did the manufacturer's web site say anything about the range of neutron energies that this device is looking for?

Thank you for your insights. Best, John

[henryhallam]

The manufacturer doesn't produce the same model any more, but they do appear to have something very similar. They states that it's capable of measuring either pulsed or continuous neutrons, this makes sense if you think about the activation method used - quite clever really but it remains to be seen whether it's accurate for low-level continuous use.

I think the one on eBay is probably very similar to the one shown on that page I linked to but without the digital readout and PC comms capabilities.

Henry

[Mark Rowley]

Henry,
How did the albatross unit turn out ? Just curious as another is for sale on e-bay.

Mark Rowley

[henryhallam]

Hi Mark
The albatross arrived fine, it was packed very well although I had to pay a hefty customs charge! It powers on fine on 120V and has some sort of self-test function which moves the needle to various ranges without any apparent problems. No way to properly test it without a neutron source though, so that will have to wait until a couple of months' time when I get a deuterium supply sorted out. I did open it up and look inside (the metal case on top, I didn't open the moderator sphere), it was certainly a custom-made job as all the electronics were on wire-wrap boards. I didn't manage to replicate the seller's claim that it showed a background neutron count, unless this was on one of the BNC connectors at the back (don't have a scope at the moment).
In retrospect, I shouldn't have paid as much as I did ($500) but at the time I had been waiting for a neutron counter to come up on eBay for a year and thought this would be a real rarity. However at a current price of $31 I'd say it's worth a shot.
If you want something more reliable, go for one of the ~$750 eberline counters that are on eBay at the moment, calibrated and guaranteed.

Henry

[Jon Rosenstiel]

Henry,

Can you post a few photos of the internals?

Jon Rosenstiel

[henryhallam]

No problem - they're on http://henryhallam.cjb.net/fusor/pumps/ (no. 487 onwards). Sorry the internals are so blurry, I'll try and take some shots with better focus after supper and I'll put them in the same place.

Henry

[henryhallam]

I can now report that after plugging in a pot that was hanging loose on the wirewrap board, it's certainly indicating some kind of background - just normal background gammas I think. During the first minute of power-on it chirps erratically, more often if I bring a weak gamma source near. Presumably it is doing a background gamma count. I'll continue investigating and let you know if I can get the needle to move.

[Jon Rosenstiel]

Yeah, it does have a "limited production run" look to it.

I would think this type detector (activation type) would have a fairly normal background count. Mostly gammas from radon daughters and K40 plus the ever present cosmic rays.

How many cpm is it clicking off?

Jon Rosenstiel

[pawinemaker]

I had contacted this seller about some of his other units, and they were not in as good condition as the one Henry got. Two of the units that were auctioned did not register any background counts, but the seller was upfront with the information.

[henryhallam]

Okay, I took it apart some more (sorry, no photos, the camera batteries are dead). All that's inside the moderator ball is a long, thin PCB (about 30cm x 1cm) that can just be slid out from above. On it are a few passive components and two geiger tubes, both wrapped in shiny metal foil of some kind. The lower one has 6 wraps and the upper 3. The lower one is the only thing that makes the meter chirp. This might be by design or because the upper is a dead tube, I don't know.. the foil is soldered on so it's not an easy matter to take the tube out and test it.
When the meter is reset with the "Channel test" switch held in the gamma position, it pretty much acts like a Geiger counter and ratemeter, though I've no idea if the displayed value in mrem is accurate. When reset in the neutron position, it thinks for a minute with the red lamp on, then switches to green and sits there with the meter needle on 0. If the gamma source is brought near, the meter does beep but the needle stays at 0. This suggested to me that it might be working as intended, i.e. subtracting the count from one tube from the other to see how many counts came from activation of the foil. HOWEVER, when I put my check source up to one tube at a time, there was still no result.
I'll keep working on it, but I'm glad I've got that proper calibrated and guaranteed Ludlum meter as well.

Henry

[henryhallam]

Probably about 10 cpm, which is more or less the same as my standard Geiger counter around here.

I would think that in the worst case, it would be possible to junk all the PCBs and make a simple circuit to do the same job run by a microcontroller using the two Geiger tubes, power supply and block o' moderator.

[Richard Hull]

I assume the foils are silver and in normal neutron service, the epithermals activate one tube group and the thermals another. Both tubes should respond to a good gamma source if placed in close proximity.

If these things are in parafin or a polyethylene chamber with over 3 inch walls there should be no epithermals bouncing around in there.

Richard Hull

[Jon Rosenstiel]

I see on the data sheet for a similar detector that one tube is wrapped in silver, the other tin.

http://www.fwt.com/hpi/hpi_2080ds.htm

Jon Rosenstiel

[Richard Hull]

This geiger/activation method of neutron detection does demand a true flux of sorts and is definitely NOT the low level detector needed for first user fusor detection. The BF3 or the scintillator-Bicron BC combo is just about it for low level detection.

He3 would be much bettter than BF3, but such devices are rarely encountered and would certainly be a "project" of sorts for the amateur. Yours truly is hot on this trail right now.

I would order the best to the worst neutron detectors as follows:

1. He3
2. BF3
3. organic-phosphor - scintillator
4. Boron lined counter
5. Fission counter tube with Pu or U235
6. Activation counter

This is based on efficiency versus neutron field intensity demanded to rise out of noise. It is tough to beat the BF3 just due to the fact that it has been around forever, is the defacto standard as nothing has ever come along to replace it and there are plenty surplus units out there at reasonable prices.

The He3 at high pressures is the most efficient of all though.

Richard Hull