Activation Type Neutron Detector, How Sensitive?

[Jon Rosenstiel]

Quite sensitive, it appears. Check this out.

I have been using an activation type of detector as my secondary neutron detector. (Anomalous event detector). This detector consists of a paraffin filled one gallon paint can with an indium wrapped GM tube at the center. (Based on Thomas Dressel’s paint can BF3 detector).

The experiment:

From the center of the paint can indium activation detector to the poissor = 21cm.
10 minute background count before run = 40cpm.
Fusor operated for 7 minutes at 19kV, 5mA. (Approximately 39,000 neutrons/second TIER). Total Isotropic Emission Rate.
Average count over the seven minute run = 181cpm. (141cpm, background subtracted).

So, with my fusor operating at an output of 39,000n/s the paint can indium activation detector is rattling off 181cpm! During the same time period my Ludlum Bonner ball detector, (located at the same distance from the poissor) is clicking off around 40cpm).

Upon power down I started a series of one minute counts. For the first minute the count was 89cpm. (Majority of the counts due to the 14.1 second beta decay of 116In). The average count for the next 5 minutes was 51cpm. (Due to the 54.3 minute beta decay of 116In).

Another run, this time at 30kV, 9mA. (387,940 n/s TIER).
Average count over a 10 minute run = 1327cpm.
First minute after power down = 410cpm.
Average count for the next 9 minutes = 141cpm.

About the paint can detector:

The detector uses a Victoreen 1B85 thin wall beta-gamma GM detector with the sensitive area wrapped in indium sheet 0.009” (0.23mm) in thickness. The indium wrapped detector is then inserted into a hole in the center of the paraffin. I mounted a BNC connector in the lid, wired up the tube and popped the lid on.

During normal fusor operation I connect the detector to my Ludlum ratemeter and position it about 22” (56cm) from my fusor. (Keeps it out of the way).
In this location the ratemeter indicates around 1300cpm with my fusor pumping out 2.0E+06 n/s TIER.

Jon Rosenstiel

[henryhallam]

Impressive! Looks like my "proper" activation-style neutron counter has a chance of actually being able to detect something.
For reference could you run again on air to see what influence xrays have? Of course the continued counts afterwards prove that activation is happening, but there's quite a difference between 181cpm and 89cpm and I wonder how much of that is because of xrays.

Henry

[ChrisSmolinski]

Henry Hallam wrote:
> Impressive! Looks like my "proper" activation-style neutron counter has a
chance of actually being able to detect something.
> For reference could you run again on air to see what influence xrays have?
Of course the continued counts afterwards prove that activation is
happening, but there's quite a difference between 181cpm and 89cpm and I
wonder how much of that is because of xrays.

The max x-ray energy for his run would be 19 keV, none of which would
make it to the GM tube in his setup.

[henryhallam]

Ah. Makes sense.

Henry

[Richard Hull]

Also remember that you will not see a single X-ray until you defeat the thinest part of your SS shell, which for most standard 60 mil shells is around 35-40kv or more. Ports for gas lines and viewing will be spewing X-rays out much earlier, so locate all detectors off the normal beam axis of these weak points. It is also important to situate the chamber so that you are not in the beams as well. By doing this you will not be bothered by false X-ray detection issues or be in harms way, plus, you can get by totally without a lead X-ray shield up to about 35kv or more. However, a good borated paraffin neutron shield would be a good idea above 20kv.

Great work Jon! How thick was your foil, approximately? I assume you pounded it out of the Indium you got from me or off E-bay?. I was thinking that a 1B85 would be the best tube to use for this and was gratified that you supplied the number.

The 1B85 is probably the best known of all the classic GM tubes, but is a bit expensive now as I bought my first 1B85 back in 1961 for $17.00 brand new from Victoreen. I still have it and several others now. I actually found new NOS 1B85's last year at a hamfest for $10.00 each! The tubes are only good for Beta and Gamma detection and most of my work now involves a need for sensing all three of the ever popular rad-rays, so I tend to only make my new systems up out of mica windowed pancake detectors.

I'll have to try your setup once I get back on line.

Richard Hull

[Jon Rosenstiel]

I neglected to mention this, but for "insurance" (and some higher power runs) I had placed a lead sheet 2.5mm in thickness between my fusor and the detector.

Richard, I used the indium I got from you, still have a fair sized lump remaining. I managed to pound and roll it out to around 0.009 inches in thickness.

Last year I bought a 1B85 replacement from LND (#72527). If I remember correctly it cost around $115.00.

The indium wrapped 1B85 has a surface area of 41cm^2. That’s a lot of area for the moderated neutrons to interact with. Probably one reason this type of detector is as sensitive as it is.

Jon Rosenstiel

[Jon Rosenstiel]

Henry,
During the one minute count immediately after power down the 14.1 second beta of 116In has gone through 4 half-lives, so the 89 counts for one minute does not accurately represent the true count at power down.

Upon power down the ratemeter hangs at the 181cpm level for a few seconds, then starts dropping like the proverbial brick. After 45 seconds it's leveled off and then over the next 3 or so hours it slowly drops as the 116In 54 minute beta decays away.

Something else to consider: The 116In 54 minute beta count increases with each run. First thing in the morning the background count was 40cpm. After several consecutive runs at increasing power levels over a one half hour period the background count was up to 600cpm! Three hours later, it’s back down to around 50cpm.

Jon Rosenstiel

[ChrisSmolinski]

Richard Hull wrote:
> The 1B85 is probably the best known of all the classic GM tubes, but is a
bit expensive now as I bought my first 1B85 back in 1961 for $17.00 brand
new from Victoreen. I still have it and several others now. I actually found
new NOS 1B85's last year at a hamfest for $10.00 each! The tubes are only
good for Beta and Gamma detection and most of my work now involves a
need for sensing all three of the ever popular rad-rays, so I tend to only make
my new systems up out of mica windowed pancake detectors.

I need to pay better attention at hamfests, that sounds like a great deal.
Speaking of hamfests, anyone here planning on attending the Howard
County, MD hamfest this Sunday?

[Richard Hull]

I was at Howard County this past Sunday.

I go up and stay with my friend Kim. She works at NRL and has been in our group for years now. The normal drive from my house would be over 4 hours. but from her place it is only about one hour.

I really made out like a bandit.

2 - 3 objective prism occulared microscopes

About $1000.00 worth of vacuum stuff for $75.00!!!
2 hydroformed 12" long bellows with KF 25's on each end..... One fabulous 6" long concertina type welded bellows coupling with 2.75 conflats on each end..... A full vacuum manifold with 10 conflat 1.33 full nipples and 2- 2.75 rotatable CF fittings...... About 20 VCR and VCO gas line fittings with many adapters and fittings....... 2 swagelock gas valves BK4 and BK6 types..... Lots of SS gas line with VCR and VCO ends on them. Two large vacuum line valves. Two 531 TC gauge tubes with 1/8 NPT to KF25 adapters. The guy also threw in 6 pounds of primo 1/2" thick pure magnesium plate stock.

I also picked up two 6 inch dial micrometer calipers for machine work. One Brown and Sharpe and the other an NSK.

Lots more small stuff was found too...WWII rad aircraft instruments, diamond files and diamond dremel tips, ten 25g tubes of Apiezon N vacuum grease normally $118.00/tube... I bought them at $1.00/tube, etc.

It was a great fest!

Richard Hull

[bwsparxz]

Sorry of the topic a bit, but great finds Richard. I see that hamfests seem to be the way to go. I understand that Carl did well at the Dayton hamfest. I shall try one soon, so I can get my hands on some of the cheap scientific loot out there.

[Dan Tibbets]

I'm curious, how far away from the tube is the indium foil? For a tube ~ 8 cm long I'm guessing the foil would be at a distance (radius) of about 1 cm, or right next to the detecter tube. First how far away from the tube could you place the foil, thereby increasing circumfrence/ area, before beta absorption by the air and inverse square law dominate. Without forcing myself to think too much I'm guessing that the increasing surface area would compensate for the greater distance so the increased beta production would be balanced by the inverse square law, and the increased air buffer would decrease your counts. But, if the the foil was waffled (like an accordian) to double or triple the surface area, but only have an average distance increase of say 1/2, would you have net increased counts?. I know the emmiting isotope has to be very near the surface of the foil for the beta particle to escape and the angle of the detected particle emmision compared to the foil surface would be smaller compared to the flat surface(meaning a smaller(?) portion of the particles would exit the foil and impact the tube), but could there be a gain overall?

Dan Tibbets

Jon Rosenstiel wrote:
> I neglected to mention this, but for "insurance" (and some higher power runs) I had placed a lead sheet 2.5mm in thickness between my fusor and the detector.
>
> Richard, I used the indium I got from you, still have a fair sized lump remaining. I managed to pound and roll it out to around 0.009 inches in thickness.
>
> Last year I bought a 1B85 replacement from LND (#72527). If I remember correctly it cost around $115.00.
>
> The indium wrapped 1B85 has a surface area of 41cm^2. That’s a lot of area for the moderated neutrons to interact with. Probably one reason this type of detector is as sensitive as it is.
>
> Jon Rosenstiel

[Jon Rosenstiel]

Hi Dan,

You're correct, the indium foil was right next to the tube. How far away could one place the foil? Good question. (And a good idea for increasing the sensitivity of this type of detector, at least up to some point). Without hitting the books I don't know the answer to your question. (And there's a good chance I still wouldn't know the answer even after hitting the books)! So what I'd do is make a wild guess as to how much the count-rate would increase and then do the actual experiment to see how close my guess was.

My guess is that if you doubled the radius to 2 cm (which would double the surface area) you would see at least 1.5 times the counts. I also don’t know how much “waffling” the foil would help, but my guess is that would help some, but not a lot... depends, I think, on the number of "pleats" and their depth.

Something else to take into account, as the radius of the foil goes up the thickness of the moderator will go down.

Jon R