Archived - 5 inch PMT/BGO Super Detector

[Frank Sanns]

Last year at the HEAS I demonstrated a half in thick by three in diameter BGO detector. Besides being ultra sensitive, it is directional by itself. This in not normally seen in common detectors and usually requires an array. BGO has such great density that it stops x-rays and low energy gammas in a very short distance. For this reason, the 3" x 1/2" BGO crystal ($25,000 for those of you that would like to buy one!) is directional in its detection capability close to its surface area in that direction. The face then has an area of pi*r^2=7.1 sq inches and the edge is thk*diameter=1.5 so the limit to the directivity would be 7.1/1.5 or around 5:1 . This is really great for finding just about ANYTHING radioactive from a great distance and by panning the detector around to pinpoint the DIRECTION that the radiation is coming from.

I decided then to use this relatively inexpensive slabs to make a super detector. The photo is of 18 of the slabs on the 5" PMT but before optical coupling fluid. The finished detector will have 38 slabs total. The surface area will be 19.6 sq. inches (almost 3 times more than my 3 inch) and the side detection will be 1.25 sq. inches for a limit to the directionality of nearly 20:1. I should have it wired up soon and will give a report.

Oh, and if you think you can pull this off with a plastic scintillator, forget about it. The stopping distance for high energy photons in plastic is so lousy that even with something of these dimensions, you would see no directivity at all or maybe even a small advantage from the side.

Frank Sanns

[richnormand]

Wow. !!!

Talk about "deja vu" This morning I e-mailed you while waiting for my plane for three of your xtals to populate my 2.5" PMT.
Cant wait to see the results of your detector.

[Jon Rosenstiel]

I just finished building a detector using a 3 inch pmt and 9 of the BGO "slabs". Thought I was "the man", but now I see I'm just an "also ran"!

Jon Rosenstiel

[Frank Sanns]

Hi Geo,

No beta window for this beast. I will keep the detection end as thin as possible but with such a large area and the cost of all of the BGO right at the end, I am a little leary of an accident that could end the life of the entire unit. It is a shame to have to do that but I do not know an easy solution unless I leave a 1" x 1" opening right to the innards of the detector. Interesting suggestion though and I will keep it in mind as the project contiues.

Frank S.

[Frank Sanns]

I completed my 5 inch PMT this weekend. It works fabulously. It behaves differently than my 3" by 0.5" BGO so it will take a while for me to gain experience with this new "Super". The directionality is around 5:1 for a radium dial watch at 5 feet. Not quite as good as I had hoped but still really easy to hear and read the directional differences.

The first picture are the thirty BGO xtals (0.5 Kg!) in place with optical coupling fluid (Visil V788) on the 5" PMT. I ended up ony using thirty because it allowed me to hold them in place using the end protective cap that came with the PMT. After the BGO crystals were in place I covered them with the reflective side of some aluminum foil and then a thin piece of PE foam for cushion and to keep the crystals from moving around. Then the cap was put on and taped to the PMT. To be sure that no outside light made it into the PMT, I layered another 2 sheets of Al foil and taped it all up near the socket. The second picture shows the PMT/BGO (before last Al foil layers and foam packing) in place in my improvised housing.

I had looked at PVC pipe and all kinds of other ways to enclose the PMT but all looked suspicious to be able to be walking around with. In fact some of the enclosures would have looked downright scary so I chose a very inexpensive, common looking, and easy to service tool box as my enclosure for $7.

The last picture shows the finished product with my Ludlum 2241-2 Scaler/Ratemeter.

One PMT@ $300 plus 30 BGO @ $1,000 each makes this a $30,000 detector if bought new. Of course with these kind of detectors I am sure there would be a big mark-up too.

Frank Sanns

[richnormand]

Your enclosure is genius!
You also have some room for some custom electronics so you do not have to have the external counter and supply.
Just don’t get caught in the cereal aisle at the supermarket with this however!!!

Will you post some sensibility results and angular measurements?

[Richard Hull]

I archived this as it shows construction and gives good detailed info.

Nice deal there Frank. A 5" pmt is not exactly a pocket item. I have some 6"X6"X6" BC 400 blocks I will soon try to link to my 5" PMT system.

Richard Hull

[Jon Rosenstiel]

Very nice, Frank. I like the toolbox idea, you can tell people you're a plumber looking for a gas leak!

Jon Rosenstiel

[DaveC]

Frank - Very nicely done!

I just have a simple question about the effects of "tiling" the detector xtals. Do you have any evidence the minute gaps between the xtal pieces cause either an elevated background signal or other noise? It seems likely that there should be heavy attenuation through multiple reflections and scattering through the gaps. So the effect could be minimal. Just curious.

I like the directionality feature, especially.

Thanks for sharing the construction methods and test results.

Dave Cooper

[Frank Sanns]

Thanks for the all of the positive comments and here are some answers (not necessarily in the order they were received!).

A 2" PMT will hold only 3 xtals to stay within the perimeter of tube. A 3" will hold 8 xtals and a 5" will hold 30. I didn't take time to figure what the equation would be to solve for it. If it were me and I had a small PMT, I would use the xtals on edge or on end to maximize the detection area for the PMT used. If on edge, the number of xtals that fit would double and on edge would quadruple or a little more.

Notice in the original picture that the light does not go through the xtals and into the PMT (or vice versa). It looks very frosted but then look the picture in the assebly with the coupling fluid and you can see how the interfaces start to dissapear and light flows much more freely between xtals and between PMT and xtals. I do not see any huge difference in noise with the setup but I guess to know for sure I would have to use the same PMT that my monster xtal is on and layer some xtals on it instead and compare. Too many variables here with different tubes, settings, shielding. Will have to work on that one a bit to get a handle on it. The 3 inch solid BGO and the 5 in mosaic BGO do behave quite differently so I will post as my experience base grows the the new setup.

I will make a crude goiniometer and use a source to map the directivity. Give me a week or so and I will post numbers for both detectors. One thing for sure they are both VERY directional.

I really like the tool box but it still scares the hell out of me to take it in the car. Can you imagine if I would get stopped and the officer see the thing with the wires coming out. And of course if he decided to open it I would HAVE to tell him to be careful since he could cause the tube to implode. And then he would open it if he or the bomb squad would and see wires going into the end of a radical looking package like a 5 inch PMT with wires going into the base. And then they x-ray it and see rectangles that do not let x-rays pass. Yes I think it is bad news no matter container it is in but at least the chance is there to be less conspicuous.

My plan is exactly to incorporate the electronics into the remaining quarter or third of the box. I would like to make it completely optical by putting in 10 standard intensity LEDS under one of the smoke plastic compartments on top of the tool box. Putting small black plastic tubes over each LED so they would only be visible only to the operator from directly above. They would be wired to a log rate meter so that the intensity would be displayed by how many LEDs would light. I was thinking a bleader circuit from the pulses with a voltage measuring IC should do it. Two LEDs light are normal background if you get 4 or 5 or more you are on to something. All ten lit and you better vacate quickly. Nobody else would know.

I just scored another batch of BGO at a significantly lower price (although they are not quite as pretty) so I may try a 3" with the xtals in another orientation. I will keep you all posted.

Frank S.

[DaveC]

Frank -

I think you really should pick a good solid scientific instrument name for it... nothing deceptive.... and get a good looking name plate engraved. Put a serial number on it or label it experimental and you are all set. A "Fragile, Extremely Delicate, Light Sensitive Electronic Instrument" tag wouldn't hurt either.

Dave Cooper

Doesn't need to be a secret.

[Frank Sanns]

That is a great idea Dave. Maybe I could call it a ballistic nuclear photonic downstater. Well I guess that would not make another person very calm. But maybe just simple a low intensity natural light detector. Great advice that I will revisit after I get some much needed sleep! Is it the weekend yet?

Frank S.

[Richard Hull]

It is all in a name!

I have an old, pre-WWII alarm clock I picked up at an antique show. In the 20's and 30's a super big deal was made about Ra dialed clocks and watches (they carried a stiff premium over there dark brothers).

The one I am talking about is labeled near the center of its 5"X5" face in big white letters.... "Radium Fireball". What a great name!

Richard Hull