Alternative magnetic shielding for PMTs.

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teslapark
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Alternative magnetic shielding for PMTs.

Post by teslapark »

Hi,

I'm just starting to wrap up my PMT/BC-720 detector, but I don't have any mu metal on hand.

I've thought about putting the tube inside of a standard steel paint can. I would center the tube down the axis of the can, so there would be at about 1.5" inches of space between the tube and the wall of the can on all sides.

I feel that this combination of distancing and the shielding effect of the steel should offer at least some protection against magnetic interference. The question is, does anyone think it will be enough?

Adam
tligon
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Re: Alternative magnetic shielding for PMTs.

Post by tligon »

For a fusor? I would think so.

PMT gain is affected by strong magnetic fields because it alters electron trajectories. But the fusor is not very magnetic. I would be sure the PMT is well away from any current-carrying inductors. Sudden discharges during grid cleanup might be picked up, but more from EMP-like interference than plain magnetic field. For that matter, you want the PMT in absolute darkness.

If arcs cause the setup to produce false counts after your provisions, more drastic measures may be needed. I wound up putting our neutron counters inside small steel cabinets, and triple-filtering their power. Noise was even getting in on the ground lead on the AC power.


On Bussard's highly magnetic machines, we used a high-nickel alloy from McMaster Carr to shield a PMT we were using for photometry. That, and distance, worked well enough that we saw no significant change in signal turning the magnets on and off.
Starfire
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Re: Alternative magnetic shielding for PMTs.

Post by Starfire »

Will work ok Adam, unless you are working very low signal at maximum gain with the pre-amp - I have used a steel bathroom rail thin walled pipe in place of a MU metal sleve. Don't use Stainless Steel - it dosen't work.

MU metal screens appear from time to time on ebay.
Richard Hester
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Re: Alternative magnetic shielding for PMTs.

Post by Richard Hester »

The mu-metal is nice to have if you can get it. Last time I looked, Oetech had shields for 2" and 5" tubes, and the prices aren't too bad. Steel actually retains a field (it has a lot higher remenance than mu-metal), but might be ok if you degauss it first.
DaveC
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Re: Alternative magnetic shielding for PMTs.

Post by DaveC »

A good thick walled low carbon steel, or wrought iron pipe will work well, sometimes even better than mu metal.

Lacking either of these, several thin sheets of steel wrapped into a can shape will also work, pretty well. How well depends on the field strengths, and frequency. Strong, low frequency magnetic fields are the most troublesome to shield, because it takes a lot of material to attenuate the flux.

A mu metal shield inside of a thick walled aluminum or even copper pipe, gives a pretty good broad spectrum shield system.

As Richard mentions, with ordinary steel, you need to de-gauss it with an AC coil, using a variac or similar control to reduce the coil voltage slowly to zero. Assuming you are careful and there are no sttrong DC fields about, a one-time treatment.should last for quite a while.

Dave Cooper
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Richard Hull
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Re: Alternative magnetic shielding for PMTs.

Post by Richard Hull »

About 7 years ago on the old songs site, someone posted of a special offered by a mu metal manufacturer offering special deals.

I sent off my $$$ and got a pretty nice selection of thin sheet and tube. Since then, I found and incredible buy at the Manassas VA hamfest. A guy had about 45lbs of mu metal but it was all in the form of a bunch of license-plate sized plates that were almost 1/8" thick!!!!! They were as shiney as a new nickel coin. I bought 'em all as the alloy is most unusual and, of course, useful.

I had the alloy formula in hand from these plates as they were gov't surplus and had the formulation on the large package label. I called a couple of mu metal fabs back then and they quoted me $256.00/ sheet of this stuff! OUCH! Uncle Sam has those deep pockets.

The actual scrap metal value of one of those plates is about $48.00 now. (mostly due to the high nickel content) I hate to think what a quote today would bring down with nickel kissing an incredible $18.00/lb. (was about $5.50/lb when I bought the stuff)

Most hamfesters are sharpies and will not sell below what a scrap yard would give them. This is the beauty of the hamfest. You can just effectively erase all fab costs from any surplus item. Fab costs are often 300% - 2000% of the material value just because they know you can't do it and they can. Naturally, there are a lot of special machines and furnaces in metal fab that you are helping pay for when you buy metals of special alloy, high purity or special shape.

Keep poking about, if you really need the Mu metal......You'll find it, ultimately.

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
tligon
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Re: Alternative magnetic shielding for PMTs.

Post by tligon »

Richard,

When is the Manassas Hamfest this year? I have not been in a while and am not on a mailing list. I need to get it on a calendar before my Activities Director schedules us for something else.
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Richard Hull
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Re: Alternative magnetic shielding for PMTs.

Post by Richard Hull »

To all in the VA, WV, MD, PA region. The Manassas fest is always the first Sunday in June. This year it would be the 3rd of June.

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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