Safecast - Kickstarter Open Source Geiger Counter

Sigurthr said:

It will sense high energy gamma radiation from all directions as it is not a shielded probe. It will sense beta and alpha from below the sensor only. Alpha only travels a few cm in air, and beta only travels a few inches (unless very high energy or a very large field).

Keep in mind pancake style probes are EXCEPTIONALLY FRAGILE. Being in a car and having the door slam shut is enough to rupture one. If a blade of grass pokes through the screen over the sensor, it will rupture it. You must be vigilant in guarding the fragile surface of the sensor.

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Pancake probes at high altitudes.

Even in the passenger compartment a Pancake detector (LND 7311, QPL 8767 et al.) is at risk. LND and Ludlum Measurements rate them only to 8000 feet altitude. Passenger compartments on commercial jets are *supposed* to be pressurized to 8000 feet equiv.

http://www.ludlums.com/ReNwsLtr/NwsLtr-4re.pdf



This is nearly the height of the mountain pass where US Hiway 40 goes over the Continental Divide (7245 ft) where I test my GEO-310 Pike's Peak Prospector High Altitude pancake probes:

http://www.qsl.net/k0ff/Pikes Peak Prospector GEO-310/MVC-014S.JPG



I designed a limited production run with Russian tubes, specifically made to withstand the lesser pressures at higher altitudes for prospector's way up in the mountains. Naturally as expected, their thicker windows block a higher proportion of Alpha particles, but they are excellent Beta detectors, and slightly surpass the 7311 in Gamma Ray detection. Since this tube works at 475 Volts and the Ludlum meter was set for 900V, an internal re-regulator sets the operating Voltage to the correct level while passing the pulse through at full height. (info available on this converter device).



Because at this altitude the mica window on a regular pancake (gas fill is at less than 1 atm.) reverses its bow direction, from slightly inwards to slightly outwards, I designed my GEO-210 standard pancake probes (LND 7311) with a foam rubber gasket between the tube face and the stainless steel screen to accommodate this motion. Designs like the 44-9 have no space to accommodate the bow, allowing the mica to contact the screen, resulting in certain popping of the sensor (they go off with a pretty loud bang).





All my standard probes are shipped inside a sealed tin can (quart sized paint can). The probe is specifically designed to fit into such a pressure vessel for shipping or transportation over mountains. High Altitude probes require no such protection. http://www.qsl.net/k0ff/GEO210/Geo 210 bottom.jpg



Bill Lehnert of LND today verified:

LND 7311-Routine operation at 5000-6000 feet no problem, i.e. Denver, Los Alamos. Above that they work fine until the reverse bow becomes a problem in the housing. Once the bow has reversed, the tube will continue to work fine. It is not a good idea to cycle between high altitude and low altitude, as eventually the seals will suffer.



LND 712- the small 500V end-window used in so many pocket-sized portables will work at full vacuum. A great selection for Alpha Particle experiments inside a vacuum jig. Gamma sensitivity 18 CPS.



LND 72344 (500V) or LND 72310 (900V) are actually described as end-window not pancake. To me they look like a small pancake. They are good somewhere between the 7311 and 712, so would be a very good candidate for a high altitude probe, still retaining a good Gamma Sensitivity number of 25.

(End of Bill's contribution)





It's really hard to beat a LND 7311 based probe for general-purpose field work (60 CPS/mR/H Co-60). The transportation issue is a real one, so make arrangements for that.

For routine high altitude usage, you might want to consider another type of tube.





Based on past bad experiences, when driving I now use an Army surplus Ammo Can fitted with a Schrader valve (like on a car tire) for over-mountain transport of thin window probes. Any numbers of Pelican etc. cases can be purchased for the same use, always look for the pressure relief valve.



Obviously I had designed purpose-built probes for actual use at those altitudes.



If anyone needs a special probe or housing built for any of the LND products for a special application, feel free to contact me.

The only charge would be for the parts if I should decide to get involved.



George Dowell

NLNL

New London Nucleonics Lab

A division of Viscom Inc.

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Benm said:

It's good to notice that they are -that- sensitive to pressure, and must be kept in some airtight container whenever shipped by airmail.

As for shielding from beta radation: You could simply glue together a box that fits the entire counter using 1cm thick plexiglass, just the stuff you can buy at a hardware store. Obviously it would have to open on one side to insert/remove the counter, but this can be done with a simple lid that slides in the open side. It is not required that this is a tight fit - beta's do not go 'around the corner', so as long as a total of 1cm of plastic is between the counter and the outside world in any direction, it will work.

Also, it doesnt matter what type of plastic you use: Polycarbonate, PMMA, ABS, its all fine since they are of comparable density and contain no elements heavier than oxygen.

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Sigurthr said:

I don't believe S_L was worried about you flying with it, I think he was more concerned about a new retailer erroneously air-shipping these completed meters all over the world with the very real possibility of them being in non hermetically sealed containers in unpressurized cargo bays of airplanes. In other words, he didn't want you to receive a broken tube.

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