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.
