Gamma only GM detectors

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Richard Hull
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Gamma only GM detectors

Post by Richard Hull »

The History

Virtually 100% of all pager type, "wear-on-your-belt" rad-alert type radiation warning devices use a gamma only detector. None are really supposed to warn of Alpha or Beta or neutron radiation. The few such belt worn systems that do warn of neutron and gamma cost many thousands of dollars and also advise of the strength and dose rate of these two radiations so detected.

Geiger tubes with mica windows easily detect large fractions of all alpha and beta radiation entering the window. Due to their metal cathode bodies, they also detect gamma radiation at much lower detection percentage levels. In other words, a much more intense gamma source is needed to have higher count rates well above background in mica windowed tubes.

The famous and ubiquitous military Geiger counter, the PDR-27 and all of its later variants could count from a few mrem per hour into the roentgen/hour range. This counter was manufactured in many alphabetic lettered variants from about the late 1940's into the late 1980's. The first 4 or 5 variants remained as purely vacuum tube based systems. Later, transistorized HV supplies and B+ HF switchers allowed D cells to replace the 7 original carbon zinc and mercury cells in the early versions, but retained vacuum tubes within. The later versions, the PDR-27J were of this type. The last variant I am aware of is the PDR-27S still manufactured in 1980, it used 100% transistorized circuitry and an identical copy civilian version made into the 1980's was the EE730 and distributed to government agencies and subcontractors outside of the military version PDR-27S.

These old guard, and long lived PDR family of counters used two GM tubes. The "pickle" probe used a mica windowed 1-inch diameter windowed, 6 inch long, 5979 tube. Inside the heavy cast aluminum body of the instrument was a much smaller GM tube that was switched in at one of the high range points. The early systems used the 5980 tube which shared the 750 volt power with the 5979 tube. Later a work-alike gamma only tube with a closed ceramic and steel body was used. Some PDR series actually had this small gamma tube mounted on the side of the pickle probe housing in its own thick, sealed aluminum tube.

These gamma-only tubes relied on their tiny gas volumes coupled with their metal shells to detect only gamma via Compton scattering of electrons from the shell into the small tube's gas volume. All PDR body cases were of thick cast aluminum which would stop all alpha and beta radiation. The typical mounting of all the 5980 gamma tubes was in a cartridge fuse clip arrangement on one of the electronic circuit boards within this heavy case.

The upshot

I obtained 46 of these small GM tubes at HEAS 2021 from one of the flea-market vendors. (super good price!) I have tested all of them and found only one to be defective. (all require 700-900 volts found in all PDR series counters) Three types are NOS from repair stock for the PDR series and one type of the tiny little pager style GM gamma tube. I was stunned to encounter 11 of a type gamma only tube from later PDRs that had a 0.3-inch mica window!! These, I discovered, would detect both alpha and beta radiation from sources placed near the small tube's mica end window. These are 7616 tubes. Strange! A couple of these tubes are still available from LND! (must be a lot of PDRs still in use!) As surplus, many of the PDRs went to police and emergency agencies to replace the rather limited and crummy civil defense yellow 700 series first surplused to these agencies.

Below I lead off with an image of my PDR-27F model, still vacuum tube based and needing D cells and the two 67.5 volt carbon-zinc batteries as well as the 22.5 volt carbon-zinc cell. They got rid of the 4 mercury RM-4 cells in this model.
Below the counter are the images of my HEAS haul of gamma tubes. I thought you might like to see them for future reference while roaming around ham-fests. I have found a few 5979 tubes and 5980 tubes at hamfests over my 45 year ham-fest adventures and questing.

Note: Naked, on my test bench, powered up properly, connected to a counter, with a 10uCi Cs137 source laying on the tube's body, I could get 400 counts per minute. Using a 2" pancake probe I get over 100,000 cpm of beta/gamma. This means that if these small gamma tubes are counting like crazy, (>1000 cpm), run away!! This tells you that you are in an intense gamma field. Background varied from one click every 4 minutes to about 1 click every two minutes.

Richard Hull
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The big universal military and industrial beast in its heavy cast aluminum case.  Used circa late 40's into the 1980's.  It was just too good to toss out in the "S" model. (D cell only) 100% transistorized model. Big, heavy, bulky and ugly, but it would take a beating in rough and tumble military service.  Battleship gray was for use by the Navy and Olive drab case color was used by the Army.
The big universal military and industrial beast in its heavy cast aluminum case. Used circa late 40's into the 1980's. It was just too good to toss out in the "S" model. (D cell only) 100% transistorized model. Big, heavy, bulky and ugly, but it would take a beating in rough and tumble military service. Battleship gray was for use by the Navy and Olive drab case color was used by the Army.
A later PDR gamma tube with mica end window that would detect alpha and beta at the end window.  Note all of the PDR series gamma tubes were designed to mount in a form of small electronic cartridge type fuse clip on the internal circuit boards.
A later PDR gamma tube with mica end window that would detect alpha and beta at the end window. Note all of the PDR series gamma tubes were designed to mount in a form of small electronic cartridge type fuse clip on the internal circuit boards.
The second PDR-27 gamma tube 5890
The second PDR-27 gamma tube 5890
The very first PDR-27 gamma tube 5890  very thick glass
The very first PDR-27 gamma tube 5890 very thick glass
Pager GM tube  Really tiny! Needs 450-550 volts to function
Pager GM tube Really tiny! Needs 450-550 volts to function
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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Dennis P Brown
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Re: Gamma only GM detectors

Post by Dennis P Brown »

Nice haul; this post made me decide to pull my 3He detector system from storage.

It has a Russian CHM-32 tube, cables, a power/computer interface box and a thumb drive software program (which I updated to the 64 bit operating system - lucky they were still in business - all new models but same core program so my software now works on the Win 10 platform.) Of course, no instructions ... but their newer stuff has a lot in common with my software package.

I understand 3He detectors are indifferent to x-rays & Gamma rays and only really respond to slow neutrons. So should just need a moderator. The connectors are all shielded coax.

That entire system now appears to run fine and gives a consistent background. The U238 sample with a moderator adds a few counts but not statistically significant over ten minutes, as expected. So, thanks to your input previously I expected that nebulous result.

Trying to get my old ST360 counter/driver to interface with my new Win 10 based computer but the original software is missing and the device is not 64 bit compliant anyway - alas, surplus tech has its limitations (when digital) in this fast moving digital age. Still, decided to order an interface cable to support the cross platform issues and will see if their newer software can still interface with this outdated model - a model they don't now support. Ultimately, not a serious issue since my ST unit has its own display but then, recording data is an issue - well, can just read and manually write it down ... LOL - wouldn't be fun if it was all turn key! That unit will only be the counter and power source for the fast neutron sensitive PMT. Not my primary counter anyway - just an experiment in its own right.

Got the Geiger counter working and I have both silver & Indium foils (and a shield tube) as well as a large moderator tube; assuming I generate enough neutrons activation detection might be possible - but that is the whole point of this exercise.

Well, everything hinges on a proper neutron source! So that will be my focus from here on out - at least till I run into my next problem ;)
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Richard Hull
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Re: Gamma only GM detectors

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I can't understand why a U238 sample with moderator would increase the neutron count at all unless you had a minimum of ten kilograms of U238 clustered around a moderator with the neutron detector buried within it...If you are talking about SF neutrons. While the SF rate in U238 is high, you have to have a large mass of it in a special setup before detecting SF neutrons.

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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Dennis P Brown
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Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Gamma only GM detectors

Post by Dennis P Brown »

Ah, ignorance is not bliss - certainly not on my part by speculating. So, the minor variation I attributed to a very few neutron events is, once more, random fluctuations from the normal electrical noise. I really need that neutron source - assuming it follows the behavior of my previous device and provides a few 100 k neutrons/sec like my first fusor.

That I would guess should be sufficient for a 3He detector to actually read a clean signal (w & w/o moderator), maybe also for my properly shielded PMT scintillation detector (Gamma and x-rays not driving the PMT) but the real question, for this effort, is it enough to activate Ag? Cart, see before horse - my new fusor hasn't even been tried with deuterium. As I not too fondly recall getting a stable run can be an interesting challenge to achieve; the seasoning of a fusor takes some time and many nerve racking runs.

Richard, again, your experience is both very helpful and also clarifies issues for someone (like me, in this case) who had no idea about the physics occurring in an experiment.
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Richard Hull
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Re: Gamma only GM detectors

Post by Richard Hull »

I seem to remember 14 SF events per Kg of U238 per second. That is quite a lot actually for a naturally self-fissioning element. Still, due to self absorption and the neutrons released being isotropically emitted, you have to have a lot of U238 on hand to start reliably, statistically counting SF neutrons.

As we all know, a natural uranium reactor is possible. It just takes tons of natural self-fissioning U in a huge moderating lattice of Graphite or heavy water and you have a low power fission reactor. There is no core in such a reactor, per se, of course. The entire edifice is active. As such some added layers of moderator help avoid killing the operators even at low power, coupled with short operational periods. Bigger graphite systems used lead walls and air or gas cooling.

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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