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Gamma dose meter Radiacmeter IM-179/U

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MinorityCarrier

Sun Apr 11 2010, 04:55AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

This one is for Proud Mary since she's expert with radiation detection instruments.

I have a few of these gamma dose meters, use to have a few more but I took several apart for the detector, that little gray box with the wires sitting in front.

There are no semiconductors in this instrument, just resistors, a thermistor, a 50uA meter and the detector. These instruments run on two batteries, a 1.3 volt one that powers the detector tube filament, and a 5.2 volt one for everything else.

I took one detector apart to see what was in it, and documented what I found in the drawing.

My question is what is the principle of operation for the detector? Does it use gas that is ionized around the tube?

Proud Mary

Sun Apr 11 2010, 02:48PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

This is a Neher-White type dosimeter designed for use in USAF survival packs of the Cold War era. It used mercury cells.

The design uses the ingenious Neher-White configuration in which the electrometer valve/tube is located inside the ionization chamber.

There is a very good explanation of Neher-White devices in the user manual of the Jordan Model AGB-10G-SR Radiation Survey Meter
here:

Get back to me if you are still unsure how it works after reading that.

In general, military dose rate meters aimed at non-technical users are designed to be insensitive to background, so they
will display nil until a dose rate of military operational significance is detected.

200 Roentgens is roughly 24 Sieverts, enough to kill you five times over.

There is another commentary on Neher-White detectors, and their vulnerability to ambient temperature change here:

Thanks for sharing that with us - a real treat to see such an ingenious device

MinorityCarrier

Sun Apr 11 2010, 04:46PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Thankyou for that comprehensive answer. I am less ignorant.

I had thought it might be an ionization chamber because of the first grid of the tube being tied to a floating coil inside the chamber. I haven't heard of a Neher-White detector before.

I noted the thermistor when I drew up the circuit for the entire instrument, now I understand what that was for.

Testing these is problematic, the only source I have access to, a 7uCi Ra226 gamma source, barely deflects the meter. I've been informed the batteries are no longer manufactured (I have a couple of spent ones) so I have to kluge some battery supplies to run these.

klugesmith

Sun Apr 11 2010, 04:58PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Proud Mary wrote ...
200 Roentgens is roughly 24 Sieverts, enough to kill you five times over.
...
Thanks for sharing that with us - a real treat to see such an ingenious device

Stella, I think your conversion is off by 1 order of magnitude.
But I agree with the final sentiment, in fact would have used the words treat and ingenious device, had they not already been taken.

Proud Mary

Sun Apr 11 2010, 05:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The lead foil on the outside of the chamber is intended to improve the linearity of the energy response, by attenuating the lower energy photons to which gas discharge devices (GM tubes, ion chambers, proportional counters etc) are much more sensitive.

If you now remove this lead foil, the chamber will be made more sensitive, at the expense of linearity.

If you hope to replace the Hg cells with something more fashionable, remember that Hg batteries have +ve on the flat end, and -ive on the convex end, the opposite of the convention for today's AA, AAA and so on.

Proud Mary

Sun Apr 11 2010, 06:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Klugesmith wrote ...

Stella, I think your conversion is off by 1 order of magnitude.

My understanding of the long obsolete and non-SI unit, the Roentgen, was that it was equal to 0.119331742243 Sv, but don't take my word for it! I might have
disnumeracy on top of everything else

klugesmith

Sun Apr 11 2010, 07:13PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Just looked it up again.
The fuzzy link is conversion from roentgen (ionization charge of 2.58e-4 C/kg of air)
to rad ("roentgen absorbed dose"): 1 rad = 100 ergs/g = 0.01 J/kg = 0.01 gray.
Roentgen to rad ratio is slightly shy of unity: 1 R -> about 0.87 rad in air, 0.96 rad in soft tissue.

Since the subject is gamma rays, with biological effectiveness taken to be 1
(unlike neutrons or alpha radiation),
1 rad = 1 rem = .01 gray = .01 sievert.
So 200 R is a bit less than 2 Sv. Taken as a full body blow it will make you very sick, but you will probably recover fully except for increased risk of cancer. Don't know how fast it would impair your soldierly duties.

Proud Mary

Sun Apr 11 2010, 07:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Well, RadPro says 1R = 0.0087 Sv, so 200R is 1.754 Sv, around 2 as you said.

The conversion calculator on the dyscalculia website must have got it wrong again!

MinorityCarrier

Sun Apr 11 2010, 08:28PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Here's what the batteries look like. I've powered the instruments up in the past with hokey not-very-portable battery setups, the most important being not to exceed 1.3 V for the tube filament. Polarity is marked internal to the instrument. These Radiacmeters are popular with Survivalist/Militia types and there are battery mods using lithium cells detailed on their websites.

Proud Mary

Sun Apr 11 2010, 09:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

MinorityCarrier wrote ...

Here's what the batteries look like. I've powered the instruments up in the past with hokey not-very-portable battery setups, the most important being not to exceed 1.3 V for the tube filament. Polarity is marked internal to the instrument. These Radiacmeters are popular with Survivalist/Militia types and there are battery mods using lithium cells detailed on their websites.

Unless a nuclear catastrophe happens conveniently close at hand, you'll never get a reading on a very insensitive meter of this type from background or low level sources.

You might try using a DC amplifier of high input impedance across the meter take-off points - say a differential FET amplifier with selectable gains of 10X and 100X - and see whether the total drift is then acceptable or not, and whether it can be improved upon.

But all in all this is really an historical curiousity rather than an instrument that can be readily converted into a useful instrument for prospecting and amateur experiments.

If you have a number of them, I'd suggest polishing them up, and selling them on ebay so you can buy something useful and relevant to our own time.

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