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Reference, Questions and Help => Nuke Q&A => Instrument Q&A => Topic started by: fnbrowning on Jul 16, 2012, 09:24

Title: 44-9 Pancake Continuous discharge.
Post by: fnbrowning on Jul 16, 2012, 09:24
Circumstances: Ludlum Model #18 Analyzer with a 44-9 pancake, recently calibrated by Ludlum.
The 44-9 was attached where specified at position one,  and the high voltage read 900V.

A reading was being taken of an isotope that is a Beta/Gamma emitter. There are NO other sources in the area.
The readings of the isotope were mid-scale on the x1000.

After several minutes, the 44-9 detector was moved away from the source, but the counts did not drop.
I moved a significant distance from any source, and pushed the RESET on the Analyzer, but the system did not respond.
Shutting the instrument OFF dropped the needle.

Returning to the source, and after a short time, it happened a second time. The counts remained steady and the instrument
needle wasn't moving even as the detector was moved quite away from the source.  This time the 44-9 was examined closely,
and a faint continuous discharge (high pitch buzzing) was heard from the G-M tube.  The system was quickly turned OFF.

I might note that a Rad list member sold the 44-9 to me in April, and he represented it has having been rebuilt with a NOS 7311 tube.

A quick check after the incidents showed the 44-9 to responded within the proper parameters to the instrument Cs-137 check source,
and another known good check source.

Question (1): How common is a coronal or continuous 'runaway' discharge within a detector of the 44-9 series?

Question (2): This having happened twice, is the 44-9 now damaged, or has the continuous discharge drastically shortened the life of the G-M tube?

Title: Re: 44-9 Pancake Continuous discharge.
Post by: GLW on Jul 16, 2012, 10:02
not common, and they don't cost much, you'll burn more money with your hourly rate troubleshooting it then you will tossing the tube and getting another,...

organic quench gas depletion (it should be halogen, but maybe it's a real old tube, mislabeled tube, or bad QC on the manufacture date),...

bad detector resistor or poor connection insulation,...

high voltage transformer in the meter is going fubar,...

just to get you started,...