*Geiger Counter Calibration*
By: wmerrin
30 January 2019

Since the subject comes up when dealing with used or surplus equipment I thought I would post a few of my opinions about calibration. My thoughts are based on my needs as an individual consumer; I am not a health physicist and I don't deal with radiation equipment for groups. If you are/do, then you already have your own opinions. Specialists for military and commercial calibration organizations have access to radiation standard sources and fixtures to ensure equipment is operating per specification. Consumers in the low-end new and used commercial equipment market usually do not have access (or budget) for this type of professional calibration.

My typical uses do not require calibration certificates traceable back to some standards organization. That means - for my purposes - if a digital counter counts accurately and if the GM tube is functioning within manufacturer's specs the counter should be reading within its expected accuracy. Health physics professionals making life-safety decisions require a higher level of traceability to defend their decisions. That's not me and never will be; deciding if/when something is "interesting" or I should take precautionary measures does not fall into the same category as making decisions which impact the health and safety of other people who may be unknown to me.

Modern digital Geiger counters do not calculate and display radiation levels the same way as analog meters. Digital meters literally count the number of radiation events the Geiger-Mueller (GM) tube detects within a certain elapsed time and then, typically, use a microprocessor to calculate the radiation level based on the number of counts per elapsed time and the GM tube's known reaction to a specified radiation type. This is how some digital units allow switching the display among counts per second (CPS) / counts per minute (CPM) / R (Roentgen) and Sv (Sievert) units.

Analog Geiger counters such as the CD-V700 do not operate this way; they are much simpler and "add up" the events, usually using a simple resistor-capacitor network. You can see this on the display during low level background conditions. The meter will show more or less zero, then a couple of hits happen and the needle moves upscale before slowly discharging back to zeroish until another hit or three happens. Consequently, the needle bounces back and forth and has an erratic appearance. The hotter the source the longer the needle stays upscale. Smaller GM tubes have lower sensitivity and are worse about this than larger, more sensitive, tubes because they experience fewer triggers per unit time.

For my purposes digital counters can be satisfactorily calibrated - maybe verified would be a better term - by introducing simulated GM tube pulses from a gadget called a Pulser. The counter is working properly if the CPS/CPM displayed by the counter matches the CPS/CPM being delivered by the Pulser. If the unit responds as expected to a known source - even a hot rock if a true test source is not available - then the GM tube is functional and the counter should be reasonably close to its normal accuracy, whatever that might be.

Digital counters tend to hold their accuracy reasonably well and (for my needs) fall into a pass/fail category. If the counter reads known pulser input correctly and the GM tube responds appropriately to a source I can satisfy myself the unit is operating within the limits of my needs. That would not be good enough for OSHA or a state health department but that isn't my concern.

Analog counters with a CPS/CPM scale can be calibrated by feeding them pulser data and adjusting until the meter reads correctly. If the fixed dial scale is calibrated in R or Sv units instead of CPS/CPM it is referenced to a specific radioisotope, usually Cesium 137 or Cobalt 60. Knowing the GM tube manufacturer and type allows correlating scale units to pulse counts. Different isotopes have different gamma energies and have different impacts on the GM tube. GM tube manufacturers rate their tube's sensitivity to a standard isotope as part of the tube's specifications.

A major concern with cabled external probes and units calibrated in R or Sv: if the original probe is replaced by a different type GM tube calibration is no longer valid and the error may be significant. Any unit other than CPS/CPM is only valid for the original GM tube. Never trust someone else's data unless they specify the measurement conditions.

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