[ RadSafe ] Calibration of handheld rad meters.
GEOelectronics at netscape.com
Thu Mar 6 21:06:40 CST 2008
Calibration can be done one of two major ways.
First, a ratemeter or scaler that has a removable probe and is used with different probes *must* be calibrated with an electronic pulse generator. In this way the CPM readouts will be accurately reported on the meter. If the meter scale also has an mR/H scale, the conversion factor *must* be correct for that type of probe. An end -window probe uses a different scale than a pancake probe, and a scintillator is different yet.
Ludlum's meters are easily exchanged so that the scale matches the probe. At NLNL we stock scales for all the different probes and meters. A typical rad lab task.
A second specific type of calibration is required in the case of ion-chambered instruments (non pulse forming), and true Geiger Counter, that is an instrument with a permanent probe attached (non removable). In these cases, a radiation range must be used and the set is calibrated to the radiation flux. Here we are calibrating the mR/H scales, not the CPM scale. Nothing would keep a lab tech from rad range calibrating a meter with a removable probe, such as a Ludlum Model 3/44-6, but the cal sticker should marry that meter to that probe alone. An exception is if a GM probe is permanently attached, it may be removed in the test lab and the meter then pulse calibrated.
Radiation range sources, almost always Cs-137 must be in the higher milliCurie level, so are by license only. LND is using gamma sensitivity figures normalized to Co-60, while Ludlum's who uses LND tube in their probes, rate those probes normalized against Cs-137. Both these isotopes are on the flat part of the energy response curve of the LND 725/ M44-6, so either method gives the same multiplication factor. Other probes show a difference in the two isotope's energies.
During either calibration regiment, the probe being used must be tested for
"High Voltage Plateau", and the metering units High Voltage control set to the correct level for that probe. Many GM probes are designed around 900V and most will work well at 900 +/- 20. I personally use 920 V when appropriate. Scintillator probes are set up per the usage; the HV, the input Lower Level Discriminator and the Upper Level Discriminator are set according to the need at hand.
Some meters have the capability of either automatically adjusting these parameters ("smart" meters and probes), while others have a manual switch to select from a few or even several different pre-established setups.
Finally the metering units input sensitivity must be adjusted for the type of probe being used. A proportional neutron probe would typically require 1680 Volts HV and 2 mV sensitivity. A GM tube would need 900 V and several 100's of mV to exclude double pulsing problems. Scintillators are set up to meet many different circumstances, no two of which are the same.
All GM probes and almost all scintillator probes respond differently according to the energy level of the radiation being detected. In almost every case, the gamma response of probes is standardized on the response to Cs-137, and it is understood that the response to different isotopes will be (maybe vastly) different.
When a GM probe is to be used on a specific isotope, say a pancake used only for I-131, the meter may be calibrated taking into account the energy level of I-131 vs. Cs-137, but the meter must be clearly marked as such.
After all that calibrating the actual operator must be made aware of "deadtime" issues in the probe being used and taught to make corrections as necessary.
With few exceptions, one must know WHAT he is measuring before the trustworthiness of that measurement can be determined. At the same time, the selection of an advantageous probe for one isotope would be contraindicated for another isotope.
There are a few advanced systems, which self calibrate in the field,
called "stabilized'. These are scintillator based and use a feedback loop to adjust the system gain to correct for (mainly) temperature drift in the probe. Some probes have Am-241 embedded into the crystal, some have Cs-137 in the battery charger base, others attach Ba-133 to the crystal, and yet others have a lamp (LED) embedded into the crystal. I've even worked with meters that only work with Bi-207 check sources! The various manufacturers use any number of schemes.
To address what has been called "functionality testing" with a "check source": Generally a good idea, but it won't always work. An example is a Cs-137 check source and a system set up with a window for Co-60 only. Such a meter won't register the Cs-137. This same system will function perfectly with Co-60.
New London Nucleonics Lab
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