When working with gamma spectroscopy it is very important to have a calibration source. Why do you need one? The answer is quite simple: to calibrate your gamma spectrometer prior to acquiring spectra.

Calibration Drift

It is true that most manufacturers (myself included) calibrate their spectrometers before shipping them out to customers. However, for most devices the calibration is not permanent and it may and in all probability will drift with time.

The most common reason for the calibration drift is temperature. Virtually all scintillators – NaI(Tl) included – are temperature dependent. Ambient temperature change on the order of few degrees C will be sufficient to shift your peaks away from their calibrated energies. Therefore most high-end spectrometers have a thermocouple mounted on scintillator and adjust calibration automatically to compensate for the temperature drift. Inexpensive devices such as Theremino-based spectrometers and GAMMA-LITE do not have this feature and therefore require recalibration.

Regardless of temperature compensation, best scientific practices requires calibration check before each measurement regardless of the nature of the measurement. This is necessary because a number of other factors may affect calibrations. These factors (in no particular order) include bias voltage drift, resistance, capacitance and gain drift in the amplifier and filter circuits, humidity (older scintillators may not be perfectly sealed), and even the quality of the RF connectors. Therefore serious enthusiasts and most definitely researcher must have one or more calibration sources.

Commercial Gamma Sources

Fortunately, there is a wide a variety of commercially available gamma sources that one can purchase without breaking a bank. E.g. for one can buy a wide variety of 1-10 uCi sources such as Cs-137, Co-57, Co-60, Ba-133, etc. at Imagesco with each source costing under $300. These sources are made my Spectrum Techniques and also can be ordered directly from the manufacturer via an email or telephone (unfortunately, Spectrum Techniques does not have an online store).

These sources are small plastic discs the size of a U.S. quarter – Fig. 1 – and they can be mailed within the U.S. with proper labeling.

Energy Ranges

Obviously one needs a source (or sources) with calibration energies in the measurement range. E.g. for a range of 30 to 1000 keV one typically uses Cs-137 for it’s main 662 keV line. For a range of 1-2 MeV one needs a Co-60 source for its 1.17 and 1.33 MeV lines. For a range of 10 to 200 keV one may consider a Co-57 source with its 122 keV line. Soft x-rays (1 to 40 keV) typically require Fe-55 source with its 5.9 keV line. Therefore one usually needs multiple calibration sources depending on the gamma energy range being studied.

Multi-Point Calibration

Is a single calibration energy enough? The answer is it depends. In a perfect world a single calibration energy should be enough because most scintillators have a highly linear response. However, practice proves to be very different from theory. Therefore I always recommend using at least two reference energies for calibration. At the very least the second reference energy will allow verifying linearity of the calibration.

Moreover, some spectrometers require two energies to properly calibrate the linear scale because the equation for the straight line y = ax + b has two parameters (a and b), which can be determined only using two distinct pairs of x and y values.

Conclusions

1. One definitely needs one or more calibration sources when acquiring gamma spectra.
2. The spectrometer calibration should be checked before each experiment, and the device has to be recalibrated if necessary.
3. Calibration sources must be chosen such that they provide reference energies in the measurement range in question. This usually means that one has to use different radioisotopes for different energy ranges.
4. Ideally one needs at least two reference energies for calibration to verify linearity or to properly set the linear calibration scale.