QCYK Calibrations - Practical Gamma-Ray Spectrometry

QCYK Reference material - Calibrations

Energy calibration
This is a straightforward energy calibration using the 13 major gamma-rays of the 12 nuclides available, with the exception of the 514 keV gamma-ray of ⁸⁵Sr. The latter is is overlapped by the annihilation peak and, unless the analysis software recognizes its Doppler broadening, deconvolution will be in error.
For reasons known only to ORTEC, GammaVision can only fit a quadratic to energy calibration data. In this particular calibration the root-mean-square difference between measured and fitted energies is 0.024 keV.

FWHM (width) calibration
This calibration was performed simultaneously with the above energy calibration. Again GammaVision insists on a quadratic fit. As I demonstrate in Chapter 6, a square root quadratic fit would be more accurate. The root-mean-square difference between measured and fitted peak widths is 0.016 keV. Note, however, that these points were derived from very well defined peaks. Even so, there is more scatter about the best fit line than was the case with the energy calibration. The uncertainty on the FWHM of poorly defined peaks would be much larger.

FEP Efficiency calibration
This calibration, performed by GammaVision using a linear function above and quadratic below the knee, set at 150 keV. In this calibration, the 136.47 keV peak of ⁵⁷Co is included to assist with fitting below the knee. In this diagram the red line is the fitted function, the blue line is a linear regression using only the nuclides not subject to true coincindence summing (TCS). This can be taken as the true efficiency fit. Although ⁵⁴Mn and ⁶⁵Zn are both electron capture nuclides and are expected to sum, their low energy X-rays are absorbed before reaching the active region of the detector. ¹⁰⁹Cd and ¹¹³In are again electron capture nuclides, but their gamma-rays are emitted by isomeric daughter states some time after the decay itself. Once again, they show no summing.

The following features are notable:

The red fitted line is below the true (blue) line. This is because of TCS of the ⁶⁰Co and ⁸⁸Y gamma-rays, in particular. The ¹³⁹Ce point is considerably lower than it should be for the same reason.
The region below the 160 keV knee is problematic because, although ²⁴¹Am and ¹⁰⁹Cd are not subject to summing, ⁵⁷Co is, with the 122.01 keV peak summing out and the 137.47 keV summing in. The fact that the latter appears to be on the true calibration line is due to chance.
Using the red, best fit, line will give inaccurate results for samples measured in this geometry for all nuclides except those that happen to have an intermediate degree of summing, which happen to fit the line, such as ⁶⁰Co apparently. Nuclides that are not subject to summing will be overestimated, those subject to large degrees of summing will be underestimated.
Even if only non-summed nuclide points are used for the efficiency calibration, it will be useless for the majority of nuclides that are subject to TCS. Some means of correcting for TCS is essential if close geometry measurements are made.

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