5.4 Uncertainty and accuracy of results
While laboratory analysis has the advantages of reproducing the same conditions for the measurement of the unknown samples and the calibration standards and attaining a better uncertainty for any individual result, it has the risk of not having analyzed a sample representative of the problem under study.
In-situ techniques, although measuring a signal produced by a larger sample, are prone to differences in measurement geometry and other effects arising from uncontrolled environmental conditions. Variations in temperature and humidity may induce instabilities in the operation of portable instruments, thus producing alterations on the measurement. For example, shift in positional peaks in gamma spectra might be caused by drifts in the operational parameters of the excitation source, the pulse processing unit and the multi-channel analyzer (MCA).
The calibration of portable detectors is made upon assumptions of flat surface and constant depth distribution of the activity, and consequently remains only as a relative measure and not as an absolute, traceable one. Rugged terrain can lead to significant departures from assumed measurement geometry, and the depth activity distribution may vary from site to site. Vegetation and other absorbers such as snow cap or leaf litter on forest floors can significantly attenuate radiation.
Additionally, most of the dose rate and gamma measurements performed with portable devices rely on the detection of high energy radiation which originates from daughter products from uranium and thorium series and do not necessarily reflect whether equilibrium exist within the different radioisotopes in a given series.