Y-activity of radioisotopes:

Activity (A) is measured in terms of peak area after subtracting background. γ-activity of radioisotopes is measured using an HPGe detector though it could also be measured by using a scintillation detector. Thus, concentration of an element may be determined by rearranging the left part of Eq. (13.8) as;

w =AM/i N Aσ φ (i - e - λti ) e - λtd ε

This way of calculating the concentration of an element is called the Absolute Method, where σ, φ , t_i, t_d and ε are the experimentally determined parameters. After calculating w, the concentration of an element in the sample, its mass may be determined. However, the absolute method has uncertainties in the values of φ and σ. It is well known that these may vary under experimental conditions depending on the location of the sample in the reactor or irradiation can. In order to eliminate these uncertainties, a Comparator Method has been suggested where a standard (primary and/or secondary) with known concentration of element is simultaneously irradiated along with the sample under similar experimental conditions. Both are counted using the same detection system in the same geometry. Thus, an equation similar to Eq.13.8 can be written for the activity of standard ( A′ ) as;

A' = ( w' iN_A/M)  σ φ (1 - e ^{- λti} ) e ^{- λtd} ′ ε γ = ( w' iN_A/M)  σ φ S D' C ε γ

Dividing LHS of Eq. 13.8 through Eq. 13.10, we get

A/ A' = w/w'× e ^{- λ td}/e ^{- λtd′}                                                                                           

This is applicable only if t1/2 of the radionuclide is comparable with delay time (td) and counting time (tc). However, if t1/2 of the radionuclide is long enough compared to delay and counting times then, it could be additional simplified to

Conc.of element in sample(S_a ) = Conc.of element in s tan dard (S_t ) × Activity in sample/ Activity in s tan dard

When many elements are to be determined in each sample, preparation of individual standards for each element becomes tedious and impractical. Some workers use synthetic multielemental standard which are difficult to prepare and have its own limitations. Thus, either multielemental standards or the use of a single comparator for all the elements becomes necessary. Within Eq. 13.12, activity is replaced through specific activity to have the uniformity while considering masses of the sample and the comparator standard. An standard/certified reference materials (SRM/CRM) are available from several agencies like as the National Institute of Standards and Technology (NIST, USA), International Atomic Energy Agency (IAEA, Vienna), National Institute of Environmental Studies (NIES, Japan), Institute of Nuclear Chemistry and Technology (INCT, Poland), wherefrom RMs of a variety of matrices such as plant leaves, tissue or dust particulate soils and rock powder are available. Therefore, these are quite expensive. Thus, several laboratories prepare their own standards by spiking with µg amounts of standard aq. solutions prepared from AR/GR/HP grade salts on to SiO₂/cellulose or Whatman filter paper strip.