Sensitivity and Detection Limits:

The sensitivity of an element in NAA is defined as counts µg^-1 for particular experimental conditions viz. t_i (irradiation), t_d (delay) and t_c (counting) durations besides neutron flux (φ) of the irradiation source, neutron absorption cross section (σ) and counting efficiency (ε) of the detector system. Depending on the nuclear properties such as isotopic abundance (i) and neutron absorption cross section (σ) of the isotope of interest, the above parameters (t_i, t_d and t_c) can be decided. From the activity Eq. (13.8), it is clear that the main parameters for sensitivity are σ, φ and ε. Since σ is fixed for a target nucleus that cannot be changed, high flux neutrons and so also a detector of high efficiency are needed. Because a sample may contain isotopes that are not active, a fraction of the atoms which are radioactive must also be known. Thus, isotopic abundance (i) of the nuclide of interest plays an important role in deciding the detection limits. Rate of production of activity of a radionuclide depends on neutron flux (φ) and the time of irradiation in a nuclear reactor (t_i). For a particular target being irradiated in a neutron device, however, activity will increase exponentially. A comparison of the production of activity within three various neutron devices is described in Figure where X-axis is the irradiation time in terms of half- life of the product and Y-axis is the activity. It is observed that highest activity is obtained for irradiation in a high flux reactor. In NAA detection limit varies from pg to mg depending on the element of interest, gamma ray background, sensitivity and sample matrix.