Neutron Activation Analysis (NAA) (Cont.)
| Comparator Method: | ||
| In this technique, the concentration (at ppm level) of an unknown sample is to compare a standard sample of known concentration. The equation used to calculate the mass of an element of an unknown sample relative to the comparator standard is2 | ||
/equ5_5.png) | ||
| Where A is the activity of the sample (sam) and standard (std), m = mass of the element, λ is the decay constant for the isotope and Td is the decay time. For short irradiation, keeping irradiation, decay and counting times normally same for samples and standards, concentration of the required element in a sample with respect to that of the standard can be written as, | ||
/equ5_6.png) | ||
| where, C is the concentration of any element and W stands for weight of the sample and standard. | ||
| The Non-Relative Method | ||
| Multi element NAA is possible in non-relative method or single comparator method2. In this technique, standards of all elements are co-irradiated each in turn with the single comparator element. Firstly, the calibration factor for all the elements relative to the comparator element are to be determined and then only the comparator element has to be used in routine measurements instead of individual standards for each element. The calibration factors of course are valid for a specific detector, a specific sample shape, a specific counting geometry and a specific irradiation facility. | ||
| Gamma Ray Detection in NAA | ||
| The gamma ray spectrum emitted from an element undergoing activation can consist of gamma ray of few MeV energy, and it may sometimes be extremely complex with some elements having several hundred gamma ray lines. In order to consider the gamma ray detection system to measure gamma rays from neutron activation, the following points should be considered: high detector efficiency, good spectral resolution and count rate. The trade-off between detection efficiency and energy resolution is the main area of uncertainty in the choice of a detection system. The instrumentation used to measure gamma rays from radioactive samples generally consists of a semiconductor, associated electronics and multichannel analyzer. Highly purified germanium (HPGe) detectors, which operate at liquid nitrogen temperature, are also used in many laboratories. Although HPGe detectors come in many different designs and sizes, the most common type of detector is the coaxial detector which are used to measure gamma ray energies over the range from about 60 keV to 3.0 MeV. The schematic of HPGe detector arrangement is given in Fig. m5.2 below: | ||
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FIGURE m5.2 Schematic of HPGe detector arrangement | ||
| For bulk material analysis, a high detection efficiency is required and NaI based scintillation detectors are used. Another important aspect in this analysis process is background subtraction. | ||
| Flowchart of NAA Analysis | ||
| For NAA gamma ray analysis, the steps which are followed are given below: | ||
| Step 1: The gamma ray spectrum of appreciable count is obtained. | ||
| Step 2: All peaks are marked as per their energies. | ||
| Step 3: The isotope corresponding to the highest peak is identified with the help of standard data table. | ||
| Step 4: Next job is to identify whether the isotope can be made by neutron activation using a standard chart3. | ||
| Step 5: Once the conditions of making the isotope are applied, the spectrum with the element (look for other intense peaks are matched, then the element has been obtained). | ||
| Step 6: If the conditions of making the isotope do not apply, or the spectrum does not match the element, then the next most intense peak has to be identified followed by step 4 and 5. This process has to be repeated until an element that matches the spectrum is obtained. | ||