**5. Data acquisition**

In order to offer a quality service, the laboratory has made important updates in its instrumentation; acquiring four state-of-the-art HPGe Canberra Detectors for the measurement of gamma radiation and a novel sample positioning system.

The NAA relative method uses gamma radiation emitted by the radioactive nuclei from activated samples and compares it to the radiation emitted by a reference material with similar characteristics.

The characteristic gamma energies of each radionuclide are measured using one of the four solid-state semiconductor detectors (GC-3018 and GC-7020) coupled to LYNX® digital signal analysers controlled by the Canberra's Genie 2000 (v3.3) software. Each of these systems is calibrated weekly in energy, and its efficiency is checked monthly using a gamma check source kit consisting of <sup>241</sup>Am, 22Na, <sup>133</sup>Ba, <sup>137</sup>Cs, <sup>155</sup>Eu and <sup>60</sup>Co electro-deposited point sources.

Depending on the irradiation, system used (**Table 1**), and once pre-determined decay times are reached, radiation measurement is performed by using one of the HPGe detectors [19].

Radiation measurement by samples coming from the pneumatic system is performed in some of the less efficient detectors (GC-1020 or GC-3018, depending upon availability). For complete analysis, two readings are carried out: the first, after 5 min of decay, positioning the vials individually at a distance of 50 mm from the detector and reading the corresponding spectra for 5 min. The second reading is done after the activity decays for an hour, at a distance of 10 mm and the spectra is read for 10 min. On the other hand, samples not going through the pneumatic transfer system are analyzed as follows: a first reading is done after a 4-day decay on one of the GC-3018 detectors, positioning the vials individually at a distance of 30 mm from the detector and reading the spectra for 3 h. A second reading is then performed after 21 days of decay on the higher efficiency GC-7020 detectors at a distance of 15 mm and the spectra is read for 4 h.

The elemental determination in the NAA is done by using the relative calibration (direct comparator method) [20]. This method uses a sample with known mass of the elements of interest (comparator or standard) and an unknown sample which is irradiated simultaneously. Taking into account that the amount of radiation emitted from the activation of the sample is proportional to the neutron flux, and this in turn to the mass of the irradiated element, it is found that the ratio between the mass fraction of the element x and the amounts of influence

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**Figure 26.** Gamma spectra from soil sample irradiated for 4 h at 30 kW and a 5-day decay.

is given by:

**Figure 25.** Detection geometry for check source verification.

Flux monitors used during sample activation for flux corrections are also analyzed by gamma spectrometry on one of the GC-3018 detectors at a distance of 50 mm for 60 min. The samples obtained its corresponding gamma spectrum (**Figures 25** and **26**).

Colombian Neutron Activation Analysis Laboratory (CNAAL): Applications and Development… http://dx.doi.org/10.5772/intechopen.74395 47

**Figure 25.** Detection geometry for check source verification.

The point-to-point adjustment option must be used to change the counting geometry on the detectors, which must be performed prior to the execution of the sequences as required by

This positioning system greatly reduces manual efforts during the analysis of radioactive samples; the only manipulation required by our staff is the setup of the 64 samples in the rack. The idea of the use of this rack is minimizing Radiation Exposure, and thus enhancing

In order to offer a quality service, the laboratory has made important updates in its instrumentation; acquiring four state-of-the-art HPGe Canberra Detectors for the measurement of

The NAA relative method uses gamma radiation emitted by the radioactive nuclei from activated samples and compares it to the radiation emitted by a reference material with similar

The characteristic gamma energies of each radionuclide are measured using one of the four solid-state semiconductor detectors (GC-3018 and GC-7020) coupled to LYNX® digital signal analysers controlled by the Canberra's Genie 2000 (v3.3) software. Each of these systems is calibrated weekly in energy, and its efficiency is checked monthly using a gamma check source kit consisting of <sup>241</sup>Am, 22Na, <sup>133</sup>Ba, <sup>137</sup>Cs, <sup>155</sup>Eu and <sup>60</sup>Co electro-deposited point

Depending on the irradiation, system used (**Table 1**), and once pre-determined decay times are reached, radiation measurement is performed by using one of the HPGe detectors [19].

Radiation measurement by samples coming from the pneumatic system is performed in some of the less efficient detectors (GC-1020 or GC-3018, depending upon availability). For complete analysis, two readings are carried out: the first, after 5 min of decay, positioning the vials individually at a distance of 50 mm from the detector and reading the corresponding spectra for 5 min. The second reading is done after the activity decays for an hour, at a distance of 10 mm and the spectra is read for 10 min. On the other hand, samples not going through the pneumatic transfer system are analyzed as follows: a first reading is done after a 4-day decay on one of the GC-3018 detectors, positioning the vials individually at a distance of 30 mm from the detector and reading the spectra for 3 h. A second reading is then performed after 21 days of decay on the higher efficiency GC-7020 detectors at a distance of 15 mm and the

Flux monitors used during sample activation for flux corrections are also analyzed by gamma spectrometry on one of the GC-3018 detectors at a distance of 50 mm for 60 min. The samples

obtained its corresponding gamma spectrum (**Figures 25** and **26**).

the operators.

the safety and well-being of personnel.

46 Advanced Technologies and Applications of Neutron Activation Analysis

gamma radiation and a novel sample positioning system.

**5. Data acquisition**

characteristics.

sources.

spectra is read for 4 h.

**Figure 26.** Gamma spectra from soil sample irradiated for 4 h at 30 kW and a 5-day decay.

The elemental determination in the NAA is done by using the relative calibration (direct comparator method) [20]. This method uses a sample with known mass of the elements of interest (comparator or standard) and an unknown sample which is irradiated simultaneously. Taking into account that the amount of radiation emitted from the activation of the sample is proportional to the neutron flux, and this in turn to the mass of the irradiated element, it is found that the ratio between the mass fraction of the element x and the amounts of influence is given by:

The subscripts indicate parameters for the unknown sample and the comparator or standard. That is, the mass fraction of the unknown sample of the element (measurand) and the mass fraction of the element in the reference material. W is the total mass of the samples. The number of net counts of the energy of interest (keV) and the counting time of the gamma radiation are decay correction factors of the peak; this factors are used to obtain the neutron flux correction factor, which quantify the gradient of the flux between the irradiation position of the sample and the comparator.

Radioactive waste is discharged as conventional waste only when its activity reaches acceptable levels established by national regulations, with previous knowledge and consent of the Regulatory Authority. Procedures for the Management of Radioactive Waste are lined up with technical and administrative requirements established by national regulations. The following schematic allows for the safe management of radioactive samples and activated materials

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**a.** Minimization: Only materials and samples directly linked to national projects are irradiated in order to generate useful information for the economic and social development of

**b.** Segregation: Separating waste generated during short and long irradiations and that col-

**d.** Classification: By activity concentration and half-lives of nuclides present in the sample. **e.** Characterization: Classification and monitoring of temporarily store waste are performed by Gamma Spectrometric analysis of the packages generated each workweek. Every package is analyzed separately in Canberra HPGe GC-7020 detectors, at a reading geometry of 10 mm during 4 h, these reading are then analyzed and the activity of each radionuclide

**f.** Storage: The NAA Lab has a decay room for the temporary storage of VSLW, equipped with the necessary shielding and equipment for its safe handling. The room is locked and permanently monitored, admission is restricted to non-operating personnel, unless authorized otherwise. The stored waste is properly labeled and grouped into packages

As part of the validation process for NAA using HPGe detectors and future accreditation of the laboratory under ISO/IEC 17025:2005 [21], the technique has been validated for the determination of rare earths such as La and Ce, and elements of interest such as U and Th in geological matrices. The following parameters were taken into account: selectivity, linearity, reproducibility, limits of detection and quantification, robustness and uncertainty

This process included the evaluation of detection limits and quantification of gamma radiation spectra obtained, according to the statistical criterion of Currie [22]. The following results show element concentrations in the sample in units of mg/kg. Ba: 129, Ce: 1.37, Co: 0.20, Cs: 0.29, La: 0.11, Rb: 10.06, Sb: 0.054, Sc: 0.024, Th: 0.27, U: 0.2. These results were comparable to those reported by other laboratories, thus demonstrating the competence of the NAA

Laboratory on multielemental analysis in geological matrices.

**c.** Pre-treatment: Waste package preparation, 16–20 samples per package.

generated during practice.

present is determined.

each workweek.

**7. Quality assurance**

estimation.

lected during decontamination activities.

the country.

The neutron flux correction factor is determined as the ratio between the flux measured with the 0.1% Au-Al flux monitors at the sample position and that of the comparator. The neutron flux is proportional to the number of counts in energy range of 198Au and depends on other factors such as the neutron capture cross-section, isotope abundance, irradiation time, 198Au half-life, detection efficiency, and number of accounts for the emission energy of the radioactive isotope. Taking the relationship between the flux readings, all the terms except for the number of counts registered for the monitor at the sample position and for the monitor at the comparators position are canceled, obtaining:

This relationship is fulfilled, assuming that the irradiation conditions and counting geometries are similar.
