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Neutron activation analysis (NAA)
Irradiation channels in the reactor pool of the FRM II. RPA: rabbit system; KBA: capsule irradiation system; KBA-1: finishing line. The single fuel element (reactor core) is placed in the central channel about 4 meters under the platform. © Xiaosong Li (FRM II/TUM)
When performing NAA, the sample is irradiated with neutrons; radioisotopes of the elements are formed (i. e. the so-called activation). The activated samples are then measured with gamma-ray detectors. From the activities, the masses of the elements can be determined.
For a routine NAA, samples with the masses of 1–100 mg are packed in polyethylene bags or sealed in quartz ampoules and irradiated in one of the following channels (Fig. 1) with different neutron fluxes (Tab. 1).
1) For irradiations samples are then packed in a polyethylene transport container (called rabbit, fig. 2 left), usually and sent into the irradiation positions near the reactor core with thermal neutron fluxes up to 4 · 1013 cm-2s-1 using pneumatic rabbit system (RPA). (The transport medium here is CO2 gas). Depending on the analytical task, the irradiation times are usually chosen between 1 minute and 2 hours. After irradiation, the samples are transported to the counting lab (fig. 3). The typical transport time on the rabbit system is about 6 – 7 minutes. This still allows for the determination of short-lived nuclides, such as 28Al, 52V, 51Ti, 27Mg etc.
Left: a PE rabbit (white) for the pneumatic system (RPA) with a few typical samples and an over-capsule (black); right: Al-rabbit for the capsule irradiation system (KBA). © Xiaosong Li (FRM II/TUM)
2) For long irradiations of several hours up to days, the hydraulic capsule irradiation system (KBA) can be used. Samples are sealed in a transport container made of aluminium (fig. 2 right) before the irradiation, because the pool water is used as the transport medium here. Due to the higher radioactivity of the transport container after long irradiations, cooling times of one or two days are usually needed. In some special cases, the container can be opened in a hot cell after irradiated. It is suitable for the analysis of long-lived radionuclides with very low concentrations in the samples.
Gamma ray spectra of a typical NAA sample with different cooling times of 1 hour (red) and 1 day (black) © TUM
3) The “fishing line” position (SDA-1) can be also chosen for some special cases, alternatively, e. g. when samples should be cooled with pool water during the irradiation. The maximal irradiation time is limited under 1 h.
The activated samples are counted in a laboratory equipped with high-purity germanium (HPGe) detectors. Normally, the samples are measured two or three times after the irradiation. Nuclides with different half-lives can be detected fairly separated when optimising cooling and counting times (Fig. 4).
A flux monitor (a gold standard) is packed and irradiated together with the samples. From the activity of the gold, one can determine the neutron flux at the irradiation position. Thanks to the highly thermalized neutron field with thermal-to-epithermal flux ratios of more than 3000 in most irradiation channels, the reactions induced by epithermal neutrons can be neglected, and thus the analysis becomes simpler and more reliable .
3 high-purity Germanium (HPGe) detectors of different types are available for the gamma-counting in the NAA-lab (fig. 5).
The detectors are placed in Pb shielding and cooled with liquid nitrogen. The detector shielding suppresses the background radiation from the environment. A lining of copper and acrylic glass on the lead walls helps to shield against the X-rays induced by the gamma radiation in the lead. All lead walls have a thickness of 10 cm.
For the sample analysis, the k0 method is mainly used . The software GENIE 2000 (Caberra/Mirion) is used for counting. The gamma spectra are evaluated using HyperLab software, and the peak lists are processed using Kayzero for Windows to obtain the elemental masses.
NAA is non-destructive. Up to 50 elements can be determined in a sample from the major components down to the trace elements in ppm (parts per million) and even to sub-ppt (part per 1012) level (Fig.6). NAA is unique for the trace-element analysis of the rare-earth elements (REEs) and several other metals (e.g. Hf, Ir, Au etc.).
The detection limits also depend on the composition of the samples. Generally, light elements (Z<11) and a few other ones (like P, Pb) can not be analysed with NAA or just with low sensitivities (Fig. 7). Here, PGAA can give better results.
 Lin, X, Baumgärtner, F., Li, X., 1996, The program „MULTINAA“ for various standardization methods in neutron activation analysis”, J. Radioanal. Nucl. Chem. Art. 215 (1996) 179.
 Li, X., Lierse von Gostomski, Ch., 2014, „Applications of k0 NAA at FRM II with high f values”, J Radioanal Nucl Chem (2014) 300:457–463 DOI: 10.1007/s10967-014-3072-7.
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