NAA

Figure 1: Irradiation channels in the reactor pool Fig. 1: Irradiation channels in the reactor pool of the FRM II. RPA: rabbit system; KBA: capsule irradiation system; KBA-1: finishing line. The fuel element is placed in the central channel about 4 meters below the platform.

Fig. 1: Irradiation channels in the reactor pool of the FRM II. RPA: rabbit system; KBA: capsule irradiation system; KBA-1: finishing line. The fuel element is placed in the central channel about 4 meters below the platform.

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 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).

Tab. 1 Tab. 1: Neutron fluxes at irradiation positions

Tab. 1: Neutron fluxes at irradiation positions

1) For irradiations, samples are then packed in a polyethylene transport container (called rabbit, fig. 2 left) and sent into the irradiation positions near the reactor core with thermal neutron fluxes up to 4 × 10¹³ n cm^-2s^-1 using a pneumatic rabbit system (RPA). (The transport medium here is CO₂ gas). Depending on the analytical task, the irradiation times are usually chosen between one minute and two hours. After irradiation, the samples are transported to the counting lab. This is located on the premises of the nearby institute Radiochemistry Munich (RCM). The typical transport time on the rabbit system is about 6 – 7 minutes. This still allows for determining short-lived nuclides, such as ²⁸Al, ⁵²V, ⁵¹Ti, ²⁷Mg, etc.

Figure 2: RPA and KBA Fig. 2: 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).

Fig. 2: 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).

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 analysing long-lived radionuclides with very low concentrations in the samples.

Figure 3: Gamma ray spectra Figure 3: Gamma ray spectra of a typical NAA sample with different cooling times of 1 hour (red) and 1 day (black)

Figure 3: Gamma ray spectra of a typical NAA sample with different cooling times of 1 hour (red) and 1 day (black)

3) The “fishing line” position (SDA-1) can also be 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 to under one hour.

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 separately when optimising cooling and counting times (fig. 3).

A flux monitor (a gold standard) is packed and irradiated with the samples. From the activity of the gold, one can determine the neutron flux at the irradiation position. Thanks to the highly thermalised 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 [1].

[1] Lin, X. et al., J. Radioanal. Nucl. Chem. Art. 215, 179 (1996).