What is Neutron Activation Analysis (NAA)?

Neutron Activation Analysis (NAA) is a sensitive multi-element analytical technique used for both qualitative and quantitative determination of elemental concentrations in a material. NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions.

To carry out an NAA analysis the sample, is placed into a suitable irradiation facility and bombarded with neutrons; this creates artificial radioisotopes of the elements present (Fig. 1). Following irradiation the artificial radioisotopes decay via the emission of particles or more importantly Gamma-rays, which are characteristic of the isotopes from which they were emitted. (Fig. 2)

How to do an NAA at FRM II?

The samples are normally, maybe of only several milligrams and without any chemical preparations, packed in a transport container (called rabbit) and irradiated at FRM-II usually in a pneumatic or a hydraulic rabbit system RPA/KBA with a typical thermal neutron flux up to 1.3E14 cm^-2s^-1.

The typical sample transport time from the irradiation position to the counting place is about 4~5 minutes on the rabbit systems at FRM II. Thus, it is under good conditions still possible to determine some short-lived isotopes, such as Al-28, V-52, Ti-51 etc. Technically, the transport time can be reduced down to 10 seconds. For this case, a special administrative permit is needed.

The sample can be measured directly in the reactor hall on a high sensitive Ge-detector after a suitable cooling time and sample handling in a glove box. Canberra Lynx digital signal analyzer and genie 2000 software are installed in the counting system. For the multi-element analysis of spectra, a homemade software package names MULTINAA operating on VMS/VAX system is used. It offers beside the k0-method also other three different standard analysis methods namely relative, flux, and mono-standard methods in one calculation procedure. Almost all usual decay schemata and coincidence corrections are included. ISOCS calibration software is used for the efficiency calculation of samples with different geometries.

Advantages of the NAA

Up to 30 or even 40 elements can be determined simultaneously. The method operates over a very wide concentration range, from the % range down to the ppt (parts per tillion e.g. 1 part in 10¹²) and sub-ppt range (Fig.3). NAA is in an unbeatable position on analysis of the rear earth elements (REE) and other metal elements in the low-level measurements. NAA can perform non-destructive analyses with no or minimal preparation. The great advantage of the multi-element analysis is that elements, which were not excepted before, are also detected. In many cases, this has been of practical meaning: especially in scientific applications, the presence of such unexpected trace elements often has enormous consequences.

Applications

Its Application can be found in many different areas. In the semi-conductor industry, NAA is used to determine the trace elements in the very pure silicon. In the Archaeology, NAA can give useful information about the origin of the findings according to the so-called "fingerprint" of the individual element composition in their raw materials. Meteorites and moon rocks were analysed by using NAA to determine their element composition. Today, NAA is also widely applied in the life science and environment monitoring.

Fig. 3: Detection limits of some trace elements in high pure silicon

Irradiation facilities at FRM II

For the neutron activation analysis (NAA) at FRM-II, following irradiation facilities can be used for different purposes:

• 6 pneumatic rabbit systems (RPA), for short time and low neutron fluence irradiation of small samples, max. 2E17 /cm²;
• 2 hydraulic irradiation systems with 10 positions, for long time irradiation of a little bit bigger samples, max. 20cm³;
• 1 mechanical position for large samples;
• 1 special position at the bottom of the reactor control rod with maximal fast neutron flux of ca. 4E14 /cm²s, irradiation time: 60d.