Neutron activation analysis

Neutron activation analysis ( NAA) is a nuclear- physical method for the quantitative analysis of the elemental or isotopic composition of samples that are irradiated with neutrons to. The nuclei to be determined ( Analytkerne ) of the sample interact with the interacting neutrons and can, depending on the type of nuclear reaction, various products are formed. This operation, which is performed in a research reactor, or by means of another neutron source is neutron activation. The resulting activation products can be radioactive and then decay with their characteristic half-life. Both activation and the decay characteristic of radiation energy is also released, which is used for analysis.

Expiration

Most neutron activation analysis carried out in four phases.

Radiotherapy

Most commonly be used for thermal neutron activation. Most target nuclei have for neutrons of this energy range to cheap capture cross sections and hence provide an easily measurable yield of activation products. It can also be activated by means of epithermal or fast neutrons.

Decay

Through the duration of the decay phase can be controlled if one prefers wants to measure activation products with long or short half-lives. By suitable choice of activation and decay spectral interferences can be eliminated in many cases.

Measurement phase

By means of gamma spectrometry is recorded a gamma spectrum of the irradiated sample. From the position of the peak-shaped signals, the nature of the existing activation products can be read. The level of the signals is related to the amount. An internal standard or calibration number can be concluded that the initial amount of analyte.

Analysis phase

Through computer will now analyze the information obtained and converted on mathematical algorithms in graph.

Variants

There are several variants of the neutron activation analysis:

• Prompt gamma activation analysis ( NAA PG or PGAA ): Here, the energies of the intended during the recording of neutrons in the nucleus freigetzten so-called prompt gamma rays and their relative intensities are measured. Use specific measuring facilities are required that allow the simultaneous neutron irradiation and gamma ray measurement.
• Instrumental neutron activation analysis ( INAA ): The classical non-destructive neutron activation analysis, in which the activity of the radionuclides produced is measured. This method allows activation in a reactor and subsequent analysis at a different location.
• Radiochemical neutron activation analysis ( RNAA ): Here, a chemical separation, before or after irradiation. Characterized the activity of other elements are excluded from the measurement. The method is thus very sensitive to isotopic low concentration.
• Delayed neutron activation analysis ( DnaA delayed eng. ): Measurement of delayed neutrons to be released after a nuclear reaction of the still excited nuclei. The method is mainly used for analysis of fissile material.

Importance and performance

Neutron activation analysis allows the quantitative determination of elements in very different types of sample materials that would partially unlock only with considerable effort ( rocks, alloys ), or their destruction would be undesirable ( archaeological artifacts and works of art). In the latter, the advantage comes into play that the NAA can find the smallest sample quantities suffice. For many elements (eg, gold and arsenic), the detection of minute traces. The excellent performance of the neutron activation analysis comes about when that is not necessarily a sample preparation ( with the attendant risk of contamination) is required.

Paintings can also be examined by a Neutronenautoradiografie, a variant of the neutron activation analysis.

A disadvantage is the high instrumental complexity. The highest performance is achieved only at high neutron flux densities. This requires a research reactor are available. Smaller neutron sources are much easier to handle. But they are limited in their performance parameters, but nevertheless suitable for many studies. Another disadvantage is that depending on the irradiation and the composition of the sample for longer time, a residual activity can be maintained.

At a hair sample of Napoléon, taken the day after his death (May 5, 1821), was founded in 1961 detected by neutron activation analysis, that he was gradually poisoned with arsenic. By sections the analysis of a 13 cm long streak, could, according to the natural growth of hair, show that he was administered during a one-year phase arsenic intermittently and at what times this happened. However, there are doubts about this theory.

In archeology, the NAA is mainly used to determine the origin of raw materials.