Monitor foils

If the hradiation is long enough to produce saturation activities in both the target and monitor foils, we have... 

Flux densities are determined by means of monitor foils for which the cross sections are well known. Sample and monitor have to be irradiated rmder exactly the same conditions. Sandwich arrangements or stacks of sample and monitor foils are preferred. 

When a relative method is used in CPAA, the ratio of the beam intensity for standard and sample has to be determined experimentally. As knowledge of the absolute beam intensity is not required, it is common practice to cover sample and standard with a thin (much less than the range) Tmonitor foil. The induced activity in the I-monitor foil is then a measure of the beam intensity. Pure metal foils are the obvious choice they are good thermal conductors, monoel-emental, and available in different thicknesses. 

To avoid recoil nuclides from the Tmonitor foil in the sample (or the standard) a recoil foil is inserted between the I-monitor foil and the sample (or the standard). As the range of the recoil nuclides is much lower than the range of the charged particles commonly used in CPAA, a few micrometers of aluminum foil is sufficient to stop the recoil nuclides completely, while the energy of the charged particles is reduced by a minor (but not negligible) fraction. 

A basic requirement for accurate NAA is the correct characterization of the irradiation facility (Becker 1987). Local and temporal neutron flux density gradients as well as gradients in the neutron energy spectrum of the irradiation position must be well understood and known for each irradiation. O Figure 30.5 illustrates the gradients in an irradiation capsule as measured by flux monitor foils. A difference of 1 mm in sample positioning will result in a 0.6% relative difference in the measured concentration. 

An indirect method for the determination of the beam intensity makes use of a beam intensity monitor foil. During the irradiation a thin metal foil is placed before the sample and the standard. The induced activity in the foil is a measure for the beam intensity. When a standard (S) and a sample (X) are both covered with a monitor foil (M) with thickness D(D < R),... 

In choosing a monitor foil the following factors must be considered ... 

The latter disadvantage is negligible for standards where the induced activity is in general high compared to the contamination. For samples with a low concentration of the element of interest and where -emitters are detected via the annihilation radiation, spectral interference may occur. Etching after irradiation allows in general to avoid this source of error (see however also 2.6.4.). In order to minimise the influence of recoil nuclei it is good practice to use a foil of the same material as the sample as beam intensity monitor or to place such a foil between the monitor foil and the sample. 

Table II-7 gives some information on beam intensity monitor foils and useful nuclear reactions.

At the same time, radionuclides formed in the beam intensity monitor foil that recoil into the sample are removed and the influence of contamination before the irradiation is overcome. 

A 25 Aim copper foil was placed before the samples and the standards, serving as a beam intensity monitor. For the standards and for the samples that were not etched after irradiation, aluminium foil (20 m thick) was placed between the copper foil and the standard or the sample. This foil stopped recoil nuclei from reaching the monitor foil. The copper and aluminium foils degraded the energy to 20 MeV. The samples that were etched after irradiation were placed directly behind the copper foil. 

To remove possible surface contamination, some samples were chemically etched after irradiation for 2 to 5 min, depending on the silicon concentration, in a 1/1 (v/v) mixture of 14 M nitric acid and 50 % hydrofluoric acid at room temperature. A 20 m surface layer was removed. The copper monitor foil and the removed surface layer degraded the energy to 20 MeV. 

The shielding tank facility, located on the south side of the reactor, will be utilized for this experiment. A plane source of fission neutrons will be generated by a number of Argonaut fuel plates (Fig. 20.2). The detecting devices are indium foils 1.9 cm in diameter, 80 mg-cm enclosed in 0.0457-cm-thick (18/mil) cadmium cans (Fig. 20.4). These foils will be mounted on a holder provided (Fig. 20.3). A pair of gold foils may be used for monitor foils and to determine the Au-Cd ratio if desired. 

Fleckenstein et al. (1960) have used a similar method to follow the turnover rates of 0 -labeled phosphate in muscle. ATP, creatine phosphate, and inorganic phosphate were separated by paper chromatography, eluted onto a platinum plate, and bombarded with 4 Mev protons. The activity of F formed is measured by an Nal scintillation counter, 2 hours after the end of bombardment. Nevertheless some difficulties were experienced due to nuclear side reactions, including activation of the platinum. The amount of can be calculated from the flux and the length of bombardment (see original paper), or may be determined by comparison with monitor foils with known concentrations of oxygen-18-labeled phosphate. 

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