Beam Intensity Monitoring

The intensity of the X-ray beam is measured by ionization chambers or pin-diodes13. Pin-diodes can only be operated in the beam stop. The variation of the beam intensity during the experiment should be measured both before and after the sample. If the beam intensity monitors are set up properly, the absorption of the primary beam by the sample can be computed for each scattering pattern. The placement of the first ionization chamber in or after the X-ray guide tube to the sample is uncritical. 

The design and placement of the second beam intensity monitor demands more attention. The definition of X-ray absorption does not discriminate between primary beam, USAXS and SAXS. So the second beam intensity monitor should guide primary beam, USAXS and SAXS through its volume, whereas the WAXS should pass outside the monitor. The optimum setup for SAXS and USAXS measurements is a narrow ionization chamber directly behind the sample. For WAXS measurement a pin-diode in the beam stop is a good solution for WAXS. For USAXS and SAXS it may be acceptable, as long as the relevant part of the primary beam is caught, the optical system is in thermal equilibrium and the synchrotron beam does not jump (cf. Sect. 4.2.3.5). 

Figure 10. Schematic layout of the X-ray source, optical components, beam intensity monitors and detectors for an undulator beamline for XSW experiments at the Advanced Photon Source (from Lee 1999).

Fig. 14 The first experimental set-up with which it was possible to collect quasi-simulta-neously SAXS/WAXS/XAFS data. 1-position sensitive WAXS detector, 2-sample position and sample environment, 3-ion chamber for incoming beam intensity monitoring, 4-fluoresence detector, 5-optical bench, 6-SAXS detector, 7-beam stop in which a photo diode for transmitted beam intensity monitoring is integrated, 9-evacuated SAXS Sight tube. 

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),... 

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. 

Strijckmans et al. (42)(46)(53) irradiate a sample, placed behind a copper beam intensity monitor, with 15 MeV protons. After chemical etching, the sample is measured repeatedly with a t- T coincidence set-up, during a 1 to 2.5 h period, starting 8 to 14 min (niobium) or 2 h (tantalum) after the irradiation. 

A sample placed behind a 25 jum copper beam intensity monitor is irradiated for 15-20 min with a 2 mA beam of 25 MeV He. By chemical etching in 6 M nitric acid a 25-50 Asurface layer is removed. The sample is dissolved in 14 M nitric acid to which some sodium fluoride, zinc nitrate and gallium oxide carrier are added. After addition of phosphoric acid, the temperature is raised to 125° and 150 ml of distillate are collected. Gallium oxide is added to the distillate and gallium hydroxide is precipitated and filtered off. Finally, fluorine is precipitated as lead chlorofluoride The precipitate is repeatedly measured with a Ge(Li) detector during a 10 h period to allow the control of the half-life. Afterwards the yield of the chemical separation is determined by activation with an Ac-Be isotopic neutron source using the F(n,a) N reaction. 

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. 

The chemical etch reduces the energy of the protons effectively incident on the samples to 11.6 - 12.0 MeV. Therefore 3 series of standards were irradiated, placed behind a beam intensity monitor and a different number of copper and aluminium foils, so that the incident energies were 12.2, 11.9 and 11.5 MeV. These energies thus span the energies effectively incident on the samples. 

As the counter moves at constant angular velocity, a recorder automatically plots the diffracted beam intensity (monitored by the counter) as a function of 20 20 is termed the diffraction angle, which is measured experimentally. Figure 3.24 shows a diffraction pattern for a powdered specimen of lead. The high-intensity peaks result when the Bragg diffraction condition is satisfied by some set of crystallographic planes. These peaks are plane-indexed in the figure. 

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