Intensity Monitors

Mini ionisation chamber

A windowless small ionisation chamber [1] was developed for monitoring the intensity of the incoming micro-beam and for correcting for the intensity variation of the excitation beam. The size of the small monitor was optimized for microprobe measurements. The small dimensions of the ionisation chamber (10 mm along the beam direction and 5 mm perpendicular to it) make it possible to place it very close, within some centimeters, to the sample. The entrance and exit windows of the chamber are defined by two Pt pinholes of 50 and 300 mm diameters, respectively without any window material. Thus the small counter can be used as an order selecting or shielding aperture while measuring simultaneously the intensity after the aperture. The 50 mm diameter of the entrance pinhole makes the positioning of the monitor possible within 30mm distance from the sample in case of using focusing elements with focal distances of >500 mm. The ionisation chamber is used with a constant Ar flow at about 1 atm absolute pressure (the gas escapes from the chamber through the aperture of the pinholes) in order to obtain higher absorption within the detector volume. Despite the very small active length of the device (4 mm) the ionisation current is well above (4 orders of magnitude) of the detection limit of the Keithley 486 current amplifier used for the current amplification. The current-voltage characteristics show that the small detector can be operated at relatively low working voltages (100-150 V). Previous investigations showed that <2% precision can be achieved while using the signal of the small monitor for normalisation.

Schematic diagram of the ionisation chamber used for focused beam intensity monitoring

An ionisation chamber and a PIN diode are used to monitor the intensity of the incoming and focused beam. A videomicroscope and a High-resolution CCD Camera facilitate sample and micro-probe alignment.

1. M. Kocsis, A. Somogyi: Miniature ionization chamber detector developed for X-ray microprobe measurements, J. Synchrotron Radiation, 10, 187-190 (2003).