Ionization Chamber Detector for X-ray with Beam Position Monitoring

Ionization Chambers have been commonly used at beamlines in many synchrotron radiation facilities. Typical ADC ion chambers are pictured in Figure 1. The chambers generate a current proportional to the incident X-ray beam intensity. The ionization chamber allows users to determine the change in beam position in a single axis by comparing two signals that are created as the beam passes through the Ion Chamber [1]. By connecting two Ion Chambers together at 90° you can determine the horizontal and vertical beam position. One unique feature of the new precision ion chambers is the incorporation of a split collector plate. The electrode is split in a saw tooth configuration with a height of approximately 10mm, 15mm, and 25mm such that, when the differential current is computed, allows use as a beam position monitor. A summary test results from the hard X-ray beamline, BL06, at bending magnet source of SAGA Light Source in Japan is presented. (a) (b) (c) (d) FIGURE 1. (a) & (b) ADC’s IC-400 Series, (c) ADC’s IC-500 Series, (d) ADC’s Micro Ion Chamber INTRODUCTION The reliable monitoring of the intensity of the X-ray beam in the case of synchrotron-radiation-induced measurements is crucial for correcting the intensity change caused by the different optical and beam influencing elements at the beam line. In order to probe precisely the change in the intensity of the X-ray radiation penetrating the sample, a counter should be introduced between the last optical or beam influencing element and the sample. FIGURE 2. Parallel Plate Geometry Typical ionization chambers are constructed with the parallel plate geometry as illustrated in Figure 2. As the voltage is increased, the resulting electric field begins to separate electron-ion pairs generated by ionizing radiation more rapidly, and the recombination process between the electrons and the ions are diminished. At a sufficiently high applied voltage, the electric field becomes strong enough to suppress the recombination process to a negligible level, and all the charges initially created through the ionization process contribute to the ionization current. Under these conditions, the current measured in the external circuit can be regarded as an indication of the formation rate of all charges due to ionization by the incident X-ray photons. Absorption of x-ray photons in materials follows the rule, I = Ioe-μt where I is the x-ray intensity after passing through a material, Io is the initial intensity, μ is the absorption coefficient for that particular photon energy and t is the thickness of the absorbing material. This photon absorption results in an electron/ion pair being produced in the absorber material. All an “ion chamber” does is to apply a potential held around these newly created electron/ion pairs, separate them and measure the resulting electric current to determine how many pairs were created and thus how many photons were absorbed [2].