Development of a portable free-air ionization chamber as an absolute intensity monitor for high-energy synchrotron radiation up to 150 keV

As an intensity monitor for high-energy synchrotron radiation, a parallel-plate free-air ionization chamber was developed. An EGS4 Monte-Carlo transport calculation considering linear polarization showed that the electron escape fraction was within 0.5% below 70 keV, 2.7–3.3% below 150 keV and 1.1% at 150 keV when the plate separation was 8.5 cm. Without polarization, the electron loss increased to 4–5% at 70–100 keV and 3% at 150 keV. Using the chamber, photon intensity was measured at SPring-8 and compared to that of another free-air ionization chamber with a 5-cm plate separation at 30 and 40 keV, which had been calibrated with a calorimeter, and to that of a Si-PIN photodiode between 50 and 150 keV. As expected by the Monte Carlo calculation considering linear polarization, agreement was obtained within 1.3% below 70 keV, 2.2% up to 100 keV and 1.0% at 150 keV. If the collection efficiency corresponding to the result is used at each energy level, the uncertainty is limited to about 3% for the photon intensity monitoring.