Online monitoring of absorbed dose in undulator magnets with RADFET dosimeters at FERMI@Elettra

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Accelerators, spectrometers, detectors and associated equipment, 2013-03, Vol.703, p.70-79
• Main Authors: Fröhlich, L., Casarin, K., Quai, E., Holmes-Siedle, A., Severgnini, M., Vidimari, R.
• Format: Article
• Language: English
• Subjects: pMOS dosimeter; RADFET; Radiation monitoring; Solid-state dosimetry; Undulator; Zero-bias mode
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2012.11.021

The FERMI@Elettra free-electron laser, based on a 1.3GeV electron linac, requires the monitoring of radiation doses up to a few kGy for the protection of sensitive equipment such as permanent magnet undulators. A new dosimetry system DOSFET-L01, employing an array of RADFETs spread throughout the accelerator, was developed. So far, the system has performed flawlessly for almost two years, taking one dose reading per minute around the clock. The REM RFT-300 sensors were set in zero-bias mode, i.e. with all electrodes grounded during exposure. This choice of mode allows the measurement of a high range of integrated doses – up to a few kGy. The paper describes the new read-out system and its application, calibration measurements in cobalt-60 and 6MeV bremsstrahlung radiation sources giving rise to a novel response function, and new data on “fade” under the zero-bias mode of use for over 300 days at room temperature. Regular readings from 28 RADFETs placed within seven undulators over the first 20 months of operation of the accelerator demonstrate how the system tracks and locates periods of high and low dose rate and thereby contributes to the protection from beam loss. The readings from the RADFET system are found to be in good agreement with Gafchromic EBT2 film dosimeters. Based on the results reported, the choice of bias mode may be revised so as to reduce fade and improve the accuracy conferred by a positive-bias mode. ► We developed a system for online dosimetry with RADFET sensors under zero bias. ► The system is calibrated for doses up to 10kGy with REM RFT-300 sensors. ► We collected data on fade for over 300 days from irradiation. ► We present undulator dose measurements for 20 months of operation of FERMI@Elettra. ► Dose measurements are in good agreement with chemical film dosimeters.