Through the injection of a Fowler-Nordheim tunnel current or the inversion of oxide fields during irradiation (Radiation-Induced Charge Neutralization), the oxide charge trapped in thick-oxide (300 ...nm) commercial RADFETs, often called Q OT could be erased. Novel trapped-hole and interface characteristics were observed after treatments of this type at high doses. With both erasure techniques, it was possible only to neutralize a fraction of the oxide trapped charge. A non negligible amount of charge and border traps is deemed here to be "intractable". That adjective an a symbol, Q IN , are introduced for the first time in this paper. Later sections discuss the possible impact of these results. The conclusion for dosimetry is that a "reusable RADFET" dosimeter, working up to an unprecedented dose before wearing out, may be a practical possibility.
Techniques based on bias switching during the irradiation allow to extend the measurement range of MOS dosimeters. The response of the REM RFT300 RADFET dosimeter during bias cycled measurements ...shows a slow shift of the quasi-steady state threshold voltage value during radiation-induced charge neutralization. This phenomenon was previously explained as due to the presence of border traps. In this work, a recently developed numerical model which included the main physical processes leading to hole trapping and neutralization in MOS oxides was used to reproduce this experiment. The application of the model shows that the slow shift of the quasi-steady state threshold voltage during neutralization stages is a consequence of the spatial redistribution of trapped charge within the oxide. The effect this phenomenon has on MOS dosimetry is analyzed.
Dosimetry, the art of dose monitoring, requires a mixture of physics, electronics and engineering. This art is of vital importance in the safety and protection of new energy machines. A case is ...presented for intensified research in the development of dosimeters suitable for the next generation of particle accelerators and fusion reactors.
Van Allen Probes A and B, launched more than a year ago (in August 2012), carried 16 p-channel metal-oxide-semiconductor Radiation-sensitive Field Effect Transistors (RadFET)s into an orbit designed ...by NASA to probe the heart of the trapped-radiation belts. Nearly 350 days of in situ measurements from the Engineering Radiation Monitor (ERM) (1) demonstrated strong variations of dose rates with time, (2) revealed a critical correlation between the ERM RadFET dosimeters and the ERM Faraday cup data on charged particles, and (3) permitted the mapping of the belts by measuring variation with orbit altitude. This paper provides an update on early results given in a NSREC2012 paper along with details and discussion of the RadFET dosimetry data analyzed .
An Engineering Radiation Monitor (ERM) has been developed as a supplementary spacecraft subsystem for NASA’s Radiation Belt Storm Probes (RBSP) mission. The ERM will monitor total dose and deep ...dielectric charging at each RBSP spacecraft in real time. Configured to take the place of spacecraft balance mass, the ERM contains an array of eight dosimeters and two buried conductive plates. The dosimeters are mounted under covers of varying shielding thickness to obtain a dose-depth curve and characterize the electron and proton contributions to total dose. A 3-min readout cadence coupled with an initial sensitivity of ∼0.01 krad should enable dynamic measurements of dose rate throughout the 9-hr RBSP orbit. The dosimeters are Radiation-sensing Field Effect Transistors (RadFETs) and operate at zero bias to preserve their response even when powered off. The range of the RadFETs extends above 1000 krad to avoid saturation over the expected duration of the mission. Two large-area (∼10 cm
2
) charge monitor plates set behind different thickness covers will measure the dynamic currents of weakly-penetrating electrons that can be potentially hazardous to sensitive electronic components within the spacecraft. The charge monitors can handle large events without saturating (∼3000 fA/cm
2
) and provide sufficient sensitivity (∼0.1 fA/cm
2
) to gauge quiescent conditions. High time-resolution (5 s) monitoring allows detection of rapid changes in flux and enables correlation of spacecraft anomalies with local space weather conditions. Although primarily intended as an engineering subsystem to monitor spacecraft radiation levels, real-time data from the ERM may also prove useful or interesting to a larger community.
RadFET and p-i-n diode semiconductor dosimeters from different manufacturers will be used for radiation monitoring at the Experiments of the CERN LHC accelerator. In this work these sensors were ...exposed over three months in the CERN-IRRAD6 facility that provides mixed high-energy particles at low rates. The aim was to validate the operation of such sensors in a radiation field where the conditions are close to the ones expected inside full working LHC particle detectors. The results of this long-term irradiation campaign are presented, discussed and compared with measurements by other dosimetric means as well as Monte Carlo simulations. Finally, the integration of several dosimetric devices in one sensor carrier is also presented.
We present irradiation experiments carried out on RadFETs in the high-intensity T2 neutron beam at the CRC-UCL in Belgium. The aim of the test was to characterize the neutron response of RadFETs in ...view of their use as an integrated part of a radiation-monitoring sensor for the CERN Large Hadron Collider (LHC) experiments. Two types of RadFETs were investigated up to a total neutron fluence of 3/spl times/10/sup 14/ cm/sup -2/ corresponding to a deposited dose of 744 Gy in silicon. The responses of bare devices to neutrons are compared to the commonly used reference measurements with gamma rays. It is found that the gamma ray calibration cannot directly be adopted to convert the RadFET signals into neutron dose. In a second experiment, the influence of a plastic packaging, simulated by polyethylene slabs of different thicknesses, was tested in the neutron beam and compared to GEANT4 Monte Carlo simulations. An increase of the RadFET neutron sensitivity by a factor of up to 7 due to the packaging is found. The influence of these findings on the conception of the radiation-monitoring sensor is discussed.
The dosimetry of X-ray microbeams using MOSFETs results in an asymmetrical beam profile due to a lack of lateral charged particle equilibrium. Monte Carlo simulations were carried out using PENELOPE ...and GEANT4 codes to study this effect and a MOSFET on a micropositioner was scanned in the microbeam. Based on the simulations a new method of microbeam dosimetry is proposed. The proposed edge-on face-to-face (EOFF) MOSFET detector, a die arrangement proposed here for the first time, should alleviate the asymmetry. Further improvement is possible by thinning the silicon body of the MOSFET.
Improvements have been made in the measurement of dose profiles in several types of X-ray beams. These include 120-kVp X-ray beams from an orthovoltage X-ray machine, 6-MV Bremsstrahlung from a ...medical LINAC in conformal mode and the 50-200 keV energy spectrum of microbeams produced at the medical beamline station of the European Synchrotron Radiation Facility. Using a quadruple metal-oxide-semiconductor field-effect transistor (MOSFET) sensor chip in "edge on" mode together with a newly developed sensor readout system, the feasibility of online scanning of the profiles of quasi-static and pulsed radiation beams was demonstrated. Measurements of synchrotron pulsed microbeams showed that a micrometer-scale spatial resolution was achievable. The use of several MOSFETs on the same chip gave rise to the correction of misalignments of the oxide films of the sensor with respect to the microbeam, ensuring that the excellent spatial resolution of the MOSFET used in "edge-on" mode was fully utilized.
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.
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