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Frohlich L.,Elettra - Sincrotrone Trieste | Casarin K.,Elettra - Sincrotrone Trieste | Quai E.,Elettra - Sincrotrone Trieste | Holmes-Siedle A.,REM Oxford Ltd. | And 2 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2013

The FERMI@Elettra free-electron laser, based on a 1.3 GeV 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 6 MeV 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. © 2012 Elsevier B.V.


Sharp R.E.,Isotron Ltd. | Hofman J.,Isotron Ltd. | Hofman J.,Brno University of Technology | Holmes-Siedle A.,REM Oxford Ltd.
Proceedings of the European Conference on Radiation and its Effects on Components and Systems, RADECS | Year: 2011

The effects of alpha irradiation on the charge in threshold voltage have been measured for standard RADFET die housed in a custom package. The intended application is for monitoring process stability in an ion implantation process. The results show that these devices exhibit good promise for use as a routine dosimeter for periodic comparison of the stability of the process. © 2011 IEEE.


Holmes-Siedle A.G.,REM Oxford Ltd. | Goldsten J.O.,Johns Hopkins University | Maurer R.H.,Johns Hopkins University | Peplowski P.N.,Johns Hopkins University
IEEE Transactions on Nuclear Science | Year: 2014

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. © 2014 IEEE.


Maurer R.,Johns Hopkins University | Goldsten J.,Johns Hopkins University | Peplowski P.,Johns Hopkins University | Holmes-Siedle A.,REM Oxford Ltd. | And 3 more authors.
IEEE Transactions on Nuclear Science | Year: 2013

The Engineering Radiation Monitor (ERM) measures dose, dose rate and charging currents on the Van Allen Probes mission to study the dynamics of earth's Van Allen radiation belts. Early results from this monitor show a variation in dose rates with time, a correlation between the dosimeter and charging current data, a map of charging current versus orbit altitude and a comparison of cumulative dose to pre-launch modeling after 260 days. Solar cell degradation monitor patches track the decrease in solar array output as displacement damage accumulates. © 2013 IEEE.


Goldsten J.O.,Johns Hopkins University | Maurer R.H.,Johns Hopkins University | Peplowski P.N.,Johns Hopkins University | Holmes-Siedle A.G.,REM Oxford Ltd. | And 2 more authors.
Space Science Reviews | Year: 2013

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 cm2) 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/cm2) and provide sufficient sensitivity (∼0.1 fA/cm2) 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. © 2012 The Author(s).


Lipovetzky J.,University of Buenos Aires | Lipovetzky J.,CONICET | Holmes Siedle A.,REM Oxford Ltd. | Garcia Inza M.,University of Buenos Aires | And 4 more authors.
IEEE Transactions on Nuclear Science | Year: 2012

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 QOT 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. © 1963-2012 IEEE.

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