DIII-D tokamak hard X-ray spectrometer with MCps counting rate

In a collaboration with Dipartimento di Fisica Università degli Studi di Milano-Bicocca and Istituto per la Scienza e Tecnologia dei Plasmi, CNR Nuclear Instruments designed the electronics of a campact t spectrometer optimized for measurements of hard X-rays generated by runaway electrons. The detector is designed to be installed in the fan-shaped collimator of the Gamma-Ray Imager diagnostic at the DIII-D tokamak. The spectrometer is based on a 1 cm x 1 cm cerium doped yttrium aluminium perovskite scintillator crystal coupled with a silicon photomultiplier. The detector dynamic energy range is in excess of 10 MeV, with an energy resolution of approximately 10% at 661.7 keV. V. The fast detector signal (≈ 70 ns full width at half maximum) allows for operation at counting rates in excess of 1 MCps. The gain stability of the system can be monitored in real-time using a light-emitting diode embedded in the instrument.


In dedicated tests, the detector proved its stability under severe counting rates, obtaining a detector gain shift smaller than 3.5% under a load of 5 of 1 MHz, improving on the results obtained by previous prototypes. This stability coupled with its fast signal provides the detector with high counting rate capability in excess of 1 MCps. The detector energy non-linearity was characterized. The spectrometer presents a ≈ 30% deviation from linearity at 10 MeV and a 50% deviation at ≈ 21 MeV. It has to be noted that once the non-linear detector response is characterized, offline correction of the measured HXR spectra can be easily performed. The spectrometer presents a wide dynamic energy range from 100 keV in excess of 10 MeV, suitable for runaway electron bremsstrahlung measurements.

Normalized output signal of the novel HXR spectromete. The signal has a FWHM of less than 70ns