Gamma Ray Spectrometry (GRS)

 

 

 

The project Gamma-ray spectrometry  is part of a EURATOM project of diagnostic enhancements to be implemented at JET, Culham, Great Britain. The objective is the installation and the availability of intense  gamma ray and neutron sources at the Tandem Accelerator of  IFIN-HH, Magurele-Bucharest, Romania, in order to carry out testing of a high rate spectrometers with large scintillator detection crystal, in collaboration with the University of Milano, Biccoca, Italy.
 

Detector tests in high neutron/gamma flux conditions

 

Giuseppe Gorini

 

Dipartimento di Fisica "G.Occhialini", Universita' degli Studi di Milano - Bicocca,piazza della Scienza 3, I-20126 Milano, Italy, and Istituto di Fisica del Plasma "Piero Caldirola", CNR, via Cozzi 53, I-20125 Milano, Italy

 

Marco Tardocchi

 

Istituto di Fisica del Plasma "Piero Caldirola", CNR, via Cozzi 53, I-20125 Milano, Italy

 

Antonino Pietropaolo, Massimo Nocente

 

Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, CNISM-Milano Bicocca, Dipartimento di Fisica "G. Occhialini" Universitΰ degli Studi Milano Bicocc, Piazza della Scienza 3, 20126 Milano, Italy

 

Silviu Olariu, Agata Olariu

 

National Institute for Physics and Nuclear Engineering, Magurele, Romania

 

 

•         The project Gamma Ray Spectrometry is part of the project of package of diagnostic enhancements to be implemented at the Joint European Torus (JET), Culham, during the Framework Programme 7. 

 

•         The objective for IFIN Magurele is the availability of intense  gamma ray and neutron sources at the Tandem accelerator of  IFIN, in order to carry out testing of  a high rate spectrometer with large LaBr3 detection crystal.

 

•         The key figure of merit is the count rate capability which shall exceed 0.5 MHz before pile-up and gain drifts begin to affect the detector response.

 

Scientific Background and motivation

 

•         In the last years gamma ray spectrometry has provided new insight into the physics of fast ions in JET.

•         Gamma spectrometry in ITER is a real challenge due to the large flux of 14 MeV neutrons; suitable neutron attenuators must be developed before gamma spectrometry can be considered as serious ITER diagnostics candidate in DT plasmas.

•         ITER-related developments should include the testing of new detector materials based on high-Z, “heavy” scintillators recently developed; these materials are fast and efficient which is promising for the development of high rate measurements with reduced neutron sensitivity.

•         These materials are not yet tested in the fusion environment with mixed neutron/gamma radiation field.

•         Present count rate limitations at JET are due, in part, to the obsolete electronics modules used for signal processing and DAQ; there is considerable room for improvement here.

•         Gamma-ray Doppler shape analysis is a newly proposed diagnostic technique that can be tested at JET with a high resolution gamma spectrometer.

 

Expected Performance/Features/Location of the new/enhanced system

 

The new gamma spectrometry diagnostics will be located in the Roof Lab. It will be a complete replacement of the two existing gamma spectrometers in use in the Roof Lab. The foreseen features are

 

•         Count-rate improvement beyond 500 kHz without pile-up losses

•         Attenuation of 14 MeV neutron flux by a factor >100 for one of the spectrometers using LiH as neutron attenuator material

•         Three new gamma spectrometers with complementary performance:

•         A high energy resolution HPGe spectrometer (resolution FWHM<2 keV for the 1.332 gamma-rays from 60Co)

•         Two high efficiency, high rate spectrometers with large detection crystal made from LaBr3 scintillators

•         Fully digitized DAQ

•         The gamma spectroscopy amplifiers for the gamma camera will be installed in the gamma camera cubicles located in the Diagnostic Hall.

 

Project Goals and Technical Objectives

 

The project has been asked to improve the performance of gamma spectrometry measurements at JET. The improvement concerns the energy resolution, which will be better that 0.2% in the case of the HPGe detector and will allow for the first ever measurements of Doppler broadening of gamma spectra from fusion experiments – due to the energy distribution of the alpha particles or other fast particles in the plasma. It also concerns the dynamic range, which is improved by using fast scintillators and matching electronics (analogue and digital). Related to improved dynamic range is the improved time resolution. This will be limited by the available signal intensity from the plasma whereas it is now limited by the high neutron/gamma background combined with slow detector response. The key figure of merit is the count rate capability which shall exceed 0.5 MHz before pile-up and gain drifts begin to affect the detector response. These effects are unavoidable but can be quantitatively controlled through the combined use of a Control and Monitoring system (C&M) and of digital data acquisition. It is expected that total throughput will exceed 2 MHz of total count rate of which a variable fraction will be from useful gamma events. Substantial neutron background reduction (factor >100) will be achieved for one of the detectors with the use of a LiH neutron attenuator. New data acquisition and analysis/simulation software will be provided for interpretation of improved measurements. The new amplifiers for the gamma camera will provide ease of use in switching between different modes of operation.

 

Production of high gamma and neutron fluxes at the Tandem accelerator

 

•         In order to produce the high-intensity gamma-ray and neutron beams, we shall use proton and alpha-particle beams incident on thick targets of Beryllium and Aluminum mounted on the MA0 experimental line of the Tandem accelerator.

•         By placing the target on the MA0 experimental line, where the incident currents can be as large as 1 mA, we have obtained gamma-ray fluxes of 6 x 108 gamma-rays / second and neutron fluxes 0.7 x 108 neutrons / second, which are large enough to produce the required count rate of about 1 MHz.

 

 

SEE ALSO: