Project

γ-ray diagnostics of energetic ions in tokamaks

Fast ions are responsible for different processes occurring in a high-temperature plasma (heating, energy transport, nuclear reactions, etc.) Moreover, classical and anomalous losses and even redistribution of these ions can have a serious effect upon final plasma heating profiles. Therefore, the study of fast ion behavior in plasmas confined by a magnetic field is of a crucial importance, especially for fusion reactor development [1, 2]. Burning plasmas e.g. in ITER will consist several groups of energetic ions: α-particles and other fusion reaction products, D – and T- ions introduced by NBI, ICRH accelerated ions etc. Diagnostic of γ-rays produced in nuclear reactions is one of the important techniques which can be used for studying the fast ions in fusion devices. Measurements performed up to now on different tokamaks have revealed an intense γ-ray emission, which come from nuclear reactions induced by fast ions interacting with fuel ions or with the plasma impurities such as Be, B, C and O. The main aim of recent experiments was to perform an experimental analysis of γ-ray energy spectra and γ-ray emission radial profiles. Such measurements were carried out using collimated spectrometers [3] and the neutron-profile monitor (camera) [4]. The measured γ-ray energy spectra provided essential data on different fast ion species and their energy distribution functions could be estimated. This was possible, since the reaction excitation functions for the relevant reactions producing γ-rays are well established and many of them exhibit a threshold and/or resonant nature. These two diagnostics’ systems (spectrometers and camera) will be upgraded on JET with new detectors and digital electronics to sustain the high count rates during the JET DT campaign. It is proposed to enhance the spectroscopic and count rate capability by replacing CsI:Tl and BGO scintillators with new modern scintillators (CeBr₃ and LaBr₃:Ce), which are much faster and have a better energy resolution.

1. V.G.Kiptily et al., Nucl. Fusion 42 (2002) 999-1007
2. V.G.Kiptily et al., Nucl. Fusion 49 (2009) 065030
3. Sadler G.J. et al., 1990 Fusion Technol. 18 556
4. Adams J.M. 1993 Nucl. Instr. Methods A 239 277.

Gamma Camera Upgrade

From a diagnostic point of view a high power DT plasma represents a rather harsh environment. For this reason, nuclear physics based diagnostics play a key role and interesting candidates are represented by high resolution Neutron Emission Spectroscopy and Gamma Ray emission spectroscopy. Neutrons of mean energy around 14 MeV are produced by the DT reaction itself while gamma rays are emitted by reactions among alpha particles or other fast ions and light impurities, typically beryllium or carbon which are present in the plasma. On JET, the α-particle diagnostic is based on the nuclear reaction ⁹Be(α,nγ)¹²C between confined α-particles and the beryllium impurity typically present in the plasma. The development of dedicated high resolution gamma ray spectrometers at JET has allowed measuring, for the first time on a fusion plasma, the Doppler Broadening of gamma ray peaks from the reactions ¹²C(³He,pγ)¹⁴N and ⁹Be(α,nγ)¹²C. These spectrometer have a single line of sight into the plasma, and their spectroscopic observation is complemented by the measurements performed by the 19 channels Gamma Ray Camera.

For operating the Gamma Ray Camera diagnostic at the high DT neutron fluxes expected in the next high-power DT campaign on JET and to improve its spectroscopic capability specific hardware improvements are needed. In particular it is proposed to enhance the existing spectroscopic and count rate capability by replacing the 19 CsI detector with new faster and better energy resolution detector modules. This is a challenging upgrade given the existing constraints in terms of available space for detectors and shielding. A possible solution is to use LaBr₃ or CeBr₃ scintillators which are characterized by short decay times (~20ns) and a high photons yields (~60000 photons/MeV) coupled to solid state photon detectors, but the final choice will be done during the project. The new detector will be able to sustain count rate in excess of 500 kHz. and energy resolution equal or better than 5% at 1.1MeV

GSU

On JET the α-particle diagnostic is based on the nuclear reaction ⁹Be(α,nγ)¹²C between confined α-particles and beryllium impurity ions typically present in the plasma. The applicability of gamma-ray diagnostics is strongly dependent on the fulfilment of rather strict requirements for the definition and characterization of the neutron and gamma radiation fields (detector Field-of-View, radiation shielding and attenuation, parasitic gamma-ray sources). For operating this diagnostic at the high DT neutron fluxes expected in the future high-power DT campaign on JET, specific improvements are needed in order to provide good quality measurements in the D-T campaign, characterized by a more challenging radiation environment.

As in the DT experiments the gamma-ray detector must fulfil requirements for high count rate measurements, the existent BGO-detector should be replaced with new detector modules (detector module 1 (DM1) based on LaBr₃ scintillator and detector module 2 (DM2) based on CeBr₃ scintillator) and an associated digital data acquisition system. The new scintillators are characterized by short decay times (~20ns) and a high photon yields. The coupling of the scintillators with photomultiplier tubes in specially designed detector modules will permit the operation at count rates over 2MHz. The high rate capability will be enabled by a dedicated pulse digitization system with a nominal 14-bit resolution.

The first option (LaBr₃) represents a reliable solution already manufactured and tested for JET gamma-ray spectrometry. The lanthanum bromide scintillation material has a good energy resolution, high sensitivity, short decay time and room temperature operability what makes it is a very good candidate for high resolution γ-ray diagnostics of JET plasmas. The main drawback of the LaBr₃ scintillator is its use in low noise measurements due to its high intrinsic background noise. The background is caused by the 0.09% natural abundance of radioactive ¹³⁸La isotope with half-life ~1011 years. The radioactive decay of the ¹³⁸La leads to γ-ray emission at ~1.4 MeV and ~0.8 MeV energies. Absorption of these gammas causes an internal activity of LaBr₃ up to ~10-3 Bq/cm³/keV. Therefore, besides ensuring an already tested component, the project attempts to provide also enhanced performance by including also a detector based on CeBr₃ scintillator. This scintillator was found to fulfill low noise measurement conditions. It shows 30 times reduction in internal activity in comparison with LaBr₃. The CeBr₃ scintillator has a similar energy resolution, sensitivity and decay time as the LaBr₃ scintillator. Moreover, the CeBr3 scintillator seems to be more resistant for gamma radiation than LaBr₃. A 1 kGy dose of gamma radiation deteriorates the yield of LaBr₃ by ~10% and worsens its energy resolution from 3.0 to 3.8%, while is almost negligible for CeBr₃. CeBr₃ may also be more resistant for neutron radiation than LaBr₃ because of the much lower neutron capture cross section in Ce than in La (at low neutron energy range, En<10keV). These features make CeBr₃ an interesting alternative for JET plasma applications in spite of the excellent spectroscopic performances of LaBr₃ scintillator.