Fusion Programme - ENEA - Fusione

1. Magnetic Confinement2007 Progress ReportCounts10 510 410 310 210 1Length (samples)Pile-upSingle10 0 0 50 100 150 200Amplitude (arb. unit)40030020010000 20 40 60SamplesFig. 1.17 - Example of distribution ofpulse lengths (AmBe neutron sourceand NE213 scintillator)Fig. 1.18 - Pile-up event with fitting of the firstpulse in the window. The fitted pulse is thensubtracted from the original signal to obtainthe second pulseon the treatment of the scintillator pile-up events (overlapping pulses). The processing software identifiesall pile-up events (fig. 1.17) and a method for their off-line digital processing was developed: pile-ups arereduced to single pulses through a pulse fitting procedure (fig. 1.18) and then correctly labelled asneutrons or gammas. If the method is applied to pile-ups with successive peaks separated by more than50 ns, ~87% of the pile-up events can be resolved and used for spectral analysis [1.60].Development of artificial diamond detectors. Of the two artificial diamond detectors grown bychemical vapour deposition (CVD) techniques, one is a 37–μm-thick polycrystalline film (p-CVD), the otheris a single 30–μm–thick crystal diamond (SCD). Both detectors are covered with a 2–μm layer of LiFenriched at 95% in 6 Li [1.61,1.62]. 6 Li converts low-energy neutrons into alphas and tritons of about 2 MeVand 2.7 MeV respectively via the well-known (n,α) reaction in 6 Li. In the case of the p-CVD detector thegoal is to measure the total neutron yield and compare it with the data of the standard 235 U fissionchambers installed at JET (KN1 diagnostics). In the case of the SCD detector the goal is more ambitiousas it was developed with the aim of having a single detector capable of measuring simultaneously the totaland the 14-MeV neutrons from DD plasma. At the moment this is achieved at JET by using fissionchambers and silicon diodes (KM-7 diagnostics). The diodes have a hardness problem and are usuallychanged each year.A peculiar aspect of the SCD detector is the conductive boron doped contact developed for thisapplication and which from preliminary studies (to be confirmed at JET) seems to allow the detector towork under very stable conditions (e.g., no polarization effects) [1.61].The different ways of detecting thermal and 14-MeV neutrons by the SCD are shown in figure 1.19, whereHPHT stands for high-pressure high-temperature layer.in Phys. Res. A 572, 355 (2007)[1.60] F. Belli et al., A method for digitalprocessing of pile-up events in organicscintillators, submitted to Nucl.Instrum. Meth. in Phys. Res.[1.61] M. Marinelli et al., Appl. Phys. Lett. 89,1(2006)[1.62] M. Marinelli et al., Appl. Phys. Lett. 90,183509 (2007)Fast neutrons are detected through the products of the reactionn + 12 C → α + 9 Be – 5.7 MeV .Thermal neutrons are detected through the products of thereaction27