Neutrons and gamma-rays emitted by thermonuclear fusion plasmas can be detected providing information on fuel and fast ions. The high neutron and gamma-ray fluxes expected at JET in DT plasmas, together with the need to provide time resolution in the range of 10-100 ms, translates into the need of spectrometers capable to operate at counting rates in the MHz range. Moreover, measurements with high energy resolution are necessary to infer detailed information on fast and fuel ions. These pose very challenging requirements on the needed detector performances. This thesis is focused on the development, characterization and implementation of new compact gamma-ray and neutron spectrometers which combine high counting rate capabilities and high energy resolutions. A prototype compact gamma-ray spectrometer has been developed within a European project which is upgrading the JET gamma ray camera (GC) detectors. The prototype detector is based on a LaBr3 scintillator crystal coupled to a Silicon PhotoMultiplier (SiPM). The developed prototype spectrometer features an energy resolution of 5% at 662 keV, which improves for higher energies and provides accurate measurements in the energy range up to a few MeVs. A suitable shaping circuit of the SiPM signal has been implemented to minimize the presence of piled up events allowing operation at counting rates in excess of 1 MCounts/s, as demonstrated with record values 3 MCounts/s at an accelerator. The 19 new detectors have been successfully calibrated and installed during 2017 in the JET GC. The enhanced energy resolution of GC will allow avoiding artifacts in the reconstructed fast ions profile, while the improved time resolution will open the possibility to track for the first time the fast ions profile changes during their slowing down times. In the neutron spectroscopy field, a 12-pixels single crystal diamond matrix has been installed on a vertical line of sight of JET allowing simultaneous measurements of 2.5 MeV and 14 MeV neutrons. The response of diamond neutron spectrometers to 2.5 and 14 MeV neutrons is very different, namely it is dominated by elastic and inelastic scattering for 2.5 MeV neutrons while for 14 MeV neutrons several nuclear reaction channels open up, offering the possibility to perform high resolution spectroscopy. The diamond matrix response function has been measured at nuclear accelerators for incoming different monoenergetic neutron energies. The data validation of the diamond matrix with 2.5 MeV neutrons has been performed by comparing with data taken by the reference 2.5 MeV neutron spectrometer at JET, namely TOFOR. The results indicated that the spectrometer works well and can provide a moderated energy resolution in D plasmas. The excellent spectroscopic capabilities for 14 MeV neutrons, instead, has been explored during the characterization of a 14 MeV DT neutron generator. Neutrons were produced by DT reactions occurring by accelerating a mixed beam of Dx+ /Tx+ /DT+ beam (x=1,2) onto a titanium target containing T/D. Diamond detectors allowed resolving for the first time the complex features of the neutron energy spectra resulting from the simultaneous presence of D+ , T+ , D2+ , T2+ , DT+ species present in the beam. These results open up to new prospects for diagnosing DT plasmas on JET and ITER. The analysis of the diamond 12C(n,α)9Be peak, in fact, will allow accurately identifying supra-thermal components in DT plasma operations and studying non classical phenomena on the beam slowing down. The results presented in this thesis represent a step forward in the development of neutron and gamma-ray spectrometers for fusion plasma diagnostics which combine the MHz counting rate capability with the enhanced energy resolution. The developed instruments feature compact size and are therefore suitable for integration in a multi line of sight camera on the next step burning plasma fusion devices such as ITER and DEMO.
I neutroni e i raggi gamma emessi da plasmi di fusione termonucleare possono essere rivelati e fornire informazioni sullo stato del plasma e sugli ioni veloci. L’alto flusso neutronico e gamma previsto al JET per plasmi DT, insieme alla necessità di misure con risoluzione temporale di 10-100 ms, richiedono l’utilizzo di spettrometri in grado di operare a tassi di conteggio nel range del MHz. Inoltre, il bisogno di misure ad alta risoluzione energetica pone ulteriori vincoli sulle prestazioni dei rivelatori. Questa tesi si è incentrata sullo sviluppo, la caratterizzazione e l’implementazione di nuovi rivelatori compatti ad alta risoluzione per la spettroscopia di neutroni e raggi gamma in grado di operare ad alti tassi di conteggio. All’interno di un progetto europeo finalizzato all’aggiornamento della gamma camera (GC) del JET, è stato sviluppato un prototipo di spettrometro gamma compatto basato su uno scintillatore LaBr3 accoppiato a un Silicon PhotoMultiplier (SiPM). Lo spettrometro offre una buona risoluzione energetica del 5% a 662 keV che migliora a energie più alte permettendo misure accurate nel range 2-6 MeV. Il rivelatore implementa un circuito di formatura del segnale dedicato in grado di accorciare la durata temporale del segnale in uscita dal SiPM. Ciò permette di minimizzare la presenza di eventi di pile-up consentendo il funzionamento a tassi di conteggio superiori a 1 MCounts/s, come dimostrato presso l’acceleratore di Legnaro fino a 3 MCounts/s. I 19 rivelatori sono stati calibrati e installati nella GC del JET nel corso del 2017. L’avanzata risoluzione energetica della GC permetterà di evitare artefatti nella ricostruzione del profilo degli ioni veloci, mentre l’alta capacità di conteggio permetterà per la prima volta di monitorare i cambiamenti del profilo ionico in tempi comparabili al loro tempo caratteristico di rallentamento. Per quanto riguarda la spettroscopia neutronica, una matrice di 12 diamanti monocristallini è stata installata al JET permettendo misure simultanee di neutroni da 2.5 MeV e 14 MeV. La risposta dei diamanti ai neutroni dipende dalla loro energia. Per neutroni da 2.5 MeV, la risposta è dominata dallo scattering elastico e anelastico mentre per neutroni da 14 MeV si aprono diversi canali di reazione, che permettono misure di spettroscopia ad alta risoluzione. La funzione di risposta della matrice è stata misurata per neutroni monoenergetici presso acceleratori nucleari a diverse energie. La validazione della matrice con neutroni da 2.5 MeV è stata eseguita confrontando i dati misurati dallo spettrometro di riferimento al JET, cioè il TOFOR. I risultati hanno evidenziato il corretto funzionamento della matrice e una moderata risoluzione energetica per plasmi di D. Le eccellenti capacità spettroscopiche dei diamanti per neutroni da 14 MeV, sono state investigate durante la caratterizzazione di un generatore di neutroni DT. I neutroni sono prodotti attraverso reazioni DT che avvengono tra il fascio di ioni misti Dx+ /Tx+ /DT+ (x=1,2) accelerati verso il bersaglio di D e T. I diamanti hanno permesso di risolvere le complesse strutture dello spettro di energia dei neutroni risultanti dalle diverse specie presenti nel fascio. Questo risultato apre nuove prospettive per la diagnostica di plasmi DT su JET e ITER. L’analisi del picco 12C(n,α)9Be del diamante, infatti, permette di identificare accuratamente le componenti sovratermiche in plasmi DT e di studiare fenomeni non classici nel rallentamento degli ioni di fascio. I risultati presentati rappresentano un avanzamento nello sviluppo di spettrometri di neutroni e raggi gamma per la diagnostica di plasmi da fusione, che combinano la capacità di operare a tassi di conteggio del MHz con la buona risoluzione energetica. I rivelatori sviluppati offrono dimensioni estremamente compatte, ideali per l’integrazione in camere con linee di vista multiple sui futuri reattori a fusione come ITER e DEMO.
(2018). Development of neutron and gamma-ray spectrometers for fusion plasma applications. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).
Development of neutron and gamma-ray spectrometers for fusion plasma applications
RIGAMONTI, DAVIDE
2018
Abstract
Neutrons and gamma-rays emitted by thermonuclear fusion plasmas can be detected providing information on fuel and fast ions. The high neutron and gamma-ray fluxes expected at JET in DT plasmas, together with the need to provide time resolution in the range of 10-100 ms, translates into the need of spectrometers capable to operate at counting rates in the MHz range. Moreover, measurements with high energy resolution are necessary to infer detailed information on fast and fuel ions. These pose very challenging requirements on the needed detector performances. This thesis is focused on the development, characterization and implementation of new compact gamma-ray and neutron spectrometers which combine high counting rate capabilities and high energy resolutions. A prototype compact gamma-ray spectrometer has been developed within a European project which is upgrading the JET gamma ray camera (GC) detectors. The prototype detector is based on a LaBr3 scintillator crystal coupled to a Silicon PhotoMultiplier (SiPM). The developed prototype spectrometer features an energy resolution of 5% at 662 keV, which improves for higher energies and provides accurate measurements in the energy range up to a few MeVs. A suitable shaping circuit of the SiPM signal has been implemented to minimize the presence of piled up events allowing operation at counting rates in excess of 1 MCounts/s, as demonstrated with record values 3 MCounts/s at an accelerator. The 19 new detectors have been successfully calibrated and installed during 2017 in the JET GC. The enhanced energy resolution of GC will allow avoiding artifacts in the reconstructed fast ions profile, while the improved time resolution will open the possibility to track for the first time the fast ions profile changes during their slowing down times. In the neutron spectroscopy field, a 12-pixels single crystal diamond matrix has been installed on a vertical line of sight of JET allowing simultaneous measurements of 2.5 MeV and 14 MeV neutrons. The response of diamond neutron spectrometers to 2.5 and 14 MeV neutrons is very different, namely it is dominated by elastic and inelastic scattering for 2.5 MeV neutrons while for 14 MeV neutrons several nuclear reaction channels open up, offering the possibility to perform high resolution spectroscopy. The diamond matrix response function has been measured at nuclear accelerators for incoming different monoenergetic neutron energies. The data validation of the diamond matrix with 2.5 MeV neutrons has been performed by comparing with data taken by the reference 2.5 MeV neutron spectrometer at JET, namely TOFOR. The results indicated that the spectrometer works well and can provide a moderated energy resolution in D plasmas. The excellent spectroscopic capabilities for 14 MeV neutrons, instead, has been explored during the characterization of a 14 MeV DT neutron generator. Neutrons were produced by DT reactions occurring by accelerating a mixed beam of Dx+ /Tx+ /DT+ beam (x=1,2) onto a titanium target containing T/D. Diamond detectors allowed resolving for the first time the complex features of the neutron energy spectra resulting from the simultaneous presence of D+ , T+ , D2+ , T2+ , DT+ species present in the beam. These results open up to new prospects for diagnosing DT plasmas on JET and ITER. The analysis of the diamond 12C(n,α)9Be peak, in fact, will allow accurately identifying supra-thermal components in DT plasma operations and studying non classical phenomena on the beam slowing down. The results presented in this thesis represent a step forward in the development of neutron and gamma-ray spectrometers for fusion plasma diagnostics which combine the MHz counting rate capability with the enhanced energy resolution. The developed instruments feature compact size and are therefore suitable for integration in a multi line of sight camera on the next step burning plasma fusion devices such as ITER and DEMO.File | Dimensione | Formato | |
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Descrizione: tesi di dottorato
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Doctoral thesis
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