The neutron Time-Of-Flight (n_TOF) facility, operational at CERN since 2000, is a pulsed neutron spallation source producing neutrons between a few meV and several GeV for two experimental stations located 185 m horizontally from the target and 20 m above the target. The neutron production target, made of pure lead, is impacted by the 20 GeV/c proton beam of the Proton Synchrotron. In 2018, the second-generation target reached the end of its lifetime, and the facility underwent a major upgrade during CERN’s Long Shutdown 2 (2019-2021), which included the installation of the new third-generation spallation target. The previous targets were based on water-cooled massive lead blocks, whereas the new target is cooled by nitrogen gas slightly above atmospheric pressure to limit erosion-corrosion and contamination of the cooling circuit with radioactive lead spallation products. A radiation protection study, performed with the FLUKA Monte Carlo code, contributed to the optimisation of the target design and to assess the safe operation of the facility. In 2021, during the commissioning of the new target, the simulation results were complemented by dedicated radiation measurements, which are described in this work: 1) Monitoring of stray radiation (neutrons and photons) in accessible areas by using fixed and mobile instrumentation (high-pressure ionisation chambers and the LUPIN extended range rem-counter, specifically conceived to measure pulsed neutrons) to ensure the compliance with the radiological area classification. 2) Monitoring of residual dose rate to provide useful information for planning and optimising accesses and interventions. 3) Monitoring of the nitrogen activation by means of a flow-through differential ionization chamber, for which a field calibration factor was calculated combining FLUKA simulations and experimental measurements. 4) Activation studies for zoning purposes by installing in specific locations of the n_TOF facility samples of materials typically used in particle accelerators (aluminium, copper, stainless steel); during the Year End Technical Stop 2021-2022, their specific activity were measured by gamma-spectrometry and compared against the applicable CERN clearance limits.
L'esperimento n_TOF (neutron Time-Of-Flight), operativo al CERN dal 2000, consiste in una sorgente pulsata di neutroni di spallazione (con uno spettro energetico da pochi meV a diversi GeV) per due stazioni sperimentali, una situata a 185 m a valle dal bersaglio e l'altra a 20 m sopra di esso. I neutroni sono prodotti dall'interazione del fascio di protoni da 20 GeV/c proveniente dal Proton Synchrotron con il bersaglio fatto di piombo puro. Nel 2018, il bersaglio di seconda generazione è stato rimosso, e la struttura è stata sottoposta ad una riqualificazione durante il cosiddetto Long Shutdown 2 (2019-2021), durante il quale è stato installato un nuovo bersaglio di spallazione (terza generazione). I bersagli precedenti erano raffreddati ad acqua, mentre il nuovo bersaglio è stato concepito per essere raffreddato da azoto in forma gassosa per limitare l'erosione e la contaminazione del circuito di raffreddamento con i prodotti di spallazione provenienti dal piombo. Uno studio preliminare di radioprotezione, realizzato con il codice Monte Carlo FLUKA, ha contribuito all'ottimizzazione del progetto e alla valutazioni dei rischi radiologici. Nel 2021, durante la messa in servizio del nuovo bersaglio, i risultati delle simulazioni sono stati completati con misure sperimentali, descritte in questo lavoro: 1) Monitoraggio della radiazione diretta (neutronica e fotonica) in aree accessibili utilizzando strumentazione fissa e mobile (camere di ionizzazione ad alta pressione e il LUPIN, un extended-range rem-counter specificamente concepito per misurare i neutroni in campi pulsati), allo scopo di garantire la conformità con la classificazione radiologica degli ambienti dove le misure sono state effetuate. 2) Monitoraggio della radiazione residua con lo scopo di fornire informazioni radiologiche rilevanti per pianificare e ottimizzare gli accessi nelle aree ove le misure sono state effettuate. 3) Monitoraggio dell'attivazione dell'azoto con una camera di ionizzazione a flusso differenziale, per la quale è stato calcolato un fattore di calibrazione, combinando simulazioni FLUKA e misure sperimentali. 4) Studi di attivazione mediante il posizionamento di campioni di materiali tipicamente utilizzati negli acceleratori di particelle (alluminio, rame, acciaio inossidabile). I campioni sono stati temporanemente rimossi durante lo stop di fine anno e la loro attivitá specifica è stata misurata attraverso spettrometria gamma e confrontata con i limiti di liberazione applicabili al CERN.
Radiation protection measurements during the commissioning of the third-generation n_TOF neutron spallation target at CERN
Lavezzari, Giacomo
2021/2022
Abstract
The neutron Time-Of-Flight (n_TOF) facility, operational at CERN since 2000, is a pulsed neutron spallation source producing neutrons between a few meV and several GeV for two experimental stations located 185 m horizontally from the target and 20 m above the target. The neutron production target, made of pure lead, is impacted by the 20 GeV/c proton beam of the Proton Synchrotron. In 2018, the second-generation target reached the end of its lifetime, and the facility underwent a major upgrade during CERN’s Long Shutdown 2 (2019-2021), which included the installation of the new third-generation spallation target. The previous targets were based on water-cooled massive lead blocks, whereas the new target is cooled by nitrogen gas slightly above atmospheric pressure to limit erosion-corrosion and contamination of the cooling circuit with radioactive lead spallation products. A radiation protection study, performed with the FLUKA Monte Carlo code, contributed to the optimisation of the target design and to assess the safe operation of the facility. In 2021, during the commissioning of the new target, the simulation results were complemented by dedicated radiation measurements, which are described in this work: 1) Monitoring of stray radiation (neutrons and photons) in accessible areas by using fixed and mobile instrumentation (high-pressure ionisation chambers and the LUPIN extended range rem-counter, specifically conceived to measure pulsed neutrons) to ensure the compliance with the radiological area classification. 2) Monitoring of residual dose rate to provide useful information for planning and optimising accesses and interventions. 3) Monitoring of the nitrogen activation by means of a flow-through differential ionization chamber, for which a field calibration factor was calculated combining FLUKA simulations and experimental measurements. 4) Activation studies for zoning purposes by installing in specific locations of the n_TOF facility samples of materials typically used in particle accelerators (aluminium, copper, stainless steel); during the Year End Technical Stop 2021-2022, their specific activity were measured by gamma-spectrometry and compared against the applicable CERN clearance limits.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/186055