The systematic control of radioactivity in scrap metal used by steel mills and foundries is of utmost importance and its relevance is certified by several international reports. Accidental or intentional dumping of radioactive sources (especially Cs-137 and Co-60) within scrap metal to be melted can produce both environmental damage and severe economic losses to involved companies. For this reason, the presence of radioactive sources must be detected as soon as possible in the production line. Nowadays, steel mills and foundries are equipped with gates scanning the trucks or the train wagons entering the scrap yard to search for radioactive sources. However, some sources, heavily shielded by the surrounding metal, can escape the detection, leading to a higher sensitivity request. In order to meet this request, the distance between the probable radioactive sources and the detection system needs to be reduced: a possible solution is represented by the placement of the detector inside the mechanical tool used to move the scrap metal (be it an electromagnet or a grapple). In this way the system operates at the shortest possible distance from the material and with the smallest volume of it, reducing its shielding effect. On the other hand, the solution leads to harsh operating conditions for the detection system like high magnetic fields, high temperatures, varying background radiation and short measurement times. This thesis work aims at developing from scratch a compact and USB powered gamma-ray spectrometer specifically designed for this purpose, based on Silicon Photomultipliers (because of their insensitivity to magnetic fields) and on a novel architecture. The detection system is based on a large NaI(Tl) cubic scintillator crystal with a side equal to 5cm optically coupled to only four SiPMs placed in each corner of the light output window of the crystal and characterized by a sensor size of 6mmx6mm. The electronics is based on two different printed circuit boards: one board hosts on one side the detectors and on the other side the circuits that perform the conditioning of the signal coming from the SiPMs so that it can be digitalised by an ADC, while the other board carries the microcontroller used to manage the whole system and to perform temperature compensation stabilizing the gain of the detectors, the circuitry that generates the variable bias voltage of the SiPMs and the USB connector used to link the system and a host computer. The analog processing chain consists of a transimpedance preamplifier, a second gain stage, an analog peak stretcher and a variable threshold comparator used to trigger the acquisition of an event. The bias voltage of the SiPMs is generated from the 5V USB power supply by means of two DC-DC boost regulators in sequence: the first boosts the voltage to 12V, while the second is a variable DC-DC regulator that generates the bias voltage of the detectors with a value that can span from 17V to 40V. The thesis starts with some considerations about gamma radiation, about spectroscopy and energy resolution and about Silicon Photomultipliers. Then, the designed detection system is presented together with the design choices made to cope with its operating conditions: both the simulations performed to choose the scintillator readout configuration and their results are presented, followed by the analysis of the PCB deputed to the analog signal conditioning, the analysis of the microcontroller board and of the firmware that manages the system. Lastly, the measurements performed in laboratory together with some on the field measurements performed in the scrap yard of a scrap dealer are presented, concluding with the results and a look towards future developments of the detection system.
Il controllo sistematico della radioattività nei rottami metallici utilizzati dalle acciaierie e dalle fonderie è della massima importanza e la sua rilevanza è certificata da numerosi rapporti internazionali. Lo smaltimento accidentale o intenzionale di sorgenti radioattive (soprattutto Cs-137 e Co-60) all'interno di rottame metallico destinato ad essere fuso può produrre sia danni ambientali che gravi perdite economiche alle imprese coinvolte. Per questa ragione, la presenza di sorgenti radioattive deve essere rilevata il prima possibile lungo la linea di produzione. Al giorno d'oggi, le acciaierie e le fonderie sono dotate di cancelli che scansionano i camion o i vagoni dei treni che entrano nel deposito rottami in cerca delle eventuali sorgenti radioattive. Tuttavia, alcune sorgenti, fortemente schermate dal metallo circostante, possono sfuggire al rivelamento, portando a una richiesta di sensibilità più elevata. Per soddisfare questa richiesta, è necessario ridurre la distanza tra le probabili sorgenti radioattive e il sistema di rivelamento: una possibile soluzione è rappresentata dal posizionamento del rivelatore all'interno dello strumento meccanico utilizzato per spostare il rottame metallico (sia esso un elettromagnete o una benna a polipo). In questo modo il sistema opera alla distanza più breve possibile dal materiale e con il suo volume più piccolo, riducendone l'effetto schermante. Per contro, la soluzione porta a difficili condizioni operative per il sistema di rilevamento come gli alti campi magnetici, le alte temperature, la radiazione di fondo variabile e i brevi tempi di misurazione. Questo lavoro di tesi mira a sviluppare da zero uno spettrometro a raggi gamma compatto e alimentato tramite USB specificamente progettato per questo scopo, basato su Silicon Photomultipliers (grazie alla loro insensibilità a campi magnetici) e su una nuova architettura. Il sistema di rivelamento è basato su un cristallo scintillatore cubico NaI(Tl) di grandi dimensioni caratterizzato da un lato di 5cm, accoppiato otticamente a soli quattro SiPM posizionati nei quattro angoli della finestra del cristallo e caratterizzati da una dimensione del sensore di 6mmx6mm. L'elettronica si basa su due diverse schede: una scheda contiene su un lato i rivelatori e sull'altro i circuiti che eseguono il condizionamento del segnale proveniente dai SiPM in modo che possa essere digitalizzato da un ADC, mentre l'altra scheda contiene il microcontrollore utilizzato per gestire l'intero sistema e per eseguire la compensazione della temperatura stabilizzando il guadagno dei rivelatori, il circuito che genera la tensione di polarizzazione variabile dei SiPM e il connettore USB utilizzato per collegare il sistema e un computer host. La catena di elaborazione analogica è costituita da un preamplificatore a transimpedenza, un secondo stadio di guadagno, un peak stretcher analogico e un comparatore a soglia variabile utilizzato per attivare l'acquisizione di un evento. La tensione di polarizzazione dei SiPM viene generata dall'alimentazione USB a 5V mediante due regolatori DC-DC boost in sequenza: il primo porta la tensione a un valore di 12V, mentre il secondo è un regolatore DC-DC variabile che genera la tensione di polarizzazione dei rivelatori con un valore compreso tra 17V e 40V. La tesi inizia con alcune considerazioni sulla radiazione di tipo gamma, sulla spettroscopia e la risoluzione energetica e sui Silicon Photomultipliers. Quindi, il sistema di rilevamento progettato viene presentato insieme alle scelte progettuali fatte per far fronte alle sue condizioni operative: vengono presentate sia le simulazioni eseguite per scegliere la configurazione di lettura dello scintillatore che i relativi risultati, seguite dall'analisi della scheda dedicata al condizionamento del segnale analogico, dall'analisi della scheda del microcontrollore e del firmware che gestisce il sistema. Infine, vengono presentate le misurazioni eseguite in laboratorio insieme ad alcune misurazioni sul campo eseguite nel deposito rottami di un rottamatore, concludendo con i risultati e uno sguardo ai futuri sviluppi del sistema di rivelamento.
SiPM-based gamma spectrometer embeddable in lifting electromagnets for scrap yard monitoring
LAVELLI, EMANUELE
2017/2018
Abstract
The systematic control of radioactivity in scrap metal used by steel mills and foundries is of utmost importance and its relevance is certified by several international reports. Accidental or intentional dumping of radioactive sources (especially Cs-137 and Co-60) within scrap metal to be melted can produce both environmental damage and severe economic losses to involved companies. For this reason, the presence of radioactive sources must be detected as soon as possible in the production line. Nowadays, steel mills and foundries are equipped with gates scanning the trucks or the train wagons entering the scrap yard to search for radioactive sources. However, some sources, heavily shielded by the surrounding metal, can escape the detection, leading to a higher sensitivity request. In order to meet this request, the distance between the probable radioactive sources and the detection system needs to be reduced: a possible solution is represented by the placement of the detector inside the mechanical tool used to move the scrap metal (be it an electromagnet or a grapple). In this way the system operates at the shortest possible distance from the material and with the smallest volume of it, reducing its shielding effect. On the other hand, the solution leads to harsh operating conditions for the detection system like high magnetic fields, high temperatures, varying background radiation and short measurement times. This thesis work aims at developing from scratch a compact and USB powered gamma-ray spectrometer specifically designed for this purpose, based on Silicon Photomultipliers (because of their insensitivity to magnetic fields) and on a novel architecture. The detection system is based on a large NaI(Tl) cubic scintillator crystal with a side equal to 5cm optically coupled to only four SiPMs placed in each corner of the light output window of the crystal and characterized by a sensor size of 6mmx6mm. The electronics is based on two different printed circuit boards: one board hosts on one side the detectors and on the other side the circuits that perform the conditioning of the signal coming from the SiPMs so that it can be digitalised by an ADC, while the other board carries the microcontroller used to manage the whole system and to perform temperature compensation stabilizing the gain of the detectors, the circuitry that generates the variable bias voltage of the SiPMs and the USB connector used to link the system and a host computer. The analog processing chain consists of a transimpedance preamplifier, a second gain stage, an analog peak stretcher and a variable threshold comparator used to trigger the acquisition of an event. The bias voltage of the SiPMs is generated from the 5V USB power supply by means of two DC-DC boost regulators in sequence: the first boosts the voltage to 12V, while the second is a variable DC-DC regulator that generates the bias voltage of the detectors with a value that can span from 17V to 40V. The thesis starts with some considerations about gamma radiation, about spectroscopy and energy resolution and about Silicon Photomultipliers. Then, the designed detection system is presented together with the design choices made to cope with its operating conditions: both the simulations performed to choose the scintillator readout configuration and their results are presented, followed by the analysis of the PCB deputed to the analog signal conditioning, the analysis of the microcontroller board and of the firmware that manages the system. Lastly, the measurements performed in laboratory together with some on the field measurements performed in the scrap yard of a scrap dealer are presented, concluding with the results and a look towards future developments of the detection system.| File | Dimensione | Formato | |
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2018_07_Lavelli.pdf
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Descrizione: Testo della tesi
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https://hdl.handle.net/10589/141720