Disputation: Laura Franconi

Master of Science Laura Franconi at Department of Physics will be defending the thesis 

"Insertable B-Layer integration in the ATLAS experiment and development of future 3D silicon pixel sensors"

for the degree of PhD

Trial lecture - time and place

Trial lecture: 10:15 am at Lille Fysiske auditorium (V232)


Adjudication committee


  • Dr. Pamela Ferrari, Nikhef, Netherlands
  • Professor Dieter Röhrich, Institute of Physics and Technology,  University of Bergen, Norway
  • Professor Andreas Görgen, Department of Physics, University of Oslo, Norway



Chair of defence




Additional information

How does a pixel detector speak? And how does radiation affect the performance of silicon

pixel sensors that will be used in the future phases of the Large Hadron Collider (LHC)?

This Ph.D. thesis aims at answering to these two questions.


The LHC is a huge machine that aims at discovering and studying the building blocks of the

universe and how they interact. To extend its discovery potential, the LHC keeps increasing

its beams’ energy and intensity. To cope with these new conditions, the LHC experiments

are upgrading their detectors. One of the recent upgrades of the ATLAS experiment is the

Insertable B-Layer (IBL), inserted in May 2014. Built with state-of-the-art technologies, this

pixel detector aims at better measuring the trajectory of particles that arise from the ATLAS

interaction region. The IBL sends the collected information as a stream of “words” made of

32 bits: each word is in fact a sequence of '0's and '1's. Some words, the so-called “hit

words”, contain two important pieces of information: the coordinates of the point at which a

particle crossed the detector and the charge collected at this position.

Part of the Ph.D. thesis work was the implementation of an offline software (the byte stream

converter) that decodes these bit words into a format readable by the ATLAS reconstruction

algorithms. Using a metaphor, detectors speak different languages. The “dialect” spoken by

the IBL detector differs from the language used by the other three layers of the pixel

detector. For example, it is able to convey more information in less words by using

“condensed words”: 4 words contain information on up to 10 pixel hits (the same number of

words in the Pixel format only transmits 4 pixel hits). Adopting this feature has the key

advantage of reducing the “detector readout bandwidth”, the number of words sent by the

detector per unit of time. Its usefulness is undeniable in the context of a large number of

words sent by each detector 40-million times per second, as it is the case of the LHC



Understanding how a high and prolonged flux of particles can damage a detector is of the

utmost importance for the silicon sensors instrumenting inner tracker at the ATLAS

experiment. Radiation reduces the amount of collected charge by modifying the silicon

structure and thus altering its electrical properties. Different approaches are under study to

counteract this degradation: for example, different materials (e.g. diamond rather than

silicon) or different geometries can be chosen.

The second part of this Ph.D. thesis work focused on the study of the 3D silicon sensor

technology. Contrary to the standard planar technology, where electrodes are implanted on

the surfaces of the silicon wafer, in the 3D layout the electrodes are columns etched through

the thickness of the device. This geometry presents several advantages: among them, it

allows for shorter inter-electrode distance, which in turn leads to better radiation hardness

(e.g. charge carriers have to travel a short distance and are less likely trapped in the

trapping centres caused by radiation damage).

The 3D sensor production process includes a series of masking, etching and doping


The devices studied in this thesis were produced by SINTEF, a Norwegian research

institute. Preliminary tests showed poor electrical performance in the vast majority of the

sensors. The origin of these shortcoming was unknown, but suspected to be related to a

faulty step in the production process.

As work for this thesis, the sensors were exposed to increasing levels of irradiation and then

tested under different conditions. Results converged to common features: sensors featuring

good electrical performance show higher charge collection even when exposed to a higher

level of radiation with respect to sensors with worse electrical characteristics. The

performed measurements also highlighted that the possible cause for such poor electrical

performance could be related to the presence of spurious structures in the silicon, caused by

a too-thin masking, that modify the electric field structure inside the silicon bulk. The same

conclusion was independently reached by researchers at SINTEF. Correcting the

shortcoming in the production process allowed SINTEF to produce new sensors that

present very encouraging results and that may be used to instrument the future generation

of pixel detectors in the ATLAS experiment.


Laura Franconi is a Ph.D. candidate in Particle Physics at the University of Oslo. She

obtained her Bachelor’s and Master’s degrees at the University of Bologna. Her work

focuses on the characterisation of silicon detector technologies used in high energy physics



Published Mar. 27, 2018 9:37 AM - Last modified Mar. 27, 2018 9:49 AM