RIPAMONTI GIANCARLO
In Memoriam
Giancarlo Ripamonti (1956-2019).
Prof. Ripamonti's research activity has been structured mainly along two lines.
During the first period, approximately from 1987 to 1990, his research was inspired by the need to design and create new SPAD (Single Photon Avalanche Diodes) devices able to offer a better resolution in measuring the arrival time of single photons. In the SPADs previously created by the group from the Politecnico di Milano in collaboration with LAMEL of Bologna, the detector response to rapid optical pulses was characterised by two components: a peak with a width at half height (full-width at half maximum FWHM) of about 100 ps, due to the photogenated carriers in the space charge zone, and a long "tail", due to the diffusion of the photogenated carriers in the neutral zone. The dependence of the shape of this tail on the wavelength of the incident photons put a serious limit to the prospects of SPADs in various applications, such as fast waveform measurements and high resolution laser ranging.
Prof. Ripamonti contributed to the design of the first epitaxial structure SPAD devices, which made it possible, on the one hand, to drastically reduce the duration of the diffusion tail and its dependence on the wavelength of the incident photons and, on the other hand, to significantly improve the temporal resolution of the device (up to about 30 ps FWHM at room temperature and 20 ps FWHM at -65 °C).
The study of the new SPAD structures has made it possible to identify new physical effects, related to the dynamics of charges in the presence of ionisation by impact. On the basis of the acquired knowledge, Prof. Ripamonti has contributed to the conception and development of new photodetectors, sensitive both to the position and to the instant of arrival of a single photon.
In the same period, Prof. Ripamonti made fundamental contributions to the creation of a single photon optical reflectometer able to combine millimetre spatial resolution, high sensitivity and wide dynamics, up to Rayleigh scattering detection. He also developed an original procedure that enables a reduction in the measurement time (up to a factor of 50) and an increase in the dynamic range in optical reflectometry measurements.
This first area of research was followed by the investigation of techniques into processing electrical signals being emitted from radiation detectors in general, in order to maximise the signal-to-noise ratio and extract the information contained in them.
In particular, in the field of optimal filtering of signals coming from nuclear particle detectors, Prof. Ripamonti's research activity focused on digital processing, and therefore initially on the analog-to-digital converter which, in order not to degrade the measurement, must have excellent linearity and resolution characteristics. One limitation of traditional converters is that it is very difficult to integrate them into VLSI technology with sufficient accuracy, stability and reproducibility. This proves to be a limitation given the tendency to integrate as much of the measurement chain as possible on a single chip, as has already been done for the detector-preamplifier block. Following these financial and technical considerations, it was then decided to verify the potential of using Sigma-Delta (oversampling) converters. Experimentally, a spectrometer based on a commercial Sigma-Delta converter was first built, of which only the modulator was used by implementing digital filtering in a DSP, with the clear advantage of thereby having complete freedom in the filtering action of the over-sampled signal. Subsequently a project was set up for the creation of a Sigma-Delta converter consisting of a configurable analog modulation section and a digital filtering section implemented in programmable logic (FPGA), in order to achieve the best synergy with the subsequent information processing.
Among the many topics addressed by Prof. Ripamonti, there undoubtedly emerges the research focused on the problem of spatial localisation of the interaction of photons at different energies with the active material of very large size (about 500 cm3) segmented hyperpure germanium detector crystals (HPGe). The problem is made extremely complex by the fact that the shape of the signal induced on the electrodes with which each crystal is electrically 'segmented' strongly depends on the spatial coordinates of the interaction point. In addition, the detectors are mounted contiguously to cover a solid angle of 4 radiants. A target is placed in the centre of the 'sphere' enclosed by the detectors and a beam of particles, collimated from the outside through a slit specifically created in the detector cap, is collided with the target and the swarm of photons is revealed and analysed and, as a result of the impact, is released, for a total of almost 3000 independent electrodes, i.e. channels whose signals are processed in parallel on-line. Prof. Ripamonti has conceived and implemented, through DSP architectures, an innovative algorithm for the estimation of the spatial coordinates of different events that interact simultaneously in the same segment of a detector (which is therefore responsible for only one overall electrical signal).
Among the most significant objects of the activity in this field was the in-depth study into innovative architectures and algorithms applied to the treatment of detector output signals, in particular complex systems for X spectroscopy and multi-channel range. First of all, Prof. Ripamonti has contributed to the development of a new system based on the digital processing of samples obtained by digitising the signal impulse at the output from a conditioning circuit. The implemented system maximises the signal-to-noise ratio in estimating the amplitude and time of signal generation, and represents a digital alternative of the analog optimal processor derived from the theory of optimal filtering. The system is able to automatically synthesise the best possible filter through the on-line measurement of the noise actually present in the experiment, and with arbitrary constraints in time and frequency, which makes it possible to obtain the best resolution and enables a complete flexibility of adaptation to variable operating conditions. Two innovative digital techniques for event triggering and precise baseline measurement have also been developed, as well as new analog signal conditioning architectures upstream of digitisation, which are aimed at simultaneously optimising the analog-to-digital conversion process and subsequent digital processing.
Prof. Ripamonti's research activity has been structured mainly along two lines.
During the first period, approximately from 1987 to 1990, his research was inspired by the need to design and create new SPAD (Single Photon Avalanche Diodes) devices able to offer a better resolution in measuring the arrival time of single photons. In the SPADs previously created by the group from the Politecnico di Milano in collaboration with LAMEL of Bologna, the detector response to rapid optical pulses was characterised by two components: a peak with a width at half height (full-width at half maximum FWHM) of about 100 ps, due to the photogenated carriers in the space charge zone, and a long "tail", due to the diffusion of the photogenated carriers in the neutral zone. The dependence of the shape of this tail on the wavelength of the incident photons put a serious limit to the prospects of SPADs in various applications, such as fast waveform measurements and high resolution laser ranging.
Prof. Ripamonti contributed to the design of the first epitaxial structure SPAD devices, which made it possible, on the one hand, to drastically reduce the duration of the diffusion tail and its dependence on the wavelength of the incident photons and, on the other hand, to significantly improve the temporal resolution of the device (up to about 30 ps FWHM at room temperature and 20 ps FWHM at -65 °C).
The study of the new SPAD structures has made it possible to identify new physical effects, related to the dynamics of charges in the presence of ionisation by impact. On the basis of the acquired knowledge, Prof. Ripamonti has contributed to the conception and development of new photodetectors, sensitive both to the position and to the instant of arrival of a single photon.
In the same period, Prof. Ripamonti made fundamental contributions to the creation of a single photon optical reflectometer able to combine millimetre spatial resolution, high sensitivity and wide dynamics, up to Rayleigh scattering detection. He also developed an original procedure that enables a reduction in the measurement time (up to a factor of 50) and an increase in the dynamic range in optical reflectometry measurements.
This first area of research was followed by the investigation of techniques into processing electrical signals being emitted from radiation detectors in general, in order to maximise the signal-to-noise ratio and extract the information contained in them.
In particular, in the field of optimal filtering of signals coming from nuclear particle detectors, Prof. Ripamonti's research activity focused on digital processing, and therefore initially on the analog-to-digital converter which, in order not to degrade the measurement, must have excellent linearity and resolution characteristics. One limitation of traditional converters is that it is very difficult to integrate them into VLSI technology with sufficient accuracy, stability and reproducibility. This proves to be a limitation given the tendency to integrate as much of the measurement chain as possible on a single chip, as has already been done for the detector-preamplifier block. Following these financial and technical considerations, it was then decided to verify the potential of using Sigma-Delta (oversampling) converters. Experimentally, a spectrometer based on a commercial Sigma-Delta converter was first built, of which only the modulator was used by implementing digital filtering in a DSP, with the clear advantage of thereby having complete freedom in the filtering action of the over-sampled signal. Subsequently a project was set up for the creation of a Sigma-Delta converter consisting of a configurable analog modulation section and a digital filtering section implemented in programmable logic (FPGA), in order to achieve the best synergy with the subsequent information processing.
Among the many topics addressed by Prof. Ripamonti, there undoubtedly emerges the research focused on the problem of spatial localisation of the interaction of photons at different energies with the active material of very large size (about 500 cm3) segmented hyperpure germanium detector crystals (HPGe). The problem is made extremely complex by the fact that the shape of the signal induced on the electrodes with which each crystal is electrically 'segmented' strongly depends on the spatial coordinates of the interaction point. In addition, the detectors are mounted contiguously to cover a solid angle of 4 radiants. A target is placed in the centre of the 'sphere' enclosed by the detectors and a beam of particles, collimated from the outside through a slit specifically created in the detector cap, is collided with the target and the swarm of photons is revealed and analysed and, as a result of the impact, is released, for a total of almost 3000 independent electrodes, i.e. channels whose signals are processed in parallel on-line. Prof. Ripamonti has conceived and implemented, through DSP architectures, an innovative algorithm for the estimation of the spatial coordinates of different events that interact simultaneously in the same segment of a detector (which is therefore responsible for only one overall electrical signal).
Among the most significant objects of the activity in this field was the in-depth study into innovative architectures and algorithms applied to the treatment of detector output signals, in particular complex systems for X spectroscopy and multi-channel range. First of all, Prof. Ripamonti has contributed to the development of a new system based on the digital processing of samples obtained by digitising the signal impulse at the output from a conditioning circuit. The implemented system maximises the signal-to-noise ratio in estimating the amplitude and time of signal generation, and represents a digital alternative of the analog optimal processor derived from the theory of optimal filtering. The system is able to automatically synthesise the best possible filter through the on-line measurement of the noise actually present in the experiment, and with arbitrary constraints in time and frequency, which makes it possible to obtain the best resolution and enables a complete flexibility of adaptation to variable operating conditions. Two innovative digital techniques for event triggering and precise baseline measurement have also been developed, as well as new analog signal conditioning architectures upstream of digitisation, which are aimed at simultaneously optimising the analog-to-digital conversion process and subsequent digital processing.
