Few Tens Of Ps Research Articles

4D-tracking with digital SiPMs

Silicon Photomultipliers (SiPMs) are the state-of-the-art technology in single-photon detection with solid-state detectors. Single Photon Avalanche Diodes (SPADs), the key element of SiPMs, can now be manufactured in CMOS processes, facilitating the integration of a SPAD array into custom monolithic ASICs. This allows implementing features such as signal digitization, masking, full hit-map readout, noise suppression, and photon counting in a monolithic CMOS chip. The complexity of the off-chip readout chain is thereby reduced.These new features allow new applications for digital SiPMs, such as 4D-tracking of charged particles, where spatial resolutions of the order of 10µm and timestamping with time resolutions of a few tens of ps are required.A prototype of a digital SiPM was designed at DESY using the LFoundry 150nm CMOS technology. Various studies were carried out in the laboratory and at the DESY II test-beam facility to evaluate the sensor performance in Minimum Ionizing Particles (MIPs) detection. The direct detection of charged particles was investigated for bare prototypes and assemblies coupling dSiPMs and thin LYSO crystals. Spatial resolution ∼20µm and a full-system time resolution of ∼50ps are measured using bare dSiPMs in direct MIP detection. Efficiency >99.5%, low noise rate and time resolution <1ns can be reached with the thin radiator coupling.

Dynamics of hydrogen bond reorganization in the S1(ππ*) state of 9-Anthracenecarboxaldehyde

Effect of solvent polarity and hydrogen bonding interaction on the excited state dynamics of 9-Anthracenecarboxaldehyde (9-ACD) have been investigated using steady state and time-resolved electronic and vibrational spectroscopic techniques. Photophysical properties and dynamics of the excited states are seen to be more sensitive to intermolecular hydrogen bonding ability of the solvent, rather than its polarity. FTIR studies reveal formation of complexes with the protic solvents via formation of conventional hydrogen bond at the carbonyl site of 9-ACD molecule. In strong hydrogen bond donating solvents, namely, 2, 2, 2-trifuoroethanol (TFE) and 1, 1, 1, 3, 3, 3-hexafluoroisopropanol (HFIP), nearly all the molecules of 9-ACD in solution remain engaged in complex formation leaving insignificant number of molecules remaining free in solution. In aprotic solvents, the S1 state is short-lived (the lifetime is only a few tens of ps) and intersystem crossing (ISC) is the major deactivation pathway (the triplet yield is near unity). However, in strong hydrogen bond donating solvents, the S1 state lifetime is long (a few ns) because the deactivation pathway for the S1 state via the triplet manifold is blocked. Both the ultrafast time-resolved electronic and vibrational spectroscopy studies in TFE and HFIP reveal that hydrogen bond reorganization process following photoexcitation of the hydrogen bonded complex is the only relaxation process undergone by the S1 state in sub-100 ps time domain. However, the time-resolved IR (TRIR) spectroscopy studies further reveal the possibility of formation of aromatic π- hydrogen bonding and reorganization of the hydrogen bond at this site makes the major contribution towards the S1 state relaxation process in strong hydrogen bond donating solvents.

Performance of an Asymmetric on–Off Keying Modulation for Space Communications Using Single-Photon Superconducting Nanowire Detectors

Communications from deep space need to overcome two major issues: the strong attenuation due to the large distances and the limited power available at the transmitter. A class of sensors which looks promising for deep space communications is provided by the superconducting nanowire single-photon detectors that are characterized by very small jitter (few tens of ps), fast rise (hundreds of ps), and comparatively slow decay (few ns). Although it could seem that the bottleneck is represented by the long reset time, in this article, we present a scheme whose effective rate is limited by the jitter and that is able to transmit, with a superconducting nanowire single-photon detector with a ratio between decay time and jitter equal to 170 (a typical value), almost six times faster than the rate that the decay time would allow.

DC-coupled resistive silicon detectors for 4D tracking

In this work, we introduce a new design concept: the DC-coupled Resistive Silicon Detectors, based on the LGAD technology. This new design intends to address a few known drawbacks of the first generation of AC-coupled Resistive Silicon Detectors (RSD). The sensor behaviour is simulated using a fast hybrid approach based on a combination of two packages, Weightfield2 and LTSpice. The simulation demonstrates that the key features of the RSD design are maintained, yielding excellent space and time resolutions: a few microns and a few tens of ps. In this report, we will outline the optimization methodology and the results of the simulation. We will also present detailed studies on the effects induced by the choice of key design parameters on the space and time resolutions provided by this sensor.

Wavelength‐dependent photochemistry of 1‐phenyl‐1‐diazopropane

Abstract1‐Phenyl‐1‐diazopropane (PDP) was photolyzed in acetonitrile/methanol mixtures using either 520‐nm light or 350‐nm light. The main photoproducts were 1‐methoxypropylbenzene and E,Z‐1‐phenylpropene. With 520‐nm light, a plot of (1‐methoxypropylbenzene)/(E,Z‐1‐phenylpropene) versus methanol concentration was linear. These results are consistent with the visible light induced fragmentation of PDP to dinitrogen and closed shell singlet (S0) ethyl, phenylcarbene (1EPC) which partitions between 1,2 hydrogen migration to form alkene, and reaction with methanol to form ether. The same plot generated with 350‐nm light significantly deviates from linearity. It is proposed that 350‐nm light generates the carbene in a vibrationally excited, open shell S1 excited state (1EPC*). This state rearranges to E,Z‐1‐phenylpropenes in competition with relaxation to ground state 1EPC over a few tens of ps. As a result, methanol cannot completely suppress alkene formation upon photolysis of PDP with 350‐nm light. The carbene excited state is a “non‐trappable carbene” route to alkenes.

R&D of a fast timing electronics board for testing SiPMs

A general-purpose SiPM electronics board has been developed in this work. Compared with commercial driver boards, this electronics board has a wider power supply range (20 V–90 V) to suit various types of SiPM. This new board enables cascading batch tests to meet the needs for high-volume SiPM testing procedures. More importantly, the new board can test SiPM timing performance down to a few tens of ps.

Timepix3 detector and Geant4-based simulations for gamma energy detection in Laser Produced Plasmas

The particular physics of Laser Produced Plasmas (LPP) needs some diagnostic requirements. Specifically, the X-ray monitoring of the plasma is known to be difficult since typically X-ray emissions are concentrated in bursts from a few tens of ps to few ns, based on the power and pulse time width of the laser. Therefore, the energy measurement of the radiation coming from a single experimental run is basically unfeasible using conventional techniques. Additional particles can be produced from LPP experiments, especially high energy gamma photons and electrons. As a case study in recent experiments, carried out on VEGA-2 laser facility (CLPU, Salamanca, Spain), the aim was to produce neutrons through photonuclear reactions on different types of solid targets. We have used the Timepix3 chip, in a “side-on” configuration, in order to produce a quick estimate of the gamma photons energy involved in the reactions. This detector, based on silicon, is realized with a single chip of 256 × 256 pixels bump-bonded with a 14 mm × 14 mm × 300 μm silicon layer. Interaction of gammas with the detector in this configuration produces some characteristic clusters of pixels and, for each cluster, a variety of physical and morphological parameters can be defined. Based on some of these parameters, we have characterized the detector response using some known laboratory gamma sources and the related Geant4 simulations. This allows quick energy discrimination for the gamma photons coming from different experimental runs.

Precision timing photon detectors using Cadmium-Telluride sensors

Precision timing properties of high energy physics detectors in high luminosity colliders has become an important requirement for the success of particle detection, of which new calorimeter with timing precision of few tens of ps is the main interest. In this paper, we presented the first calorimetry and timing performance measurement of Cadmium-Telluride (CdTe) as the active material of sampling calorimeter, using 2 GeV to 200 GeV electron beam from CERN beam lines. A good energy performance is achieved due to the better sensitivity of CdTe to X ray photons in the secondary shower than other semi-conducting sensors such as silicon. The timing resolution of the tested CdTe sensor is measured to be down to 21 ps under the best conditions.

Particle IDentification with the ALICE Time-Of-Flight detector at the LHC

The main physics goal of ALICE (A Large Ion Collider Experiment) is the study of the physics of strongly interacting matter at extreme energy densities, where the formation of a new phase of matter, the Quark-GluonPlasma (QGP), is expected. One of the most important aspect of ALICE is the Particle IDentification (PID) performed by combining different identification techniques. The time measurement with the Time-Of-Flight (TOF) detector, in conjunction with the momentum and track length measured by the tracking detectors is used to calculate the particle mass in the intermediate momentum range from 0.3 up to a few GeV/c. The TOF system covers an area of 141 m2 and consists of Multi-gap Resistive Plate Chambers (MRPC), that allow excellent performances in terms of intrinsic time resolution (of the order of few tens of ps) and overall efficiency (close to 100%).The ALICE TOF system has shown very stable operation during the firstthree years of collisions at the LHC, reaching the design goal of 80ps time resolution. The performance and main physics results will bereported.

On the reliability of NMR relaxation data analyses: A Markov Chain Monte Carlo approach

The analysis of NMR relaxation data is revisited along the lines of a Bayesian approach. Using a Markov Chain Monte Carlo strategy of data fitting, we investigate conditions under which relaxation data can be effectively interpreted in terms of internal dynamics. The limitations to the extraction of kinetic parameters that characterize internal dynamics are analyzed, and we show that extracting characteristic time scales shorter than a few tens of ps is very unlikely. However, using MCMC methods, reliable estimates of the marginal probability distributions and estimators (average, standard deviations, etc.) can still be obtained for subsets of the model parameters. Thus, unlike more conventional strategies of data analysis, the method avoids a model selection process. In addition, it indicates what information may be extracted from the data, but also what cannot.

Particle identification with the ALICE Time-Of-Flight detector at the LHC

High performance Particle Identification system (PID) is a distinguishing characteristic of the ALICE experiment at the CERN Large Hadron Collider (LHC). Charged particles in the intermediate momentum range are identified in ALICE by the Time-Of-Flight (TOF) detector. The TOF exploits the Multi-gap Resistive Plate Chamber (MRPC) technology, capable of an intrinsic time resolution at the level of few tens of ps with an overall efficiency close to 100% and a large operation plateau. The full system is made of 1593 MRPC chambers with a total area of 141m2, covering the pseudorapidity interval [−0.9,+0.9] and the full azimuthal angle.The ALICE TOF system has shown very stable operation during the first 3years of collisions at the LHC. In this paper a summary of the system performance as well as main results with data from collisions will be reported.

Can Transient Phenomena Help Improving Time Resolution in Scintillators?

The time resolution of a scintillator-based detector is directly driven by the density of photoelectrons generated in the photodetector at the detection threshold. At the scintillator level it is related to the intrinsic light yield, the pulse shape (rise time and decay time) and the light transport from the gamma-ray conversion point to the photodetector. When aiming at 10 ps time resolution, fluctuations in the thermalization and relaxation time of hot electrons and holes generated by the interaction of ionization radiation with the crystal become important. These processes last for up to a few tens of ps and are followed by a complex trapping-detrapping process, Poole-Frenkel effect, Auger ionization of traps and electron-hole recombination, which can last for a few ns with very large fluctuations. This paper will review the different processes at work and evaluate if some of the transient phenomena taking place during the fast thermalization phase can be exploited to extract a time tag with a precision in the few ps range. A very interesting part of the sequence is when the hot electrons and holes pass below the limit of the ionization threshold. The only way to relax their energy is then through collisions with the lattice resulting in the production of optical and acoustic phonons with relatively high energy (up to several tens of meV) near the ionization threshold. As the rate of such electron/phonon exchange is about 100 events/ps/electron or hole and as the number of electrons/holes generated after mutiplication in a high light yield scintillator like LSO can be as high as 100,000 or more, we end up with an energy deposition rate of about 100 KeV/ps. This energy deposition rate contributes to many fast processes with a characteristic time in the ps range such as band-to-band luminescence, hot intraband luminescence, acoustic shock wave generation, fast local variation of index of refraction, etc. We will discuss if the part of the total energy which is released this way, and which can represent between 50% and 90% of the energy of the incoming ionization radiation, can be efficiently exploited to improve the time resolution of scintillators, presently limited to the 100 ps range.

Stability of fluctuating and transient aggregates of amphiphilic solutes in aqueous binary mixtures: Studies of dimethylsulfoxide, ethanol, and tert-butyl alcohol

In aqueous binary mixtures, amphiphilic solutes such as dimethylsulfoxide (DMSO), ethanol, tert-butyl alcohol (TBA), etc., are known to form aggregates (or large clusters) at small to intermediate solute concentrations. These aggregates are transient in nature. Although the system remains homogeneous on macroscopic length and time scales, the microheterogeneous aggregation may profoundly affect the properties of the mixture in several distinct ways, particularly if the survival times of the aggregates are longer than density relaxation times of the binary liquid. Here we propose a theoretical scheme to quantify the lifetime and thus the stability of these microheterogeneous clusters, and apply the scheme to calculate the same for water-ethanol, water-DMSO, and water-TBA mixtures. We show that the lifetime of these clusters can range from less than a picosecond (ps) for ethanol clusters to few tens of ps for DMSO and TBA clusters. This helps explaining the absence of a strong composition dependent anomaly in water-ethanol mixtures but the presence of the same in water-DMSO and water-TBA mixtures.

New Phonon Dynamics in a Diamond Crystal during Post-Annealing

A new mesoscopic response in a crystal during post-annealing (PA) after ion implantation has been discovered. PA is an indispensable procedure for activating the implanted dopant center by retrieving the lost crystallinity of the host crystal, which usually elapses over periods longer than a few milliseconds. By using an empirical molecular-dynamics simulation, we examined what actually happened during extended PA for up to one nanosecond in case of sub-keV N2 beam implantation into bulk diamond at room temperature with annealing at 1000 K. While observing the changes in the long-range-order (LRO) parameter, a notable phase transition from the crystalline to the amorphous state (CA transition) occurred synergistically at approximately a few picoseconds (ps). After the CA transition, PA was initiated. A few tens of ps later, a sequence of restoration spikes was observed in LRO parameter with a regular time interval of 70–80 ps. Each sharp peak showed the spontaneous restoration of “amorphous -> crystal -> amorphous” transitions. This process may trigger a “critical state” that occurs during the slow and steady recovery of the lost crystallinity in the early stages of PA. It was found that annealing not only enhances the vibration amplitude of the individual atoms in the bulk but also causes phonon-assisted restoration in a mesoscopic space.

Effects of Surface Recombination on Exciton Dynamics in GaN Nanorods

We have investigated the optical properties of GaN nanorods with different diameters by micro-photoluminescence (PL) and time-resolved PL measurements grown on (111) Si substrates by using radio-frequency plasma-assisted molecular-beam epitaxy. In order to confirm the formation of the GaN nanorods and their physical dimension, we employed high resolution scanning electron microscopy. As the diameters of the nanorods decreased, the donor bound exciton (D 0 X) emission, which is mainly observed at low temperature for samples with larger diameters, rapidly quenched compared with the free exciton (FX). The activation energy of the FX for GaN nanorods with different diameters was also evaluated. The decay times are a few tens of ps for both the D 0 X and the FX transitions. The overall decay times decrease linearly with decreasing average diameter of the nanorods. Time-resolved PL studies show that surface recombination is dominant in determining the excitonic dynamics.

Micro- and Time-resolved Photoluminescence in GaN Nanorods with Different Diameters

We have investigated the optical properties of GaN nanorods with different diameters by using microand time-resolved photoluminescence measurements. Donor-bound and free exciton peaks are observed in GaN nanorods with diameters larger than 100 nm. While the relative magnitude of the free exciton emission gradually increases with decreasing nanorod diameter, there is a relative decrease in the emission from the donor bound exciton. Using time-resolved photoluminescence, the diameter dependence of the decay times for these exciton peaks is measured to be a few tens of ps. With decreasing diameter, the decay time decreases due to surface recombination.

Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases

We demonstrate that the nanoparticle formation during laser ablation of metals by short (of a few tens of ps) laser pulses strongly depends on the concentration of surrounding gas. While, at vacuum conditions, nanoparticle formation shows very “sharp” atomic force microscope images of aggregated clusters, following with clear appearance of plasmon resonance on the absorption spectra of deposited films, an addition of gas particles starts to decrease the probability of cluster formation. This process shows a threshold for both helium (33 torr) and xenon (12 torr) above which no surface plasmon resonance and correspondingly no observable nanoparticles on the deposited surfaces were detected. The destruction of nanoparticle formation was attributed to the negative influence of surrounding gas particles on ablated particles aggregation.

X radiation sources based on accelerators

Light sources based on accelerators aim at producing very high brilliance coherent radiation, tuneable from the infrared to X-ray range, with picosecond or femtosecond light pulses. The first synchrotron light sources were built around storage rings in which a large number of relativistic electrons produce “synchrotron radiation” when their trajectory is subjected to a magnetic field, either in bending magnets or in specific insertion devices (undulators), made of an alternating series of magnets, allowing the number of curvatures to be increased and the radiation to be reinforced. These “synchrotron radiation” storage rings are now used worldwide (there are more than thirty), and they simultaneously distribute their radiation to several tens of users around the storage ring. The most effective installations in term of brilliance are the so-called 3rd generation synchrotron radiation light sources. The radiation produced presents pulse durations of the order of a few tens of ps, at a high rate (of the order of MHz); it is tuneable over a large range, depending on the magnetic field and the electron beam energy and its polarisation is adjustable (in the VUV-soft-X range). Generally, a very precise spectral selection is made by the users with a monochromator. The single pass linear accelerators can produce very short electron bunches ( ∼ 100 femtosecond ). The beam of very high electronic density is sent into successive undulator modules, reinforcing the radiation's longitudinal coherence, produced according to a Free Electron Laser (FEL) scheme by the interaction between the electron bunch and a light wave. The very high peak brilliance justifies their designation as 4th generation sources. The number of users is smaller because an electron pulse produces a radiation burst towards only one beamline. Energy Recovery Linacs (ERL) let the beam pass several times in the accelerator structures either to recover the energy or to accelerate the electrons during several turns, and thus provide subpicosecond beams for a greater number of users. A state-of-the-art of X sources using conventional (and not laser plasma based) accelerators is given here, underlying the performance already reached or forecast and the essential challenges. To cite this article: M.-E. Couprie, J.-M. Filhol, C. R. Physique 9 (2008).

1∕ffluctuations of DNA temperature at thermal denaturation

We theoretically investigated the temperature fluctuations of DNA close to denaturation and observed a strong enhancement of these fluctuations at the critical temperature. Although in a much lower frequency range, such a sharp increase was also reported in the recent experimental work of Nagapriya [Phys. Rev. Lett. 96, 038102 (2006)]. We showed that there is instead no enhancement of temperature fluctuations when the dissipation coefficient gamma in Langevin equations is assumed to be larger than a few tens of ps;{-1} , and pointed out the possible role of the solvent in real experiments. We sought for a possible correlation between the growth of large bubbles and the enhancement of temperature fluctuations but found no direct evidence thereof. Finally, we showed that neither the enhancement of fluctuations nor the 1f dependence are observed at the scale of a single base pair, while these properties show up when summing the contributions of a large number of base pairs. We therefore conclude that both effects result from collective motions that are facilitated by the divergence of the correlation length at denaturation.

Effects of Pulse Delay in Metal Micromachining with Femtosecond Laser Pulses

Single and double femtosecond laser pulses with different time delays were used to investigate surface morphology changes in Cr thin films. A double pulse was generated by varying the beam path length of a split pulse and recombining the two beams. With a single pulse, a nanometer-sized droplet was formed on the irradiated surface; the area was dependent on the laser power. With mul- tiple pulses at a fixed repetition rate, the subwavelength ripples aligned perpendicular to the polari- zation direction of the laser light with a spacing of about 200 nm. Curiously, the initial ripple was in the same direction as the laser polarization for the first few pulses. Using a double pulse with differ- ent polarization directions and a time separation of zero, the ripples formed in the sum vector direc- tion of both beams. Increasing the time separation removed the ripples effectively. After a few tens of ps, the second pulse did not affect the surface changes. The changes in surface morphology were attributable to interference or disturbances between the hot electron oscillations induced by the two pulses and the diffusion time of the electron energy. A double pulse with different polarization direc- tions within the diffusion time of the electron energy is a useful method for decreasing the size of the ablated area and eliminating ripples in micromachining.