J. Rosado

Analysis of the fluorescence emission from atmospheric nitrogen by electron excitation, and its application to fluorescence telescopes

A microscopic analysis of the processes involved in the fluorescence emission of nitrogen molecules induced by electronic collisions is carried out for a large range of incident energies (eV to GeV) and pressures (Pa to atmospheric conditions). The contribution of secondary electrons to that fluorescence is calculated by means of detailed Monte Carlo simulations. For this purpose, a novel analytical approximation of the energy spectrum of secondary electrons is used. The results of the simulations are shown to be useful for the interpretation of available experimental data. For instance, they account for the pressure dependence of the fluorescence observed in laboratory experiments. The conditions under which emitted fluorescence is proportional to deposited energy are also studied. Finally, these calculations provide an absolute value of the fluorescence yield consistent with available experimental data.

Modeling crosstalk in silicon photomultipliers

Optical crosstalk seriously limits the photon-counting resolution of silicon photomultipliers. In this work, realistic analytical models to describe the crosstalk effects on the response of these photodetectors are presented and compared with experimental data. The proposed models are based on the hypothesis that each pixel of the array has a finite number of available neighboring pixels to excite via crosstalk. Dead-time effects and geometrical aspects of the propagation of crosstalk between neighbors are taken into account in the models for different neighborhood configurations. Simple expressions to account for crosstalk effects on the pulse-height spectrum as well as to evaluate the excess noise factor due to crosstalk are also given. Dedicated measurements were carried out under both dark-count conditions and pulsed illumination. Moreover, the influence of afterpulsing on the measured pulse-height spectrum was studied, and a measurement of the recovery time of pixels was reported. High-resolution pulse-height spectra were obtained by means of a detailed waveform analysis, and the results have been used to validate our crosstalk models.

Modeling crosstalk and afterpulsing in silicon photomultipliers

An experimental method to characterize the crosstalk and afterpulsing in silicon photomultipliers has been developed and applied to two detectors fabricated by Hamamatsu. An analytical model of optical crosstalk that we presented in a previous publication has been compared with new measurements, confirming our results. Progresses on a statistical model to describe afterpulsing and delayed crosstalk are also shown and compared with preliminary experimental data

Characterization and modeling of crosstalk and afterpulsing in Hamamatsu silicon photomultipliers

The crosstalk and afterpulsing in Hamamatsu silicon photomultipliers, called Multi-Pixel Photon Counters (MPPCs), have been studied in depth. Several components of the correlated noise have been identified according to their different possible causes and their effects on the signal. In particular, we have distinguished between prompt and delayed crosstalk as well as between trap-assisted and hole-induced afterpulsing. The prompt crosstalk has been characterized through the pulse amplitude spectrum measured at dark conditions. The newest MPPC series, which incorporate isolating trenches between pixels, exhibit a very low prompt crosstalk, but a small component remains likely due to secondary photons reflected on the top surface of the device and photon-generated minority carriers diffusing in the silicon substrate. We present a meticulous procedure to characterize the afterpulsing and delayed crosstalk through the amplitude and delay time distributions of secondary pulses. Our results indicate that both noise components are due to minority carriers diffusing in the substrate and that this effect is drastically reduced in the new MPPC series as a consequence of an increase of one order of magnitude in the doping density of the substrate. Finally, we have developed a Monte Carlo simulation to study the different components of the afterpulsing and crosstalk. The simulation results support our interpretation of the experimental data. They also demonstrate that trenches longer than those employed in the Hamamatsu MPPCs would reduce the crosstalk to a much greater extent.

Improved model for the analysis of air fluorescence induced by electrons

Abstract A model recently proposed for the calculation of air-fluorescence yield excited by electrons is revisited. Improved energy distributions of secondary electrons and a more realistic Monte Carlo simulation including some additional processes have allowed us to obtain more accurate results. The model is used to study in detail the relationship between fluorescence intensity and deposited energy in a wide range of primary energy (keVs–GeVs). In addition, predictions on the absolute value of the fluorescence efficiency in the absence of collisional quenching will be presented and compared with available experimental data.

Comparison of available measurements of the absolute air‐fluorescence yield and determination of its global average value

Experimental results of the absolute air‐fluorescence yield are given very often in different units (photons/MeV or photons/m) and for different wavelength intervals. In this work we present a comparison of available results normalized to its value in photons/MeV for the 337 nm band at 1013 hPa and 293 K. The conversion of photons/m to photons/MeV requires an accurate determination of the energy deposited by the electrons in the field of view of the experimental set‐up. We have calculated the energy deposition for each experiment by means of a detailed Monte Carlo simulation and the results have been compared with those assumed or calculated by the authors. As a result, corrections to the reported fluorescence yields are proposed. These corrections improve the compatibility between measurements in such a way that a reliable average value with uncertainty at the level of 5% is obtained.

On the energy deposition by electrons in air and the accurate determination of the air-fluorescence yield

The uncertainty in the absolute value of the air-fluorescence yield still puts a severe limit on the accuracy in the primary energy of ultra-high-energy cosmic rays. The precise measurement of this parameter in laboratory is in turn conditioned by a careful evaluation of the energy deposited in the experimental collision chamber. In this work we discuss on the calculation of the energy deposition and its accuracy. Results from an upgraded Monte Carlo algorithm that we have developed are compared with those obtained using Geant4, showing excellent agreement. These updated calculations of energy deposition are used to apply some corrections to the available measurements of the absolute fluorescence yield, allowing us to obtain a reliable world average of this important parameter.

A Monte Carlo study of the relevance of fluorescence radiation in VHE gamma ray observations with Cherenkov telescopes

It is generally assumed that fluorescence radiation does not play a significant role in the performance of Cherenkov telescopes. However, this assumption is required to be verified using detailed Monte Carlo simulations. In order to do this, we have implemented the production and tracking of fluorescence radiation inside the CORSIKA code, and simulated gamma-ray induced showers in the very high energy range. The most accurate fluorescence-yield data available so far was used for this purpose. The distribution of both light components on the ground has been studied as a function of various parameters affecting the detection and reconstruction of gamma-ray showers such as the angular aperture. From these distributions, we determined the conditions under which fluorescence radiation becomes significant. These results will also be useful to estimate the corresponding systematic errors in Cherenkov telescope observations. The full simulation results have been cross-checked, on a small sample of events, against numerical calculations based on a one-dimension shower profile and found to be compatible with each other. Both tools can be used for further investigations, like studying the possibility to modify Cherenkov telescopes for the measurement of fluorescence induced by extensive air showers.

The effect of the fluorescence yield selection on the energy scales of Auger, HiRes and TA

The fluorescence yield data used for shower reconstruction in the Auger, HiRes and TA experiments are different, not only in the overall absolute value but also in the wavelength spectrum and the various atmospheric dependencies. The effect on the energy reconstruction of using different fluorescence yield parameterizations is discussed. In addition, the impact of a change in the fluorescence spectrum depends on the optical efficiency of the telescopes. A simple analytical procedure allows us to evaluate the combined effect of fluorescence yield and optical efficiency showing a non-negligible deviation between the energy scales of TA and Auger. However no relevant effect is found in the comparison between HiRes and Auger. Finally we show that a similar procedure could also be applied with real data. The accurate measurement of the calorimetric energy of UHECR showers by means of fluorescence telescopes relies on the precise knowledge of the air-fluorescence yield. In practice, the fluorescence yield is described by a set of parameters that allows us to convert the fluorescence intensity recorded by the telescope into energy deposited in the atmosphere at given atmospheric conditions. Auger, HiRes and Telescope Array (TA) use different parameterizations of the fluorescence yield (absolute value, wavelength spectrum and atmospheric dependency). The comparison of the energy spectra of Auger and HiRes/TA indicates a possible disagreement in the energy scale (1), which could be partly due to using different fluorescence yield data. In this work we evaluate the effect of the fluorescence yield choice on the relative energy scale of these experiments. While deviations in the absolute value of the fluorescence yield translate directly into deviations in the reconstructed energy, the effect of differences in the relative wavelength spectra depends on the optical efficiency of the telescope. Therefore, in order to evaluate the actual influence of the fluorescence yield on the energy scales of these experiments, the combined effect of the assumed Y values (i.e., absolute spectral values) and the optical efficiency has to be analysed. The dependence of the fluorescence yield with the atmospheric parameters is determined by the characteristic pressures P � . Neglecting the humidity contribution to quenching and the temperature dependence of the collisional cross section, the differences in the assumed Pvalues do not lead to significant disagreements in the energy reconstruction, since most of the shower energy is deposited in the lower atmospheric layers where quenching effects saturate. However these humidity and temperature contributions to Pgive rise to a non-negligible effect on the reconstructed energy (2). The wavelength spectrum assumed by Auger, HiRes and TA, as well as the corresponding optical efficiencies of these experiments are described in section 2. The analytical method we used to compute the differences in the energy scales is described in section 3 and the corresponding results are presented in section 4. This theoretical method overestimate somewhat the energy deviations with respect to those This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A new design for a high-voltage pulsed electron gun

Characterization of a very simple pulsed electron gun operating in the tens of keV range is presented. Electron emission is based on laser-produced plasma and electrostatic acceleration, generating a homogeneous beam of circular cross section. Electron bunches up to 0.5 nC in 20 ns pulses with 10 mA peak intensity were observed. A continuous energy spectrum was measured and analyzed.