An intense positron sources is a demanding element in the design of future lepton colliders. A crystal-based hybrid positron source could be an alternative to a more conventional scheme based on the ...electron conversion into positron in a thick amorphous target. The conceptual idea of the hybrid source is to have two separate objects, a photon radiator and a photon-to-positron converter target. In such a scheme an electron beam crosses a thin axially oriented crystal with the emission of a channeling radiation, characterized by a considerably larger amount of photons if compared to Bremsstrahlung. The net result is an increase in the number of produced positrons at the converter target. In this paper we present the results of a beam test conducted at the DESY TB 21 with 5.6 GeV electron beam and a crystalline tungsten radiator. Experimental data clearly highlight an increased production of photons and they are critically compared with the outcomes of novel method to simulate the number of radiated photons, showing a very good agreement. Strong of this, the developed simulation tool has been exploited to design a simple scheme for a positron source based on oriented crystal, demonstrating the advantages in terms of reduction of both deposited energy and the peak energy deposition density if compared to conventional sources. The presented work opens the way for a realistic and detailed design of a hybrid crystal-based positron source for future lepton colliders.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The paper reports on elaboration of plasma‐assisted molecular beam epitaxy (MBE) of AlxGa1 − xN‐based quantum‐well (QW) structures with high Al content (up to 50% in the QW) grown directly on ...c‐sapphire. Different elements of the structure design are considered consecutively in detail along with the advanced growth approaches developed for each element. Special attention is paid to the growth conditions of (i) AlN nucleation layers with suppressed generation of threading dislocations (TDs), (ii) 2‐µm thick AlN buffer layers with atomically smooth droplet‐free morphology (rms = 0.46 nm) grown under the strongly metal‐rich conditions, (iii) cladding and waveguide AlGaN layers also possessing the atomically smooth droplet‐free morphology that is ensured by the accurately established phase diagram of metal(Ga)‐rich growth conditions within the temperature range 660–780 °C. Employing several 3‐nm thick strained GaN insertions in the AlN buffer layer and a AlGaN/AlN superlattice (SL) on top of it is shown to result in a significant decrease of TD's density down to 108–109 cm−2 in the top QW region fabricated by a submonolayer digital alloying (SDA) technique. Finally, advanced AlGaN‐based QW structures are presented, which demonstrate optically pumped lasing within the deep‐ultraviolet (UV) wavelength range with the threshold power density below 600 kW cm−2 (at 289 nm).
Plasma‐assisted molecular beam epitaxy of AlxGa1 − xN‐based quantum well (QW) structures with high Al content (up to 50% in the QW) grown directly on c‐sapphire is described in this Feature Article. Special attention is paid to the growth conditions of (i) AlN nucleation layers with suppressed generation of threading dislocations (TDs), (ii) atomically‐smooth 2‐μm‐thick AlN buffer layers with several 3‐nm‐thick strained GaN insertions and AlGaN/AlN superlattice to decrease TD's density down to 108–109cm−2, (iii) cladding and waveguide AlGaN layers with atomically‐smooth droplet‐free morphology and QW region fabricated by a sub‐monolayer digital alloying technique. These QW structures demonstrate optically‐pumped lasing within the deep‐UV (sub‐300 nm) wavelength range with threshold power density below 600 kW/cm2 (at 289 nm).
•MBE growth and characterization of AlGaN heterostructures.•Effect of the Ga surfactant during high temperature AlN growth.•Effect of ammonia flow at different growth temperature.•Observation of AlN ...surface with atomically smooth terraces and RMS below 1 nm.•Increasing 2DEG HEMT mobility up to 2000 cm2/V·s.
It is shown that the use of high-temperature AlN/AlGaN buffer layers grown by NH3-MBE at extremely high temperatures (up to 1200 °C) allows one to improve drastically the structural quality of topmost GaN layer. The influence of an ammonia flow in the temperature range 1000–1200 °C was investigated. AlN surface with atomically smooth terraces and RMS below 1 nm has been achieved. This approach along with using Ga as a surfactant during the buffer growth allows one to increase the 2DEG mobility in GaN/AlGaN up to 2000 cm2/V·s.
The ENUBET project is aimed at designing and experimentally demonstrating the concept of monitored neutrino beams. These novel beams are enhanced by an instrumented decay tunnel, whose detectors ...reconstruct large-angle charged leptons produced in the tunnel and give a direct estimate of the neutrino flux at the source. These facilities are thus the ideal tool for high-precision neutrino cross-section measurements at the GeV scale because they offer superior control of beam systematics with respect to existing facilities. In this paper, we present the first end-to-end design of a monitored neutrino beam capable of monitoring lepton production at the single particle level. This goal is achieved by a new focusing system without magnetic horns, a 20 m normal-conducting transfer line for charge and momentum selection, and a 40 m tunnel instrumented with cost-effective particle detectors. Employing such a design, we show that percent precision in cross-section measurements can be achieved at the CERN SPS complex with existing neutrino detectors.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Selected composition of structural heat-insulating ash tuff concrete class B15-20 on tuff rubble and ash using plasticizer D5, determined the calculated heat transfer resistance of the two-layer ...structure, the dependencies of the density and strength of the mixture from cement consumption for 1 m3.
This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate ...low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258-290 nm) with the threshold power density below 600 kW cm−2. The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 108 and 3 × 109 cm−2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 m−2). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30-70) nm thick AlN nucleation layer on c-Al2O3, followed by a 2 m thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (∼3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm−2 at 289 nm for a single quantum well laser structure has been demonstrated.
Abstract
This article describes the design, assembly and
characterization of a portable cosmic ray detector, developed by the
INSULAB group and suitable for teaching activities aimed at high
school ...students. It consists of a compact aluminum suitcase
containing three plastic scintillator modules coupled to
photomultipliers, readout by a custom compact electronics chain and
powered by a power bank. The modules operate in coincidence and the
system records the arrival time of each particle and the time over
threshold of the signal of each scintillator module. The data are
acquired and processed by a Raspberry PI connected to a touch screen
display for online monitoring. The procedure implemented for the
determination of the detector efficiency is reported, along with the
results of the measurements of the cosmic ray rate as a function of
the altitude and the zenith angle, performed in the laboratory and
in different locations outdoors.
We investigated the radiative properties of InGaN/GaN heterostructures with multiple quantum wells (MQWs) grown on silicon substrates with different thicknesses of quantum wells at optical ...excitation. The correlation of laser and photoluminescent properties with the surface morphology of the gallium nitride coating layers and the density of V-defects has been established. It is shown that, with a growth in the density of V-defects, the threshold power density of the excitation of the generation of InGaN/GaN heterostructures with MQWs increases.
Polysiloxane-based scintillators for shashlik calorimeters Acerbi, F.; Branca, A.; Brizzolari, C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2020, Letnik:
956
Journal Article
Recenzirano
Odprti dostop
We present the first application of polysiloxane-based scintillators as active medium in a shashlik sampling calorimeter. These results were obtained from a testbeam campaign of a ∼6×6×45 cm3 (13 X0 ...depth) prototype. A Wavelength Shifting fiber array of 36 elements runs perpendicularly to the stack of iron (15 mm) and polysiloxane scintillator (15 mm) tiles with a density of about one over cm2. Unlike shashlik calorimeters based on plastic organic scintillators, here fibers are optically matched with the scintillator without any intermediate air gap. The prototype features a compact light readout based on Silicon Photo-Multipliers embedded in the bulk of the detector. The detector was tested with electrons, pions and muons with energies ranging from 1 to 7 GeV at the CERN-PS. This solution offers a highly radiation hard detector to instrument the decay region of a neutrino beam, providing an event-by-event measurement of high-angle decay products associated with neutrino production (ENUBET, Enhanced NeUtrino BEams from kaon Tagging, ERC project). The results in terms of light yield, uniformity and energy resolution, are compared to a similar calorimeter built with ordinary plastic scintillators.
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