Particle identification studies with a full-size 4-GEM prototype for the ALICE TPC upgrade

A large Time Projection Chamber is the main device for tracking and charged-particle identification in the ALICE experiment at the CERN LHC. After the second long shutdown in 2019/20, the LHC will deliver Pb beams colliding at an interaction rate of about 50 kHz, which is about a factor of 50 above the present readout rate of the TPC. This will result in a significant improvement on the sensitivity to rare probes that are considered key observables to characterize the QCD matter created in such collisions. In order to make full use of this luminosity, the currently used gated Multi-Wire Proportional Chambers will be replaced. The upgrade relies on continuously operated readout detectors employing Gas Electron Multiplier technology to retain the performance in terms of particle identification via the measurement of the specific energy loss by ionization d$E$/d$x$. A full-size readout chamber prototype was assembled in 2014 featuring a stack of four GEM foils as an amplification stage. The performance of the prototype was evaluated in a test beam campaign at the CERN PS. The d$E$/d$x$ resolution complies with both the performance of the currently operated MWPC-based readout chambers and the challenging requirements of the ALICE TPC upgrade program. Detailed simulations of the readout system are able to reproduce the data.Comment: Submitted to NIM

The upgrade of the ALICE TPC with GEMs and continuous readout

The upgrade of the ALICE TPC will allow the experiment to cope with the high interaction rates foreseen for the forthcoming Run 3 and Run 4 at the CERN LHC. In this article, we describe the design of new readout chambers and front-end electronics, which are driven by the goals of the experiment. Gas Electron Multiplier (GEM) detectors arranged in stacks containing four GEMs each, and continuous readout electronics based on the SAMPA chip, an ALICE development, are replacing the previous elements. The construction of these new elements, together with their associated quality control procedures, is explained in detail. Finally, the readout chamber and front-end electronics cards replacement, together with the commissioning of the detector prior to installation in the experimental cavern, are presented. After a nine-year period of R&D, construction, and assembly, the upgrade of the TPC was completed in 2020.publishedVersio

High-pressure behaviour along the jadeite NaAlSi2O6 – aegirine NaFeSi2O6 solid solution up to 10 GPa.

The unit-cell parameters evolution along the jadeite – aegirine solid solution was determined up to pressures between 6.5 and 9.7 GPa and room temperature by single-crystal X-ray diffraction. The pressure-volume data have been fitted using a third-order Birch-Murnaghan equation of state. The bulk modulus, KT0, is 134.0(7) GPa for pure jadeite and 116.1(5) for pure aegirine. Its evolution with composition along the join is not linear and can be described by the following weighted 2nd order polynomial: KT0 = 116.2(5) + [0.25(3)*(mol % Jd)] - [0.0008(3)*(mol % Jd)2] (1) The equation (1) is not affected by the values of the first pressure derivative K’. Such EoS coefficient shows no dependence upon composition and values between 3.9 and 4.5. The higher compressibility is always observed along the a axis, bnt compression along the b axis is most affected by compositional changes. Our unit-cell volume vs composition at room temperature data indicate a slight but significant deviation from linearity and, therefore, from the ideal solid solution between jadeite and aegirine reported by previous investigations

Combined 147,146 Sm- 143,142 Nd constraints on the longevity and residence time of early terrestrial crust

International audiencePrimordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited Nd-142 anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here we report new whole-rock Sm-147,Sm-146-Nd-143,Nd-142 data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our Sm-147-Nd-143 data combined with literature data define an age of 3371 +/- 141 Ma (2 SD) and yield an initial epsilon Nd-143 of -5.6 +/- 2.1. These results are at odds with the Acasta zircon U-Pb record, which comprises emplacement ages of 3920-3960 Ma. Ten of our thirteen samples show Nd-142 deficits of -9.6 +/- 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between Nd-142 anomalies and a mid-Archean Sm-147-Nd-143 age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early enrichment of the Acasta source. Two compositional end-members can be identified: a felsic component with Nd-142/Nd-144 identical to the modern Earth and a mafic component with Nd-142/Nd-144 as low as -14.1 ppm. The ca. 4310 Ma AGC source is similar to 200 Myr younger than those estimated for Nuvvuagittuq (northern Quebec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd-Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to Nd-142 evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode

Ultralow viscosity of carbonate melts at high pressures

Knowledge of the occurrence and mobility of carbonate-rich melts in the Earth's mantle is important for understanding the deep carbon cycle and related geochemical and geophysical processes. However, our understanding of the mobility of carbonate-rich melts remains poor. Here we report viscosities of carbonate melts up to 6.2 GPa using a newly developed technique of ultrafast synchrotron X-ray imaging. These carbonate melts display ultralow viscosities, much lower than previously thought, in the range of 0.006-0.010 Pa s, which are ~2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. As a result, the mobility of carbonate melts (defined as the ratio of melt-solid density contrast to melt viscosity) is ~2 to 3 orders of magnitude higher than that of basaltic melts. Such high mobility has significant influence on several magmatic processes, such as fast melt migration and effective melt extraction beneath mid-ocean ridges

Particle identification studies with a full-size 4-GEM prototype for the ALICE TPC upgrade

A large Time Projection Chamber is the main device for tracking and charged-particle identification in the ALICE experiment at the CERN LHC. After the second long shutdown in 2019/20, the LHC will deliver Pb beams colliding at an interaction rate of about 50 kHz, which is about a factor of 50 above the present readout rate of the TPC. This will result in a significant improvement on the sensitivity to rare probes that are considered key observables to characterize the QCD matter created in such collisions. In order to make full use of this luminosity, the currently used gated Multi-Wire Proportional Chambers will be replaced. The upgrade relies on continuously operated readout detectors employing Gas Electron Multiplier technology to retain the performance in terms of particle identification via the measurement of the specific energy loss by ionization dE/dx. A full-size readout chamber prototype was assembled in 2014 featuring a stack of four GEM foils as an amplification stage. The performance of the prototype was evaluated in a test beam campaign at the CERN PS. The dE/dx resolution complies with both the performance of the currently operated MWPC-based readout chambers and the challenging requirements of the ALICE TPC upgrade program. Detailed simulations of the readout system are able to reproduce the data