With the upgrade of the CEBAF accelerator, high intensity electron beams up to 12 GeV will soon be available at the Jefferson Laboratory (USA). Within this project, the CLAS collaboration has initiated a major modification of the Hall B spectrometer in order to run experiments at luminosities as high as 10^35/cm²/s. In particular, a new vertex detector will be installed in the vicinity of the target. Because of the particle flux and the required spatial resolution, Irfu has proposed in 2006 to equip this tracker with thin Micromegas detectors. The feasibility study confirmed the interest of these detectors and in 2009 JLab officially entrusted to Irfu the construction of the forward vertex detector and a part of the central one. In 2011, the realization of an additional Micromegas tracker in the very forward region was decided from a collaboration with INFN.



The vertex detector will ensure the reconstruction of particles from 5 to 125° with respect to the electron beam. The forward part, built with 6 Micromegas disks, will cover angles from 5 to 35° in combination with a set of Drift Chambers. The Barrel region will provide the reconstruction from 35 to 125°, and will be equipped with 6 cylinders of Micromegas.


Hadron structure / Nucleon structure


Schematic view of the vertex tracker

Status of the project:

During the R&D phase, several innovating aspects has been studied, due to the specifity of this tracker:

  • Several curved Micromegas prototypes have been built and characterized with a cosmic bench. The use of the bulk technology indeed allows since 2006 to manufacture detectors on thin PCB (100-200 micron thick). These tests showed that the Micromegas performance is not degraded when curved, down to radius of around 10 cm.
  • The presence of a strong magnetic field (5 T) requires modifying significantly the working point of the detectors. Tests performed at Saclay and JLab, as well as Garfield simulations, determined the modifications to be made on the gas mixture, the drift electric fields, and the choice of the micro-mesh.
  • Because of the lack of space close to the vertex tracker, the electronics needs to be placed 1.5 to 2 meters upstream. Long cables thus have to ensure the signal transmission between the detectors and the Front-End electronics. Several types of low capacitance cables have therefore been tried, and the best performance was achieved with 43 pF/m, micro-coaxial cables from Hitachi
  • The expected spark rate is relatively high, because of the hadron flux in the vicinity of the target. Geant4 simulations and beam tests at CERN and JLab suggested that this spark rate will be of the order of several Hz per detector. In order to minimize the impact on the performance and to prepare a potential luminosity upgrade, it has been decided to use the resistive strip technology invented at CERN in 2010. Three resistive prototypes – two disks and one curved – has then been characterized with cosmics, and revealed an efficiency higher than 97%.

In parallel, a new Asic (DREAM) has been developed to meet the nominal requirements of CLAS12 – input capacitances up to 200 pF, particle flux above 10 MHz, trigger rate around 20 kHz. The first DREAM prototypes have been tested in 2012, and showed an increase of the S/N ratio of about 25% compared to the chip developed for the T2K experiment. A first Front-End card has been characterized in mid-2013, and operated in a 1.5 T magnetic field.


CLAS12 spectrometer


The production of the 1,000 DREAM Asics has just started, and will be ended by the 2nd trimester of 2014. The production of the 6 forward detectors will begin in spring 2014, and will be followed by the construction of the 18 cylindrical Micromegas. The forward detectors as well as 2 layers of the barrel tracker will be installed in Hall B from February to September 2015. After a commissioning of CLAS12 at the end of 2015, the 4 remaining layers of the barrel Micromegas will be assembled in 2016.



Scientific leader: Franck Sabatié
Project leader: Stephan Aune

Last update : 10/04 2017 (3412)


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