Nuclear Physics Division

In view of the energy increase of the electron accelerator at Jefferson Laboratory, the CLAS spectrometer should be significantly upgraded for 2014. The project, named CLAS12, plans in particular the construction a Central Tracker, installed in a 5 T solenoid, and dedicated to the detection of protons and pions of average momentum (between 0.2 and 1.5 GeV/c).

Initially, this tracker was supposed to be composed of silicon detectors, coming from an American-Russian collaboration, having a large total surface area (0.8 m²) and consisting of 35,000 electronic channels. However, in order to improve the angular resolution, whilst reducing the quantity of material and the cost of such an equipment, the CLAS group from SPhN has suggested to replace 80% of these silicons with Micromegas gaseous detectors. This suggestion was made following the R&D performed at Sédi on the next generation of Micromegas, known as "bulks". Simulations, based on the Garfield and Magboltz programs, have confirmed the potential of such a mixed setup in terms of spatial and angular resolutions. There is, however, a major difficulty related to the use of gaseous detectors in a high magnetic field. Indeed, the electrons, created by the passage of a charged particle in the detector, drift with a high Lorentz angle, with respect to the direction of the electric field. In standard conditions, this angle is around 75°, and the detector is unusable. Simulations have shown that it could be possible to reduce this angle below 20°, thanks to a judicious choice of gas and by increasing the drift field of the detector, that would have a cost in terms of micromesh transparency. With such an angle, the detector could reconstruct tracks with an efficiency greater than 95%.

In the presence of a magnetic field the signals due to drift electrons and those due to electrons from the micromesh thus give rise to two separate peaks on the strips, and the position difference between these peaks provides a direct measurement of the Lorentz angle seen by the electrons in the drift space. A first set of measurements of the Lorentz angles was performed at Saclay, where a hot dipole from SACM allowed to collect data up to 1.5 T on a small Micromegas detector. The agreement with the simulation was found to be fairly good, and these tests have therefore validated both the measurement setup as well as the relevance of the simulation in intermediate magnetic fields.

The same setup has been installed in real conditions, inside the CLAS solenoid magnet in the Hall B of the Jefferson Laboratory (Figure 2). The used detector had an active surface area of 53 x 12 cm² on a 100 microns thick PCB, with a drift electrode of aluminised mylar located several millimeters above the micromesh. The measurements were performed in a 10% argon-isobutane mixture, in a field ramped from 0 to 4.2 T, and for various values of drift electric field (Figure 3).