Precise reconstruction of particle tracks in the TPC requires a thorough understanding of the drift velocity and any inhomogeneities in the drift field. A non-uniform electron drift can be caused by mechanical or electrical imperfections in the field cage and readout chambers, whereas deviations of the electron drift from the ideal paths inside the gas volume are caused by temperature variations, relative misalignment of the electrical and magnetic fields (ExB effects) and local variations of the electric field from moving charges (space-charge effects). To calibrate the drift field parameters against a known standard, a laser calibration system was built, using a large number of narrow ultraviolet rays at predefined positions inside the drift volume to generate tracks. The system was designed to make fast and accurate measurements of time varying drift velocities. It will run every half hour interspersed between physics events to measure the drift velocity and assess space charge effects. The laser system was used extensively during the detector commissioning for testing of the electronics and the alignment of the readout chambers and central electrode.
We use pulsed monochromatic laser beams of 266 nm wavelengthand about 5 ns pulse duration with approximately gaussian cross section with 400 micrometer. The ionization in the gas volume along the laser path occurs via two photon absorption by organic impurities with ionization potentials in the range 5-8 eV. The molecules of the pure NeCO2N2 drift gas have ionization potentials above 10eV and are not ionized by the laser. The aim is to measure the response of the TPC to several hundred laser tracks generated simultaneously throughout the TPC drift volume at predefined positions. The laser events can be generated in special calibration runs or interspersed between physics events. To obtain the best precision of the measured tracks, the preferred geometry is one where the tracks have constant drift times and are perpendicular to the wires. For this configuration, clusters are smallest and the electronics and reconstruction programs give the best possible single point resolution. Simultaneously, a extensive coverage of the full drift volume is desired. This led us to provide tracks in planes at constant z, of which some radiate with approximately constant phi. Tracks generated at different z throughout the drift volume allow easy determination of drift velocities from single laser events. Most metallic surfaces have work functions below 4.66 eV and emit electrons by photoelectric effect when hit by UV light above this energy. Being a first order effect in the light intensity, a considerable amount of low energy electrons are seen from the diffusely scattered, time correlated UV light produced by reflections. The signal from the aluminum surface of the central electrode is used to give a precise picture at the maximum drift time across the electrode.
A commercial laser outside the TPC generates an energetic pulsed beam of UV light with 25 mm diameter and very low divergence. Through an optical system of semitransparent beam splitters, mirrors and bending prisms, this wide beam is split in several lower intensity beams and guided into the TPC at different entry points through quartz windows. The wide beams travel along the inside of the hollow outer rods of the field cage, used for holding the mylar strips that define the electric field. Inside the rods, the wide beams are intersected by a number of very small mirrors (1 mm diameter) that each deflect a small part of the wide laser beam into the TPC drift volume. The dimensions, points of origin and directions of the narrow beams are given by the size, positions and angles of the micromirrors and only to a very minor degree by the parameters of the wide beam. The micromirrors are grouped in small bundles and placed along the length of the rod so that they do not shadow each other. The undeflected part of the wide beam is used for position and intensity monitoring by cameras placed at the far end of the rod. All elements of the optical guidance and splitting system are static, except for a few remotely controllable mirrors used to fine tune the beam path. Six rods in each half of the TPC were equipped with four micromirror bundles each. Each mirror bundle contains seven small mirrors. The wide beam originates from one laser for each TPC half and is split and guided into the six rods. The two lasers are synchronized to provide simultaneous laser pulses in the full TPC, thus resulting in a total of 336 simultaneous narrow laser rays in the TPC volume. It is also possible to operate the system with just one laser for the full TPC using an additional beam-splitter near the laser.
Click here for the homepage of the ALICE TPC Laser Calibration Group.