NEW METHOD TO PRODUCE LASER CALIBRATION BEAMS IN GASEOUS DETECTORS .

1. NEW METHOD TO PRODUCE LASER CALIBRATION BEAMS IN GASEOUS DETECTORS. Alexei Lebedev, Brookhaven National Laboratory, Upton, NY, 11973, USA ABSTRACT A new method utilizing diffraction of UV laser beams on annular diaphragms provides very narrow laser beams with full diameter 100-400 mm, divergence ~0.05mrad and effective length up to 10 meters, which exceeds existing methods with focusing optics. The characteristics of laser beams and linear ionization created with different diaphragm sizes are present. Optics schemes proposed to create a system with new beams. Introduction Narrow laser beams widely used in many experiments to simulate charge particles and align detectors[1]. For LHC or future linear collider the requirements to correct the position and to measure coordinates of particles are about 10-20mm for whole detector with the size ~ 10 meters. Different approaches are proposed to create laser beams [2-4]. The direct way for alignment is directing of narrow laser beam going through all detectors. The ability to resolve the location of beam is proportional to the beam radius and it is attractive to use calibration beams with less diameters. Current approach is to use focusing telescopes or diffraction through hole(fig.1). There are physical limitations due to diffraction limit. Divergence of the beam q ~0.64l /w, where l-wavelength and w- waist of the beam. It is obvious that it is impossible to create long beams with sub millimeter diameter. It is known the creation of bright spot, called Poisson spot(fig.2), when wide wave plane illuminates an opaque sphere. This line created in the shadow of sphere and could be used to monitor positioning of different objects, because the divergence of Poisson line is significantly smaller q ~l /d, where d is diameter of the ball [5,6]. We propose to use “Poisson beam” to simulate straight tracks in gaseous detectors. This beam could create ionization due to two-photon ionization process. Additional diaphragm installed after the ball or annular diaphragms eliminates background ionization from bright field. We present results with different balls and diaphragm sizes to form very narrow laser beams with 100-500mm diameter and 3-10 meters long with an ionization exceeding relativistic particle(fig.5). Note that our beam size measurements were made on base of beam profile(fig. 4). Special optics with cone lens proposed to create a system of these beams (fig.6). References: 1. W. Blum, L. Rolandi. Particle detection with Drift Chambers, Springer-Verlag, 1994. 2.RASNIK for ATLAS http://www.nikhef.nl/pub/departments/et/experiments/atlas/rasnik/atlas_rasnik/atlas_rasnik.html 3. W. Blum, H. Kroha, P. Widman, NIM, A 267 (1995) 413-417. 4. T.Akesson et al. An alignment method for the ATLAS end-cup TRD detector using a narrow monochromatic X-ray beam, NIM A463 (2001), 129-141. 5. L.V.Griffith et al. Magnetic alignment and the Poisson alignment reference system. Rev. Sci. Instr. 61(8), Aug 1990, 2138-2154. 6. A.Lebedev et al, A laser calibration system for the STAR TPC, NIM A478 (2002) 163, NIM A499 (2003), 692. • Focusing through telescope 2. Diffraction through hole- Airy disk q q w d~1mm q= 2l/p w, ~.18mrad, 266nm q = 1.22l/ d, ~.32mrad, 266nm Fig. 1 Principles to create laser beams Fig. 2 Principles to create Poisson beams d Laser D L D1 or Fe55 Drift chamber screen with scale Microscope Fig. 3. Experimental setup UV laser beam creates electrons via 2-photon ionization • = 1.28 l/ d mrad, mm, mm He-Ne laser, 633nm q = 1.02 l/ d mrad, mm, mm UV laser, 266nm Fig. 5 Electron linear density and beam profile for 3.12 and 5.0 mm balls.. Fig 4. Beam profile for different ball sizes and wavelengths. wide laser beam micromirror . cone lens annular beam micromirror’s bundle Fig. 6. Proposed optics to create annular beams. VCI 2007