Specification for active photon collimator system

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Notice: this document is currently in draft form, and is expected to change over the coming weeks - Richard Jones, 2/7/2008


  1. Peak polarization: provide for beam-collimator alignment at the level required to maintain the polarization at the coherent peak within 1% of its value at nominal alignment while running at beam currents Ie > 100 nA.
  2. Peak tagging efficiency: provide for beam-collimator alingment at the level required to maintain the tagging efficiency at the coherent peak within 1% of its value at nominal alignment while running at Ie > 100 nA.
  3. Spot enlargement from beam motion: provide for virtual beam spot stability at the level to required to maintain peak polarization and tagging efficiency within 1% of their values at nominal alignment while running at Ie > 100 nA.
  4. Finding the beam during setup: provide x and y spot offset signals that can be used by the electron beam controls system to steer the beam into nominal alignment from anywhere within a circle of radius 2 cm centered on the collimator axis.
  5. Allowance for radiator spot moves: ability in the electron beam controls upstream of the radiator to put the center of the electron beam anywhere within ±5 mm of its nominal position at the radiator, while maintaining the virtual spot alignment at the collimator position within the tolerances indicated above. Spot moves might be requested as frequently as every 600 hours at the maximum beam intensity for GlueX.
  6. Alignment with GlueX target axis: ability to position the active collimator axis remotely so that, after beam-collimator alignment, the collimated photon beam passes within 1 mm of the GlueX target geometric axis.
  7. Secondary and alternate collimators: ability to remotely move the secondary collimator into alignment with the primary/active collimator, or to remove it during normalization runs; ability to exchange the primary collimator with an alternate larger-aperture primary collimator (> 1 m/E), and to remove all collimation out to the boundaries of the collimator cave beamline shielding, for tagger and beamline commissioning studies and tagging efficiency measurements.

Active collimator specification

Table 1: Characteristics of the active front-end portion of the primary collimator on the Hall D photon beam line.

number of active collimators1
aperture diameter5 mm
number of x readout channels4
number of y readout channels4
position resolution1200 μm rms
detector typetungsten pins shower charge integrator
readout electronicscharge-sensitive preamplifier, ADC

1Assumes nominal beam intensity of 107 or more tagged photons per second in the primary coherent peak.

Primary collimators

Table 2: Characteristics of the primary collimators on the Hall D photon beam line.

 small aperturelarge aperture
approximate acceptance0.5 m/E2 m/E
aperture diameter3.4 mm13.6 mm
outer diameter10 cm10 cm
length30 cm30 cm
materialtungsten (>90%)tungsten (>90%)
range of vertical motion±5 mm remotely controlled uniform translation2
range of horizontal motion ±30 cm remotely controlled uniform translation3
translation system step size 0.5 mm on both horizontal and vertical axes4

1Active collimator described above in Table 1 is positioned on the entry face of this primary collimator, with its aperture aligned with the primary collimator axis.
2Sufficient to cover the maximum differential settling of the collimator and Hall D proper, and keep the photon beam centered on the GlueX target axis.
3Sufficient to move either primary collimator into nominal position, or remove both completely from the beam.
4Sufficient to guarantee co-alignment with secondary collimator to within ±0.3 mm.

Secondary collimators

Table 3: Characteristics of the secondary collimator on the Hall D photon beam line.