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ROC

Rotating Oscillating Collimator

Features

  • Stepper motor controlled rotating oscillating collimator

  • Maximum horizontal opening 45o.

  • Front and back shields limit horizontal acceptance to 20o or 30o and corresponding vertical acceptance to 13.4o or 20o.

  • 35 0.005” (0.13 mm) thick stainless steel collimator blades separated by 1.25o.

  • Each blade 3.4” (86.3 mm) deep x 4.16” (105.6 mm) high.

  • Blades covered with GdO paint.

  • Front of collimator 7.9” (200 mm) from sample (without front shield) 5.8” (147.3 mm) with front shield installed.
  • Transmission greater than 90%.

Description

In order to perform experiments on specimens in special environment chambers (cryorefrigerators, cryostats , furnaces, …) as well as to reduce background from the environment, it is necessary to suppress the scattering from vacuum and heat shield walls that are external to the specimen itself. The rotating oscillating collimator (ROC) is used to accomplish this task.

The figure above shows the ROC for a 15-element detector array. The individual collimator blades are not shown in this drawing.

In normal operation, the ROC is placed directly in front of the opening of the detector shield. Additional neutron shields are placed at the front and back of the ROC to limit the vertical and horizontal angular acceptance of the ROC in order to further reduce background.

The central element of the ROC is driven with a stepper motor through a speed reducing gearbox so that the blades rotate about the specimen position, reversing direction at the end of travel. The span of rotation and direction switching is controlled by limit switches. In this manner, any mechanical differences between the blades and blade openings is averaged and the blades themselves do not create shadows on the detector plane.

The radial orientation of the ROC blades ensures that neutrons that reach the detector are scattered from a region within 2 cm of the specimen axis. In this manner, scattering from heat shields and vacuum enclosures is suppressed.

In the case of a 2-position detector shield, two sets of ROC shields are supplied. When the detector array is moved from one sample-detector distance to another, the appropriate ROC shield must be put in place.

The ROC shown above is designed for the IA 15-element LPSD detector array. Other configurations can be supplied.