Assembly for Rutherford backscattering at exactly 180 degree

We describe a facility which allows us to measure projectiles scattered at an angle arbitrarily near 180 degree. The backscattered projectiles are deflected in the horizontal plane out of the incoming beam by a combination of electric and magnetic fields. The charged particles, which are spatially dispersed according to their energy, are detected by a cooled 300 mm2 ion implanted silicon detector. By a judicious choice of the relative strength of both fields we ensure that the essential parts of the spectrum will not be distorted due to an energy dependent percentage of projectiles missing the detector. As the projectiles are also dispersed according to their charge state we are able to distinguish between, e.g., singly and doubly charged He projectiles as they hit the detector at different positions. The detector can be moved in the vertical direction to continuously vary the backscattering angle between 180 degree and 178.5 degree. This experimental result shows for the first time the angular range to which the 180 degree enhancement is confined. Such a device is ideally suited for investigating single crystals by uniaxial channeling blocking. copyright 1996 American Institute of Physics