ASSEMBLY FOR RUTHERFORD BACKSCATTERING AT EXACTLY 180-DEGREES

We describe a facility which allows us to measure projectiles scattere d at an angle arbitrarily near 180 degrees. The backscattered projecti les are detected in the horizontal plane out of the incoming beam by a combination of electric and magnetic fields. The charged particles, w hich are spatially dispersed according to their energy, are detected b y a cooled 300 mm(2) ion implanted silicon detector. By a judicious ch oice of the relative strength of both fields we ensure that the essent ial Darts of the spectrum will not be distorted due to an energy depen dent percentage of projectiles missing the detector. As the projectile s are also dispersed according to their charge state we are able to di stinguish between, e.g., singly and doubly charged He projectiles as t hey hit the detector at different positions. The detector can be moved in the vertical direction to continuously vary the backscatteling ang le between 180 degrees and 178.5 degrees. This experimental result sho ws for the first time the angular range to which the 180 degrees enhan cement is confined. Such a device is ideally suited for investigating single crystals by uniaxial channeling blocking. (C) 1996 American Ins titute of Physics.