DETECTION LIMIT FOR A VERY THIN BACKING FILM

Detection limit for thin backing films is formulated. A polyvinyl formal film of ~ 0.14µm was bombarded with 3 MeV protons and continuos backgrounds were measured with a Si(Li) detector. A large constant background of a high energy region (≥6 keV) was interpreted as the Compton tail due to the 440 keV γ-rays from the excited state of ²³Na which contaminated the surface of a carbon collimator placed just in front of a target. The detection limits of this thin film, under the condition of the beam spot size 6 mmϕ without collimator, are estimated for the elements of atomic number Z=10~55 and the minimum detection limit is calculated to be 4×10⁻¹²g for Z=28~34.