An accelerator produces an He+ ion current of 50 nA at 1.0 MeV. Using a 1 cm2 detector 5 cm from the target at a scattering angle of 170, determine the smallest amount of Au (atoms/cm2) that can be detected. Detectability is arbitrarily defined as 100 counts in 1 hr. Under similar conditions, what is the detection limit for oxygen? Compare these limits with the number of atoms/cm2 in a monolayer (=1015 atoms/cm2)

Please use the following equation for the first part:

where () is usually called the scattering cross section. For a small detector of area A, at distance l from the target, the solid angle is given by A/l2 in steradians. For the geometry of Fig. 2.5, the number Ns of target atoms /cm2 is related to the yield Y or the number QD of detected particles (in an ideal, 100\%-efficient detector that subtends a solid angle ) by Y=QD=()QNs