A magnetic immersion lens apparatus includes an outer pole piece and an inner pole piece with a gap therebetween proximate a common axis of the first and second pole pieces. The outer pole piece has an opening that permits energetic particles from a target in front of the immersion lens to pass through the outer pole piece to an external detector. The outer or inner pole piece has one or more notches near the gap, including at least one notch that expands cone of acceptance through which the energetic particles can pass from the target to the external detector. This abstract is provided to comply with rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.