We present two methods of independently mapping the dimensions of the
sensitive volume in an ion-counting nanodosimeter. The first method is based on
a calculational approach simulating the extraction of ions from the sensitive
volume, and the second method on probing the sensitive volume with 250 MeV
protons. Sensitive-volume maps obtained with both methods are compared and
systematic errors inherent in both methods are quantified.Comment: 27 pages, 8 figures. Submitted to JINST, Jan. 16 200