A narrow beam of 400-keV photons is incident normally on a 2 mm iron liner. a) What fraction of the photons has an interaction in the liner? b) What thickness of Fe is needed to reduce the fraction of photons that are transmitted without interaction to 10%? c) If Al were used instead of Fe, what thickness would be needed in (b)? d) How do the answers in (b) and (c) compare when expressed in g cm²²? e) If lead were used in (b), how would its thickness in g cm-2 compare with those for Al and Fe?