cross section vs cross-sectional

cross section is a noun but cross-sectional is not a noun.

Word	Noun	Adjective	Verb	Adverb
cross section	Yes	No	No	No
cross-sectional	No	Yes	No	No

cross section (noun)	cross-sectional (noun)
(physics) the probability that a particular interaction (as capture or ionization) will take place between particles; measured in barns
a sample meant to be representative of a whole population
a section created by a plane cutting a solid perpendicular to its longest axis

cross section (adjective)	cross-sectional (adjective)
	of or relating to a cross section
	representing a plane made by cutting across something at right angles to its length