I'd agree: quadrant is it...
Unless the size is unimportant. Then "sector" is probably the best word.
Stephen La Rocque>
Quadrant has my vote also. I checked in the Oxford English Dictionary and got:
A quarter of a circle or circular body, viz. (a) an arc of a circle, forming one fourth of the circumference; (b) one fourth of the area of a circle, contained within two radii at right angles.
A thing having the form of a quarter-circle.