on balancing forces we have
normal rxn. N = mg.cosx
f = mg.sinx
where f = friction coeff. * N
f = mgcosx /31/2
mgsinx = mgcosx /31/2so tan x = 1/31/2
x = 30 degrees
height = R - Rcos30
so height at which particle remains stationary is ( 1 - 31/2 /2 )R