FUNCTION PR(eta,p,Bo,l)
REAL(rp), INTENT(IN) :: eta ! in radians
REAL(rp), INTENT(IN) :: p ! dimensionless (in units of mc)
REAL(rp), INTENT(IN) :: Bo
REAL(rp), INTENT(IN) :: l
REAL(rp) :: PR
REAL(rp) :: g
REAL(rp) :: v
REAL(rp) :: k
REAL(rp) :: lc
REAL(rp) :: z
REAL(rp) :: Pi
g = SQRT(p**2 + 1.0_rp)
v = C_C*SQRT(1.0_rp - 1.0_rp/g**2)
k = C_E*Bo*SIN(deg2rad(eta))/(g*C_ME*v)
lc = (4.0_rp*C_PI/3.0_rp)/(k*g**3) ! Critical wavelength
z = lc/l
call P_integral(z,Pi)
PR = (C_C*C_E**2)*Pi/(SQRT(3.0_rp)*C_E0*g**2*l**3)
END FUNCTION PR