A collimator incorporating shielding shaped according to the formula L 0 2 = y 2 + [ y 2 ⁢ z 2 ( y + D ) 2 ] where y is the minimum thickness of encased shielding needed to shield the collimator from un-collimated radiation entering the collimator at a distance, z, along the longitudinal axis of the collimator, z measured from the bottom of a cylindrical detector, and D is the inner diameter of the collimator as established by the outer diameter of the detector. Select embodiments may be employed for collecting collimated high energy gamma rays from soil using a gamma ray detector.