US Patent 8198600 Localization of a radioactive source

An angle-responsive sensor, comprising: a radiation detector adapted to detect ionizing radiation; at least one collimator arranged to block radiation from reaching the detector in a manner dependent on a relative orientation of a radiation source, the detector and the collimator, the detector and the collimator defining an aim for the sensor; and circuitry coupled to the detector and which generates an output signal which varies as a function of the relative orientation, wherein the detector and the collimator are arranged to have a working volume of at least 10 cm in depth and having an angular width, such that the slope of the signal as a function of angle varies by less than a factor of 2 over the working volume.