Background: In nuclear monitoring applications

a gamma-ray imager based on coded aperture (CA) technology can get a 2-D source intensity distribution map

which is superimposed on a corresponding video camera image

to visualize an object scene indicating the location

activity and even the type of radionuclide of radioactive sources. Though the CA imager has some advantages

it is still limited to imaging sources at large standoff distance or with high energy photon emission. There are two main barriers. One is that increasing the area of position-sensitive detector brings unacceptable additional system complexity and cost. Another is that thicker detector induces image blurring. Purpose: In order to overcome the disadvantages of CA imager

a novel gamma-ray imager prototype based on rotating modulator (RM) technique is built in our laboratory. Methods: The laboratory prototype consists of a rotating collimator

a NaI(Tl) detectors array

readout and control circuit

a stepping motor

and their supporting frame. Experimental tests were carried out. Results: The field of view (FOV) of this prototype is ±14.6?. An angular resolution of 2? is achieved. The energy resolution of the NaI(Tl) detectors is 7%@662 keV and their average detection efficiency to 662-keV gamma-rays is 22.2%. Conclusion: A RM imager is capable of imaging gamma-rays and measuring the incident photon spectrum simultaneously. Thus it can form radioactive map of different radionuclide. Through temporal modulation and image reconstruction

the angular resolution of RM imager is comparable to that of a CA imager. Using non-position-sensitive detectors

the detector area and thickness can be increased to improve the detection efficiency with relatively low cost. The study is still in progress and the performance of the RM imager is expected to be improved for further development.Cited