A method for detecting special nuclear materials , comprising the steps of - irradiating a detection region of an object to be detected with a first X-ray beam , the energy of the first X-ray beam being selected to enable the special nuclear materials that possibly exist in the object to be detected to undergo an observable photofission; - measuring a first fission ray signal emitted from the object to be detected due to the photofission, wherein if the measured first fission ray signal exceeds a first threshold value, it is determined that the detection region of the object to be detected contains fissile materials; characterized by the further steps of: - irradiating the detection region of the object to be detected with low energy neutrons , the energy of the low energy neutrons being selected to enable the special nuclear materials that possibly exist in the fissile materials in the object to be detected to undergo a thermal neutron induced fission; - measuring a second fission ray signal emitted from the object to be detected due to the thermal neutron induced fission, wherein if the measured second fission ray signal exceeds a second threshold value, it is determined that the fissile materials in the object to be detected contain special nuclear materials ; - generating a first electron beam by an electron accelerator, the electron beam having a first energy; - producing the first X-ray beam using the first electron beam ; - generating a second electron beam by the electron accelerator if it is determined that the detection region of the object to be detected contains fissile materials, the second electron beam having a second energy different than the first energy; - producing a second X-ray beam by bombarding an electronic target with the second electron beam, wherein the energy of the second X-ray beam is lower than the energy of the first X-ray beam and the second X-ray beam is used to generate low energy neutrons. - irradiating a detection region of an object to be detected with a first X-ray beam , the energy of the first X-ray beam being selected to enable the special nuclear materials that possibly exist in the object to be detected to undergo an observable photofission; - measuring a first fission ray signal emitted from the object to be detected due to the photofission, wherein if the measured first fission ray signal exceeds a first threshold value, it is determined that the detection region of the object to be detected contains fissile materials; - irradiating the detection region of the object to be detected with low energy neutrons , the energy of the low energy neutrons being selected to enable the special nuclear materials that possibly exist in the fissile materials in the object to be detected to undergo a thermal neutron induced fission; - measuring a second fission ray signal emitted from the object to be detected due to the thermal neutron induced fission, wherein if the measured second fission ray signal exceeds a second threshold value, it is determined that the fissile materials in the object to be detected contain special nuclear materials ; - generating a first electron beam by an electron accelerator, the electron beam having a first energy; - producing the first X-ray beam using the first electron beam ; - generating a second electron beam by the electron accelerator if it is determined that the detection region of the object to be detected contains fissile materials, the second electron beam having a second energy different than the first energy; - producing a second X-ray beam by bombarding an electronic target with the second electron beam, wherein the energy of the second X-ray beam is lower than the energy of the first X-ray beam and the second X-ray beam is used to generate low energy neutrons. <EOS>