The illicit trafficking of special nuclear materials (SNMs) poses a grave threat to global security and necessitates the development of effective nuclear material identification methods. This study investigated a method to isotopically identify the SNMs, including \(^{233,235,238}\) U, \(^{239-242}\) Pu, and \(^{232}\) Th, based on the detection of delayed \(\gamma\) -rays from photofission fragments. The delayed \(\gamma\) -ray spectra resulting from the photofission of SNMs irradiated by a 14 MeV \(\gamma\) beam with a total of 10 \(^{9}\) were simulated using Geant4. Three high-yield fission fragments, namely \(^{138}\) Cs, \(^{89}\) Rb, and \(^{94}\) Y, were selected as candidate fragments for SNM identification. The yield ratios of these three fragments were calculated, and the results from the different SNMs were compared. The yield ratio of \(^{138}\) Cs/ \(^{89}\) Rb was used to identify most SNMs, including \(^{233,235,238}\) U, \(^{242}\) Pu, and \(^{232}\) Th, with a confidence level above 95 \(\%\) . To identify \(^{239-241}\) Pu with the same confidence, a higher total number of 1011 \(\gamma\) beams is required. However, although the \(^{94}\) Y/ \(^{89}\) Rb ratio is suitable for elementally identifying SNMs, isotopic identification is difficult. In addition, the count rate of the delayed \(\gamma\) above 3 MeV can be used to rapidly detect the presence of nuclear materials.
