9Be convertor has been successfully used in photoneutron and x-ray radiography system (PNXR) to achieve material identification by combining X-ray imaging and photoneutron imaging technologies together within one 7 MeV LINAC. The photoneutron convertor, a 2.6 kg 9Be cylinder of 16cm × 7 cm that converts photons to photoneutrons, has the advantages of high neutron yield (1010∼1011 n/s), robustness and low cost. However, this source also suffers from its intrinsic disadvantage that it is not a point source but a volume source that deteriorates the spatial resolution of neutron image. To enhance the performance of PNXR, the 9Be source must be researched in detail to know its neutron emitting process. This paper presents the general research method of evaluating photoneutron source from aspects of the effective source volume, size and characteristics of emitted neutrons. Simulation with MCNP5 code tracks all the physics process of photoneutron from its origination to emission. The position and time information of each photoneutron's birth and collisions (including scattering and (n,2n) reaction) inside 9Be are recorded. Totally 1,178,918 histories are tallied with PTRAC card of MCNP5. An algorithm is developed to extract all the data and integrate them to calculate the effective source volume. Upon the simulation result, the 16cm ×7 cm 9Be cylinder shows as volume source of hourglass shape with average equivalent radius of 3.96 cm. Each neutron undergoes 6.24 collisions before its successful escape from 9Be convertor. The average energy of photoneutrons is decreased from 0.74 MeV at birth to 0.34 MeV by the collisions. The method presented in this paper will help the future optimization of 9Be convertor for PNXR. It can also be used for the research of other (γ,n) or (γ,fission) based neutron sources.
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