Fast neutron radiography using protoluminescent imaging plates

Fast neutron radiography (FNR) and resonance neutron radiography (RNR) are complementary to the conventional radiography with high energy gamma-rays or brems-strahlung radiation used for the inspection of thick metal objects. In both non-destructive methods, the contrast sensitivity and the penetration power can be improved by using higher energy neutrons. At present direct techniques based either on Solid State Nuclear Track detectors (SSNTDs) or scintillating screens and transfer techniques using activation threshold detectors and radiographic films are applied for the detection of fast neutron images. Rather low detection sensitivity, of film and SSNTD based fast neutron imaging methods and also rather poor inherent image contrast of SSNTD pose a problem for FNR in the fast neutron energy region 1-15 MeV interesting for NDT. For more efficient detection of fast neutron images the use of novel highly sensitive photoluminescent imaging plates (IP) in combination with threshold activation detectors as converter screens in transfer imaging technique was proposed by authors and tested at the KFKI research reactor. The conventional IP produced by FUJI Photo Film Co. for the detection of beta and X-ray radiation were used. The threshold activation detectors were the reactions In-115(n, n)In-115m, Zn-64(n,p) Cu-64, Fe-56(n,p)Mn-56, Mg-24(n,p)Na-24 and Al-27(n,alpha)Na-24. These threshold reactions cover the fast neutron energy region between 0.7 MeV and 12 MeV. Pure, commercially available metals 0.1 nun to 0.25 mm thick made of In, Zn, Fe, Mg and Al were used as converter screens. The very high sensitivity of IP, the linearity of their response over 5 decades of exposure dose and the igh dynamic digitalisation latitude enabled fast neutron radiography of image quality comparable to the quality of thermal NR. In our experimental conditions (Phi(n)approximate to 10(8) n/cm(2)s, R(Cd)approximate to 2) the neutron exposure and IP exposure periods were still practical and comparable to the half life of the corresponding reaction products (half an hour to several hours). Even with the Al-27(n,alpha)Na-24 reaction having a U-235 fission spectrum avaraged cross-section of 0.65 mb useful fast neutron images were obtained. These preliminary experiments clearly demonstrated the useful imaging properties of IP in transfer fast neutron radiography with threshold activation detectors. On the basis of this new fast neutron imaging technique and using an appropriate set of threshold activation converter screens FNR in energy window between the 0.1 MeV to 20 MeV could be conceived.