System overview
In Fig. 1, we illustrate the principle to produce neutrons and photons used for the bimodal imaging driven by a single e-LINAC system. The 9 MeV electrons are very energetic, and hence the bremsstrahlung photons generated on the tungsten target are forward emitted. A heavy water convertor is placed ahead of the tungsten target to generate both the imaging neutrons and photons. Heavy water is chosen as the material to convert bremsstrahlung photons to neutrons due to (1) the low (γ,n) threshold of 2H (Eth = 2.223 MeV) and (2) the superb neutron moderation capability of 2H and 16O. The neutron moderation capability is critical in this study because the fast photoneutrons produced by the 2H(γ,n)1H reaction, in general, should be decelerated to slow neutrons to improve the imaging sensitivity. The orbital electrons of the 16O atoms and those of the 2H atoms can induce the scattering of bremsstrahlung photons. When the emitting angle of scattered photons is chosen as 90° with respect to the direction of bombarding electrons, the energy of incident photons interrogating the inspected object will typically be less than 511 keV (due to the Compton scattering), as shown in Fig. 2 (a). Photons with such an energy spectrum mainly interact with atoms via Compton scattering, which shows an almost constant mass attenuation coefficient for different elements13, and hence are helpful to analyze the mass thickness of the inspected object compared with x-ray tube measurements, as shown in Fig. 2 (b).
The neutron emission direction should be the same as the photons to conform the photon imaging geometry. In fact, the energy spectrum of emitted neutrons is not sensitive to the emission direction due to the almost isotropic moderation process of neutrons within the heavy water converter. In Fig. 3, we present the neutron energy spectrum measured with the time of flight (TOF) method by a 3He counter placed 10 meters away from the heavy water converter at the angle of 90°. The simulated neutron energy spectrum is also shown, and the two spectra match fairly well. The results show that when the 9 MeV e-LINAC works at 100 µA current, a 2500 neutron/cm2/s thermal neutron flux at 10 meters away can be anticipated for the neutron imaging. Its counterpart for photons is 108 photon/cm2/s.
Imaging sequence
Although imaging photons and neutrons produced by the bremsstrahlung photons share the same imaging geometry, their imaging processes’ interference should be considered. As e-LINAC works at a pulse mode of 5 µs duration and 100 Hz repetition rate, the photon flight time from the heavy water converter to the detector is merely 5.033 µs, in which the 5 µs is the pulse width of photons and the 0.033 µs is the photon’s flight time across 10 meters. Considering the decay time for the light emitted by the scintillation screen is 0.2 µs, in order to let the photons’ influence on the detection system die away, an additional time delay of 2 µs after the last photon bombarding the nMCP detector should be set for the photon imaging and before triggering the acquisition of neutron imaging. Therefore, in principle, the duration of [7.033 µs, 10 ms] after each electron pulse can be assigned to neutrons for neutron imaging. In the experiments, we chose [50 µs, 9.95 ms] as the duration for neutron imaging to avoid the mutual interference between the two imaging processes. Thermal neutrons used for neutron imaging have a characteristic speed of 2200 m/s and require 4.5 ms for the 10-meter flight. Thus, both the photon imaging and neutron imaging can be perfectly accommodated by the [33ns, 7.033 µs] and [50 µs, 9.95 ms] durations, respectively, as shown in Fig. 4.
The spatial distributions of imaging neutrons and imaging photons
Data acquisition of the last collision positions of neutrons inside the heavy water converter indicates the heavy water converter acts as a volume neutron source. As the detector system is typically placed 10 meters from the heavy water converter, this volume neutron source will be reduced to a surface source with a disk shape (the diameter is 10 cm, determined by the flight tube in Fig. 1(b)), as shown in Fig. 5 (a), with its counterpart for photons shown in Fig. 5 (b). The centers of gravity for neutrons and photons are (Y = − 0.29 cm, Z = − 0.0061 cm) and (Y = − 1.31 cm, Z = 0.0037 cm), respectively. The 1.02 cm distance between them is caused by the different scattering physics of neutrons and photons. This difference results in a mismatching error between the neutron image and the photon image, which is about 20 µm and can be neglected when the distance between the inspected object and the detector is only 1/500 of the heavy water converter to the detector. The full width at half maximum (FWHM) along the Y or Z directions for neutrons and photons is calculated as FHWMn = 5.5 cm, and FWHMp = 5.2 cm, which can introduce a penumbra blurring of 110 µm and 104 µm for neutrons imaging and photons imaging, respectively
Fusion of the neutron image and the photon image
With the imaging sequence shown in Fig. 4, the same inspected object’s neutron image and photon image can be acquired successively within a single e-LINAC operation. As shown in Fig. 6(a), two keys clamped by the aluminium holder are inspected. Fig. 6(b) and Fig. 6(c) show the photon image and neutron image, respectively. The difference between Fig. 6(b) and Fig. 6(c) is apparent. The key’s plastic handle can hardly be noticed in Fig. 6(b), while it is evident in Fig. 6(c). On the contrary, the aluminium key is clear in Fig. 6(b) while almost transparent in Fig. 6(c). The underlying principle is that the cross section of 1H is large (30.4 [email protected] meV) for neutrons but very small for photons (0.406 barn@200 keV, for 12C is 2.452barns@200 keV), while the opposite is true for 27Al(1.68 [email protected] meV for neutrons and 5.48 barns@200 keV for photons). Fig. 7 (a) shows the fused image from Fig. 6(b) and Fig. 6(c), in which the color indicates the type of the material, while the shade may reflect the mass thickness of the inspected object. There are 2048 × 2048 pixels of 200-μm size in the image. For each pixel, its neutron attenuation and photon attenuation will determine the coordinate of a point in Fig. 7(b). All the pixels in Fig. 6(b) and Fig. 6(c) thus help form Fig. 7(b), in which we can see six clusters. Cluster (1) shows a large neutron attenuation and a small photon attenuation, and Fig. 7(c)(1) indicates it is the plastic handle of the key. Cluster (2) shows both strong attenuation for neutrons and photons, and Fig. 7(c)(2) indicates this zone has both plastic and aluminium. Clusters (3) to (6) have the same slope in Fig. 7(c)(2), implying that they are the same material because the ratios between the neutron attenuation and photon attenuation are the same. Their different distances to the origin reflect the various mass thicknesses of the aluminium material in the key. The results shown in Fig. 7 indicate that bimodal imaging can be a very effective method to identify different materials with various mass thicknesses.
Benefitting from the drastic difference between the attenuation coefficients for neutrons and photons, this technology can help find the residual core material in cast turbine blade14. Fig. 8 (b) and (d) are the photon image and neutron image for a blade shown in Fig. 8 (a) with residual gadolinium tracer (gadolinium oxide powder in this study), respectively, while Fig. 8(c) and (e) are that for a blade without residual gadolinium tracer, respectively. There is no significant difference that can be noticed between the Fig. 8 (c) and (e), indicating the inability for photons to investigate the residual gadolinium tracer inside the blade. On the contrary, the difference between Fig. 8 (b) and (d) is evident, implying that the blade with residual gadolinium tracer can be effectively discriminated by neutrons. By fusing the images of Fig. 8 (b) to (e), a new image reflecting the position distribution of residual gadolinium tracer inside the blade is formed and shown in Fig. 9(a). To conduct a more quantitative comparison between the blades with or without gadolinium tracer, the distributions of the value, which is the ratio between the mass attenuation coefficient of neutrons and that of photons, of each pixel in the six squares of Fig. 9(a) are calculated and shown in Fig. 9(b)(1)~(6), with their counterparts for blade without gadolinium tracer are also shown for comparison. Because of the existence of gadolinium tracer, the separation between the two curves in Fig. 9(b)(1)(2)(5) are evident. Due to the lack of gadolinium tracer in Fig. 9(b)(3)(4)(6), the two curves in which conform to each other and does not show significant difference. Fig. 9(c)(1)(2) show the bivariate histograms of turbine blades without or with gadolinium tracer. The turbine blade with gadolinium tracer differs from that without gadolinium tracer obviously.