Imaging of low Z masked with high Z (Pb, U) materials using 14 MeV neutron

An experimental study has been performed using 14 MeV neutrons for imaging of low Z material (particularly composed of C, H, O elements) masked with thick layers of dense and high Z materials. The experimental setup consists of a D-T neutron generator, a metallic collimator and an imaging system. The imaging system is designed with a polypropylene zinc sulphide scintillator screen integrated with a lens coupled 16-bit ICCD camera. Imaging capability of the system was investigated using iron test samples with holes and line pair features. The minimum hole size of 2 mm could be imaged at a contrast of 36% and a line of 2 mm width visible at a contrast of 24% indicating the system's resolution of ∼ mm. Low Z samples such as water (H2O) and polyethylene (C2H2) n placed behind thick layers of Pb (40 mm) and Uranium (35 mm), were imaged successfully. These images reveal the system's ability towards low Z material imaging in the presence of heavier metals. Good contrast images acquired at a lower neutron yield of ∼ 5 × 108 n/sec of D-T neutron generator has provided a possibility to realise fast neutron imaging having moderate resolution (∼ mm) with a smaller footprint and an economical system design for field applications.


Introduction
Fast neutron radiography (FNR) is a powerful and rapidly growing non-destructive evaluation (NDE) tool with wide applications [1]- [4].Fast neutrons are highly penetrating compared to most of the other existing NDE probes such as X/gamma-rays, thermal or cold neutrons etc., due to their low interaction cross section.Because of its strong penetrating capability and weak dependency on the atomic number (Z) of the material, FNR is the preferred approach for imaging and investigation of low Z materials to simultaneously be observed with heavier metals.Another important feature of fast neutrons is that they do not transmute nuclei as readily as thermal neutrons, since the capture cross-sections of fast neutrons are relatively very small.This feature allows objects to be released immediately after inspection and investigation without waiting too long for radioactive nuclei to decay to an acceptable safe radiation level.
Due to these unique and distinctive features, FNR has been explored in different industrial fields, such as security applications (cargo inspection for contraband such as narcotics, explosives, and illicit drugs) [5,6] Imaging of special nuclear materials (SNM) [7,8], radiography and tomography of encapsulated heavy shielded low Z compound materials [9,10], cultural heritage investigation [11], etc. FNR can be performed with a variety of neutron sources, extending from fission neutrons to neutron generators, accelerators or spallation neutron sources, and nuclear reactors.Among them, reactors and large accelerators have been successfully used to develop advanced fast neutron imaging facilities, by virtue of their high neutron fluxes.Fast neutron imaging beam lines or test facilities for example, NECTAR: radiography and tomography station using fission neutrons [12], fast neutron tomography system designed at 500 kW research reactor, Ohio State University [13], and Fast neutron tomography (FNCT) experiments at an accelerator facility of PTB, Germany [14] have proven the potential use of these sources in producing high quality images.The neutron radiography facility relying on large neutron source has good beam quality, high imaging collimation ratio, and high beam intensity at the imaging plane, but the disadvantage is that the facilities are too large and the cost is high, which prevents them from meeting the users local testing needs.Other sources, like neutron generators (NGs), provide an advantage in designing FNR systems with a smaller footprint and higher neutron energies.The NGs are usually deuterium-deuterium (D-D) and deuterium-tritium -1 -(D-T), which are based on T(d, n) 4 He and D(d, n) 3 He nuclear fusion reactions producing fast neutrons of energy 14.1 MeV and 2.45 MeV, respectively.Commercially available compact and portable NGs, offer the advantage of being able to take fast neutron radiography systems to the investigating object, rather than vice versa.Thus, enabling the use of non-destructive FNR inspection method for field applications.NG has the added advantage of safety, as it can be switched on or off whenever required [15,16].FNR with D-T neutrons is preferred over D-D neutrons because neutrons with energy above ∼ 10 MeV have the high ability to penetrate samples with a thickness of tens of centimeters.Various FNR studies, performed with D-T neutrons have been reported using high-yield (> 10 9 n/sec) NGs by different groups of researchers [17][18][19].
An ideal source for radiography would be a high-yield, small spot size source with a small foot print enough to be transportable.A higher yield means a high neutron flux, offering high collimation ratio as required for achieving high-quality images and resolution.The efforts to increase fast neutron flux will then in turn increase the engineering complexity of adequate shielding for protection of personnel, the imaging sensor, and the associated electronics.Thereby, developing a FNR system with D-T NG of lower yield (∼ 10 8 n/sec), smaller foot print and a moderate resolution of ∼ mm would be most useful for industrial use with minimal shielding.The resulting FNR system would then also be widely acceptable as a practical NDT tool for field applications.But FNR with lower neutron yield (∼ 5 × 10 8 n/sec or less) D-T NG is challenging and very few such studies have been reported that to with longer image acquisition time (∼ hours) [20,21].However, the technological advancement in scintillator development with high detection efficiency for fast neutrons accompanied by improved novel methods of digital imaging and image processing tools [22]- [25] have ensured a high success rate of FNR towards compact system development with low neutron yield sources.
In our earlier study, FNR feasibility experiments were carried out using a 30 mm thick plastic scintillator screen, a D-T NG of ∼ 2 × 10 9 n/s neutron yield, and with that minimum 5 mm hole size in different materials could be imaged [19].The present study has been performed using a four times lower neutron yield (∼ 5 × 10 8 n/sec) of D-T NG and a thin (3 mm) scintillation screen.Its aim is to image smaller (< 5 mm) features in thick metal samples thereby improving the system's performance and detection of H, C, and O elements based low Z materials masked with thick layers of dense, high Z materials.System performance was evaluated by imaging test samples with holes and line pair patterns in a 25 mm thick Fe plate.Configurations containing low and high Z materials were also imaged.Moderate quality images acquired with ∼ 5 min exposure time and analysis showed that a minimum of 2 mm hole size and 2 mm width lines could be detected in a 25 mm thick Fe plate.In spite of low neutron yield, the presence of low Z materials (Water and Polyethylene (C 2 H 2 )  ) masked with thick layers of (35 mm) Uranium (U) and (40 mm) Pb were imaged successfully.The experimental results indicate that the FNR system with D-T NG of lower neutron yield is feasible, and it enables the imaging of small features (∼ mm) in thick metals as well as low Z material imaging in complex configurations.This research work is a demonstration step in FNR technology development towards portable radiography applications.

Experimental set-up
A lab-based D-T neutron generator [26] has been used for the FNR study.It is a 300 kV DC accelerator, where d + ions produced in an RF ion source are extracted, focused, accelerated, and bombarded -2 -on the target.The target is maintained at ground potential.The deuteron ions impinge on the titanium-tritiated (TiT) target providing 14.1 MeV neutrons via T(d, n) 4 He nuclear fusion reactions.NG was operated at a yield of ∼ 5 × 10 8 n/sec for the FNR experiments.The yield was measured using foil activation method [26,27].The neutron emitting area on the target is about 20 mm in diameter.Since the small emitting spot is essential to minimize the source-induced blurring to the image, thus to reduce the beam spot, a custom designed metallic (MS-mild steel) collimator of L/D = 43 (L = collimator length = 430 mm and D = entrance opening diameter = 10 mm) was coupled with neutron target (more detail on collimator at reference [19]).The cuboidal-shaped MS collimator has a conical cutout inside it throughout its length (L) 43 cm with an inlet opening (D) of 10 mm (figure 1(c)).A part of the collimator that surrounded the NG target was of dimension 200 mm (length) × 200 mm (height) × 100 mm (thickness) with a cut-out of 100 mm × 100 mm × 100 mm at centre for coupling the NG target part.
The collimator was positioned at a 90-degree angle with respect to the d + ion beam (figure 1) to avoid and reduce the high contribution of the scattered neutrons at the image plane.The major source of scattered neutrons is the concrete walls of the experimental room, which are a ta close distance (of ∼ 1 to 2.5 meters) to the neutron target.This geometrical arrangement of the collimator was also necessary due to the limited space available (for accommodating collimator, sample, imaging box, and shielding material around it) between the neutron target and the front wall in the forward (0-degree angle) direction.
The imaging system is composed of a 3 mm thick scintillator screen, a front-coated aluminum mirror, and a lens-coupled ICCD camera system, all incorporated into a light-tight box.From neutron source to the scintillator screen, the distance was ∼ 85 cm.A mirror was placed at 45 • with respect to the scintillator, directing scintillation light to the camera avoiding radiation damage at ICCD due to direct neutron beam exposure.A commercially available PP/ZnS: Cu scintillator screen of dimensions 300 mm (length) × 300 mm (width) used for fast neutron detection [28,29].It is a homogeneous mixture of a hydrogen-rich material (polypropylene-PP) with a certain amount of zinc sulphide phosphor activated with copper (ZnS:Cu), providing high light output for fast neutrons, and the maximum emission wavelength is approximately 520 nm.The hydrogen nuclei of PP act as a converter for the fast neutrons, producing a proton recoil reaction to yield photons within the scintillator.The scintillator produced light photons are collected using a lens of f = 50 mm, f/0.85 coupled to a 16-bit ICCD.ICCD has 1 k × 1 k pixels, a pixel size of 13 μm × 13 μm, and was operated at a gain of 2200.The camera temperature was kept at −10 • C to reduce thermal noise.Scintillator to lens distance was ∼ 47 cm, and in this condition the field of view (FOV) was 130 mm (H) × 130 mm (V), providing an effective pixel size of ∼ 132 micron.The camera was surrounded with Pb, Fe, and concrete blocks to protect it from scattered high energy neutrons and neutron-induced gamma radiation.The FNR experimental setup and its schematic are shown in figure 1.

Sample and methodology
Different test samples and configurations containing low and high Z materials used for investigation are described here.First, for system performance evaluation, test samples of iron (Fe) material were imaged.Iron material was chosen because of its weak scattering for fast neutrons compared to that of polyethylene, graphite, etc.Also, Fe is easily available at a reasonable lower price compared to -3 - dense and high Z materials like tungsten (W) or tantalum (Ta) [13].Also, Fe is a most commonly used structural material.Test samples were fabricated using a 25 mm thick Fe plate with dimensions of 120 mm (length) × 50 mm (wide).Test Sample 1 (TS1) was a hole sample having five cylindrical holes of diameter 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm each drilled to a depth of 25 mm.Test Sample 2 (TS2) was a line pair pattern, with a group of four lines having a line width of 2 mm, 3 mm, 4 mm, and 5 mm with the same line spacing of 2 mm, 3 mm, 4 mm, and 5 mm respectively.Next, the imaging performance of the FNR system towards complex samples containing low and high Z material was performed using another set of samples.In the first configuration, a water sample was masked with a 35 mm thick circular disk (150 mm in diameter) of Natural Uranium (U), and in the second configuration, a high-density polyethylene (HDPE) block [(50 mm (width) × 30 mm (height) × 100 mm (depth)] having a 5 mm hole at its center] masked with the Natural uranium (U) disk and a 40 mm thick Pb block (200 mm (length) × 100 mm (wide)).A photograph of the different samples is provided in figure 2.
For the experiments, samples were placed in contact with the scintillator, and images were acquired for an exposure time of ∼ 300 seconds.Note that a support base was used to position the sample in line with the neutron beam.Due to different sample configurations, bases of different geometry (and of different materials like paraffin blocks, iron, Pb plates, etc.) were required and used accordingly.Open beam (OB-ref, without sample and support material) and dark field -4 -

Results and discussion
TS1-sample with holes.To image small structures in thick samples, TS1 with hole features was imaged.A flat field image of the hole sample with an intensity plot profile across the holes is shown in figure 3. The plot profile shows four major peaks, revealing 2 mm, 3 mm, 4 mm, and 5 mm holes.For quantitive analysis, hole contrast was evaluated and found to vary from 36% to 46%, as presented in -5 - A significant variation in intensity has been observed in the image, even though the sample has a constant thickness throughout.The possible cause could be the scattered neutrons due to the sample itself [31].Neutrons are scattered from the sample itself or from the support materials, which are used as a base to lift the sample height and bring it in line with the neutron beam (figure 1(b)).Depending upon the geometry of the sample and its base structure, their material type, and their positions with respect to the scintillator screen and neutron beam, the scattered neutron contribution varies, resulting in a non-uniform contribution at the image plane.Where I Line and I Fe are the intensity computed at line region (no material) and the Fe region (next to that respective line in the image), respectively.These regions are indicated in the image (figure 4) with rectangles (red color).Contrast values obtained for different lines of width (5 mm-3 mm) were in the range of ∼ 32%-40% and a line of 2 mm width was visible at a contrast of ∼ 24%.Thus, it can be stated that the spatial resolution of the imaging system with respect to resolving the line -6 -features is ∼ 2 mm, or in other words 0.25 lp/mm.The major factor (other than the low neutron source intensity, i.e., smaller L/D) that limits the system's capability of minimum detectable feature size is the scattered neutrons arising mainly from the experimental surroundings (including collimator materials, accelerator tube, shielding blocks, vacuum pumps etc.) and experimental room walls, as well as from the investigated thick samples itself.For a portable FNR system, these factors can be eliminated or improved by using a portable compact D-T NG with a smaller (∼ 1-2 mm) neutron emission spot [17].In that case, no additional collimator is required, and its smaller component size would contribute to almost negligible scattering of neutrons.Also, its use in the field would eliminate the possibility of scattered neutrons caused by experimental hall walls.Thus, overall performance would be improved, and resulting in further better quality imaging.

Imaging of low Z material
After the imaging of test samples of structural material, this section focuses on the imaging of low-Z materials.Fast neutrons can provide high contrast good quality images of objects containing low-Z/high-Z materials simultaneously [13,14].This is due to the fact that in the high energy (≳ MeV) range, the neutron interaction cross section (macroscopic) is in general low and does not exhibit significant element-to-element variations.Imaging of low Z materials composed of lighter elements (C, H, N, O etc.) is not only important for industrial applications but also from a security point of view it is most essential.Because most of the present potential threats (chemical explosives like TNT, RDX, etc.) are primarily composed of lighter (C, H, N, O etc.) elements [32].X-rays or thermal neutrons fail to image and detect such low Z if hidden or shielded with thick layers of dense, high Z materials like Fe, Pb, U, etc.In practical scenarios, for example, imaging and detection of threat materials (chemical explosives) hidden inside large volumes of cargo, trucks etc. FNR can be a potential tool as a primary source of investigation [33].It can be used as a first level of screening prior to the other confirmatory neutron interrogative techniques such as associated particle imaging or fast neutron resonance radiography [34,35] etc. for the scanning of transport vehicles at entry points.
In order to image C, H, O element based low Z material, water (H 2 O) and HDPE samples were chosen.Water (1.5 liters) filled in a glass beaker was imaged with and without a U-disk mask.Similarly, HDPE block was imaged in different configurations masking by U and Pb.The results are discussed below.
Water sample + U. Flat field images of the water sample with and without mask material are shown in figure 5, along with an intensity plot profile across the water-U interface region.The water is clearly visible in both images at different contrasts.The evaluated contrast at the water-glass interface was ∼ 27%, which was reduced to 23.7% in the presence of U-disk.[Interface Contrast = (I glass − I water )/(I water + I glass ), where I water = intensity at the water region of glass (marked 1) and I glass = intensity at the no water region of glass (marked 2)].Contrast values illustrate that the FNR system has a high probability of imaging a water like liquid sample in such configuration.It also reveals the high penetration ability of 14 MeV neutrons through uranium disk and its sufficient attenuation in water sample.HDPE sample + U + Pb. Figure 6 shows the FNR images of the HDPE sample with and without masked materials (Pb, U).The presence of HPDE and its hole feature can be seen clearly in all the  images.Plot profiles (figure 6) across the hole region (yellow marked) also shows the presence of hole feature in HDPE block in all three images.Contrast values calculated for the hole and at the interface of HDPE -masked materials (Pb, U) are listed in table 2. Hole contrast reduces from 58% to 42% in the presence of uranium and it goes further down to 26% in the presence of an additional Pb block making hole visibility difficult, as it can be seen in the images (figure 6).The contrasts at the interface of HDPE and mask materials (U, Pb) obtained were ∼ 42% and 26% for U and U + Pb respectively.
The ability to penetrate thick layers of Pb-U and reveal the presence of water or HDPE highlights the imaging potential of the FNR system towards the imaging of materials made of lighter elements (H, C, O etc.) in environments having layers of high Z materials.Photon (X-/gamma rays) as well as thermal neutron based imaging may not be able to provide sufficient transmission intensity through thick layers of high Z metals (U, Pb) and sufficient contrast for hydrogenous materials (water/HDPE) in such scenarios [31].Since photons being strongly attenuated by high Z materials and may give -8 -

Conclusions
Fast neutron radiography using D-T NG with neutron yield of ∼ 5×10 8 n/sec has been realized, intended for the investigation of low Z material (involving C, H, and O elements) in environments containing low and high Z materials.System performance was evaluated by imaging the test samples of Fe with different sized holes and line pairs.The minimum hole size of 2 mm and the lines of 2 mm width could be imaged at a contrast of 36% and 24%, respectively.Imaging of such features in a 25 mm thick Fe sample has demonstrated the possibility of FNR for the examination of large structural objects where iron is one of the major components.Here, the main challenge faced was the scattered neutrons from the surroundings, collimator wall material, and the sample itself.However, the use of a thin scintillator screen with an ICCD based digital imaging system and collimator placed in a proper geometry overall made it possible to reduce scattered neutron effects and obtain reasonably good quality FNR images.
In spite of the low neutron yield, images of water and HDPE shielded by high Z materials could be achieved.Water sample images in the presence of a 35 mm thick uranium disk witness the potential of the FNR system in applications where low Z materials are hidden in environments having low and high Z simultaneously.The experimental results of HDPE images (at a contrast of 44% along with a 5 mm hole at ∼ 26% hole contrast) in the presence of thick layers of 35 mm U + 40 mm Pb indicate the significance of FNR for bulk sample investigations.
Such FNR system with low yield NG is worthful to use in public/field areas with minimal shielding and smaller footprint.Although, the image quality or resolution due to low flux is not at par with a reactor or accelerator based FNR system, which provides large L/D.However, it does provide an alternative for portable system with adequate resolution and simplicity for applications where such capability is needed but resources are limited.System performance can be further improved by using portable compact size NG with smaller (∼1-2 mm) neutron emission spot, resulting in collimator removal and reduction of the scattered radiation fraction, thus enhancing the image quality for smaller size features detection.

Figure 1 .
Figure 1.(a) FNR experimental set-up; (b) zoom view of imaging box, and (c) schematic of the FNR experimental set-up.

Figure 3 .
Figure 3. Flat field FNR Image of the TS1 having different size holes and the intensity plot across hole region (marked region).

TS2-sample with
lines.Next, to find the resolving capability of the system for closely spaced line features in the Fe plate, a sample (TS2) with a group of lines was imaged.From the flat field image of the TS2 presented in figure 4, it can be observed that lines of different widths from 5 mm to 2 mm, are all visible and clearly resolved.The intensity plot across the lines in the sample image reveals sets of four peaks and four valleys, indicating different groups of lines.The contrast parameters were calculated for few lines (marked by arrows in figure 4) of different width via [(I Line − I Fe )/(I Line + I Fe )].

Figure 4 .
Figure 4. Flat field FNR Image of TS2 Sample with lines of different width and the intensity plot across lines (yellow marked region).

Figure 5 .
Figure 5. Flat Field FNR images of water sample with (left) no mask material, and (middle) masked with U-disk and (right) respective intensity plot profile across the marked region.

Figure 6 .
Figure 6.Flat Field FNR images of HDPE (top-left) without mask, (top-middle) with U-disk and (top-right) with U + Pb, (bottom) respective plot profile across the hole region (marked).

Table 1 .
Hole contrast evaluated for hole sample.

table 1 .
[Hole contrast = (I hole − I Fe )/(I hole + I Fe ), where I hole = intensity inside the hole region and I Fe = intensity in the Fe region next to the respective hole].The Fe regions used for computing I Fe for different holes are indicated with 1, 2, 3, and 4 in the image.It can be noted that the minimum hole size imaged is 2 mm, while a 1 mm diameter hole was not visible at all.The FWHM (full width and half maxima) of the 2 mm hole obtained was 2.05 mm from the plot profile.The result shows the system's imaging capability of detecting up to a 2 mm size hole in 25 mm thick Fe plate, indicating ∼ mm order spatial resolution of the system.

Table 2 .
Contrast values evaluated for HDPE sample, where I hole = intensity at hole region, I HDPE = intensity at HDPE region (location marked-1) and I mask = intensity at region of mask material location (mark-2).Mark 1 and 2 in last image indicates the location of region of interest (for all three images) taken for intensity (I mask ) computation to evaluate the respective contrast.