A Fast Neutron Radiography System Using a High Yield Portable DT Neutron Source

Journal of Imaging

Article A Fast Neutron Radiography System Using a High Yield Portable DT Neutron Source

David L. Williams *, Craig M. Brown, David Tong, Alexander Sulyman and Charles K. Gary Adelphi Technology Inc., 2003 E. Bayshore Rd, Redwood City, CA 94063, USA; [email protected] (C.M.B.); [email protected] (D.T.); [email protected] (A.S.); [email protected] (C.K.G.) * Correspondence: [email protected]; Tel.: +1-650-474-2750

Received: 20 October 2020; Accepted: 24 November 2020; Published: 26 November 2020

Abstract: Resolution measurements were made using 14.1 MeV neutrons from a high-yield, portable DT neutron generator and a neutron camera based on a scintillation screen viewed by a digital camera. Resolution measurements were made using a custom-built, plastic, USAF-1951 resolution chart, of dimensions 125 98 25.4 mm3, and by calculating the modulation transfer function from × × the edge-spread function from edges of plastic and steel objects. A portable neutron generator with a yield of 3 109 n/s (DT) and a spot size of 1.5 mm was used to irradiate the object with neutrons for × 10 min. The neutron camera, based on a 6LiF/ZnS:Cu-doped polypropylene scintillation screen and digital camera was placed at a distance of 140 cm, and produced an image with a spatial resolution of 0.35 cycles per millimeter.

Keywords: fast neutron radiography; FNR; imaging; deuterium-tritium; DT; 14.1 MeV; USAF-1951; modulation transfer function; MTF

1. Introduction Kallmann and Kuhn [1,2] are credited with taking the first neutron radiographs shortly after the discovery of the neutron by Chadwick in 1932. They used a low-yield neutron generator with a yield of 4 107 n/s and exposure times of 4 to 5 h [3]. Higher-quality neutron radiography images were then × produced by Thewlis [4] using reactors and also by Peters [5] using a higher-yield neutron generator than that used by Kallmann, which yielded images of reasonable quality in 1–3 min [3]. High-yield portable neutron generators offer the promise of being able to take a useful neutron radiography system to the object to be imaged rather than vice versa, potentially enabling use of this technique for non-destructive inspection applications. This early neutron imaging work focused on thermal neutrons, which were either produced by a reactor, or were produced by moderating the neutrons from a neutron generator. Fast neutron radiography (FNR), directly using the 14.1 MeV neutrons arising from the deuterium–tritium (DT) reaction, has been reported using lower-yield portable neutron generators by Andersson [6], and high-yield neutron generators that are not portable, by Wu [7,8]. With incremental improvement in DT set up reported by Bishnoi [9] and Kam [10]. It should be noted that FNR using fast, 2.45 MeV, neutrons arising from the deuterium–tritium (DD) reaction has previously been reported using neutron generators [11–13]. However, the DD fusion reaction rate is approximately 1% that of the DT reaction rate [14]. Consequently, more power is required to achieve a given neutron yield, so this technology is not suited to truly portable radiography applications. An ideal source for radiography would be a high-yield, small-spot-size source with a small enough footprint to be transportable. To date, some have said that neutron radiography remains constrained by the lack of availability of high-intensity portable neutron sources [15], without which the object to be imaged must be taken to a reactor or fixed neutron generator source, which is impractical for some applications such as building inspection.

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J. Imaging 2020, 6, 128 2 of 11 sources [15], without which the object to be imaged must be taken to a reactor or fixed neutron generator source, which is impractical for some applications such as building inspection. TheThe radiographicradiographic contrasts obtained obtained by by 14.1 14.1 MeV MeV FNR FNR are are similar similar to those to those obtained obtained with withhigh- high-energyenergy X-ray X-ray and and gamma gamma-ray-ray radiography radiography and and in in general general are are smaller smaller than those those achieved achievedwith with thermalthermal neutron neutron radiography. radiography. However, However, unlike unlike with with X-ray X-ray and and gamma-ray gamma-ray radiography, radiography, the the broader broader contrastcontrast latitude latitude allows allows low-atomic-number low-atomic-number materials materials to to simultaneously simultaneously be be observed observed with with heavier heavier metalsmetals [ 16[16].]. ImagingImaging performance performance using using a a high-yield, high-yield small-spot-size,, small-spot-size, DT DT generator generator is is presented. presented. A A 10 10 min min exposureexposure time time resulted resulted in in an an image image with with a a resolution resolution of of 0.35 0.35 cycles cycles per per mm. mm. A A custom-fabricated custom-fabricated plasticplastic phantom phantom imaging imaging object object was was built built to to investigate investigate the the performance performance of of a a neutron neutron imaging imaging camera camera consistingconsisting of of a a scintillation scintillation screen screen viewed viewed with with a a cooled cooled digital digital camera camera charge-coupled charge-coupled device device (CCD) (CCD) array.array. The The resolution resolution was was determined determined using using a USAF-1951 a USAF resolution-1951 resolution target [17 target] and also [17] by and calculating also by thecalculating modulation the transfer modulation function transfer (MTF) function by analyzing (MTF) the by edges analyzing of object the images edges [18 of]. objectThese calculations images [18]. wereThese performed calculations for bothwere steel performed and plastic. for both The advantagesteel and plastic. of the neutron-basedThe advantage technique of the neutron over X-rays-based istechnique the ability over to image X-rays “low-Z” is the ability (such asto hydrogenous)image “low-Z” materials, (such as hydrogenous) hence the use ofmaterials, the plastic hence resolution the use phantomof the plastic object. resolution phantom object. TheThe advent advent of of portable portable neutron neutron generators generators with with both both high high yields, yields,> 10>109 9n n/s,/s, and and small small neutron neutron emissionemission spot spot sizes, sizes, of of 1–2 1–2 mm, mm, could could potentially potentially enable enable wider wider adoption adoption of of this this technique. technique.

2.2. Materials Materials and and Methods Methods ® AnAn imaging imaging phantom phantom object was object fabricated was fromfabricated a stack fromof patterned a stack Delrin of [Polyoxymethylene, patterned Delrin® (CH[Polyoxymethylene,2O)n] sheets with (CH a combined2O)n] sheets thickness with a ofcombined 25 mm. Thethickness sheets of were 25 mm. fabricated The sheets using were a Computer fabricated Numericallyusing a Computer Controlled Numerically (CNC) milling Controlled machine (CNC) patterned milling with machine a USAF-1951-compliant patterned with a USAF resolution-1951- chartcomp consistingliant resolution of diﬀ erentchart sizedconsisting vertically of different and horizontally sized vertically oriented and groups horizontally of three oriented bars of groups equal widthof three and bars separation. of equal The width object and also separation. contained The six boltobject holes also which contained aligned six the bolt sheets holes with which respect aligned to eachthe sheets other. Thewith imaging respect to phantom each other. is shown The imaging in Figure phantom1. is shown in Figure 1.

(a) (b)

FigureFigure 1. 1.Imaging Imaging phantom phantom object—125 object—125 mm mm ×98 98 mm mm ×25.4 25.4 mm. mm.The The spatialspatial frequency frequency of of each each tri-bar tri-bar × × featurefeature is is given given in in cycles cycles per per millimeter. millimeter (.a ()a View) View from from above; above; (b ()b design.) design.

The completed object measured 125 mm 98 mm 25 mm. The stacked-sheet approach was The completed object measured 125 mm× × 98 mm× × 25 mm. The stacked-sheet approach was usedused because because of of the the ﬁne fine detail detail required required in thein the pattern. pattern. An An object object with with a feature a feature size lesssize thanless 1than mm 1 and mm aand depth a depth of 25 mmof 25 would mm would otherwise otherwise be diﬃ becult difficult to fabricate to fabricate from a singlefrom a block single of block plastic of since plastic it wouldsince it requirewould arequir small-diametere a small-diameter tool (submillimeter tool (submillimeter diameter) diameter) to cut through to cut the through entire thicknessthe entire of thickness the object, of likelythe object, causing likely the toolcausing to break. the tool The to completed break. The object completed is shown object in Figureis shown2, which in Figure shows 2, which the overall shows thicknessthe overall of thethickness object. of the object.

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FigureFigure 2.2. ImagingImaging phantomphantom object—125object—125 mmmm × 9898 mmmm × 25.25.44 m mm.m. × × The imaging phantom object was placed in direct contact with a “250 200 mm X-ray Neutron The imaging phantom object was placed in direct contact with a “250 ×× 200 mm X-ray Neutron Tomography Camera” Camera” from from the the company company Neutron Neutron Optics Optics [17], [17], almost almost flush flush with the with scintillator the scintillator (aside from(aside the from thickness the thickness of metal of forming metal forming the camera’s the camera’s light-tight light box,-tight and box, a thin and sheet a thin of lead sheet used of lead to screen used X-rays). The camera consists of a cooled Sony ICX694ALG CCD (2752 2200 pixels = 6 Megapixel) to screen X-rays). The camera consists of a cooled Sony ICX694ALG CCD× (2752 × 2200 pixels = 6 that images a 250 mm 200 mm scintillator using a high-resolution f/1.4 lens. The scintillator is Megapixel) that images× a 250 mm × 200 mm scintillator using a high-resolution f/1.4 lens. The viewedscintillator in reflection is viewed via in areflection mirror. This via a configuration mirror. This allowsconfiguration the camera allows electronics the camera tobe electronics shielded fromto be radiationshielded from because radiation it is not because in direct it lineis not of in sight direct with line the of neutron sight with source. the neutron A 2 mm source. thick polypropylene A 2 mm thick 6 phosphorpolypropylene plate, phosphor doped with plate,LiF / dopedZnS:Cu with from 6LiF/ZnS:Cu RC Tritec Ltd., from was RC used Tritec as a Ltd fast., imagingwas used scintillator. as a fast Thisimaging phosphor scintillator. emits This 530 nmphosphor light in emits response 530 n tom irradiation light in response with fast to (irradiation>0.8 MeV) with neutrons fast (>0.8 [19]. MeV) neutronsThe camera[19]. was oriented such that the neutron generator’s target was positioned at the scintillator’s zenithThe at a camera distance was of 140 oriented cm. A such 3 mm that thickness the neutron of lead was generator placed’s between target was the imaging positioned phantom at the objectscintillator flush’s againstzenith at the a distance camera. of This 140 thicknesscm. A 3 m ofm leadthickness was determinedof lead was placed to be e betweenffective inthe screening imaging outphantom unwanted object X-rays flush arisingagainst fromthe camera the neutron. This generator’sthickness of 150 lead kV was acceleration determined voltage. to be Leadeffective bricks in werescreening placed out around unwanted the neutronX-rays arising camera’s from CCD the arrayneutron to reducegenerator’s background 150 kV acceleration signals arising voltage. from neutron-inducedLead bricks were prompt placed gamma-ray around the emission neutron from camera’s materials CCD in array the walls to reduce and the background floor of the signals room. Thearising neutron from generatorneutron-induced and camera prompt were gamma positioned-ray inemission the center from of materials an empty in room, the walls approximately and the floor 1 m fromof the the room. floor. The The neutron room had generator a high roofand andcamera a low were mass positioned ceiling. E ffinorts the were center taken of an to minimizeempty room, any massapproximately close to the 1 cameram from tothe avoid floor. neutron The room scatter, had and a high minimize roof and gamma a low rays mass produced ceiling.from Efforts neutron were interactionstaken to minimize with extraneous any mass close material to the in camera the laboratory. to avoid A neutron 20 cm thicknessscatter, and of lithiumminimize carbonate gamma rays was usedproduced to attempt from neutron to shield interactions the detector with from extraneous scattered, material thermalized in the neutrons.laboratory. A A compact,20 cm thickness portable, of Adelphilithium carbonate Technology was DT used generator to attempt was to used shield as a the neutron detector source. from Thescattered, generator thermalized was operated neutrons. with A a 9 yieldcompact, of 3 portable,10 n/s DTAdelphi for 10 Technology min. This yield DT generator is estimated was from used neutron as a neutron measurements source. The made generator using a × Bonnerwas operated ball neutron with detector. a yield ofThis 3 neutron× 109 n/s generator DT for had 10 min an emission. This yield spot is size estimated of 1.5 mm, from as observed neutron bymeasurements measuring the made burn using pattern a Bonner on an aluminum ball neutron foil detector placed over. This the neutron solid metal generator target, had prior an to emission loading thespot generator size of 1.5 with mm, tritium. as observed The target by measuring was oriented the burn at 45 pattern◦ to both on the an ionaluminum beam and foil imaging placed over camera, the sosolid the metal projection target, of prior the target to loading on the the scintillator generator plane with istritium. circular. The The target expected was oriented angular divergenceat 45° to both of the beamion beam is consequently and imaging L /camera,D = 1400 so mm the/1.5 projection mm 900, of the otherwise target on expressed the scintillator as a divergence plane is circular. angle of 1 ≈ TanThe− expected(1.5/1400) angular= 0.061 divergence◦. of the beam is consequently L/D = 1400 mm/1.5 mm ≈ 900, otherwise expressedThe USAF as a divergence resolution chartangle wasof Tan analyzed−1(1.5/1400) by rotating = 0.061°. the image such that the bars were oriented parallelThe to USAF the axes resolution (i.e., ‘vertical’ chart was lines analyzed in the image by rotating parallel the to theimagey axis such and that ‘horizontal’ the bars were lines oriented parallel toparallel the x axis). to the The axes average (i.e., ‘vertical’ value of lines the pixelsin the atimage a given parallel x position to the was y axis then and taken ‘horizontal’ by taking lines the averageparallel valueto the ofx theaxis pixels). Thein average a ‘vertical’ value strip. of the These pixels results at a were given then x position plotted andwas the then observation taken by oftaking maxima the andaverage minima value in of the the expected pixels in locations a ‘vertical’ indicates strip. These that a results given resolution,were then plotted could be and imaged. the observation of maximaAdditionally, and minima the in modulation the expected transfer locations function indicates (MTF) that was a calculatedgiven resolution, by taking could a one-dimensional be imaged. FourierAdditionally, transform of the the modulation line-spread function transfer (LSF) function [20]. The(MTF) LSF was is the calculated derivative byof the taking edge-spread a one- functiondimensional (ESF), Fourier which transform is the intensity of the asline a- functionspread function of position (LSF) along [20]. a The line LSF perpendicular is the derivative to the of edge the edge-spread function (ESF), which is the intensity as a function of position along a line perpendicular to the edge of the object. After first rotating the line edge to be parallel to the y axis, the x values were

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J. Imaging 2020, 6, x FOR PEER REVIEW 4 of 12 of the object. After first rotating the line edge to be parallel to the y axis, the x values were averaged averagedacross the across portion the of portion the image of the that image straddles that thestraddles edge to the derive edge theto derive LSF. This the wasLSF. smoothedThis was smoothed by taking bythe taking average the of average the surrounding of the surrounding pixels over pixels a 2 mm over region a 2 m eitherm region side ofeither each side point. of each This point. was found This wasto be found sufficient to be to sufficient allow a to useful allow derivative a useful derivative to be calculated. to be calculated. The resolution The resolution of the imaging of the imaging system systemis conservatively is conservatively defined defin as theed reciprocal as the reciprocal of the spatial of the frequencyspatial frequency where the where modulation the modulation transfer transferfunction (MTF)function falls (MTF) to 0.15 falls [21 ]. to The 0.15 resolution [21]. The could, resolution alternatively, could, be alternatively, reported as the be reciprocal reported as of this the quantity.reciprocal These of this MTF quantity. measurements These MTF were measurements taken at different were locations taken at within different the imagelocations and wit comparedhin the withimage the and USAF-1951 compared results. with the USAF-1951 results.

3. Results The image shownshown inin FigureFigure3 3was was produced produced when when the the imaging imaging phantom phantom object object was was irradiated irradiated for 10for min.10 min A. 12.5A 12. mm5 mm thick thick steel steel block block was was irradiated irradiated with with the the sample sample for for comparison,comparison, andand isis located above the plastic phantom in the image.

Figure 3. ImageImage from from the the scintillation scintillation camera camera produced by irradiating the sample for 10 min.min.

Based on on the the neutron neutron yield, yield, the the duration duration of ofthe the exposure exposure and and the the solid solid angle angle subtended subtended by the by scintillatorthe scintillator from from the the neutr neutronon source, source, the the number number of of neutrons neutrons contributing contributing to to the the image image can can be estimated toto bebe (250(250 × 200200 mm)mm)/(4π/(4π ×1400 140022 mmmm22)) × 3 × 109 n/sn/s × 606000 s = 3.73.7 × 101099 neutronsneutrons across across the × × × × × × 250 × 202000 m mmm imaging imaging area area,, i.e., i.e., 73,000 73,000 neutrons per mm 2.. × Variations ofof overalloverall brightness brightness across across the the image image (darker (darker bottom bottom left left corner corner in Figurein Figure3) were 3) were due dueto the to presence the presence of structures of structures within within the generator the generator itself. itself. Moving Moving the camera the camera moved moved the position the position of the of the dark spot across the image. Dark-field corrections (background subtraction) was not performed on this image.

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3.1. Analysis of the USAF-1951 Resolution Chart dark spot across the image. Dark-ﬁeld corrections (background subtraction) was not performed on 3.1.1.this image. Visual Inspection J. Imaging 2020, 6, x FOR PEER REVIEW 5 of 12 3.1. AnalysisInspection ofthe of USAF-1951the images Resolutionshows that Chart a spatial frequency of 0.35 cycles/mm is observable. The results are recorded3.1. in Analysis Table of 1. the USAF-1951 Resolution Chart 3.1.1. Visual Inspection 3.1.1. Visual Inspection Inspection of the imagesTable 1. shows Visibility that of a the spatial resolution frequency standard’s of 0.35 tri cycles-bar patterns./mm is observable. The results Inspection of the images shows that a spatial frequency of 0.35 cycles/mm is observable. The are recorded in Table1. 1 resultsSpatial are recorded Frequency in Table 1. Horizontal Vertical Image 0.25 c/mm Visible Visible Table 1. VisibilityTable of 1. the Visibility resolution of the resolution standard’s standard’s tri-bar tri patterns.-bar patterns. 0.28 c/mm Visible Visible Spatial0.31 Frequency c/mmSpatial 1 FrequencyHorizontalVisible 1 HorizontalVisible Vertical Vertical ImageImage 0.35 c/mm 0.25 c/mmVisible VisibleVisible Visible 0.25 c/mm0.28 c/mm Visible Visible VisibleVisible 0.280.39 c/mm c/mm 0.31 c/mmNot Visible visible VisibleVisible VisibleVisible 0.31 c/mm0.35 c/mm Visible Visible VisibleVisible 0.350.44 c/mm c/mm 0.39 c/mmNot Visible VisibleNot visibleNot Visiblevisible Visible 0.39 c/mm Not visible Visible 0.44 c/mm0.441 Cycles c/mm Not per Visible millimeterNot Visible Not. visibleNot visible 1 Cycles per1 millimeter.Cycles per millimeter. Plotting the pixel intensity as a function of position on a 3D graph can make the pattern clearer, as seenPlotting in Figure the pixel4. Plotting intensity the pixel as a intensity function as ofa function position of position on a 3D on grapha 3D graph can can make make the the patternpattern clearer, clearer, as seen in Figure4.as seen in Figure 4.

(a) (b)

Figure 4. The use of 3D plots can make the images more visible: (a) 0.25 c/mm vertical; (b) 0.35 c/mm vertical. Units in pixels; averaging was performed to reduce the statistical noise. (a) (b) 3.1.2. Brightness as a Function of Position across the USAF-1951 Pattern Figure 4. TheThe use use of ofBy 3D 3D averaging plots plots can can the make make pixel thevalues images across more each xvisible visible: or y direction: ( (aa)) 0.2 0.25,5 a c cgraph/mm/mm vertical;of the average ( b)) 0.3 0.35 intensity5 c/mm/mm may vertical. Units inbe pixels pixels;constructed.; averaging Plotting was these performed on a graph to as reduce a function the of statistical position noise.in the cycle, rather than absolute distance, allows the three intensity peaks to line up for different line pair spacings in the pattern. This 3.1.2. Brightness Brightness as asis a ashown Function Function in Figures of Position 5 and 6 for across horizontal the and USAF USAF-1951 vertical-1951 bars Pattern , respectively. By averaging the pixel values across each x or y direction direction,, a graph of the average intensity may be constructed. Plotting Plotting these these on on a a graph graph as as a function of position in the cycle, rather than absolute distance, allows thethe threethree intensityintensity peakspeaks to to line line up up for for di differentﬀerent line line pair pair spacings spacings in in the the pattern. pattern. This This is isshown shown in in Figures Figures5 and 5 and6 for 6 for horizontal horizontal and and vertical vertical bars, bars respectively., respectively.

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Figure 5. Intensity distribution for the horizontal bars in the USAF-1951 pattern. Figure 5. IntensityIntensity distribution distribution for for the the horizontal horizontal bars bars in in the the USAF USAF-1951-1951 pattern pattern..

Figure 6. Intensity distribution for the vertical bars in thethe USAF-1951USAF-1951 pattern.pattern. Figure 6. Intensity distribution for the vertical bars in the USAF-1951 pattern. The graphsgraphs shownshown in in Figures Figures5 and5 and6 show 6 show a central a central peak peak and and two two side side lobes lobes corresponding corresponding to the to The graphs shown in Figures 5 and 6 show a central peak and two side lobes corresponding to threethe three bars bars comprising comprising each each pattern. pattern. This This periodicity periodicity is clearly is clearly visible visible down down to 0.35to 0.35 cycles cycles/mm/mm and and is the three bars comprising each pattern. This periodicity is clearly visible down to 0.35 cycles/mm and marginalis marginal at 0.39at 0.39 cycles cycles/mm./mm. This This is inis in agreement agreement with with visual visu inspectional inspection of theof the image. image. is marginal at 0.39 cycles/mm. This is in agreement with visual inspection of the image. 3.2. Modulation Transfer Function 3.2. Modulation Transfer Function 3.2. Modulation Transfer Function The ESF, LSF, and MTF for the plastic imaging phantom object are shown in Figures7–10. Figure7 The ESF, LSF, and MTF for the plastic imaging phantom object are shown in Figures 7–10. Figure is theThe left ESF, (vertical) LSF, and edge MTF of the for plastic, the plastic Figures imaging8 and phantom9 are taken object from are left shown to right in acrossFigures the 7–10. (vertical) Figure 7 is the left (vertical) edge of the plastic, Figures 8 and 9 are taken from left to right across the (vertical) 7transition is the left of(vertical) Adelphi’s edge logo of the and plastic, Figure Figures10 is taken 8 and across 9 are the taken bottom from (horizontal) left to right across edge. the (vertical) transition of Adelphi’s logo and Figure 10 is taken across the bottom (horizontal) edge. transitionFor comparison, of Adelphi’s the logo same and parameters Figure 10 are is taken plotted across for the the 12.5 bottom mm thick (horizontal) steel block edge. shown, as shown in Figure 11.

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(a) (b) (c) (a) (b) (c) Figure 7. Vertical left edge of the plastic phantom: (a) ESF, (b) LSF (inverted for clarity), and (c) MTF. FigureFigure 7. 7.Vertical Vertical left left edge edge of of the the plastic plastic phantom: phantom: ( a(a)) ESF, ESF, ( b(b)) LSF LSF (inverted(inverted forfor clarity),clarity), and (c) MTF.

(a) (b) (c) (a) (b) (c) FigureFigure 8. 8. VerticalVertical edge, edge, top top part part of of Adelphi’s Adelphi’s logo logo:: ( (aa)) ESF, ESF, ( (b)) LSF, LSF, and (c) MTF. Figure 8. Vertical edge, top part of Adelphi’s logo: (a) ESF, (b) LSF, and (c) MTF.

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(a) (b) (c) (a) (b) (c) FigureFigure 9. Vertical 9. Vertical edge, edge, bottom bottom part part of of Adelphi’s Adelphi’s logo logo:: (a) ESF, (a )(b ESF,) LSF ((invertedb) LSF (invertedfor clarity), forand clarity),(c) MTF. and Figure 9. Vertical edge, bottom part of Adelphi’s logo: (a) ESF, (b) LSF (inverted for clarity), and (c) MTF. (c) MTF.

(a) (b) (c) (a) (b) (c) Figure 10. Bottom edge of the phantom: (a) ESF, (b) LSF (inverted for clarity), and (c) MTF. FigureFigure 10. Bottom10. Bottom edge edge of of the the phantom: phantom: (a) ESF, ESF, ( (bb) )LSF LSF (inverted (inverted for for clarity), clarity), and and (c) MTF. (c) MTF. J. Imaging 2020, 6, x FOR PEER REVIEW 9 of 12 For comparison, the same parameters are plotted for the 12.5 mm thick steel block shown, as For comparison, the same parameters are plotted for the 12.5 mm thick steel block shown, as shown in Figure 11. shown in Figure 11.

(a) (b) (c)

FigureFigure 11. 11.Steel Steel block: block: ( a(a)) ESF, ESF, ((bb)) LSF (inverted (inverted for for clarity), clarity), and and (c) MTF. (c) MTF.

A summary of the results is given in Table 2.

Table 2. MTF measurements from different locations from within the image.

Material Location Orientation Spatial Frequency 1 Plastic Left edge Vertical 0.17/mm Plastic Logo top center Vertical 0.18/mm Plastic Logo bottom, center Vertical 0.16/mm Plastic Bottom edge Horizontal 0.42/mm Steel Left edge Vertical 0.2/mm 1 Smallest spatial frequency at which the MTF is 0.15.

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A summary of the results is given in Table2.

Table 2. MTF measurements from diﬀerent locations from within the image.

4. Discussion The USAF-1951 image gave a resolution of 0.35 cycles/mm. Measurements of the MTF at various locations within the image provide results of 16 to 18 cycles per mm for vertical lines, and 0.42 cycles per mm for a horizontal line. The best result (0.42 cycles per millimeter) was at the horizontal bottom edge of the phantom (which unfortunately is the only long horizontal edge in the image). The neutron emission spot has a circular projection of diameter 1.5 mm when viewed from the position of the camera (orthogonal to the neutron generator’s ion beam) because the neutron generator’s target is positioned at 45◦ to the ion beam. The difference between calculated MTF values for horizontal and vertical lines in the object is consequently not attributable to the neutron generator and can be attributed to misalignment of the phantom with respect to the direction of the neutrons, exacerbated by parallax effects. This effect is clearly visible in the optical photograph of the phantom shown in Figure1a and is particularly noticeable in the curved semicircles forming Adelphi’s logo. The top surface of the phantom is white, the absence of material is black, and the curved semicircular side walls can be seen as a grey color, indicating that the (visible light) camera was not coincident with the projection of the curved surface, as would be the case if the optical camera was positioned at infinity. The same grey color is not observed on the adjacent vertical lines of the logo, as is expected from parallax effects. Consistent with this, the USAF-1951 horizontal tri-bars for 0.31 and 0.39 cycles per millimeter appear darker than the others in the optical image, because of their orientation with respect to the camera. These parallax effects will similarly frustrate the USAF-1951 neutron image and will result in a less pronounced transition in pixel intensity across an interface, resulting in worse MTF measurements. The origin of the resolution limit in this image is most likely geometrical effects of phantom alignment and parallax caused by the relatively close distance of the detector. The 6 megapixel camera is not the limiting factor. Resolution is likely also hampered by neutron interactions, mostly from the floor and walls of the building. The experiment is likely under-reporting the achievable resolution. Additional work could lead to improvements: Greater number of neutrons in the image, either higher neutron yield (the generator is capable of • 1 1010 n/s) or increase the exposure duration (longer than 10 min); × Careful alignment of the phantom (taking a series of measurements at slightly different angles to • optimize for each structure being imaged); Use of a different standard that minimizes the geometrical/parallax effects (for example a • spoke wheel); Acquiring multiple shorter exposures and adding or averaging them with appropriate filtering • methods (e.g., median); Improving neutron and gamma-ray shielding around the neutron camera’s scintillator to reduce • the effects of scattered neutrons and gamma rays produced by neutron interactions within the scene which subsequently scatter onto the detector; Use of a scintillator that is more sensitive to fast neutrons and less sensitive to gamma rays could • also be explored. J. Imaging 2020, 6, 128 10 of 11

This work could be extended by repeating this study with imaging phantoms made from diﬀerent materials in order to illustrate the improved contrast latitude aﬀorded by this technique.

5. Conclusions The resolution of a portable neutron imaging system which was comprised of an Adelphi Technology DT generator and a Neutron Optics camera was investigated. A spatial resolution of 0.35 cycles per mm was measured via a plastic USAF-1951 target. and via MTF calculations, which gave a resolution between 0.16 and 0.42 cycles per mm depending upon the location and orientation of the measurement within the image.

Author Contributions: CAD design of the phantom, taking the measurements, analysis of the results, and manuscript writing—original draft preparation, D.L.W.; set up of the generator, C.M.B.; set up of the neutron camera, D.T.; fabrication of the USAF-1951 imaging phantom, A.S.; set up of the generator and MTF calculation methodology, C.K.G. All authors have read and agreed to the published version of the manuscript. Funding: This material is based upon work supported by the DARPA under Contract No. HR001117C0020. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of DARPA. Conflicts of Interest: Adelphi Technology manufactures neutron generators, including the DT neutron generator used in this work.

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