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

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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. J. Imaging 2020, 6, 128; doi:10.3390/jimaging6120128 www.mdpi.com/journal/jimaging J. Imaging 2020, 6, x FOR PEER REVIEW 2 of 12 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 thosethose achievedachievedwith 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]. object These calculationsimages [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 wasobject fabricated was fromfabricated a stack from of 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. J. Imaging 2020, 6, 128 3 of 11 J. Imaging 2020, 6, x FOR PEER REVIEW 3 of 12 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 thewith 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).
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