- Email: [email protected]
Large area imaging of hydrogenous materials using fast neutrons from a DD fusion generator
Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Large area imaging of hydrogenous materials using fast neutrons from a DD fusion generator J.T. Cremer a,n, D.L. Williams a, C.K. Gary a, M.A. Piestrup a, D.R. Faber a, M.J. Fuller a, J.H. Vainionpaa a, M. Apodaca a, R.H. Pantell b, J. Feinstein b a b
Adelphi Technology Inc., 2003 East Bayshore Road, Redwood City, California 94063, USA Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
a r t i c l e i n f o
abstract
Article history: Received 10 May 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Available online 8 February 2012
A small-laboratory fast-neutron generator and a large area detector were used to image hydrogenbearing materials. The overall image resolution of 2.5 mm was determined by a knife-edge measurement. Contact images of objects were obtained in 5–50 min exposures by placing them close to a plastic scintillator at distances of 1.5 to 3.2 m from the neutron source. The generator produces 109 n/s from the DD fusion reaction at a small target. The combination of the DD-fusion generator and electronic camera permits both small laboratory and field-portable imaging of hydrogen-rich materials embedded in high density materials. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fast neutron radiography Neutron imaging system Fast neutron generator for imaging DD fusion generator for imaging
Radiography using fast neutrons from accelerators has been known as an excellent method to penetrate high-density, thick objects. Fast neutron radiography was demonstrated by Hall [1,2] at Lawrence Livermore National Laboratory (LLNL) using a 30 cm by 30 cm area and 4 cm thick plastic scintillator (BC-408) plates, which was viewed indirectly by mirror reflection using a high resolution CCD camera. Hall obtained multiple view and ‘‘mock’’, tomographic, fast neutron images with a variety of test objects via a CCD camera, which imaged fast neutron induced light emission from a Bicron (BC-408) plastic scintillator via a 451 turning mirror. We followed the Hall approach to produce a large area, fast neutron imaging detector at modest cost. In this paper we report our fast neutron imaging results using a fast neutron generator source of 2.5 MeV, DD fusion neutrons and fast neutron imaging detector developed at Adelphi Technology. The 2.5 MeV fast neutrons are produced from the deuterium– deuterium (DD) fusion reaction in the diode of a RF plasma based neutron generator [6]. Single-beam, DD generators can produce neutron yields of up to 1010 n/s or greater [7]. An Adelphi Technology Inc. model DD-109 neutron generator with neutron yield 109 n/s was used in this work. The generator housing was approximately 91.44 cm long, and the generator HDPE shielding weighed about 90 kg. To reduce the background x-ray and gamma radiation, a 6.35 mm thick sheet of lead was placed over the
n
Corresponding author. Tel.: þ1 650 474 2750; fax: þ1 650 474 2755. E-mail address: [email protected] (J.T. Cremer).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.02.003
source port of the DD generator. The lead strongly attenuates the bremsstrahlung from accelerated electrons in the diode region, and attenuates x-rays emitted from electrons that fill K-shell vacancies in the copper and titanium target created by the DD reaction’s 3 MeV protons. Fig. 1 is an illustration of the DD neutron generator. The deuteron ions are accelerated by an applied DC voltage and strike a Ti-coated, hollow copper target with deuterium atoms embedded in the Ti from impact of prior Dþ ions. Water flows through channels in the hollow copper target to provide cooling from the 1 kW of deposited energy of the 120 kV, 9.0 mA, Dþ ion beam on the 4.5 cm2 target area, which produces total DD neutron yield 109 n/s. Each leg of the vertically oriented V Ti-coated copper target intercepts the 4.0 mm diameter, cylindrical Dþ beam to produce a half-ellipse-shaped source with a horizontal (x), major axis of 5.6 cm and vertical (y) minor axis of 0.4 cm. For Monte Carlo simulation, the DD source is represented by a rectangle in the z¼ 0 plane with dimensions horizontal dimension x¼2.4 cm and vertical dimension y¼0.4 cm as illustrated in Fig. 1(b). The emission of neutron flux and energy is slightly anisotropic for the DD and DT reactions relative to the direction of the Dþ ion beam that strikes the Ti-coated DD target [6]. Forward-emitted neutrons have the largest flux with maximum emitted energy 2.9 MeV. Back-emitted neutrons have a mid-range flux at the minimum emitted energy of 2.1 MeV. Neutrons emitted to the side toward the detector have the minimum flux with emitted average energy 2.5 MeV. We calculated a brightness of 8.3 106 n/sr/mA in comparison to our measured brightness 6.6 106 n/sr/mA.
52
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
Plasma Ion Source
Accelerator Section
Tapered Fiber Optic
CCD
Multi-Channel Plate BC-408 Scintillator
D+ HV
Object
"V" Radiator
n
Plasma Gate Electrode
Ti-Target
Electron Shroud
mirror 7.5 x 7.5 cm2 d = 5 or 38 mm
0.6
o = 152 or 320 cm
0.4
0 -1.5
-1
-0.5
-0.2
0
y (cm)
x (cm)
0.2 -2
15 cm
n 1
-2.5
Photons
5 cm
D+ 0.8
-3
Lens
m = 15 or 124 cm
Fig. 2. Shown is a diagram of the fast neutron radiographic imaging system comprised of the fast 2.5 MeV neutron source and imaging detector with object. The thick knife edge experiment and the large object imaging used the two configurations shown.
-0.4 -0.6 -0.8 -1 Fig. 1. (a) Shown are the principal components of the DD neutron radiographic imaging system. The DD-109 neutron generator produces a yield of 109 to1010 n/s. (b) Fast neutron source on each leg of the vertically oriented V target is half-ellipse shape with horizontal, x-directed, major axis 5.7 cm and vertical, y-directed, minor axis of 0.4 cm, which for Monte Carlo simulations can be approximated by rectangle in z ¼ 0 plane with horizontal x ¼2.4 cm and vertical y¼0.4 cm dimensions, shown by dotted line.
Thermal neutrons, x-rays, or low energy gammas have an attenuation cross section dominated by absorption, which allows the use of relatively thin thermal neutron phosphors scintillators. Fast neutron attenuation is dominated by elastic scatter with hydrogen atoms in hydrogen-rich materials. The attenuation by scatter requires the use of hydrogen-rich plastic scintillators, which are much thicker than the thermal neutron phosphors. The average 50% decrease of neutron energy per elastic collision with a scintillator hydrogen nucleus increases the neutron cross section for subsequent scatter and absorption, and significantly increases the probability of a second scatter or absorption interaction in sufficiently thick plastic scintillator sheets [8,9]. The recoil protons create a cascade of scatter events in the scintillator, which lead to excited electron states, which transition to ground with the emission of light. The imaging system is schematically illustrated in Fig. 2. The camera box allows the Stanford Photonics CCD camera [5] and its electronics to be placed outside the direct neutron beam exposure. This prevents direct neutron beam radiation damage of the camera CCD sensor and electronics. At the base of the light-tight camera box is a removable, 11.25 cm 11.25 cm area, aluminum cover that is 3 mm thick. A thick knife edge plate or object can be placed on the outer cover surface and the plastic scintillator on the inner surface. Numerous fast neutron imaging trials were performed with various thicknesses of BC-408 plastic scintillators, which ranged from 1.0 mm to 25 mm. As a result, the 5.0 mm thickness scintillator [10] appeared to provide the best imaging resolution and contrast for 5 to 60 min exposure times with 108 109 n/s DD neutron source yields. As shown in Fig. 2, the emitted scintillator light is intercepted by a 451 mirror (with reflectivity 495%). The mirror reflects the
light upward out of the neutron path to a 5 cm diameter lens with f-stop f# ¼ 1.0. The lens focuses the light onto the dual micro channel plate (MCP). The dual MCP light amplifier and CCD chip are thermoelectrically cooled to 20 1C. The dual MCP amplifier produces high photon gains up to 106 at a very low dark count level of 1 or 2 electron counts per frame at 15 frames per second. In our experiments the dual MCP amplifier was set to an overall photon amplification gain of Gmcp ¼5.0 105. The Stanford Photonics CCD camera [5] uses a Sony XX285 scientific grade image sensor, which has a full frame pixel count of 1380 K by 1024 K in which the np ¼1.41 106 pixels are 6.47 mm square. The addition of a 1.6 to 1 reducing fiber optic taper between the smaller CCD sensor and the larger dual MCP image intensifier output creates an effective square pixel with 10.35 mm sides at the input image plane. The dual MCP image intensifier has a 50 linepairs/mm resolution, which corresponds to 10 mm effective pixel size. An image is created by summing multiple frames. Each frame is 66 ms exposure (15 frames per second). The short frame times reduce the dark count accumulation to 1 or 2 dark counts per 66 ms frame, and the Piper software [5] for camera control, image acquisition, and image signal processing further reduces noise by filtering out cosmic ray noise and spot noise from each of the incoming 66 ms frames. The dark count of the dual MCA with 1.4 106 pixels is then 2.1 10 5 electrons per pixel per second. The frames can be stacked to form images comprised of 5 104 frames or more, which enabled image times from 66 ms to 8 h or more. The image times in our experiments ranged from 5 to 50 min. The dual MCP CCD night-vision camera used in our neutron imaging experiment, which stacks short-time-exposure frames, is designed to be limited by the photon noise (shot noise) rather than the dark noise and camera electronic noise. The photon noise is the natural variation of the photon flux emitted from the scintillator. That is, the dark count levels of the CCD pixels can be ignored compared to the assumed Poisson uncertainty of the photon emission from the scintillator, and the input neutron signal fluence. The dark noise and camera electronic noise levels were tested by operation of the imaging system for the 5 min to 1 h exposure times without the neutron generator. The nogenerator testing showed each frame and the stacked frames to be dark. Speckle and noise were readily apparent only when the generator was emitting fast neutrons.
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
A thin knife edge is commonly used to determine resolution of optical, thermal, and x-ray imaging systems [11]. The fast neutron scatter cross sections for the hydrogen and carbon atoms in the HDPE are only a few barns. The small, fast neutron cross section thus requires a thick plate of HDPE to act as an effective fast neutron knife edge. In fact a more appropriate term would be thick-edge test or abrupt boundary test. To confirm detector resolution during general imaging experiments, one can measure the intensity profiles of thick, hydrogen-rich, object features, which have a sufficiently abrupt change in thickness. Alternatively, the thick knife edge test can also be performed with plate of tungsten, which also strongly attenuates neutrons. In our thick knife edge tests, a 2.54 cm thick HDPE plate blocked half the neutrons from striking the scintillator. This produced an image with sharp contrast between the dark and light image regions. An intensity profile was taken across this image. The object resolution in the horizontal and vertical directions was measured by the fast neutron imaging of a 2.54 cm thick HDPE ‘‘quasi knife edge’’ that covered half the plastic BC-408 scintillator [3,4]. In the horizontal and vertical directions, the measured object resolution was compared to Monte Carlo simulation results. The thick knife edge was set on the outer surface of the 3 mm thick aluminum cover of the camera box, which was located 1.52 m from the fast neutron source. On the aluminum cover inner surface was attached the 5 mm thick, BC-408 scintillator with 10 cm by 10 cm area. The horizontal and vertical neutron divergence angles at the thick knife edge were 15.6 and 2.6 mrad, respectively. The thick knife edge plate transmission T for fast neutrons changes the image intensity from the covered to uncovered region by a factor of DI given by:
DI ¼ 1T
ð1Þ
The thick knife-edge measurements produced line intensity profiles in the horizontal I(x) and vertical I(y) directions. The corresponding line spread function along the horizontal, x-direction LSF(x) and vertical, y-direction LSF(y) were given by: LSFðxÞ ¼
dIðxÞ dIðyÞ and LSFðyÞ ¼ dx dy
ð2Þ
The normalized Fourier transform of the horizontal and vertical LSF yielded the corresponding horizontal MTF(fx) and vertical MTF(fy) modulation transfer functions, which were functions of the horizontal fx ¼ 1/x and vertical fy ¼1/y spatial frequencies [12,13]. The MTF(f) is the Fourier transform of the impulse response, or point spread function PSF(y), which is the pattern seen from a perfect point source, which in one dimension is the line spread function LSF(y). If we put an attenuating object with a sharp line transition in the image, the resulting knife edge image, I(y) will be determined by the integral of the LSF(y) over the transmitting part of the image, or Z y IðyÞp LSFðgÞdg ð3Þ 1
In one dimension, the MTF(f) is the normalized Fourier transform of the LSF(y), and R1 g 1 LSFðgÞcos 2p f dg R1 ð4Þ MTFðf Þ ¼ 1 LSFðgÞdg We can now calculate the MTF(f) from the knife edge using, LSFðyÞ ¼
dIðyÞ dy
ð5Þ
53
Thus,
R1 MTFðf Þ ¼
dIðgÞ 1 dg cos
R1
2p gf dg
dIðgÞ 1 dg d
g
ð6Þ
The differentiation of the line intensity profile I(y) to obtain the line spread function LSF(y)exaggerates errors in the data curve. This can be avoided by integrating the MTF(f) equation by parts to give: Rd IðdÞcos 2p df IðcÞcos 2p cf þ 2pp c IðgÞsin 2p gf dg ð7Þ MTFðf Þ ¼ IðdÞIðcÞ Here c and d have replaced the limits of integration at N and N, respectively, and are points on the intensity profile curve I(y), where dI(y)/dy has become zero, and the regions outside c and d do not contribute to the integrals. The calculated MTF(f) is independent of the fast neutron transmission T of the HDPE knife edge plate. As the spatial frequency f increases from zero, the MTF(f) decreases from 1 to zero. The spatial resolution s[mm] of the imaging system is conservatively defined as the reciprocal of the spatial frequency fs[mm 1] in which the MTF falls to 0.05, where MTF(fs)¼0.05 and s ¼1/f. In Fig. 3(a) is shown a fast neutron image of the sharp knife edge of the 2.54 cm thick HDPE plate taken with the 5.0 mm thick BC-408 scintillator. The HDPE knife edge plate and scintillator are separated by the 3.0 mm thick aluminum lid of the camera box. Also shown is the software output of the video monitor, which presents the neutron image of the sample rotated clockwise by 901 with the 2.54 cm thick HDPE block on top and the 5.08 cm thick HDPE block on bottom and separated by a horizontal air gap. Fig. 3(b) is a diagram of the knife edge experiments. The scintillator face is half covered by the 2.54 cm thick HDPE block with the knife edge along the horizontal x-axis and centered on vertical y-axis at y¼0. The HDPE block covers the bottom half of the scintillator yo0, the top scintillator half y 40 is uncovered, and the knife edge is at y¼0. In Fig. 3(c) is plotted the measured, y-directed, vertical line intensity profile I(y) vs. pixel number for the single knife edge image shown in (a). The raw I(y) data curve was translated along the y-direction so the midpoint of the rise is at the origin. Each pixel is 0.09 mm in width in the x and y directions. The raw data was then smoothed using a Gaussian kernel that had a standard deviation of 21 pixels. The smoothed raw data curve is also plotted in Fig. 3(c) in which a histogram of the knife edge image with ImageJ software [14] showed an average of 71.5 counts per pixel with a standard deviation of about 8.6 counts per pixel. The standard deviation of counts per pixel is approximately the square root of the mean counts per pixel, which indicates a Poisson distribution of counts. The expression for the MTF(f) in Eq. (7) is used to compute the measured MTF(fy) along the vertical, y-direction, which is plotted in Fig. 3(d). Based upon MTF(fy) ¼0.05 in Fig. 3(d), the measured resolution along the vertical, y-direction is given by sy ¼ 1/fy that computes to sy ¼2.40 mm at spatial frequency fy ¼0.42 mm 1. The experimentally measured resolution was approximately the same in both horizontal x and vertical y directions. In our experiments resolution was dominated by fast neutron elastic scatter in the object and scintillator, rather than by the incident neutron divergence angles. The resolution contribution due to the divergence of the incident neutrons at the thick knife edge and scintillator were relatively small compared to the measured 2.4 mm resolution contribution from neutron scatter in the 2.54 cm thick HDPE thick knife edge and 5 mm thick plastic scintillator. The 2.54 cm thick knife edge produced a calculated blur or resolution
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
Iexp (y)
54
Pixels (y)
1.0
MTF
0.8 0.6 0.4 0.2 0.2
0.4 0.6 f (mm-1)
0.8
1.0
Fig. 3. (a) Shown is fast neutron image of sharp knife edge of 2.54 cm thick HDPE with the 5.0 mm thick BC-408 scintillator. The measured thick knife edge neutron image was acquired in 50 min exposure with detector and adjacent HDPE knife edge set 152 cm from DD source. The software output of video monitor shows neutron image of sample rotated clockwise by 901 with 2.54 cm HDPE on top and 5.08 cm HDPE on bottom, separated by horizontal air gap. (b) This diagram depicts the knife edge experimental setup to determine the MTF(f) and resolution of the fast neutron imaging system. The 5 mm thick, Bicron BC-408 scintillator face is half covered by a 2.54 cm thick HDPE block. (c) Plotted is the measured, raw data, intensity profile I(y) as a function of pixel number along a vertical, y-directed line, which spans the uncovered and HDPE-covered halves of the scintillator. The 2.54 cm thick HDPE block in (a) forms the horizontal knife edge along the x-direction. Also plotted is the intensity profile obtained by smoothing the raw data curve. The I(y) raw data curve was translated along the y-direction so the midpoint of the rise is at the origin. Each pixel is 0.09 mm width in the x and y directions. The raw data was then smoothed using a Gaussian kernel that had a standard deviation of 21 pixels. (d) Plotted is the measured MTF(f) for the fast neutron imaging system obtained from the knife edge data in (b). Based on MTF(f)¼ 0.05, the measured resolution is s ¼ 1/f, which computes to s ¼ 2.4 mm.
contribution of 0.40 mm in the horizontal, x-direction and of 0.07 mm in the vertical, y-direction, which is due to the smaller neutron divergence angle in the vertical, y-direction. The 5 mm thick plastic scintillator produced a calculated resolution contribution of 0.01 mm in the x-direction and 0.08 mm in the y-direction. The calculated resolution contributions of the 2.54 cm thick knife edge and 5 mm thick scintillator were added in quadrature. The resulting, calculated contribution to resolution due to neutron divergence angles was only 0.41 mm in the horizontal x-direction and 0.07 mm resolution contribution in the vertical, y-directions. The measured, total resolution of 2.40 mm dominate over the calculated horizontal and vertical neutron divergence resolution contributions of 0.41 mm and 0.07 mm in the knife edge and scintillator. The resolution in the horizontal direction was expected to be slightly larger (more blur) than the resolution in the vertical direction due to the larger horizontal source size. The resolution contribution from neutron scatter in the knife edge and prism are obtained by subtracting in quadrature horizontal and vertical neutron divergence resolution contributions of 0.41 mm and 0.07 mm from the measured, total resolution of 2.40 mm in the horizontal and vertical directions. This yields knife edge and scintillator neutron scatter resolution contributions of 2.36 mm and 2.40 mm in the horizontal and vertical directions, respectively. The slightly larger resolution contribution (more blur) in the horizontal direction due to the larger source size in the horizontal direction was not measurable due to the limits of our imaging system resolution. Next, we performed imaging experiments of large objects, which required a larger area scintillator. Consequently, a large area BC-408 scintillator [4] that was 38 mm thick was available, but required placement outside the CCD camera box and required fabrication of a light-tight tunnel between the open CCD camera
Table 1 Thick knife edge measurements and imaging experiments. Object at scintillator
Source to scintillator distance o (cm)
Scintillator thickness d (cm)
Scintillator to mirror distance m (cm)
Mirror to detector distance (cm)
Thick-edge Bottle/purse
152 320
0.5 38
15 124
20 20
box and the large area scintillator. The scintillator itself was lighttight wrapped with aluminum-coated mylar on all sides except the inside scintillator surface that faced the CCD camera. The camera lens was refocused to accommodate the larger field of view of the large scintillator. In this fast neutron imaging experiment the cover of the camera box was removed and a large, 63.5 cm by 99 cm area BC-408 scintillator (38 mm thick) and object (wine bottle or purse) were placed at a distance of 124 cm from the 451 camera mirror and 3.2 m from the DD source (see Table 1 and Fig. 2). The scintillator was connected to the CCD camera box via a light-tight coupling, black-taped cardboard channel with 25 cm horizontal by 33 cm vertical camera field of view of the scintillator. Two 5.5-minute exposures were taken of a half-filled wine bottle and a leather purse containing a cell phone. In Fig. 4(a) is a photo of the half-filled wine bottle. The water level in the photo is hidden by the wine company label; however, the water level appears in the fast neutron radiograph. In Fig. 4(b) is the fast 2.5 MeV neutron background-only radiographic image that was acquired in a 5.5 min exposure. And in Fig. 4(c) is the fast 2.5 MeV neutron radiographic image of the half-filled wine bottle that was also acquired in a 5.5 min exposure.
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
55
Fig. 4. (a) Photo shows half-filled wine bottle placed in front of large field of view (25 cm horizontal 38 cm vertical area), 38 mm thick BC-408 plastic scintillator set 3.2 m from the DD source and 1.09 m from the CCD camera box. Water level in photo that is hidden by label appears in fast neutron radiograph. (b) Fast 2.5 MeV neutron background-only radiographic image acquired in 5.5 min exposure. (c) Fast 2.5 MeV neutron radiographic image of half-filled wine bottle acquired in 5.5 min exposure. (d) Shown is the background-subtracted radiographic image of half-filled wine bottle in which (b) is subtracted from (c). (e) Fast neutron image of a purse with concealed cell phone acquired in 5.5 min exposure with background subtraction and use of false color.
With equal exposures times and the same conditions for a background-only image and a background with half-filled wine bottle neutron image, one can perform background subtraction in an attempt to obtain a better resolved and high contrast fast neutron image. In Fig. 4(d) is the background-subtracted, contact radiographic image of the half-filled wine bottle in which background-only image (b) was subtracted from background and halffilled wine bottle image (c). The pixel by pixel subtraction is done with the CCD camera software, but can also be performed using ImageJ, which is available from NIH online [14]. Finally, in Fig. 4(e) is a fast neutron image of a purse with concealed cell phone acquired in 5.5 min exposure. This image is obtained by background subtraction and use of false color with ImageJ. In summary, the fast neutron images in Figs. 4(c) and (d) of the half-filled wine bottle clearly emphasizes the location of the liquid through the label and glass. The second fast neutron image in Fig. 4(e) distinctly shows the concealed cell phone inside the purse. In both the thick knife edge and imaging experiments the resolution was dominated by neutron scatter rather than by the neutron divergence angles.
Acknowledgments We acknowledge the valued assistance of J. Reijonen and M. King of Lawrence Berkeley National Laboratory. This work
was supported in part by the US National Science Foundation, Grant no. IIP-0724503, and US Department of Energy, Grant no. DE-FG02–04ER86177. References [1] J. Hall, F. Dietrich, C. Logan, B. Rusnak, AIP Conf. Proc. 576 (2001) 1113. [2] J. Hall, F. Dietrich, C. Logan, B. Rusnak, Lawrence Livermore National Lab, UCRL-MI-140345 (Sept. 11, 2000). [3] C. Hurlbut, ELJEN Technology, Sweetwater, Texas. [4] Saint-Gobain Crystals and Detectors, Newbury, Ohio, USA. [5] M. Buchin, Stanford Photonics, Palo Alto, CA, USA. [6] S.S. Nargolwalla, E.P. Przybylowicz, Activation Analysis with Neutron Generators, Wiley and Sons, New York, 1973. [7] J. Reijonen, F. Gicquel, S.K. Hahto, M. King., T.-P. Lou, K.-N. Leung, Appl. Radiat. Isot. 63 (2005) 757. [8] G. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York, 1989. [9] N. Tsoulfanidis, Measurement and Detection of Radiation, McGraw-Hill, New York, 1983. [10] J.I.W. Watterson, R.M. Ambrosi, H. Rahmanian, The CAARI 2000: Sixteenth international conference on the application of accelerators in research and industry, AIP Conference Proceedings, Volume 576, pp. 1087–1090 (June 12, 2001), doi:10.1063/1.1395494. [11] D. Attwood, Soft X-rays and Extreme Ultraviolet Radiation, Cambridge University Press, Cambridge, UK, 1999. [12] J.R. Meyer-Arendt, Introduction to Classical and Modern Optics, Prentice Hall, 1984. [13] F.L. Pedrotti, L.S. Pedrotti, Introduction to Optics, Prentice-Hall, 1993. [14] ImageJ software downloaded for free from US National Institute of Health —Image Processing and analysis in Java, /http://imagej.nih.gov/ij/docs/S.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Large area imaging of hydrogenous materials using fast neutrons from a DD fusion generator J.T. Cremer a,n, D.L. Williams a, C.K. Gary a, M.A. Piestrup a, D.R. Faber a, M.J. Fuller a, J.H. Vainionpaa a, M. Apodaca a, R.H. Pantell b, J. Feinstein b a b
Adelphi Technology Inc., 2003 East Bayshore Road, Redwood City, California 94063, USA Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
a r t i c l e i n f o
abstract
Article history: Received 10 May 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Available online 8 February 2012
A small-laboratory fast-neutron generator and a large area detector were used to image hydrogenbearing materials. The overall image resolution of 2.5 mm was determined by a knife-edge measurement. Contact images of objects were obtained in 5–50 min exposures by placing them close to a plastic scintillator at distances of 1.5 to 3.2 m from the neutron source. The generator produces 109 n/s from the DD fusion reaction at a small target. The combination of the DD-fusion generator and electronic camera permits both small laboratory and field-portable imaging of hydrogen-rich materials embedded in high density materials. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fast neutron radiography Neutron imaging system Fast neutron generator for imaging DD fusion generator for imaging
Radiography using fast neutrons from accelerators has been known as an excellent method to penetrate high-density, thick objects. Fast neutron radiography was demonstrated by Hall [1,2] at Lawrence Livermore National Laboratory (LLNL) using a 30 cm by 30 cm area and 4 cm thick plastic scintillator (BC-408) plates, which was viewed indirectly by mirror reflection using a high resolution CCD camera. Hall obtained multiple view and ‘‘mock’’, tomographic, fast neutron images with a variety of test objects via a CCD camera, which imaged fast neutron induced light emission from a Bicron (BC-408) plastic scintillator via a 451 turning mirror. We followed the Hall approach to produce a large area, fast neutron imaging detector at modest cost. In this paper we report our fast neutron imaging results using a fast neutron generator source of 2.5 MeV, DD fusion neutrons and fast neutron imaging detector developed at Adelphi Technology. The 2.5 MeV fast neutrons are produced from the deuterium– deuterium (DD) fusion reaction in the diode of a RF plasma based neutron generator [6]. Single-beam, DD generators can produce neutron yields of up to 1010 n/s or greater [7]. An Adelphi Technology Inc. model DD-109 neutron generator with neutron yield 109 n/s was used in this work. The generator housing was approximately 91.44 cm long, and the generator HDPE shielding weighed about 90 kg. To reduce the background x-ray and gamma radiation, a 6.35 mm thick sheet of lead was placed over the
n
Corresponding author. Tel.: þ1 650 474 2750; fax: þ1 650 474 2755. E-mail address: [email protected] (J.T. Cremer).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.02.003
source port of the DD generator. The lead strongly attenuates the bremsstrahlung from accelerated electrons in the diode region, and attenuates x-rays emitted from electrons that fill K-shell vacancies in the copper and titanium target created by the DD reaction’s 3 MeV protons. Fig. 1 is an illustration of the DD neutron generator. The deuteron ions are accelerated by an applied DC voltage and strike a Ti-coated, hollow copper target with deuterium atoms embedded in the Ti from impact of prior Dþ ions. Water flows through channels in the hollow copper target to provide cooling from the 1 kW of deposited energy of the 120 kV, 9.0 mA, Dþ ion beam on the 4.5 cm2 target area, which produces total DD neutron yield 109 n/s. Each leg of the vertically oriented V Ti-coated copper target intercepts the 4.0 mm diameter, cylindrical Dþ beam to produce a half-ellipse-shaped source with a horizontal (x), major axis of 5.6 cm and vertical (y) minor axis of 0.4 cm. For Monte Carlo simulation, the DD source is represented by a rectangle in the z¼ 0 plane with dimensions horizontal dimension x¼2.4 cm and vertical dimension y¼0.4 cm as illustrated in Fig. 1(b). The emission of neutron flux and energy is slightly anisotropic for the DD and DT reactions relative to the direction of the Dþ ion beam that strikes the Ti-coated DD target [6]. Forward-emitted neutrons have the largest flux with maximum emitted energy 2.9 MeV. Back-emitted neutrons have a mid-range flux at the minimum emitted energy of 2.1 MeV. Neutrons emitted to the side toward the detector have the minimum flux with emitted average energy 2.5 MeV. We calculated a brightness of 8.3 106 n/sr/mA in comparison to our measured brightness 6.6 106 n/sr/mA.
52
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
Plasma Ion Source
Accelerator Section
Tapered Fiber Optic
CCD
Multi-Channel Plate BC-408 Scintillator
D+ HV
Object
"V" Radiator
n
Plasma Gate Electrode
Ti-Target
Electron Shroud
mirror 7.5 x 7.5 cm2 d = 5 or 38 mm
0.6
o = 152 or 320 cm
0.4
0 -1.5
-1
-0.5
-0.2
0
y (cm)
x (cm)
0.2 -2
15 cm
n 1
-2.5
Photons
5 cm
D+ 0.8
-3
Lens
m = 15 or 124 cm
Fig. 2. Shown is a diagram of the fast neutron radiographic imaging system comprised of the fast 2.5 MeV neutron source and imaging detector with object. The thick knife edge experiment and the large object imaging used the two configurations shown.
-0.4 -0.6 -0.8 -1 Fig. 1. (a) Shown are the principal components of the DD neutron radiographic imaging system. The DD-109 neutron generator produces a yield of 109 to1010 n/s. (b) Fast neutron source on each leg of the vertically oriented V target is half-ellipse shape with horizontal, x-directed, major axis 5.7 cm and vertical, y-directed, minor axis of 0.4 cm, which for Monte Carlo simulations can be approximated by rectangle in z ¼ 0 plane with horizontal x ¼2.4 cm and vertical y¼0.4 cm dimensions, shown by dotted line.
Thermal neutrons, x-rays, or low energy gammas have an attenuation cross section dominated by absorption, which allows the use of relatively thin thermal neutron phosphors scintillators. Fast neutron attenuation is dominated by elastic scatter with hydrogen atoms in hydrogen-rich materials. The attenuation by scatter requires the use of hydrogen-rich plastic scintillators, which are much thicker than the thermal neutron phosphors. The average 50% decrease of neutron energy per elastic collision with a scintillator hydrogen nucleus increases the neutron cross section for subsequent scatter and absorption, and significantly increases the probability of a second scatter or absorption interaction in sufficiently thick plastic scintillator sheets [8,9]. The recoil protons create a cascade of scatter events in the scintillator, which lead to excited electron states, which transition to ground with the emission of light. The imaging system is schematically illustrated in Fig. 2. The camera box allows the Stanford Photonics CCD camera [5] and its electronics to be placed outside the direct neutron beam exposure. This prevents direct neutron beam radiation damage of the camera CCD sensor and electronics. At the base of the light-tight camera box is a removable, 11.25 cm 11.25 cm area, aluminum cover that is 3 mm thick. A thick knife edge plate or object can be placed on the outer cover surface and the plastic scintillator on the inner surface. Numerous fast neutron imaging trials were performed with various thicknesses of BC-408 plastic scintillators, which ranged from 1.0 mm to 25 mm. As a result, the 5.0 mm thickness scintillator [10] appeared to provide the best imaging resolution and contrast for 5 to 60 min exposure times with 108 109 n/s DD neutron source yields. As shown in Fig. 2, the emitted scintillator light is intercepted by a 451 mirror (with reflectivity 495%). The mirror reflects the
light upward out of the neutron path to a 5 cm diameter lens with f-stop f# ¼ 1.0. The lens focuses the light onto the dual micro channel plate (MCP). The dual MCP light amplifier and CCD chip are thermoelectrically cooled to 20 1C. The dual MCP amplifier produces high photon gains up to 106 at a very low dark count level of 1 or 2 electron counts per frame at 15 frames per second. In our experiments the dual MCP amplifier was set to an overall photon amplification gain of Gmcp ¼5.0 105. The Stanford Photonics CCD camera [5] uses a Sony XX285 scientific grade image sensor, which has a full frame pixel count of 1380 K by 1024 K in which the np ¼1.41 106 pixels are 6.47 mm square. The addition of a 1.6 to 1 reducing fiber optic taper between the smaller CCD sensor and the larger dual MCP image intensifier output creates an effective square pixel with 10.35 mm sides at the input image plane. The dual MCP image intensifier has a 50 linepairs/mm resolution, which corresponds to 10 mm effective pixel size. An image is created by summing multiple frames. Each frame is 66 ms exposure (15 frames per second). The short frame times reduce the dark count accumulation to 1 or 2 dark counts per 66 ms frame, and the Piper software [5] for camera control, image acquisition, and image signal processing further reduces noise by filtering out cosmic ray noise and spot noise from each of the incoming 66 ms frames. The dark count of the dual MCA with 1.4 106 pixels is then 2.1 10 5 electrons per pixel per second. The frames can be stacked to form images comprised of 5 104 frames or more, which enabled image times from 66 ms to 8 h or more. The image times in our experiments ranged from 5 to 50 min. The dual MCP CCD night-vision camera used in our neutron imaging experiment, which stacks short-time-exposure frames, is designed to be limited by the photon noise (shot noise) rather than the dark noise and camera electronic noise. The photon noise is the natural variation of the photon flux emitted from the scintillator. That is, the dark count levels of the CCD pixels can be ignored compared to the assumed Poisson uncertainty of the photon emission from the scintillator, and the input neutron signal fluence. The dark noise and camera electronic noise levels were tested by operation of the imaging system for the 5 min to 1 h exposure times without the neutron generator. The nogenerator testing showed each frame and the stacked frames to be dark. Speckle and noise were readily apparent only when the generator was emitting fast neutrons.
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
A thin knife edge is commonly used to determine resolution of optical, thermal, and x-ray imaging systems [11]. The fast neutron scatter cross sections for the hydrogen and carbon atoms in the HDPE are only a few barns. The small, fast neutron cross section thus requires a thick plate of HDPE to act as an effective fast neutron knife edge. In fact a more appropriate term would be thick-edge test or abrupt boundary test. To confirm detector resolution during general imaging experiments, one can measure the intensity profiles of thick, hydrogen-rich, object features, which have a sufficiently abrupt change in thickness. Alternatively, the thick knife edge test can also be performed with plate of tungsten, which also strongly attenuates neutrons. In our thick knife edge tests, a 2.54 cm thick HDPE plate blocked half the neutrons from striking the scintillator. This produced an image with sharp contrast between the dark and light image regions. An intensity profile was taken across this image. The object resolution in the horizontal and vertical directions was measured by the fast neutron imaging of a 2.54 cm thick HDPE ‘‘quasi knife edge’’ that covered half the plastic BC-408 scintillator [3,4]. In the horizontal and vertical directions, the measured object resolution was compared to Monte Carlo simulation results. The thick knife edge was set on the outer surface of the 3 mm thick aluminum cover of the camera box, which was located 1.52 m from the fast neutron source. On the aluminum cover inner surface was attached the 5 mm thick, BC-408 scintillator with 10 cm by 10 cm area. The horizontal and vertical neutron divergence angles at the thick knife edge were 15.6 and 2.6 mrad, respectively. The thick knife edge plate transmission T for fast neutrons changes the image intensity from the covered to uncovered region by a factor of DI given by:
DI ¼ 1T
ð1Þ
The thick knife-edge measurements produced line intensity profiles in the horizontal I(x) and vertical I(y) directions. The corresponding line spread function along the horizontal, x-direction LSF(x) and vertical, y-direction LSF(y) were given by: LSFðxÞ ¼
dIðxÞ dIðyÞ and LSFðyÞ ¼ dx dy
ð2Þ
The normalized Fourier transform of the horizontal and vertical LSF yielded the corresponding horizontal MTF(fx) and vertical MTF(fy) modulation transfer functions, which were functions of the horizontal fx ¼ 1/x and vertical fy ¼1/y spatial frequencies [12,13]. The MTF(f) is the Fourier transform of the impulse response, or point spread function PSF(y), which is the pattern seen from a perfect point source, which in one dimension is the line spread function LSF(y). If we put an attenuating object with a sharp line transition in the image, the resulting knife edge image, I(y) will be determined by the integral of the LSF(y) over the transmitting part of the image, or Z y IðyÞp LSFðgÞdg ð3Þ 1
In one dimension, the MTF(f) is the normalized Fourier transform of the LSF(y), and R1 g 1 LSFðgÞcos 2p f dg R1 ð4Þ MTFðf Þ ¼ 1 LSFðgÞdg We can now calculate the MTF(f) from the knife edge using, LSFðyÞ ¼
dIðyÞ dy
ð5Þ
53
Thus,
R1 MTFðf Þ ¼
dIðgÞ 1 dg cos
R1
2p gf dg
dIðgÞ 1 dg d
g
ð6Þ
The differentiation of the line intensity profile I(y) to obtain the line spread function LSF(y)exaggerates errors in the data curve. This can be avoided by integrating the MTF(f) equation by parts to give: Rd IðdÞcos 2p df IðcÞcos 2p cf þ 2pp c IðgÞsin 2p gf dg ð7Þ MTFðf Þ ¼ IðdÞIðcÞ Here c and d have replaced the limits of integration at N and N, respectively, and are points on the intensity profile curve I(y), where dI(y)/dy has become zero, and the regions outside c and d do not contribute to the integrals. The calculated MTF(f) is independent of the fast neutron transmission T of the HDPE knife edge plate. As the spatial frequency f increases from zero, the MTF(f) decreases from 1 to zero. The spatial resolution s[mm] of the imaging system is conservatively defined as the reciprocal of the spatial frequency fs[mm 1] in which the MTF falls to 0.05, where MTF(fs)¼0.05 and s ¼1/f. In Fig. 3(a) is shown a fast neutron image of the sharp knife edge of the 2.54 cm thick HDPE plate taken with the 5.0 mm thick BC-408 scintillator. The HDPE knife edge plate and scintillator are separated by the 3.0 mm thick aluminum lid of the camera box. Also shown is the software output of the video monitor, which presents the neutron image of the sample rotated clockwise by 901 with the 2.54 cm thick HDPE block on top and the 5.08 cm thick HDPE block on bottom and separated by a horizontal air gap. Fig. 3(b) is a diagram of the knife edge experiments. The scintillator face is half covered by the 2.54 cm thick HDPE block with the knife edge along the horizontal x-axis and centered on vertical y-axis at y¼0. The HDPE block covers the bottom half of the scintillator yo0, the top scintillator half y 40 is uncovered, and the knife edge is at y¼0. In Fig. 3(c) is plotted the measured, y-directed, vertical line intensity profile I(y) vs. pixel number for the single knife edge image shown in (a). The raw I(y) data curve was translated along the y-direction so the midpoint of the rise is at the origin. Each pixel is 0.09 mm in width in the x and y directions. The raw data was then smoothed using a Gaussian kernel that had a standard deviation of 21 pixels. The smoothed raw data curve is also plotted in Fig. 3(c) in which a histogram of the knife edge image with ImageJ software [14] showed an average of 71.5 counts per pixel with a standard deviation of about 8.6 counts per pixel. The standard deviation of counts per pixel is approximately the square root of the mean counts per pixel, which indicates a Poisson distribution of counts. The expression for the MTF(f) in Eq. (7) is used to compute the measured MTF(fy) along the vertical, y-direction, which is plotted in Fig. 3(d). Based upon MTF(fy) ¼0.05 in Fig. 3(d), the measured resolution along the vertical, y-direction is given by sy ¼ 1/fy that computes to sy ¼2.40 mm at spatial frequency fy ¼0.42 mm 1. The experimentally measured resolution was approximately the same in both horizontal x and vertical y directions. In our experiments resolution was dominated by fast neutron elastic scatter in the object and scintillator, rather than by the incident neutron divergence angles. The resolution contribution due to the divergence of the incident neutrons at the thick knife edge and scintillator were relatively small compared to the measured 2.4 mm resolution contribution from neutron scatter in the 2.54 cm thick HDPE thick knife edge and 5 mm thick plastic scintillator. The 2.54 cm thick knife edge produced a calculated blur or resolution
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
Iexp (y)
54
Pixels (y)
1.0
MTF
0.8 0.6 0.4 0.2 0.2
0.4 0.6 f (mm-1)
0.8
1.0
Fig. 3. (a) Shown is fast neutron image of sharp knife edge of 2.54 cm thick HDPE with the 5.0 mm thick BC-408 scintillator. The measured thick knife edge neutron image was acquired in 50 min exposure with detector and adjacent HDPE knife edge set 152 cm from DD source. The software output of video monitor shows neutron image of sample rotated clockwise by 901 with 2.54 cm HDPE on top and 5.08 cm HDPE on bottom, separated by horizontal air gap. (b) This diagram depicts the knife edge experimental setup to determine the MTF(f) and resolution of the fast neutron imaging system. The 5 mm thick, Bicron BC-408 scintillator face is half covered by a 2.54 cm thick HDPE block. (c) Plotted is the measured, raw data, intensity profile I(y) as a function of pixel number along a vertical, y-directed line, which spans the uncovered and HDPE-covered halves of the scintillator. The 2.54 cm thick HDPE block in (a) forms the horizontal knife edge along the x-direction. Also plotted is the intensity profile obtained by smoothing the raw data curve. The I(y) raw data curve was translated along the y-direction so the midpoint of the rise is at the origin. Each pixel is 0.09 mm width in the x and y directions. The raw data was then smoothed using a Gaussian kernel that had a standard deviation of 21 pixels. (d) Plotted is the measured MTF(f) for the fast neutron imaging system obtained from the knife edge data in (b). Based on MTF(f)¼ 0.05, the measured resolution is s ¼ 1/f, which computes to s ¼ 2.4 mm.
contribution of 0.40 mm in the horizontal, x-direction and of 0.07 mm in the vertical, y-direction, which is due to the smaller neutron divergence angle in the vertical, y-direction. The 5 mm thick plastic scintillator produced a calculated resolution contribution of 0.01 mm in the x-direction and 0.08 mm in the y-direction. The calculated resolution contributions of the 2.54 cm thick knife edge and 5 mm thick scintillator were added in quadrature. The resulting, calculated contribution to resolution due to neutron divergence angles was only 0.41 mm in the horizontal x-direction and 0.07 mm resolution contribution in the vertical, y-directions. The measured, total resolution of 2.40 mm dominate over the calculated horizontal and vertical neutron divergence resolution contributions of 0.41 mm and 0.07 mm in the knife edge and scintillator. The resolution in the horizontal direction was expected to be slightly larger (more blur) than the resolution in the vertical direction due to the larger horizontal source size. The resolution contribution from neutron scatter in the knife edge and prism are obtained by subtracting in quadrature horizontal and vertical neutron divergence resolution contributions of 0.41 mm and 0.07 mm from the measured, total resolution of 2.40 mm in the horizontal and vertical directions. This yields knife edge and scintillator neutron scatter resolution contributions of 2.36 mm and 2.40 mm in the horizontal and vertical directions, respectively. The slightly larger resolution contribution (more blur) in the horizontal direction due to the larger source size in the horizontal direction was not measurable due to the limits of our imaging system resolution. Next, we performed imaging experiments of large objects, which required a larger area scintillator. Consequently, a large area BC-408 scintillator [4] that was 38 mm thick was available, but required placement outside the CCD camera box and required fabrication of a light-tight tunnel between the open CCD camera
Table 1 Thick knife edge measurements and imaging experiments. Object at scintillator
Source to scintillator distance o (cm)
Scintillator thickness d (cm)
Scintillator to mirror distance m (cm)
Mirror to detector distance (cm)
Thick-edge Bottle/purse
152 320
0.5 38
15 124
20 20
box and the large area scintillator. The scintillator itself was lighttight wrapped with aluminum-coated mylar on all sides except the inside scintillator surface that faced the CCD camera. The camera lens was refocused to accommodate the larger field of view of the large scintillator. In this fast neutron imaging experiment the cover of the camera box was removed and a large, 63.5 cm by 99 cm area BC-408 scintillator (38 mm thick) and object (wine bottle or purse) were placed at a distance of 124 cm from the 451 camera mirror and 3.2 m from the DD source (see Table 1 and Fig. 2). The scintillator was connected to the CCD camera box via a light-tight coupling, black-taped cardboard channel with 25 cm horizontal by 33 cm vertical camera field of view of the scintillator. Two 5.5-minute exposures were taken of a half-filled wine bottle and a leather purse containing a cell phone. In Fig. 4(a) is a photo of the half-filled wine bottle. The water level in the photo is hidden by the wine company label; however, the water level appears in the fast neutron radiograph. In Fig. 4(b) is the fast 2.5 MeV neutron background-only radiographic image that was acquired in a 5.5 min exposure. And in Fig. 4(c) is the fast 2.5 MeV neutron radiographic image of the half-filled wine bottle that was also acquired in a 5.5 min exposure.
J.T. Cremer et al. / Nuclear Instruments and Methods in Physics Research A 675 (2012) 51–55
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Fig. 4. (a) Photo shows half-filled wine bottle placed in front of large field of view (25 cm horizontal 38 cm vertical area), 38 mm thick BC-408 plastic scintillator set 3.2 m from the DD source and 1.09 m from the CCD camera box. Water level in photo that is hidden by label appears in fast neutron radiograph. (b) Fast 2.5 MeV neutron background-only radiographic image acquired in 5.5 min exposure. (c) Fast 2.5 MeV neutron radiographic image of half-filled wine bottle acquired in 5.5 min exposure. (d) Shown is the background-subtracted radiographic image of half-filled wine bottle in which (b) is subtracted from (c). (e) Fast neutron image of a purse with concealed cell phone acquired in 5.5 min exposure with background subtraction and use of false color.
With equal exposures times and the same conditions for a background-only image and a background with half-filled wine bottle neutron image, one can perform background subtraction in an attempt to obtain a better resolved and high contrast fast neutron image. In Fig. 4(d) is the background-subtracted, contact radiographic image of the half-filled wine bottle in which background-only image (b) was subtracted from background and halffilled wine bottle image (c). The pixel by pixel subtraction is done with the CCD camera software, but can also be performed using ImageJ, which is available from NIH online [14]. Finally, in Fig. 4(e) is a fast neutron image of a purse with concealed cell phone acquired in 5.5 min exposure. This image is obtained by background subtraction and use of false color with ImageJ. In summary, the fast neutron images in Figs. 4(c) and (d) of the half-filled wine bottle clearly emphasizes the location of the liquid through the label and glass. The second fast neutron image in Fig. 4(e) distinctly shows the concealed cell phone inside the purse. In both the thick knife edge and imaging experiments the resolution was dominated by neutron scatter rather than by the neutron divergence angles.
Acknowledgments We acknowledge the valued assistance of J. Reijonen and M. King of Lawrence Berkeley National Laboratory. This work
was supported in part by the US National Science Foundation, Grant no. IIP-0724503, and US Department of Energy, Grant no. DE-FG02–04ER86177. References [1] J. Hall, F. Dietrich, C. Logan, B. Rusnak, AIP Conf. Proc. 576 (2001) 1113. [2] J. Hall, F. Dietrich, C. Logan, B. Rusnak, Lawrence Livermore National Lab, UCRL-MI-140345 (Sept. 11, 2000). [3] C. Hurlbut, ELJEN Technology, Sweetwater, Texas. [4] Saint-Gobain Crystals and Detectors, Newbury, Ohio, USA. [5] M. Buchin, Stanford Photonics, Palo Alto, CA, USA. [6] S.S. Nargolwalla, E.P. Przybylowicz, Activation Analysis with Neutron Generators, Wiley and Sons, New York, 1973. [7] J. Reijonen, F. Gicquel, S.K. Hahto, M. King., T.-P. Lou, K.-N. Leung, Appl. Radiat. Isot. 63 (2005) 757. [8] G. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York, 1989. [9] N. Tsoulfanidis, Measurement and Detection of Radiation, McGraw-Hill, New York, 1983. [10] J.I.W. Watterson, R.M. Ambrosi, H. Rahmanian, The CAARI 2000: Sixteenth international conference on the application of accelerators in research and industry, AIP Conference Proceedings, Volume 576, pp. 1087–1090 (June 12, 2001), doi:10.1063/1.1395494. [11] D. Attwood, Soft X-rays and Extreme Ultraviolet Radiation, Cambridge University Press, Cambridge, UK, 1999. [12] J.R. Meyer-Arendt, Introduction to Classical and Modern Optics, Prentice Hall, 1984. [13] F.L. Pedrotti, L.S. Pedrotti, Introduction to Optics, Prentice-Hall, 1993. [14] ImageJ software downloaded for free from US National Institute of Health —Image Processing and analysis in Java, /http://imagej.nih.gov/ij/docs/S.