CALCULUS FOR A NEUTRON IMAGING SYSTEM BASED ON A CCD CAMERA

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1 NUCLEAR PHYSICS INSTRUMENTS AND METHODS CALCULUS FOR A NEUTRON IMAGING SYSTEM BASED ON A CCD CAMERA M. DINCA 1, M. PAVELESCU 2 1 Institute for Nuclear Research, P.O. Box 078, Pitesti, Romania, dinca@scn.ro 2 Romanian Scientist Academy, Bucharest, Romania, mpavelescu@pcnet.ro Received October 12, 2005 The detector based on a CCD camera intended for the dry neutron radiography facility located at the tangential channel of the ACPR reactor from Pitesti is presented. The calculus of the established parameters, the design and the components of the detector: scintillator, mirror, CCD camera, etc. are described in this paper. The detector has two interchangeable scintillators, a scintillator for gamma radiation and one scintillator for the neutron beam. Key words:neutron radiography, CCD camera, detector. 1. INTRODUCTION An arrangement for investigation through neutron radiography consists of a neutron source, a collimator, an investigated object and a detector of neutrons. The detector is a key component in neutron radiography. It is a plane integrator device that registers the projection of the object done with neutrons. There are more types of detectors for neutron radiography and all of them use materials that strongly interact with neutrons and generate, possibly in more steps, an output image in a variety of methods. When all parts of the detector are together in the neutron beam, is involved a direct method, and when a part of the detector is exposed to neutrons and connected subsequently at the rest of the detector, is involved an indirect method (named transfer method also). The transfer method is used when is present a strong gamma field of radiations. The direct method uses converters that promptly emit a or b radiations or scintillators that emit visible light. The light of the scintillators can be captured with CCD video cameras and digitally stored in a PC. Among all combinations, that using radiological films and metallic converter foils and track-etch method assure the best geometrical resolution (up to 20 µm) for the image. The deficiencies of these methods are low contrast (track-etch method) limited linearity and reproducibility (radiological films), long exposure times, etc. Rom. Journ. Phys., Volume 51, Nos. 3 4, p , Bucharest, 2006

2 364 M. Dinca, M. Pavelescu 2 The method with scintillator and CCD camera offers some advantages: good linearity and reproducibility, high sensitivity, high dynamical range (up to ), high temporal resolution (offers real time imaging). The geometrical resolution of the CCD cameras is poorer then geometrical resolution of the methods with radiological films. The dynamical range and reproducibility of the image for a CCD camera can be improved cooling the sensor by Peltier effect, water or air circulating or liquid nitrogen. The cooling is efficient when the time of integration of the image is longer and is necessary to reduce the dark current. If the light of the scintillator is under the sensitivity level of the CCD camera or it is necessary to image real time events (actually with a poorer geometrical resolution), an image intensifier prior the CCD camera is used. Can be used scintillators that contain Li 6 (in LiF), B 10 (in B 4 C) or Gd 157 (Gd 2 OS 2 :Tb) that capture the thermal neutron and emit ionizing radiation. 3Li n 1 2 α H 3 + 4,78 MeV 5B n 1 2 α Li 7 + 2,792 MeV (or 2,310 MeV) (1) 64Gd n 1 e Gd KeV (main) The ionizing radiations produce scintillations in a proper substance (ZnS or Gd 2 OS 2 :Tb itself). In 94% of nuclear reactions of the 5 B 10 with thermal neutrons results 3 Li 7 in its first excited state followed by a gamma radiation emission with energy of 0,482 MeV. Although 64 Gd 157 has a capture section bigger then 3 Li 6 or 5 B 10, the number of photons emitted/captured neutron is smaller then for lithium or boron and is more sensible to gamma radiations. Because of bigger reaction cross-section of boron with thermal neutrons should be preferable instead of lithium but is more difficult to obtain a screen with B 4 C then with LiF. As a consequence are used granular scintilators made of Li 6 F-ZnS (Ag) or Li 6 F-ZnS (Al,Cu,Au) with blue, respective green emitting light. ZnS is the ideal scintillator for a particles. For a bigger efficiency is used enriched lithium in 3 Li 6 isotope up to 93%. The light is captured by the CCD camera from scintillator using an aluminized mirror that deflects the light to 90 0 degree to avoid the exposure of the camera in the radiation beam. The CCD sensor has pixels with 9 µm 9 µm pixel sizes. The neutron beam has 30 cm in diameter and an intensity of n/cm 2 /s in the plane of the detector at the neutron radiography facility from tangential channel of the ACPR reactor from INR Pitesti. The scintillator was chosen to have 30 cm x 30 cm dimensions and Li 6 F-ZnS(Ag) type with the peak of the light emission at 450 nm. The aluminized mirror has 300 mm 400 mm dimensions and was obtained on a floated glass with thickness of 2.3 mm. The aluminum layer (100 nm) is covered by a SiO 2 (3-4 nm) protective layer.

3 3 Neutron imaging system 365 The lens has the F number equal with F#1.3 for visualization of entire scintillator. 2. PROJECTING BASES OF THE DETECTOR 2.1 THE EFFICIENCY OF THE NEUTRON CONVERSION The number of the photoelectrons/captured neutron The investigated object modifies the incident neutron beam according to neutron cross-sections of its constituents. A calculus is made for the maximum intensity of n/cm 2 /s. A dynamic range of indicates that a good CCD camera can put in evidence intensities of n/cm 2 /s and 100 n/cm 2 /s in the same image. We consider a neutron captured in the 6 Li nucleus. Are emitted 2 α 4 and 1 H 3 particles with total kinetic energy of ev that is transformed in light energy with an efficiency of η l = 28%. To emit a photon in ZnS it is necessary a mean energy of 3 ev. A captured neutron determines an emission of photons, n p, in 4π equal with: n p = /3 = ( 2 ) The CCD signal in photoelectrons per detected neutron [1] is given by: 2 n = η η η n / [2F(m+1)] = pe og Le CCD e 5 2 = / [2F(m+1)] = (3) 5 2 =1.62 x 10 / [2F(m+1)] where: - η 0g = optical efficiency of mirror (reflection) - η Le = optical efficiency of lens (transmission) - η CCD = quantum efficiency of CCD - F = F number of the lens - m = minification ratio (l obj /l CCD = m) The factor [2F(m+1)] 2 is the fraction of solid angle inside the light is collected by lens. If the projection of the entire surface of the scintillator is done on the CCD sensor, then m = 300/9.2 = 32.6 and for F=1.3 it is obtained n pe = 21.2 photoelectrons / captured neutron. There are authors that done experiments and consider the number of photons emitted by scintillator being five times smaller because of the bigger emission of photons in backwards direction (55%) and self-absorption of the scintillator. Result

4 366 M. Dinca, M. Pavelescu 4 a number of 4.24 photoelectrons/captured neutron. This number is less than 10 and seems that it is not possible to have a signal well defined above background noise [1]. If there is projected an area of 50 mm 50 mm of the scintillator on the CCD sensor (m = 50/9.2 = 5.43) a similar calculus like that for entire surface leads to n pe = photoelectrons/captured neutron. In this configuration it is possible to have a registration of a captured neutron. The light emitted by scintillator is considered to be punctually because of short range of ionizing particles of the 2 α 4 and 1 H 3 and the light is collected on a pixel that emits the photoelectrons The number of the photoelectrons/pixel/second A calculus of the number of photoelectrons/pixel/s that appears in the sensor of the CCD camera is made. For a typical AST scintillator the efficiency of neutron detection is η S = 15%. The neutron beam intensity is n/cm 2 /s. The area of the scintillator corresponding to a pixel of a pixels is D 2 = (30 cm/1024) 2 = cm 2 when entire scintillator is visualized. Result: N pe = I x D 2 η S x n pe / 5 = = photoelectrons/pixel/s For a visualized area of 50 mm x 50 mm we have d 2 = (5 cm/1024) 2 = cm 2 and results: N pe = I d 2 η S n pe = = photoelectrons/pixel/s To perform dynamic neutron radiography with 30 frames/s there is imposed a minimum limit of 300 photoelectrons/s. Because the calculus was made for a neutron beam unaffected by object there is obvious that for this arrangement it is not possible to do real time neutron radiography without image intensifier. 2.2 THE LENS The image from scintillator is projected on the photocathode of the image intensifier that has 18 mm in diameter. There are considered the classical formulae in optics: ' p + p = f (4) ' ' ' y p m = y = p (5)

5 5 Neutron imaging system 367 where: - p and p are the distances object-lens and lens-image; - y and y are the length of object and image; - f is the focal distance; - m is linear magnification. With y and y established, the side of scintillator and diameter of the photocathode (the image on the scintillator is round like round neutron beam), there is determined p for different values of the f from formulae: ' f ( y y) p = (6) y that represents the distance from virtual image of the scintillator in the mirror and lens. For y=300 mm and y =18 mm there are obtained for p, for example, some values shown in the Table 1. Table 1 The values of p for some arbitrary values of f (y = 300 mm, y =18 mm) f (mm) p (mm) ,33 441, , , If f = 50 mm it is obtained a reasonable distance p = 885 mm, so that the camera is sufficiently far away from radiation beam. When a little surface of the scintillator is viewed and projected on the photocathode there are analyzed two situations with y=100 mm and y=50 mm. The necessary values for p are shown in the Table 2. Table 2 The value of p for some arbitrary values of f and two values of y (50 mm and 100 mm) and y =18 mm f (mm) p (mm) 327,7 393,3 458,8 524, ,5 721,1 786,6 (y =100 mm) p (mm) (y= 50 mm) 188,8 226,6 264,4 302, ,7 415,5 453, ,2 917,7 983,3 1048,8 1114, ,1 1442,2 1573,3 491,1 528,8 566,6 604, From the data from Table 2 there is obvious that for y=100 mm there are obtained reasonable values of the p for values of f up to 130 mm. But the value f=120 mm assures an optimum. For y=50 mm it is a set of reasonable values, p = 450 mm and f=120 mm. As a consequences should be proper lens with zoom with f = (40 120) and the values of p in the range 400 mm 900 mm.

6 368 M. Dinca, M. Pavelescu GEOMETRICAL RESOLUTION OF THE DETECTOR The resolution of the detector is a combination of the CCD camera resolution and the resolution of the lens. The total resolution of the detector is: Detector / resolution (µm) = CCD camera resolution (µm) / PMAG where, - CCD camera resolution (µm) = 2 x Pixel size (µm) - PMAG = CCD sensor size / Field of view, is the magnification power of the lens. Result: Detector resolution (µm) = CCD camera resolution (µm) / (Sensor size/field of view) = (2 x Pixel size) / (Sensor size / Field of view) = (2 x Field of view) / (Sensor size / Pixel size) = (2 x Field of view) / Number of pixels For a field of view (FOV) = 300 mm (entire scintillator is visualized) the geometrical resolution is (2 300 mm)/1024 pixels = 0,586 mm. For FOV = 50 mm, the geometrical resolution is (2 50 mm)/1024 pixels = 0,097 mm. For a CCD camera with pixels the resolution is double, 1,172 mm, respectively 0,194 mm. 3. THE PROJECT OF THE DETECTOR The calculus done for resolution, photoelectrons production and the size of the scintillator, other space constrains etc. have lead to a design of the detector based on a CCD camera. The chain of components consists in: scintillator-mirror-first lens-image intensifier-second lens-ccd camera-interface-pc-software. The scintillator has 30 cm x 30 cm dimensions and is a Li 6 F-ZnS (Ag) type with the peak of the light emission at 450 nm. The aluminized mirror deflects the light to 90 0 degree to avoid the exposure of the camera in the radiation beam. It has 300 mm 400 mm dimensions and was obtained on a floated glass with thickness of 2.3 mm. The aluminum layer (100 nm) is covered by a SiO 2 (3-4 nm) protective layer. The first lens consist in an objective with F number equal with F#1.3 for visualization of entire scintillator and additionally in its front another objective to obtain a bigger focal distance to visualize a smaller area of the scintillator. Very important is the image intensifier XD4 type with 18 mm diameter of the photocathode. The second lens optically couples the image intensifier to CCD sensor. CCD camera is one without cooling to assure real time imaging and has a sensor with pixels and 9 µm x 9 µm pixel size (2/3 ). There is chosen a proper interface for CCD camera to PC. For aquisition it is used the software supplied with the camera and free or custom made software for analyses. In Figure 1 it is shown schematically the detector with CCD camera.

7 7 Neutron imaging system 369 The housing of the detector is made from an aluminum frame covered with aluminum sheets 2.2 mm thickness. The external dimensions are 1275 mm 634 mm 564 mm. The scintillator for neutrons is inter-changeable with a scintillator for gamma radiations. The change is made by a step-by-step motor. The detector is sustained vertically by the metallic frame of the facility. The holder of the detector assures movement forward-back based on a DC motor and the vertical rotation. There is also possible to rise the detector horizontally to take out from the neutron beam to protect the CCD camera and also to perform neutron radiography with other methods. Neutron beam PC Scintillator Lens and image intensifier CCD Turning mirror Mobile platform 1 Mobile platform 2 Fig. 1. Sketch of the detector for neutrons based on a CCD camera. Inside of detector other two step-by-step motors assure the control of distance between mirror and CCD camera and between mirror and scintillator. It is possible to adjust the focalization of the image for entire surface of the scintillator or on a small part. 4. CONCLUSIONS The CCD camera with pixels assures a geometrical resolution of 0.6 mm for the visualization of the entire scintillator, in close concordance with the geometrical resolution that is assured by the assembly collimator-object-detector for objects with the thickness about 5 cm. For thin objects under 1 cm is assured a geometrical resolution of 0.1 mm in the condition that the FOV = 5 cm. The geometrical resolution of the image on the monitor depends also of: - the geometrical resolution of the assembly collimator-object-detector;

8 370 M. Dinca, M. Pavelescu 8 - the scattering of the neutrons on the path in the collimator, in the investigated object and detection system prior detection; - the intrinsic geometrical resolution of the scintillator; - another aleatory processes; There is not practical to exaggerate with the resolution of the detector if the other components of the neutron radiography facility have inferior resolutions. The neutron beam intensity, detection efficiency, necessary zoom, quantum efficiency, etc. have established the working distance between 400 mm and 900 mm with an principal objective with f = (40-120). Because the intensity of the neutron beam is relatively small it is necessary to use an image intensifier with an 18 mm diameter of the photocathode. The image intensifier assures a multiplication factor up to and make possible to have real time investigations. The coupling of the image intensifier to CCD camera is made by a secondary lens. Is used this solution to be possible subsequently to change the CCD camera with other better even a cooled camera. This detector assures a tool for a variety of investigations at INR Pitesti in different industrial or non-industrial fields. There are obtained statically and dynamical images. The next target is to do tomography with neutrons but this will be possible better with a cooled CCD camera. REFERENCES 1. R.C. Lanza, et al A Cooled Based Neutron Imaging System for Low Fluence Neutron Sources, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, Vol. 43, No. 3, June Nuclear Instruments & Methods in Physics Research A, vol. 377 (nr. 1) şi 424 (nr. 1). 3. M. Dinca, C. Iorgulis, M. Pavelescu, Development of a new radiography facility with extended applications at the INR TRIGA reactor, 7 th World Conference on Neutron Radiography, September 15 to 21, 2002, Roma-Italy. 4. M. Dinca, E. Anghel, M. Preda, M. Pavelescu, Computed Image Analysis of Neutron Radiographs, 16 th European TRIGA Users Conference, Pitesti, September, 2000.