LiF crystals as high spatial resolution neutron imaging detectors

Nuclear Instruments and Methods in Physics Research A 651 (2011) 90–94

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

LiF crystals as high spatial resolution neutron imaging detectors M. Matsubayashi a,n, A. Faenov b, T. Pikuz b, Y. Fukuda c, Y. Kato d, R. Yasuda a, H. Iikura a, T. Nojima a, T. Sakai a a

Quantum Beam Science Directorate, Japan Atomic Energy Agency, Shirane 2-4, Shirakata, Tokai, Ibaraki 319-1195, Japan Joint Institute for High Temperatures of Russian Academy of Sciences, Izhorskaja Street 13/19, Moscow, Russia c Kansai Photon Science Institute, Japan Atomic Energy Agency, Kizugawa, Kyoto 619-0215, Japan d The Graduate School for the Creation of New Photonics Industries, Hamamatsu, Shizuoka 431-1202, Japan b

a r t i c l e i n f o

a b s t r a c t

Available online 25 January 2011

Neutron imaging by color center formation in LiF crystals was applied to a sensitivity indicator (SI) as a standard samples for neutron radiography. The SI was exposed to a 5 mm pinhole-collimated thermal neutron beam with an LiF crystal and a neutron imaging plate (NIP) for 120 min in the JRR-3M thermal neutron radiography facility. The image in the LiF crystal was read out using a laser confocal microscope. All gaps were clearly observed in images for both the LiF crystal and the NIP. The experimental results showed that LiF crystals have excellent characteristics as neutron imaging detectors in areas such as high spatial resolution. & 2011 Elsevier B.V. All rights reserved.

Keywords: Neutron radiography Neutron imaging detector Spatial resolution LiF crystal JRR-3M

1. Introduction

2. Materials and methods

Spatial resolution, temporal resolution and image gradation resolution are listed as significant performance items for neutron imaging detectors. As for the traditional imaging system for neutron radiography using a single emulsion type X-ray film and a 25 mm thick gadolinium screen with a vacuum cassette, a spatial resolution of 17 mm as a line spread function has been reported [1]. Commercially available neutron imaging plates (NIPs) [2] have been reported to offer spatial resolution with a measured line spread function of 58 mm [3]. The cooled CCD camera detection system with the combination of an ultra-thin scintillator and an optimized lens system has been developed by researchers in PSI and has achieved the resolving power of 20 line pairs/mm in spatial frequency at 10% of the modulation transfer function [4]. The neutron imaging detectors using Gd- or B-doped microchannel plates with cross-delay readout have recorded an estimated spatial resolution of 17 mm [5]. For further improvement of spatial resolution in neutron imaging detectors, the authors applied the color center (CC) formation in LiF crystals to neutron imaging and obtained a high spatial resolution of 5.4 mm with little granular noise, good linearity and a large dynamic range [6]. Here the authors report on the performance of LiF crystals as neutron imaging detectors evaluated by using a sensitivity indicator (SI) [7] for neutron radiography.

Neutron imaging by CC formation in LiF crystals was applied to the SI as the standard sample for neutron radiography using the thermal neutron radiography facility (TNRF-2) [8] in the research reactor JRR-3M.

n

Corresponding author. Tel.: +81 29 284 3751; fax: + 81 29 284 3555. E-mail address: [email protected] (M. Matsubayashi).

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.072

2.1. LiF crystal In LiF, CCs are formed by the aggregation of F centers, which are produced by irradiation of ionizing radiation [9]. The CCs emit visible luminescence peaking at 520 and 680 nm when these color centers are illuminated by an optical beam at 450 nm. Since the CCs are not destroyed by this readout process, the recorded images remain stable unless the LiF is heated up to 4120 1C. The exposed LiF can be reused for imaging by heating up to 400 1C [10]. 2.2. Sensitivity indicator The SI was originally designed for characterization of neutron radiography facilities based on ASTM (ASTM–SI) [11–13]. Although the use of the ASTM–SI is restricted to silver halide films exposed with metallic conversion screens used in the direct method according to ASTM E545-86 [14], the well-established specification of the ASTM–SI was considered useful for characterizing other neutron imaging methods. The ASTM–SI combined a hole gage and gap gage into an aluminum case. In the experiment, the self-produced SI for the JRR-3M TNRF (R3M–SI) according to the ASTM–SI was used. Fig. 1 shows the internal configuration of

M. Matsubayashi et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 90–94

Aluminum Lead

Cast Acrylic Resin

91

Aluminum 2.66

0.56 255 141 (µm) (µm)

1.21 25.5

263 131 (µm) (µm)

2.49 263 129 (µm) (µm)

5.00 267 149 (µm) (µm)

25.5 Shim: [thickness(mm)

B

A

0.25

0.13]

0.3 mm thick Aluminum dust cover

Gap size: 268 22 59 108 (µm) 131 33 90

Aluminum Shim

side view

Fig. 1. Internal configuration of the R3M–SI.

the R3M–SI. It consists of eight four-steps (0.56, 1.21, 2.49, 5.00 mm in thickness) of cast acrylic resin, which have holes and gaps (268, 131, 22, 33, 59, 90, 108 mm in width) and a foursteps of lead. Two thin shims of A and B with four small holes (141, 131, 129, 149 mm in diameter for the shim A, 255, 263, 263, 267 mm in diameter for the shim B) were placed under the cast acrylic resin steps. The gaps were formed by aluminum shims between the cast acrylic resin steps.

Cassette Aluminum foil

Neutrons

2.3. TNRF-2 The beam line for TNRF-2 provides a 25 cm wide 30 cm high rectangular-shaped thermal neutron beam with a flux of 1.2  108 n/cm2 s and an L/D ratio of 4 150 at the imaging position. In order to improve the L/D ratio for reducing geometric unsharpness, a pinhole 5 mm in diameter was placed at a distance of 2.3 m from the imaging position as the collimator. Thermal neutrons through the pinhole provided a beam around 20 mm square with a flux of up to 107 n/cm2 s and an L/D ratio of 460. 2.4. Experimental procedure As shown in Fig. 2, the R3M–SI was placed in close contact with an LiF single crystal 20 mm in diameter and 3 mm thick with an illumination area 18 mm in diameter, covered with a 12-mm thick aluminum foil to protect the LiF crystal and they were stuck to the cassette of a NIP. Because the R3M–SI was larger than the LiF crystal, the NIP was applied to confirm the imaging area covered by the LiF crystal. The sample and detectors were exposed to the pinhole-collimated thermal neutron beam for 120 min. After recording, the image in the LiF crystal was read out using a laser confocal microscope (Olympus model FV-300). The LiF crystal under the microscope was illuminated with the 488 nm line of an argon ion laser and the luminescence from the CCs at 4510 nm was observed. In this microscope, the illumination laser beam is scanned at the object plane to record a 2-dimensional image of fluorescence as 1024  1024 digital data. In the readout, different objectives with magnifications of 4  and 10  were used. The pixel size and field of view were (3.45 mm)2 and

Neutron imaging plate LiF crystal R3M-SI Fig. 2. Schematic layout of neutron imaging using a LiF crystal of 20 mm in diameter and 3 mm thick. In image recording, the R3M–SI was placed in close contact with an LiF crystal covered by Al foil and they were stuck to the cassette of a neutron imaging plate.

(3.5 mm)2 for the 4  objective and (1.38 mm)2 and (1.41 mm)2 for the 10  objective, respectively. The objectives were focused on the surface of the LiF crystal while reading out the image. The image in the NIP was read out using an imaging plate reader (Fuji film BAS-2500) with a readout resolution of 50 mm. To retrieve the neutron image of the R3M–SI from the over-exposed NIP, the successive repeated readout technique [3] was applied.

3. Results and discussion Fig. 3(a) shows the neutron image of the R3M–SI and LiF crystal captured by the NIP. Perturbed neutron flux distribution by the alignment of pinhole collimator induced inhomogeneous shading in the image. The darker circular region shows the LiF crystal. Fig. 3(b) gives the relative position of the LiF crystal. It is recognized that the LiF crystal covers the main part of the

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R3M–SI. Fig. 3(c) shows the neutron image of the R3M–SI captured by the LiF crystal. An objective with 4  magnification was used for the readout. The readout 3.5-mm-square images were manually combined for obtaining the whole image of the R3M–SI captured by the LiF crystal. As indicated in Fig. 3(a), the perturbed neutron flux

distribution also caused inhomogeneous shading especially on the right side in this image. All gaps formed with sizes of 268, 131, 22, 33, 59, 99 and 108 mm were clearly observed. For further analysis of the image, the small regions around gaps and holes were read out using an objective with 10  magnification.

Step tichkness (mm) 0.56

1.21

2.49

5.00

Gap size (µm): 268

131

22 33 59 90 108

Fig. 3. Neutron image of the R3M–SI captured by the neutron imaging plate (a). The darker circular region shows the LiF crystal covered area. The drawing shows the relative position of the LiF crystal (b). Neutron image of the R3M–SI captured by the LiF crystal (c). In image readout, luminescence from the LiF crystal was observed with a laser scanning confocal microscope with 4  magnification.

Step thickness: 1.21 mm

45000 62 µm

40000

40000

35000

2.49 mm

30000

Intensity, a.u

Intensity, a.u

45000

35000

62 µm

30000

25000 0

200

400

600

800

Microns

700

1000 1200 1400

750

800

850

900

Microns

Nonuniform neutron flux Step thickness:1.21 mm

55000 55000 2.49 mm

45000

Intensity, a.u

Intensity, a.u

29 µm 50000

50000

45000

40000 40000 35000 0

200

400

600 800 Microns

1000 1200 1400

700

750

800 Microns

850

900

30 µm

Fig. 4. Gap size measurements for gaps with a formed distance of 59 mm (a) and 22 mm (b). Line profiles for the full width (left) of the gap image (center) and the profiles for the enlarged region (right) around the gap.

M. Matsubayashi et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 90–94

3.1. Gap size measurements

Table 1 Summary of gap size measurements. Unsharpness Step thickness [Ug]a (mm) [tst] (mm)

0.56 1.21

7.0 8.4

2.49

11.2

5.00

16.7

Gap size (mm)b

22

33

59

90

108

131

268

–/25 / 30 / 33 33/ 

–/38 40/ 38 37/ 29 31/ 

–/66 65/ 62 62/ 52 50/ 

–/92 89/ 85 81/ 79 78/ 

–/– 105/ 103 96/96

–/140 140/ 133 133/ 122 127/ 

–/263 263/ 263 263/ 260 247/

96/

a The size of the geometric unsharpness Ug at each step thickness is given by Ug ¼ (tst + tb)/(L/D), where tst is the step thickness, tb ¼ 2.66 mm is the thickness of the R3M–SI basement, L/D ¼460. b Gap sizes at step thickness of 1.21 and 2.49 mm were measured at two points near next steps.

The images of all gaps between the steps were obtained and used for measurements of gap size. Fig. 4(a) and (b) shows the magnified image and line profiles of gaps formed to be 59 and 22 mm wide, respectively. The upper half of the images corresponds to a step thickness of 1.21 mm and lower half to 2.49 mm. As shown in Fig. 3(c), the 59 mm wide gap is positioned in the nonuniform region. Therefore, the line profiles in Fig. 4(a) show the effect of nonuniform neutron flux distribution. The profiles on the right are enlarged ones near the gap and were measured as a gap size of 62 mm for both step thickness. The layout of Fig. 4(b) is the same as that of Fig. 4(a). Apparent nonuniform neutron flux distribution is not observed in the line profiles. The line profiles on the right give a wider gap size of 30 mm than the formed value of 22 mm. The measured gap sizes are summarized in Table 1. The size of the geometric unsharpness Ug at each step thickness is given by Ug ¼ ðtst þ tb Þ=ðL=DÞ

Step

Step thickness: tst tb = 2.66 (mm) Gap

93

Basement of R3M-SI

Fig. 5. Schematic layout for the calculation of the geometric unsharpness.

ð1Þ

where tst is the step thickness, tb ¼2.66 mm is the thickness of the R3M–SI basement, L/D¼460. The schematic layout is shown in Fig. 5. The discrepancies between the gap sizes of the formed and the measured are almost less than the sizes of the geometric unsharpness. 3.2. Hole size measurements Enlarged images of regions around the holes prepared in the shim A and B are shown in Fig. 6. Fig. 6(a)–(d) corresponds to

Fig. 6. Hole size measurements for shim B (a)–(d) and the shim A (e) and (f). Hole size almost same as the formed diameter was obtained for the shim B. Meanwhile, the measured hole size for the shim A was smaller than the formed diameter.

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Table 2 Summary of hole size measurements. Transmittance [B]a

Absorber (step) thickness [tst] (mm)

0.56 1.21 2.49 5.00

Measured hole size (mm)

Inside hole

Shim A

Shim B

Shim A

Shim B

tsh ¼0 mm

tsh ¼ 0.13 mm

tsh ¼0.25 mm

Hole size

Measured

Hole size

Measured

0.86 0.71 0.50 0.25

0.82 0.68 0.48 0.24

0.80 0.66 0.46 0.23

141 131 129 149

 128  130 – –

255 263 263 267

255 252 250–255 262

a The transmittance B at the position inside hole and outside hole at each step thickness is given by B¼ exp{  (tst + tsh)S}, where tst is the step thickness, tsh is the shim thickness, S ¼2.8 cm  1 is the macroscopic cross-section of Perspex [15].

Absorber (step) Absorber (step) thickness: tst

Shim thickness: tsh

tb = 2.66 (mm) Shim

Hole Basement of R3M-SI

Fig. 7. Schematic layout for the calculation of the transmittance.

holes with a formed diameter of 255, 263, 263 and 267 mm at step thickness of 0.56, 1.21, 2.49 and 5.00 mm, respectively. Fig. 6(e) and (f) corresponds to holes with a formed diameter of 141 and 131 mm at step thickness of 0.56 and 1.21 mm, respectively. The holes in the images are clearly observed in Fig. 6(a)–(c). Although the image in Fig. 6(d) shows only homogeneous shading, the line profile shows the existence of a 262 mm diameter hole. Not so large discrepancies between hole diameters of the formed and the measured are observed for the shim B. The smaller holes produced in the thinner shim A compared with those in the shim B are hardly recognizable in the images as shown in Fig. 6(e) and (f). Their line profiles indicate the existence of holes 130 mm diameter in the images. The measured hole sizes are summarized in Table 2. The geometric unsharpness of 5.8 mm is given by substituting tst ¼0.00 mm into Eq. (1). The transmittance B at the position inside hole and outside hole at each step thickness is given by B ¼ expfðtst þ tsh ÞSg

ð2Þ

where tst is the step thickness, tsh is the shim thickness, S ¼2.8 cm  1 [15] is the macroscopic cross-section of Perspex. The schematic layout is shown in Fig. 7. The discrepancies between the hole sizes of the formed and the measured exceed the size of the geometric unsharpness at several holes. When taking the uncertainty of 5.4 mm due to the spatial resolution of the LiF crystal into consideration, these discrepancies are considered to be acceptable. The difference in transmittance between at the positions inside hole and outside hole affect the detectability. Smaller differences in transmittance for the shim A than the shim B make detection of holes difficult, especially in the case of less than 2% transmittance difference.

4. Conclusion It has been demonstrated that the LiF crystal performs efficiently as neutron imaging detectors in areas such as high spatial resolution with high image gradation resolution. The neutron

image recorded with LiF was examined with use of the R3M–SI. All seven gaps with sizes from 22 to 268 mm were clearly observed and their sizes were almost measured within the discrepancies determined by the geometric unsharpness due to step thicknesses. Six of eight small holes around 130 and 260 mm in diameters were observed and their diameters were almost measured within the intrinsic resolution of LiF. Two holes around 130 mm in diameters with less than 2% transmittance differences were rarely observed. Even with a longer exposure time (more than 100 times) than the other neutron imaging detectors, superior spatially resolved neutron imaging using LiF crystals is expected to lead to the development of new research fields in neutron imaging. If enriched 6LiF is used in place of natural Li, the sensitivity is expected to be improved by up to 13 times while exposure time can be reduced to less than 1/10.

Acknowledgement The authors thank Prof. Hisao Kobayashi for his useful information on the spatial resolution of film method neutron imaging. References [1] H. Kobayashi, Neutron Radiography, vol. 3, Kluwer Academic Publishers, Dordrecht, 1990, p. 893. [2] K. Takahashi, S. Tazaki, J. Miyahara, Y. Karasawa, N. Niimura, Nucl. Instr. and Meth. A 377 (1996) 119. [3] H. Kobayashi, M. Satoh, Nucl. Instr. and Meth. A 424 (1999) 1. ¨ [4] E.H. Lehmann, G. Frei, G Kuhne, P Boillat, Nucl. Instr. and Meth. A 576 (2007) 389. [5] O.H.W. Siegmund, J.V. Vallerga, A. Martin, B. Feller, M. Arif, D.S. Hussey, et al., Nucl. Instr. and Meth. A 579 (2007) 188. [6] M. Matsubayashi, A. Faenov, T. Pikuz, Y. Fukuda, Y. Kato, Nucl. Instr. and Meth. A 622 (2010) 637. [7] J.W.F. Markgraf, R. Matfield, in: J.C. Domanus (Ed.), Practical Neutron Radiography, Kluwer Academic Publishers, Dordrecht, 1992, p. 175. [8] M. Matsubayashi, H. Kobayashi, T. Hibiki, K. Mishima, et al., Nucl. Technol. 132 (2000) 309. [9] J.H. Schulman, W.D. Compton, Color Centers in Solids, Pergamon, Oxford, 1962. [10] G. Baldacchini, S. Bollanti, F. Bonfigli, F. Flora, P. Di Lazzaro, A. Lai, et al., Rev. Sci. Instrum. 76 (2005) 113104. [11] ANSI/ASTM E142-77, Standard method for controlling quality of radiographic testing. [12] ASTM E757-80, Standard method for controlling quality of radiographic testing using wire penetrometers. [13] ASTM E545-75, Standard method for determining image quality in thermal neutron radiographic testing. [14] ASTM E545-86, Standard method for determining image quality in direct thermal neutron radiographic testing. [15] J.W.F. Markgraf, R. Matfield, in: J.C. Domanus (Ed.), Practical Neutron Radiography, Kluwer Academic Publishers, Dordrecht, 1992, p. 40.