ELSEVIER

Nuclear Instruments and Methods in Physics Research A 377 (1996) 133 136 NUCLEAR INSTRUMENTS

& METHODS IN PHYSICS RESEARCH

$ech(~ A

Neutron radiography with a one-dimensional image sensor having a high linearity range

Budi Santoso ", Masayoshi Tamaki b'*, Masahiro Oda b, Masahiko Honda b, Yasushi Ikeda c

"National Atomic Energy Ageney, Nuclear Technology Assessment Center, BATAN Jakarta 12710, Indonesia hl)epartment of Energy Engineering and Science, Nagoya Universitv Furo-¢ho, Chikusa-ku, Nagoya 464-01, Japan

¢Japan Fine Ceramics Center, Rokuno 2-4-1, Atsuta-ku, Nagoya 4.~6, Japan

Abstract By a combination of a linear image sensor and a honeycomb collimator, neutron radiography imaging system with wide

dynamic range was constructed. Using this system, the effective thermal neutron attenuation coefficients for several materials were evaluated systematically. The imaging system was also used to perform neutron computed tomography as well as to evaluate the CT-value for several metallic samples. The obtained CT values agreed well with the effective neutron attenuation coefficients.

Keywords: Neutron radiography; Linear image sensor; Neutron attenuation coefficient: Neutron computed tomography

1. Introduction

Neutron radiography (NR) and computed tomography (NCT) have been investigated with various methods to date. The film method [1] had a high spatial resolution, but image processing was very elaborate and time-consuming. A conventional electronic imaging method (television) method was easy for obtaining many projection data and subsequent image processing [2], but the spatial resolution and the range of dynamic sensitivity were still inferior to the former method. A cooled charge-coupled device (C- CCD) system was used [3] for its excellently high dynamic range and succeeded to yield high quality NCT images. However, Kobayashi only performed a relative evaluation of CT values for various materials by using CT values of aluminum and iron as references. In addition, the charac- teristics of the C-CCD sensor were sensitive to radiation effects caused by gamma-ray and fast neutron exposure.

• In order to improve the system characteristics of the narrow dynamic range for film and television methods, and radiation effects for C-CCD sensors, an electronic imaging device having a wide dynamic range has been developed and used for neutron radiography and NCT [4,5]. The imaging system was composed of a cooled linear image sensor (Hamamatsu Photonics Co.) and a personal com- puter. The photo-diode cell detects weak scintillation light and has a much wider latitude (16 bit) than the convention- al electronic imaging system (8 bit ADC) and even the

*Corresponding author: Tel. +81 52 789 4693, fax +81 52 789 4692, e-mail a40507a@nucc.cc.nagoya-u.ac.jp.

film method. The latitude of the cooled imaging system was equivalent to that of the C-CCD system.

To evaluate neutron transmittance through objects, it is important to eliminate the influences of a divergent incident beam from the neutron source and of scattered neutrons from samples, as well as a neutron exposure field, on an neutron radiograph. For this purpose, a neutron absorbing honeycomb collimator was developed [6].

This study evaluates the performance of a combined neutron radiography system with a cooled linear image sensor and a honeycomb collimator. The effective neutron attenuation coefficients of various materials were deter- mined systematically from neutron radiographs, and the reliability of CT values obtained by this system was also evaluated.

2. Experiments and image processing

2.1. Neutron radiography system

A cooled linear image sensor was used to detect neutron-induced photons from the scintillation converter. This system was tested as an imaging device for neutron radiography and neutron computed tomography. The linear image sensor is composed of a series of linearly arrayed photo-sensitive P - N junction cells. Photons hitting the cell produce a number of photon-induced electron-hole pairs. The photo-electron charge is stored in each cell until a next reading signal scans the cell. During the storage period, background current due to thermal noise occurs in the cell.

0168-9002/96/$15.00 Copyright ©1996 Elsevier Science B.V. All rights reserved PII S0168-9002( 96)00 149-0 IX. ELECTRONIC IMAGING

134 B. Santoso et al. / Nucl. Instr. and Meth. in Phys. Res. A 377 (1996) 133 136

By cooling the cell to a low temperature, the background current is maintained at an acceptably low level. As a matter of fact, such background current is subtracted from each sensor output. The sensor output shows a linear relationship with neutron exposure. The range of linearity has been determined experimentally to be from 5 × l0 5 Ix s (detection limit) to 5 × 10 ~ Ix s (saturation limit) in units of luminous exposure [5].

The photo-sensitive area of the cell is a narrow rectan- gular in shape and has a dimension of 25 l,~m X 2.5 ram. The image sensor consisted of 512 cells which were arrayed linearly. Because of the large area of the cell, each cell can detect more luminous photons and store a much larger photo-electric charge than the cells in the usual 2-dimensional CCD camera, which cell size is about 15 p,m × 15 #m [3]. This results in a higher signal/noise ratio of the image signal. The large cell area has an additional important advantage in that the cell has no appreciable degradation from radiation eflects caused by gamma-rays and fast neutrons coming from the sample or the exposure field environment in the neutron radiography facility as compared to the damage of small size cell in a CCD camera.

A honeycomb collimator (HC) made of neutron absorb- ing material (gadolinium-coated thin aluminum structure) was used for the reduction of scattered neutrons [6]. It was set between a sample and the imaging system. Scattered neutrons from the sample and the neutron exposure field were absorbed by the HC before reaching the imaging converter. Consequently, the image on the converter was formed by neutrons transmitted directly through the sam- ple. However, the image of the sample with the HC contains the image of HC itself. Elimination of the superimposed HC image will be imperative to use this system for quantitative neutron radiography and NCT. The details of this correction are discussed in a later section.

2.2. Exper imental procedures

Neutron radiography and NCT experiments were con- ducted using the Thermal Neutron Radiography Facility-2 (TNRF-2) in the Japan Research Reactor-3M (JRR-3M) at the Japan Atomic Energy Research Institute (JAERI) [7]. Experimental conditions are briefly summarized as foi- lows. The thermal neutron flux is 1.5 X 10 ~ n cm 2s '. A scintillation neutron radiography converter of ~'LiF-ZnS (Ag) (Kasei Optonics Co.) was used. The present neutron radiography system, used a set of the neutron radiography converter, an optical mirror, a lens and the linear sensor, enclosed in a dark box. The linear sensor was cooled to 251 K with a Peltier element. In this geometrical setup of the image system, the overall image magnification of the optical system was determined to be 0.078. The cell width of 25 p~m corresponded to an image size of 320 l~m on the converter. This value was used in the CT image recon- struction calculation and to determine the absolute intrinsic

CT value of samples. The saturation exposure time of the photo-electric charge in the photo cell was determined to be 71 s. The sensor output was digitized with a 16 bit ADC and stored in a memory device. The image data was processed and analyzed by computer (FACOM M-1800/ 20, Fujitsu) at the Nagoya University Computation Center.

In the neutron radiography experiments, the HC was set between sample and converter. Samples used for NR imaging were step wedges of Fe, Cu, Pb, C (graphite) and Ni. The step wedge sample was arranged so that incident neutron beam was exposed vertically against each flat face of the steps. Neutrons transmitted the sample in the direction of the step height. As a result, an array of the steps was imaged simultaneously as a one dimensional profile of the sensor output. The exposure time (storing time of photo-electron charge) was chosen to be 50 s for an imaging of the neutron radiograph.

The samples used for the NCT were cylindrical AI, Pb, Ti, Cu, Bs (brass), SUS (stainless steel), Fe and Ni metals. For taking NCT data, a set of samples was mounted on a rotatory stepping motor that was controlled by the personal computer. The sample was rotated from 0 ° to 360 ° with a stepwise rotation angle. The exposure time was 40 s for each projection image. For an NCT reconstruction of a set of samples, 50 projections were taken as a data set. The CT reconstruction algorithm used in this study was the conw)lution integral method, which was developed in our laboratory [8,9,2,4,5]. Shepp and Logan filters [10] were used during the CT calculation process.

3. Results and discussions

3.1. Reduct ion o f the scat tered componen t

Fig. la shows one dimensional profiles of the sensor output for a step wedge object (Fe) with HC, background (HC image) and dark current. In the background (HC) profile, two characteristic features were clearly observed. One is that the profile curve has a systematic fluctuation due to overlapping of the image of HC partition walls. Another is that the profile seems to be appreciably distorted due to a position dependence of an effective solid angle of every unit HC cell and of a probability of self absorption effect passing through the honeycomb walls. It comes from characteristics of the divergent type neutron beam from the 7R horizontal tube at the JRR-3M. The aperture size of the 7R beam tube is 30 mm × 30 mm in square [7]. Thus, only the beam size corresponded to the 7R aperture can be used intrinsically as a parallel neutron beam, easily seen in the background profile (Fig. la). Therefore, the neutron intensity profile of the peripheral portion of this aperture size is apparently distorted. The image profile of the iron step wedge with HC was also systematically distorted and fluctuated. Therefore, the apparent image data (l~,p) of the step wedge with HC has

B. Santo~o et al. / Nucl. Instr. and Meth. in Phys. Res. A 377 (1996) 133-136 135

3 . 0

2 .0 121 O

~o 1.0 1-

09

1.2 (D 0 c 1.0

~ o.B o'}

0.6

0.4

o.2 z

0.(

Dark curren

O.0 0 1 O0 200 300 400 500

Pixel

(a) Measured Prof i les

1 O0 200 300 400 501 Pixel

(b) Corrected profile of Fe stepwedge

Fig. I. Typical output profiles of a linear sensor NR image. (a) background (HC), step wedge Fe with HC, and dark current, (b) corrected transmittance profile of step wedge Fe.

range of the imaging system for neutron fluence was experimentally determined to be about 500 (exposure time range from 0.1 to 50 s for the present experimental setup), which is appreciably high compared to those of conven- tional film and television imaging systems ( < 100),

3.2. Efjective neutron attenuation coeJficient

Using this imaging system and subsequent data process- ing, effective attenuation coefficients were determined for various materials. Table 1 shows the effective neutron attenuation coefficients for Fe, Cu, Pb, C (graphite) and Ni step wedge samples. The effective total macroscopic cross sections by Kobayashi [11], where a combination system of a C-CCD imaging device and a correction method of the scattered neutron term by the umbra condition had been used, were tabulated together as reference values. By taking a ratio of the present result to reference value for each material, degrees of relative agreement were calcu- lated. An averaged ratio of present case to reference one was evaluated to be 1.00 with _+12% error. Systematic deviation was not observed. This reveals that the combined cooled linear sensor and HC imaging system, developed in the present work, is useful in obtaining the effective neutron attenuation coefficient of the object materials.

3.3. N C T image and its C T value

to be corrected by using the background (HC) and dark current data. The background profile (IHc) is the data of the HC image without sample. The dark current profile (1pc) is the data without neutron beam. Corrected image data (normalized neutron transmittance) is given by (lAp -- 1Dc)/(l .c IDc). Fig. l b shows the corrected profile of the neutron transmittance tot the Fe step wedge. It shows 10 steps very clearly without appreciable HC noise and any beam divergence distortion. This comes from the high linearity and the wide dynamic range of the linear sensor. In the present experimental setup case, the relationship between the sensor output V in volts and the neutron exposure time t in seconds was represented as follows,

V = (0.042 +0.002)t ' ~""~ '> 015)

This indicates that the present imaging system has a highly linear response with the neutron fluence. The linearity

The imaging system was used to collect projection data for the reconstruction of the NCT image and to measure the intrinsic CT-values for various materials. Fig. 2 shows a typical CT image obtained using the present system. The reconstructed image contained systematic artifacts, which might be induced by the limited projection number (50 projections) and the HC-induced trace noise. From the results, CT values of samples were determined as shown in Table 2. Effective total macroscopic cross sections [11] were also tabulated together as reference values. By taking a ratio of the CT value to the reference value for each material, the degree of relative agreement was also esti- mated. The averaged ratio was evaluated to be 0.99 with -+5% error. Only a 1% systematic deviation was observed.

The CT values (Table 2) seemed to be fairly consistent with the effective neutron attenuation coefficients (Table 1 ) for each sample material. In order to evaluate the degree of projection data degradation data by image processing

Table 1 Effective neutron attenuation coefficients calculated from the corrected NR image of step wedge materials

Material Effective neutron attenuation coefficient [cm '] Reference case [11] [cm I ] BNL-325 [12] [cm ~]

Lead (Pb) 0.311 _+0.020 0.380_+0.031 0.369 Graphite (C) 0.382_+0.003 0.372_+0.072 0.385 Copper (Cu) 0.882_+0.011 0.873-+0.048 0.96 Iron (Fe) 1.050-+0.028 1.044_+0.015 1.12 Nickel (Ni) 1.861 _+0.104 1.63_+0.12 1.98

IX. ELECTRONIC IMAGING

136 B. Santoso et al. / Nucl. Instr. and Meth. in Phys. Res. A 577 (1996) 1,7,?-136

Table 2 CT values obtained from reconstructed NCT images of cylindrical materials

Material CT value [cm ~] Reference case I I l] [cm J] BNL-325 [12l [cm ~]

Aluminum (All 0.090+0.0l 5 0.088+0.002 0.10 lead (Pb) 0.342-0.027 0.380+0.031 0.369 Titanium (Ti) 0.51 I + 0.053 0.526_+_0.053 0.57 Brass (Cu/Zn) 0.616+_0.054 - 0.74 Copper (Cu) 0.832+0.050 0.873+0.048 0.96 Stainless steel 0.974+0.020 0.977-0.034 1,14 Iron (Fe) 1.049+0.030 1.044-0.015 1.15 Nickel (Nil 1.772 +0.029 1.63 +0.12 1,98

Ti Cu ®

Fe Pb

Fig. 2. Typical NCT image of cylindrical samples with a diameter of 10 mm (Ti, Cu, Fe and Pb metals).

and CT calculation, a ratio of the measured CT value to the effective neutron attenuation coefficient was calculated for each material. The average of the ratios was evaluated to

be 1.00 with _+7% error. No systematic deviation was introduced in the total experimental and analyzing system. This indicates that the combined imaging system, estab-

lished in the present work, is also useful for obtaining quantitative CT-values.

4. Conclusions

Through this study of neutron radiography and NCT using the combined system of a one dimensional sensor with wide linearity range and a HC for scattered neutron reduction, a quantitative neutron radiography system has been established to determine an effective neutron attenua- tion coefficient and subsequently applied to NCT to

determine a CT value for a given material. Concluding remarks can be made as follows:

(1) By utilizing the high linearity range of the NR system, a procedure to eliminate the systematically superimposed HC fluctuation in neutron radiograph was realized by simple image processing scheme using normal- ized transmittance.

(2) The effective attenuation coefficients were deter-

mined for various step wedge materials. (3) The CT values were also obtained from the NCT

image of several cylindrical materials and agreed with the

effective neutron attenuation coefficients, respectively.

Acknowledgments

One of the authors (B.S.) is grateful to the late Prof. Dr.

Kanji Tasaka for his kind advices related to this work when he stayed at Nagoya University to study as a master course student. The authors also acknowledge Mr. Akira Tsuruno (JAERI) for his kind permission for their coopera-

tion.

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