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NEUTRON COMPUTED TOMOGRAPHY

Mardiyanto* and Mohtar*

ABSTRACT

NEUTRON COMPUTED TOMOGRAPHY. Computed tomography is anon-destructive testing method which can visualize cross-section of materials based on theirnuclear characteristics. In the previous work, X-ray was used as its radiation media. Theaim of this experiment was to improve the computed tomography technique using neutronbeam. For reconstructing the cross-section image of materials, a filtered back projectionwas used. Results indicated that a minimum hole shown was 3 mm in diameter using ablack and white presentation. While using eight colour levels, a hole of 2 mm in diametercould be seen clearly. It is expected that neutron computed tomography can improve theresults of non-destructive testing.

ABSTRAK

"COMPUTED TOMOGRAPHY" DENGAN NEUTRON. Computed tomographyadalah metode uji tidak merusak yang bisa menampakkan tam pang lintang suatu bahanyang berdasarkan pada sifat-sifat nuklirnya. Pada penelitian sebelumnya, sinar-X digunakansebagai medium radiasi. Tujuan daTi eksperimen ini adalah untuk menambah kemampuanteknik computed tomography dengan menggunakan berkas neutron. Untuk merekonstruksicitra tam pang Jintang suatu bahan. digunakan proyeksi balik yang di filter ("filtered backprojection"). Dari hasil penelitian ditunjukkan bahwa lubang minimum yang tampak adalahyang berdiameter 3 mm bila dipresentasikan dengan warna hitam putih. Apabila digunakan8 warna, lubang dengan diameter 2 mm dapat dilihat dengan jelas. Dengan penelitian inidiharapkan bahwa computed tomography dengan neutron dapat melengkapi informasihasil-hasil dari uji tidak merusak.

INTRODUCTIONThe inspection of critical industrial components is similar as routine

medical check-ups. Two methods can be used to examine the existingdefects within an object, these are destructive and non-destructive testing(NOT). In destructive testing, the sample object needs to be destructed andfollowed by visual exmination, while in the latter destruction is notnecessary.

Computed tomography (CT) is one of NOT methods which can visualizecross-section images of materials based on nuclear characteristics. CT wasoriginated in medical radiology and is becoming increasingly prevalent inindustrial NOT. Several CT algorithms have been presented in order toobtain the accurate reconstruction as well as the minimum reconstructiontime.

CHO [1] presented general thoughts on the physical aspects with

Materials Science Research Centre, SATAN

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reference to X-rays below 100 KeV, specifically, the interaction of theX-rays with materials and a brief review of a number of computer

algorithms emphasizing the linear superposition technique with com-

pensation [I]. SHEPP and LOGAN [2] used a simulated phantom to

compare the Fourier and the search algorithm. CHO and CHAN [3] madea comparative study of 3-dimensional image reconstruction algorithms with

reference to a number of projections and noise filtering. MARTZ et al

[4] proposed computed tomography systems and their industrial application

using X-rays, while FUJINE et al [5] introduced the utilization of online

video image processing system in the neutron radiography facility.Although a lot of work has been done in this field, more studies on

CT using neutron beam need to be carried out in order to improve themethod.

The objective of this research is to study the application of CT usingneutron beam for industrial applications. Because of the limited number

of projections, it is hoped that this method is able to represent at least

macroscopic defects in the order of 10-1 mm.

MATERIAL AND METHOD

In this experiment, the Kyoto University Research Reactor neutron

radiography facility equiped with on-line video image processing systemwas used. This system consisted of a neutron TV system, video image

processing and personal computer. For CT purposes a stepping motor for

turning the object sample, a stepper motor control and driving unit which

could be operated manually as well as by using a PC were added to the

neutron radiography facility. The PC was a NEC PC-980 1 type, and wasused for arithmetic calculation in CT application and receiving image data

through an RS-232C interface from the video image processing system.

The whole system is illustrated in Figure 1.The sample object was a cylindrical aluminum. In contrast with X-rays,

neutron can penetrate aluminum easily since aluminum has a very lowneutron attenuation coefficient. Artificial defects were made in the form

of holes with different diameter and filled with paraffin in order to increase

their neutron attenuation (Figure 2). The diameters of the holes were 1.5;2.0; 3.0; 3.5; 4.0; 4.5; 4.9; 5.5; and 6.0 mm. This sample was mounted

on the sample table which was rotated with a stepping motor.This research was carried out using the neutron radiography facility at

the Kyoto University Research Reactor E-2 beam tube as thermal neutronsource, with thermal neutron flux was 1.2 x 106 n.Cm -2 .sec-1. The cadmium

ratio measured with a gold foil was 400 and the neutron-gamma ratio was106n.Cm-2.R-I. An aluminium plug was used to prevent the release of

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(1)

radioactive Ar-4l. As a gamma filter, a bismuth single crystal I cm thickwas placed in the aluminium plug. The neutron shutler was made fromboron carbide (B4C) and was situated at the middle of the collimator.

In this experiment four steps were performed, i.e. computer codepreparation, system arrangement, data collection and image reconstruction.

A computer code for collecting, refining data and reconstructing animage was developed and written in BASIC language.

Mathematically, the reconstruction can be described as an estimationof neutron attenuation, g(x,y), of its real value g(x,y) in one fixed plane.

Suppose an integral :

Pg(L) = ~ g ds

along a certain line L in a plane is a linear attenuation coefficient alongthe line which is measured through neutron transmission along L. Theattenuation Pg(L) can be represented as

Pg(L(t,e = I gdsL (t,e)where L(t,e) is the line whose normal axis passes through the origin asshown in Figure 3.Based on the figure, t can be written as :

Pg(t,e) = (2)

t = x cas e + y sin e (3)

Let p(w,e) be the Fourier transform of p(t,e) that is equal to theFourier transform of g in polar-coordinates, so that :

P (w,e) = g (w,e)

where

g(w,e) = f_~f~ g(x,y)e-iw(x cos 8+y sin 0) dxdy

(4)

(5)

In this case if p(t,e) is taken for all lines L, then :

g(x,y) = 1/(4n2) i7tde f_~p(w,e)eiW(X cos O+y sin 0) Iwl dw (6)is the inverse Fourier transform of g(w,e), where the Iwl comes from theJacobian of the transformation into polar coordinates. The projection datap(t,e) is taken from measurements so it has discrete values of p(tk,ej).Where: tk= ka, k= 0, I, 2, 3, .... ; 8j=jn/n; j= 0, I, 2, .... n-I; a is

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the f:lY sp:lcing Md n is the number of projections. The approximatereconstruction in the discrete domain of Equation (6) can be representedas :

n-I 00g~(x,y) = a/(2n) L: L: P(tk,8j) /l (x cos 8j+y sin 8j-tk) (7)

j=O k=-oowhere /l(w) is Fourier transform of /let) and is a filter function. The sumon k in (7) is finite, namely k ~ lIa and if m rays cover the unit in eachprojection, then a=2/m.

The code for reconstructing the image was made based on the abovealgorithm. The reconstruction can be represensted by a simple way as :

(Reconstructed image )=(Projection data)* {W(Ka)}Where : W(Ka) = the weighting function,

k = 0; 1; 2; anda = the ray spacing.In order to save computing time of the image reconstruction SHEPP

and LOGAN proposed a linear discrete weighting function :')

W (0) - 4/ (4a-) k=O') 2

W (ka) = -4 {na- (4k -I)} k=1; 2;The system as shown in Figure I was arranged and, the recording of

the object image was carried out using a video tape recorder. The steppingmotor and the object sample were located in front of the outlet apertureof the collimator. The object sample was arranged at the axis of thecollimator, so that, the whole object was irradiated. The neutron beampassed through the sample was attenuated by the atoms in the sample andthe degree of attenuation depended on the length of the neutron path andthe attenuation coefficient of the atoms. The transmitted neutrons which

carried the information of the object sample were converted into visiblelight. A NE-426 neutron scintillator, a LiF+ZnS(Ag) type, was used asthe converter. The light was, then, detected using a TV camera, displayedin a TV monitor, and the image was recorded by a video tape recorder.The sample was turned in 21 steps of 9 each, and recorded for two minutes.For this purpose it was considered enough to turn the sample only onehalf cycle.

To reduce the collection time, the data was stored digitally in a floppy

diskette. The analog data was converted into digital using analog to digitalconverter before being stored in the floppy diskette. The data was obtainedfrom the line 330 of each image frame and the image was the result of1000 frames integration.

Finally, the collected data was refined by subtracting its background

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noise before the image reconstruction. For refinement the data obtainedfrom the image was subtracted by the data obtained from the backgroundimage, and the image reconstruction was carried out using filter backprojection algorithm.

RESULTS AND DISCUSSIONOne of the digital data is illustrated in Figure 4. The datum was the

result of integrating 1000 image frames. The curv