Developments in Radiographic Inspection Methods

Novel X-ray Backscatter Technique for Detecting Crack below Deposit S. Naito, S. Yamamoto, Toshiba Corporation, Japan

ABSTRACT

We are studying X-ray backscattering technology with uncollimated X-ray irradiation (XBU), which

is expected to detect a crack below deposit without removing the deposit and to inspect a large area of

an object surface at once. As a first step of this study, we evaluated fundamental XBU performances in

the viewpoint of application to Visual Testing (VT). Using a pinhole X-ray 2D camera and an

industrial X-ray tube, we measured artificial cracks in stainless steel test pieces. Those results were as

follows. (1) The artificial slits of greater than 0.05 mm width were detected. Theoretical analysis

showed that the targeted 0.025 mm width crack can be detected by increasing an intensity of

irradiation X-rays. (2) From the analysis based on the experiments, crack-detectable deposit thickness

is 0.7 mm when the crack is 0.025 mm width and 0.5 mm depth and the deposit is metal-oxide deposit

of 1.2 g/cm3. (3) An artificial stress corrosion cracking in a curved test piece below artificial deposit of

iron oxide was detected. From its measurement time, requirement for the X-ray generator to apply

XBU to VT was evaluated. In the case of setting the X-ray irradiation head within 50 mm from an

object surface, about 38 mA of the X-ray tube current is required to measure a crack in 5 minutes (one

third of the VT measurement time including removing the deposit).

INTRODUCTION

X-ray backscatter technology (XBT) 1) is a method of obtaining the spatial density distribution of an

object by irradiating it with X-rays and measuring the intensity distribution of scattered X-rays. XBT

has the following features attractive for on-site structural crack detection: (1) it is a nondestructive and

non-contact method; (2) it can detect a crack below the surface; (3) it is applicable to composite skins;

(4) it is not susceptible to surface roughness and material properties, except their densities; and (5) an

X-ray source and a detector can be located on the same side of the object, enabling testing of massive

extended structures.

Several studies of XBT have been conducted.1–15)

For detection and sizing of near-surface

cracks under weld-deposited cladding, Babot et al.2)

showed experimentally that XBT can detect an

artificial crack of 0.02 mm width located in steel at 3 mm depth. Regarding high spatial resolution,

Lawson3)

achieved material identification of sub-surface layers with a depth resolution of 0.025 mm.

Although XBT has not yet been put to practical use in crack inspection, XBT apparatus accelerating

scanning of an object has been used for aircraft corrosion inspection4) and baggage inspection5), etc.

These features and revealing studies indicate that XBT is likely to be an excellent method for detecting

cracks in structures below deposit, without removing the deposit or performing other surface

preparations.

However, XBT requires a very long measurement time to detect a micro crack, because a

narrow pitch scanning of an object surface is needed. That is an important obstacle hindering practical

use of XBT for crack inspection. To resolve that problem, we are studying a XBT using uncollimated

X-ray irradiation (XBU), which enables to inspect a large area of an object surface at once by large

area X-ray irradiation and X-ray 2D detection.

As a first step of this study, we evaluated the fundamental XBU performances both

experimentally and analytically in the viewpoint of application to Visual Testing (VT). For X-ray 2D

detection, we used a pinhole X-ray camera consisting of a pinhole and an X-ray image intensifier. In

recent years, development of a small and high intensive X-ray generator is being advanced.16) We

supposed the use of such X-ray generator in this evaluation. In Secs. III and IV, under the use of the

pinhole X-ray camera, we evaluated detectable crack width (0.025 mm of the target value) and crack-

detectable deposit thickness. In Sec V, we demonstrated detection of artificial stress corrosion

cracking (SCC) in a curved test piece. In Sec VI, we discussed the measurement time and requirement

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for the X-ray generator to apply XBU to VT, XBU measurement in gamma radiation environment, and

that in water.

EQUIPMENT CONFIGURE AND MEASUREMENT PRINCIPLE

Figure 1 shows XBU equipment configure for VT application and its measurement principle. The

XBU equipment consists of a pinhole X-ray camera and a small and high intensive X-ray generator. A

structure is irradiated with X-rays from the X-ray generator. X-rays transmitting the deposit are

scattered at the structure. Intensity distribution of scattered X-rays is measured with the pinhole X-ray

camera. Figure 2 shows an example of a measured image of the scattered X-rays. Because X-rays are

hardly scattered at the crack, a region where the crack exits is dark and that where the crack does not

exist is light on the image of the scattered X-rays. The crack can be detected from dark and light on

the image.

DETECTABLE CRACK WIDTH

Detectable crack width in XBU is determined from the spatial resolution of the 2D X-ray detector. The

spatial resolution in the pinhole X-ray camera is determined from spatial resolutions of the X-ray

image intensifier and the pinhole. The recent high-performance X-ray image intensifiers have a good

spatial resolution (~�m). On the other hand, X-rays transmitting the pinhole material limit the spatial

resolution of the pinhole. Hence, the detectable crack width was evaluated in several pinhole

configures.

Figure 3 shows the experimental setup. An X-ray image intensifier (X-ray II, TOSHIBA

E5889BE-P1K) with a pinhole was positioned at a distance of 15 mm from the test piece surface. The

distance from the pinhole to the X-ray sensitive region of the X-ray II (50 mm in diameter) was 15

mm. The X-ray II was connected to a CCD camera (Bitran BS-42, 40M pixels), which acquired the

output image from the X-ray II and displayed it. We used an industrial X-ray tube (tube voltage of 80

kV and tube current of 4 mA). The X-ray tube was located at a distance of 300 mm from a position

just below the pinhole. The irradiation X-rays were in the form of an uncollimated conical beam. The

angle between the central axis of the cone and the test piece surface (the X-ray irradiation angle) was

about 10 degrees. We used a test piece made of stainless steel (SUS304) with a slit width of 0.05 – 0.5

mm and the slit depth of 1.0 mm. Each measurement time was 1 - 80 minutes. In the experiment, three

pinholes were used. One was a lead plate of 2.0 mm thickness with a hole of 1.0 mm in diameter at the

center of the plate. Others are shown in Fig. 3. The pinhole type 1 was a tungsten disk with a conical

hole. The minimum pinhole diameter was 0.1 mm. The pinhole type 2 was a stacked plate of two

pinholes of type 1.

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Scattered

X-rays

Crack

Irradiation

X-rays

Deposit

Structure

X-ray

generator

Head

Pinhole X-ray

camera

Scattered

X-rays

Crack

Irradiation

X-rays

Deposit

Structure

X-ray

generator

Head

Pinhole X-ray

camera

Figure 1 - XBU equipment configure

and its measurement principle.

Fig. 2. Example of measured image

of scattered X-rays when using the

pinhole of 1.0 mm diameter.

Figures 5 and 6 show the measured image for the pinhole type 2 and the slit of 0.05 mm width and its

brightness distribution. The x position in Fig. 6 is a rough guide, because the sensitive region of the X-

ray II was a spherical surface and the position was distorted. Figure 7 shows the brightness drop rate

BD in all of the measured images. We defined BD as a ratio of A to B, which are indicated in Fig. 6.

The detectable slit width

with the pinhole type 1 was 0.08 mm due to X-rays transmitting the pinhole material. When the slit

was less than 0.08 mm, the brightness could not be distinguished from the brightness fluctuation (BD

< ~0.04). In the pinhole type 2, the slit of 0.050 mm width could be detected as the brightness drop in

the brightness distribution (Fig. 6), although it was not directly observed in the image (Fig. 5) due to

the spatial resolution of X-ray II that we used (~0.050 mm) and low X-ray intensity.

Irradiation

X-rays

Slit

Lead

shielding

15 mm

300 mm 15 mm

X-ray image intensifier

Test piece

Pinhole

10°

Scattered X-rays

Central axis

of irradiation

X-rays

X-ray tube

Irradiation

X-rays

Slit

Lead

shielding

15 mm

300 mm 15 mm

X-ray image intensifier

Test piece

Pinhole

10°

Scattered X-rays

Central axis

of irradiation

X-rays

X-ray tube

Fig. 3. Experimental setup

Extended figure4.0 mm

8.0 mmTungsten disk of

25.0 mm in diam.

0.1 mm

0.05 mm

Type 1

Type 2

Conical hole

8.0 mmType 1

Type 1

Extended figure4.0 mm

8.0 mmTungsten disk of

25.0 mm in diam.

0.1 mm

0.05 mm

Type 1

Type 2

Conical hole

8.0 mmType 1

Type 1

Fig 4. Pinhole configure

0.00.20.40.60.81.0

9 10 11 12 13 14 15 16 17 18x position [mm]

Bri

ghtn

ess

[a.u

.]

x

y

A

B0.00.20.40.60.81.0

9 10 11 12 13 14 15 16 17 18x position [mm]

Bri

ghtn

ess

[a.u

.]

x

y

A

B

Figure 5 - Example of measured

image (pinhole type 2, slit of 0.05

mm width). The red dot circle

indicates slit position.

Figure 6 - Brightness distribution of Fig. 5.

The values are those integrated for y

direction. The red arrow shows slit

position.

Figure 7 - Brightness drop rate. Red circles, blue

diamonds, and yellow rectangles show values in

pinhole types of 1 and 2 and those in 0.10 lead

plate of 1.0 mm diameter, respectively.

0.00.10.20.30.40.50.60.7

0.01 0.1 1Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

on

less

]

Slit width [mm]

Pinhole type 2

Pinhole type 1

1.0 mm in diam.

0.00.10.20.30.40.50.60.7

0.01 0.1 1Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

on

less

]

Slit width [mm]

Pinhole type 2

Pinhole type 1

1.0 mm in diam.

Next, we predicted XBU response for the slit of 0.025 mm with the pinhole type 2. Figure 8 shows

calculation result of the brightness distribution with the scattered X-ray response calculation code.

This code calculates scattered X-ray intensity from X-ray transmissivities of materials in all of X-ray

flight paths, which was verified to be in good agreement with the measured data (Fig. 9). The

distribution was calculated under a condition that the spatial resolution of the X-ray II was negligible

small and the X-ray intensity was sufficiently high to neglect statistical fluctuation of detected X-ray

number. The result predicts that BD is about 0.1 and the slit of 0.025 mm width can be detected with

the pinhole type 2 by using an X-ray II of a high spatial resolution and increasing an intensity of

irradiation X-rays.

In conclusion, the artificial slits of greater than 0.05 mm width was detected with the pinhole

type 2. The analytical result showed that the slit of 0.025 mm width can be detected with the pinhole

type 2 by using an X-ray II of a high spatial resolution and increasing an intensity of irradiation X-

rays.

CRACK DETECTALBLE DEPOSIT THICKNESS

In XBU, parameters of a measurement condition are deposit thickness, crack width, crack depth,

spatial resolution of the pinhole X-ray camera, X-ray irradiation angle with respect to an object

surface, and X-ray energy. These parameters are roughly reduced to a ratio of the volume of the crack

to that of the measurement region (RCM) (see Appendix). The volume of the crack is that of the void

generated by the crack. The measurement region is the spatial resolution of the pinhole X-ray camera

on the object surface × X-ray reachable depth determined from the X-ray irradiation angle and the X-

ray energy. We experimentally evaluated the lower limit of a crack detectable RCM. We evaluated

crack detectable deposit thickness from the lower limit.

The experimental setup was almost the same as that in Sec. III. Using the pinhole X-ray

camera, we measured the intensity distribution of scattered X-rays from a stainless steel test piece

covered with a stainless plate. The X-ray irradiation angle was set to be 30 degrees. The pinhole plate

was a lead plate of 8.0 mm thickness with a hole of 1.0 mm in diameter at the center of the plate. The

pinhole was positioned at a distance of 50 mm from the test piece surface. The distance from the

0 0.2 0.4 0.6 0.8 1.0 1.20

0.5

1.0

Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

on

less

]

Silt width [mm]

experimental

calculation

0 0.2 0.4 0.6 0.8 1.0 1.20

0.5

1.0

Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

on

less

]

Silt width [mm]

experimental

calculation

x position [mm]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.5 0.0 0.5

Bri

ghtn

ess

[a.u

.]

x position [mm]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.5 0.0 0.5

Bri

ghtn

ess

[a.u

.]

Figure 8 - Calculation result of

brightness distribution for the slit of

0.025 mm in width and 0.5 mm in

depth when using the pinhole type 2.

Figure 9 - Comparison of calculated BDs

with experimental values. The calculation

and experimental values was those when

using the lead plate of 2 mm thickness

with a pinhole of 1 mm diameter. Red

squares and blue circles show

experimental BDs and calculated BDs,

respectively.

pinhole to the X-ray sensitive region of the X-ray II was 74 mm. These provided 1.7 mm of the spatial

resolution of the pinhole X-ray camera on the object surface. The bias voltages of the X-ray tube were

80 kV, 120 kV, and 160 kV. The slit width of the test piece was 0.5 mm. The thickness of the stainless

steel plate was 0.1 – 1.1 mm. The measurement time of each image was 10 – 20 minutes.

Table I shows the experimental results. Figure 10 shows an example of the measured image.

Figure 11 shows RCM vs. the brightness drop rate BD. The BD increased with increasing the RCM.

Under the same RCM, almost the same BDs were obtained. When RCM was less than 0.18, detection

of the slit was difficult due to statistical fluctuation of the scattered X-rays and electrical noise.

Therefore, as a rough indication, the lower limit of a crack detectable RCM can be evaluated to

be 0.18.

Table 1 - Experimental results

0.00

0.10

0.20

0.30

0.40

0.50

0.00 0.10 0.20 0.30 0.40

RCM [dimensionless]

Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

onle

ss]

0.00

0.10

0.20

0.30

0.40

0.50

0.00 0.10 0.20 0.30 0.40

RCM [dimensionless]

Bri

ghtn

ess

dro

p r

ate

BD

[dim

ensi

onle

ss]

Figure 11 - RCM vs. brightness drop rate BD.

Figure 10 - Example of measured image.

The stainless plate thickness was 0.3

mm. The red dot circle indicates slit

position. RCM and BD were 0.28. and

0.22, respectively.

Item

Tube

voltage

[V]

X-ray

mean free

path [mm]

X-ray

reachable

depth

[mm]

Slit depth

[mm]

Stainless

steel plate

thickness

[mm]

RCM

[dimensionless]

BD

[dimensionless]

1 80 0.65 0.22 0.7 0.0 0.37 0.45

2 80 0.65 0.22 0.7 0.1 0.20 0.15

3 80 0.65 0.22 0.7 0.2 0.03 undetectable

4 120 2.15 0.72 2.3 0.0 0.37 0.4

5 120 2.15 0.72 2.3 0.4 0.17 0.11

6 120 2.15 0.72 2.3 0.6 0.06 undetectable

7 160 3.45 1.15 1.0 0.0 0.33 0.31

8 160 3.45 1.15 1.0 0.3 0.28 0.19

9 160 3.45 1.15 1.0 0.6 0.18 0.13

10 160 3.45 1.15 2.3 0.0 0.37 0.36

11 160 3.45 1.15 2.3 0.3 0.28 0.15

12 160 3.45 1.15 2.3 0.6 0.18 0.16

13 160 3.45 1.15 3.5 0.0 0.37 0.39

14 160 3.45 1.15 3.5 0.3 0.28 0.22

15 160 3.45 1.15 3.5 0.6 0.18 0.17

16 160 3.45 1.15 3.5 0.8 0.11 0.11

17 160 3.45 1.15 3.5 1.1 0.02 undetectable

Next, we evaluate the crack detectable deposit thickness when sizes of the crack are 0.025 mm in

width and 0.5 mm in depth. The thickness is calculated as deposit thickness satisfying 0.18 of RCM.

The object material is set to be stainless steel. The spatial resolution of the pinhole camera on the

object surface, the X-ray tube voltage, and the X-ray irradiation angle are set to be 0.1 mm, 80 kV, and

30 degrees, respectively. Those give the measurement region of 0.1 mm in diameter and 0.22 mm in

depth. When a crack of w in width is covered with a stainless steel plate of d in thickness, RCM is

expressed by

mm22.0mm/2)1.0(

)mm22.0(mm1.0

regiont measuremen of Volume

Crack of VolumeRCM

2⋅⋅

−⋅⋅==

π

dw

. (1)

d at RCM=0.18 is the crack detectable thickness of the stainless steel plate. Assuming that the deposit

is metal-oxide deposit of 1.2 g/cm3, the crack detectable deposit thickness is approximated as the

product of d at RCM=0.18 and a ratio of stainless steel density (8.0 g/cm3) to the metal-oxide density

(1.2 g/cm3). Figure 12 shows a calculation result of the crack detectable deposit thickness in metal-

oxide deposit of 1.2 g/cm3. The crack detectable deposit thickness for a crack of 0.025 mm in width is

0.7 mm.

MEASUREMENT OF ARTIFICIAL STRESS CORROSION CRACKING

To evaluate whether XBU can detect stress corrosion cracking (SCC) or not, we measured an artificial

SCC in a curved test piece with a compact pinhole X-ray camera we fabricated.

Figure 13 shows the compact pinhole X-ray camera. It consists of the pinhole type 2 and a

compact X-ray image intensifier (HAMAMATSU C10569-01) of 0.010 mm of spatial resolution. The

C10569-01 can directly obtain detected X-ray number. The sizes of the camera are 100 mm ×100 mm

×170 mm, which give object accessibility comparable to a CCD camera for VT (50 mm in diameter

and 120 mm length). Figure 14 shows a photograph of SCC of the test piece. The SCC was

Figure 12 - Calculation result of crack detectable deposit thickness in metal-oxide

deposit (1.2 g/cm3)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.000 0.025 0.050 0.075 0.100

Cra

ck d

etec

tab

le d

epo

sit

thic

kn

ess

[mm

]

Crack width [mm]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.000 0.025 0.050 0.075 0.100

Cra

ck d

etec

tab

le d

epo

sit

thic

kn

ess

[mm

]

Crack width [mm]

fabricated through chemical processing. The SCC had inhomogeneous width. The maximum width

was about 0.1 mm. Figure 15 shows a photograph of the SCC covered with iron oxide powder. We

measured this covered SCC within the detection area indicated in Fig. 14.

Figure 16 shows the experimental setup. The compact pinhole X-ray camera was positioned at a

distance of 22 mm from the test piece surface. The camera was connected to an image processing unit

(HAMAMATSU C9851), which acquired the output image from the X-ray II and displayed it. We

used an industrial X-ray tube (tube voltage of 80 kV and tube current of 4 mA). The X-ray tube was

located at a distance of 230 mm from a position just below the pinhole. The irradiation X-rays were in

the form of an uncollimated conical beam. The X-ray irradiation angle was about 8 degrees. The

measurement time was 390 minutes.

Figure 17 shows the measured image. Due to low X-ray intensity the SCC could not be clearly

observed in the image. Figure 18 shows distribution of the scattered X-ray intensity, which was

derived by accumulating the intensity in 4 mm width. This width was selected as that in which the

SCC extending direction seemed to be consistent with the accumulation direction (vertical direction).

The SCC could be clearly observed as the drop of the detected X-ray number. Therefore, we consider

that the SCC is also clearly observed in the image by increasing the X-ray intensity. This time, only

one sample of a SCC was measured. It is necessary to measure several shaped and sized SCCs to

探傷ヘッドピンホール170mm100mm

External

Pinhole type 2170mm100mm X-ray image

intensifier

Internal探傷ヘッドピンホール170mm100mm

External

Pinhole type 2170mm100mm X-ray image

intensifier

Internal

Figure 13 - Photographs of compact pinhole X-ray camera

SCC

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Iron

oxide

powder

SCC

���������

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Iron

oxide

powder

Figure 14 - Photograph of SCC of

the test piece

Figure 15 - Photograph of SCC

covered with iron oxide powder

evaluate detectability for a SCC quantitatively. However, at least we obtained prospect of detecting a

SCC.

DISCUSSION

Measurement time and requirement for X-ray generator

We estimated the measurement time when using a small and high intensive X-ray generator and

evaluated requirement for the X-ray generator as follows. The measurement time was 390 minutes in

the artificial SCC measurement. The X-ray tube current was 4 mA. Distance of the object surface from

the X-ray tube was 230 mm. The X-ray irradiation angle was 7.5 degrees. If using the small and high

intensive X-ray generator, the distance of the object surface from the X-ray tube will be about 50 mm

230mm

Central axis of

irradiation X-rays Irradiation

X-rays

~8º

22 mm

Pinhole X-ray

camera

Curved test piece

230mm

Central axis of

irradiation X-rays Irradiation

X-rays

~8º

22 mm

Pinhole X-ray

camera

Curved test piece

Figure 16 - Experimental setup

17 mm

13

mm

SCC

0100200300400500600700S

catt

ered

X-r

ay n

um

ber

[cou

nt]

17 mm

Accumulation

area (4mm

width)

17 mm

13

mm

SCC

0100200300400500600700S

catt

ered

X-r

ay n

um

ber

[cou

nt]

17 mm

17 mm

13

mm

SCC

0100200300400500600700S

catt

ered

X-r

ay n

um

ber

[cou

nt]

17 mm

Accumulation

area (4mm

width)

Figure 17 - Measured image Figure 18 - Horizontal position distribution of

detected scattered X-rays

and the X-ray irradiation angle will be more than 30 degrees. X-ray intensity on the object surface is

inversely proportional to the second power of the distance and proportional to sine of the irradiation

angle. Therefore, the X-ray intensity increases by a factor of 81 (=sin(30°)/sin(7.5°)×(230/50)2) at the

same tube current. The image in Fig. 17 will be obtained in about 4.8 minutes. Here, current VT takes

about 15 minutes for removing the deposit in the area where the VT can inspect at once (25 mm ×25

mm). When the measurement time is 5 minutes (one third of the 15 minutes) and a tenfold X-ray

intensity is needed to obtain a clear image, the necessary X-ray tube current is 38 mA

(=4.8min./5min.×10×4 mA).

Gamma radiation environment

A typical gamma dose rate of reactor internals during inspection is 1 mSv/h (=5 × 105 /cm2/s). The

sensitivity of the pinhole X-ray camera in Fig. 13 for 1.25 MeV gamma-rays (60Co) is 1/250 of that for

50 keV X-rays. X-ray sensitive area of the camera is 25 mm in diameter. The pinhole diameter is 0.10

mm. The scattered X-ray flux can be estimated to be about 2 × 108 /cm2/s when using the small and

high intensive X-ray generator of 38 mA of the tube current. Therefore, a signal-to-noise ratio as a

ratio of the detected number of scattered X-rays to that of gamma-rays is estimated to be 1.6

(=2×108×0.10

2/(5×10

5×25

2) ×250), when all gamma-rays transmit the pinhole material and the lead

shielding of the pinhole camera. To obtain more than 10 of the signal-to-noise ratio, lead shielding of

more than 30 mm thickness will be needed.

Measurement in water

Transmissivity of 50 keV X-rays for water is about 60 % in 20 mm water thickness. The scattered X-

rays are reachable to the pinhole X-ray camera within such a distance. The issue of the measurement

in water is that X-rays scattered at water are detected and they add to the measured image as

background. In our experimental investigation, the brightness distribution of this background was flat.

Therefore, it can be subtracted from the measured image when statistical fluctuation of the brightness

is sufficient low. We consider that such subtraction is possible, for example, by using the small and

high intensive X-ray generator as described above.

SUMMARY AND CONCLUSION

We evaluated fundamental performances of X-ray backscattering technology with uncollimated X-ray

irradiation (XBU) in the viewpoint of application to Visual Testing (VT). Using a pinhole X-ray 2D

camera and an industrial X-ray tube, we measured artificial cracks in stainless steel test pieces.

Summaries of those results are as follows. (1) The artificial slits of greater than 0.05 mm width were

detected by the pinhole type 2. Theoretical analysis showed that the targeted 0.025 mm width crack

can be detected by increasing an intensity of irradiation X-rays. (2) From the analysis based on

experiments using test pieces covered with a stainless plate, crack-detectable deposit thickness is 0.7

mm when the crack is 0.025 mm width and 0.5 mm depth and the deposit is metal-oxide deposit of 1.2

g/cm3. (3) An artificial stress corrosion cracking in a curved test piece below artificial deposit of iron

oxide was detected. From its measurement time, the necessary X-ray tube current was estimated to be

38 mA when using a small and high intensive X-ray generator under development.

In conclusion, although strongly depending on the X-ray generator performance, the pinhole

X-ray camera has potential for detecting the crack of 0.025 mm width under the deposit. Therefore, we

consider that prospect for application of XBU to VT was shown.

REFERENCES

1) Stokes J A, Alvar K R, Corey R L, “Some New Applications of Collimated Photon Scattering

for Nondestructive Examination”, Nucl. Inst. Meth., 1982 193 261-267.

2) Babot D, Berodias G, Peix G, “Detection and Sizing by X-ray Compton Scattering of Near-

surface Cracks under Weld Deposited Cladding”, NDT&E International, 1994 24 5 247-251.

3) Lawson L, “Compton X-ray Backscatter Depth Profilometry for Aircraft Corrosion

Inspection”, Materials Evaluation, 1995 8 936-941.

4) Harding G, “Inelastic Photon Scattering: Effects and Applications in Biomedical Science and

Industry”, Radiat. Phys. Chem., 1997 50 1 91-111.

5) American Science and Engineering, Inc., http://www.as-

e.com/products_solutions/z_backscatter.asp

6) Niemann W, Zahorodny S, “Status and Future Aspects of X-ray Backscattering Imaging”,

Review of Progress in Quantitative Nondestructive Evaluation, 1998 17 379-385.

7) Bridge B, “A Theoretical Feasibility Study of the Use of Compton Backscatter Gamma-ray

Tomography (CBGT) for Underwater Offshore NDT”, British Journal of NDT, 1985 27 357-

363.

8) Dunn W L, Yacout A M, “Corrosion Detection in Aircraft by X-ray Backscatter Methods”,

Applied Radiation and Isotopes, 2000 53 625-632.

9) Duvauchelle P, Girier P, Peix G, “Development of High resolution Focusing Collimators

Intend for Nondestructive Testing by the Compton Scattering Tomography Technique”, Appl.

Radiat. Isot., 1990 41 2 199-205.

10) Olkkonen H, Karjalainen P, “A 170Tm Gamma Scattering Technique for the Determination of

Absolute Bone Density”, British Journal of Radiography, 1975 48 594-597.

11) Clarke R L, Milne E N C, Van Dyk G, “The Use of Compton Scattered Gamma Rays for

Tomography”, Investigative Radiography, 1976 11 225-235.

12) Niemann W, Zahorodny S, “Status and Future Aspects of X-ray Backscattering Imaging”,

Review of Progress in Quantitative Nondestructive Evaluation, 1998 17 379-385.

13) Kaufman L, Gamsu G, Savoca C, et al., “Measurement of Absolute Lung Density by

Compton-scatter Densitometry”, IEEE Trans. Nucl. Sci., 1976 NS-23 1 599-605.

14) Gautam S R, Hopkins F F,Klinksiek R, et al., “Compton Interaction Tomography I. Feasibility

Studies for Applications in Earthquake Engineering ”, IEEE Trans. Nucl. Sci., 1983 NS-30 2

1680-1684.

15) Harding G, Strecker H, Tischler R, “X-ray Imaging with Compton-scatter Radiation”, Philips

Tech. Rev., 1983 41 46-59.

16) AET Inc., http://www.aetjapan.com/english/hardware/xray.html

Figures 19(a) and 19(b) respectively show measurement regions of X-ray backscatter technology

(XBT) and XBT using uncollimated X-ray irradiation (XBU) in an object. In XBT, the measurement

region is an overlapped region of a collimated X-ray beam and a collimation region of a detector. In

XBU, the measurement region is an overlapped region of an X-ray reachable depth and a collimation

region of a detector.

In XBT, as presented in Fig. 19(c), when simultaneously irradiating multiple positions on an

object surface, one detector detects not only scattered X-rays from a target measurement region but

also those from an unwanted, un-targeted measurement region. Therefore, simultaneous measurement

of the surface (plane-by-plane measurement) is difficult. XBT requires scanning of the surface in a

narrow pitch with a narrow collimated X-ray beam, where sizes of the pitch and the beam collimation

are comparable to the crack size. Such scanning requires a very long measurement time. In contrast, in

XBU, as presented in Fig. 19(d), one detector views only a target measurement region. Therefore, a

large area of the surface can be inspected at once by irradiating a large area of an object surface with

X-rays and measuring the scattered X-rays using a two-dimensional (2D) X-ray detector. Drastic

reduction of the measurement time is expected.

Appendix. Comparison of XBU with XBT

XBT has been developed as a method to measure a three-dimensional (3D) spatial density

distribution of an object. XBU obtains a 2D spatial density distribution because the measurement

region cannot be moved in a depth direction of the object because the irradiating X-rays are not

collimated. The goal of XBU is not 3D sizing of a crack, but inspection of the presence or absence of a

crack, which is a priority on site. Here, in a pinhole X-ray camera, one measurement region is an

overlapped region of an X-ray reachable depth and a spatial resolution of the pinhole camera. X-ray

reachable depth l can be expressed by

1)sin

11( −

+=θ

λl

, (2)

where � �is the X-ray irradiation angle. � is the mean free path of X-ray, which is determined from X-

ray energy and material. A crack is detected as a ratio of the scattered X-ray intensity from the

measurement region including the crack to that not including the crack. In a rough approximation, this

ratio is expressed as a ratio of the volume of the crack to that of the measurement region, although

there is actually a depth direction dependence of the outgoing scattered X-ray intensity inside the

measurement region.

(a) XBT (b) XBU

(c) XBT (d) XBU

SourceDetector

Measurement region

Collimated X-ray beam-

Measurement region

X-ray reachable

depth

Object

Detector

Object

Focal region of the detector collimator

Collimator

Measurement region

-

X-ray cone

beam

Source

Object

Detector

Target measurement

regions

Unwanted measurement

regions Target measurement

regionsTarget

measurement regions

Target measurement

regions

(a) XBT (b) XBU

(c) XBT (d) XBU

SourceDetector

Measurement region

Collimated X-ray beam-

Measurement region

X-ray reachable

depth

Object

Detector

Object

Focal region of the detector collimator

Collimator

Measurement region

-

X-ray cone

beam

Source

Object

Detector

Target measurement

regions

Unwanted measurement

regions Target measurement

regionsTarget

measurement regions

Target measurement

regions

Figure 19 - Measurement regions of XBT and XBU