Material identification using dual particle interrogation

Accepted Manuscript Material identification using dual particle interrogation Tongyuan Cui, Yangyi Yu, Yigang Yang, Zhi Zhang, Xuewu Wang

PII: DOI: Reference:

S0168-9002(19)30106-8 https://doi.org/10.1016/j.nima.2019.01.053 NIMA 61827

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Nuclear Inst. and Methods in Physics Research, A

Received date : 31 July 2018 Revised date : 11 January 2019 Accepted date : 16 January 2019 Please cite this article as: T. Cui, Y. Yu, Y. Yang et al., Material identification using dual particle interrogation, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.01.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Manuscript Click here to view linked References

1

1

Material identification using dual particle interrogation

2

Tongyuan Cui1,2, Yangyi Yu1,2, Yigang Yang1,2*, Zhi Zhang1,2, Xuewu Wang1,2

3

1

Department of Engineering Physics, Tsinghua University, Beijing, P. R. China

2

4

Ministry of Education, Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Beijing, P. R. China

5

Abstract— In this paper, the application of bi-modal analysis, which fuses the X-ray attenuation and photoneutron attenuation

6

together to enhance the capability of material identification, is presented. Within a 7 MeV e-LINAC system, both X-rays and

7

photoneutrons, which are produced by the (γ, n) reaction of 9Be nuclei with X-rays, are delivered simultaneously. A moderation

8

process is introduced to help retard the measurement time of photoneutrons to avoid the X-ray pulse’s interference to the neutron

9

detector. A V(t) factor is derived from the attenuation coefficients of X-rays and photoneutrons to separate different materials. The

10

hydrogen-wealthy materials, organic materials, low-Z, middle-Z and high-Z metals could be effectively discriminated. The

11

preliminary bi-modal imaging result, realized with a neutron detector array of 32 3He counters and an X-ray detector array, is also

12

presented.

13

Keywords: Materials identification; X-ray attenuation; Photoneutron attenuation; Bi-modal analysis; Moderation;

14

I. INTRODUCTION 15

The detection of illicit materials has been the research interest of homeland security

[1]

. As one of the most popularly used

16

technologies, MV X-ray imaging can realize excellent spatial resolution with good penetrating capability, however, it cannot

17

provide atomic number information for the inspected materials, unless the scattering X-ray spectra are analyzed[2]. Because of the

18

different reaction properties, the neutron technologies can play a complimentary role to the X-ray imaging[3][4][5][6][12][13]. The

19

combination of X-ray and neutron penetration information would effectively improve the capability for material identification.

20

FNGR (fast neutron and gamma-ray radiography), in which 14 MeV neutrons and 60Co 1.17/1.33 MeV γ-rays are applied, is an

21

implementation of integrating different penetrating capability of neutrons and photons together to improve the materials

22

identification capability[7][8][9][10]. Because of the monochromaticity of (D, T) neutrons and 60Co γ-rays (the energies of γ-rays of

23

60

24

coefficients for neutrons and photons, hence a ratio can be derived from the two coefficients to identify different materials.

25

However, there are three disadvantages in this system. First, the neutron yield of the neutron generator used in FNGR system is as

26

low as 108 n/s and can hardly reach 1010 n/s, resulting in a very poor cargo inspection speed of ~ mm/second, which is unacceptable

Co are very close so they can be approximately deemed as single energy photons), each material has the different attenuation

2 27

for the real application. Second, the short life-span of the neutron generator, usually 2000 to 4000 hours, is another handicap

28

hindering its application in reality. Third, the regular operation of the FNGR system is always challenged by the system complexity

29

due to the two radiation sources involved, of which the working stability of the neutron generator is of primary concern.

30

Photoneutrons are liberated as the byproduct of several MeV photons if a suitable photon-to-neutron-convertor is applied.

31

Because the (γ, n) reaction is the threshold reaction (Eth=1.67 MeV for 9Be, Eth=2.223 MeV for 2H and higher for other nuclides)

32

and has a smaller (about 2~3 orders lower) cross section than competing photoatomic reactions, the efficiency of converting

33

photons to neutrons is quite low. Usually the converting efficiency ranges from 10-5 to 10-3 neutron/photon for an optimally shaped

34

9

35

by the e-LINAC compensates for the low efficiency. A photoneutron yield of 109~1011 n/s can be readily delivered by a 7~9 MeV

36

e-LINAC[11]. Unlike the neutron generator, the photoneutron source can be easily maintained because beryllium is a strong metal

37

and heavy water can be held in a well-designed aluminum vessel to prevent leakage and evaporation. Thus, the advantages of

38

large neutron yield, approximately infinite life-span, low cost and robustness can be obtained with an e-LINAC photoneutron

39

source. With a 7 MeV e-LINAC working at 400rad/min@1m dose rate and a 2.6 kg beryllium convertor, the neutron yield of more

40

than 1010n/s has been realized[13].

Be or D2O convertor with 7~9 MeV bremsstrahlung photons. However, the very large number (1014~1015/s) of photons produced

41

Before making use of photoneutrons, two problems caused by the photoneutron production process should be addressed in

42

advance. First, as the byproduct of photons, the emission of photoneutrons cannot be free of the interference of the preceding X-

43

ray pulse. The low conversion efficiency implies that the detection of each photoneutron is always accompanied by thousands of

44

~ MeV photons (although the e-LINAC works in pulse mode of 5 μs pulse width, the flight time difference between photons and

45

neutrons from source to detector is too trivial to separate them). Hence the response of neutron detector might be predominately

46

affected by photons but not photoneutrons, although the neutron detector can be designed to less sensitive to photons. Therefore,

47

the first problem of applying photoneutrons in neutron imaging is how to measure photoneutrons under the interference of very

48

intense X-ray pulse. Second, in the process of photoneutrons emission, the energy of a photoneutron comes from the excess of

49

photon energy over (γ, n) threshold. The continuous energy spectrum of the bremsstrahlung photons leads to a continuous energy

50

spectrum of the photoneutrons. Hardening effect will modify the spectra of both photons and photoneutrons when they penetrate

51

the inspected material, so the second problem is that no constant ratio between photoneutron attenuation coefficient and photon

52

attenuation coefficient is available for each material.

53

In this paper, the research of how to measure photoneutron penetration information under the intense interference of X-ray pulse

54

is presented. The V value, defined as a ratio between the photoneutron attenuation coefficient and the photon attenuation coefficient,

55

is investigated by simulation and experiments to identify different materials. The preliminary result of the bi-modal imaging, which

3 56

fuses the X-ray penetrating image and the photoneutron penetrating image, is also presented.

57

II. SYSTEM OVERVIEW

58

Fig. 1 is the overview of a 7 MeV e-LINAC based system for the bi-modal analysis. A 7 MeV e-LINAC is used to produce

59

bremsstrahlung photons, the intensity of which after penetrating the inspected material will be measured by an X-ray detector to

60

reveal the mass thickness information of the penetrated object. Polyethylene, aluminum, iron, copper and lead are selected as

61

simulant materials to cover the low-Z to high-Z range. A Φ160 mm×70 mm cylindrical beryllium convertor is positioned in front

62

of the tungsten target to convert photons that are not used for X-ray imaging to photoneutrons for the purpose of neutron

63

interrogation. Though beryllium and heavy water are both convertor candidates for their low threshold energies, beryllium is used

64

here to acquire higher neutron energies. There is an 11ºangle offset between the directions of penetrating neutrons and photons

65

because the photoneutron detector and the X-ray detector cannot share the same beam path. Lead is used to collimate photons and

66

paraffin is used to collimate photoneutrons. A 3He counter surrounded by polyethylene is used to measure photoneutrons. A thin

67

B4C layer is positioned in front of the 3He counter to absorb scattered thermal neutrons. There is also a benchmark 3He detector

68

surrounded by paraffin to monitor the dose rate fluctuation of the e-LINAC. Fig. 2 gives the simulated photoneutron spectrum at

69

the position of the neutron detector. polyethylene

beryllium 70mm

scattered neutron thermalized neutron

tungsten 7 MeV e 

( , n )

photoneutron

3

fast neutron

X  ray ( , n )

B4C

160mm

3He

Neutron collimator 7MeV e-

He(n, p )3 H

detector

11

X-ray detector

X-ray collimator

D Plywood, PE, Al, Fe, Cu, Pb

70 71

Benchmark 3He counter surrounded by paraffin

Fig. 1 The layout of the 7 MeV e-LINAC based bi-modal analysis system.

4

percentage(%) per 5keV

10

9

Be photoneutron spectrum

1

0.1

0.01 7.371MeV 10 of Be

7.542MeV 10 of Be

1E-3

0

1

2

3

4

En (MeV)

72 73

Fig. 2 The 7 MeV e-LINAC induced photoneutron spectrum with the beryllium convertor.

74

III. V VALUE FOR MATERIALS IDENTIFICATION

75 76

The V value, which is defined by the ratio of the attenuation coefficients for photoneutrons and photons, varies with different materials and their thicknesses,

V (t )  ln( I n (t ) / I n 0 ) / ln( I X (t ) / I X 0 )

77

I n 0 and I n (t ) are count rates for photoneutrons before and after penetration,

78

where t is the thickness of the inspected material;

79

respectively;

80

hardening effect, no constant V value can be assigned for each material. Instead, the value of V varies because of variable

81

penetrating thickness; hence, V (t ) is used instead of V to separate different materials.

82

I X 0 and I X (t ) are that for photons; and V (t ) is the derived parameter for materials identification. Because of the

IV. MEASUREMENT OF PHOTONEUTRON PENETRATION INFORMATION UNDER THE INTENSE X-RAY PULSE INTERFERENCE

83

The intense interference of X-ray pulse to photoneutrons was first addressed in this experiment. Photoneutrons are emitted in a

84

pulse mode of 5 μs width and 50 Hz to 250 Hz repetition frequency, which is the same as X-ray pulses. In the direction of neutron

85

transmission, the ratio of photons to photoneutrons is approximately 28000 (calculated by simulation), thus the response of 3He

86

detector would be dominated by photons rather than photoneutrons, although 3He is less sensitive to photons.

87

The solution is to shift the measuring time of photoneutrons by a moderation process. It may take tens to hundreds of

88

microseconds to slow down the fast photoneutrons to thermal neutrons for improving the detection efficiency by the 3He counter

89

(1.9 cm diameter and 11.3 cm length, with 3.8 atm of 3He and 0.2 atm of Ar), which is surrounded by polyethylene (moderation

90

material). Before the photoneutrons are moderated and captured by 3He nuclei, photons have disappeared because they can only

5

92

photoneutrons can be free of the interference of X -ray pulse.

0.30

1.2

(1ms,0.9985)

1.1

0.25

1.0 0.9

0.20

0.8 0.7

0.15

0.6 0.5

0.10

0.4 0.3

0.05

0.2 0.1

0.00

0.0

10

-1

10

0

1

2

3

4

10 10 10 10 10 -6 Time delay after each pulse (10 s)

5

10

6

cumulative probability of neutron detection before time delay after each pulse

undergo several collisions before being absorbed or escaping that usually takes several nanoseconds. Thus the detection of

differential probability of neutron detection at each time interval (normalized to 1)

91

93 94

Fig. 3 Simulation results of photoneutron detection within each time interval after X-ray pulse (The “□” curve gives the neutron

95

detection probability of each time interval. Data is normalized to unity to show the fraction of each time interval. The “○” curve

96

signifies the cumulative probability).

97

Fig. 3 exhibits the simulation results of neutron detection probability at different time delays after each X-ray pulse. It can be

98

seen that no neutrons are detected before 1 μs because photoneutrons are not quickly moderated to thermal neutrons and absorbed

99

by the 3He counter. Moreover, no photoneutrons are detected after 10 ms because all photoneutrons have been absorbed in or

100

escape from polyethylene, thus no photoneutrons enter the 3He counter. The differential probability curve of Fig. 3 reveals that

101

most of the photoneutrons are detected in the time range of 10 μs to 1 ms and <0.15% of photoneutrons are detected after 1 ms.

102

Therefore, the time window for photoneutron detection can be selected as [10 μs, 1 ms]. 3He

Saturation signal of 5μs X-ray pulse Neutron signal

~ 10  s

~ 125 s Recovery time from X-ray pulse saturation

103

(Left)

(Right)

104

Fig. 4 (Right): Preamplifier waveform of 3He counter under pulsed X-rays and photoneutrons, (Left): Expanded view of the box

105

frame on the right.

106

Fig. 4 exhibits the preamplifier waveform of 3He counter under the mixed radiation field of pulsed X-rays and photoneutrons.

107

In the right figure, a saturation signal of X-ray pulse and several photoneutron signals are presented. The left figure displays an

6 108

expanded view of X-ray pulse saturation signal. Although the duration of X-ray pulse is only 5 μs, the saturation signal lasts ~10

109

μs, indicating that 3He counter is deeply saturated by a large number of photons. Moreover, the recovery time of 3He counter from

110

the deep saturation caused by the X-ray pulse is about 125 μs. So the 3He counter is affected by the X-ray pulse for almost 135 μs

111

during which the neutron signal cannot be registered. The amplitudes of neutron signals superimposed on the slope of recovery

112

process may be reduced; thus the pulse height spectrum of the 3He counter is broadened and the amplitudes of some neutron signals

113

cannot surpass the threshold, which is set for discriminating electronic noise and γ-ray signals.

114

The signals from the preamplifier of 3He counter are fed into an ORTEC 855 amplifier and then analyzed by the ORTEC 919E

115

MCA (Multi-Channel Analyzer). Fig. 5 displays a pulse height distribution of 3He counter with high voltage of 900 V supplied by

116

an ORTEC 556. The spectrum does not look like the typical 3He counter spectrum, which contains one full energy peak and two

117

additional continuums due to the wall effect. In order to eliminate unwanted small signals, the lower threshold is set as channel

118

No. 100 and the upper threshold is set as channel No. 1024.

Count per channel within 1 minute

140

HV=900V

120 100 80 60 40 20 0 0

119 120

200

400

600

Channel number

800

1000

Fig. 5 A 3He counter pulse height spectrum of photoneutrons with high voltage of 900 V.

121

The count rate of the 3He counter with respect to different time delays after each pulse was measured using the above-mentioned

122

parameters. Fig. 6 illustrates the results of three different experimental scenarios, one is nothing but 4.5-meter of air before

123

photoneutrons are detected, and the other two are 2-cm-thick polyethylene and 3-cm-thick iron, respectively. Polyethylene

124

manifests a lower count rate curve as compared to iron because of its stronger neutron attenuation capability. The curves of Fig. 6

125

are of the same shapes as the simulation results in Fig. 3; the only difference is that no neutrons are detected before 50 μs in Fig.

126

6. The absence of a neutron signal from 1 μs to 50 μs can be attributed to the long recovery time after the X-ray flash in Fig. 4.

Count rate of detected neutron (1/s)

7

nothing 2cm_PE 3cm_Fe

10000

8000

6000

4000

2000

0

1

10

2

10

3

10

4

10

Time delay after each pulse(s)

127 128

Fig. 6 The count rate of photoneutrons varies with the time delay after X-ray pulse. This figure shows the curves of count rate

129

when air, 2-cm-thick polyethylene or 3-cm-thick iron are penetrated.

130

Although the 3He counter is collimated and shielded, unwanted scattered neutrons, which do not carry the information of

131

inspected materials, could still be detected. In order to ensure that the curves detected in Fig. 6 are only of penetrating neutrons,

132

the data are reprocessed and shown in Fig. 7. There are two attenuation curves in Fig. 7, one is the attenuation curve of 2-cm-thick

133

polyethylene and the other is that of 3-cm-thick iron. Both curves are derived from the count rate curve of polyethylene or iron

134

divided by the air count rate curve. The attenuation curves of polyethylene and iron behaves in a similar fashion; they are almost

135

constant from 100 μs to 1 ms, gradually increasing to unity from 1 ms to 2 ms, and statistically staying at unity after 2 ms. The

136

constant values from 100 μs to 1 ms reflect the attenuation capability of polyethylene or iron to photoneutrons, and their behavior

137

after 2 ms indicates that no penetrating photoneutrons are detected by the detector; however, background neutrons are still detected

138

in this time region. The attenuation values before 50 μs are found to be larger than unity; this erroneous behavior could be due to

139

the deep saturation of 3He counter and preamplifier caused by the intense X-ray pulse. The number of photoneutrons before 50 μs

140

is less than 5% of the total photoneutrons (Fig. 3); therefore, ignoring photoneutron signals before 50 μs might be a reasonable

141

choice.

Photoneutron attenuation

8 attenuation of 2cm_PE attenuation of 3cm_Fe

2.4

2.0

1.6

1.2

0.8

0.4

0.0 0

100 200 300 400 500 600 700 800 900 1000 2000

4000

Time delay after each X-ray pulse(s)

142 143

Fig. 7 The photoneutron attenuation of 2-cm-thick polyethylene or 3-cm-thick iron relative to air, with the time delay varied. The

144

erroneous results before 50 μs, which are larger than unity, are due to the influence of X-ray flash, because the neutron pulses

145

within this region are superimposed on the recovery slope and might not be registered for the lowered pulse height. When the 2-

146

cm-thick polyethylene or 3-cm-thick iron is placed in front of the 3He detector, the number of photons in the X-ray flash will be

147

reduced, leading to a lower recovery slope and more neutrons are registered for the relative larger pulse height. Therefore, the

148

photoneutron attenuations are calculated as larger than unity, indicating the time before 50 μs shoud be ignored.

149

Fig. 8 depicts the time scheme for the detection of photons and photoneutrons.

t X is the time duration for photon detection; t r t n is the time

150

is the recovery time that the neutron detector should wait until the influence of X-ray pulse disappears completely;

151

duration for the neutron detector to detect moderated photoneutrons. No penetrating neutrons are detected after

152

system works at a frequency of 160 Hz, so the period of T is 6.25 ms. After the measurement of photoneutrons, the neutron detector

153

needs to wait about 5 ms for measuring the photoneutrons of next pulse. T

X-pulse 1

tX tr 154

1  6.25ms 160 Hz

X-pulse 2

tn

t X  5 s

tr  45 s

tn ~ 1000  s

155

Fig. 8 Time scheme for X-rays and moderated photoneutrons detection.

156

V. MEASUREMENT OF V CURVES FOR DIFFERENT MATERIALS

157

t n . The e-LINAC

The experiment was conducted to measure the V curves of various materials. The parameters of eight different materials,

9 158

including polyethylene, melamine, urea, sugar, aluminum, iron, copper, and lead, which were penetrated by photoneutrons, are

159

presented in Table 1. The background neutron count rate was also measured when penetrating neutrons were blocked by 50-cm-

160

thick polyethylene. In order to rectify the fluctuation of dose rate of the e-LINAC among different measurements, the count rate

161

of an additional benchmark 3He counter was used to correct the count rate of the attenuated photoneutrons measured by the 3He

162

counter array. Melamine and urea were used to simulate explosives, and sugar was used to simulate common goods, and these

163

materials were contained in aluminum cases of wall thickness 0.5 mm.

164

Table 1 the samples for photoneutron transmission experiments Materials

Polyethylene

Melamine*

W ×L

Thickness

(mm)

(mm)

400 × 400

400 × 400

Comment

1, 2, 3, 5, 7, 10, 20, 30, 40, 50, 60, 70, 80, 90,

Density:

100

0.95 g/cm3

10, 20, 30, 50, 80, 130

Net mass (g): 1665, 3330, 4980, 6080, 11060, 17145

Urea*

400 × 400

10, 20, 30, 50, 80, 130

Net mass (g): 1550, 3125, 4675, 6125, 10800, 16895

Sugar*

400 × 400

10, 20, 30, 50, 80, 130

Net mass (g): 1835, 3665, 5505, 7295, 12800, 20180

Aluminum

Iron

Copper

400 × 400

400 × 400

400 × 400

1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 150, 200,

Density (g/cm3):

300, 400, 500

2.7

1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 150, 200,

Density (g/cm3):

250, 300

7.86

1, 2, 3, 5, 7, 10, 20, 30, 50, 80, 130, 200

Density (g/cm3): 8.96

Lead

Background

400 × 400

1, 2, 3, 5, 7, 10, 20, 30, 40, 50, 60, 70, 80, 90,

Density (g/cm3):

100, 140, 180, 220, 260

11.34

Neutron count rate: 0.0201 cps no material; neutrons were blocked by 50-cm-thick polyethylene.

165

* These materials were contained in aluminum cases of wall thickness 0.5 mm.

10

Photoneutron attenuation

1

Polyethylene Melamine Urea Sugar Aluminum Iron Copper Lead

0.1

0.01

0.1

1

10

100 2

Mass thickness (g/cm )

166 167

Fig. 9 The photoneutron attenuation of different materials.

168

Fig. 9 displays the attenuation curves of photoneutrons when penetrating different materials with variable mass thicknesses. All

169

neutron counts were accumulated to be greater than 10000 in order to ensure <1% statistical error. The count rate of photoneutrons

170

decreases to <3% after penetrating 10 cm of polyethylene, 20 cm of copper, and 26 cm of lead. Polyethylene manifests the strongest

171

attenuation because of its large hydrogen concentration, whereas metals demonstrate smaller attenuation capabilities. The

172

attenuation difference among organic materials, such as melamine, urea, and sugar, is inconspicuous, thus the separation of these

173

materials is difficult. It is noticed that an acceptable “gap” in attenuation capability exists between these organic materials and

174

polyethylene, and an even bigger “gap” is present between organic materials and metals.

175

Fig. 10 illustrates the results of X-ray attenuation for different materials (sugar, melamine and urea are not measured). All

176

materials manifest almost the same attenuation behavior, thus indicating that the Compton scattering is the dominating photoatomic

177

reaction in the MeV energy range for low and middle Z elements. Lead is an exception because the cross sections of photoelectric

178

absorption and pair production are proportional to Z5 or Z2, respectively. If the difference among these materials is neglected, the

179

mass thickness can be determined by the 7 MV X-ray attenuation.

180

11

7MeV X-ray attenuation

1

0.1

Polyethylene Aluminium Iron Copper Lead

0.01

1E-3 0

181 182

20

40

60

80

100

120

2

Mass thickness(g/cm )

Fig. 10 7 MV X-ray attenuation of different materials.

183

Fig. 11 illustrates the V curves of eight materials by combining the attenuation information of photoneutrons and photons

184

(background neutron count rate was subtracted from the curves in Fig. 9). It is obvious that as a hydrogen-wealthy material (14.3

185

wt% H), polyethylene produces the highest V curve, whereas melamine and urea, explosive simulants, and sugar manifests lower

186

V curves (melamine: 4.76 wt% H, urea: 6.67 wt% H, sugar: 6.43 wt% H). Lead demonstrates the lowest V curve for its large

187

photon cross section and modest neutron cross section. Aluminum yields the largest V value curve among metals, whereas the V

188

curve of copper is a little lower than aluminum and higher than iron.

189

Fig.11 also gives the simulation results for the comparison. All the experimental curves conform to the simulation curves well

190

except for aluminum. There is an appreciable difference between the simulation and experimental results. The actually used

191

aluminum sample in experiments is an alloy, not pure aluminum, which may explain this deviation. This explanation may also

192

work for the curves of copper. In the small mass thickness range, experimental results are always larger than simulation results. It

193

may be caused by the neutron spectral difference between experiments and simulations. There are more scattered low energy

194

neutrons in experiments than in simulations. The larger attenuation coefficients of slow neutrons increase the V values for small

195

mass thicknesses. The effect of scattered slow neutrons diminishes for thicker samples because they are fully attenuated.

12

PE_simu_V PE_exp_V

15

15

10

10

5

5

0

0

2

4

6

8

0

10

Urea_simu_V Urea_exp_V

15

15

10

10

0 0

2

4

6

8

10

2

Mass thickness(g/cm )

5

12

V

V

8

10

2

12

Sugar_simu_V Sugar_exp_V

0

2

4

6

8

10 2

12

Mass thickness(g/cm )

14

Fe_simu_V Fe_exp_V

1.5

2 1

1.0 0.5

0

25

50

75

100

2

Mass thickness(g/cm )

2.5

125

150

0.0

0

50

100

150

200

250

Pb_simu_V Pb_exp_V

2.0

1.5

2

Mass thickness(g/cm )

2.5

Cu_simu_V Cu_exp_V

2.0

1.5

V

V

6

2.0

3

1.0 0.5

196

4

Mass thickness(g/cm )

2.5

Al_simu_V Al_exp_V

4

0.0

2

5

5

0

0

20

V

V

2

Mass thickness (g/cm )

20

0

Melamine_simu_V Melamine_exp_V

20

V

V

20

1.0 0.5

0

50

100

150

2

Mass thickness(g/cm )

200

0.0

0

50

100

150

200

2

250

Mass thickness(g/cm )

300

197

Fig. 11 V value curves of eight different materials (“□” represents simulation results and ”○” denotes experimental results).

198

VI. DISCUSSION

199

The results of Fig. 11 suggest that hydrogen-wealthy materials, heavy metals, and organic materials can be effectively separated

200

from other metals based on their V values. The calibration curves of Fig. 10 posit that mass thicknesses of different materials could

201

be estimated by X-ray attenuation. Although there are some systematic errors, especially for high Z materials, this method of

202

determining mass thickness is practically acceptable. By combining the attenuation information of photoneutrons and photons, V

203

values of different materials can be derived. With V value and mass thickness, the objects can be identified via the curves in Fig.

204

11.

205

The current experiments were carried out to analyze only pure materials. If two or more materials are mixed together, new V

206

curves will be produced by weighing different curves; however, the main identification difficulties will depend on the

207

concentrations of different materials.

13

208 209

Fig. 12 Bi-modal imaging of 2-cm-thick steel and 1-cm-thick polyethylene: (1) Layout of overlapped 2-cm-thick steel and 1-cm-

210

thick polyethylene; (2) 7 MV X-ray image; (3) Photoneutron image; (4) Fusion of X-ray image and photoneutron image.

211

Introducing a moderation process in order to address the problem of X-ray pulse interference to photoneutrons detection can

212

cause some loss in positional information of incident neutrons. Preliminary research was conducted to build a neutron detector

213

array, which has 32 3He counters as shown in Fig. 1, together with an X-ray detector array (480 CsI detectors of 5 mm height, 10

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mm width and 20 mm length), to realize the bi-modal imaging, with the results shown in Fig. 12. It is evident that the attenuation

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of steel under the effects of X-rays and photoneutrons differs significantly from that of polyethylene. As 32 3He counters are used

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to measure neutrons from 0 cm to 160 cm in the vertical direction, the spatial resolution of the photoneutron image thus is 5 cm,

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worse than that of the X-ray image, <5 mm. Nevertheless, the performance of the system to identify materials can be significantly

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enhanced by the bi-modal imaging. In a future study, a new configuration of the neutron detector array should be investigated to

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ensure the effective measurement of photoneutrons while keeping the positional information of incident neutrons.

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VII. CONCLUSION

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The underlying principle of the current research is to combine both X-ray and photoneutron images together to enhance the

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capability of material identification within an e-LINAC system. Simulation and experimental results both confirm the feasibility

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of using two radiations coming from one e-LINAC system for the identification of different materials. Therefore, it can be inferred

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that bi-modal analysis is a promising method to separate hydrogen-wealthy materials, organic materials, low-Z metals, middle-Z

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metals, and high-Z metals, which is helpful for homeland security.

14 226

ACKNOWLEDGMENT

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This research is supported by the National Natural Science Foundation of China (Grant No. 11735008).

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The authors also thank Nuctech Company Limited for providing the accelerator and other experimental facilities for the

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experiments performed in this research.

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