- Email: [email protected]
Chemical composition identification using fast neutrons
ARTICLE IN PRESS
Applied Radiation and Isotopes 61 (2004) 73–79
Chemical composition identification using fast neutrons Davorin Sudac*, Sasha Blagus, Vladivoj Valkovic Rudjer Boskovic Institute, P.O. Box 180, Zagreb 10 002, Croatia Received 15 February 2004; accepted 27 February 2004
Abstract Measurements of characteristic gamma ray spectra resulting from A(n, n’g)A reactions induced by 14 MeV neutrons on different bulk samples (graphite, water, melamin) were performed. Of particular importance are results of experiments with objects buried in the soil. It has been shown that samples of graphite can be identified when buried under 10 cm of soil. NaI(Tl) and BaF2 scintilators were used as gamma ray detectors. Background radiation was reduced by the associated alpha particle technique. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hidden substances; Associated alpha particle technique; Nonelastic neutron scattering; Gamma spectroscopy
1. Introduction Nuclear analytical methods for location and chemical composition identification of hidden objects (explosives, mines, drugs, etc) in various consignments (soil, baggage, shipping containers) show a number of advantages in comparison with other methods. The penetrating ability of neutrons and g-rays provide an effective way for measuring the elemental content of interrogated material. In particular, nonelastic scattering of 14 MeV neutrons and detection of characteristic g-rays generated by A(n, n’g)A reactions on the constituent’s chemical elements (C, N, O, Si, Cl, etc), can be an efficient method for identification and location of hidden substances. Since neutrons interact not only with interrogated material, but also with all surrounding materials, the reduction of background radiation is a key point in the practical use of fast neutrons. This background radiation can be strongly suppressed (Nebbia et al., 2001), using the associated alpha particle method which was proposed and used many years ago (Miljanic et al., 1969; Valkovic et al., 1969) in neutron-
charged particle coincidence measurements from 14 MeV neutron-induced reactions. The associated particle technique is based on the electronic ‘‘collimation’’ of the neutron beam generated by the 3H(d, n)4He reaction by detecting alpha particles emitted into a known solid angle. The neutrons ‘‘tagged’’ in this way interact with interrogated objects and produce g-rays by A(n, n’g)A processes. The measurement of the time difference in the detection of g-rays and alpha particles allows determinaton of the distance traveled by the neutron. As its direction is also known, three-dimensional spatial position of the hidden substance can be discovered in the interrogated object. In comparison with the previous work (Blagus et al., 2004), the electronic setup was improved and in addition with NaI(Tl) scintilator, a BaF2 scintilator, having better timing characteristics, was used. In this work, the measured energy spectra of different substances and comparision between two g-ray detectors are presented.
2. Experimental setup *Corresponding author. Tel.: +385-1-4561-111; fax: +3851-4680-239. E-mail address: [email protected] (D. Sudac).
3
The neutron beam was obtained by means of the H(d,n)4He reaction, by bombarding a water-cooled
0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.02.019
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74
571 SA 473 A CFD
454 TFA
Delay amp. 427 A
Preamp. 113 Delay line 425 A STOP 100 ns range
PHILIPS XP-2020
PCI-9812/10 and Computer
LG 1 567 TAC
416 A GDG
LG 2
START Delay amp. 427 A
Preamp. 113 473 A CFD
TN1705 MCA or Oscilloscope
571 SA
Fig. 1. The electronic setup.
120
C 4.44
1 escape
NaI(Tl)
Number of counts
100
80
60
2 escape 40
20
(a)
0
Carbon
100
Number of counts
standard tritium target with a 150 keV deuteron beam using a Texas Nuclear Corporation 300 keV electrostatic accelerator. The active part of the target was 6 mm in diameter and consisted of titanium (0.95 mg/cm2) deposited on a copper backing (7 mm in diameter and 0.3 mm thickness). The tritium activity was B30 GBq/ cm2. The associated alpha particle detector for fast coincidence measurement was a 0.5 mm thin Pilot B fast organic scintillator (Nuclear Enterprises Inc., Edinburgh, Scotland) with decay time 1.8 ns, coupled via a perspex light guide to a Philips XP2020 photomultiplier tube which has a much better timing and gain characteristics than the previously used RCA-6342A tube (Blagus et al., 2004). Its position (81.7 relative to the deuteron beam) was fixed in such a way that the associated neutron beam was in the vertical plane at 90 relative to the deuteron beam. The 18 mm diameter slit in front of the scintillator was used to define the geometrical dimensions of the neutron beam on the interrogated object. In front of the scintillator a 7 mm Al foil was mounted to prevent the scattered deuterons, light and secondary electrons from reaching the scintillator. Since the scintillation detector was maintained at a distance of 110 mm from the tritium target, for the slit diameter of 18 mm, the diameter of the neutron spot at the position of the interrogated object, at a distance of 500 mm from tritium target was about 100 mm. The distance between the tritium target and interrogated object was 500 mm. The position and
BaF2
80
60
40
20
0 0
(b)
1
2
3
4
5
6
7
8
Energy (MeV)
Fig. 2. The g-ray spectra of graphite as obtained by using (a) NaI(Tl) detector (NT=0.83 108) and (b) BaF2 detector (NT=0.57 108).
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79
approximate dimension of the ‘‘neutron beam’’ was checked by means of an NE-213 neutron detector in coincidence with the associated alpha particle detector. Neutron beam monitoring was accomplished by means of a silicon photodiode maintained at 135 relative to the deuteron beam. In the first series of experiments a commercial, cylindrical 10.16 cm NaI(Tl) detector attached to a 7.62 cm RCA-8054 photomultiplier was used as the g-ray detector. The detector was placed at a distance of 400 mm from the interrogated object. Its energy calibration and energy resolution measurement were performed with a standard g calibration source 60Co (1.17 and 1.33 MeV) and sum peak 2.5 MeV, of 319 kBq activity. The energy resolution for 60Co (1.33 MeV) was about 6%. During the irradiation, the g-ray detector was only shielded by 70 mm of lead. In the second series of experiments, the hexagonal BaF2 crystal (a=50 mm, d=100 mm, v=160 mm) coupled to a photomultiplier tube was used as the g-ray detector. Again, its energy calibration and energy resolution (17.5%) measurement were performed with the standard g calibration 60Co source. Standard Ortec NIM modules were used. In the experimental set-up a XP2020 photomultiplier was
attached to Model 269 photomultiplier base and g-ray detectors were attached to a Model 276 photomultiplier base and preamplifier, in the case of NaI(Tl) detector, or to a Model 113 preamplifier in the case of the BaF2 detector. For the purpose of fast timing the pulses from the anodes of the g-ray detector and associated alpha particle detector were extracted, while the linear pulses were collected from the preamplifier output. In order to measure the a–g coincidence spectra, the signal from the anode of the alpha particle detector photomultiplier was fed via a Model 473A constant fraction discriminator to the Model 567 TAC (time to amplitude converter) as the start signal, and in the same way, the signal from the anode of the g-ray detector was fed via a Model 454 timing filter amplifier through a Model 473A constant fraction amplifier to the TAC as the stop signal. The selection of a–g coincidences was performed within 100 ns TAC range. The energy signal from the g-ray detector and the time spectrum from the TAC were gated using the Model 426 linear gates by the logic pulse from the TAC and recorded via Nu DAQ Card PCI9812/10 (AD Link Technology Inc., Taiwan) in two parameter list mode in the memory of the computer for on-line analysis. The experimental setup is presented in Fig. 1.
120
NaI(Tl)
O 6.13
100
C 1 escape
N 2.31
100
O 3.79, 3.84
150
NaI(Tl)
1 escape
Number of counts
Number of counts
O 1 escape 3.44
O 2.74
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N 1.635 80
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N 5.105
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N 6.2
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(a)
(a)
0 120
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BaF2
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Oxygen
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Nitrogen
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20 20
0
0 0
(b)
1
2
3
4
5
6
7
0
8
Energy (MeV)
Fig. 3. The g-ray spectra of water as obtained by using (a) NaI(Tl) detector (NT=1.4 108) and (b) BaF2 detector (NT=2.9 108).
(b)
1
2
3
4
5
6
7
8
Energy (MeV)
Fig. 4. The g-ray spectra of melamine (C3H6N6) as obtained by using (a) NaI(Tl) detector (NT=1.4 108) and (b) BaF2 detector (NT=1.1 108).
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form the bump in the region between 5 and 7 MeV.On the lower energy part of the spectrum, the oxygen at the 2.74 MeV line can be easily observed. The 3.79 and 3.84 MeV lines are not separated because of detector resolution, but together form one clearly visible peak. Their 1st escape peak is folded over the 3.44 MeV peak. Fig. 3b shows the g-ray spectrum of water as obtained by the BaF2 detector we used. Only two bumps can be observed. The one is in the region of 2–4 MeV and the second one in the region of 5–7 MeV. Fig. 4 shows the g-ray spectra of the 2 kg sample of melamine (C3H6N6). In the case of the Na(Tl) detector (Fig 4a), a very prominent peak caused by C 4.44 MeV line, together with associated 1st escape peak, is observed. N 1.635 MeV and N 2.31 MeV peaks are also clearly visible. In the energy region above 4.5 MeV broad features which could be ascribed to N 5.105 MeV and N 6.2 MeV lines can be observed. As can be seen from the spectrum in Fig. 4b, obtained by the BaF2 detector, only broad features in the place of the abovementioned carbon and nitrogen peaks can be observed. Fig. 5 shows the g-ray spectrum of graphite (0.86 kg, 10 10 5 cm3 ) inserted into a box of soil and the g-ray spectrum of the box of soil alone. Two g-ray spectra were collected. The first one was obtained with graphite inserted into the box of soil and the second one was
3. Measurement and results Different bulk samples: 4.44 kg of graphite, 2 l of water contained in a bottle with a thin plastic wall 2 kg of melamine (C3 H6 N6 ) were irradiated by the 14.12 MeV neutron flux (B107 /4ps) and coincidence gamma ray spectra were collected. The experiment with a sample of 0.86 kg graphite (10 10 5 cm3 ) under 10 cm of soil was also performed. The results of these measurements are presented in Figs. 2–7. In all figures the total number of tagged neutrons is marked as NT. Fig. 2 shows the g-ray spectra of the 4.44 kg sample of carbon. In the case of Na(Tl) detector (Fig. 2a), a very prominent peak caused by C 4.44 MeV line together with associated 1st and 2nd escape peaks is observed. In the case of the BaF2 detector used (Fig. 2b), only broad features in the place of the above-mentioned carbon peaks can be observed. Fig. 3 shows the g-ray spectra of water. Fig. 3a is the g-ray spectrum of water obtained by NaI(Tl) detector. Only peaks associated with oxygen can be observed. On the higher-energy part of the spectrum, the 6.13 MeV peak together with the associated 1st escape peak are clearly visible. The 7.11 and 6.92 MeV lines and their associated escape peaks are not clearly visible, but together with the 2nd escape peak of the 6.13 MeV line
300
250
1.8 Silicon
Number of counts
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150
Oxygen
Carbon
100
Oxygen 50
0 0
1
2
3
4
5
6
7
8
Energy (MeV)
Carbon in soil Soil alone Carbon in soil - Soil alone
Fig. 5. The result of experiment with 10 10 5 cm3 graphite cube inserted into the box of soil. The gamma ray spectra were collected with BaF2 detector (NT=1.4 108). The whole TAC peak was taken considerated.
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79 350
1
3
Neutron beam
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250
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77
Box of soil
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Graphite
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100
50
(a)
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2 1
Box of soil Number of counts
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Neutron beam
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1
200
2 3
100
0 0
10
20
30
40
(b)
50
60
70
80
90
100
Time (ns)
Fig. 6. TAC spectra for a–g coincidences from neutron bombardment of (a) graphite (0.86 kg, 10 10 5 cm3) inserted into the box of soil and (b) the box of soil alone.
obtained as background with the soil alone. The g-ray spectra were separately analyzed in the usual way (Blagus et al., 2004) and then subtracted. Fig. 5 shows the final result. The broad features in the place of the carbon peaks are clearly visible although the box full of soil is not a ‘‘target out’’ situation since the graphite is now replaced by soil. Fig. 6a shows the TAC spectrum of graphite (0.86 kg, 10 10 5 cm3 ) inserted into the box of soil and Fig. 6b shows the TAC spectrum of the box of soil alone. The part of timing spectrum belonging to the g–a coincidence was divided into 1 ns wide slices corresponding
to 5 cm of the flight path for 14.12 MeV neutrons and each was analyzed in the usual way (but only the random BGD was subtracted from the sample spectra). The insert in Fig. 6 designates the position of the flight path for each time slice. Fig. 7(a)–(f), shows the change of the g-ray spectra for different slices of the TAC peak. For TAC regions 1 and 2 only the broad features belonging to oxygen lines from soil are visible in both of the cases. In the TAC region 3 oxygen is also present in the case of the soil alone, but in the second case broad features belonging to oxygen is replaced with the broad feature belonging to carbon.
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79
78 30
(a) carbon in soil
(b) soil alone
interval 1
25
interval 1
Number of counts
Oxygen Oxygen 20
15
10
5
0
(c) carbon in soil
Number of counts
40
Oxygen
interval 2
(d) soil alone interval 2
Oxygen 30
20
10
0
(e) carbon in soil
50
Number of counts
(f ) soil alone
interval 3
Carbon
interval 3
40
Oxygen 30
20
10
0 0
1
2
3
4
5
6
7
Energy (MeV)
8
1
3
5
7
Energy (MeV)
Fig. 7. The g-ray spectra obtained for different windows on TAC peaks. Graphite cube was placed under 10 cm of dry soil (NT=1.4 108).
4. Conclusions The results presented here show that it is quite possible to perform strong background reduction using the associated alpha particle technique. Carbon and oxygen lines are clearly visible in all spectra. Nitrogen lines are clearly visible in melamine. Relatively high value of oxygen in dry soil (B40% depend on soil type) (Obhodas et al., 2002) and soil moisture in a real minefield (B20% depend on location and season) (Obhodas et al., 2003) can cause a problem with oxygen identification. The BaF2 detector used here has a very bad resolution, but its timing characteristic is very good (Sudac et al., 2003). There are other problems connected
with this approach as well. One is sensitivity variation with geometry because of the inverse square dependence of neutron flux and intensity of detected gammas. Second is the limitation of using the low neutron flux in order to reduce random coincidences. This means that the measurements would be time consuming. The possible solution of these problems is to combine associated alpha particle technique and irradiation with nanosecond pulsed fast neutron beam. During the pulse duration, the g-ray spectra from inelastic neutron scattering could be collected, while between the neutron pulses the activation gamma rays could be counted. Future efforts should proceed in this direction.
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Acknowledgements This work was supported by IAEA Research Contract No 10855/RO ‘‘A feasibility study of landmine detection with 14 MeV neutrons’’.
References Blagus, S., Sudac, D., Valkovic, V., 2004. Hidden substances identification by detection of fast neutron induced g rays using associated a particle technique. Nucl. Instrum. Methods B 213, 434–438. Miljanic, Dj., Antolkovic, B., Valkovic, V., 1969. Applications of time measurements to charged particle detection in reactions with 14.4 Mev neutrons. Nucl. Instrum. Methods 76, 23–428. Nebbia, G., Lunardon, M., Pesente, S., Viesti, G., Barbui, M., Cinausero, M., Fioretto, E., Prete, G., Pantaleo, A., D’Erasmo, G., Palomba, M., Coco, I., Filippini, V., Tiana, M., Lazzizziera, I., 2001. Development of a new, large area scanning system for the detection of hidden explosive using tagged 14 MeV Neutron. Presented at Seventh International Conference on Applications of Nuclear Techniques ‘‘Nuclear
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and Atomic Industrial & Analytical Applications’’, Crete, Greece, June 17–23. Obhodas, J., Sudac, D., Nad, K., Valkovic, V., Nebbia, G., Viesti, G., 2002. The role of soil in NBT applications to landmine detection problem, presented at 17th International Conference on the Application of Accelerators in Research and Industry CAARI2002, November 12–16, Denton, TX, USA. Obhodas, J., Sudac, D., Valkovic, V., 2003. The role of soil in landmine detection problem. Proceedings from the International Conference on Requirements and Technologies for the Detection, Removal and Neutralization of Landmines and UXO, Vol. 1, Brussels, Belgium, September 15–18, pp. 101–106. Sudac, D., Blagus, S., Matika, D., Kollar, R., Grivicic, T., Valkovic, V., 2003. The Use of the 14 MeV neutrons in the explosive detaction. Proceedings from the International Conference on Requirements and Technologies for the Detection, Removal and Neutralization of Landmines and UXO, Vol.2, Brussels, Belgium, September 15–18, pp. 749–754 Valkovic, V., Miljanic, Dj., Tomas, P., Antolkovic, B., Furic, M., 1969. Neutron-charged particle coincidence measurements from 14.1 Mev neutron induced reactions. Nucl. Instrum. Methods 76, 29–34.
Applied Radiation and Isotopes 61 (2004) 73–79
Chemical composition identification using fast neutrons Davorin Sudac*, Sasha Blagus, Vladivoj Valkovic Rudjer Boskovic Institute, P.O. Box 180, Zagreb 10 002, Croatia Received 15 February 2004; accepted 27 February 2004
Abstract Measurements of characteristic gamma ray spectra resulting from A(n, n’g)A reactions induced by 14 MeV neutrons on different bulk samples (graphite, water, melamin) were performed. Of particular importance are results of experiments with objects buried in the soil. It has been shown that samples of graphite can be identified when buried under 10 cm of soil. NaI(Tl) and BaF2 scintilators were used as gamma ray detectors. Background radiation was reduced by the associated alpha particle technique. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hidden substances; Associated alpha particle technique; Nonelastic neutron scattering; Gamma spectroscopy
1. Introduction Nuclear analytical methods for location and chemical composition identification of hidden objects (explosives, mines, drugs, etc) in various consignments (soil, baggage, shipping containers) show a number of advantages in comparison with other methods. The penetrating ability of neutrons and g-rays provide an effective way for measuring the elemental content of interrogated material. In particular, nonelastic scattering of 14 MeV neutrons and detection of characteristic g-rays generated by A(n, n’g)A reactions on the constituent’s chemical elements (C, N, O, Si, Cl, etc), can be an efficient method for identification and location of hidden substances. Since neutrons interact not only with interrogated material, but also with all surrounding materials, the reduction of background radiation is a key point in the practical use of fast neutrons. This background radiation can be strongly suppressed (Nebbia et al., 2001), using the associated alpha particle method which was proposed and used many years ago (Miljanic et al., 1969; Valkovic et al., 1969) in neutron-
charged particle coincidence measurements from 14 MeV neutron-induced reactions. The associated particle technique is based on the electronic ‘‘collimation’’ of the neutron beam generated by the 3H(d, n)4He reaction by detecting alpha particles emitted into a known solid angle. The neutrons ‘‘tagged’’ in this way interact with interrogated objects and produce g-rays by A(n, n’g)A processes. The measurement of the time difference in the detection of g-rays and alpha particles allows determinaton of the distance traveled by the neutron. As its direction is also known, three-dimensional spatial position of the hidden substance can be discovered in the interrogated object. In comparison with the previous work (Blagus et al., 2004), the electronic setup was improved and in addition with NaI(Tl) scintilator, a BaF2 scintilator, having better timing characteristics, was used. In this work, the measured energy spectra of different substances and comparision between two g-ray detectors are presented.
2. Experimental setup *Corresponding author. Tel.: +385-1-4561-111; fax: +3851-4680-239. E-mail address: [email protected] (D. Sudac).
3
The neutron beam was obtained by means of the H(d,n)4He reaction, by bombarding a water-cooled
0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.02.019
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79
74
571 SA 473 A CFD
454 TFA
Delay amp. 427 A
Preamp. 113 Delay line 425 A STOP 100 ns range
PHILIPS XP-2020
PCI-9812/10 and Computer
LG 1 567 TAC
416 A GDG
LG 2
START Delay amp. 427 A
Preamp. 113 473 A CFD
TN1705 MCA or Oscilloscope
571 SA
Fig. 1. The electronic setup.
120
C 4.44
1 escape
NaI(Tl)
Number of counts
100
80
60
2 escape 40
20
(a)
0
Carbon
100
Number of counts
standard tritium target with a 150 keV deuteron beam using a Texas Nuclear Corporation 300 keV electrostatic accelerator. The active part of the target was 6 mm in diameter and consisted of titanium (0.95 mg/cm2) deposited on a copper backing (7 mm in diameter and 0.3 mm thickness). The tritium activity was B30 GBq/ cm2. The associated alpha particle detector for fast coincidence measurement was a 0.5 mm thin Pilot B fast organic scintillator (Nuclear Enterprises Inc., Edinburgh, Scotland) with decay time 1.8 ns, coupled via a perspex light guide to a Philips XP2020 photomultiplier tube which has a much better timing and gain characteristics than the previously used RCA-6342A tube (Blagus et al., 2004). Its position (81.7 relative to the deuteron beam) was fixed in such a way that the associated neutron beam was in the vertical plane at 90 relative to the deuteron beam. The 18 mm diameter slit in front of the scintillator was used to define the geometrical dimensions of the neutron beam on the interrogated object. In front of the scintillator a 7 mm Al foil was mounted to prevent the scattered deuterons, light and secondary electrons from reaching the scintillator. Since the scintillation detector was maintained at a distance of 110 mm from the tritium target, for the slit diameter of 18 mm, the diameter of the neutron spot at the position of the interrogated object, at a distance of 500 mm from tritium target was about 100 mm. The distance between the tritium target and interrogated object was 500 mm. The position and
BaF2
80
60
40
20
0 0
(b)
1
2
3
4
5
6
7
8
Energy (MeV)
Fig. 2. The g-ray spectra of graphite as obtained by using (a) NaI(Tl) detector (NT=0.83 108) and (b) BaF2 detector (NT=0.57 108).
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79
approximate dimension of the ‘‘neutron beam’’ was checked by means of an NE-213 neutron detector in coincidence with the associated alpha particle detector. Neutron beam monitoring was accomplished by means of a silicon photodiode maintained at 135 relative to the deuteron beam. In the first series of experiments a commercial, cylindrical 10.16 cm NaI(Tl) detector attached to a 7.62 cm RCA-8054 photomultiplier was used as the g-ray detector. The detector was placed at a distance of 400 mm from the interrogated object. Its energy calibration and energy resolution measurement were performed with a standard g calibration source 60Co (1.17 and 1.33 MeV) and sum peak 2.5 MeV, of 319 kBq activity. The energy resolution for 60Co (1.33 MeV) was about 6%. During the irradiation, the g-ray detector was only shielded by 70 mm of lead. In the second series of experiments, the hexagonal BaF2 crystal (a=50 mm, d=100 mm, v=160 mm) coupled to a photomultiplier tube was used as the g-ray detector. Again, its energy calibration and energy resolution (17.5%) measurement were performed with the standard g calibration 60Co source. Standard Ortec NIM modules were used. In the experimental set-up a XP2020 photomultiplier was
attached to Model 269 photomultiplier base and g-ray detectors were attached to a Model 276 photomultiplier base and preamplifier, in the case of NaI(Tl) detector, or to a Model 113 preamplifier in the case of the BaF2 detector. For the purpose of fast timing the pulses from the anodes of the g-ray detector and associated alpha particle detector were extracted, while the linear pulses were collected from the preamplifier output. In order to measure the a–g coincidence spectra, the signal from the anode of the alpha particle detector photomultiplier was fed via a Model 473A constant fraction discriminator to the Model 567 TAC (time to amplitude converter) as the start signal, and in the same way, the signal from the anode of the g-ray detector was fed via a Model 454 timing filter amplifier through a Model 473A constant fraction amplifier to the TAC as the stop signal. The selection of a–g coincidences was performed within 100 ns TAC range. The energy signal from the g-ray detector and the time spectrum from the TAC were gated using the Model 426 linear gates by the logic pulse from the TAC and recorded via Nu DAQ Card PCI9812/10 (AD Link Technology Inc., Taiwan) in two parameter list mode in the memory of the computer for on-line analysis. The experimental setup is presented in Fig. 1.
120
NaI(Tl)
O 6.13
100
C 1 escape
N 2.31
100
O 3.79, 3.84
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NaI(Tl)
1 escape
Number of counts
Number of counts
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O 2.74
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N 1.635 80
60
N 5.105
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N 6.2
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Number of counts
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40
Nitrogen
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20 20
0
0 0
(b)
1
2
3
4
5
6
7
0
8
Energy (MeV)
Fig. 3. The g-ray spectra of water as obtained by using (a) NaI(Tl) detector (NT=1.4 108) and (b) BaF2 detector (NT=2.9 108).
(b)
1
2
3
4
5
6
7
8
Energy (MeV)
Fig. 4. The g-ray spectra of melamine (C3H6N6) as obtained by using (a) NaI(Tl) detector (NT=1.4 108) and (b) BaF2 detector (NT=1.1 108).
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form the bump in the region between 5 and 7 MeV.On the lower energy part of the spectrum, the oxygen at the 2.74 MeV line can be easily observed. The 3.79 and 3.84 MeV lines are not separated because of detector resolution, but together form one clearly visible peak. Their 1st escape peak is folded over the 3.44 MeV peak. Fig. 3b shows the g-ray spectrum of water as obtained by the BaF2 detector we used. Only two bumps can be observed. The one is in the region of 2–4 MeV and the second one in the region of 5–7 MeV. Fig. 4 shows the g-ray spectra of the 2 kg sample of melamine (C3H6N6). In the case of the Na(Tl) detector (Fig 4a), a very prominent peak caused by C 4.44 MeV line, together with associated 1st escape peak, is observed. N 1.635 MeV and N 2.31 MeV peaks are also clearly visible. In the energy region above 4.5 MeV broad features which could be ascribed to N 5.105 MeV and N 6.2 MeV lines can be observed. As can be seen from the spectrum in Fig. 4b, obtained by the BaF2 detector, only broad features in the place of the abovementioned carbon and nitrogen peaks can be observed. Fig. 5 shows the g-ray spectrum of graphite (0.86 kg, 10 10 5 cm3 ) inserted into a box of soil and the g-ray spectrum of the box of soil alone. Two g-ray spectra were collected. The first one was obtained with graphite inserted into the box of soil and the second one was
3. Measurement and results Different bulk samples: 4.44 kg of graphite, 2 l of water contained in a bottle with a thin plastic wall 2 kg of melamine (C3 H6 N6 ) were irradiated by the 14.12 MeV neutron flux (B107 /4ps) and coincidence gamma ray spectra were collected. The experiment with a sample of 0.86 kg graphite (10 10 5 cm3 ) under 10 cm of soil was also performed. The results of these measurements are presented in Figs. 2–7. In all figures the total number of tagged neutrons is marked as NT. Fig. 2 shows the g-ray spectra of the 4.44 kg sample of carbon. In the case of Na(Tl) detector (Fig. 2a), a very prominent peak caused by C 4.44 MeV line together with associated 1st and 2nd escape peaks is observed. In the case of the BaF2 detector used (Fig. 2b), only broad features in the place of the above-mentioned carbon peaks can be observed. Fig. 3 shows the g-ray spectra of water. Fig. 3a is the g-ray spectrum of water obtained by NaI(Tl) detector. Only peaks associated with oxygen can be observed. On the higher-energy part of the spectrum, the 6.13 MeV peak together with the associated 1st escape peak are clearly visible. The 7.11 and 6.92 MeV lines and their associated escape peaks are not clearly visible, but together with the 2nd escape peak of the 6.13 MeV line
300
250
1.8 Silicon
Number of counts
200
150
Oxygen
Carbon
100
Oxygen 50
0 0
1
2
3
4
5
6
7
8
Energy (MeV)
Carbon in soil Soil alone Carbon in soil - Soil alone
Fig. 5. The result of experiment with 10 10 5 cm3 graphite cube inserted into the box of soil. The gamma ray spectra were collected with BaF2 detector (NT=1.4 108). The whole TAC peak was taken considerated.
ARTICLE IN PRESS D. Sudac et al. / Applied Radiation and Isotopes 61 (2004) 73–79 350
1
3
Neutron beam
300
250
Number of counts
2
77
Box of soil
1 2
200
Graphite
3
150
100
50
(a)
0
2 1
Box of soil Number of counts
3
Neutron beam
300
1
200
2 3
100
0 0
10
20
30
40
(b)
50
60
70
80
90
100
Time (ns)
Fig. 6. TAC spectra for a–g coincidences from neutron bombardment of (a) graphite (0.86 kg, 10 10 5 cm3) inserted into the box of soil and (b) the box of soil alone.
obtained as background with the soil alone. The g-ray spectra were separately analyzed in the usual way (Blagus et al., 2004) and then subtracted. Fig. 5 shows the final result. The broad features in the place of the carbon peaks are clearly visible although the box full of soil is not a ‘‘target out’’ situation since the graphite is now replaced by soil. Fig. 6a shows the TAC spectrum of graphite (0.86 kg, 10 10 5 cm3 ) inserted into the box of soil and Fig. 6b shows the TAC spectrum of the box of soil alone. The part of timing spectrum belonging to the g–a coincidence was divided into 1 ns wide slices corresponding
to 5 cm of the flight path for 14.12 MeV neutrons and each was analyzed in the usual way (but only the random BGD was subtracted from the sample spectra). The insert in Fig. 6 designates the position of the flight path for each time slice. Fig. 7(a)–(f), shows the change of the g-ray spectra for different slices of the TAC peak. For TAC regions 1 and 2 only the broad features belonging to oxygen lines from soil are visible in both of the cases. In the TAC region 3 oxygen is also present in the case of the soil alone, but in the second case broad features belonging to oxygen is replaced with the broad feature belonging to carbon.
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78 30
(a) carbon in soil
(b) soil alone
interval 1
25
interval 1
Number of counts
Oxygen Oxygen 20
15
10
5
0
(c) carbon in soil
Number of counts
40
Oxygen
interval 2
(d) soil alone interval 2
Oxygen 30
20
10
0
(e) carbon in soil
50
Number of counts
(f ) soil alone
interval 3
Carbon
interval 3
40
Oxygen 30
20
10
0 0
1
2
3
4
5
6
7
Energy (MeV)
8
1
3
5
7
Energy (MeV)
Fig. 7. The g-ray spectra obtained for different windows on TAC peaks. Graphite cube was placed under 10 cm of dry soil (NT=1.4 108).
4. Conclusions The results presented here show that it is quite possible to perform strong background reduction using the associated alpha particle technique. Carbon and oxygen lines are clearly visible in all spectra. Nitrogen lines are clearly visible in melamine. Relatively high value of oxygen in dry soil (B40% depend on soil type) (Obhodas et al., 2002) and soil moisture in a real minefield (B20% depend on location and season) (Obhodas et al., 2003) can cause a problem with oxygen identification. The BaF2 detector used here has a very bad resolution, but its timing characteristic is very good (Sudac et al., 2003). There are other problems connected
with this approach as well. One is sensitivity variation with geometry because of the inverse square dependence of neutron flux and intensity of detected gammas. Second is the limitation of using the low neutron flux in order to reduce random coincidences. This means that the measurements would be time consuming. The possible solution of these problems is to combine associated alpha particle technique and irradiation with nanosecond pulsed fast neutron beam. During the pulse duration, the g-ray spectra from inelastic neutron scattering could be collected, while between the neutron pulses the activation gamma rays could be counted. Future efforts should proceed in this direction.
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Acknowledgements This work was supported by IAEA Research Contract No 10855/RO ‘‘A feasibility study of landmine detection with 14 MeV neutrons’’.
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