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Performance characteristics of a prompt gamma-ray activation analysis (PGAA) system equipped with a new compact D–D neutron generator
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Performance characteristics of a prompt gamma-ray activation analysis (PGAA) system equipped with a new compact D–D neutron generator Yong Joon Park, Byung Chul Song, Hee-Jung Im, Jong-Yun Kim Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Dukjin-dong 150-1, Yuseong-gu, Daejeon 305-353, Republic of Korea
a r t i c l e in fo
abstract
Article history: Received 3 July 2008 Received in revised form 21 April 2009 Accepted 21 April 2009 Available online 3 May 2009
A new prompt gamma-ray activation analysis (PGAA) system equipped with a compact deuterium– deuterium (D–D) neutron generator has been developed for fast detection of explosives and chemical warfare agents. The PGAA system was built based on a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency (RF)-driven ion source. The ionic current of the compact neutron generator was determined as a function of the acceleration voltage at various RF powers. Monoenergetic neutrons (2.45 MeV) with a neutron yield of 41 107 n/s were obtained at a deuterium pressure of 8.0 mTorr, an acceleration voltage of 80 kV, and an RF power of 1.1 kW. The performance of the PGAA system was examined by studying the dependence of a prompt gamma-ray count rate on crucial operating parameters. & 2009 Elsevier B.V. All rights reserved.
Keywords: Neutron generator Deuterium–deuterium fusion reaction Prompt gamma-ray activation analysis Neutron yield
1. Introduction The development of a non-destructive technology that can detect illicit materials within a very short period of time has become an important issue for analytical chemists in the area of practical cargo and luggage inspection, since seaports, airports and road border crossing points are very attractive targets for terrorists and contraband activities [1]. Among the many nondestructive methods, neutron-based methods are the most promising because they can provide information about the elemental composition of bulk materials for the detection of drugs, toxic agents, fissile materials, explosives, and chemical warfare agents [2,3]. Neutron-based methods can be categorized into three main classes: thermal-neutron analysis (TNA), fast neutron analysis (FNA), and neutron moderation. The prompt gamma-ray activation analysis (PGAA) technique used both in FNA and TNA is an attractive quantitative method especially for determining the light elements in bulk materials. Two types of neutron sources are generally used in a practical PGAA system: radioisotope neutron sources or electronic neutron generators. Compared to the radioisotope as a neutron source, a neutron generator is more environmentally friendly, safer for operators, and more sensitive for an elemental analysis; it is also easier to control the neutron characteristics such as the neutron yield, the pulse repetition rate, and the duration [4]. In a field application, equipment size, the stability of the neutron generation, analytical
Corresponding author.
E-mail address: [email protected] (J.-Y. Kim). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.04.022
speed and cost should also be taken into account. The thermalneutron yield is critical for the neutron capture reaction in TNA, while the neutron energy needed to induce the nuclear reaction by incident fast neutrons is important in FNA. According to a previous study [5], the deuterium–deuterium (D–D) neutron generator and 252Cf are much more efficient in TNA compared to the deuterium-tritium (D–T) neutron generator and the 241Am–Be source for the detection of elements of interest. This is because the lower energy neutrons produced by the D–D neutron generator and 252Cf are easily moderated by the presence of hydrogenous materials surrounding the element of interest. Consequently, they are widely used as lower energy neutron sources in TNA, whereas the D–T neutron generators are not adequate for use in TNA because the fast neutrons are not easily slowed by the surrounding medium. Based on our experience with the HANARO PGAA facility at the Korea Atomic Energy Research Institute (KAERI) [6–8], we have recently developed a neutron-induced prompt gamma system (NIPS) utilizing 252Cf as a neutron source [9,10]. Recently, we have developed a new PGAA system at KAERI (KAERI-PGAA), which is equipped with a fully high-voltage-shielded, axial deuterium– deuterium neutron generator (Fig. 1) developed by Plasma and Ion Source Technology Group (P&ISTG) at Lawrence Berkeley National Laboratory (LBNL) [11–14]. In the present study, the neutron yield of a D–D neutron generator as a safer neutron source compared to the radioactive deuterium–tritium or tritium–tritium (T–T) neutron generators was measured using 3He proportional counters to determine the optimum operating conditions for PGAA. The intensity of the prompt gamma-ray peak of chlorine (Cl) at 1951 keV under the optimum conditions for neutron
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Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
Explosively bonded titanium on copper target
Water-cooling lines for the target cooling External antenna driven, water-cooled RF-induction ion source
High voltage feed-through
Fig. 1. Design of compact axial, RF-driven D–D neutron generator [14].
generation was measured as a representative example for the performance test of the PGAA system.
2. Experiment and methods 2.1. Structural design of the KAERI-PGAA system In this study, neutrons were generated via the deuterium– deuterium fusion reaction by an axial D–D neutron generator composed of three main components: a radio frequency (RF)driven ion source with an external antenna, a diode ion beam accelerator, and an explosively-bonded titanium-on-copper target. The layout of the neutron generator is shown in Fig. 1. Atomic and molecular deuterium ions were produced by an RF discharge of 99.995% pure deuterium gas (Messer Gas Technologies, Malvern, PA, USA). The deuterium ions were accelerated onto a titanium target with a single gap DC accelerator to produce nearly monoenergetic 2.5 MeV neutrons by the reaction shown in Eq. (1). 2 1H
þ 21 H ! 32 He þ 10 n Q -value ¼ 3:27 MeV
Shielded neutron generator
vacuum pump
PE Shielding ion source
(1)
Fig. 2 shows a compact D–D neutron generator placed in the centre of a radiation shield/moderator structure consisting of three layers of 300-mm-thick polyethylene, 7-mm-thick polyethylene doped with 9% boron, and a 1.5-mm-thick lead sheet in order to shield both the neutron and secondary gammarays. A high-purity germanium (HPGe) detector was axially placed at a distance of 25 cm from the titanium target in order to acquire the gamma-ray spectra. The HPGe detector was shielded with 5-cm-thick lead blocks. 2.2. Measurement of the ion current and the neutron yield of the D–D neutron generator The ion current is a function of many parameters such as the gas pressure in the ion source, the RF frequency, and the RF power. These parameters affect the atomic ion ratio of the molecular gases, such as hydrogen and its isotopes, noise related to the RF leakage, high-voltage breakdown, and plasma density. The ionic current was measured as a function of the accelerator voltage
gamma detector
neutron generator Fig. 2. KAERI-NIPS system. (a) Installation of KAERI-NIPS system in the controlled area (b) equipped with neutron generator placed inside the shield and (c) gammaray detector shielded with polyethylene and Pb.
(from 40 to 100 kV), and the RF power (from 0.6 to 1.5 kW) was measured at a fixed RF frequency of 13.56 MHz. The D–D neutron yield was measured using a 3He gas proportional counter, and comparing the count rate to that of a 252Cf source measured in the same configuration of the radiation shield/moderator structure [15]. This measurement was used because both neutron sources are frequently used in TNA, and the average neutron energy values of a 252Cf source (2.3 MeV) with a peak maximum between 0.5 and 1.0 MeV is very similar to that of our monoenergetic 2.5 MeV D–D neutron generator. If the count rate of the D–D neutron generator and 252Cf with a known neutron yield is measured, the neutron yield of the neutron generator can then be obtained using
ARTICLE IN PRESS Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
the following simple equations: with f ¼
N Cf C Cf
where NDD and CDD refer to the neutron yield and the count rate of the D–D neutron generator, respectively, and f is a conversion factor obtained from the measurement of CCf referring to a count rate of 252Cf with known yield NCf. A 3He proportional counter (GE Reuter Stokes, RS-P4-0806-207) was used to measure the count rates of our D–D neutron generator and 252Cf source. Since the neutron energy distribution of the D–D neutron generator is very different from that of 252Cf, and the thermal-neutron crosssection for the 3He(n, p) reaction falls off rapidly with neutron energy, the neutron yield obtained in this study is not the actual neutron yield. However, from a practical point of view, it is quite reasonable to accept the neutron yield values obtained using the 3 He proportional counter for the detection of slow neutrons useful in TNA. Since the mass of the 252Cf neutron source (K868) purchased from the IAEA was 2.9 ng on February 19, 1999, the neutron yield NCf in Eq. (2) was 2.6 103 n/s as of December 20, 2006. The neutron yield of the source is traceable to the NIST reference source CR-5. The neutron count rate of the 252Cf reference source positioned next to the target was 0.17 counts/s, and, consequently, the conversion factor f was 1.53 104.
0.6kW 0.8kW 1.0kW 1.2kW 1.5kW
5
(2)
4 Ion Current (mA)
NDD ¼ fC DD
245
3
2
1
0 40
60 Acceleration Voltage (kV)
80
Fig. 3. Deuterium ion current versus acceleration voltage and RF discharge power at 8 mTorr.
2.3. Prompt gamma-ray spectroscopy of KCl
7 4
6 5
3 4 2
3
Ion Current (mA)
Neutron Yield (x106 neutron/s)
The KAERI-PGAA system was tested by measuring the prompt gamma-ray peak intensity of Cl as a representative example, because Cl is a key element for the detection of narcotics and some explosives. One kilogram of a KCl powder sample (99%, Shinyo Pure Chemical Co., Japan) in a polyethylene bottle was used as a sample. An HPGe detector (43% efficiency relative to a 7.6 cm 7.6 cm sodium iodide crystal, EG&G Ortec, USA) connected to a computer-based 16k channel MCA (919 MCB, EG&G Ortec, USA) was used to measure the gamma spectra. The resolution of the detector was 2.2 keV at 1332 keV 60Co. The effects of the RF power and count time on the gamma-ray intensity of the Cl peak were investigated to evaluate the performance of the KAERI-PGAA.
2 1 1
3. Results and discussion
0
0 3.1. Optimum conditions for a neutron generation of the KAERI-PGAA The neutron yield is directly proportional to the ionic current, which increases with the acceleration voltage and the applied RF power. Fig. 3 shows that the ion current increases linearly with the acceleration voltage when the deuterium gas pressure in the ion source is 8 mTorr. A maximum ion current of 4.59 mA was achieved at an acceleration voltage of 80 kV when the RF power was 1.5 kW. The neutron yield of the D–D neutron generator deduced from Eq. (2) with a conversion factor f of 1.53 104 and a count rate CDD of 3803 counts/s measured using the neutron detector was 5.82 107 n/s at 80 kV, 1.1 kW, and 8 mTorr. Theoretically, the higher neutron yield could be obtained by increasing RF power. However, due to the overheating of the beam spot [13], the neutron yield reached a maximum at an RF power of 1.15 kW and then decreased as the RF power increased. In contrast, the ion current increased monotonically with the RF power as shown in Fig. 4. Deuterium gas pressure in the acceleration column is also an important parameter for the operation of a neutron generator, and it should be as low as possible in order to avoid a voltage breakdown in the accelerator column and to obtain a higher fraction of the atomic deuterium ions relative to
0.6
0.8 1.0 1.2 RF Power (kW)
1.4
Fig. 4. Effect of RF power on the neutron yield and deuterium ion count rate at an acceleration voltage of 30 kV and a deuterium gas pressure of 8 mTorr.
the molecular deuterium ions. A low pressure in the column decreases the plasma density in the ion source, and plasma density is related to the ion current. In this study, instead of directly measuring the column pressure, the pressure in the ion source was measured using the pressure sensor that was connected directly to the ion source chamber. Fig. 5 shows that the neutron yield increased as the pressure in the ion source increased, and the yield reached a constant value after 8 mTorr. As shown in Fig. 6, the neutron yield increased sigmoidally with the acceleration voltage toward a maximum yield value of 80 keV. In conclusion, the optimum conditions for the KAERI-PGAA parameters such as the acceleration voltage, the RF power and the gas pressure in the ion source were determined as 80 kV at 1.1 kW and 8 mTorr. Consequently, all the experimental results in PGAA were obtained under the optimum conditions.
ARTICLE IN PRESS
1.4
104 103
1950.9
1959.1
105
1.6
H (2223.3 keV)
106
Counts
Neutron Yield (x106 neutron/s)
1.8
CI (1950.9 keV)
Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
anihilation (511 keV)
246
102
1.2
101
1.0 100
4
6
8 10 Pressure (mTorr)
12
14
0
4.000
6.000
8.000
Energy (keV)
Fig. 5. Neutron yield as a function of deuterium gas pressure at an acceleration voltage of 30 kV and a RF power of 0.6 kW.
Fig. 7. Typical gamma-ray spectrum for the KCl powder sample. Inset: enlargement of the region with two chlorine peaks at 1951 and 1959 keV.
5x103
6 4
3
2 2
0
1
0
20
40 60 Acceleration Voltage (kV)
80
100
Fig. 6. Acceleration voltage versus neutron yield and deuterium ion current at a RF power of 1.1 kW and a gas pressure of 8 mTorr.
Net Peak Area
4
4x103
Ion Current (mA)
Neutron Yield (x107 neutron/s)
2.000
3x103 2x103 1x103
0
0
20
40 60 Acceleration Voltage (kV)
80
Fig. 8. Net peak area of chlorine for KCl powder sample measured at 1951 keV versus acceleration voltage of neutron generator affecting the neutron yield and consequently the gamma-ray yield.
3.2. Performance characteristics of the KAERI-PGAA system Fig. 7 shows the full-energy prompt gamma-ray spectra of KCl. The thermal-neutron capture cross-section of 35Cl is 33.1 b, which is relatively large compared with that of other elements such as hydrogen (0.33 b) and Si (0.17 b) [3,9]. Among many full-energy peaks from Cl, which are free of interference, the intensity of the most prominent peak at 1951 keV was selected and measured at a resolution of 2.2 keV as a representative example in this study [9]. The net peak area of Cl at 1951 keV in the prompt gamma-ray spectra varied sigmoidally with the acceleration voltage, as shown in Fig. 8, in the same manner that the neutron yield varied sigmoidally with the acceleration voltage in the previous result shown in Fig. 6; therefore, the acceleration voltage was fixed at 80 kV. Fig. 9 shows the plot of the net peak area versus the counting time with a standard error of the mean (SEM) values of net peak area. The net peak area of the Cl in the KCl samples
depended linearly on the count time. In 1993, the Federal Aviation Administration (FAA) set the key standard for the processing rate of the explosive detection systems deployed at US airports. In order to be certified, a machine must be able to process 450 people or bags per hour, i.e. 1 bag per 8 s. To meet this 450-bag-per-hour throughput rate, total counts as a function of measurement time should be kept as large as possible and, therefore, is a very important parameter in a current PGAA. In our present study, a rough minimum of 6 min for a data acquisition is required to achieve a relative standard error of less than 10%. The performance of our current KAERI-PGAA system for cargo inspection in terms of relative standard error and signal-to-noise ratio can be improved by a multivariate data processing such as the principal component analysis and the noise reduction technique [16]; however, this is beyond the scope of the current study.
ARTICLE IN PRESS Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
containing illicit materials by using the PGAA technique and a multivariate chemometric data analysis. It may also have applications in various industrial fields such as an on-line or inline real-time monitoring of the production process. At the moment, however, our current PGAA system needs at least 6 min to achieve a relative standard error of less than 10%. In order to meet the FAA’s standard for the inspection, we will need to improve the processing rate of our KAERI-PGAA system.
R2 = 0.9997 1,000
Net Peak Area
247
100
Acknowledgement This work was supported by the Radiation Technology Development program through the Korea Science and Engineering Foundation funded by the Korean Ministry of Education, Science and Technology. The authors thank Dr. K.-N. Leung and Dr. J. Reijonen for advice. We also would like to thank Yong Suk Choi and Sun Kyung Ji for their support in preparing this manuscript.
10
1 1
10
100
1.000
References
Counting time (s) Fig. 9. Net peak area of chlorine for KCl powder samples measured at 1951 keV versus counting time.
4. Conclusions In this study, the performance characteristics of the KAERIPGAA system using an RF-induction-based D–D neutron generator as a non-destructive inspection tool were examined. For a PGAA, the optimum experimental conditions for the required neutron generation such as the deuterium ion current, the deuterium pressure, the acceleration voltage, and the RF power were determined for the performance test of the KAERI-PGAA system equipped with a compact D–D neutron generator system. Monoenergetic neutrons (2.5 MeV) with a neutron yield of 41 107 n/s were used to test our newly-developed KAERI-PGAA system. The effects of the count time and acceleration voltage on the prompt gamma-ray spectroscopy of potassium chloride were investigated. Our KAERI-PGAA system, which is equipped with a newly-developed D–D neutron generator obtained from LBNL, will be applied to a security check for the screening of luggage
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
[15] [16]
S. Singh, M. Singh, Signal Process. 83 (2003) 31. E.M.A. Hussein, E.J. Waller, Appl. Radiat. Isot. 53 (2000) 557. B.C. Song, Y.J. Park, K.Y. Jee, Anal. Sci. Technol. 16 (5) (2003) 41A. V.D. Aleksandrov, E.P. Bogolubov, O.V. Bochkarev, L.A. Korytko, V.I. Nazarov, Yu.G. Polkanov, V.I. Ryzhkov, T.O. Khasaev, Appl. Radiat. Isot. 63 (2005) 537. H.-H. Hsu, K.J. Kearfott, Nucl. Instr. and Meth. A 422 (1999) 914. H.-J. Im, H.-J. Cho, B.C. Song, Y.J. Park, Y.-S. Jung, W.-H. Kim, Nucl. Instr. and Meth. A 566 (2006) 442. Y.S. Chung, H.J. Cho, J.H. Moon, S.H. Kim, Y.J. Kim, Anal. Sci. Technol. 16 (5) (2003) 391. S.C. Son, H.Y. Pyo, Y.J. Park, K.Y. Jee, W.H. Kim, Anal. Sci. Technol. 17 (1) (2004) 8. Y.J. Park, B.C. Song, M.I. Chowdhury, K.Y. Jee, J. Radioanal. Nucl. Chem. 260 (3) (2004) 585. Y.J. Park, B.C. Song, K.Y. Jee, Anal. Sci. Technol. 16 (1) (2003) 12. J. Reijonen, K.-N. Leung, R.B. Firestone, J.A. English, D.L. Perry, A. Smith, F. Gicquel, M. Sun, H. Koivunoro, T.-P. Lou, B. Bandong, G. Garabedian, Zs. Revay, L. Szentmiklosi, G. Molnar, Nucl. Instr. and Meth. A 522 (2004) 598. J. Reijonen, F. Gicquel, S.K. Hahto, M. King, T.-P. Lou, K.-N. Leung, Appl. Radiat. Isot. 63 (2005) 757. J. Reijonen, Nucl. Instr. and Meth. B 261 (2007) 272. Plasma and Ion Source Technology Group in Lawrence Berkley National Laboratory, /http://www-ibt.lbl.gov/PIS/ng_research.htmlS (accessed September 20, 2008). I.J. Kim, N.S. Jung, H.D. Choi, Nucl. Eng. Technol. 40 (4) (2008) 299. H. Neamintara, V. Mangclaviraj, S. Punnachaiya, Nucl. Eng. Technol. 39 (6) (2007) 717.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Performance characteristics of a prompt gamma-ray activation analysis (PGAA) system equipped with a new compact D–D neutron generator Yong Joon Park, Byung Chul Song, Hee-Jung Im, Jong-Yun Kim Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Dukjin-dong 150-1, Yuseong-gu, Daejeon 305-353, Republic of Korea
a r t i c l e in fo
abstract
Article history: Received 3 July 2008 Received in revised form 21 April 2009 Accepted 21 April 2009 Available online 3 May 2009
A new prompt gamma-ray activation analysis (PGAA) system equipped with a compact deuterium– deuterium (D–D) neutron generator has been developed for fast detection of explosives and chemical warfare agents. The PGAA system was built based on a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency (RF)-driven ion source. The ionic current of the compact neutron generator was determined as a function of the acceleration voltage at various RF powers. Monoenergetic neutrons (2.45 MeV) with a neutron yield of 41 107 n/s were obtained at a deuterium pressure of 8.0 mTorr, an acceleration voltage of 80 kV, and an RF power of 1.1 kW. The performance of the PGAA system was examined by studying the dependence of a prompt gamma-ray count rate on crucial operating parameters. & 2009 Elsevier B.V. All rights reserved.
Keywords: Neutron generator Deuterium–deuterium fusion reaction Prompt gamma-ray activation analysis Neutron yield
1. Introduction The development of a non-destructive technology that can detect illicit materials within a very short period of time has become an important issue for analytical chemists in the area of practical cargo and luggage inspection, since seaports, airports and road border crossing points are very attractive targets for terrorists and contraband activities [1]. Among the many nondestructive methods, neutron-based methods are the most promising because they can provide information about the elemental composition of bulk materials for the detection of drugs, toxic agents, fissile materials, explosives, and chemical warfare agents [2,3]. Neutron-based methods can be categorized into three main classes: thermal-neutron analysis (TNA), fast neutron analysis (FNA), and neutron moderation. The prompt gamma-ray activation analysis (PGAA) technique used both in FNA and TNA is an attractive quantitative method especially for determining the light elements in bulk materials. Two types of neutron sources are generally used in a practical PGAA system: radioisotope neutron sources or electronic neutron generators. Compared to the radioisotope as a neutron source, a neutron generator is more environmentally friendly, safer for operators, and more sensitive for an elemental analysis; it is also easier to control the neutron characteristics such as the neutron yield, the pulse repetition rate, and the duration [4]. In a field application, equipment size, the stability of the neutron generation, analytical
Corresponding author.
E-mail address: [email protected] (J.-Y. Kim). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.04.022
speed and cost should also be taken into account. The thermalneutron yield is critical for the neutron capture reaction in TNA, while the neutron energy needed to induce the nuclear reaction by incident fast neutrons is important in FNA. According to a previous study [5], the deuterium–deuterium (D–D) neutron generator and 252Cf are much more efficient in TNA compared to the deuterium-tritium (D–T) neutron generator and the 241Am–Be source for the detection of elements of interest. This is because the lower energy neutrons produced by the D–D neutron generator and 252Cf are easily moderated by the presence of hydrogenous materials surrounding the element of interest. Consequently, they are widely used as lower energy neutron sources in TNA, whereas the D–T neutron generators are not adequate for use in TNA because the fast neutrons are not easily slowed by the surrounding medium. Based on our experience with the HANARO PGAA facility at the Korea Atomic Energy Research Institute (KAERI) [6–8], we have recently developed a neutron-induced prompt gamma system (NIPS) utilizing 252Cf as a neutron source [9,10]. Recently, we have developed a new PGAA system at KAERI (KAERI-PGAA), which is equipped with a fully high-voltage-shielded, axial deuterium– deuterium neutron generator (Fig. 1) developed by Plasma and Ion Source Technology Group (P&ISTG) at Lawrence Berkeley National Laboratory (LBNL) [11–14]. In the present study, the neutron yield of a D–D neutron generator as a safer neutron source compared to the radioactive deuterium–tritium or tritium–tritium (T–T) neutron generators was measured using 3He proportional counters to determine the optimum operating conditions for PGAA. The intensity of the prompt gamma-ray peak of chlorine (Cl) at 1951 keV under the optimum conditions for neutron
ARTICLE IN PRESS 244
Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
Explosively bonded titanium on copper target
Water-cooling lines for the target cooling External antenna driven, water-cooled RF-induction ion source
High voltage feed-through
Fig. 1. Design of compact axial, RF-driven D–D neutron generator [14].
generation was measured as a representative example for the performance test of the PGAA system.
2. Experiment and methods 2.1. Structural design of the KAERI-PGAA system In this study, neutrons were generated via the deuterium– deuterium fusion reaction by an axial D–D neutron generator composed of three main components: a radio frequency (RF)driven ion source with an external antenna, a diode ion beam accelerator, and an explosively-bonded titanium-on-copper target. The layout of the neutron generator is shown in Fig. 1. Atomic and molecular deuterium ions were produced by an RF discharge of 99.995% pure deuterium gas (Messer Gas Technologies, Malvern, PA, USA). The deuterium ions were accelerated onto a titanium target with a single gap DC accelerator to produce nearly monoenergetic 2.5 MeV neutrons by the reaction shown in Eq. (1). 2 1H
þ 21 H ! 32 He þ 10 n Q -value ¼ 3:27 MeV
Shielded neutron generator
vacuum pump
PE Shielding ion source
(1)
Fig. 2 shows a compact D–D neutron generator placed in the centre of a radiation shield/moderator structure consisting of three layers of 300-mm-thick polyethylene, 7-mm-thick polyethylene doped with 9% boron, and a 1.5-mm-thick lead sheet in order to shield both the neutron and secondary gammarays. A high-purity germanium (HPGe) detector was axially placed at a distance of 25 cm from the titanium target in order to acquire the gamma-ray spectra. The HPGe detector was shielded with 5-cm-thick lead blocks. 2.2. Measurement of the ion current and the neutron yield of the D–D neutron generator The ion current is a function of many parameters such as the gas pressure in the ion source, the RF frequency, and the RF power. These parameters affect the atomic ion ratio of the molecular gases, such as hydrogen and its isotopes, noise related to the RF leakage, high-voltage breakdown, and plasma density. The ionic current was measured as a function of the accelerator voltage
gamma detector
neutron generator Fig. 2. KAERI-NIPS system. (a) Installation of KAERI-NIPS system in the controlled area (b) equipped with neutron generator placed inside the shield and (c) gammaray detector shielded with polyethylene and Pb.
(from 40 to 100 kV), and the RF power (from 0.6 to 1.5 kW) was measured at a fixed RF frequency of 13.56 MHz. The D–D neutron yield was measured using a 3He gas proportional counter, and comparing the count rate to that of a 252Cf source measured in the same configuration of the radiation shield/moderator structure [15]. This measurement was used because both neutron sources are frequently used in TNA, and the average neutron energy values of a 252Cf source (2.3 MeV) with a peak maximum between 0.5 and 1.0 MeV is very similar to that of our monoenergetic 2.5 MeV D–D neutron generator. If the count rate of the D–D neutron generator and 252Cf with a known neutron yield is measured, the neutron yield of the neutron generator can then be obtained using
ARTICLE IN PRESS Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
the following simple equations: with f ¼
N Cf C Cf
where NDD and CDD refer to the neutron yield and the count rate of the D–D neutron generator, respectively, and f is a conversion factor obtained from the measurement of CCf referring to a count rate of 252Cf with known yield NCf. A 3He proportional counter (GE Reuter Stokes, RS-P4-0806-207) was used to measure the count rates of our D–D neutron generator and 252Cf source. Since the neutron energy distribution of the D–D neutron generator is very different from that of 252Cf, and the thermal-neutron crosssection for the 3He(n, p) reaction falls off rapidly with neutron energy, the neutron yield obtained in this study is not the actual neutron yield. However, from a practical point of view, it is quite reasonable to accept the neutron yield values obtained using the 3 He proportional counter for the detection of slow neutrons useful in TNA. Since the mass of the 252Cf neutron source (K868) purchased from the IAEA was 2.9 ng on February 19, 1999, the neutron yield NCf in Eq. (2) was 2.6 103 n/s as of December 20, 2006. The neutron yield of the source is traceable to the NIST reference source CR-5. The neutron count rate of the 252Cf reference source positioned next to the target was 0.17 counts/s, and, consequently, the conversion factor f was 1.53 104.
0.6kW 0.8kW 1.0kW 1.2kW 1.5kW
5
(2)
4 Ion Current (mA)
NDD ¼ fC DD
245
3
2
1
0 40
60 Acceleration Voltage (kV)
80
Fig. 3. Deuterium ion current versus acceleration voltage and RF discharge power at 8 mTorr.
2.3. Prompt gamma-ray spectroscopy of KCl
7 4
6 5
3 4 2
3
Ion Current (mA)
Neutron Yield (x106 neutron/s)
The KAERI-PGAA system was tested by measuring the prompt gamma-ray peak intensity of Cl as a representative example, because Cl is a key element for the detection of narcotics and some explosives. One kilogram of a KCl powder sample (99%, Shinyo Pure Chemical Co., Japan) in a polyethylene bottle was used as a sample. An HPGe detector (43% efficiency relative to a 7.6 cm 7.6 cm sodium iodide crystal, EG&G Ortec, USA) connected to a computer-based 16k channel MCA (919 MCB, EG&G Ortec, USA) was used to measure the gamma spectra. The resolution of the detector was 2.2 keV at 1332 keV 60Co. The effects of the RF power and count time on the gamma-ray intensity of the Cl peak were investigated to evaluate the performance of the KAERI-PGAA.
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3. Results and discussion
0
0 3.1. Optimum conditions for a neutron generation of the KAERI-PGAA The neutron yield is directly proportional to the ionic current, which increases with the acceleration voltage and the applied RF power. Fig. 3 shows that the ion current increases linearly with the acceleration voltage when the deuterium gas pressure in the ion source is 8 mTorr. A maximum ion current of 4.59 mA was achieved at an acceleration voltage of 80 kV when the RF power was 1.5 kW. The neutron yield of the D–D neutron generator deduced from Eq. (2) with a conversion factor f of 1.53 104 and a count rate CDD of 3803 counts/s measured using the neutron detector was 5.82 107 n/s at 80 kV, 1.1 kW, and 8 mTorr. Theoretically, the higher neutron yield could be obtained by increasing RF power. However, due to the overheating of the beam spot [13], the neutron yield reached a maximum at an RF power of 1.15 kW and then decreased as the RF power increased. In contrast, the ion current increased monotonically with the RF power as shown in Fig. 4. Deuterium gas pressure in the acceleration column is also an important parameter for the operation of a neutron generator, and it should be as low as possible in order to avoid a voltage breakdown in the accelerator column and to obtain a higher fraction of the atomic deuterium ions relative to
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0.8 1.0 1.2 RF Power (kW)
1.4
Fig. 4. Effect of RF power on the neutron yield and deuterium ion count rate at an acceleration voltage of 30 kV and a deuterium gas pressure of 8 mTorr.
the molecular deuterium ions. A low pressure in the column decreases the plasma density in the ion source, and plasma density is related to the ion current. In this study, instead of directly measuring the column pressure, the pressure in the ion source was measured using the pressure sensor that was connected directly to the ion source chamber. Fig. 5 shows that the neutron yield increased as the pressure in the ion source increased, and the yield reached a constant value after 8 mTorr. As shown in Fig. 6, the neutron yield increased sigmoidally with the acceleration voltage toward a maximum yield value of 80 keV. In conclusion, the optimum conditions for the KAERI-PGAA parameters such as the acceleration voltage, the RF power and the gas pressure in the ion source were determined as 80 kV at 1.1 kW and 8 mTorr. Consequently, all the experimental results in PGAA were obtained under the optimum conditions.
ARTICLE IN PRESS
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104 103
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H (2223.3 keV)
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Neutron Yield (x106 neutron/s)
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Y.J. Park et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 243–247
anihilation (511 keV)
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8 10 Pressure (mTorr)
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Fig. 5. Neutron yield as a function of deuterium gas pressure at an acceleration voltage of 30 kV and a RF power of 0.6 kW.
Fig. 7. Typical gamma-ray spectrum for the KCl powder sample. Inset: enlargement of the region with two chlorine peaks at 1951 and 1959 keV.
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Fig. 6. Acceleration voltage versus neutron yield and deuterium ion current at a RF power of 1.1 kW and a gas pressure of 8 mTorr.
Net Peak Area
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Ion Current (mA)
Neutron Yield (x107 neutron/s)
2.000
3x103 2x103 1x103
0
0
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40 60 Acceleration Voltage (kV)
80
Fig. 8. Net peak area of chlorine for KCl powder sample measured at 1951 keV versus acceleration voltage of neutron generator affecting the neutron yield and consequently the gamma-ray yield.
3.2. Performance characteristics of the KAERI-PGAA system Fig. 7 shows the full-energy prompt gamma-ray spectra of KCl. The thermal-neutron capture cross-section of 35Cl is 33.1 b, which is relatively large compared with that of other elements such as hydrogen (0.33 b) and Si (0.17 b) [3,9]. Among many full-energy peaks from Cl, which are free of interference, the intensity of the most prominent peak at 1951 keV was selected and measured at a resolution of 2.2 keV as a representative example in this study [9]. The net peak area of Cl at 1951 keV in the prompt gamma-ray spectra varied sigmoidally with the acceleration voltage, as shown in Fig. 8, in the same manner that the neutron yield varied sigmoidally with the acceleration voltage in the previous result shown in Fig. 6; therefore, the acceleration voltage was fixed at 80 kV. Fig. 9 shows the plot of the net peak area versus the counting time with a standard error of the mean (SEM) values of net peak area. The net peak area of the Cl in the KCl samples
depended linearly on the count time. In 1993, the Federal Aviation Administration (FAA) set the key standard for the processing rate of the explosive detection systems deployed at US airports. In order to be certified, a machine must be able to process 450 people or bags per hour, i.e. 1 bag per 8 s. To meet this 450-bag-per-hour throughput rate, total counts as a function of measurement time should be kept as large as possible and, therefore, is a very important parameter in a current PGAA. In our present study, a rough minimum of 6 min for a data acquisition is required to achieve a relative standard error of less than 10%. The performance of our current KAERI-PGAA system for cargo inspection in terms of relative standard error and signal-to-noise ratio can be improved by a multivariate data processing such as the principal component analysis and the noise reduction technique [16]; however, this is beyond the scope of the current study.
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containing illicit materials by using the PGAA technique and a multivariate chemometric data analysis. It may also have applications in various industrial fields such as an on-line or inline real-time monitoring of the production process. At the moment, however, our current PGAA system needs at least 6 min to achieve a relative standard error of less than 10%. In order to meet the FAA’s standard for the inspection, we will need to improve the processing rate of our KAERI-PGAA system.
R2 = 0.9997 1,000
Net Peak Area
247
100
Acknowledgement This work was supported by the Radiation Technology Development program through the Korea Science and Engineering Foundation funded by the Korean Ministry of Education, Science and Technology. The authors thank Dr. K.-N. Leung and Dr. J. Reijonen for advice. We also would like to thank Yong Suk Choi and Sun Kyung Ji for their support in preparing this manuscript.
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10
100
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References
Counting time (s) Fig. 9. Net peak area of chlorine for KCl powder samples measured at 1951 keV versus counting time.
4. Conclusions In this study, the performance characteristics of the KAERIPGAA system using an RF-induction-based D–D neutron generator as a non-destructive inspection tool were examined. For a PGAA, the optimum experimental conditions for the required neutron generation such as the deuterium ion current, the deuterium pressure, the acceleration voltage, and the RF power were determined for the performance test of the KAERI-PGAA system equipped with a compact D–D neutron generator system. Monoenergetic neutrons (2.5 MeV) with a neutron yield of 41 107 n/s were used to test our newly-developed KAERI-PGAA system. The effects of the count time and acceleration voltage on the prompt gamma-ray spectroscopy of potassium chloride were investigated. Our KAERI-PGAA system, which is equipped with a newly-developed D–D neutron generator obtained from LBNL, will be applied to a security check for the screening of luggage
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
[15] [16]
S. Singh, M. Singh, Signal Process. 83 (2003) 31. E.M.A. Hussein, E.J. Waller, Appl. Radiat. Isot. 53 (2000) 557. B.C. Song, Y.J. Park, K.Y. Jee, Anal. Sci. Technol. 16 (5) (2003) 41A. V.D. Aleksandrov, E.P. Bogolubov, O.V. Bochkarev, L.A. Korytko, V.I. Nazarov, Yu.G. Polkanov, V.I. Ryzhkov, T.O. Khasaev, Appl. Radiat. Isot. 63 (2005) 537. H.-H. Hsu, K.J. Kearfott, Nucl. Instr. and Meth. A 422 (1999) 914. H.-J. Im, H.-J. Cho, B.C. Song, Y.J. Park, Y.-S. Jung, W.-H. Kim, Nucl. Instr. and Meth. A 566 (2006) 442. Y.S. Chung, H.J. Cho, J.H. Moon, S.H. Kim, Y.J. Kim, Anal. Sci. Technol. 16 (5) (2003) 391. S.C. Son, H.Y. Pyo, Y.J. Park, K.Y. Jee, W.H. Kim, Anal. Sci. Technol. 17 (1) (2004) 8. Y.J. Park, B.C. Song, M.I. Chowdhury, K.Y. Jee, J. Radioanal. Nucl. Chem. 260 (3) (2004) 585. Y.J. Park, B.C. Song, K.Y. Jee, Anal. Sci. Technol. 16 (1) (2003) 12. J. Reijonen, K.-N. Leung, R.B. Firestone, J.A. English, D.L. Perry, A. Smith, F. Gicquel, M. Sun, H. Koivunoro, T.-P. Lou, B. Bandong, G. Garabedian, Zs. Revay, L. Szentmiklosi, G. Molnar, Nucl. Instr. and Meth. A 522 (2004) 598. J. Reijonen, F. Gicquel, S.K. Hahto, M. King, T.-P. Lou, K.-N. Leung, Appl. Radiat. Isot. 63 (2005) 757. J. Reijonen, Nucl. Instr. and Meth. B 261 (2007) 272. Plasma and Ion Source Technology Group in Lawrence Berkley National Laboratory, /http://www-ibt.lbl.gov/PIS/ng_research.htmlS (accessed September 20, 2008). I.J. Kim, N.S. Jung, H.D. Choi, Nucl. Eng. Technol. 40 (4) (2008) 299. H. Neamintara, V. Mangclaviraj, S. Punnachaiya, Nucl. Eng. Technol. 39 (6) (2007) 717.