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Optimizing the signal-to-noise ratio for the PELAN system
Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
Optimizing the signal-to-noise ratio for the PELAN system Phillip C. Womble*, George Vourvopoulos, Jon Paschal, Ivan Novikov, Gongyin Chen Applied Physics Institute, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42101, USA
Abstract Pulsed elemental analysis with neutrons is a portable system for the detection of explosives, weighing less than 45 kg. It is based on the principle that explosives and other contraband contain various chemical elements such as H, C, N, O, etc. in quantities and ratios that differentiate them from other innocuous substances. Neutrons are produced with a pulsed 14 MeV (d-T) neutron generator. Separate gamma-ray spectra from fast neutron, thermal neutron and activation reactions are accumulated and analyzed to determine elemental content. Currently, a 7.6 cm 7.6 cm BGO detector is incorporated into the design. However, the high g-ray efficiency of this detector has a drawback in that it detects g-rays from the environment surrounding the object under interrogation. These g-rays from the environment are essentially noise that must be filtered from the signal from the object under interrogation. Since there is no practical way to focus the neutrons, the signal-to-noise ratio of the detector must be modified. In the past 2 years, we have tried several approaches to solve this problem. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.30.Kv; 82.80.Jp; 83.85.Hf; 29.40.Mc Keywords: Explosive; Neutrons; Gamma rays; Non-destructive
1. Introduction Explosives can be differentiated from innocuous materials and from other contraband materials through the examination of the elemental content. In particular, the ratios of chemical elements have been used to perform this differentiation with great success [1]. Pulsed Elemental Analysis with Neutrons (PELAN) is a system that determines the elemental content of objects in an automatic manner and was *Corresponding author. Tel.: +1-270-781-3859; fax: +1270-781-1104. E-mail address: [email protected] (P.C. Womble).
designed for the characterization of explosives. The system consists of a pulsing d-T neutron generator and a bismuth germanate (BGO) gamma-ray detector. PELAN is a small man-portable device, composed of two units which interlock to form the shape shown in Fig. 1. In the first unit the neutron generator along with the computer and the various power supplies are housed. In the second unit the BGO g-ray detector and the necessary material to shield the detector from the neutrons are housed. The total mass of PELAN is less than 45 kg. The d-T neutron generator provides 14 MeV neutrons in 10 ms duration pulses. During the neutron pulse, the g-ray spectrum is primarily
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01123-9
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
Neutron Generator
Computer and Power Supplies
Battery Pack Detector Shielding
471
Double-blind field trials [2] with high explosives proved PELAN’s ability to detect explosives in different scenarios. PELAN was operated in an automatic mode, without interference from the operator. The data acquisition and analysis were performed automatically by the PELAN software and a result was then returned to the operator.
2. Changing the signal-to-noise (SNR) ratio Palm-top Control Detector
Fig. 1. The PELAN system.
composed of g-rays from the (n; n0 g) and (n; pg) reactions on elements such as C and O, and is stored at a particular memory location within the data acquisition system. Between pulses, some of the fast neutrons that are still within the object lose energy by collisions with light elements composing the object. When the neutrons have an energy less than 1 eV, they are captured by elements such as H, N, and Fe through (n; g) reactions. The g-rays from this set of reactions are detected by the same set of detectors but stored at a different memory address within the data acquisition system. This procedure is repeated with a frequency of approximately 10 kHz. After a predetermined number of pulses, there is a longer pause that allows the detection of g-rays emitted from elements such as Si and P that have been activated. Therefore, by utilizing fast neutron reactions, neutron capture reactions, and activation analysis, a large number of elements contained in an object can be identified in a continuous mode without sampling. PELAN is controlled with a palm-top computer (Fig. 1), connected via an RF network. The palm top provides fully automatic operation of PELAN. With a single touch command, neutrons are produced and data are accumulated for a predetermined time. Upon the completion of data acquisition, the data are automatically reduced, analyzed, and the results of the interrogation are displayed on the palm-top screen.
Currently, a 3 3 BGO detector is incorporated into the design. However, the high g-ray efficiency of this detector has a drawback in that it detects grays from the environment surrounding the object under interrogation. These g-rays from the environment are essentially noise that must be filtered from the signal from the object under interrogation. Since there is no practical way to focus the neutrons, the SNR ratio of the detector must be modified. In the past 2 years, we have tried several approaches to solve this problem. The traditional way of removing the noise from the ground or from the neutron generator is to place a very heavy, dense material such as lead between the objects creating the noise and the detector. Unfortunately, this has two drawbacks: (1) for a portable system, this increases the weight, and (2) these materials contribute to the noise. Another way of removing the noise from the spectrum is to use an active filtering device. For example, Compton-suppression systems are often used in nuclear physics to eliminate the continuum caused by Compton scattering within the detector. Carbon and oxygen measurements are most likely to be affected by the noise from the environment and the neutron generator. Thus, an active filter must be optimized for g-rays with energies between 4 and 6 MeV. In this energy range, the two dominant photon interactions with matter are pair production and scattering (primarily Compton scattering). Fig. 2 is a schematic drawing showing g-rays entering the detector. On the left-hand side (A), three g-rays are entering PELAN’s detector. The top-most g-ray is scattered and there is an appreciable probability that the Compton scattered g-ray will leave the detector. The bottom-most g-ray
472
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
6000
Carbon, 4.4 MeV
5000 Oxygen, 6.9 MeV
Counts
4000
(A)
(B)
Fig. 2. Schematic representation of high-energy g-rays entering the PELAN detector (A). The PELAN detector surrounded by a veto detector (B). See text for details.
3000
2000
Oxygen, 6.1 MeV Oxygen Single escape of 6.1 MeV
1000
0 4000
4500
5000
5500
6000
6500
7000
7500
8000
Energy (keV)
Fig. 4. The spectrum from the PELAN detector in anticoincidence mode with veto detector (—) and a spectrum taken with the PELAN detector alone. Both spectra were taken with the detector near the ground.
Fig. 3. The PELAN detector surrounded by the veto detector.
undergoes pair production with a 511 keV g-ray escaping the detector. Only the g-ray that is perpendicular to the front surface of the detector deposits its entire energy within the detector. On the right-hand side of Fig. 2, the same detector is now surrounded by another detector. The two g-rays that deposited only a fraction of their energy have a high probability of being absorbed in this second detector. Data are collected in the PELAN detector only when there is no corresponding signal from this second detector (anti-coincidence mode). Since this second detector vetoes data in the PELAN detector, we describe it as a ‘‘veto detector’’. Fig. 3 shows a veto detector designed for the PELAN project. The PELAN detector is a 7.6 cm 7.6 cm (3 in. 3 in.) BGO detector. The veto detector is also composed of BGO with a
thickness of 2.5 cm (1 in.) with an inner diameter of just over 7.6 cm. The veto detector is composed of four optically isolated pieces of BGO. Each piece has two photomultiplier tubes in order to efficiently gather the light from the scintillations. In practice, the signals from the pair of the photomultipliers from each piece are tied together. There is no attempt to gather position information from the veto detector. An anti-coincidence circuit for the veto detector has also been designed. The timing resolution between PELAN’s detector and the veto detector is approximately 10 ns when using a 22Na g-ray source. The veto detector is not fully annular and only completes three-quarters of the circumference. Tests completed during the design phase of the veto detector construction indicated that a complete annulus was not required. This is due to the shielding intrinsic to PELAN (see Fig. 1) which is sufficient to limit the g-ray noise emanating from the neutron generator. Fig. 4 shows two g-ray spectra: the dashed line spectrum is taken without the veto detector, and the solid line spectrum is taken with the veto detector surrounding the PELAN detector. The shielding effect of the veto detector is apparent, since the solid line spectrum is lower than the dashed line one. In the region around 5.1 MeV, the second escape peak of the 6.1 MeV g-ray is totally
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
473
4000 w/o veto w/ veto
3500 3000
Counts
2500 2000 1500 1000
Fig. 5. Gamma rays (represented by arrows) whose direction is along the long axis of the detector are preferentially selected.
absent from the spectrum taken with the veto detector. The veto detector does indeed reduce the ‘‘noise’’ generated by the background. However, the change in the SNR was equivalent to the SNR produced by surrounding the BGO with a 1 in. thick piece of Pb. We could not justify the expense and the complexity in utilizing the veto detector. A third way is to change the geometry of the detector. The gamma rays that PELAN analyzes range in energy from 1.0 to 8 MeV. These are highly penetrating and require a long free mean path within the BGO for full energy deposition in the crystal. If the geometry is changed so that the cross-sectional area of the detector is smaller than the length of the detector, then the higher-energy gamma rays will deposit preferentially their full energy along the long axis of the detector (see Fig. 5). A large detector can then be created having a close-packed array of these detectors. To suppress scattering, any event in which two or more detectors are in coincidence is vetoed out. In this manner, the solid angle is maintained and gamma rays in the direction of the long axis are preferentially selected. We refer to this design as a ‘‘segmented’’ detector. However, there are several technical problems associated with segmented detector. First, the resolution of the detector decreases due to the fact that smaller diameter photo-multiplier tubes have a poorer light collection efficiency. Because of this limitation, the segments could not have a diameter much smaller than 2.5 cm (1 in.) with 7.6 cm (3 in.) length. Second, the detectors must be in close proximity to increase the efficiency of the anti-
500 0 150
200
Channel
250
300
Fig. 6. Spectra from segmented detector tests at 4.4 MeV. Lighter colored spectrum is in anti-coincidence mode and darker is without anti-coincidence.
coincidence circuit. Finally, compact PC-based electronics must be designed. A proof of principle test was performed utilizing four NaI detectors which matched the desired shape. They were connected to an anti-coincidence circuit and the resulting spectra are shown in Fig. 6. The analysis of the effectiveness of this detector is still going on.
3. Conclusions For PELAN, we are still seeking an improved detection system. We desire improvements in directionality which we believe will change the SNR ratio and eliminate the problems caused by clutter.
Acknowledgements This work is supported in part by the Department of Defense under Contracts DAAD07-98-C0116, DAAD05-98-C-022 and DACA72-01-C0017.
References [1] G. Vourvopoulos, Chemistry and Industry, 18 April 1994, pp. 297–300. [2] G. Vourvopoulos, P.C. Womble, Talanta 54 (2001) 459.
Optimizing the signal-to-noise ratio for the PELAN system Phillip C. Womble*, George Vourvopoulos, Jon Paschal, Ivan Novikov, Gongyin Chen Applied Physics Institute, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42101, USA
Abstract Pulsed elemental analysis with neutrons is a portable system for the detection of explosives, weighing less than 45 kg. It is based on the principle that explosives and other contraband contain various chemical elements such as H, C, N, O, etc. in quantities and ratios that differentiate them from other innocuous substances. Neutrons are produced with a pulsed 14 MeV (d-T) neutron generator. Separate gamma-ray spectra from fast neutron, thermal neutron and activation reactions are accumulated and analyzed to determine elemental content. Currently, a 7.6 cm 7.6 cm BGO detector is incorporated into the design. However, the high g-ray efficiency of this detector has a drawback in that it detects g-rays from the environment surrounding the object under interrogation. These g-rays from the environment are essentially noise that must be filtered from the signal from the object under interrogation. Since there is no practical way to focus the neutrons, the signal-to-noise ratio of the detector must be modified. In the past 2 years, we have tried several approaches to solve this problem. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.30.Kv; 82.80.Jp; 83.85.Hf; 29.40.Mc Keywords: Explosive; Neutrons; Gamma rays; Non-destructive
1. Introduction Explosives can be differentiated from innocuous materials and from other contraband materials through the examination of the elemental content. In particular, the ratios of chemical elements have been used to perform this differentiation with great success [1]. Pulsed Elemental Analysis with Neutrons (PELAN) is a system that determines the elemental content of objects in an automatic manner and was *Corresponding author. Tel.: +1-270-781-3859; fax: +1270-781-1104. E-mail address: [email protected] (P.C. Womble).
designed for the characterization of explosives. The system consists of a pulsing d-T neutron generator and a bismuth germanate (BGO) gamma-ray detector. PELAN is a small man-portable device, composed of two units which interlock to form the shape shown in Fig. 1. In the first unit the neutron generator along with the computer and the various power supplies are housed. In the second unit the BGO g-ray detector and the necessary material to shield the detector from the neutrons are housed. The total mass of PELAN is less than 45 kg. The d-T neutron generator provides 14 MeV neutrons in 10 ms duration pulses. During the neutron pulse, the g-ray spectrum is primarily
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)01123-9
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
Neutron Generator
Computer and Power Supplies
Battery Pack Detector Shielding
471
Double-blind field trials [2] with high explosives proved PELAN’s ability to detect explosives in different scenarios. PELAN was operated in an automatic mode, without interference from the operator. The data acquisition and analysis were performed automatically by the PELAN software and a result was then returned to the operator.
2. Changing the signal-to-noise (SNR) ratio Palm-top Control Detector
Fig. 1. The PELAN system.
composed of g-rays from the (n; n0 g) and (n; pg) reactions on elements such as C and O, and is stored at a particular memory location within the data acquisition system. Between pulses, some of the fast neutrons that are still within the object lose energy by collisions with light elements composing the object. When the neutrons have an energy less than 1 eV, they are captured by elements such as H, N, and Fe through (n; g) reactions. The g-rays from this set of reactions are detected by the same set of detectors but stored at a different memory address within the data acquisition system. This procedure is repeated with a frequency of approximately 10 kHz. After a predetermined number of pulses, there is a longer pause that allows the detection of g-rays emitted from elements such as Si and P that have been activated. Therefore, by utilizing fast neutron reactions, neutron capture reactions, and activation analysis, a large number of elements contained in an object can be identified in a continuous mode without sampling. PELAN is controlled with a palm-top computer (Fig. 1), connected via an RF network. The palm top provides fully automatic operation of PELAN. With a single touch command, neutrons are produced and data are accumulated for a predetermined time. Upon the completion of data acquisition, the data are automatically reduced, analyzed, and the results of the interrogation are displayed on the palm-top screen.
Currently, a 3 3 BGO detector is incorporated into the design. However, the high g-ray efficiency of this detector has a drawback in that it detects grays from the environment surrounding the object under interrogation. These g-rays from the environment are essentially noise that must be filtered from the signal from the object under interrogation. Since there is no practical way to focus the neutrons, the SNR ratio of the detector must be modified. In the past 2 years, we have tried several approaches to solve this problem. The traditional way of removing the noise from the ground or from the neutron generator is to place a very heavy, dense material such as lead between the objects creating the noise and the detector. Unfortunately, this has two drawbacks: (1) for a portable system, this increases the weight, and (2) these materials contribute to the noise. Another way of removing the noise from the spectrum is to use an active filtering device. For example, Compton-suppression systems are often used in nuclear physics to eliminate the continuum caused by Compton scattering within the detector. Carbon and oxygen measurements are most likely to be affected by the noise from the environment and the neutron generator. Thus, an active filter must be optimized for g-rays with energies between 4 and 6 MeV. In this energy range, the two dominant photon interactions with matter are pair production and scattering (primarily Compton scattering). Fig. 2 is a schematic drawing showing g-rays entering the detector. On the left-hand side (A), three g-rays are entering PELAN’s detector. The top-most g-ray is scattered and there is an appreciable probability that the Compton scattered g-ray will leave the detector. The bottom-most g-ray
472
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
6000
Carbon, 4.4 MeV
5000 Oxygen, 6.9 MeV
Counts
4000
(A)
(B)
Fig. 2. Schematic representation of high-energy g-rays entering the PELAN detector (A). The PELAN detector surrounded by a veto detector (B). See text for details.
3000
2000
Oxygen, 6.1 MeV Oxygen Single escape of 6.1 MeV
1000
0 4000
4500
5000
5500
6000
6500
7000
7500
8000
Energy (keV)
Fig. 4. The spectrum from the PELAN detector in anticoincidence mode with veto detector (—) and a spectrum taken with the PELAN detector alone. Both spectra were taken with the detector near the ground.
Fig. 3. The PELAN detector surrounded by the veto detector.
undergoes pair production with a 511 keV g-ray escaping the detector. Only the g-ray that is perpendicular to the front surface of the detector deposits its entire energy within the detector. On the right-hand side of Fig. 2, the same detector is now surrounded by another detector. The two g-rays that deposited only a fraction of their energy have a high probability of being absorbed in this second detector. Data are collected in the PELAN detector only when there is no corresponding signal from this second detector (anti-coincidence mode). Since this second detector vetoes data in the PELAN detector, we describe it as a ‘‘veto detector’’. Fig. 3 shows a veto detector designed for the PELAN project. The PELAN detector is a 7.6 cm 7.6 cm (3 in. 3 in.) BGO detector. The veto detector is also composed of BGO with a
thickness of 2.5 cm (1 in.) with an inner diameter of just over 7.6 cm. The veto detector is composed of four optically isolated pieces of BGO. Each piece has two photomultiplier tubes in order to efficiently gather the light from the scintillations. In practice, the signals from the pair of the photomultipliers from each piece are tied together. There is no attempt to gather position information from the veto detector. An anti-coincidence circuit for the veto detector has also been designed. The timing resolution between PELAN’s detector and the veto detector is approximately 10 ns when using a 22Na g-ray source. The veto detector is not fully annular and only completes three-quarters of the circumference. Tests completed during the design phase of the veto detector construction indicated that a complete annulus was not required. This is due to the shielding intrinsic to PELAN (see Fig. 1) which is sufficient to limit the g-ray noise emanating from the neutron generator. Fig. 4 shows two g-ray spectra: the dashed line spectrum is taken without the veto detector, and the solid line spectrum is taken with the veto detector surrounding the PELAN detector. The shielding effect of the veto detector is apparent, since the solid line spectrum is lower than the dashed line one. In the region around 5.1 MeV, the second escape peak of the 6.1 MeV g-ray is totally
P.C. Womble et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 470–473
473
4000 w/o veto w/ veto
3500 3000
Counts
2500 2000 1500 1000
Fig. 5. Gamma rays (represented by arrows) whose direction is along the long axis of the detector are preferentially selected.
absent from the spectrum taken with the veto detector. The veto detector does indeed reduce the ‘‘noise’’ generated by the background. However, the change in the SNR was equivalent to the SNR produced by surrounding the BGO with a 1 in. thick piece of Pb. We could not justify the expense and the complexity in utilizing the veto detector. A third way is to change the geometry of the detector. The gamma rays that PELAN analyzes range in energy from 1.0 to 8 MeV. These are highly penetrating and require a long free mean path within the BGO for full energy deposition in the crystal. If the geometry is changed so that the cross-sectional area of the detector is smaller than the length of the detector, then the higher-energy gamma rays will deposit preferentially their full energy along the long axis of the detector (see Fig. 5). A large detector can then be created having a close-packed array of these detectors. To suppress scattering, any event in which two or more detectors are in coincidence is vetoed out. In this manner, the solid angle is maintained and gamma rays in the direction of the long axis are preferentially selected. We refer to this design as a ‘‘segmented’’ detector. However, there are several technical problems associated with segmented detector. First, the resolution of the detector decreases due to the fact that smaller diameter photo-multiplier tubes have a poorer light collection efficiency. Because of this limitation, the segments could not have a diameter much smaller than 2.5 cm (1 in.) with 7.6 cm (3 in.) length. Second, the detectors must be in close proximity to increase the efficiency of the anti-
500 0 150
200
Channel
250
300
Fig. 6. Spectra from segmented detector tests at 4.4 MeV. Lighter colored spectrum is in anti-coincidence mode and darker is without anti-coincidence.
coincidence circuit. Finally, compact PC-based electronics must be designed. A proof of principle test was performed utilizing four NaI detectors which matched the desired shape. They were connected to an anti-coincidence circuit and the resulting spectra are shown in Fig. 6. The analysis of the effectiveness of this detector is still going on.
3. Conclusions For PELAN, we are still seeking an improved detection system. We desire improvements in directionality which we believe will change the SNR ratio and eliminate the problems caused by clutter.
Acknowledgements This work is supported in part by the Department of Defense under Contracts DAAD07-98-C0116, DAAD05-98-C-022 and DACA72-01-C0017.
References [1] G. Vourvopoulos, Chemistry and Industry, 18 April 1994, pp. 297–300. [2] G. Vourvopoulos, P.C. Womble, Talanta 54 (2001) 459.