- Email: [email protected]
Performance of a tagged neutron inspection system (TNIS) based on portable sealed generators
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480 www.elsevier.com/locate/nima
Performance of a tagged neutron inspection system (TNIS) based on portable sealed generators G. Nebbiaa,, S. Pesentea, M. Lunardona, G. Viestia, P. LeTourneurb, F. Heuvelineb, M. Mangeardb, C. Tchengb a
Dipartimento di Fisica dell’Universita` di Padova and INFN Sezione di Padova, via Marzolo 8, I-35131 Padova, Italy b EADS-SODERN, 20 Av. Descartes, 94451 Limeil-Brevannes, France Received 22 March 2004; received in revised form 24 May 2004; accepted 24 May 2004 Available online 26 July 2004
Abstract A portable sealed neutron generator has been modified to produce 14 MeV tagged neutron beams with an embedded YAP:Ce scintillation detector. The system has been tested by detecting the coincident gamma-rays produced in the irradiation of a graphite sample by means of a standard NaI(Tl) scintillator. Time resolution of about dt ¼ 425 ns (FWHM) has been measured. The sealed neutron tube has been operated up to 107 neutron/s. Possible applications in non-destructive assays and future developments of the Tagged Neutron Inspection System concept are discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.Dz; 29.40.Mc; 82.80.JP; 89.20.Bb Keywords: Tagged neutrons; Explosive detection
1. Introduction Non-destructive assay by fast neutrons is a technique widely employed in several fields [1]. As an example, the detection of hidden explosives [2] is achieved by determining the elemental ratio of C, N, O nuclei from the intensity of the characteristic gamma rays produced in fast neuCorresponding author. Tel.: +39-49-82-77-134; Fax: +39-
49-876-26-41. E-mail address: [email protected] (G. Nebbia).
tron-induced reactions. This technique, indicated as Fast Neutron Analysis (FNA), can be used to detect explosive in landmines or to inspect unexploded ordnance (UXO) [3]. The use of fast neutron inspection techniques is mainly limited by the fact that the neutron source and the gamma-ray detectors are placed as close as possible to the inspected object. In this condition, the signal-to-noise ratio depends on the fraction of neutrons that reach the sample and the solid angle subtended by the gamma-ray detector. The spectra are thus dominated by the gammas originated
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.05.137
ARTICLE IN PRESS 476
G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
close to the neutron source and to the gamma-ray detector. Consequently, it is difficult to search for hidden objects inside a large volume of background material since the spectra would be dominated by the background gammas. Such limitations might be avoided by using a pulsed, collimated neutron beam and by measuring the time delay between the production of the neutron burst and the arrival of the gamma rays in a detector array. This technique is indicated as Pulsed Fast Neutron Analysis (PFNA) [4]. While FNA systems usually employ small portable neutron generators, PFNA requires the use of particle accelerators to produce fast neutrons. Consequently, PFNA systems can be used only in specific application when the cost of the entire setup and its portability are not a relevant issue. In the last decade, efforts have been devoted to producing tagged neutron beams with compact sealed neutron generators. Such a task is, indeed, achieved routinely in open-end accelerators by using the well-known Associated Particle Technique where the T(D,n)4He or the D(D,n)3He neutron source reactions are used [5]. Examples of systems using the associated particle technique with sealed neutron generators are reported in Refs. [6,7]. Although the interest in obtaining a sealed neutron generator with embedded associated particle detector is a long-standing issue in neutron applications [8], such interest has increased significantly in recent times for all applications related to civil security after the 9/11 events. Within the EXPLODET (EXPLosive DETection) project [9,10], a new sealed neutron generator for the production of tagged neutron beams has been developed in a collaboration between INFN, the Universities of Padova and Bari (Italy) and EADS-SODERN (France). The results obtained in the development of the Tagged Neutron Inspection System are reported in this paper.
2. The tagged neutron sealed generator A standard SODERN neutron-emitting module has been modified to allow the insertion of an alpha particle detector [11]. Such a detector is mounted on a stainless steel CF63 flange, its inner
surface being located at 5.5 cm from the target. The total tritium activity of the sealed generator is about 1 GBq. A YAP:Ce scintillator [12] of 4 cm diameter and 0.5 mm thickness has been selected for the present application. The YAP:Ce exhibits, indeed, several characteristics that are very well suited for this task: excellent mechanical and chemical properties, radiation hardness, fast response and high light output [13]. The scintillator has been mounted on the CF63 flange equipped with a UV-extended sapphire window (3 mm thick, 48 mm diameter) without any optical grease, as required in the neutron generator manufacturing procedure [14]. Preliminary laboratory tests, performed with an 241 Am alpha particle source, demonstrated that the light output was reduced only by 30% with such mounting as with respect to the direct coupling between scintillator and photomultiplier (PMT). This reduction is not critical for the present application due to the excellent light output of the YAP:Ce scintillator and the use of the signal only for fast timing. Since the counting rate capability and time resolution are the major goals of the present application, we used a small diameter fast PMT Hamamatsu R1450, without a light guide. With such PMT, typical energy resolution on the order of dE a =E a ¼ 6% (FWHM) has been measured with small diameter YAP:Ce scintillators using alpha particles from an 241Am source. The energy resolution degrades when the whole 4 cm diameter crystal is irradiated; this fact will be further discussed in the following. Finally, the surface of the YAP:Ce crystal was coated with a layer of 1 mg/cm2 of metallic silver to maximize the light collection, stop the elastically scattered deuterons and protect the crystal from the light glow inside the neutron generator. The tagging system (i.e. the YAP:Ce mounted on the optical flange with the PMT and the associated front-end electronics) has been tested with neutrons produced in the D+T reaction making use of the 150 kV electrostatic neutron generator at the Institute Ruder Boskovic (IRB) in Zagreb. In this test, a 5.5 mm diameter collimator was used in front of the YAP:Ce crystal that was placed at about 5.5 cm from the T-target. It was
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
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Tests of the new system have been performed placing the neutron generator at about 50 cm from a graphite sample with dimensions 20 20 10 cm3. A standard 300 300 NaI(Tl) scintillator from Crysmatec (France) was placed on a side of the graphite sample, at a distance of about 28 cm from its center. The NaI(Tl) was protected from direct neutrons by a heavy metal shadow bar. Standard NIM electronics was used to process the signals from YAP:Ce and NaI(Tl) scintillators. In particular, energy signals were formed by using amplifiers with t ¼ 0:5 ms and t ¼ 1 ms shaping time for the YAP:Ce and NaI(Tl) scintillators, respectively. The anode signals were processed using standard Constant Fraction Discriminators. A fast coincidence between the YAP:Ce and the NaI(Tl) was used to start a Time to Amplitude Converter (TAC) and to provide the Master Gate to the acquisition system. The TAC was operated with a range of 200 ns, the stop signal being provided by the NaI(Tl) detector. The two energy signals and the analog output from the TAC were recorded on a four-channel Flash ADC card hosted on a portable PC [16]. Typical results obtained with this system are shown in Fig. 1, relative to a measurement run in which the counting rates of the YAP:Ce and NaI(Tl) detectors were 105 and 5 103 c/s, respectively. When the solid angle of the tagging detector (DO/O ¼ 3 102 ) is taken into account, the YAP:Ce counting rate corresponds to the operation of the neutron generator at a flux of about 0.3 107 neutron/s. In such conditions, the coincidence rate was 140 c/s, the data shown in the figures refer to an irradiation of about 45 min. The pulse-height spectrum of the YAP:Ce detector is shown in Fig. 1c. The continuous pulse-height distribution is due to the combination of several effects: the alpha particles produced in the D+T
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shown that the tagging system in the above conditions is capable of detecting the associated alpha particle with an energy resolution of about dE a =E a ¼ 25%. Results from the IRB tests are the subject of a separate paper [15].
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Fig. 1. (a) TAC spectrum; (b) pulse-height spectrum of the NaI(Tl) detector; (c) pulse-height spectrum of the YAP:Ce detector.
reaction have a finite energy distribution before entering the detector mainly due to the energy straggling of the alpha particles in the T–Ti target and in the Ag coating of the YAP:Ce. Such effects have been estimated by Monte Carlo calculations to account for an energy spread of about DE ¼ 0:2 MeV [14]. Moreover, since the PMT photocathode diameter is only 18 mm compared to the 40 mm of the scintillator, the alpha particles reaching the periphery of the detector produce lower pulse heights due to the reduced light collection efficiency of the system. The TAC spectrum, shown in Fig. 1a exhibits a peak with a width of dt ¼ 10:5 ns (FWHM) upon a continuous background of uncorrelated events. In this peak there are two classes of events: the
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
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gamma-rays emitted in the neutron irradiation of the graphite sample and the neutrons scattered from the sample into the NaI(Tl) detector. They are not resolved in time because of the short distance between the sample and the detector. As a consequence, in the coincident gamma spectrum the characteristic gamma-rays from the graphite sample (E g ¼ 4:44 MeV and its first escape at E g ¼ 3:94 MeV) are embedded on a large background, as reported in Fig. 1b. In this spectrum the peak-to-background ratio is P/B=0.5 at E g ¼ 4:4 MeV. One should notice that the spectrum of Fig. 1b refers to data taken in the full TAC range of 200 ns. In order to verify the performances of the system we have selected a region of the TAC spectrum corresponding to the first half of the time peak with a width of 6 ns (see Fig. 2a) as representative of the gammas that originated from the graphite sample irradiation. The second half of the time peak being roughly associated to the neutron scattered by the graphite inside the NaI(Tl) detector. By subtracting the background contribution selected in the time window on the left of the peak (see Fig. 2a), one obtains the energy spectrum shown in Fig. 2b. The spectrum now shows a very clear signature of the Eg ¼ 4:44 MeV gamma ray with its escape peaks. The peak-to-background ratio is now B/P=7 at E g ¼ 4:4 MeV. Moreover the selection of the graphite sample by the right time of flight gate results in a reduction of the low pulse-height region in the associated particle spectrum (see Fig. 2c). This is due to the smaller contribution of alpha particles in the periphery of the YAP:Ce scintillator . A second part of the analysis was devoted to the study of the time resolution of the system. The time resolution is important since it determines the minimum volume unit (voxel) that can be inspected using the TNIS. For this purpose we set a narrow gate on the Eg ¼ 4:44 MeV gamma-rays and on the high pulse-height part of the YAP:Ce spectrum, a typical measured time resolution is dt ¼ 4:4 ns (FWHM). Such measured time resolutions include several contributions that have to be determined to extract the intrinsic performance of the system. The intrinsic time resolution of the
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Fig. 2. (a) TAC spectrum with selected time gates; (b) background subtracted NaI(Tl) energy spectrum; (c) background subtracted YAP:Ce energy spectrum (see text for details).
NaI(Tl) detector has been measured in laboratory conditions to be dt ¼ 2:7 ns (FWHM) for 1.3 MeV gamma-rays. Moreover, the geometry of the experimental set-up induces an additional uncertainty of dt ¼ 2:1 ns (FWHM) due to the combined effects of the uncertainty in the interaction time of the fast neutrons inside the graphite sample and the jitter associated with the finite energy distribution of the alpha particles. The intrinsic time resolution of the tagged neutron system results are about dt ¼ 2:8 ns when both the geometry and the NaI(Tl) contributions are properly subtracted. We notice that this time
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
resolution is much larger compared to the limiting value of the YAP:Ce detectors [7,17], indicating that the performances of the system could be further optimized. Finally, it is worth mentioning that the system has been tested up to a YAP:Ce count rate of 3 105 c/s and the time resolution measured at such a rate is close to that reported above. A further increase of the counting rate would require, however, the use of specific front-end electronics to minimize random coincidences.
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the inspected volume typically of a cylinder of 35 cm diameter and 25 cm height at a distance of 50 cm from the neutron source. As an example such geometry can be applied in inspection of hand luggage or small parcels at customs or in counter-terrorism applications. Moreover, a substantial improvement of the voxel resolution of the system can be obtained by replacing the 4 cm diameter crystal with a matrix of smaller YAP:Ce scintillators giving the possibility to tailor the voxel dimensions to the specific inspection task. Further developments along this line are planned for the near future.
4. Summary and conclusions A Tagged Neutron Inspection System using a portable sealed neutron generator based on the D+T reaction has been developed for nondestructive inspections. The use of a tagged neutron beam is proposed as the means of improving the performance of Fast Neutron Analysis systems based on standard portable neutron generators, without having the large cost and complexity of the Pulsed Fast Neutron Analysis Systems that make use of a traditional particle accelerator. The prototype of the sealed neutron generator with an embedded charged particle detection system has been developed using standard, long lifetime generators and newly developed YAP:Ce scintillation detectors. Such material matches very well the requirements of this specific application in terms of light output, count rate capability and radiation hardness. Test of the neutron generator have been performed in a typical geometry for inspection of small parcels by using a standard NaI(Tl) detector, up to a neutron flux of 107 neutron/s. The gammaray spectrum from a graphite sample shows an impressing improvement in the signal to noise ratio when the proper gates on the measured coincidence time have been set. The measured time resolution of the system using NaI(Tl) detectors has been measured to be about dt ¼ 5 ns, which implies a resolution along the neutron path of about 25 cm. The performance of the present system is already suitable for several applications, namely
Acknowledgements This work is part of the collaboration between the Dipartimento di Fisica of the Padova University, the INFN Sezione di Padova and the EADS SODERN. It was partially funded by the INFN EXPLODET and AENEAS projects and by the MURST COFIN-9902195937 project ‘‘Detection of anti-personnel landmines by using a tagged beam of 14 MeV neutrons’’
References [1] T. Gozani, Nucl. Instr. and Meth. B 213 (2004) 460. [2] G. Vourvopoulos, P.C. Womble, Talanta 54 (2001) 459 (and references therein). [3] P. Bach, J.L. Ma, D. Froment, J.C. Jaureguy, Nucl. Instr. and Meth. B 79 (1993) 605. [4] D.R. Brown, T. Gozani, Nucl. Instr. and Meth. B 99 (1995) 753. [5] see for example V. Valkovic, et al., Nucl. Instr. and Meth. 76 (1969) 29. [6] A. Beyerle, et al., Nucl. Instr. and Meth. A 299 (1990) 458. [7] S. Mitra, et al., Appl. Radiat. Isot. 49 (1998) 537. [8] see for example J.D.L.H. Wood, Nucl. Instr. and Meth. 21 (1963) 49. [9] G. Viesti, et al., Nucl. Instr. and Meth. A 422 (1999) 918. [10] M. Lunardon, et al., Nucl. Instr. and Meth. B 213 (2004) 544. [11] www.sodern.fr. [12] from CRYTUR Turnov (CK). [13] see for example S. Baccaro, et al., Nucl. Instr. and Meth. A 361 (1995) 209.
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[14] S. Pesente, Ph.D. Thesis, University of Padova, 2003, unpublished. [15] S. Pesente, et al., Nucl. Instr. and Meth. A, (2004) in press.
[16] M. Lunardon, et al., Development of a low-cost PC based acquisition system using Flash ADC, Report DFPD 01/ NP/45 unpublished. [17] M. Moszynski, et al., Nucl. Instr. and Meth. A 404 (1998) 157.
Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480 www.elsevier.com/locate/nima
Performance of a tagged neutron inspection system (TNIS) based on portable sealed generators G. Nebbiaa,, S. Pesentea, M. Lunardona, G. Viestia, P. LeTourneurb, F. Heuvelineb, M. Mangeardb, C. Tchengb a
Dipartimento di Fisica dell’Universita` di Padova and INFN Sezione di Padova, via Marzolo 8, I-35131 Padova, Italy b EADS-SODERN, 20 Av. Descartes, 94451 Limeil-Brevannes, France Received 22 March 2004; received in revised form 24 May 2004; accepted 24 May 2004 Available online 26 July 2004
Abstract A portable sealed neutron generator has been modified to produce 14 MeV tagged neutron beams with an embedded YAP:Ce scintillation detector. The system has been tested by detecting the coincident gamma-rays produced in the irradiation of a graphite sample by means of a standard NaI(Tl) scintillator. Time resolution of about dt ¼ 425 ns (FWHM) has been measured. The sealed neutron tube has been operated up to 107 neutron/s. Possible applications in non-destructive assays and future developments of the Tagged Neutron Inspection System concept are discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.Dz; 29.40.Mc; 82.80.JP; 89.20.Bb Keywords: Tagged neutrons; Explosive detection
1. Introduction Non-destructive assay by fast neutrons is a technique widely employed in several fields [1]. As an example, the detection of hidden explosives [2] is achieved by determining the elemental ratio of C, N, O nuclei from the intensity of the characteristic gamma rays produced in fast neuCorresponding author. Tel.: +39-49-82-77-134; Fax: +39-
49-876-26-41. E-mail address: [email protected] (G. Nebbia).
tron-induced reactions. This technique, indicated as Fast Neutron Analysis (FNA), can be used to detect explosive in landmines or to inspect unexploded ordnance (UXO) [3]. The use of fast neutron inspection techniques is mainly limited by the fact that the neutron source and the gamma-ray detectors are placed as close as possible to the inspected object. In this condition, the signal-to-noise ratio depends on the fraction of neutrons that reach the sample and the solid angle subtended by the gamma-ray detector. The spectra are thus dominated by the gammas originated
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.05.137
ARTICLE IN PRESS 476
G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
close to the neutron source and to the gamma-ray detector. Consequently, it is difficult to search for hidden objects inside a large volume of background material since the spectra would be dominated by the background gammas. Such limitations might be avoided by using a pulsed, collimated neutron beam and by measuring the time delay between the production of the neutron burst and the arrival of the gamma rays in a detector array. This technique is indicated as Pulsed Fast Neutron Analysis (PFNA) [4]. While FNA systems usually employ small portable neutron generators, PFNA requires the use of particle accelerators to produce fast neutrons. Consequently, PFNA systems can be used only in specific application when the cost of the entire setup and its portability are not a relevant issue. In the last decade, efforts have been devoted to producing tagged neutron beams with compact sealed neutron generators. Such a task is, indeed, achieved routinely in open-end accelerators by using the well-known Associated Particle Technique where the T(D,n)4He or the D(D,n)3He neutron source reactions are used [5]. Examples of systems using the associated particle technique with sealed neutron generators are reported in Refs. [6,7]. Although the interest in obtaining a sealed neutron generator with embedded associated particle detector is a long-standing issue in neutron applications [8], such interest has increased significantly in recent times for all applications related to civil security after the 9/11 events. Within the EXPLODET (EXPLosive DETection) project [9,10], a new sealed neutron generator for the production of tagged neutron beams has been developed in a collaboration between INFN, the Universities of Padova and Bari (Italy) and EADS-SODERN (France). The results obtained in the development of the Tagged Neutron Inspection System are reported in this paper.
2. The tagged neutron sealed generator A standard SODERN neutron-emitting module has been modified to allow the insertion of an alpha particle detector [11]. Such a detector is mounted on a stainless steel CF63 flange, its inner
surface being located at 5.5 cm from the target. The total tritium activity of the sealed generator is about 1 GBq. A YAP:Ce scintillator [12] of 4 cm diameter and 0.5 mm thickness has been selected for the present application. The YAP:Ce exhibits, indeed, several characteristics that are very well suited for this task: excellent mechanical and chemical properties, radiation hardness, fast response and high light output [13]. The scintillator has been mounted on the CF63 flange equipped with a UV-extended sapphire window (3 mm thick, 48 mm diameter) without any optical grease, as required in the neutron generator manufacturing procedure [14]. Preliminary laboratory tests, performed with an 241 Am alpha particle source, demonstrated that the light output was reduced only by 30% with such mounting as with respect to the direct coupling between scintillator and photomultiplier (PMT). This reduction is not critical for the present application due to the excellent light output of the YAP:Ce scintillator and the use of the signal only for fast timing. Since the counting rate capability and time resolution are the major goals of the present application, we used a small diameter fast PMT Hamamatsu R1450, without a light guide. With such PMT, typical energy resolution on the order of dE a =E a ¼ 6% (FWHM) has been measured with small diameter YAP:Ce scintillators using alpha particles from an 241Am source. The energy resolution degrades when the whole 4 cm diameter crystal is irradiated; this fact will be further discussed in the following. Finally, the surface of the YAP:Ce crystal was coated with a layer of 1 mg/cm2 of metallic silver to maximize the light collection, stop the elastically scattered deuterons and protect the crystal from the light glow inside the neutron generator. The tagging system (i.e. the YAP:Ce mounted on the optical flange with the PMT and the associated front-end electronics) has been tested with neutrons produced in the D+T reaction making use of the 150 kV electrostatic neutron generator at the Institute Ruder Boskovic (IRB) in Zagreb. In this test, a 5.5 mm diameter collimator was used in front of the YAP:Ce crystal that was placed at about 5.5 cm from the T-target. It was
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
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Tests of the new system have been performed placing the neutron generator at about 50 cm from a graphite sample with dimensions 20 20 10 cm3. A standard 300 300 NaI(Tl) scintillator from Crysmatec (France) was placed on a side of the graphite sample, at a distance of about 28 cm from its center. The NaI(Tl) was protected from direct neutrons by a heavy metal shadow bar. Standard NIM electronics was used to process the signals from YAP:Ce and NaI(Tl) scintillators. In particular, energy signals were formed by using amplifiers with t ¼ 0:5 ms and t ¼ 1 ms shaping time for the YAP:Ce and NaI(Tl) scintillators, respectively. The anode signals were processed using standard Constant Fraction Discriminators. A fast coincidence between the YAP:Ce and the NaI(Tl) was used to start a Time to Amplitude Converter (TAC) and to provide the Master Gate to the acquisition system. The TAC was operated with a range of 200 ns, the stop signal being provided by the NaI(Tl) detector. The two energy signals and the analog output from the TAC were recorded on a four-channel Flash ADC card hosted on a portable PC [16]. Typical results obtained with this system are shown in Fig. 1, relative to a measurement run in which the counting rates of the YAP:Ce and NaI(Tl) detectors were 105 and 5 103 c/s, respectively. When the solid angle of the tagging detector (DO/O ¼ 3 102 ) is taken into account, the YAP:Ce counting rate corresponds to the operation of the neutron generator at a flux of about 0.3 107 neutron/s. In such conditions, the coincidence rate was 140 c/s, the data shown in the figures refer to an irradiation of about 45 min. The pulse-height spectrum of the YAP:Ce detector is shown in Fig. 1c. The continuous pulse-height distribution is due to the combination of several effects: the alpha particles produced in the D+T
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shown that the tagging system in the above conditions is capable of detecting the associated alpha particle with an energy resolution of about dE a =E a ¼ 25%. Results from the IRB tests are the subject of a separate paper [15].
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Fig. 1. (a) TAC spectrum; (b) pulse-height spectrum of the NaI(Tl) detector; (c) pulse-height spectrum of the YAP:Ce detector.
reaction have a finite energy distribution before entering the detector mainly due to the energy straggling of the alpha particles in the T–Ti target and in the Ag coating of the YAP:Ce. Such effects have been estimated by Monte Carlo calculations to account for an energy spread of about DE ¼ 0:2 MeV [14]. Moreover, since the PMT photocathode diameter is only 18 mm compared to the 40 mm of the scintillator, the alpha particles reaching the periphery of the detector produce lower pulse heights due to the reduced light collection efficiency of the system. The TAC spectrum, shown in Fig. 1a exhibits a peak with a width of dt ¼ 10:5 ns (FWHM) upon a continuous background of uncorrelated events. In this peak there are two classes of events: the
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
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gamma-rays emitted in the neutron irradiation of the graphite sample and the neutrons scattered from the sample into the NaI(Tl) detector. They are not resolved in time because of the short distance between the sample and the detector. As a consequence, in the coincident gamma spectrum the characteristic gamma-rays from the graphite sample (E g ¼ 4:44 MeV and its first escape at E g ¼ 3:94 MeV) are embedded on a large background, as reported in Fig. 1b. In this spectrum the peak-to-background ratio is P/B=0.5 at E g ¼ 4:4 MeV. One should notice that the spectrum of Fig. 1b refers to data taken in the full TAC range of 200 ns. In order to verify the performances of the system we have selected a region of the TAC spectrum corresponding to the first half of the time peak with a width of 6 ns (see Fig. 2a) as representative of the gammas that originated from the graphite sample irradiation. The second half of the time peak being roughly associated to the neutron scattered by the graphite inside the NaI(Tl) detector. By subtracting the background contribution selected in the time window on the left of the peak (see Fig. 2a), one obtains the energy spectrum shown in Fig. 2b. The spectrum now shows a very clear signature of the Eg ¼ 4:44 MeV gamma ray with its escape peaks. The peak-to-background ratio is now B/P=7 at E g ¼ 4:4 MeV. Moreover the selection of the graphite sample by the right time of flight gate results in a reduction of the low pulse-height region in the associated particle spectrum (see Fig. 2c). This is due to the smaller contribution of alpha particles in the periphery of the YAP:Ce scintillator . A second part of the analysis was devoted to the study of the time resolution of the system. The time resolution is important since it determines the minimum volume unit (voxel) that can be inspected using the TNIS. For this purpose we set a narrow gate on the Eg ¼ 4:44 MeV gamma-rays and on the high pulse-height part of the YAP:Ce spectrum, a typical measured time resolution is dt ¼ 4:4 ns (FWHM). Such measured time resolutions include several contributions that have to be determined to extract the intrinsic performance of the system. The intrinsic time resolution of the
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Fig. 2. (a) TAC spectrum with selected time gates; (b) background subtracted NaI(Tl) energy spectrum; (c) background subtracted YAP:Ce energy spectrum (see text for details).
NaI(Tl) detector has been measured in laboratory conditions to be dt ¼ 2:7 ns (FWHM) for 1.3 MeV gamma-rays. Moreover, the geometry of the experimental set-up induces an additional uncertainty of dt ¼ 2:1 ns (FWHM) due to the combined effects of the uncertainty in the interaction time of the fast neutrons inside the graphite sample and the jitter associated with the finite energy distribution of the alpha particles. The intrinsic time resolution of the tagged neutron system results are about dt ¼ 2:8 ns when both the geometry and the NaI(Tl) contributions are properly subtracted. We notice that this time
ARTICLE IN PRESS G. Nebbia et al. / Nuclear Instruments and Methods in Physics Research A 533 (2004) 475–480
resolution is much larger compared to the limiting value of the YAP:Ce detectors [7,17], indicating that the performances of the system could be further optimized. Finally, it is worth mentioning that the system has been tested up to a YAP:Ce count rate of 3 105 c/s and the time resolution measured at such a rate is close to that reported above. A further increase of the counting rate would require, however, the use of specific front-end electronics to minimize random coincidences.
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the inspected volume typically of a cylinder of 35 cm diameter and 25 cm height at a distance of 50 cm from the neutron source. As an example such geometry can be applied in inspection of hand luggage or small parcels at customs or in counter-terrorism applications. Moreover, a substantial improvement of the voxel resolution of the system can be obtained by replacing the 4 cm diameter crystal with a matrix of smaller YAP:Ce scintillators giving the possibility to tailor the voxel dimensions to the specific inspection task. Further developments along this line are planned for the near future.
4. Summary and conclusions A Tagged Neutron Inspection System using a portable sealed neutron generator based on the D+T reaction has been developed for nondestructive inspections. The use of a tagged neutron beam is proposed as the means of improving the performance of Fast Neutron Analysis systems based on standard portable neutron generators, without having the large cost and complexity of the Pulsed Fast Neutron Analysis Systems that make use of a traditional particle accelerator. The prototype of the sealed neutron generator with an embedded charged particle detection system has been developed using standard, long lifetime generators and newly developed YAP:Ce scintillation detectors. Such material matches very well the requirements of this specific application in terms of light output, count rate capability and radiation hardness. Test of the neutron generator have been performed in a typical geometry for inspection of small parcels by using a standard NaI(Tl) detector, up to a neutron flux of 107 neutron/s. The gammaray spectrum from a graphite sample shows an impressing improvement in the signal to noise ratio when the proper gates on the measured coincidence time have been set. The measured time resolution of the system using NaI(Tl) detectors has been measured to be about dt ¼ 5 ns, which implies a resolution along the neutron path of about 25 cm. The performance of the present system is already suitable for several applications, namely
Acknowledgements This work is part of the collaboration between the Dipartimento di Fisica of the Padova University, the INFN Sezione di Padova and the EADS SODERN. It was partially funded by the INFN EXPLODET and AENEAS projects and by the MURST COFIN-9902195937 project ‘‘Detection of anti-personnel landmines by using a tagged beam of 14 MeV neutrons’’
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