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The characterization of legacy radioactive materials by gamma spectroscopy and prompt gamma activation analysis (PGAA)
Nuclear Instruments and Methods in Physics Research B 213 (2004) 410–413 www.elsevier.com/locate/nimb
The characterization of legacy radioactive materials by gamma spectroscopy and prompt gamma activation analysis (PGAA) Gerald A. English a,*, Richard B. Firestone a, Dale L. Perry a, Jani Reijonen a, Bernhard Ludewigt a, Ka-Ngo Leung a, Glenn Garabedian b, Gabor Molnar c, Zsolt Revay c a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 51-208, Berkeley, CA 94720, USA b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Institute of Isotope and Surface Chemistry, H-1525, Budapest, Hungary
Abstract To characterize legacy radioactive materials, it is necessary to determine both the radioactive and, in the case of carrier-based materials, the stable, non-radioactive chemical constituents. Reputable process knowledge may afford some insight but, absent such information, gamma spectroscopy and (non-destructive) prompt gamma activation analysis (PGAA) cover essentially all of the analytical needs, with the former addressing most radionuclides with the exception of the pure b -emitters and the latter addressing the stable chemical constituents. This paper integrates both methods into a general analytical protocol based upon radioanalytical work performed at Lawrence Berkeley National Laboratory (LBNL) and PGAA work performed collaboratively by the various groups. A new LBNL-developed neutron generator is also discussed. 2003 Elsevier B.V. All rights reserved. PACS: 25.40.Lw; 29.25.Dz; 29.30.Kv; 81.70.Jb Keywords: Neutron; Gamma; Spectroscopy; Analysis; Radioactive; Chemical
1. Introduction To characterize legacy radioactive materials, it is necessary to determine the entrained radioactivity and, if present, any associated stable chemical constituents. If reputable historical records are
*
Corresponding author. Tel.: +1-510-486-4054/7673; fax: +1-510-495-2384. E-mail address: [email protected] (G.A. English).
available, process knowledge may provide important insight in this regard. However, for most legacy materials, such information is either lacking or incomplete and analysis is required. With respect to the analysis of stable chemical constituents, current methods employ wet-chemical techniques that may expose the analyst to potentially hazardous conditions including unnecessary radiation fields and the laboratory to potential radiological problems including radioactive contamination and the generation of radwaste with its
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01665-3
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413
attendant costs. To address the analysis of entrained radioactivity, gamma spectroscopy is used to determine all radionuclides except the pure b emitters. To address the analysis of stable chemical constituents, prompt gamma activation analysis (PGAA) may be used. This latter method circumvents the difficulties attendant with wet-chemical methods because PGAA is non-destructive and may be used to assay materials in situ, without any preparatory work. The following sections describe the process to perform a comprehensive legacy analysis.
2. Experimental Analysis of LBNL legacy radioactive materials requires the use of a high-efficiency, high-purity germanium (HPGe) detector/multichannel analyzer (MCA) system, with a calibrated energy range of 5 keV to 2 MeV [1] for decay gammas or 11 MeV for PGAA. LBNL has not used PGAA to assay legacy materials to date but has performed an experiment to determine the utility of the new LBNL-developed, self-replenishing, low power (600 W on target), deuterium–deuterium (i.e. D + D)-based, neutron generator (3 · 107 neutrons/s, isotropic) to conduct PGAA. Polyethylene is used as the moderator where En ðmaxÞ 2:5 MeV. Fig. 1 provides the relevant data from that experiment. The lines at 478 and 2223 keV represent true prompt gamma lines from
411
thermal neutron capture in 10 B and 1 H, respectively. The line at 1778 keV is the main decay gamma from the decay of radioactive 28 Al, the activation product of stable Al. This latter observation indicates that the LBNL neutron generator may be used to produce radioactive activation products that may themselves be used to assay stable elements, especially if the half-lives are relatively short, as is the case for 28 Al (t1=2 2:24 min). The prompt gamma library consists of 33,000 literature-based lines representing all isotopes from 1 H to 238 U, inclusive, where the relative cross section yields have been normalized to precise measurements obtained for all elements at the Budapest reactor. PGAA library data from all reliable sources were consequently merged into the final database.
3. Results and discussion Gamma spectroscopic analyses at LBNL have been relatively straightforward with the exception of the analysis of the actinides. In the 5f-series of elements, care must be exercised to account for complex decay schemes that are not generally
29.1y 29.1y 243 243
Cm
ε (0.29%) (0.29%) α (99.71%) (99.71%) 7370y 243Am
α
2.3565d 239
Np
24110y 239Pu
Fig. 1. LBNL PGAA spectrum.
Fig. 2.
243
Cm and
243
α
Am decay relationship.
412
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413 68.9y 232U
α 1.405 x 1010y 232Th
α 5.75y 228
Ra β-
6.15h 228
Ac β1.9131y 228Th
Fig. 3.
232
U and
232
α
Th decay relationship.
found in the decay of non-actinide radionuclides. In particular, special decay modes involving convergent decay pathways with related or unrelated parents may present a serious problem to the analyst (Figs. 2 and 3). With respect to PGAA, Fig. 1 indicates that the LBNL-developed neutron generator does have utility in the (non-destructive) PGAA process. Of special importance, the D + D mode affords the analyst with a means to generate neutrons using an accelerator but without employing tritium (i.e. a D + T system) and may, therefore, avoid the attendant difficulties that accompany the use of this radionuclide.
4. Conclusion and future plans The PGAA data in Fig. 1 indicate that PGAA may be performed on the stable chemical constituents of radioactive legacy materials. LBNL plans to perform additional experiments to optimize PGAA analysis of legacy radioactive materials. If a multitude of decay and prompt gammas is present, special techniques to reduce background like the use of bismuth germanate (Bi4 Ge3 O12 )-based anti-coincidence for Compton-suppression or
generator pulsing may be employed. Since PGAA lines cover a range of energies from relatively low to a maximum of 11 MeV, software may be developed to identify and eliminate pair-production peaks that may significantly complicate spectral analysis. Improvements in detector configurations and system optimization may offer capabilities for enhancing line intensities especially for those lines with relatively weak PGAA sensitivities. While the neutron output of the existing generator type may be substantially increased, LBNL has been developing new neutron generator designs capable of producing much higher neutron fluxes. The next generation devices are expected to yield neutron fluxes >1010 neutrons/s and, thus, should afford better PGAA results. A sealed version of the neutron generator may be operated with a deuterium–tritium gas mixture, that is, a D + T generator which may have utility in the characterization of denser materials like cemented waste where the higher En ðmaxÞ of 14.5 MeV affords better penetration. Also, D + T systems generate an 100 times higher fast neutron flux compared to D + D with the potential to impart a factor of 10 to K 100 times the thermal neutron flux on target. LBNLÕs neutron generator design is scalable to very high neutron outputs, which could eventually exceed 1012 neutrons/s (D + D) and 1014 neutrons/s (D + T). To increase neutron capture cross sections further, ÔcoldÕ neutrons may be used. In this approach, thermal neutrons are cooled with any of various cryogenic agents (e.g. liquid hydrogen) placed outside a (internally nickel-coated) guide tube and, in accordance with the so-called ‘‘1=v law’’, the capture cross section increases as neutron temperature and, consequently, p neutron veffiffiffiffi locity decreases, since rn;c / 1=v / 1= T . Studies using fast neutrons (e.g. from a D + T generator) to excite carbonaceous materials, whether lightly or highly concatenated aliphatic or aromatic organics, by fast-neutron reactions [e.g. ðn; aÞ, ðn; 2nÞ, ðn; pÞ, etc.] will be important to the characterization of legacy materials of an organic nature. Timing circuitry involving the use of advanced timing software and the LBNL neutron generator operated in pulsed mode may provide capabilities for imaging (three-dimensional) legacy items inside containers that are better assayed
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413
413
closed due to potential hazards to personnel, i.e. prompt gamma tomography (PGT).
US Department of Energy under contract no. DEAC03-76SF00098.
Acknowledgements
Reference
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the
[1] K. Debertin, R.G. Helmer, Gamma- and X-Ray Spectrometry with Semiconductor Detectors, North Holland, 1988.
The characterization of legacy radioactive materials by gamma spectroscopy and prompt gamma activation analysis (PGAA) Gerald A. English a,*, Richard B. Firestone a, Dale L. Perry a, Jani Reijonen a, Bernhard Ludewigt a, Ka-Ngo Leung a, Glenn Garabedian b, Gabor Molnar c, Zsolt Revay c a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 51-208, Berkeley, CA 94720, USA b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Institute of Isotope and Surface Chemistry, H-1525, Budapest, Hungary
Abstract To characterize legacy radioactive materials, it is necessary to determine both the radioactive and, in the case of carrier-based materials, the stable, non-radioactive chemical constituents. Reputable process knowledge may afford some insight but, absent such information, gamma spectroscopy and (non-destructive) prompt gamma activation analysis (PGAA) cover essentially all of the analytical needs, with the former addressing most radionuclides with the exception of the pure b -emitters and the latter addressing the stable chemical constituents. This paper integrates both methods into a general analytical protocol based upon radioanalytical work performed at Lawrence Berkeley National Laboratory (LBNL) and PGAA work performed collaboratively by the various groups. A new LBNL-developed neutron generator is also discussed. 2003 Elsevier B.V. All rights reserved. PACS: 25.40.Lw; 29.25.Dz; 29.30.Kv; 81.70.Jb Keywords: Neutron; Gamma; Spectroscopy; Analysis; Radioactive; Chemical
1. Introduction To characterize legacy radioactive materials, it is necessary to determine the entrained radioactivity and, if present, any associated stable chemical constituents. If reputable historical records are
*
Corresponding author. Tel.: +1-510-486-4054/7673; fax: +1-510-495-2384. E-mail address: [email protected] (G.A. English).
available, process knowledge may provide important insight in this regard. However, for most legacy materials, such information is either lacking or incomplete and analysis is required. With respect to the analysis of stable chemical constituents, current methods employ wet-chemical techniques that may expose the analyst to potentially hazardous conditions including unnecessary radiation fields and the laboratory to potential radiological problems including radioactive contamination and the generation of radwaste with its
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01665-3
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413
attendant costs. To address the analysis of entrained radioactivity, gamma spectroscopy is used to determine all radionuclides except the pure b emitters. To address the analysis of stable chemical constituents, prompt gamma activation analysis (PGAA) may be used. This latter method circumvents the difficulties attendant with wet-chemical methods because PGAA is non-destructive and may be used to assay materials in situ, without any preparatory work. The following sections describe the process to perform a comprehensive legacy analysis.
2. Experimental Analysis of LBNL legacy radioactive materials requires the use of a high-efficiency, high-purity germanium (HPGe) detector/multichannel analyzer (MCA) system, with a calibrated energy range of 5 keV to 2 MeV [1] for decay gammas or 11 MeV for PGAA. LBNL has not used PGAA to assay legacy materials to date but has performed an experiment to determine the utility of the new LBNL-developed, self-replenishing, low power (600 W on target), deuterium–deuterium (i.e. D + D)-based, neutron generator (3 · 107 neutrons/s, isotropic) to conduct PGAA. Polyethylene is used as the moderator where En ðmaxÞ 2:5 MeV. Fig. 1 provides the relevant data from that experiment. The lines at 478 and 2223 keV represent true prompt gamma lines from
411
thermal neutron capture in 10 B and 1 H, respectively. The line at 1778 keV is the main decay gamma from the decay of radioactive 28 Al, the activation product of stable Al. This latter observation indicates that the LBNL neutron generator may be used to produce radioactive activation products that may themselves be used to assay stable elements, especially if the half-lives are relatively short, as is the case for 28 Al (t1=2 2:24 min). The prompt gamma library consists of 33,000 literature-based lines representing all isotopes from 1 H to 238 U, inclusive, where the relative cross section yields have been normalized to precise measurements obtained for all elements at the Budapest reactor. PGAA library data from all reliable sources were consequently merged into the final database.
3. Results and discussion Gamma spectroscopic analyses at LBNL have been relatively straightforward with the exception of the analysis of the actinides. In the 5f-series of elements, care must be exercised to account for complex decay schemes that are not generally
29.1y 29.1y 243 243
Cm
ε (0.29%) (0.29%) α (99.71%) (99.71%) 7370y 243Am
α
2.3565d 239
Np
24110y 239Pu
Fig. 1. LBNL PGAA spectrum.
Fig. 2.
243
Cm and
243
α
Am decay relationship.
412
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413 68.9y 232U
α 1.405 x 1010y 232Th
α 5.75y 228
Ra β-
6.15h 228
Ac β1.9131y 228Th
Fig. 3.
232
U and
232
α
Th decay relationship.
found in the decay of non-actinide radionuclides. In particular, special decay modes involving convergent decay pathways with related or unrelated parents may present a serious problem to the analyst (Figs. 2 and 3). With respect to PGAA, Fig. 1 indicates that the LBNL-developed neutron generator does have utility in the (non-destructive) PGAA process. Of special importance, the D + D mode affords the analyst with a means to generate neutrons using an accelerator but without employing tritium (i.e. a D + T system) and may, therefore, avoid the attendant difficulties that accompany the use of this radionuclide.
4. Conclusion and future plans The PGAA data in Fig. 1 indicate that PGAA may be performed on the stable chemical constituents of radioactive legacy materials. LBNL plans to perform additional experiments to optimize PGAA analysis of legacy radioactive materials. If a multitude of decay and prompt gammas is present, special techniques to reduce background like the use of bismuth germanate (Bi4 Ge3 O12 )-based anti-coincidence for Compton-suppression or
generator pulsing may be employed. Since PGAA lines cover a range of energies from relatively low to a maximum of 11 MeV, software may be developed to identify and eliminate pair-production peaks that may significantly complicate spectral analysis. Improvements in detector configurations and system optimization may offer capabilities for enhancing line intensities especially for those lines with relatively weak PGAA sensitivities. While the neutron output of the existing generator type may be substantially increased, LBNL has been developing new neutron generator designs capable of producing much higher neutron fluxes. The next generation devices are expected to yield neutron fluxes >1010 neutrons/s and, thus, should afford better PGAA results. A sealed version of the neutron generator may be operated with a deuterium–tritium gas mixture, that is, a D + T generator which may have utility in the characterization of denser materials like cemented waste where the higher En ðmaxÞ of 14.5 MeV affords better penetration. Also, D + T systems generate an 100 times higher fast neutron flux compared to D + D with the potential to impart a factor of 10 to K 100 times the thermal neutron flux on target. LBNLÕs neutron generator design is scalable to very high neutron outputs, which could eventually exceed 1012 neutrons/s (D + D) and 1014 neutrons/s (D + T). To increase neutron capture cross sections further, ÔcoldÕ neutrons may be used. In this approach, thermal neutrons are cooled with any of various cryogenic agents (e.g. liquid hydrogen) placed outside a (internally nickel-coated) guide tube and, in accordance with the so-called ‘‘1=v law’’, the capture cross section increases as neutron temperature and, consequently, p neutron veffiffiffiffi locity decreases, since rn;c / 1=v / 1= T . Studies using fast neutrons (e.g. from a D + T generator) to excite carbonaceous materials, whether lightly or highly concatenated aliphatic or aromatic organics, by fast-neutron reactions [e.g. ðn; aÞ, ðn; 2nÞ, ðn; pÞ, etc.] will be important to the characterization of legacy materials of an organic nature. Timing circuitry involving the use of advanced timing software and the LBNL neutron generator operated in pulsed mode may provide capabilities for imaging (three-dimensional) legacy items inside containers that are better assayed
G.A. English et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 410–413
413
closed due to potential hazards to personnel, i.e. prompt gamma tomography (PGT).
US Department of Energy under contract no. DEAC03-76SF00098.
Acknowledgements
Reference
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the
[1] K. Debertin, R.G. Helmer, Gamma- and X-Ray Spectrometry with Semiconductor Detectors, North Holland, 1988.