- Email: [email protected]
Determination of the gamma emission intensities of 111Ag
Applied Radiation and Isotopes 87 (2014) 107–111
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Determination of the gamma emission intensities of
111
Ag
Sean Collins a,n, John Keightley a, Chris Gilligan b, Joel Gasparro a, Andy Pearce a a b
National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom Atomic Weapons Establishment, Aldermaston, Reading, RG7 4PR, United Kingdom
H I G H L I G H T S
Gamma emission probabilities of 111Ag determined. Absolute intensity of 342.1 keV of 111Ag determined. Gamma emission of 450.0 keV confirmed and measured. Significant discrepancies in published gamma emission probabilities outlined.
art ic l e i nf o
a b s t r a c t
Available online 12 November 2013
A radioactive solution of 111Ag, standardised by the absolute measurement methods 4π(PC)-γ and 4π(LS)-γ coincidence counting at the National Physical Laboratory (NPL), was measured by two independently calibrated HPGe γ spectrometers in order to estimate the γ emission intensities and to determine the absolute intensity, with the aim of improving the currently published values. An absolute intensity value of 6.68 (7)% was obtained for the 342.1 keV γ emission, which is in agreement with previously reported values, but greatly reduces the uncertainty. Additionally, this work proposes a new emission intensity for the 450.0 keV γ emission that has not been previously reported, with an absolute intensity of 0.000482 (12)%. An investigation of the published γ emission intensities shows significant discrepancies that require resolution. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Ag-111 Gamma emission intensities Spectroscopy Radioactivity
1. Introduction The radionuclide 111Ag is a short-lived product of the fission of uranium and plutonium isotopes, with a half-life of 7.45 (1) days (Blachot, 2009). It is used as a minor component in the verification scheme of the Comprehensive Test-Ban Treaty, (De Geer, 1999), and has also been proposed as a potential radiopharmaceutical due to its retention in synovial joints (Chattopadhyay et al., 2008). According to the decay scheme by Firestone et al. (1996), decay of this nuclide proceeds entirely by beta-minus emissions to six excited levels of 111Cd (Fig. 1). The three major beta-minus branches consist of a 92% intensity branch direct to the ground state of 111Cd, a 7.1% intensity branch to the 342.1 keV excited level and 1% branch to the 245.4 keV excited level. Investigation of the current state of recently published data (Blachot, 2009, p. 1311) revealed that the evaluation of the γ emission intensities have been derived from a small selection of publications exhibiting a large variation in their reported values. Additionally, only one measurement of the absolute intensity of the 342.1 keV γ emission has been made by Nethaway and Prindle (1977), the measured value has a large uncertainty of 4.5% making
n
Corresponding author. Tel.: þ 44 208 943 8508. E-mail addresses: [email protected], [email protected] (S. Collins).
the use of this nuclear data for the measurement of the activity of 111 Ag by γ spectrometry less than ideal. Preliminary γ spectrometry measurements (utilising the published data of Blachot (2009)) on the stock solution used in this work yielded variations between the activities determined using the 342.1 keV γ emission and those determined utilising the 96.7 keV and 245.4 keV γ emissions, with a low bias of approximately 14% and 18%, respectively. Therefore it was decided to investigate these anomalies via a series of measurements of a solution of 111Ag that was standardised by the two independent primary standardisation techniques, 4π(LS)-γ digital coincidence counting and 4π(PC)-γ coincidence counting, to provide improved estimates of the γ emission intensities. 2. Experimental 2.1. Preparation of
111
Ag
The 111Ag solution was produced by the neutron irradiation of a 1.1 g palladium wire, prepared at NPL, in the 2 MW ‘swimming pool’ type Research reactor at Technische Universiteit Delft, Netherlands. To remove the palladium radioisotopes of 103Pd, 107 Pd and 109Pd produced during the irradiation, the palladium wire was dissolved in hot concentrated HNO3 with 1 drop of HCl
0969-8043/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.007
108
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
Fig. 1. Decay scheme for
111
Ag (Firestone et al., 1996).
added. Inactive AgNO3 was added to increase the total mass of silver in the solution. NaCl was introduced to produce AgCl which was separated by micro precipitation and filtration, the AgCl was then dissolved in ammonia and reduced back to silver metal using ascorbic acid before finally being re-dissolved using 8 mol dm 3 HNO3. This solution was diluted to produce a final 1 mol dm 3 HNO3 solution that was used to prepare the measurement samples. 2.2. Primary standardisation of the activity concentration of
111
Ag
The primary standardisation of the material was performed utilising two of the NPL coincidence counting systems. The first incorporated 4π(LS)-γ Digital Coincidence Counting (Keightley and Watt, 2002) utilising an NPL built Liquid Scintillation (LS) counter (with two photomultiplier tubes operating in coincidence for background reduction) along with a 10 cm NaI(Tl) detector for registration of γ photons. The series of LS sources consisted of accurately weighed aliquots of the stock solution (between 30 and 70 mg) added to 10 g of Ultima Gold AB and 0.5 g of 1 mol dm 3 HNO3. The second system incorporated 4π(PC)-γ coincidence counting with the pill-box type proportional counter (PC) operating at atmospheric pressure with P10 gas used as the counting medium. A 10 cm NaI(Tl) detector was utilised for registration of γ photons. Accurately weighed aliquots of the solution were dispensed onto a series of gold-coated VYNS foils (intrinsic VYNS thickness approximately 30 mg cm 2; gold thickness approximately 10 mg cm 2). For both systems, a γ gate was set to encompass the 342.1 keV photopeak, and corrections for “out-of-channel” γ photons were employed. The range of beta-channel efficiencies utilised in the efficiency extrapolations were 96.5%–89.8% for the 4π(LS)-γ system, and 93.1%–89.0% for the 4π(PC)-γ system. Both systems yielded similar results for the activity concentration: 599 (6) and 601 (6) kBq g 1 (k¼1), respectively. The impurities of 110mAg and 198 Au (discussed in the next section) made a negligible contribution to the final estimate of the absolute disintegration rate. An unweighted mean was used to estimate of the activity concentration of the solution, and was calculated as 600 (6) kBq g 1 (k ¼1). 2.3. Gamma emission intensity measurements of
111
Ag
Two sets of samples were prepared gravimetrically, with one aliquot samples dispensed to two 2 ml ISO ampoules (ISO, 2010) and two aliquot samples dispensed to four 20 ml plastic scintillation vials. These solutions were measured by two independent HPGe γ spectrometers. The p-type detector ‘BART’ with a beryllium window and a relative efficiency of 28.9% was used to measure the
solution dispensed to the ISO ampoules. The semi-planar detector ‘LANCELOT’ with a carbon fibre detector window and a relative efficiency of 65% was used to measure the solution contained in the plastic scintillation vials. Both detectors used a graded shield of Pb, Cd and Cu and utilised identical electronic set-ups, incorporating a Canberra AFT research amplifier (6 ms shaping time) and utilising the “PUR/DT” circuitry to correct the Live Time for pile-up and dead time. The detectors were energy calibrated using a 152Eu source with peak centroids set within 0.1 keV for γ emissions with intensities in excess of 1%, using the evaluated energies from the Decay Data Evaluation Project (BIPM, 2004). The photon detection efficiency calibration for each detector was performed using solutions of a mixed radionuclide standard containing 241Am, 109Cd, 57Co, 139Ce, 51 Cr, 113Sn, 85Sr, 137Cs, 54Mn, 88Y, 65Zn and 60Co in 4 mol dm 3 HCl. Additional calibration points for BART were added using solutions of single radionuclides of 51Cr, 75Se, 85Sr, 109Cd, 125I, 141Ce and 210 Pb, where the individual radionuclides had been standardised at NPL using the NPL re-entrant Ionisation Chamber which is directly traceable to primary standards. Each calibration sample was in a matched geometry for the measurement samples. The samples measured by ‘BART’ were placed at a distance of 25 cm from the detector window, for a live time of between 12,000 and 85,000 s. The samples measured by ‘LANCELOT’ were placed at 10 cm from the detector window, for a live time ranging from 14,400 to 86,400 s. The acquisition times were of sufficient length to ensure uncertainty on the counts for the 96.7 keV, 245.4 keV and 342.1 keV photopeaks were less than 0.3%. The distances that were used for the measurements allowed for the assumption that the effect of cascade summing to be negligible for both detectors. A total of eight spectra were acquired: two for ‘BART’ and six for ‘LANCELOT’. The peak areas were analysed using the CANBERRA GENIE 2000 software, with all peaks manually checked using the Interactive Peak Fitting application within this software. Dead time, pulse pile-up and background peak area corrections were all applied using the CANBERRA GENIE 2000 software. All the photopeaks in the spectra were readily identifiable (Fig. 2), with 110m Ag and 198Au identified as impurities with activity concentrations of 37.4 (4) Bq g 1 and 11.0 (5) Bq g 1 respectively (k ¼1). Corrections were also made for the interference of 620.3 keV γ photon emissions (from the decay of 110mAg) with those of 111Ag at similar energies (619.3 keV, 620.2 keV and 621.2 keV). The convoluted peaks of 620.3 keV and 621.2 keV were separated using the interactive peak fitting application and reported as individual values. There was no indication of a photopeak at 619.3 keV. The photopeak at 450.0 keV was identified as the γ emission of 111Ag that has been identified in the decay scheme determined by Firestone et al. (1996) but has no published γ emission values. Additionally there was no indication of a photopeak at 865.9 keV and as such the 866.7 keV γ emission has been reported separately. Due to the difference in the density of the solutions between the efficiency calibration and the sample of interest, corrections were made to compensate for the different attenuations of the γ emissions. This required two different methods due to the different sample mounting mechanisms used. For the measurements performed on ‘BART’ a far-field model for cylindrical samples (Parker, 1984) was used. While for ‘LANCELOT’ a simple 1dimensional model was utilised (Parker, 1984). The linear attenuation coefficient was calculated for each of the different chemistries using data from the NIST XCOM database (Berger et al., 2010) These corrections amounted to less than 0.5% for ‘BART’ and 0.9% for ‘LANCELOT’ for any individual γ emission, with the largest correction applied to the 96.7 keV photopeak detection efficiency, with the size of the correction reducing in size with the increase of the γ emission energy.
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
Fig. 2. Example of acquired gamma spectrum of
Table 1 The absolute γ emission intensities of 111Ag per 100 decays. The photon energies derived from the energy calibration described in Section 2.3. Photon energy /keV
γ Emission intensity /%
96.7 245.4 342.1 374.8 450.0 524.5 620.2 621.2 866.7
0.1018 1.113 6.68 0.00240 0.000482 0.00376 0.00883 0.0161 0.00773
(5) (5) (6) (5) (5) (6) (6) (6) (7)
(14) (14) (7) (7) (12) (10) (14) (4) (11)
3. Results The absolute γ emission intensities were derived from the activity determination by primary standardisation methods and the measurement of the 111Ag γ emissions by two HPGe γ spectrometers. These results are presented in Table 1 as the absolute γ emission intensities in per cent of disintegrations. The absolute intensity of the 342.1 keV γ emission was deduced as 6.68 (7)% with the uncertainty budget presented in Table 3. This is in excellent agreement with the previously reported value of 6.68 (33)% (Nethaway and Prindle, 1977). An earlier value of 6.0 (15)% has been reported by Robinson and Langer (1958), however they reported that a NaI(Tl) scintillation detector was used, with count-rates compared to the beta counting rate using a 4πβ spectrometer (thus not utilising the power of the coincidence method). This may be the cause of the large uncertainty attributed to this result. The relative γ emission intensities were calculated as the arithmetic mean of the corrected normalised values (the 342.1 keV intensity being taken equal to 100), the results of which can be viewed in Table 2, which includes a comparison with the evaluated values of (Blachot, 2009) and the experimental values of Goswamy et al. (1992) which were not included in the evaluation. As can be seen in Fig. 2, there were no indications in the observed spectra, of the γ emissions of 278.3 keV, 509.4 keV, 522.4 keV and 754.6 keV, additionally (not seen in Fig. 2) no convoluting photopeaks were found at 619.3 keV or 865.9 keV. The lack of an observable 278.3 keV photopeak may be explained
109
111
Ag.
Table 2 Comparison of the previously evaluated intensities by Blachot (2009) and Goswamy et al. (1992), normalised to the 342.1 keV transition and those determined in this work. The uncertainties represent one standard deviation, coverage factor k ¼ 1. Further explanations of these results are given in Section 3. Photon energy /keV
This work
Goswamy et al. (1992)
Blachot (2009)
96.7 245.4 342.1 374.8 449.8 522.4 524.5 619.3 620.2 621.2 754.6 865.1 866.7
1.524 (14) 16.66 (11) 100 0.0360 (9) 0.00721 (17) – 0.0563 (13) – 0.1322 (14) 0.242 (4) – – 0.1157 (11)
1.56 (2) 16.35 (20) 100 0.039 (2) – 0.059 (3)
1.73 (9) 18.5 (10) 100 0.047 (2) – 0.014 (2) 0.031 (2) 0.008 (4) 0.164 (12) 0.09 (3) 0.04 (-) 0.023 (4) 0.054 (4)
0.38 (1)
0.0026 (5) 0.112 (3)
Table 3 Uncertainty budget for the absolute γ emission intensity of the 342.1 keV γ emission. Uncertainty component
Relative uncertainty % (k ¼ 1)
Standard deviation Dead-time, pile-up Attenuation corrections Photopeak efficiency Activity concentration Gravimetric
0.05 0.10 0.010 0.5 1.0 0.10
Total
1.3
by the combination of the small γ emission intensity and the fact that it appears in the Compton continuum stemming from 342.1 keV γ emissions. The γ emissions at 509.4 keV reside in the area of the background 511 keV annihilation photopeak, though it would be expected to still be able to see a distortion in the photopeak due to its presence. The 522.4 keV is not thought to be part of the 524.5 keV photopeak in the acquired spectra; the FWHM of the 524.5 keV photopeak being 1.42 keV, as the photopeak does not show any distortion in the shape of the photopeak
110
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
that would be apparent if present. The 754.6 keV transitions were also not visible in the acquired spectra. At the currently evaluated intensities these would have been expected to be visible within the spectra. Further investigation reveals that the 509.4 keV has only been detected by one previous author (Burmistrov and Didorenko, 1974) and the 754.6 keV γ emissions has only been detected by two of the previous authors (Burmistrov and Didorenko, 1974 and Goswamy et al., 1992). The lack of the 754.6 keV and 509.4 keV and 619.3 keV photopeaks could possibly indicate that the 765.9 keV level does not exist or that its feeding is significantly smaller than currently reported. 3.1. Differences between evaluated values and present work There are large differences between the values proposed in this work and the currently evaluated data, as seen in Table 2. Though the values presented in this work are in good agreement with the values proposed by Goswamy et al. (1992). These values have not been incorporated in the evaluation performed by Blachot (2009). Additionally, as previously mentioned the measured results exhibit significant ranges in the values and are discrepant as seen in Table 4. These values and their spread are indicative of the paucity of the γ emission measurements currently available for 111Ag. 3.2. Validity of the 450 keV gamma emission The 450.0 keV γ emission reported in this work has not been reported previously, while it has been proposed in the decay scheme, shown in Fig. 1, by Firestone et al. (1996), The photopeak was present in four of the eight spectra collected, from both detectors used, being visible in the early spectra before the decay reduced the count rate to below the limit of detection. The determined γ emission intensity was relatively consistent across the five spectra after decay and all other corrections were made with a standard deviation of 3.2% (Fig. 3). Considering the large uncertainty due to the counting statistics of between 8 and 15% due to the small number of counts collected (less than 500 in 86,400 s), the consistency of the results are very good, therefore reducing the possibility of the photopeak coming from random or true summing. The remaining possibility is that the photopeak originates from another radionuclide, though no other known γ emission that is possible exists at the same energy. Both of these points of evidence therefore indicate that this is a real γ emission of 111Ag. 3.3. Validation of proposed gamma emission intensities The validity of the newly estimated γ emission intensities from our work was confirmed by utilising the ‘SIRIC’ software (Michotte
et al., 2006) to predict the calibration factor of the NPL re-entrant ionisation chamber of type TPA-Mk II, for 2 ml ampoules. Using the γ emission intensities of Blachot (2009) yielded a predicted calibration factor of 1.22 (6) pA MBq 1 (k ¼1), and when applied to the measured current from a series of 2 ml ampoules measured in the NPL ionisation chamber yielded an estimate of the activity concentration of the stock solution of 582 (30) kBq g 1. Incorporating the newly derived emission intensities from this work yielded a predicted calibration factor of 1.182 (14) pA MBq 1 (k ¼1), which resulted in an estimate of the activity concentration of the stock solution of 598 (7) kBq g 1. This compares favourably to the primary standardisation result of 600 (6) kBq g 1 (k ¼1). A calibration factor was measured for the ionisation chamber and resulted in a calibration factor of 1.178 (12) pA MBq 1 which is in agreement with the predicted values of ‘SIRIC’ using the new values presented in this work.
4. Conclusions In this work, the principal γ emission of 342.1 keV in 111Ag decay was experimentally determined to be 6.68 (7)% (k ¼1). The use of absolute counting techniques to determine the activity and two calibrated independent HPGe γ detectors has allowed the precision measurement of the γ emissions with confidence in the values reported in this work. This work also proposes the first γ emission intensity for the γ emission of 450.0 keV of 111Ag, which was calculated with an absolute intensity of 0.000482 (12)%.
Fig. 3. Relative emission intensity for each measurement of the 450.0 keV γ emission. The error bars show the uncertainty of the photopeak area for each measurement and the dashed line indicating the arithmetic mean of the measurements.
Table 4 Comparison of the published relative intensities. The value reported for this work for the 619.3, 620.2 and 621.2 keV γ emissions is the combined value of the 620.2 and 621.2 keV emissions. The γ emissions not reported in this work have been omitted from the table. Ref.
Robinson and Langer (1958) Hnatowich and Coryell (1970) Heath (1974) Burmistrov and Didorenko (1974) Purdum et al (1975) Shevelev et al (1975) Goswamy et al. (1992) This work Critical value χ2
Photon energy /keV 96.7
245.4
374.8
524.5
619.3, 620.2 and 621.2
865.1 and 866.7
– 4.2 (1) 1.6 (2) 3.8 (-) 3.4 (2) 1.8 (2) 1.56 (2) 1.524 (14) 3.02 158
16 (8) 20.6 (2) 16.9 (20) – 19.1 (13) 18.4 (4) 16.35 (20) 16.66 (11) 2.80 4.75
– 0.048 (2) – 0.04 (-) 0.043 (4) – 0.039 (2) 0.0360 (9) 3.79 10.5
– – – – – 0.050 (9) 0.059 (3) 0.0563 (13) 4.61 0.62
– – – – – 0.42 (4) 0.38 (1) 0.374 (4) 4.61 0.79
– 0.127 (11) – 0.08 (-) 0.077 (10) 0.165 (18) 0.112 (3) 0.1157 (11) 3.32 6.18
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
The ionisation chamber calibration factor prediction software ‘SIRIC’ was used as a method to test the validity of the new γ emission values presented in this work. This method showed an improvement in the predicted response against the real response, therefore providing some proof of the reliability of the values presented. Additionally, the published values of the relative γ emissions of 111 Ag were compared pointing out large discrepancies within the sets of data for the individual photon intensities. These discrepancies indicate problems with the currently available γ emission intensity data for 111Ag (Blachot, 2009) and therefore it is suggested that further investigations are carried out to confirm these new values. Future work on this radionuclide will incorporate an examination of the X-ray emission intensities. Acknowledgements The authors would like to thank Andrew Fenwick for the preparation of the samples. This work was partly supported by the National Measurement System Programmes Unit of the UK's Department for Innovation, Universities and Skills. References Berger, M.J., Hubbell, J.H., Seltzer, S.M., Change, J., Coursey, J.S., Sukumar, R., Zucker, D.S., Olsen, K., 2010. XCOM: Photon Cross Sections Database, NIST Standard Reference Database 8. 〈http://physics.nist.gov/PhysRefData/Xcom/html/xcom1. html〉. BIPM, 2004. Monographie BIPM-5—Table of Radionuclides, Seven Volumes. CEA/ LNE-LNHB, 91191 Gif-sur-Yvette, France and BIPM, Pavillon de Breteuil, 92312 Sèvres, France. And 〈http://www.nucleide.org/DDEP.htm〉.
111
Blachot, J., 2009. Nucl. Data Sheets 110, 1239–1407. Burmistrov, V.R., Didorenko, V.A., 1974. Study of the decay of 111Ag (7.5 days) by the anticoincidence method, Program and Theses. In: Proceedings of 24th Ann. Conf. Nucl. Spectrosc. Struct. At. Nuclei, Kharkov, p. 73. Chattopadhyay, S., Vimalnath, K.V., Saha, S., Korde, A., Sarma, H.D., Pal, S., Das, M.K., 2008. Preparation and evaluation of a new radiopharmaceutical for radiosynovectomy, 111Ag-labelled hydroxyapatite (HA) particles. Appl. Radiat. Isot. 66, 334–339. De Geer, L.E., 1999. CTBT Relevant Radionuclides. Technical Report PTS/IDC-1999/ 02, April 1999. CTBTO, Vienna. Firestone, R.B., Shirley, V.S.S., Baglin, C.M., Zipkin, J., Chu, S.Y.F., 1996. Table of Isotopes, eighth ed. CD ROM Edition, Version 1.0. Goswamy, J., Chand, B., Mehta, D., Singh, N., Trehan, P.N., 1992. Study of 99Mo and 111 Ag Decays. Appl. Radiat. Isot. 43, 1456–1473. Heath, R.L., 1974. Gamma-Ray Spectrum Catalogue. Ge(Li) and Si(Li) Spectrometry, ANCR-1000-2. Hnatowich, D.J., Coryell, C.D., 1970. Decay of 1.2 min 111mAg and 7.5 day 111gAg. Priv. Comm. ISO, 2010. ISO 9187-1:2010 – Injection Equipment for Medical Use – Part 1: Ampoules for Injectables. Keightley, J.D., Watt, G.C., 2002. Digital coincidence counting (DCC) and its uses in the corrections for out-of-channel gamma events in 4πβ-γ coincidence counting. Appl. Radiat. Isot. 56, 205–210. Michotte, C., Pearce, A.K., Cox, M.G., Gostely, J.J., 2006. An approach based on the SIR measurement model for determining the ionization chamber efficiency curves, and a study of 65Zn and 201Tl photon emission intensities. Appl. Radiat. Isot. 64, 1147–1155. Nethaway, D.R., Prindle, A.L., 1977. Fission of 240Pu with 14.8-MeV neutrons. Phys. Rev. C: Nucl. Phys. 16, 1907–1918. Parker, J.L., 1984. The Use of Calibration Standards and the Correction for Sample Self-attenuation in Gamma-ray Nondestructive Assay. Los Alamos National Laboratory. Purdum, L.E., Agin, G.P., Potnis, V.R., Madeville, C.E., 1975. Gamma rays emitted in decay of 111Ag. Bull. Am. Phys. Soc. 20 (1), 73. (HH8). Robinson, R.L., Langer, L.M., 1958. Inner beta spectra of 111Ag and 86Rb. Phys. Rev. 112, 481–485. Shevelev, G.A., Troitskaya, A.G., Kartashov, V.M., 1975. The excited states of 111Cd. Izv. Akad. Nauk SSSR, Ser. Fiz. 39, 2038.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Determination of the gamma emission intensities of
111
Ag
Sean Collins a,n, John Keightley a, Chris Gilligan b, Joel Gasparro a, Andy Pearce a a b
National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom Atomic Weapons Establishment, Aldermaston, Reading, RG7 4PR, United Kingdom
H I G H L I G H T S
Gamma emission probabilities of 111Ag determined. Absolute intensity of 342.1 keV of 111Ag determined. Gamma emission of 450.0 keV confirmed and measured. Significant discrepancies in published gamma emission probabilities outlined.
art ic l e i nf o
a b s t r a c t
Available online 12 November 2013
A radioactive solution of 111Ag, standardised by the absolute measurement methods 4π(PC)-γ and 4π(LS)-γ coincidence counting at the National Physical Laboratory (NPL), was measured by two independently calibrated HPGe γ spectrometers in order to estimate the γ emission intensities and to determine the absolute intensity, with the aim of improving the currently published values. An absolute intensity value of 6.68 (7)% was obtained for the 342.1 keV γ emission, which is in agreement with previously reported values, but greatly reduces the uncertainty. Additionally, this work proposes a new emission intensity for the 450.0 keV γ emission that has not been previously reported, with an absolute intensity of 0.000482 (12)%. An investigation of the published γ emission intensities shows significant discrepancies that require resolution. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Ag-111 Gamma emission intensities Spectroscopy Radioactivity
1. Introduction The radionuclide 111Ag is a short-lived product of the fission of uranium and plutonium isotopes, with a half-life of 7.45 (1) days (Blachot, 2009). It is used as a minor component in the verification scheme of the Comprehensive Test-Ban Treaty, (De Geer, 1999), and has also been proposed as a potential radiopharmaceutical due to its retention in synovial joints (Chattopadhyay et al., 2008). According to the decay scheme by Firestone et al. (1996), decay of this nuclide proceeds entirely by beta-minus emissions to six excited levels of 111Cd (Fig. 1). The three major beta-minus branches consist of a 92% intensity branch direct to the ground state of 111Cd, a 7.1% intensity branch to the 342.1 keV excited level and 1% branch to the 245.4 keV excited level. Investigation of the current state of recently published data (Blachot, 2009, p. 1311) revealed that the evaluation of the γ emission intensities have been derived from a small selection of publications exhibiting a large variation in their reported values. Additionally, only one measurement of the absolute intensity of the 342.1 keV γ emission has been made by Nethaway and Prindle (1977), the measured value has a large uncertainty of 4.5% making
n
Corresponding author. Tel.: þ 44 208 943 8508. E-mail addresses: [email protected], [email protected] (S. Collins).
the use of this nuclear data for the measurement of the activity of 111 Ag by γ spectrometry less than ideal. Preliminary γ spectrometry measurements (utilising the published data of Blachot (2009)) on the stock solution used in this work yielded variations between the activities determined using the 342.1 keV γ emission and those determined utilising the 96.7 keV and 245.4 keV γ emissions, with a low bias of approximately 14% and 18%, respectively. Therefore it was decided to investigate these anomalies via a series of measurements of a solution of 111Ag that was standardised by the two independent primary standardisation techniques, 4π(LS)-γ digital coincidence counting and 4π(PC)-γ coincidence counting, to provide improved estimates of the γ emission intensities. 2. Experimental 2.1. Preparation of
111
Ag
The 111Ag solution was produced by the neutron irradiation of a 1.1 g palladium wire, prepared at NPL, in the 2 MW ‘swimming pool’ type Research reactor at Technische Universiteit Delft, Netherlands. To remove the palladium radioisotopes of 103Pd, 107 Pd and 109Pd produced during the irradiation, the palladium wire was dissolved in hot concentrated HNO3 with 1 drop of HCl
0969-8043/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.007
108
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
Fig. 1. Decay scheme for
111
Ag (Firestone et al., 1996).
added. Inactive AgNO3 was added to increase the total mass of silver in the solution. NaCl was introduced to produce AgCl which was separated by micro precipitation and filtration, the AgCl was then dissolved in ammonia and reduced back to silver metal using ascorbic acid before finally being re-dissolved using 8 mol dm 3 HNO3. This solution was diluted to produce a final 1 mol dm 3 HNO3 solution that was used to prepare the measurement samples. 2.2. Primary standardisation of the activity concentration of
111
Ag
The primary standardisation of the material was performed utilising two of the NPL coincidence counting systems. The first incorporated 4π(LS)-γ Digital Coincidence Counting (Keightley and Watt, 2002) utilising an NPL built Liquid Scintillation (LS) counter (with two photomultiplier tubes operating in coincidence for background reduction) along with a 10 cm NaI(Tl) detector for registration of γ photons. The series of LS sources consisted of accurately weighed aliquots of the stock solution (between 30 and 70 mg) added to 10 g of Ultima Gold AB and 0.5 g of 1 mol dm 3 HNO3. The second system incorporated 4π(PC)-γ coincidence counting with the pill-box type proportional counter (PC) operating at atmospheric pressure with P10 gas used as the counting medium. A 10 cm NaI(Tl) detector was utilised for registration of γ photons. Accurately weighed aliquots of the solution were dispensed onto a series of gold-coated VYNS foils (intrinsic VYNS thickness approximately 30 mg cm 2; gold thickness approximately 10 mg cm 2). For both systems, a γ gate was set to encompass the 342.1 keV photopeak, and corrections for “out-of-channel” γ photons were employed. The range of beta-channel efficiencies utilised in the efficiency extrapolations were 96.5%–89.8% for the 4π(LS)-γ system, and 93.1%–89.0% for the 4π(PC)-γ system. Both systems yielded similar results for the activity concentration: 599 (6) and 601 (6) kBq g 1 (k¼1), respectively. The impurities of 110mAg and 198 Au (discussed in the next section) made a negligible contribution to the final estimate of the absolute disintegration rate. An unweighted mean was used to estimate of the activity concentration of the solution, and was calculated as 600 (6) kBq g 1 (k ¼1). 2.3. Gamma emission intensity measurements of
111
Ag
Two sets of samples were prepared gravimetrically, with one aliquot samples dispensed to two 2 ml ISO ampoules (ISO, 2010) and two aliquot samples dispensed to four 20 ml plastic scintillation vials. These solutions were measured by two independent HPGe γ spectrometers. The p-type detector ‘BART’ with a beryllium window and a relative efficiency of 28.9% was used to measure the
solution dispensed to the ISO ampoules. The semi-planar detector ‘LANCELOT’ with a carbon fibre detector window and a relative efficiency of 65% was used to measure the solution contained in the plastic scintillation vials. Both detectors used a graded shield of Pb, Cd and Cu and utilised identical electronic set-ups, incorporating a Canberra AFT research amplifier (6 ms shaping time) and utilising the “PUR/DT” circuitry to correct the Live Time for pile-up and dead time. The detectors were energy calibrated using a 152Eu source with peak centroids set within 0.1 keV for γ emissions with intensities in excess of 1%, using the evaluated energies from the Decay Data Evaluation Project (BIPM, 2004). The photon detection efficiency calibration for each detector was performed using solutions of a mixed radionuclide standard containing 241Am, 109Cd, 57Co, 139Ce, 51 Cr, 113Sn, 85Sr, 137Cs, 54Mn, 88Y, 65Zn and 60Co in 4 mol dm 3 HCl. Additional calibration points for BART were added using solutions of single radionuclides of 51Cr, 75Se, 85Sr, 109Cd, 125I, 141Ce and 210 Pb, where the individual radionuclides had been standardised at NPL using the NPL re-entrant Ionisation Chamber which is directly traceable to primary standards. Each calibration sample was in a matched geometry for the measurement samples. The samples measured by ‘BART’ were placed at a distance of 25 cm from the detector window, for a live time of between 12,000 and 85,000 s. The samples measured by ‘LANCELOT’ were placed at 10 cm from the detector window, for a live time ranging from 14,400 to 86,400 s. The acquisition times were of sufficient length to ensure uncertainty on the counts for the 96.7 keV, 245.4 keV and 342.1 keV photopeaks were less than 0.3%. The distances that were used for the measurements allowed for the assumption that the effect of cascade summing to be negligible for both detectors. A total of eight spectra were acquired: two for ‘BART’ and six for ‘LANCELOT’. The peak areas were analysed using the CANBERRA GENIE 2000 software, with all peaks manually checked using the Interactive Peak Fitting application within this software. Dead time, pulse pile-up and background peak area corrections were all applied using the CANBERRA GENIE 2000 software. All the photopeaks in the spectra were readily identifiable (Fig. 2), with 110m Ag and 198Au identified as impurities with activity concentrations of 37.4 (4) Bq g 1 and 11.0 (5) Bq g 1 respectively (k ¼1). Corrections were also made for the interference of 620.3 keV γ photon emissions (from the decay of 110mAg) with those of 111Ag at similar energies (619.3 keV, 620.2 keV and 621.2 keV). The convoluted peaks of 620.3 keV and 621.2 keV were separated using the interactive peak fitting application and reported as individual values. There was no indication of a photopeak at 619.3 keV. The photopeak at 450.0 keV was identified as the γ emission of 111Ag that has been identified in the decay scheme determined by Firestone et al. (1996) but has no published γ emission values. Additionally there was no indication of a photopeak at 865.9 keV and as such the 866.7 keV γ emission has been reported separately. Due to the difference in the density of the solutions between the efficiency calibration and the sample of interest, corrections were made to compensate for the different attenuations of the γ emissions. This required two different methods due to the different sample mounting mechanisms used. For the measurements performed on ‘BART’ a far-field model for cylindrical samples (Parker, 1984) was used. While for ‘LANCELOT’ a simple 1dimensional model was utilised (Parker, 1984). The linear attenuation coefficient was calculated for each of the different chemistries using data from the NIST XCOM database (Berger et al., 2010) These corrections amounted to less than 0.5% for ‘BART’ and 0.9% for ‘LANCELOT’ for any individual γ emission, with the largest correction applied to the 96.7 keV photopeak detection efficiency, with the size of the correction reducing in size with the increase of the γ emission energy.
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
Fig. 2. Example of acquired gamma spectrum of
Table 1 The absolute γ emission intensities of 111Ag per 100 decays. The photon energies derived from the energy calibration described in Section 2.3. Photon energy /keV
γ Emission intensity /%
96.7 245.4 342.1 374.8 450.0 524.5 620.2 621.2 866.7
0.1018 1.113 6.68 0.00240 0.000482 0.00376 0.00883 0.0161 0.00773
(5) (5) (6) (5) (5) (6) (6) (6) (7)
(14) (14) (7) (7) (12) (10) (14) (4) (11)
3. Results The absolute γ emission intensities were derived from the activity determination by primary standardisation methods and the measurement of the 111Ag γ emissions by two HPGe γ spectrometers. These results are presented in Table 1 as the absolute γ emission intensities in per cent of disintegrations. The absolute intensity of the 342.1 keV γ emission was deduced as 6.68 (7)% with the uncertainty budget presented in Table 3. This is in excellent agreement with the previously reported value of 6.68 (33)% (Nethaway and Prindle, 1977). An earlier value of 6.0 (15)% has been reported by Robinson and Langer (1958), however they reported that a NaI(Tl) scintillation detector was used, with count-rates compared to the beta counting rate using a 4πβ spectrometer (thus not utilising the power of the coincidence method). This may be the cause of the large uncertainty attributed to this result. The relative γ emission intensities were calculated as the arithmetic mean of the corrected normalised values (the 342.1 keV intensity being taken equal to 100), the results of which can be viewed in Table 2, which includes a comparison with the evaluated values of (Blachot, 2009) and the experimental values of Goswamy et al. (1992) which were not included in the evaluation. As can be seen in Fig. 2, there were no indications in the observed spectra, of the γ emissions of 278.3 keV, 509.4 keV, 522.4 keV and 754.6 keV, additionally (not seen in Fig. 2) no convoluting photopeaks were found at 619.3 keV or 865.9 keV. The lack of an observable 278.3 keV photopeak may be explained
109
111
Ag.
Table 2 Comparison of the previously evaluated intensities by Blachot (2009) and Goswamy et al. (1992), normalised to the 342.1 keV transition and those determined in this work. The uncertainties represent one standard deviation, coverage factor k ¼ 1. Further explanations of these results are given in Section 3. Photon energy /keV
This work
Goswamy et al. (1992)
Blachot (2009)
96.7 245.4 342.1 374.8 449.8 522.4 524.5 619.3 620.2 621.2 754.6 865.1 866.7
1.524 (14) 16.66 (11) 100 0.0360 (9) 0.00721 (17) – 0.0563 (13) – 0.1322 (14) 0.242 (4) – – 0.1157 (11)
1.56 (2) 16.35 (20) 100 0.039 (2) – 0.059 (3)
1.73 (9) 18.5 (10) 100 0.047 (2) – 0.014 (2) 0.031 (2) 0.008 (4) 0.164 (12) 0.09 (3) 0.04 (-) 0.023 (4) 0.054 (4)
0.38 (1)
0.0026 (5) 0.112 (3)
Table 3 Uncertainty budget for the absolute γ emission intensity of the 342.1 keV γ emission. Uncertainty component
Relative uncertainty % (k ¼ 1)
Standard deviation Dead-time, pile-up Attenuation corrections Photopeak efficiency Activity concentration Gravimetric
0.05 0.10 0.010 0.5 1.0 0.10
Total
1.3
by the combination of the small γ emission intensity and the fact that it appears in the Compton continuum stemming from 342.1 keV γ emissions. The γ emissions at 509.4 keV reside in the area of the background 511 keV annihilation photopeak, though it would be expected to still be able to see a distortion in the photopeak due to its presence. The 522.4 keV is not thought to be part of the 524.5 keV photopeak in the acquired spectra; the FWHM of the 524.5 keV photopeak being 1.42 keV, as the photopeak does not show any distortion in the shape of the photopeak
110
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
that would be apparent if present. The 754.6 keV transitions were also not visible in the acquired spectra. At the currently evaluated intensities these would have been expected to be visible within the spectra. Further investigation reveals that the 509.4 keV has only been detected by one previous author (Burmistrov and Didorenko, 1974) and the 754.6 keV γ emissions has only been detected by two of the previous authors (Burmistrov and Didorenko, 1974 and Goswamy et al., 1992). The lack of the 754.6 keV and 509.4 keV and 619.3 keV photopeaks could possibly indicate that the 765.9 keV level does not exist or that its feeding is significantly smaller than currently reported. 3.1. Differences between evaluated values and present work There are large differences between the values proposed in this work and the currently evaluated data, as seen in Table 2. Though the values presented in this work are in good agreement with the values proposed by Goswamy et al. (1992). These values have not been incorporated in the evaluation performed by Blachot (2009). Additionally, as previously mentioned the measured results exhibit significant ranges in the values and are discrepant as seen in Table 4. These values and their spread are indicative of the paucity of the γ emission measurements currently available for 111Ag. 3.2. Validity of the 450 keV gamma emission The 450.0 keV γ emission reported in this work has not been reported previously, while it has been proposed in the decay scheme, shown in Fig. 1, by Firestone et al. (1996), The photopeak was present in four of the eight spectra collected, from both detectors used, being visible in the early spectra before the decay reduced the count rate to below the limit of detection. The determined γ emission intensity was relatively consistent across the five spectra after decay and all other corrections were made with a standard deviation of 3.2% (Fig. 3). Considering the large uncertainty due to the counting statistics of between 8 and 15% due to the small number of counts collected (less than 500 in 86,400 s), the consistency of the results are very good, therefore reducing the possibility of the photopeak coming from random or true summing. The remaining possibility is that the photopeak originates from another radionuclide, though no other known γ emission that is possible exists at the same energy. Both of these points of evidence therefore indicate that this is a real γ emission of 111Ag. 3.3. Validation of proposed gamma emission intensities The validity of the newly estimated γ emission intensities from our work was confirmed by utilising the ‘SIRIC’ software (Michotte
et al., 2006) to predict the calibration factor of the NPL re-entrant ionisation chamber of type TPA-Mk II, for 2 ml ampoules. Using the γ emission intensities of Blachot (2009) yielded a predicted calibration factor of 1.22 (6) pA MBq 1 (k ¼1), and when applied to the measured current from a series of 2 ml ampoules measured in the NPL ionisation chamber yielded an estimate of the activity concentration of the stock solution of 582 (30) kBq g 1. Incorporating the newly derived emission intensities from this work yielded a predicted calibration factor of 1.182 (14) pA MBq 1 (k ¼1), which resulted in an estimate of the activity concentration of the stock solution of 598 (7) kBq g 1. This compares favourably to the primary standardisation result of 600 (6) kBq g 1 (k ¼1). A calibration factor was measured for the ionisation chamber and resulted in a calibration factor of 1.178 (12) pA MBq 1 which is in agreement with the predicted values of ‘SIRIC’ using the new values presented in this work.
4. Conclusions In this work, the principal γ emission of 342.1 keV in 111Ag decay was experimentally determined to be 6.68 (7)% (k ¼1). The use of absolute counting techniques to determine the activity and two calibrated independent HPGe γ detectors has allowed the precision measurement of the γ emissions with confidence in the values reported in this work. This work also proposes the first γ emission intensity for the γ emission of 450.0 keV of 111Ag, which was calculated with an absolute intensity of 0.000482 (12)%.
Fig. 3. Relative emission intensity for each measurement of the 450.0 keV γ emission. The error bars show the uncertainty of the photopeak area for each measurement and the dashed line indicating the arithmetic mean of the measurements.
Table 4 Comparison of the published relative intensities. The value reported for this work for the 619.3, 620.2 and 621.2 keV γ emissions is the combined value of the 620.2 and 621.2 keV emissions. The γ emissions not reported in this work have been omitted from the table. Ref.
Robinson and Langer (1958) Hnatowich and Coryell (1970) Heath (1974) Burmistrov and Didorenko (1974) Purdum et al (1975) Shevelev et al (1975) Goswamy et al. (1992) This work Critical value χ2
Photon energy /keV 96.7
245.4
374.8
524.5
619.3, 620.2 and 621.2
865.1 and 866.7
– 4.2 (1) 1.6 (2) 3.8 (-) 3.4 (2) 1.8 (2) 1.56 (2) 1.524 (14) 3.02 158
16 (8) 20.6 (2) 16.9 (20) – 19.1 (13) 18.4 (4) 16.35 (20) 16.66 (11) 2.80 4.75
– 0.048 (2) – 0.04 (-) 0.043 (4) – 0.039 (2) 0.0360 (9) 3.79 10.5
– – – – – 0.050 (9) 0.059 (3) 0.0563 (13) 4.61 0.62
– – – – – 0.42 (4) 0.38 (1) 0.374 (4) 4.61 0.79
– 0.127 (11) – 0.08 (-) 0.077 (10) 0.165 (18) 0.112 (3) 0.1157 (11) 3.32 6.18
S. Collins et al. / Applied Radiation and Isotopes 87 (2014) 107–111
The ionisation chamber calibration factor prediction software ‘SIRIC’ was used as a method to test the validity of the new γ emission values presented in this work. This method showed an improvement in the predicted response against the real response, therefore providing some proof of the reliability of the values presented. Additionally, the published values of the relative γ emissions of 111 Ag were compared pointing out large discrepancies within the sets of data for the individual photon intensities. These discrepancies indicate problems with the currently available γ emission intensity data for 111Ag (Blachot, 2009) and therefore it is suggested that further investigations are carried out to confirm these new values. Future work on this radionuclide will incorporate an examination of the X-ray emission intensities. Acknowledgements The authors would like to thank Andrew Fenwick for the preparation of the samples. This work was partly supported by the National Measurement System Programmes Unit of the UK's Department for Innovation, Universities and Skills. References Berger, M.J., Hubbell, J.H., Seltzer, S.M., Change, J., Coursey, J.S., Sukumar, R., Zucker, D.S., Olsen, K., 2010. XCOM: Photon Cross Sections Database, NIST Standard Reference Database 8. 〈http://physics.nist.gov/PhysRefData/Xcom/html/xcom1. html〉. BIPM, 2004. Monographie BIPM-5—Table of Radionuclides, Seven Volumes. CEA/ LNE-LNHB, 91191 Gif-sur-Yvette, France and BIPM, Pavillon de Breteuil, 92312 Sèvres, France. And 〈http://www.nucleide.org/DDEP.htm〉.
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Blachot, J., 2009. Nucl. Data Sheets 110, 1239–1407. Burmistrov, V.R., Didorenko, V.A., 1974. Study of the decay of 111Ag (7.5 days) by the anticoincidence method, Program and Theses. In: Proceedings of 24th Ann. Conf. Nucl. Spectrosc. Struct. At. Nuclei, Kharkov, p. 73. Chattopadhyay, S., Vimalnath, K.V., Saha, S., Korde, A., Sarma, H.D., Pal, S., Das, M.K., 2008. Preparation and evaluation of a new radiopharmaceutical for radiosynovectomy, 111Ag-labelled hydroxyapatite (HA) particles. Appl. Radiat. Isot. 66, 334–339. De Geer, L.E., 1999. CTBT Relevant Radionuclides. Technical Report PTS/IDC-1999/ 02, April 1999. CTBTO, Vienna. Firestone, R.B., Shirley, V.S.S., Baglin, C.M., Zipkin, J., Chu, S.Y.F., 1996. Table of Isotopes, eighth ed. CD ROM Edition, Version 1.0. Goswamy, J., Chand, B., Mehta, D., Singh, N., Trehan, P.N., 1992. Study of 99Mo and 111 Ag Decays. Appl. Radiat. Isot. 43, 1456–1473. Heath, R.L., 1974. Gamma-Ray Spectrum Catalogue. Ge(Li) and Si(Li) Spectrometry, ANCR-1000-2. Hnatowich, D.J., Coryell, C.D., 1970. Decay of 1.2 min 111mAg and 7.5 day 111gAg. Priv. Comm. ISO, 2010. ISO 9187-1:2010 – Injection Equipment for Medical Use – Part 1: Ampoules for Injectables. Keightley, J.D., Watt, G.C., 2002. Digital coincidence counting (DCC) and its uses in the corrections for out-of-channel gamma events in 4πβ-γ coincidence counting. Appl. Radiat. Isot. 56, 205–210. Michotte, C., Pearce, A.K., Cox, M.G., Gostely, J.J., 2006. An approach based on the SIR measurement model for determining the ionization chamber efficiency curves, and a study of 65Zn and 201Tl photon emission intensities. Appl. Radiat. Isot. 64, 1147–1155. Nethaway, D.R., Prindle, A.L., 1977. Fission of 240Pu with 14.8-MeV neutrons. Phys. Rev. C: Nucl. Phys. 16, 1907–1918. Parker, J.L., 1984. The Use of Calibration Standards and the Correction for Sample Self-attenuation in Gamma-ray Nondestructive Assay. Los Alamos National Laboratory. Purdum, L.E., Agin, G.P., Potnis, V.R., Madeville, C.E., 1975. Gamma rays emitted in decay of 111Ag. Bull. Am. Phys. Soc. 20 (1), 73. (HH8). Robinson, R.L., Langer, L.M., 1958. Inner beta spectra of 111Ag and 86Rb. Phys. Rev. 112, 481–485. Shevelev, G.A., Troitskaya, A.G., Kartashov, V.M., 1975. The excited states of 111Cd. Izv. Akad. Nauk SSSR, Ser. Fiz. 39, 2038.