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Determination of the absolute photon emission intensities of some gamma rays of 166mHo
Author’s Accepted Manuscript Determination of the absolute photon emission intensities of some gamma rays of 166mHo Virginia Peyres, Eduardo García-Toraño
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S0969-8043(17)30344-5 http://dx.doi.org/10.1016/j.apradiso.2017.06.035 ARI7939
To appear in: Applied Radiation and Isotopes Received date: 10 March 2017 Revised date: 22 June 2017 Accepted date: 22 June 2017 Cite this article as: Virginia Peyres and Eduardo García-Toraño, Determination of the absolute photon emission intensities of some gamma rays of 166mHo, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.06.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Determination of the absolute photon emission intensities of some gamma rays of 166mHo Virginia Peyres*1, Eduardo García-Toraño CIEMAT, Metrología de Radiaciones Ionizantes, Avda. Complutense 40, 28040 Madrid, Spain *
Corresponding author: Tel.: +34 313466225 Fax: +34 913466442.
Email address: [email protected]
Abstract
This paper presents the results of the absolute measurement of some photon emission intensities in the decay of 166mHo. Point sources from a reference solution standardized in the frame of the EURAMET.RI (II)-K2.Ho-166m activity comparison were measured by gamma spectrometry. The detection efficiency was obtained by Monte Carlo calculations including the complete decay scheme. Results obtained for 27 gamma and X-ray emissions are compared to reference values. Keywords: 166mHo; Photon emission intensities; Gamma-ray spectrometry.
1. Introduction The nuclide 166mHo disintegrates by beta minus emission to seventeen excited levels of 166 Er. Most disintegrations feed two excited levels at 1828 keV and 1787 keV with intensities of 17.2 % and 74.8 % respectively. Deexcitation of the excited levels of 166Er gives rise to a large number of gamma-ray cascades (Schönfeld and Dersch, 2004) as shown in figure 1. The combination of a relative long half-life (1200 years) and the emission of a large number of well separated and intensive gamma-rays in a wide range of energy (between 80 and 1400 keV) makes it a candidate for calibration of germanium detectors (Danilenko et al., 1989; Morel et al., 1996; Hino et al., 2000; Bernardes et al., 2002). Its characteristics are also suitable for checking the stability of ionization chambers, in replacement of 226Ra sources which are commonly used to this purpose (Hino et al., 2000). 1
Present address: CIEMAT, Avda. Complutense 40, 28040 Madrid (Spain)
Figure1. Simplified decay scheme of 166mHo. Data are taken from Nucleide (Schönfeld and Dersch, 2004).
Absolute and relative photon emission intensities for this nuclide have been measured by several authors (Reich and Cline, 1970; Gehrke et al., 1977; Sooch et al., 1982; Danilenko et al., 1989; Wang and Yang, 1992; Miyahara et al., 1994; Morel et al., 1996; Hino et al., 2000; Bernardes et al., 2002; Moreira et al., 2010). Absolute results obtained in early works exhibit a large dispersion and high uncertainties (Reich and Cline, 1970; Danilenko et al., 1989). More recent measurements present lower uncertainties (Moreira et al., 2010; Bernardes et al., 2002; Hino et al., 2000; Morel et al., 1996; Miyahara et al., 1994) but with a large dispersion of the results. Among all results available, the works published by Morel et al. (1996) and Bernardes et al. (2002) deserve special attention for their careful setup. In both cases, two detectors, one for the low energy range and other for intermediate and higher energy range were used. Efficiency calibration was also performed by both authors with a large number of experimental points (more than 70 for Morel). Measurements were carried out in such a way that coincidence-summing corrections are either negligible (Bernardes et al., 2002) or were checked at several source-to-detector distances (Morel et al., 2002). The radioactive solutions used to prepare the samples were, in both cases, standardized in the frame of international comparisons or sent for measurement to the SIR (BIPM) (Ratel et al., 2003; Ratel et al., 2015). Therefore, both sets of results of the photon emission intensities are based on absolute activity measurements, performed in the frame of international comparisons. Other recent measurements, also absolute, were presented by Hino et al. (2000) and Moreira et al. (2010). The first author does not provide details on the efficiency calibration or the measurement geometry. In the paper by Moreira, no information is given on the magnitude of the coincidence-summing correction and efficiency calibration is carried out with 6 radionuclides, from which two (133Ba and 152Eu) are subject to important coincidence-summing effects. The latest evaluation of 166mHo performed in the frame of the Decay Data Evaluation Project (DDEP) by Schönfeld and Dersch (2004), does not include the most recent measurements among which there is a lack of consistence. This paper presents new absolute measurements of photon emission intensities of 166mHo. The reference solution used was standardized in the framework of the EURAMET.RI(II)K2.Ho-166m (Kossert et al, 2014) activity comparison. The procedure followed to purify and characterize the solution has been described in detail elsewhere (Guéguen et al., 2014). Potential gamma-ray emitting contaminants were determined in the comparison process
[Kossert et al., 2014) and their relative contribution was considered < 1.5 x 10-4. CIEMAT participated in the comparison with the CIEMAT/NIST and 4-counting methods, but the results of the photon emission intensities are based on the comparison reference value (119.27(10) kBq g-1), which differs from the value declared by CIEMAT in less than 0.07 %
2. Materials and methods 2.1
Experimental setup and efficiency determination
Six sources were prepared for gamma-ray spectrometry measurement by dropping aliquots of about 10 mg of the solution onto 75 µm polyethylene disk foils, allowing drying, covering with similar foils and thermally sealing. The diameter of the polyethylene disks was 2 cm and that of the active sources was about 3 mm. The experimental setup was formed by an extended-range coaxial high purity germanium detector (HPGe), with diameter and height close to 61 mm, equipped with a carbon-epoxy window and a very thin contact (about 300 nm) in the front surface. The electronic setup includes a high-voltage power supply from BERTAN, preamplifier, spectroscopy amplifier and pulse generator from CANBERRA and a successive-approximation analog-to-digital converter model 7423 from SILENA. A cubic shielding structure of about 80 × 80 × 80 cm x cm x cm surrounds the detector. It is composed by layers of Pb (5 cm), Cd (3 mm) and Cu (1.5 mm). The Ge detector was originally calibrated by Monte Carlo simulation in an energy range from 14 to 1800 keV (Peyres and García-Toraño, 2007). Point sources from monoenergetic and multi-gamma emitters were measured at 15 cm from the detector window and provided a set of 26 experimental values to which the results of the simulations were compared. Discrepancies between simulated and experimental values were, in most cases, below 1%. Since then, experimental data have been completed by adding new results using selected gamma emissions of 133Ba, 129I, 210Pb and 113Sn to complete a total of 38 experimental points. That allowed improving the reliability of the efficiency determination. Nuclear decay data for 133Ba, 129I and 113Sn were taken from Nucléide database (2017). For the gamma ray of 46.54 keV of 210Pb, emission intensities recently measured by Rodrigues et al. (2016) were used. A numerical detector model was used to obtain the efficiencies for the gamma-ray lines of 166m Ho. To this purpose, the Monte Carlo code PENELOPE 2014 was used (Salvat, 2015). The uncertainty of full-energy peak efficiencies is composed by the inherent statistics component of the calculation, which has remained very low (between 0.23% and 0.85%),
and by that due to the input values of the Monte Carlo simulation, basically a function of the detector and measurement parameters (Lépy et al., 2015), the latter being always difficult to assess. Nevertheless, the procedure has been validated in the course of several interlaboratory comparisons in which excellent results have been obtained (Tzika et al., 2015; Tzika et al., 2016; Peyrés and García-Toraño, 2011). That reflects the fact that Monte Carlo calculation can provide robust efficiency values if geometrical parameters have been optimized by comparing calculations and experimental. An accurate interpolation in a wide range of energies is supported by the fact that Monte Carlo simulation takes into account the physical processes taking place in detector and source, including the photon and electron cross sections that allow a physics-based interpolation. As an additional estimation of the uncertainty attainable with this method, experimental efficiency data have also been fitted using two log-log polynomial functions, one for lowenergy range (15 keV to 150 keV) and another one for the high-energy range (120 keV to 2 MeV). In this case, relative standard combined uncertainties are about 1.0 % from 48 to 120 keV, 0.6 % in the 120 keV to 500 keV energy range and 0.5 % above 500 keV. Since the Monte Carlo method is based on the same experimental data and is not dependent on a predefined functional expression, these figures can be considered as an upper limit to the uncertainty of data obtained in this study. The efficiency curve is presented in figure 2. It shows the experimental points and corresponding values calculated for the validation of the Monte Carlo model; the calculated values of the efficiency of X-rays and gamma-ray lines of 166mHo of interest to this study and the z-score statistics used to compare both sets of results. Figure 2. Full-energy peak efficiency curve obtained for point sources placed at 15 cm. from the detector. Results of the Monte Carlo simulation and experimental data used to validate the model are shown as well as the efficiencies of the emission lines of 166m Ho studied in this paper. Also shown are the z-score values. 2.2
Measurements of photon emission intensities
Six point sources from the reference solution were measured at source-to-detector distances of 15 cm, matching the geometry used to calibrate the detector. Each source was measured during 200 000 s. A set of measurements with shorter acquisition times (50 000 s each) were also taken for checking purposes, but they were not retained for analysis. The stability of the equipment was considered satisfactory as neither FWHM (Full Width at Half Maximum) values, nor the energy calibration suffered significant changes during the measurements. Photon emission intensities were calculated according to the formula:
̇
Where are the emission intensities of the X- or gamma rays; ̇ are the net peak counting rates after background subtraction (net peak areas divided by the measurement time, expressed in seconds); are the full-energy peak efficiencies; A is the activity of the source at the measurement time and are the coincidence summing corrections. Correction by decay during the measurements was negligible. Gamma-ray peak areas were determined using the IAEA software GRILS, a non-linear fitting code that provides accurate uncertainty estimations and is a part of the GANAAS suite (IAEA, 1991). The analysis of X-ray peaks was done with the COLEGRAM software (Ruellan et al., 1996). Background spectra only presented measurable structures under the gamma lines of 300.74 and 611.6 keV Coincidence summing corrections are a major source of concern for these calculations, as they can reach values up to 3% for some emissions. All calculations were done by a combination of simulations made with the PENELOPE (Salvat, 2015) and PENNUC codes (García-Toraño et al., 2017). The combination of both codes allows simulating the decay of a nucleus as a random sequence of transitions; each transition involving the release of electrons or photons or the production of vacancies in atomic subshells followed by subsequent relaxation of the atomic electron. When the decay path includes a metastable level, the set of transitions prior to that level is considered as an independent emission event (in which several particles may be released). The emitted particles are fed to PENELOPE. Nuclear data are taken from the Nucléide database (Nucléide, 2017) from which data sets can be exported in a compatible format. A powerful feature of the program is the ability to deal with nuclear level lifetimes as well as detector times. In principle, a certain resolution value is assigned to the detector, and all particles reaching the detector within this time are treated as a single detection event, even if they correspond to a complete cascade. When the lifetime exceeds the detector resolution time (a sampling algorithm is used), the level is considered as metastable and a new cascade is generated. A similar situation can arise when lifetimes of nuclear levels are considered. Therefore, if the original data set of the 166mHo scheme is fed, calculated efficiencies for all gamma-ray emissions will include coincidence summing effects, as this is an integral part of the simulation. On the contrary, by assigning a negative value to the detector time, all particles emitted in the disintegration are considered as individual events from the point of view of the detector response. In both cases, results are dependent on the emission intensities whose values are target values for this study. Nevertheless, as both sets of values are combined to obtain the coincidence-summing correction, the influence of the emission intensities is, at least partially, cancelled. Coincidence-summing correction factors
obtained in this way were then applied to efficiency values calculated as explained in 2.1. An additional check was made with the ETNA software (Lépy et al., 2012).
3. Results and discussion The final results of the absolute photon intensities of 166mHo are presented in table I, together with the results of recent measurements and recommended values from the Decay Data Evaluation Project (DDEP) by Schönfeld and Dersch (2004). Each result corresponds to the average of the measurement of six sources. A total of 26 intensities have been obtained. For the sake of clarity, only the most relevant data are presented in Figure 3. Complete details of the uncertainty components considered are shown in table II.
Figure. 3. Photon intensities determined in this work for gamma emissions with energies 80.57, 184.41, 280.46, 410.96, 711.7, 752.28, 810.29 and 830.56 keV. Recommended values as well as some recently published values are presented for comparison.
A complete analysis of the X-ray region of spectra is hampered by the limited energy resolution of our experimental system. The system is not able to discriminate between components of XK and XKrays groups. Hence, only aggregated data are presented for both groups. For XKthe agreement with recommended data is good, while for XKrays, subject to high uncertainties in the present measurements, differences are not covered by the stated uncertainties. For the lines with energies 80.57, 184.41, 280.46 keV, coincidence-summing corrections are in the order of 3% in the measurement geometry used. Results for the 80.57 keV line are not in good agreement with recommended values, but agree well with absolute data obtained by Hino, Morel and Bernardes and their co-workers which, in turn, do not match the recommended value which was not obtained only as a combination of experimental values, but also taking into account balance conditions. For the emission at 184.41 keV, a small discrepancy is found with the reference value, while results for the gamma emission at 280.46 keV agree well with recommended values. Results for the 410.96, 711.7 and 752.28 keV emissions are compatible with recommended values, which have typical uncertainties within 1.5 and 2.4 %. For the 410.96 keV emission, discrepancies are found with results from Morel and Bernardes, but a good agreement is reached with those from Moreira and Hino as well as with recommended
values. Discrepancies cannot be attributed to coincidence-summing effects, as the total correction for this line is 1.038. Values for the other two gamma emissions at 711.7 and 752.28 keV agree well with results from Morel and Bernardes. Finally, the results for the 810.29 and 830.56 keV emissions. They are compatible with recommended values as well as with data from Morel and Bernardes. Coincidencesumming corrections for these lines are 1.02 and 1.024 respectively. Results for the other, less intense, emissions, agree well with recommended data with uncertainties between 1% for the 529.82 keV emission and 4% for the 1427.24 keV emissions. Overall, 24 from the 27 results presented are compatible with the recommended values.
4. Conclusions A new set of photon-emission intensities of 166mHo has been measured using a solution previously standardized in the frame of an international activity comparison. A total of 27 X-ray and gamma intensities have been determined by a combination of measurements and Monte Carlo calculations. Uncertainties depend on the line intensities with a minimum of 0.8% for the 184.4 keV emission and a maximum of 5.6% for the weak emission at 1400.8 keV. Most results are compatible with recommended values. Nevertheless, given the unresolved discrepancies (also evident in other absolute determinations) with the important 80.57 keV emission, a reevaluation of the decay scheme of this nuclide should be considered.
References Bernardes, E.M.O., Delgado, J.U., Tauhata, L., da Silva, C.J., Iwahara, A., Poledna, R., Paschoa, A.S., 2002. 166mHo: a multi-γ standard for the calibration of Ge spectrometers. Appl. Radiat. Isot. 56, 157-161. Danilenko, V.N., Gromova, N.P., Kostantinov, A.A., Kurenkov, N.V., Malinin, A.B., Mamelin, A.V., Matveev, S.V., Sazonova, T.E., Stepanov, E.K., Sepman, S.V., Toporov, Yu.G., Tronova., I.N., 1989. Methods of producing radionuclides for spectrometric gammaray sources and their standardization, 3. Holmium-166m. Int. J. Appl. Radiat. Isot., 40, 789792.
García-Toraño, E., Peyrés, V., Bé, M.M., Lépy, M.-C., Dulieu, C., V., Salvat, F., 2017. Simulation of decay processes and radiation transport times in radioactivity measurements. Nucl. Instrum. Methods B 396, 43–49. Gehrke R. J., Helmer R. G. and Greenwood R. C., 1977. Precise relative y-ray intensities for calibration of Ge semiconductor detectors. Nucl. Inst. Meth. 147, 405-423. Guéguen, F., Isnard, H., Kossert, K., Bresson, C., Caussignac, C., Stadelmann, G., Nonell, A., Mialle, S., Cartier, F., 2014. Purification of 166mHo solution by successive highperformance liquid chromatography and gravitational chromatography for half-life determination. J. Radioanal. Nucl. Chem. 302, 289-295. Hino, Y., Matui, S., Yamada, T., Takeuchi, N., Onoma, K., Iwamoto, S., Kogure, H., 2000. Absolute measurements of 166mHo radioactivity and development of sealed sources for standardization of -ray emitting nuclides. Appl. Radiat. Isot. 52, 545–549. IAEA, International Atomic Energy Agency, 1991. Nuclear Analysis Software, Part 2: Gamma Spectrum Analysis, Activity Calculations and Neutron Activation Analysis (GANAAS). Computer Manual Series no. 3. Kossert, K., Altzitzoglou, T., Auerbach, P., Bé, M.M., Bobin, C., Cassette, P., GarcíaToraño, E., Grigaut-Desbrosses, H., Isnard, H., Lourenço, V., Nähle, O., Paepen, J., Peyrés, V., Pommé, S., Rozkov, A., Sanchez-Cabezudo, A.I.,, Sochorová, J., Thiam, C., Van Ammel, R., 2014. Results of the EURAMET.RI(II)-K2.Ho-166m activity comparison. Metrologia 51 Tech. Suppl. 06022. Lépy, M.-C, Ferreux, L., Pierre, S., 2012. Coincidence summing corrections applied to volume sources. Appl. Radiat. Isot. 70, 2137–2140. Lépy , M.C., Pearce, A., Sima, O., 2015. Uncertainties in gamma-ray spectrometry. Metrologia 52, S123-S145. Miyahara, H., Matumoto, H., Mori, C., Takeuchi, N., Genka, T., 1994. Determination of gamma-ray emission probabilities for 75Se, 166mHo and 192Ir by a self-consistent method. Nucl. Instrum. Methods A 339, 203-208. Moreira, D. S., Koskinas, M. F., Dias, M. S., Takeda, M.N., 2010. Disintegration Rate and Gamma Ray Probability per Decay Measurement of 166mHo. 2010 IEEE Nuclear Science Symposium Conference Record, 482-485. Morel, J., Etcheverry, M., Plagnard, J., 1996. Emission probabilities of KX- and -rays following 166mHo decay. Appl. Radiat. Isot. 47, 529–534. Nucléide, http://www.nucleide.org/DDEP_WG/DDEPdata.htm. (updated: 4th October 2017), hosted by CEA/LNE-LNHB, Saclay, France.
Peyrés, V., García-Toraño, E., 2007, Efficiency calibration of an extended-range Ge detector by a detailed Monte Carlo, Nucl. Instrum. Methods A, 580, Issue 1, 296-298. Peyrés, V., García-Toraño, E., 2011, On the measurement of positron emitters with Ge detectors, Nucl. Instrum. Methods A 637, 100–104. Ratel, R., Michotte, C. and Hino, Y.,2003. BIPM comparison BIPM.RI(II)-K1.Ho-166m of activity measurements of the radionuclide 166Hom and links for the 2000 international comparison APMP.RI(II)-K2.Ho-166m, Metrologia 40, Technical Supplement. Ratel, R., Michotte, C., Courte, S. and Kossert, K., 2015. Update of the BIPM comparison BIPM.RI(II)-K1.Ho-166m activity measurements of the radionuclide 166mHo for the PTB (Germany), with linked results for the EURAMET.RI(II)-K2.Ho-166m comparison, Metrologia 52, Technical Supplement 06006. Reich, C.W., Cline, J.E., 1970. Gamma-ray studies of the -vibrational band of 166Er. Nucl. Phys. A 159, 181-201. Rodrigues, M., Cassette, P., Lépy, M.C., Loidl, M., Ménesguen, Y., 2016, Determination of absolute photon emission intensities of 210Pb, Appl. Radiat. Isot. 109, 500–506. Ruellan, H., Lépy, M.-C., Etcheverry, M., Plagnard, J., Morel, J., 1996. A new spectra processing code applied to the analysis of 235U and 238U in the 60–200 keV energy range. Nucl. Instrum. Methods Phys. Res. A 369, 651-656. Salvat, F., 2015. PENELOPE-2014: A code System for Monte Carlo Simulation of Electron and Photon Transport, OECD/NEA Data Bank, NEA/NSC/DOC(2015)3, Issy-lesMoulineaux, France, available from http://www.nea.fr/lists/penelope.html. Schönfeld, E. and Dersch, R., 2004. In: M.-M. Bé et al., Monographie BIPM-5 - Table of Radionuclides (Vol. 2 – A=151 to 242), 75-84, ISBN 92-822-2207-1. Sooch, S.S., Kaur, R., Singh, N., Trehan, P.N., 1982. Precise measurements of gamma ray energies and intensities in the 166mHo decay. Nucl. Instrum. Methods 203, 339–341. Tzika, F., Burda, O., Hult, M., Arnold, D., Caro Marroyo, B., Dryák, P., Fazio, A., Ferreux, L., García-Toraño, E., Javornik, A., Klemola, S., Luca, A., Moser, H., Nečemer, M., Peyrés, V., Reis, M., Silva, L., Šolc, J., Svec, A., Tyminski, Z., Vodenik, B., Wätjen, U., 2016. 60Co in cast steel matrix: A European interlaboratory comparison for the characterisation of new activity standards for calibration of gamma-ray spectrometers in metallurgy. Appl. Radiat. Isot. 114, 167–172. Tzika, F., Hult, M., Burda, O., Arnold, D., Sibbens, G., Caro Marroyo, B., Gómez– Mancebo, M-B., Peyrés, V., Moser, H., Ferreux, L., Šolc, J., Dryák, P., Fazio, A., Luca, A.,
Vodenik, B. Reis, M., Tyminski, Z., Klemola. S., 2015. Interlaboratory comparison on 137 Cs activity concentration in fume dust. Radiation Physics and Chemistry 116, 106–110. Wang, X.L., Yang, J.X., 1992. Relative gamma-ray emission probabilities of 166mHo. Nucl. Instrum. Methods A 312, 385-389.
Table I. Photon emission intensities of 166mHo decay. Values obtained in this work as well as recommended values and selected data from the literature. Energy (keV)
This work
X Kα2 48.22 X Kα1 49.12 X Kα2 48.22
Recommended
Moreira et al.,2010
10.81
Bernardes et al., 2002 9.98 (5)
19.2
19.55 (28)
Hino et al., 2000
Morel et al., 1996 10.29 (8) 18.56 (13)
29.56 (32)
28.85 (17)
X K1,3,5 55.7
6.24 (14)
5.83 (9)
5.84 (5)
X K,4,KO 57.2
1.62 (5)
1.51 (3)
1.544 (20)
X Kβ 56.06
6.95 (42)
80.5725
11.86 (14)
12.66 (23)
12.31 (14)
11.68 (10)
11.84 (16)
12.06 (8)
184.41
71.45 (58)
72.5 (3)
71.64 (96)
72.60 (47)
72.4 (7)
70.21 (35)
214.79
7.38(5)
0.445 (11)
215.87
0.421 (6)
2.66 (17)
2.504 (13)
214.79 + 215.87
3.10 (5)
3.009 (24)
259.736
1.07 (3)
1.078 (10)
280.46
29.19 (23)
29.54 (25)
300.741
3.65 (5)
3.73 (3)
3.633 (20)
3.593 (18)
365.768
2.46 (5)
2.46 (4)
2.433 (15)
2.390 (12)
410.956
11.45 (12)
11.35 (17)
11.27 (9)
11.17 (6)
451.54
2.98 (5)
2.915 (14)
3.01 (5)
2.905 (16)
464.798
1.22 (4)
1.25 (4)
529.825
9.50 (10)
9.4 (4)
9.65 (8)
9.35 (5)
9.63 (11)
9.36 (5)
570.995
5.39 (8)
5.43 (20)
5.49 (6)
5.42 (3)
5.54 (8)
5.41 (3)
611.579
1.47 (5)
1.31 (21)
670.526
5.40 (7)
5.31 (21)
691.253
1.28 (4)
1.32 (7)
711.697
54.39 (47)
54.9 (9)
54.43 (32)
53.8 (2)
56.0 (5)
53.6 (3)
752.28
12.01 (13)
12.2 (3)
12.16 (11)
11.98 (6)
12.27 (15)
11.92 (6)
1.049 (7) 29.01 (22)
29.30 (15)
1.039 (6) 29.7 (3)
11.39 (13)
1.169 (12)
1.343 (16) 5.32 (3)
11.10 (6) 2.852 (26)
1.152 (8)
5.35 (6)
28.55 (14)
1.342 (24) 5.65 (9)
1.31 (1)
5.31 (3) 1.307 (13)
778.827
3.07 (5)
3.01 (8)
2.96 (5)
3.019 (18)
810.286
56.90 (52)
57.3 (11)
57.31 (40)
56.6 (3)
58.2
2.978 (18) 56.4 (3)
830.565
9.68 (11)
9.72 (18)
9.75 (9)
9.56 (5)
9.77 (12)
9.58 (5)
875.663
0.74 (3)
0.721 (9)
950.988
2.72 (5)
2.744 (19)
1241.52
0.84 (3)
0.85 (3)
1400.79
0.53 (3)
1427.24
0.48 (2)
0.722 (7)
0.713 (12)
2.693 (19)
2.663 (16)
0.798 (9)
0.787 (9)
0.508 (6)
0.506 (7)
0.484 (4)
0.498 (6)
0.482 (7)
0.489 (4)
2.71 (5)
Table II. Uncertainty budget on the absolute photon emission intensities Uncertainty component
Relative standard uncertainty (%)
Type
Peak area (statistics, determination and background)
0.15 (184.4 keV) to 4.7 (1400.79 keV)
A and B
1.0 (E<120 keV) to 0.5 (E>500 keV) 0.23 to 0.85
B
0.1 (711.7 keV) to 1.07 (1427.24 keV) < 0.001 <0.001 0.24
A
Efficiency: Interpolation Statistics MC calculation Coincidence summing corrections Correction for acquisition time Nuclear data input Activity determination
A
B B B
Figure 1 (single-column fitting fig)
Figure 2 (double-column fitting fig)
Figure 3 (single-column fitting fig)
Highlights
Photon emission intensities of 166mHo measured by gamma-ray spectrometry.
Monte Carlo numerical model used to calculate counting efficiencies.
Gamma-spectrometry carried out using a calibrated high purity germanium detector
Measured photon emission intensities are compared with previously published values.
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S0969-8043(17)30344-5 http://dx.doi.org/10.1016/j.apradiso.2017.06.035 ARI7939
To appear in: Applied Radiation and Isotopes Received date: 10 March 2017 Revised date: 22 June 2017 Accepted date: 22 June 2017 Cite this article as: Virginia Peyres and Eduardo García-Toraño, Determination of the absolute photon emission intensities of some gamma rays of 166mHo, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.06.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Determination of the absolute photon emission intensities of some gamma rays of 166mHo Virginia Peyres*1, Eduardo García-Toraño CIEMAT, Metrología de Radiaciones Ionizantes, Avda. Complutense 40, 28040 Madrid, Spain *
Corresponding author: Tel.: +34 313466225 Fax: +34 913466442.
Email address: [email protected]
Abstract
This paper presents the results of the absolute measurement of some photon emission intensities in the decay of 166mHo. Point sources from a reference solution standardized in the frame of the EURAMET.RI (II)-K2.Ho-166m activity comparison were measured by gamma spectrometry. The detection efficiency was obtained by Monte Carlo calculations including the complete decay scheme. Results obtained for 27 gamma and X-ray emissions are compared to reference values. Keywords: 166mHo; Photon emission intensities; Gamma-ray spectrometry.
1. Introduction The nuclide 166mHo disintegrates by beta minus emission to seventeen excited levels of 166 Er. Most disintegrations feed two excited levels at 1828 keV and 1787 keV with intensities of 17.2 % and 74.8 % respectively. Deexcitation of the excited levels of 166Er gives rise to a large number of gamma-ray cascades (Schönfeld and Dersch, 2004) as shown in figure 1. The combination of a relative long half-life (1200 years) and the emission of a large number of well separated and intensive gamma-rays in a wide range of energy (between 80 and 1400 keV) makes it a candidate for calibration of germanium detectors (Danilenko et al., 1989; Morel et al., 1996; Hino et al., 2000; Bernardes et al., 2002). Its characteristics are also suitable for checking the stability of ionization chambers, in replacement of 226Ra sources which are commonly used to this purpose (Hino et al., 2000). 1
Present address: CIEMAT, Avda. Complutense 40, 28040 Madrid (Spain)
Figure1. Simplified decay scheme of 166mHo. Data are taken from Nucleide (Schönfeld and Dersch, 2004).
Absolute and relative photon emission intensities for this nuclide have been measured by several authors (Reich and Cline, 1970; Gehrke et al., 1977; Sooch et al., 1982; Danilenko et al., 1989; Wang and Yang, 1992; Miyahara et al., 1994; Morel et al., 1996; Hino et al., 2000; Bernardes et al., 2002; Moreira et al., 2010). Absolute results obtained in early works exhibit a large dispersion and high uncertainties (Reich and Cline, 1970; Danilenko et al., 1989). More recent measurements present lower uncertainties (Moreira et al., 2010; Bernardes et al., 2002; Hino et al., 2000; Morel et al., 1996; Miyahara et al., 1994) but with a large dispersion of the results. Among all results available, the works published by Morel et al. (1996) and Bernardes et al. (2002) deserve special attention for their careful setup. In both cases, two detectors, one for the low energy range and other for intermediate and higher energy range were used. Efficiency calibration was also performed by both authors with a large number of experimental points (more than 70 for Morel). Measurements were carried out in such a way that coincidence-summing corrections are either negligible (Bernardes et al., 2002) or were checked at several source-to-detector distances (Morel et al., 2002). The radioactive solutions used to prepare the samples were, in both cases, standardized in the frame of international comparisons or sent for measurement to the SIR (BIPM) (Ratel et al., 2003; Ratel et al., 2015). Therefore, both sets of results of the photon emission intensities are based on absolute activity measurements, performed in the frame of international comparisons. Other recent measurements, also absolute, were presented by Hino et al. (2000) and Moreira et al. (2010). The first author does not provide details on the efficiency calibration or the measurement geometry. In the paper by Moreira, no information is given on the magnitude of the coincidence-summing correction and efficiency calibration is carried out with 6 radionuclides, from which two (133Ba and 152Eu) are subject to important coincidence-summing effects. The latest evaluation of 166mHo performed in the frame of the Decay Data Evaluation Project (DDEP) by Schönfeld and Dersch (2004), does not include the most recent measurements among which there is a lack of consistence. This paper presents new absolute measurements of photon emission intensities of 166mHo. The reference solution used was standardized in the framework of the EURAMET.RI(II)K2.Ho-166m (Kossert et al, 2014) activity comparison. The procedure followed to purify and characterize the solution has been described in detail elsewhere (Guéguen et al., 2014). Potential gamma-ray emitting contaminants were determined in the comparison process
[Kossert et al., 2014) and their relative contribution was considered < 1.5 x 10-4. CIEMAT participated in the comparison with the CIEMAT/NIST and 4-counting methods, but the results of the photon emission intensities are based on the comparison reference value (119.27(10) kBq g-1), which differs from the value declared by CIEMAT in less than 0.07 %
2. Materials and methods 2.1
Experimental setup and efficiency determination
Six sources were prepared for gamma-ray spectrometry measurement by dropping aliquots of about 10 mg of the solution onto 75 µm polyethylene disk foils, allowing drying, covering with similar foils and thermally sealing. The diameter of the polyethylene disks was 2 cm and that of the active sources was about 3 mm. The experimental setup was formed by an extended-range coaxial high purity germanium detector (HPGe), with diameter and height close to 61 mm, equipped with a carbon-epoxy window and a very thin contact (about 300 nm) in the front surface. The electronic setup includes a high-voltage power supply from BERTAN, preamplifier, spectroscopy amplifier and pulse generator from CANBERRA and a successive-approximation analog-to-digital converter model 7423 from SILENA. A cubic shielding structure of about 80 × 80 × 80 cm x cm x cm surrounds the detector. It is composed by layers of Pb (5 cm), Cd (3 mm) and Cu (1.5 mm). The Ge detector was originally calibrated by Monte Carlo simulation in an energy range from 14 to 1800 keV (Peyres and García-Toraño, 2007). Point sources from monoenergetic and multi-gamma emitters were measured at 15 cm from the detector window and provided a set of 26 experimental values to which the results of the simulations were compared. Discrepancies between simulated and experimental values were, in most cases, below 1%. Since then, experimental data have been completed by adding new results using selected gamma emissions of 133Ba, 129I, 210Pb and 113Sn to complete a total of 38 experimental points. That allowed improving the reliability of the efficiency determination. Nuclear decay data for 133Ba, 129I and 113Sn were taken from Nucléide database (2017). For the gamma ray of 46.54 keV of 210Pb, emission intensities recently measured by Rodrigues et al. (2016) were used. A numerical detector model was used to obtain the efficiencies for the gamma-ray lines of 166m Ho. To this purpose, the Monte Carlo code PENELOPE 2014 was used (Salvat, 2015). The uncertainty of full-energy peak efficiencies is composed by the inherent statistics component of the calculation, which has remained very low (between 0.23% and 0.85%),
and by that due to the input values of the Monte Carlo simulation, basically a function of the detector and measurement parameters (Lépy et al., 2015), the latter being always difficult to assess. Nevertheless, the procedure has been validated in the course of several interlaboratory comparisons in which excellent results have been obtained (Tzika et al., 2015; Tzika et al., 2016; Peyrés and García-Toraño, 2011). That reflects the fact that Monte Carlo calculation can provide robust efficiency values if geometrical parameters have been optimized by comparing calculations and experimental. An accurate interpolation in a wide range of energies is supported by the fact that Monte Carlo simulation takes into account the physical processes taking place in detector and source, including the photon and electron cross sections that allow a physics-based interpolation. As an additional estimation of the uncertainty attainable with this method, experimental efficiency data have also been fitted using two log-log polynomial functions, one for lowenergy range (15 keV to 150 keV) and another one for the high-energy range (120 keV to 2 MeV). In this case, relative standard combined uncertainties are about 1.0 % from 48 to 120 keV, 0.6 % in the 120 keV to 500 keV energy range and 0.5 % above 500 keV. Since the Monte Carlo method is based on the same experimental data and is not dependent on a predefined functional expression, these figures can be considered as an upper limit to the uncertainty of data obtained in this study. The efficiency curve is presented in figure 2. It shows the experimental points and corresponding values calculated for the validation of the Monte Carlo model; the calculated values of the efficiency of X-rays and gamma-ray lines of 166mHo of interest to this study and the z-score statistics used to compare both sets of results. Figure 2. Full-energy peak efficiency curve obtained for point sources placed at 15 cm. from the detector. Results of the Monte Carlo simulation and experimental data used to validate the model are shown as well as the efficiencies of the emission lines of 166m Ho studied in this paper. Also shown are the z-score values. 2.2
Measurements of photon emission intensities
Six point sources from the reference solution were measured at source-to-detector distances of 15 cm, matching the geometry used to calibrate the detector. Each source was measured during 200 000 s. A set of measurements with shorter acquisition times (50 000 s each) were also taken for checking purposes, but they were not retained for analysis. The stability of the equipment was considered satisfactory as neither FWHM (Full Width at Half Maximum) values, nor the energy calibration suffered significant changes during the measurements. Photon emission intensities were calculated according to the formula:
̇
Where are the emission intensities of the X- or gamma rays; ̇ are the net peak counting rates after background subtraction (net peak areas divided by the measurement time, expressed in seconds); are the full-energy peak efficiencies; A is the activity of the source at the measurement time and are the coincidence summing corrections. Correction by decay during the measurements was negligible. Gamma-ray peak areas were determined using the IAEA software GRILS, a non-linear fitting code that provides accurate uncertainty estimations and is a part of the GANAAS suite (IAEA, 1991). The analysis of X-ray peaks was done with the COLEGRAM software (Ruellan et al., 1996). Background spectra only presented measurable structures under the gamma lines of 300.74 and 611.6 keV Coincidence summing corrections are a major source of concern for these calculations, as they can reach values up to 3% for some emissions. All calculations were done by a combination of simulations made with the PENELOPE (Salvat, 2015) and PENNUC codes (García-Toraño et al., 2017). The combination of both codes allows simulating the decay of a nucleus as a random sequence of transitions; each transition involving the release of electrons or photons or the production of vacancies in atomic subshells followed by subsequent relaxation of the atomic electron. When the decay path includes a metastable level, the set of transitions prior to that level is considered as an independent emission event (in which several particles may be released). The emitted particles are fed to PENELOPE. Nuclear data are taken from the Nucléide database (Nucléide, 2017) from which data sets can be exported in a compatible format. A powerful feature of the program is the ability to deal with nuclear level lifetimes as well as detector times. In principle, a certain resolution value is assigned to the detector, and all particles reaching the detector within this time are treated as a single detection event, even if they correspond to a complete cascade. When the lifetime exceeds the detector resolution time (a sampling algorithm is used), the level is considered as metastable and a new cascade is generated. A similar situation can arise when lifetimes of nuclear levels are considered. Therefore, if the original data set of the 166mHo scheme is fed, calculated efficiencies for all gamma-ray emissions will include coincidence summing effects, as this is an integral part of the simulation. On the contrary, by assigning a negative value to the detector time, all particles emitted in the disintegration are considered as individual events from the point of view of the detector response. In both cases, results are dependent on the emission intensities whose values are target values for this study. Nevertheless, as both sets of values are combined to obtain the coincidence-summing correction, the influence of the emission intensities is, at least partially, cancelled. Coincidence-summing correction factors
obtained in this way were then applied to efficiency values calculated as explained in 2.1. An additional check was made with the ETNA software (Lépy et al., 2012).
3. Results and discussion The final results of the absolute photon intensities of 166mHo are presented in table I, together with the results of recent measurements and recommended values from the Decay Data Evaluation Project (DDEP) by Schönfeld and Dersch (2004). Each result corresponds to the average of the measurement of six sources. A total of 26 intensities have been obtained. For the sake of clarity, only the most relevant data are presented in Figure 3. Complete details of the uncertainty components considered are shown in table II.
Figure. 3. Photon intensities determined in this work for gamma emissions with energies 80.57, 184.41, 280.46, 410.96, 711.7, 752.28, 810.29 and 830.56 keV. Recommended values as well as some recently published values are presented for comparison.
A complete analysis of the X-ray region of spectra is hampered by the limited energy resolution of our experimental system. The system is not able to discriminate between components of XK and XKrays groups. Hence, only aggregated data are presented for both groups. For XKthe agreement with recommended data is good, while for XKrays, subject to high uncertainties in the present measurements, differences are not covered by the stated uncertainties. For the lines with energies 80.57, 184.41, 280.46 keV, coincidence-summing corrections are in the order of 3% in the measurement geometry used. Results for the 80.57 keV line are not in good agreement with recommended values, but agree well with absolute data obtained by Hino, Morel and Bernardes and their co-workers which, in turn, do not match the recommended value which was not obtained only as a combination of experimental values, but also taking into account balance conditions. For the emission at 184.41 keV, a small discrepancy is found with the reference value, while results for the gamma emission at 280.46 keV agree well with recommended values. Results for the 410.96, 711.7 and 752.28 keV emissions are compatible with recommended values, which have typical uncertainties within 1.5 and 2.4 %. For the 410.96 keV emission, discrepancies are found with results from Morel and Bernardes, but a good agreement is reached with those from Moreira and Hino as well as with recommended
values. Discrepancies cannot be attributed to coincidence-summing effects, as the total correction for this line is 1.038. Values for the other two gamma emissions at 711.7 and 752.28 keV agree well with results from Morel and Bernardes. Finally, the results for the 810.29 and 830.56 keV emissions. They are compatible with recommended values as well as with data from Morel and Bernardes. Coincidencesumming corrections for these lines are 1.02 and 1.024 respectively. Results for the other, less intense, emissions, agree well with recommended data with uncertainties between 1% for the 529.82 keV emission and 4% for the 1427.24 keV emissions. Overall, 24 from the 27 results presented are compatible with the recommended values.
4. Conclusions A new set of photon-emission intensities of 166mHo has been measured using a solution previously standardized in the frame of an international activity comparison. A total of 27 X-ray and gamma intensities have been determined by a combination of measurements and Monte Carlo calculations. Uncertainties depend on the line intensities with a minimum of 0.8% for the 184.4 keV emission and a maximum of 5.6% for the weak emission at 1400.8 keV. Most results are compatible with recommended values. Nevertheless, given the unresolved discrepancies (also evident in other absolute determinations) with the important 80.57 keV emission, a reevaluation of the decay scheme of this nuclide should be considered.
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Table I. Photon emission intensities of 166mHo decay. Values obtained in this work as well as recommended values and selected data from the literature. Energy (keV)
This work
X Kα2 48.22 X Kα1 49.12 X Kα2 48.22
Recommended
Moreira et al.,2010
10.81
Bernardes et al., 2002 9.98 (5)
19.2
19.55 (28)
Hino et al., 2000
Morel et al., 1996 10.29 (8) 18.56 (13)
29.56 (32)
28.85 (17)
X K1,3,5 55.7
6.24 (14)
5.83 (9)
5.84 (5)
X K,4,KO 57.2
1.62 (5)
1.51 (3)
1.544 (20)
X Kβ 56.06
6.95 (42)
80.5725
11.86 (14)
12.66 (23)
12.31 (14)
11.68 (10)
11.84 (16)
12.06 (8)
184.41
71.45 (58)
72.5 (3)
71.64 (96)
72.60 (47)
72.4 (7)
70.21 (35)
214.79
7.38(5)
0.445 (11)
215.87
0.421 (6)
2.66 (17)
2.504 (13)
214.79 + 215.87
3.10 (5)
3.009 (24)
259.736
1.07 (3)
1.078 (10)
280.46
29.19 (23)
29.54 (25)
300.741
3.65 (5)
3.73 (3)
3.633 (20)
3.593 (18)
365.768
2.46 (5)
2.46 (4)
2.433 (15)
2.390 (12)
410.956
11.45 (12)
11.35 (17)
11.27 (9)
11.17 (6)
451.54
2.98 (5)
2.915 (14)
3.01 (5)
2.905 (16)
464.798
1.22 (4)
1.25 (4)
529.825
9.50 (10)
9.4 (4)
9.65 (8)
9.35 (5)
9.63 (11)
9.36 (5)
570.995
5.39 (8)
5.43 (20)
5.49 (6)
5.42 (3)
5.54 (8)
5.41 (3)
611.579
1.47 (5)
1.31 (21)
670.526
5.40 (7)
5.31 (21)
691.253
1.28 (4)
1.32 (7)
711.697
54.39 (47)
54.9 (9)
54.43 (32)
53.8 (2)
56.0 (5)
53.6 (3)
752.28
12.01 (13)
12.2 (3)
12.16 (11)
11.98 (6)
12.27 (15)
11.92 (6)
1.049 (7) 29.01 (22)
29.30 (15)
1.039 (6) 29.7 (3)
11.39 (13)
1.169 (12)
1.343 (16) 5.32 (3)
11.10 (6) 2.852 (26)
1.152 (8)
5.35 (6)
28.55 (14)
1.342 (24) 5.65 (9)
1.31 (1)
5.31 (3) 1.307 (13)
778.827
3.07 (5)
3.01 (8)
2.96 (5)
3.019 (18)
810.286
56.90 (52)
57.3 (11)
57.31 (40)
56.6 (3)
58.2
2.978 (18) 56.4 (3)
830.565
9.68 (11)
9.72 (18)
9.75 (9)
9.56 (5)
9.77 (12)
9.58 (5)
875.663
0.74 (3)
0.721 (9)
950.988
2.72 (5)
2.744 (19)
1241.52
0.84 (3)
0.85 (3)
1400.79
0.53 (3)
1427.24
0.48 (2)
0.722 (7)
0.713 (12)
2.693 (19)
2.663 (16)
0.798 (9)
0.787 (9)
0.508 (6)
0.506 (7)
0.484 (4)
0.498 (6)
0.482 (7)
0.489 (4)
2.71 (5)
Table II. Uncertainty budget on the absolute photon emission intensities Uncertainty component
Relative standard uncertainty (%)
Type
Peak area (statistics, determination and background)
0.15 (184.4 keV) to 4.7 (1400.79 keV)
A and B
1.0 (E<120 keV) to 0.5 (E>500 keV) 0.23 to 0.85
B
0.1 (711.7 keV) to 1.07 (1427.24 keV) < 0.001 <0.001 0.24
A
Efficiency: Interpolation Statistics MC calculation Coincidence summing corrections Correction for acquisition time Nuclear data input Activity determination
A
B B B
Figure 1 (single-column fitting fig)
Figure 2 (double-column fitting fig)
Figure 3 (single-column fitting fig)
Highlights
Photon emission intensities of 166mHo measured by gamma-ray spectrometry.
Monte Carlo numerical model used to calculate counting efficiencies.
Gamma-spectrometry carried out using a calibrated high purity germanium detector
Measured photon emission intensities are compared with previously published values.