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Precise determination of photon emission probabilities for the main X- and γ-rays of 226Ra in equilibrium with daughters
ARTICLE IN PRESS
Applied Radiation and Isotopes 60 (2004) 341–346
Precise determination of photon emission probabilities for the main X- and g-rays of 226Ra in equilibrium with daughters J. Morela,*, S. Sepmanb, M. Raskob, E. Terechtchenkob, J.U. Delgadoc a CEA-DIMRI-BNM-LNHB, B.P. 52, F-91193 Gif-sur-Yvette Cedex, France D.I. Mendeleyev Institute for Metrology (VNIIM), 19 Moskowsky Prospect, St. Petersburg 198005, Russia c LNMRI, IRD, Commissao de Energia Nuclear, Av. Salvador Allende, S/N–Recreio, 22780 160 Rio de Janeiro CEP, Brazil b
Abstract Within the context of a joint project between VNIIM (D.I. Mendeleyev Institute for Metrology) and LNHB (Laboratoire National Henri Becquerel), special 226Ra sources were prepared by VNIIM in order to determine as accurately as possible the absolute photon emission probabilities for the main X- and g-rays following the decay of 226 Ra and daughters. The main purpose of this work was to supplement a previous joint study by Laboratorio Nacional de Metrologia das Radia@oes Ionizantes (LNMRI) and LNHB to determine their relative values. Some specific point sources were produced for a-spectrometry measurements that were undertaken at VNIIM and also for g-ray spectrometry studies at VNIIM and LNHB. The 226Ra activity for the g-spectrometric sources was measured relative to the a-spectrometric sources by comparing the counts of the main g-rays. The total uncertainty of the activity for these sources was 0.2% (k ¼ 1). Using calibrated germanium detectors, several X- and g-ray spectra were analyzed to determine the absolute photon emission probabilities of 226Ra in radioactive equilibrium with daughters. The results are presented and compared to other published values. r 2003 Elsevier Ltd. All rights reserved. Keywords: g-Ray spectrometry; Alpha counting;
226
Ra and daughters; Photon emission probabilities
1. Introduction Over recent years, the methods used to measure the activity of 222Rn have been arousing increasing interest that can be explained by the impact of this radionuclide on environmental measurements. Two comparison campaigns were organized by the National Physical Laboratory (NPL) as part of the European Collaboration in Metrology (Euromet) programme, the first in 1992 and the second in 1994 (Dean and Burke, 1994). The most striking observation of the second campaign was a bias of a few percent between what were said to be the absolute measurements and those made using gamma-ray spectrometry. Additional experiments were *Corresponding author. Tel.: +33-1-69-08-4178; fax: +33-169-08-9529. E-mail addresses: [email protected], [email protected] (J. Morel).
carried out to determine the main cause of this bias, and it was discovered that radioactive material tended to settle on the two tubes and valves of the vials containing 222 Rn gas. An original method for measuring activity had been developed in parallel to these studies (Picolo, 1995), based on the condensation of 222Rn on a cold surface and counting the alpha particles emitted by the deposit in a pre-defined geometry. Taking advantage of the experiments carried out previously, a set of nuclear data measurements characterizing the gamma-ray emissions of 222Rn in equilibrium with its daughters was made (Morel et al., 1998) using two spherical vials with no valves and two others with only one. The results obtained for the major rays had a relative uncertainty of between 1.3% and 1.5%; these uncertainties were considered too high, with 0.9% affecting the activity measurement and between 0.5% and 1.0% affecting calibration precision. Thus, to lower the uncertainty and improve the quality of the gamma-ray spectrometry
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measurements, a similar task was carried out using 222 Rn point sources or sources considered as such. The relative intensities of the X- and g-rays of 226Ra and its daughters had an uncertainty of between 0.3% and 0.5% for the major emissions in the studies carried in collaboration with LNMRI (Delgado et al., 2002). However, for these data to be expressed as absolute values, an emission probability of 44.8370.27 had to be adopted for the 609-keV gamma ray, deduced by combining three published values, two obtained from . measurements: 44.670.5 (Schotzig and Debertin, 1983) and 44.870.6 (Morel et al., 1998) and the other from an evaluation: 45.070.4 (Coursol et al., 1990). This arbitrary assignment of the absolute emission probability was not satisfactory; therefore, in the context of an agreement between our laboratory and the VNIIM, the latter used their expertise in source preparation and made available two 226Ra point sources, characterized by a lack of radon emanation and with a certified activity of 70.2% (at 1s confidence level). Thus, by referring to the work carried out earlier and using these sources, the absolute emission probabilities of X- and g-ray emissions could be determined in a non-arbitrary manner; the results are described here.
2. Sample preparation 2.1. Description Two 40 kBq sources for gamma-ray spectrometry and another two 13 kBq sources for alpha-particle spectrometry were prepared by VNIIM from the same radium sample (Kharitonov et al., 2002), based on their experience and that of the Khlopin Radium Institute (St. Petersburg). A gamma-ray source comprised a disk of two sheets of stainless steel, each with a mass per unit surface area of 18 mg/cm2; the radioactive material was placed between these two sheets at the center of a circle with a diameter of less than 4 mm and the whole assembly was sealed with epoxy resin. The disk was then secured to an aluminum ring for ease of handling. The alpha sources comprised a deposit on a polished stainless steel disk, protected by 0.12-mm layer of titanium dioxide and 0.07-mm layer of aluminum. The slight emanation of radon was monitored by placing the sources in a 210 l stainless steel container and observing the formation of radon using an alpha monitor; the emanation coefficient did not exceed 0.1% for the gamma sources. 2.2. Certification The gamma sources were certified at VNIIM in two stages. First the activity of the alpha sources had to be determined using two independent methods, and secondly the activity of the gamma sources had to be
determined by comparison with the alpha sources. The comparison was made by gamma-ray spectrometry using a Ge–Li detector and monitoring the 186.2, 242.0, 295.2, 351.9, 609.3, 1120.3 and 1764.5 keV peaks. All sources were placed 10 and 15 cm from the detector. The activity of the alpha sources was first determined by means of the defined solid-angle method, using a ZnS scintillation detector. Thereafter, the measurement was checked using an alpha spectrometer (with 20 keV resolution) to compare the emission of alpha particles from 226Ra to the emission from an 241Am calibration source. On the basis of these two methods, an activity uncertainty of less than 0.2% was observed at the 1s confidence level. Making allowance for the uncertainties associated with comparison, the total uncertainty associated with the gamma source activity was finally set at 0.2%.
3. Photon emission probabilities 3.1. Relative emission probabilities Since the results of the work carried out previously were considered satisfactory (Morel et al., 1998; Delgado et al., 2002), there was no point in redetermining the relative X- and g-ray emission probabilities. Consequently, the results were used as a reference. The photon emission probabilities corresponding to these publications are listed in Table 1 relative to the 609.3-keV gamma ray of 100. The corresponding uncertainties include no allowance for the uncertainty affecting the activity. From these results we adopted a final value equal to the weighted mean of each of the values in the two series of measurements, and a relative uncertainty equal to the maximum value of the difference between the internal relative uncertainty and the relative standard deviation for the weighted mean. The results obtained are shown in the ‘‘Current work’’ column, and are compared with the recent values obtained from a study of high-energy calibration gamma rays (Molnar et al., 2002). These two sets of data are in very good agreement; in particular, the relative uncertainties of several intense gamma rays are 0.3%, which is highly satisfactory. 3.2. Determination of absolute emission probabilities The relative photon emission probabilities were converted into absolute values by means of several operations that involved the counting of a few intense peaks characteristic of 226Ra daughters and known to high accuracy. The energies of these gamma rays were 186.2, 295.2, 351.9, 609.3, 1120.3 and 1764.5 keV. The photon emission probabilities following the decay of 226Ra were measured at VNIIM by using three
ARTICLE IN PRESS J. Morel et al. / Applied Radiation and Isotopes 60 (2004) 341–346 Table 1 X- and gamma-ray emission probabilities for gamma ray) Energy (keV)
53.2 74.8 Bi XKa2 76.7 Po XKa2 77.1 Bi XKa1 79.3 Po XKa1 81.1 Rn XKa2 83.8 Rn XKa1 87.2 Bi XKb3,1,5 89.6 Po XKb3,1,5 89.9 Bi XKb2,4,O 92.4 Po XKb2,4,O 94.7 Rn XKb3,1,5 97.7 Rn XKb2,4,O 186.2 242.0 258.8 273.7+274.7 273.7 274.7 295.2 351.9 387.0+389.0 387.0 389.0 455.0 480.5 487.1 580.3 609.3 665.4 768.4 785.8+785.9 806.2 934.1 1120.3 1155.2 1238.1 1281.0 1377.7 1385.3 1401.5 1408.0 1509.2 1661.3 1729.6 1764.5 1847.4 2118.5 2204.1 2293.4 2447.7
Morel et al. (1998)
343
226
Ra and daughters expressed as relative values (normalized to 100 for the 609.3-keV Delgado et al. (2002)
Current work
Molnar et al. (2002)
Value
Uncertainty (%) Value
Uncertainty (%) Value
Uncertainty (%) Value
Uncertainty (%)
— — — — — — — — — — — — — — 15.74 1.170 1.025 0.292 0.732 39.96 77.46 1.511 0.612 0.900 0.656 0.752 0.962 0.837 100.0 3.415 10.83 2.395 2.813 6.815 33.06 3.612 12.97 3.194 8.91 1.817 2.964 5.384 4.750 2.375 6.275 33.93 4.513 2.549 10.89 — 3.455
— — — — — — — — — — — — — — 0.9 2.3 2.8 9.0 3.0 0.9 0.9 1.8 4.9 3.5 5.3 4.8 4.4 4.4 0.9 1.7 1.3 3.3 2.1 1.2 1.1 2.2 1.1 3.0 1.3 3.5 2.7 1.7 2.8 4.0 1.9 1.2 2.0 2.5 1.6 — 1.9
1.0 0.7 5.2 0.7 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.8 20.0 0.4 0.3 0.8 1.0 8.5 2.9 0.3 0.3 1.4 1.8 1.6 1.9 1.3 1.3 1.3 0.3 0.5 0.5 0.9 0.8 0.5 0.3 1.0 0.5 0.9 0.5 1.0 0.7 0.5 1.4 1.5 0.5 0.3 0.8 0.8 0.5 2.5 0.7
1.0 0.7 5.2 0.7 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.8 20.0 0.4 0.3 0.8 0.9 6.2 3.6 0.3 0.3 1.1 1.7 1.5 1.8 1.3 1.2 1.2 0.3 0.5 0.5 0.9 0.7 0.5 0.3 0.9 0.4 0.9 0.5 1.1 0.7 0.8 1.2 1.5 0.5 0.3 0.7 0.8 0.5 2.5 0.7
0.8 — — — — — — — — — — — — 0.6 0.4 — — — — 0.3 0.3 — — — — — — — 0.3 0.6 0.3 — 0.5 0.6 0.4 0.5 0.3 0.5 0.3 0.9 0.4 0.4 0.6 0.6 0.5 0.3 0.7 0.8 0.8 1.5 1.2
2.329 13.20 1.164 22.22 2.119 0.348 0.480 7.08 0.770 2.204 0.185 0.176 0.045 7.812 15.90 1.171 1.053 0.265 0.787 40.36 78.16 1.539 0.651 0.888 0.640 0.749 0.961 0.823 100.0 3.359 10.66 2.447 2.788 6.783 32.71 3.594 12.83 3.147 8.69 1.744 2.924 5.233 4.606 2.271 6.226 33.54 4.448 2.536 10.74 0.665 3.402
2.329 13.20 1.164 22.22 2.119 0.348 0.480 7.08 0.770 2.204 0.185 0.176 0.045 7.812 15.88 1.171 1.050 0.278 0.760 40.32 78.10 1.528 0.647 0.890 0.642 0.750 0.961 0.824 100.0 3.364 10.68 2.443 2.791 6.788 32.74 3.597 12.85 3.151 8.72 1.750 2.927 5.245 4.636 2.284 6.229 33.56 4.457 2.537 10.76 0.665 3.408
2.384 — — — — — — — — — — — — 7.85 15.98 — — — — 40.61 78.34 — — — — — — — 100.0 3.386 10.77 — 2.777 6.83 32.77 3.595 12.80 3.159 8.79 1.755 2.934 5.250 4.670 2.299 6.25 33.63 4.42 2.548 10.75 0.677 3.410
ARTICLE IN PRESS
0.6 0.4 0.4 0.4 0.4 0.5 0.27 0.4567 0.4568 0.4557 0.4538 0.4548 0.4573 0.4557 a
b
Correlated components are eliminated for this analysis. Activity uncertainty of 0.2% is not taken into account for this weighted mean evaluation, but is added at the end of the calculation.
0.45b 0.3b 0.3b 0.3b 0.3b 0.4b 0.5a 0.4a 0.4a 0.4a 0.4a 0.5a 3.568a 18.37 a 35.55 a 45.44 a 14.89 a 15.32 a 3.569 0.5 3.528 0.7 18.36 0.4 18.49 0.6 35.54 0.4 35.80 0.6 45.43 0.4 45.57 0.6 14.88 0.4 15.04 0.6 15.28 0.5 15.45 0.8 Adopted ratio (weighted mean and uncertainty) 0.5 0.4 0.4 0.4 0.4 0.5 3.586 18.38 35.65 45.44 14.93 15.26 — 0.6 0.6 0.6 0.6 0.7 — 18.55 35.79 45.22 14.89 15.41 186.2 295.2 351.9 609.3 1120.3 1764.5
Uncertainty (%) Value
VNIIM
3.568b 18.42b 35.62b 45.38b 14.89b 15.35b
Value Uncertainty (%) Uncertainty (%) Value Uncertainty (%) Value Uncertainty (%) Value Uncertainty (%)
Series B (12 cm) Series A (12 cm)
LNHB
Value
Series C (22 cm)
Synthesis A, B, C
Value
Adopted values from VNIIM & LNHB
Ra and daughters, and determination of relative-absolute conversion term
Ratio of absolute to relative values Energy
The results obtained from one VNIIM set of studies and for the series of three LNHB experiments are shown in Table 2 for the six major gamma rays. All these results
Photon emission probabilities
3.3. Results
226
semiconductor detectors: HPGe coaxial-n-type detector (relative efficiency: 30%, resolution at 122 and 1332 keV: 1.3 and 2.6 keV, respectively), Ge–Li coaxial-type detector (relative efficiency: 15%, with resolutions at 122 and 1332 keV of 1.7 and 2.9 keV, respectively); and HPGe coaxial n-type detector (volume: 40 cm3, with resolutions at 122 and 1332 keV of 2.2 and 2.9 keV, respectively). Each detector was calibrated on the basis of peak-efficiency for source-to-entrance distances of 10, 20 and 30 cm using the following reference point sources: 54Mn, 57Co, 60Co, 88Y, 133Ba, 134Cs, 137Cs, 139 Ce, 152Eu, 154Eu, 198Au and 241Am. The activity of these sources was measured by absolute methods with an uncertainty from 0.5% to 0.8% (K ¼ 1). Corrections for coincidence losses of the order 1.0, 0.5 and 0.2%, and dependence on the source to detector distance were taken into account. The fitting of the experimental efficiency data is described in another paper (Terechtchenko et al., 2004). A general uncertainty of the efficiency response in the 60 to 1900 keV energy range is estimated to range from 0.5% to 1.0%. Concerning the measurements at LNHB, both the sources prepared and certified by VNIIM were submitted to three counting campaigns, the first in September 2001, the second and the third in February 2002; on average, each counting campaign lasted 30,000 s. Source–detector distance was 12 cm in the first two studies, and 22 cm in the third. The coaxial n-type HPGe detector (volume of 100 cm3, resolution of 0.66 keV at 122 keV and 1.80 keV at 1332 keV) was the same as that described in previous publications (Morel et al., 1998; Delgado et al., 2002); however, recent treatment of the crystal surface by the manufacturer necessitated a recalibration using the following calibration sources: 22Na, 56Co, 57Co, 60Co, 65Zn, 110Agm, 133 Ba, 137Cs, 152Eu, 166Hom and 241Am. The nuclear data for some of the multi-gamma emitters were taken from the most recent work of Plagnard et al. (1993) for 133Ba, 152 Eu and 241Am, and Morel et al. (1996) for 166Hom. Nuclear data for all of the other radionuclides were taken from the ‘‘Nuclide’’ file (B!e et al., 1998), with the exception of 56Co, for which suitable data were in the personal possession of the authors. Some aspects of the counting operations were corrected slightly, including decay, count losses due to pile-up effects (even though slight), and summing effects. The latter were more significant for the 609.3 and 1120.3 keV g-rays than for the four other emissions, i.e., 0.7 and 1.0% at a distance of 12 cm, and 0.2% and 0.3% at a distance of 22 cm, respectively.
Uncertainty (%)
J. Morel et al. / Applied Radiation and Isotopes 60 (2004) 341–346
Table 2 Absolute photon emission probabilities for the most intense g-rays emitted by
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are given with a relative uncertainty estimated at the 1s confidence level, and the adopted value corresponds to the weighted mean (only non-correlated components Table 3 Absolute X- and gamma-ray emission probabilities for Energy (keV)
53.2 74.8 Bi XKa2 76.7 Po XKa2 77.1 Bi XKa1 79.3 Po XKa1 81.1 Rn XKa2 83.8 Rn XKa1 87.2 Bi XKb3,1,5 89.6 Po XKb3,1,5 89.9 Bi XKb2,4,O 92.4 Po XKb2,4,O 94.7 Rn XKb3,1,5 97.7 Rn XKb2,4,O 186.2 242.0 258.8 273.7+274.7 273.7 274.7 295.2 351.9 387.0+389.0 387.0 389.0 455.0 480.5 487.1 580.3 609.3 665.4 768.4 785.8+785.9 806.2 934.1 1120.3 1155.2 1238.1 1281.0 1377.7 1385.3 1401.5 1408.0 1509.2 1661.3 1729.6 1764.5 1847.4 2118.5 2204.1 2293.4 2447.7
Present work
345
were used to calculate the weighting). The last column shows the factor used to convert from relative to absolute emission probabilities, corresponding to the
226
Ra and daughters
. Schotzig and Debertin (1983)
Lin and Harbottle (1991)
Morel et al. (1998)
Value Uncertainty (%) Value
Uncertainty (%)
Value
Uncertainty (%)
Value Uncertainty (%)
1.061 6.02 0.531 10.13 0.966 0.159 0.219 3.227 0.351 1.004 0.084 0.080 0.020 3.560 7.24 0.534 0.478 0.127 0.347 18.37 35.59 0.697 0.295 0.406 0.293 0.342 0.438 0.376 45.57 1.533 4.87 1.113 1.272 3.093 14.92 1.639 5.86 1.436 3.972 0.797 1.334 2.390 2.113 1.041 2.839 15.29 2.031 1.156 4.90 0.303 1.553
4.5 10 10 10 10 11 11 10 — 10 — 11 11 1.7 1.5 — — — — 1.6 1.1 — — — — — — — 1.1 — 1.5 1.9 — 1.3 1.4 — 1.2 — — — — — 2.4 — — 2.0 — 2.6 2.4 3.0 2.6
— — — — — — — — — — — — — — 7.43 0.524 0.474 — — 19.3 37.6 — — 0.417 — 0.32 0.422 0.352 46.1 1.46 4.94 1.09 1.22 3.03 15.1 1.63 5.79 1.43 4.00 0.757 1.27 2.15 2.11 1.15 2.92 15.4 — — — — —
— — — — — — — — — — — — — — 1.5 2.1 2.3 — — 1.0 1.1 — — 6.2 — 1.2 3.8 4.0 1.1 2.0 1.2 1.8 1.6 1.3 1.3 1.2 1.4 1.4 1.5 2.4 1.6 2.8 1.9 2.6 1.4 1.3 — — — — —
— — — — — — — — — — — — — — 7.05 0.52 0.46 0.13 0.33 17.90 34.70 0.68 0.27 0.40 0.29 0.34 0.43 0.38 44.84 1.53 4.85 1.07 1.26 3.05 14.81 1.62 5.81 1.43 3.99 0.81 1.33 2.41 2.13 1.06 2.81 15.20 2.02 1.14 4.88 — 1.55
1.0 0.8 5.2 0.8 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.7 20.0 0.6 0.4 0.8 1.0 6.2 3.6 0.4 0.4 1.1 1.7 1.5 1.8 1.3 1.3 1.3 0.4 0.6 0.5 0.9 0.8 0.5 0.4 1.0 0.5 0.9 0.5 1.0 0.7 0.6 1.3 1.5 0.6 0.4 0.8 0.8 0.6 2.5 0.7
1.11 6.1 0.66 10.2 1.11 0.158 0.26 3.82 — 1.11 — 0.111 0.034 3.51 7.12 — — — — 18.2 35.1 — — — — — — — 44.6 — 4.76 1.04 — 3.07 14.7 — 5.78 — — — — — 2.08 — — 15.1 — 1.17 4.98 0.301 1.55
— — — — — — — — — — — — — — 1.3 2.5 2.9 — 3.1 1.3 1.3 2.0 — 3.6 5.4 4.9 4.5 4.5 1.3 1.9 1.6 3.4 2.3 1.5 1.4 2.4 1.4 3.1 1.6 3.6 2.8 1.9 2.9 4.1 2.1 1.5 2.2 2.7 1.8 — 2.1
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absolute emission probability of the 609.3-keV gamma ray; the adopted value of 0.4557 is the weighted average of the results obtained for each of the six gamma rays, and has been allocated a relative uncertainty of 0.27%. This value was obtained using special point sources certified to within 70.2%, and is considerably different (i.e., 1.5%) from the 0.4483 standard of our previous work (Delgado et al., 2002). However, the latter is a calculated value obtained in an arbitrary manner by combining three published values, and justifies the current work. The value of 0.4557 was used to deduce the absolute photon emission probabilities for all the X- and gamma rays emitted by 226Ra in equilibrium with daughters, as listed in Table 3 and compared with other measurements . (Schotzig and Debertin, 1983; Lin and Harbottle, 1991) and our previous values (Morel et al., 1998). With the exception of a few g-ray emissions, the results are in agreement with our previous studies; although those earlier data were the result of an extremely different activity measurement with 222Rn gas samples. Furthermore, the most intense gamma rays have a far lower uncertainty of between 0.4% and 0.6%.
4. Conclusions 226
Ra and daughters are present in environmental and building materials, and precise nuclear data are required to improve the evaluation of the risk to human health due to exposure to this natural radiation. Complementary gamma-ray spectrometry measurements have been made using 226Ra point sources prepared specially and certified by VNIMM in order to supplement our earlier work; these measurements produce absolute X- and gray photon emission probabilities for 226Ra in equilibrium with daughters. The resulting data were assigned an uncertainty from 0.4% to 0.6% for the most intense emissions, which should improve considerably the quality of certain environmental measurements. Another advantage could be the possibility of developing 226 Ra as a multi-gamma reference source, not only for conventional liquid and solid sample measurement systems, but also for gaseous discharge monitoring facilities.
Acknowledgements The authors wish to express their sincere gratitude for the support of the Minist"ere des Affaires Etrang"eres, France, which helped strengthen the collaboration
between VNIIM and the LNHB, improved our knowledge of the nuclear data, and developed the corresponding techniques.
References B!e, M.M., Duchemin, B., Morillon, C., Browne, E., Chechev, . V., Egorov, A., Helmer, R., Schonfeld, E., 1998. Nucl!eide: Nuclear and Atomic Decay Data, version 1-98, BNM-CEALPRI. Saclay, France. Dean, J.C.J., Burke, M., 1994. An intercomparison of 222Rn measurement systems in European laboratories. Nucl. Instrum. Methods Phys. Res. A 339, 264. Delgado, J.U., Morel, J., Etcheverry, M., 2002. Measurements of photon emission probabilities from the decay of 226Ra and daughters. Appl. Radiat. Isot. 56, 137. Coursol, N., Lagoutine, F., Duchemin, B., 1990. Evaluation of non-neutron nuclear data for uranium-238 decay chain. Nucl. Instrum. Meth. Phys. Res. A 286, 589. Kharitonov, I.A., Rasko, M.A., Sepman, S.V., Terechtchenko, E.E., Hejdelman, A.M., 2002. A source for measurement of the absolute intensities of 226Ra gamma-radiation in equilibrium with decay products. Appl. Radiat. Isot. 56, 37. Lin, W., Harbottle, G., 1991. Gamma ray emission intensities of 226Ra in equilibrium with its daughters. J. Radioanal. Nucl. Chem. 153 (2), 137. Molnar, G.L., R!evay, Z., Belgya, T., 2002. New intensities for high energy gamma-ray standards. Proceedings of the 11th International Symposium on Capture Gamma-ray Spectroscopy and Related Topics, Pruhonice near Prague, Czech Republic, September 2002. World Scientific, Singapore, to be published. Morel, J., Etcheverry, M., Plagnard, J., 1996. Emission probabilities of KX and g-rays following 166Hom decay. Appl. Radiat. Isot. 47 (5–6), 529. Morel, J., Etcheverry, M., Picolo, J.L., 1998. Emission probabilities of the main g-rays following the decay of 222 Rn and daughters. Appl. Radiat. Isot. 49 (9–11), 1387. Picolo, J.L., 1995. Etude et r!ealisation d’un dispositif cryog!enique permettent d’effectuer des mesures absolues d’activit!e du radon 222 en vue de l’!elaboration d’un e! talon primaire. Rapport CEA-R-5696, Saclay, France. Plagnard, J., Etcheverry, M., Morel, J., 1993. Etalonnage des spectrom"etres avec le baryum 133, l’europium 152 et l’am!ericium 241, r!eactualisation des donn!ees caract!erisant l’!emission X et gamma. Journ!ees de Spectrom!etrie gamma et X 93, Note CEA-N-2756, ISSN 0429-3460, 205. . Schotzig, U., Debertin, K., 1983. Photon emission probabilities per decay of 226Ra and 232Th in equilibrium with their daughter products. Int. J. Appl. Radiat. Isot. 14, 533. Terechtchenko, E., Rasko, M., Sepman, S., Zanevsky, A., Tran Tuan, A., Amiot, M.N., Bobin, C., Morel, J., 2004. Study of XK and g photon emission following decay of 154Eu. Appl. Radiat. Isot., accepted for publication in this issue.
Applied Radiation and Isotopes 60 (2004) 341–346
Precise determination of photon emission probabilities for the main X- and g-rays of 226Ra in equilibrium with daughters J. Morela,*, S. Sepmanb, M. Raskob, E. Terechtchenkob, J.U. Delgadoc a CEA-DIMRI-BNM-LNHB, B.P. 52, F-91193 Gif-sur-Yvette Cedex, France D.I. Mendeleyev Institute for Metrology (VNIIM), 19 Moskowsky Prospect, St. Petersburg 198005, Russia c LNMRI, IRD, Commissao de Energia Nuclear, Av. Salvador Allende, S/N–Recreio, 22780 160 Rio de Janeiro CEP, Brazil b
Abstract Within the context of a joint project between VNIIM (D.I. Mendeleyev Institute for Metrology) and LNHB (Laboratoire National Henri Becquerel), special 226Ra sources were prepared by VNIIM in order to determine as accurately as possible the absolute photon emission probabilities for the main X- and g-rays following the decay of 226 Ra and daughters. The main purpose of this work was to supplement a previous joint study by Laboratorio Nacional de Metrologia das Radia@oes Ionizantes (LNMRI) and LNHB to determine their relative values. Some specific point sources were produced for a-spectrometry measurements that were undertaken at VNIIM and also for g-ray spectrometry studies at VNIIM and LNHB. The 226Ra activity for the g-spectrometric sources was measured relative to the a-spectrometric sources by comparing the counts of the main g-rays. The total uncertainty of the activity for these sources was 0.2% (k ¼ 1). Using calibrated germanium detectors, several X- and g-ray spectra were analyzed to determine the absolute photon emission probabilities of 226Ra in radioactive equilibrium with daughters. The results are presented and compared to other published values. r 2003 Elsevier Ltd. All rights reserved. Keywords: g-Ray spectrometry; Alpha counting;
226
Ra and daughters; Photon emission probabilities
1. Introduction Over recent years, the methods used to measure the activity of 222Rn have been arousing increasing interest that can be explained by the impact of this radionuclide on environmental measurements. Two comparison campaigns were organized by the National Physical Laboratory (NPL) as part of the European Collaboration in Metrology (Euromet) programme, the first in 1992 and the second in 1994 (Dean and Burke, 1994). The most striking observation of the second campaign was a bias of a few percent between what were said to be the absolute measurements and those made using gamma-ray spectrometry. Additional experiments were *Corresponding author. Tel.: +33-1-69-08-4178; fax: +33-169-08-9529. E-mail addresses: [email protected], [email protected] (J. Morel).
carried out to determine the main cause of this bias, and it was discovered that radioactive material tended to settle on the two tubes and valves of the vials containing 222 Rn gas. An original method for measuring activity had been developed in parallel to these studies (Picolo, 1995), based on the condensation of 222Rn on a cold surface and counting the alpha particles emitted by the deposit in a pre-defined geometry. Taking advantage of the experiments carried out previously, a set of nuclear data measurements characterizing the gamma-ray emissions of 222Rn in equilibrium with its daughters was made (Morel et al., 1998) using two spherical vials with no valves and two others with only one. The results obtained for the major rays had a relative uncertainty of between 1.3% and 1.5%; these uncertainties were considered too high, with 0.9% affecting the activity measurement and between 0.5% and 1.0% affecting calibration precision. Thus, to lower the uncertainty and improve the quality of the gamma-ray spectrometry
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measurements, a similar task was carried out using 222 Rn point sources or sources considered as such. The relative intensities of the X- and g-rays of 226Ra and its daughters had an uncertainty of between 0.3% and 0.5% for the major emissions in the studies carried in collaboration with LNMRI (Delgado et al., 2002). However, for these data to be expressed as absolute values, an emission probability of 44.8370.27 had to be adopted for the 609-keV gamma ray, deduced by combining three published values, two obtained from . measurements: 44.670.5 (Schotzig and Debertin, 1983) and 44.870.6 (Morel et al., 1998) and the other from an evaluation: 45.070.4 (Coursol et al., 1990). This arbitrary assignment of the absolute emission probability was not satisfactory; therefore, in the context of an agreement between our laboratory and the VNIIM, the latter used their expertise in source preparation and made available two 226Ra point sources, characterized by a lack of radon emanation and with a certified activity of 70.2% (at 1s confidence level). Thus, by referring to the work carried out earlier and using these sources, the absolute emission probabilities of X- and g-ray emissions could be determined in a non-arbitrary manner; the results are described here.
2. Sample preparation 2.1. Description Two 40 kBq sources for gamma-ray spectrometry and another two 13 kBq sources for alpha-particle spectrometry were prepared by VNIIM from the same radium sample (Kharitonov et al., 2002), based on their experience and that of the Khlopin Radium Institute (St. Petersburg). A gamma-ray source comprised a disk of two sheets of stainless steel, each with a mass per unit surface area of 18 mg/cm2; the radioactive material was placed between these two sheets at the center of a circle with a diameter of less than 4 mm and the whole assembly was sealed with epoxy resin. The disk was then secured to an aluminum ring for ease of handling. The alpha sources comprised a deposit on a polished stainless steel disk, protected by 0.12-mm layer of titanium dioxide and 0.07-mm layer of aluminum. The slight emanation of radon was monitored by placing the sources in a 210 l stainless steel container and observing the formation of radon using an alpha monitor; the emanation coefficient did not exceed 0.1% for the gamma sources. 2.2. Certification The gamma sources were certified at VNIIM in two stages. First the activity of the alpha sources had to be determined using two independent methods, and secondly the activity of the gamma sources had to be
determined by comparison with the alpha sources. The comparison was made by gamma-ray spectrometry using a Ge–Li detector and monitoring the 186.2, 242.0, 295.2, 351.9, 609.3, 1120.3 and 1764.5 keV peaks. All sources were placed 10 and 15 cm from the detector. The activity of the alpha sources was first determined by means of the defined solid-angle method, using a ZnS scintillation detector. Thereafter, the measurement was checked using an alpha spectrometer (with 20 keV resolution) to compare the emission of alpha particles from 226Ra to the emission from an 241Am calibration source. On the basis of these two methods, an activity uncertainty of less than 0.2% was observed at the 1s confidence level. Making allowance for the uncertainties associated with comparison, the total uncertainty associated with the gamma source activity was finally set at 0.2%.
3. Photon emission probabilities 3.1. Relative emission probabilities Since the results of the work carried out previously were considered satisfactory (Morel et al., 1998; Delgado et al., 2002), there was no point in redetermining the relative X- and g-ray emission probabilities. Consequently, the results were used as a reference. The photon emission probabilities corresponding to these publications are listed in Table 1 relative to the 609.3-keV gamma ray of 100. The corresponding uncertainties include no allowance for the uncertainty affecting the activity. From these results we adopted a final value equal to the weighted mean of each of the values in the two series of measurements, and a relative uncertainty equal to the maximum value of the difference between the internal relative uncertainty and the relative standard deviation for the weighted mean. The results obtained are shown in the ‘‘Current work’’ column, and are compared with the recent values obtained from a study of high-energy calibration gamma rays (Molnar et al., 2002). These two sets of data are in very good agreement; in particular, the relative uncertainties of several intense gamma rays are 0.3%, which is highly satisfactory. 3.2. Determination of absolute emission probabilities The relative photon emission probabilities were converted into absolute values by means of several operations that involved the counting of a few intense peaks characteristic of 226Ra daughters and known to high accuracy. The energies of these gamma rays were 186.2, 295.2, 351.9, 609.3, 1120.3 and 1764.5 keV. The photon emission probabilities following the decay of 226Ra were measured at VNIIM by using three
ARTICLE IN PRESS J. Morel et al. / Applied Radiation and Isotopes 60 (2004) 341–346 Table 1 X- and gamma-ray emission probabilities for gamma ray) Energy (keV)
53.2 74.8 Bi XKa2 76.7 Po XKa2 77.1 Bi XKa1 79.3 Po XKa1 81.1 Rn XKa2 83.8 Rn XKa1 87.2 Bi XKb3,1,5 89.6 Po XKb3,1,5 89.9 Bi XKb2,4,O 92.4 Po XKb2,4,O 94.7 Rn XKb3,1,5 97.7 Rn XKb2,4,O 186.2 242.0 258.8 273.7+274.7 273.7 274.7 295.2 351.9 387.0+389.0 387.0 389.0 455.0 480.5 487.1 580.3 609.3 665.4 768.4 785.8+785.9 806.2 934.1 1120.3 1155.2 1238.1 1281.0 1377.7 1385.3 1401.5 1408.0 1509.2 1661.3 1729.6 1764.5 1847.4 2118.5 2204.1 2293.4 2447.7
Morel et al. (1998)
343
226
Ra and daughters expressed as relative values (normalized to 100 for the 609.3-keV Delgado et al. (2002)
Current work
Molnar et al. (2002)
Value
Uncertainty (%) Value
Uncertainty (%) Value
Uncertainty (%) Value
Uncertainty (%)
— — — — — — — — — — — — — — 15.74 1.170 1.025 0.292 0.732 39.96 77.46 1.511 0.612 0.900 0.656 0.752 0.962 0.837 100.0 3.415 10.83 2.395 2.813 6.815 33.06 3.612 12.97 3.194 8.91 1.817 2.964 5.384 4.750 2.375 6.275 33.93 4.513 2.549 10.89 — 3.455
— — — — — — — — — — — — — — 0.9 2.3 2.8 9.0 3.0 0.9 0.9 1.8 4.9 3.5 5.3 4.8 4.4 4.4 0.9 1.7 1.3 3.3 2.1 1.2 1.1 2.2 1.1 3.0 1.3 3.5 2.7 1.7 2.8 4.0 1.9 1.2 2.0 2.5 1.6 — 1.9
1.0 0.7 5.2 0.7 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.8 20.0 0.4 0.3 0.8 1.0 8.5 2.9 0.3 0.3 1.4 1.8 1.6 1.9 1.3 1.3 1.3 0.3 0.5 0.5 0.9 0.8 0.5 0.3 1.0 0.5 0.9 0.5 1.0 0.7 0.5 1.4 1.5 0.5 0.3 0.8 0.8 0.5 2.5 0.7
1.0 0.7 5.2 0.7 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.8 20.0 0.4 0.3 0.8 0.9 6.2 3.6 0.3 0.3 1.1 1.7 1.5 1.8 1.3 1.2 1.2 0.3 0.5 0.5 0.9 0.7 0.5 0.3 0.9 0.4 0.9 0.5 1.1 0.7 0.8 1.2 1.5 0.5 0.3 0.7 0.8 0.5 2.5 0.7
0.8 — — — — — — — — — — — — 0.6 0.4 — — — — 0.3 0.3 — — — — — — — 0.3 0.6 0.3 — 0.5 0.6 0.4 0.5 0.3 0.5 0.3 0.9 0.4 0.4 0.6 0.6 0.5 0.3 0.7 0.8 0.8 1.5 1.2
2.329 13.20 1.164 22.22 2.119 0.348 0.480 7.08 0.770 2.204 0.185 0.176 0.045 7.812 15.90 1.171 1.053 0.265 0.787 40.36 78.16 1.539 0.651 0.888 0.640 0.749 0.961 0.823 100.0 3.359 10.66 2.447 2.788 6.783 32.71 3.594 12.83 3.147 8.69 1.744 2.924 5.233 4.606 2.271 6.226 33.54 4.448 2.536 10.74 0.665 3.402
2.329 13.20 1.164 22.22 2.119 0.348 0.480 7.08 0.770 2.204 0.185 0.176 0.045 7.812 15.88 1.171 1.050 0.278 0.760 40.32 78.10 1.528 0.647 0.890 0.642 0.750 0.961 0.824 100.0 3.364 10.68 2.443 2.791 6.788 32.74 3.597 12.85 3.151 8.72 1.750 2.927 5.245 4.636 2.284 6.229 33.56 4.457 2.537 10.76 0.665 3.408
2.384 — — — — — — — — — — — — 7.85 15.98 — — — — 40.61 78.34 — — — — — — — 100.0 3.386 10.77 — 2.777 6.83 32.77 3.595 12.80 3.159 8.79 1.755 2.934 5.250 4.670 2.299 6.25 33.63 4.42 2.548 10.75 0.677 3.410
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0.6 0.4 0.4 0.4 0.4 0.5 0.27 0.4567 0.4568 0.4557 0.4538 0.4548 0.4573 0.4557 a
b
Correlated components are eliminated for this analysis. Activity uncertainty of 0.2% is not taken into account for this weighted mean evaluation, but is added at the end of the calculation.
0.45b 0.3b 0.3b 0.3b 0.3b 0.4b 0.5a 0.4a 0.4a 0.4a 0.4a 0.5a 3.568a 18.37 a 35.55 a 45.44 a 14.89 a 15.32 a 3.569 0.5 3.528 0.7 18.36 0.4 18.49 0.6 35.54 0.4 35.80 0.6 45.43 0.4 45.57 0.6 14.88 0.4 15.04 0.6 15.28 0.5 15.45 0.8 Adopted ratio (weighted mean and uncertainty) 0.5 0.4 0.4 0.4 0.4 0.5 3.586 18.38 35.65 45.44 14.93 15.26 — 0.6 0.6 0.6 0.6 0.7 — 18.55 35.79 45.22 14.89 15.41 186.2 295.2 351.9 609.3 1120.3 1764.5
Uncertainty (%) Value
VNIIM
3.568b 18.42b 35.62b 45.38b 14.89b 15.35b
Value Uncertainty (%) Uncertainty (%) Value Uncertainty (%) Value Uncertainty (%) Value Uncertainty (%)
Series B (12 cm) Series A (12 cm)
LNHB
Value
Series C (22 cm)
Synthesis A, B, C
Value
Adopted values from VNIIM & LNHB
Ra and daughters, and determination of relative-absolute conversion term
Ratio of absolute to relative values Energy
The results obtained from one VNIIM set of studies and for the series of three LNHB experiments are shown in Table 2 for the six major gamma rays. All these results
Photon emission probabilities
3.3. Results
226
semiconductor detectors: HPGe coaxial-n-type detector (relative efficiency: 30%, resolution at 122 and 1332 keV: 1.3 and 2.6 keV, respectively), Ge–Li coaxial-type detector (relative efficiency: 15%, with resolutions at 122 and 1332 keV of 1.7 and 2.9 keV, respectively); and HPGe coaxial n-type detector (volume: 40 cm3, with resolutions at 122 and 1332 keV of 2.2 and 2.9 keV, respectively). Each detector was calibrated on the basis of peak-efficiency for source-to-entrance distances of 10, 20 and 30 cm using the following reference point sources: 54Mn, 57Co, 60Co, 88Y, 133Ba, 134Cs, 137Cs, 139 Ce, 152Eu, 154Eu, 198Au and 241Am. The activity of these sources was measured by absolute methods with an uncertainty from 0.5% to 0.8% (K ¼ 1). Corrections for coincidence losses of the order 1.0, 0.5 and 0.2%, and dependence on the source to detector distance were taken into account. The fitting of the experimental efficiency data is described in another paper (Terechtchenko et al., 2004). A general uncertainty of the efficiency response in the 60 to 1900 keV energy range is estimated to range from 0.5% to 1.0%. Concerning the measurements at LNHB, both the sources prepared and certified by VNIIM were submitted to three counting campaigns, the first in September 2001, the second and the third in February 2002; on average, each counting campaign lasted 30,000 s. Source–detector distance was 12 cm in the first two studies, and 22 cm in the third. The coaxial n-type HPGe detector (volume of 100 cm3, resolution of 0.66 keV at 122 keV and 1.80 keV at 1332 keV) was the same as that described in previous publications (Morel et al., 1998; Delgado et al., 2002); however, recent treatment of the crystal surface by the manufacturer necessitated a recalibration using the following calibration sources: 22Na, 56Co, 57Co, 60Co, 65Zn, 110Agm, 133 Ba, 137Cs, 152Eu, 166Hom and 241Am. The nuclear data for some of the multi-gamma emitters were taken from the most recent work of Plagnard et al. (1993) for 133Ba, 152 Eu and 241Am, and Morel et al. (1996) for 166Hom. Nuclear data for all of the other radionuclides were taken from the ‘‘Nuclide’’ file (B!e et al., 1998), with the exception of 56Co, for which suitable data were in the personal possession of the authors. Some aspects of the counting operations were corrected slightly, including decay, count losses due to pile-up effects (even though slight), and summing effects. The latter were more significant for the 609.3 and 1120.3 keV g-rays than for the four other emissions, i.e., 0.7 and 1.0% at a distance of 12 cm, and 0.2% and 0.3% at a distance of 22 cm, respectively.
Uncertainty (%)
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Table 2 Absolute photon emission probabilities for the most intense g-rays emitted by
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are given with a relative uncertainty estimated at the 1s confidence level, and the adopted value corresponds to the weighted mean (only non-correlated components Table 3 Absolute X- and gamma-ray emission probabilities for Energy (keV)
53.2 74.8 Bi XKa2 76.7 Po XKa2 77.1 Bi XKa1 79.3 Po XKa1 81.1 Rn XKa2 83.8 Rn XKa1 87.2 Bi XKb3,1,5 89.6 Po XKb3,1,5 89.9 Bi XKb2,4,O 92.4 Po XKb2,4,O 94.7 Rn XKb3,1,5 97.7 Rn XKb2,4,O 186.2 242.0 258.8 273.7+274.7 273.7 274.7 295.2 351.9 387.0+389.0 387.0 389.0 455.0 480.5 487.1 580.3 609.3 665.4 768.4 785.8+785.9 806.2 934.1 1120.3 1155.2 1238.1 1281.0 1377.7 1385.3 1401.5 1408.0 1509.2 1661.3 1729.6 1764.5 1847.4 2118.5 2204.1 2293.4 2447.7
Present work
345
were used to calculate the weighting). The last column shows the factor used to convert from relative to absolute emission probabilities, corresponding to the
226
Ra and daughters
. Schotzig and Debertin (1983)
Lin and Harbottle (1991)
Morel et al. (1998)
Value Uncertainty (%) Value
Uncertainty (%)
Value
Uncertainty (%)
Value Uncertainty (%)
1.061 6.02 0.531 10.13 0.966 0.159 0.219 3.227 0.351 1.004 0.084 0.080 0.020 3.560 7.24 0.534 0.478 0.127 0.347 18.37 35.59 0.697 0.295 0.406 0.293 0.342 0.438 0.376 45.57 1.533 4.87 1.113 1.272 3.093 14.92 1.639 5.86 1.436 3.972 0.797 1.334 2.390 2.113 1.041 2.839 15.29 2.031 1.156 4.90 0.303 1.553
4.5 10 10 10 10 11 11 10 — 10 — 11 11 1.7 1.5 — — — — 1.6 1.1 — — — — — — — 1.1 — 1.5 1.9 — 1.3 1.4 — 1.2 — — — — — 2.4 — — 2.0 — 2.6 2.4 3.0 2.6
— — — — — — — — — — — — — — 7.43 0.524 0.474 — — 19.3 37.6 — — 0.417 — 0.32 0.422 0.352 46.1 1.46 4.94 1.09 1.22 3.03 15.1 1.63 5.79 1.43 4.00 0.757 1.27 2.15 2.11 1.15 2.92 15.4 — — — — —
— — — — — — — — — — — — — — 1.5 2.1 2.3 — — 1.0 1.1 — — 6.2 — 1.2 3.8 4.0 1.1 2.0 1.2 1.8 1.6 1.3 1.3 1.2 1.4 1.4 1.5 2.4 1.6 2.8 1.9 2.6 1.4 1.3 — — — — —
— — — — — — — — — — — — — — 7.05 0.52 0.46 0.13 0.33 17.90 34.70 0.68 0.27 0.40 0.29 0.34 0.43 0.38 44.84 1.53 4.85 1.07 1.26 3.05 14.81 1.62 5.81 1.43 3.99 0.81 1.33 2.41 2.13 1.06 2.81 15.20 2.02 1.14 4.88 — 1.55
1.0 0.8 5.2 0.8 6.8 25.1 14.8 2.5 15.4 5.3 5.4 6.7 20.0 0.6 0.4 0.8 1.0 6.2 3.6 0.4 0.4 1.1 1.7 1.5 1.8 1.3 1.3 1.3 0.4 0.6 0.5 0.9 0.8 0.5 0.4 1.0 0.5 0.9 0.5 1.0 0.7 0.6 1.3 1.5 0.6 0.4 0.8 0.8 0.6 2.5 0.7
1.11 6.1 0.66 10.2 1.11 0.158 0.26 3.82 — 1.11 — 0.111 0.034 3.51 7.12 — — — — 18.2 35.1 — — — — — — — 44.6 — 4.76 1.04 — 3.07 14.7 — 5.78 — — — — — 2.08 — — 15.1 — 1.17 4.98 0.301 1.55
— — — — — — — — — — — — — — 1.3 2.5 2.9 — 3.1 1.3 1.3 2.0 — 3.6 5.4 4.9 4.5 4.5 1.3 1.9 1.6 3.4 2.3 1.5 1.4 2.4 1.4 3.1 1.6 3.6 2.8 1.9 2.9 4.1 2.1 1.5 2.2 2.7 1.8 — 2.1
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absolute emission probability of the 609.3-keV gamma ray; the adopted value of 0.4557 is the weighted average of the results obtained for each of the six gamma rays, and has been allocated a relative uncertainty of 0.27%. This value was obtained using special point sources certified to within 70.2%, and is considerably different (i.e., 1.5%) from the 0.4483 standard of our previous work (Delgado et al., 2002). However, the latter is a calculated value obtained in an arbitrary manner by combining three published values, and justifies the current work. The value of 0.4557 was used to deduce the absolute photon emission probabilities for all the X- and gamma rays emitted by 226Ra in equilibrium with daughters, as listed in Table 3 and compared with other measurements . (Schotzig and Debertin, 1983; Lin and Harbottle, 1991) and our previous values (Morel et al., 1998). With the exception of a few g-ray emissions, the results are in agreement with our previous studies; although those earlier data were the result of an extremely different activity measurement with 222Rn gas samples. Furthermore, the most intense gamma rays have a far lower uncertainty of between 0.4% and 0.6%.
4. Conclusions 226
Ra and daughters are present in environmental and building materials, and precise nuclear data are required to improve the evaluation of the risk to human health due to exposure to this natural radiation. Complementary gamma-ray spectrometry measurements have been made using 226Ra point sources prepared specially and certified by VNIMM in order to supplement our earlier work; these measurements produce absolute X- and gray photon emission probabilities for 226Ra in equilibrium with daughters. The resulting data were assigned an uncertainty from 0.4% to 0.6% for the most intense emissions, which should improve considerably the quality of certain environmental measurements. Another advantage could be the possibility of developing 226 Ra as a multi-gamma reference source, not only for conventional liquid and solid sample measurement systems, but also for gaseous discharge monitoring facilities.
Acknowledgements The authors wish to express their sincere gratitude for the support of the Minist"ere des Affaires Etrang"eres, France, which helped strengthen the collaboration
between VNIIM and the LNHB, improved our knowledge of the nuclear data, and developed the corresponding techniques.
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