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Decay scheme study of 126Sn and 126Sb
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 1571–1577
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Decay scheme study of
126
Sn and
126
Sb
L. Ferreux a,, M.-C. Le´py a, M.-M. Be´ a, P. Cassette a, P. Bienvenu b, G. Andreoletti c a b c
CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), F-91191 Gif-sur-Yvette, France CEA Cadarache DEN/DEC/SA3C/LARC, 13115 St Paul Lez Durance, France AREVA Cogema SL/UP2-800, 50444 Beaumont Hague, France
a r t i c l e in fo
Keyword: 126 Sn 126 Sb Photon emission intensities Kb/Ka intensity ratio Gamma spectrometry
abstract To study the decay scheme of 126Sn, two samples of a purified solution were measured by gamma-ray spectrometry and the relative photon emission intensities were determined. The 126Sbm isomeric branching ratio was derived to be 18.6 (6) %. The maximum beta energy of the 126Sn decay was checked by liquid scintillation. The Kb/Ka intensity ratio of Sb was determined being 0.226 (11). These new experimental results were used to re-examine the whole decay scheme of 126Sn and its daughters. & 2010 Published by Elsevier Ltd.
1. Introduction Sn-126 is a long-lived radionuclide which decays through beta transitions. Its half-life is in the order of 2 105 years with associated uncertainty about 10%. There are two short-lived daughter products (126Sbm (19.15 (8) min) and 126Sb (12.4 (1) days), and Sn–Sb equilibrium is assumed to be reached 75 days after separation. The decay scheme of the tin/antimony-126 ensemble is poorly understood. The decay scheme obtained from two references (Katakura and Kitao, 2002; Be´ et al., 2004) is given in Fig. 1. It is characterized by the presence of a metastable excited level (126Sbm) with a half-life of 19.15 min which decays, for a part, by beta minus transitions directly to 126Te excited levels and, for another part, to 126Sb ground state (12.4 days) which in turn disintegrates via beta minus decay to excited levels of 126Te. This scheme shows that the ground state of 126Te is finally reached by a single gamma transition. Therefore, measurements of the associated photon emission intensities and the knowledge of the internal conversion coefficients enable to deduce the activity of 126Sn. The initial aim of this work was to determine the half-life of 126 Sn by the dual measurements of mass and activity. This was the objective of a cooperative investigation with AREVA NC and CEA LARC who ensured purification of the solution and mass measurement, respectively. LNHB carried out the activity measurement using gamma spectrometry on the 666.1 and 695.0 keV spectral lines common to the two daughter products (126Sb and 126 Sbm). This resulted in a half-life value of 1.98(6) 105 a (Bienvenu et al., 2009). In a second step, the relative photon Corresponding author. Tel.: + 33 1 69 08 56 08; fax: +33 1 69 08 26 19.
E-mail address: [email protected] (L. Ferreux). 0969-8043/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.11.055
emission intensities were measured and these new experimental data were used to re-examine the whole decay scheme of 126Sn and its daughters.
2. Experimental protocol Photon emission intensities were measured by gamma spectrometry. On account of the low 126Sn specific activity, the measurements were made directly on purified solutions in suitable containers. Since the detectors were calibrated for point sources, for exploiting the measurements it was necessary to calculate a transfer efficiency factor to get from the calibration geometry to the geometrical conditions of the measurement. 2.1. Sources Two samples (A and B) were provided, along with an inert matrix corresponding to the nitric medium used. They were repackaged in small cylindrical sealed Plexiglas containers of 38 mm in external diameter, with internal dimensions 14 mm diameter and 10 mm height, containing solution with masses of 1.7446(6) and 1.7618(6) g, respectively. The inert matrix was set up in the same way as samples to record a background in the same configuration as the sample measurements. 2.2. Detector Samples were measured using a high-purity germanium (HPGe) detector at 10.35 cm from its beryllium window. The germanium crystal has a diameter of 48 mm and is 52.7 mm thick; it is equipped with 500 mm beryllium window. The pulses were processed using conventional analogue electronics and the
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Fig. 1.
126
Sn decay scheme (Katakura and Kitao, 2002; Be´ et al., 2004).
resulting full widths at half maximum at 122 and 1332 keV were 0.77 and 1.73 keV, respectively. The efficiency calibration of the detector was carried out using point sources corresponding to various radionuclides whose activity was measured using primary methods, with relative standard uncertainties ranging between 0.2 and 0.5%. This enabled experimental calibration points (energy-efficiency pair) to be obtained with relative standard uncertainties generally between 0.4 and 2% over the energy range from 50 keV to 2 MeV. The overall calibration curve R(E) was obtained by fitting a polynomial function to the experimental points on a logarithmic scale using the EFFIGIE software (Morel and Valle´e, 1996). The final relative standard uncertainty in the calculated efficiency was about 0.5%.
2.3. Activity measurement Each container was measured over a total acquisition time of 650 000 s. Fig. 2 shows the spectrum obtained for solution A for region 200–1800 keV that is characterised by a significant number of peaks. The activity was determined from the two peaks at 666.1 and 695.0 keV. The peak corresponding to an energy of 666.1 keV is nicely isolated, and its area is calculated by the MAESTROs software (ORTEC, 2008). On the other hand, photon emission at 695.0 keV suffers from interference with the emission at 697.0 keV. For this doublet, the COLEGRAM software (Ruellan et al., 1996) was used to separate the contributions from the two components as shown in Fig. 2.
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1573
126Sn_region 200-1800 keV
Counts per channel
Counts per channel
100000
10000
10000 6000 1000
1000
690
695 Energy (keV)
700
100
10 200
700
1200
1700
Energy (keV) Fig. 2. Spectrum of
Table 1 Results of
126
126
Sn in the 200–1800 keV region and deconvolution of the 695–697 keV doublet.
Sb activity measurement for samples A and B.
Energy (keV)
N(E)
u(N(E))
u(N(E)) (%)
e(E)
u(e(E)) (%)
I(E) (%)
u(I) (%)
CR
u(CR) (%)
666.1 695.0
204611 197826
486 495
0.24 0.25
1.87 10 3 1.80 10 3
0.30 0.30
99.623 99.661
0.006 0.005
0.90 0.90
666.1 695.0
191903 184254
463 474
0.24 0.26
1.87 10 3 1.80 10 3
0.30 0.30
99.623 99.661
0.006 0.005
0.90 0.90
1.14 1.03 0.33 1.13 1.03 0.33 Mean activity for sample A 1.14 1.03 0.33 1.13 1.03 0.33 Mean activity for sample B
Cc
u(Cc) (%)
A (Bq)
uc(A)
uc(A) (%)
193.69 194.73 194.21 181.66 181.37 181.51
2.16 2.16 2.72 2.03 2.01 2.54
1.11 1.11 1.4 1.12 1.11 1.4
Columns 2 to 4 show the net count of peaks and corresponding absolute and relative uncertainties. Columns 5 and 6 contain the efficiency values of the detector for a point source and associated relative uncertainty. Photon emission intensities and their relative uncertainties appear in columns 7 and 8. Columns 9 and 10 on the one hand, and columns 11 and 12 on the other, contain the correction factors and associated relative uncertainties for efficiency and coincidence correction respectively. Activities calculated for each peak are given in column 13, with columns 14 and 15 showing the associated absolute and relative uncertainties, respectively.
The 126Sn activity in the measured solution, provided by the full-energy peak (FEP) with energy E is calculated as: A¼
NðEÞ C ðEÞ CP CTM CC ðEÞ eðEÞ IðEÞ t R
ð1Þ
A (Bq) is the 126Sn activity in the solution at the reference date, N(E) is the number of pulses recorded in the FEP with energy E, e(E) is the FEP efficiency of the detector used for the same energy, I(E) is the corresponding emission intensity, t (s) is the acquisition time (live time). The various correction factors are:
CR(E): geometrical transfer correction factor for efficiency. This
depends on energy and is calculated using the ETNA software (Piton et al., 2000). For the two neighbouring energies being considered, the correction factor is 0.90. CP: decay correction to reconcile the pulse count with the reference date: Dt ð2Þ CP ¼ exp -lnð2Þ T
Dt is the time between the reference date and the measurement starting, and T is the half-life of 126Sn. CTM: decay correction during measurement, tR T ð3Þ CTM ¼ 1-expð-lnð2Þ Þ T lnð2Þ tR tR is the total measurement time (real time)
Taking account of the half-life of 126Sn, these last two corrections are negligible. Coincidence corrections, CC(E), for the different peaks were calculated using ETNA (Piton et al., 2000). For the present measurement conditions, they are of the order of 3%.
The activity was measured for each of samples A and B using the two peaks at 666.1 and 695.0 keV. Since no b transition is expected to the first excited level and the ground state level of 126 Te, the absolute emission intensity I(E) is deduced from the decay scheme by the relation: IðEÞ ð1 þTÞ ¼ 100
ð4Þ
where aT is the internal conversion coefficient (ICC) for the gamma transition being considered. The ICCs were interpolated from the tables of Band et al., 2002, with the BrIcc program (Kibe´di et al., 2008) in the ‘‘Frozen Orbital’’ approximation that give a666 = 0.00378(6) and a695 =0.00340(5), respectively. The absolute emission intensities (I(E) 100) of the two gamma rays used here are thus 99.623(6) and 99.6612(50), respectively. The raw data and parameters used with associated uncertainties and final results are presented in Table 1. Relative combined standard uncertainties are calculated according to the variance propagation law. The relative standard uncertainty associated with the counting process is the same for the two peaks, and is in the order of 0.25%. In particular case, the factor dominating the uncertainty is related to transfer efficiency, and it is
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approximately 1.1%. The coincidence corrections have a relative standard uncertainty of 0.3%. The 126Sn activity in each sample was obtained from the mean of the activities calculated for each of the peaks; the relative uncertainty corresponds to the relative uncertainty of the individual values. Finally, the mass activity for each sample is obtained by dividing the measured activity by the mass of the corresponding solution. Hence, mass activity for samples A and B are 111.3(1.2) Bq g 1 and 103.0 (1.1) Bq g 1, respectively. These results were used for the determination of the 126Sn half-life that is described in details by Bienvenu et al. (2009).
3. Improving understanding of the
126
Sn decay scheme
Following on from measurement of the activity of 126Sn, the spectra were also processed to calculate relative photon emission intensities for this radionuclide in an attempt to improve understanding of its decay scheme, and in particular, the branching ratio to 126Te and 126Sb from the metastable level of 19.15 min. The reference values of 126Sb gamma transitions characteristics are taken from the evaluation by Lagoutine presented in NUCLE´IDE (Be´ et al., 2004). This evaluation is based on publications from 1975 and before, as well as the Nuclear Data Sheets assessment (Katakura and Kitao, 2002). The total sum of beta transitions probabilities is 104.3% in the NDS evaluation while the same sum is 102% in the Lagoutine’s evaluation. This emphasises a need in improving the 126Sn/126Sb decay scheme. This determination of photon emission intensities was completed by controlling impurities and quantifying the 121Sbm activity. A specific liquid beta scintillation measurement also enabled the maximum energy of beta disintegrations to be determined. 3.1. Determination of
121
Sbm concentration
In gamma spectrometry, the presence of 121Sbm is indicated in particular by the 37 keV photon emission. This peak is clearly
present and its area enables to predict the activity contained in each of the samples. The activities determined for samples A and B are 14.7 and 13.6 kBq, respectively, to within roughly 10%. We may note that the ratio of impurities in solutions A and B is of the same order of magnitude. 3.2. Verification of maximum beta decay energy In the publication of Murty et al. (1970) a beta emission with an end-point energy of 2.5 MeV was reported. In order to verify the existence of such a transition which, if it exists, would populate the 1361 keV level, a liquid scintillation beta spectrometry was carried out. The spectral resolution of this type of measurement, plus various artefacts appearing at high energies, means that a usable beta spectrum cannot be obtained to deduce a Kurie line. However, quantitative indications as to the maximum beta emission can be derived. In this particular case, the information sought is the presence or otherwise of beta emission above 2 MeV. 3.2.1. Preparation of scintillation sources A scintillating source was prepared by mixing 270 mg of a 126 Sn solution with 10 ml of Ultima Gold AB scintillator in a 20 ml glass vial. A blank was also prepared using an inactive tin solution of similar chemical composition to that of the active solution. To calibrate the energy scale of the spectrometer, we prepared two calibration sources with 89Sr and 90Sr-90Y using the inactive tin solution so that the chemical composition of these sources was as close as possible to that of the 126Sn solution. 3.2.2. Spectral analysis On the spectrum obtained with a Wallac 1414 spectrometer, a distribution related to the presence of 121Snm in the sample is mainly observed in the central portion, followed by a distribution resulting from the presence of 126Sn in the high energy portion. The maximum energy of the 121Snm spectrum is at 359 keV and
Table 2 Relative photon emission intensities obtained for each sample, mean value and standard uncertainty (normalization at 100 photons per 100 disintegrations for the 666.1 keV line). Sample A
Sample B
Energy(keV)
Relative emission intensities
Rel. uncert. (%)
Relative emission intensities
Rel. uncert. (%)
MEAN
uc (%)
666.1 695.0 222.45 278.5( +279.5 ?) 296.6 + 296.8 414.4 + 414.9 555.2 573.5 591.8( +593.4?) 620.5 + 620.9? 639.4 656.5 673.5( +674.8?) 697 720.5 856.4 928.12 953.3( +954.3?) 959.6 989.1 1 034.9(+ 1 035.4?) 1 062.9 1 212.3 1 477.3
100.00 100.58 0.28 0.17 0.95 92.80 0.38 1.24 1.63 1.95 0.21 0.39 0.77 4.58 10.10 3.06 1.52 0.19 0.08 1.33 1.76 0.50 0.39 0.54
1.22 1.22 3.31 3.76 2.60 1.29 3.61 2.27 2.16 2.06 4.54 3.43 2.72 1.06 1.51 1.81 2.24 4.45 7.09 2.63 2.33 4.37 3.80 3.16
100.00 99.97 0.23 0.14 0.98 92.25 4.34 0.33 1.60 1.88 0.22 0.17 0.89 5.03 10.32 3.11 1.66 0.18 Not detected 1.16 1.74 0.64 0.47 0.65
1.22 1.22 3.54 4.03 2.60 1.30 2.05 3.90 2.21 2.12 4.52 4.34 2.61 1.06 1.52 1.91 2.40 4.68
100.00 100.27 0.26 0.15 0.96 92.52 2.36 0.79 1.62 1.91 0.21 0.28 0.83 4.81 10.21 3.08 1.59 0.19 0.08 1.25 1.75 0.57 0.43 0.60
0.4 16 14 2.0 0.4 119 81.9 1.1 2.6 5 55 11 7 1.5 1.1 6 2.9 7.09 9 0.8 17 13 13
2.88 2.39 3.81 4.26 3.24
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provides a calibrating point in the spectrum which is completed by the locations corresponding to the maximum energies of 89Sr (1495 keV) and 90Y (2280 keV). The location of the channel corresponding to the maximum beta energy of 121Snm, 89Sr and 90Y is estimated graphically by extrapolating the high energy front, and corresponds to the 140, 600 and 975 channels, respectively. This leads to an equation for the linear calibration curve: E ¼ 35:79 þ 2:33 c
ð5Þ
where E is the energy in keV and c is the channel number. Using this calibration, the spectrum attributed to 126Sn comes to a halt at an energy of (1800 7100) keV. The uncertainty in this energy is mainly the result of calibration uncertainty, but also of the uncertainty in the location of the line corresponding to the 126 Sn maximum beta energy. Enlargement of the part of the energy spectrum greater than 2 MeV shows no electron emission in this region. In conclusion, the 126Sn beta spectrum reveals no emission of energy greater than 2 MeV, which enables to conclude that a 2.5 MeV maximum beta transition energy in the decay scheme of 126 Sn is absent. 3.3. Relative photon emission intensities in decay of equilibrium in the energy range more than 100 keV
126
Sn/126Sb in
Within this range of energies we find essentially those lines corresponding to photon emissions from the daughter products of 126 Sn, viz. 126Sb and 126Sbm. To provide relative values we make use of the 666.1 keV line as a reference, and its photon emission intensity is therefore normalized to 100. Relative intensities have been calculated as ratios (Table 2); the radionuclide activity is therefore not involved, and the stated relative uncertainty is taken as equal to the uncertainty associated with the corrected counting rates. Four of the lines reported in NUCLE´IDE (Be´ et al., 2004) were not detected: these correspond to energies 149.3, 208.6, 296.8, and Table 3 Energies and relative emission intensities of additional detected peaks. Energy (keV)
1063.9 keV. The presence of the 928.12 keV line from the 2703.6 keV level was confirmed with a relative intensity of 1.59 (9), this level is populated only from the 126Sbm decay. By contrast, several low intensity lines measured in the two spectra and not given in NUCLE´IDE (Be´ et al., 2004) are listed in Table 3; such peaks are of low intensity and the energies have an absolute uncertainty of around 1 keV. Lines at 1081, 1110 and 1362 keV correspond to the summed peaks. Lines at 684.7 and 934 keV might correspond to transitions from the 3450.3 keV level to levels of 2764.6 and 2514.3 keV, respectively. 3.4. Photon emission analysis in the energy range less than 100 keV 3.4.1. Absolute emission intensities in decay of 126Sn Photon emissions following the disintegration of 126Sn are in the energy range between 18 and 88 keV. This region is also characterised by intense Sb and Te KX-ray peaks which are strongly distorted to the left by scattering effects. Moreover, the region includes background peaks (X-rays from Pb, etc.) which are identified by comparison with a spectrum acquired using a container filled with a neutral matrix. For the doublet at 86.9 and 87.6 keV the spectrum was processed with COLEGRAM (Ruellan et al., 1996) to enable efficiency and coincidence corrections associated with each of the peaks to be applied. However, the deconvolution associated with these two peaks was quite simple. The absolute intensity values were obtained using the activity value of 194.2 (2.5) Bq. Details of their determination are given in Table 4 and the results are compared in Table 5 with the values of the NDS evaluation (Katakura and Kitao, 2002). These latter values are based only on one measurement result (Smith et al,. 1976). The results obtained in the current study are in accordance and with better accuracy. 3.4.2. Antimony XKb/XKa intensity ratio Within the spectra we observe mainly the Sb X-ray emissions: for these we find a Kb/Ka ratio of 0.238 and 0.228 using two different detectors. The uncertainty is of the order of 3 to 4% (in particular on account of the peak area which is somewhat fragile, Table 5 126 Sn absolute photon emission intensities, I(E), and associated uncertainties.
Relative emission intensities
684.7 726 730.7 934 1 081 1 110 1 191 1 290 1 362 1 589
1575
Sample A
Sample B
0.19 0.07 0.12 0.17 0.44 0.37 0.25 0.19 0.32 0.06
0.14 0.03 0.14 0.10 0.47 0.41 0.28 0.27 0.16 0.11
NDS value
Measured value
E (keV)
I(E) (%)
uc(I(E))
I(E) (%)
uc(I(E))
17.7 21.6 22.6 23.3 42.7 64.3 86.9 87.6
0.0000455 1.26 0.1 6.4 0.5 9.6 8.9 37
0.0000049 0.18 0.015 0.9 0.07 1.5 1.3 4
0.47 7.80 8.59 38.4
0.01 0.18 0.19 0.9
(3) (4) (4) (4) (4) (4) (4) (4)
Table 4 Details of determination of absolute photon emission intensities in the low-energy range (decay of
126
Sn).
Energy (keV)
N(E)
u(N(E)) (%)
e(E)
ue(E)) (%)
CR
u(CR) (%)
Cc
u(Cc) (%)
I(E) (%)
u(I(E)) (%)
42.7 64.3 86.9 87.6
4521 80023 88334 394296
1.49 0.35 0.34 0.16
9.413E–03 9.685E–03 9.594E–03 9.585E–03
1 1 1 1
0.8141 0.8392 0.8492 0.8494
1.9 1.6 1.5 1.5
1.004 1.001 1.001 0.999
0.04 0.01 0.01 0.01
0.47 7.80 8.59 38.36
2.9 2.3 2.3 2.2
Columns 2 and 3 show the net count of peaks and relevant relative uncertainties. Columns 4 and 5 contain the efficiency values of the detector for a point source and associated relative uncertainty. Columns 6 and 7 on the one hand, and columns 8 and 9 on the other, contain the correction factors and associated relative uncertainties for efficiency and coincidence correction respectively. Photon emission intensities and their relative standard uncertainties are given in the last two columns.
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Table 6 Comparison of measured and evaluated relative emission intensities of
149.3 208.6 223.3 278.6 296.6 296.8 414.4 414.9 555 573.8 593 620.1 639.7 656.3 666.1 675 695 697 720.3 856.7 928.2 954 959.6 989.3 1034.9 1061.6 1063.9 1213 1476.2
Relative emission intensities (%) NUCLEIDE
Relative uncertainty (%)
0.40 0.50 1.40 2.21 4.92 0.50 83.87 1.00 1.81 6.72 7.52 0.90 0.90 2.21 100.00 3.71 100.00 32.10 53.97 17.56
50.00 40.00 14.29 9.09 8.16 40.00 2.51 30.00 11.11 4.48 6.67 11.11 11.11 4.55 0.07 21.62 0.07 18.75 4.46 5.14
1.50 0.50 6.82 1.00 0.90
33.33 20.00 4.41 5.00 0.05 6.67 8.70 16.67
2.31 0.30
126
Sn.
Measured relative emission intensities (%) (Mean)
0.26 0.15 0.96
16 14 2.6
92.52
1.3
2.36 0.79 1.62 1.91 0.21 0.28 100.00 0.83 100.27 4.81 10.21 3.08 1.59 0.19 0.08 1.25 1.75 0.57
120 82.0 2.2 2.6 5 60
3.5. Discussion 3.5.1. 126Sb metastable level Following 126Sn disintegration, the first excited level of antimony 126 is metastable, with a half-life of 19.15 min. The branching ratio between the gamma transition to the ground state of 126Sb and the beta transition to the excited levels of 126Te is poorly understood. The results shown in Table 6 enable to comment certain assumptions on the overall scheme of the two nuclides in equilibrium. The data reported in Table 6 are as follows:
Columns 2 and 3 are the relative intensities and associated
11 1.2 7 1.6 2.0 6 4.5 8 9 1.8 17
0.43 0.60
and efficiency correction factors). Hence these results are mutually compatible and we obtain a final ratio of 0.233(5), which may be compared with the only measured value of 0.226 (11) (Close et al. 1973) and the theoretical value of 0.2266 from a Hartree–Fock relativistic calculation (Scofield, 1974).
Uncertainty (%)
uncertainties for 126Sb alone, i.e. for 100 126Sb disintegrations as deduced from Nucle´ide (Be´ et al, 2004); Columns 4 and 5 list the relative intensities and uncertainties for 126Sn and 126Sb in equilibrium measured in the present work; Columns 6 and 7 show the ratio between the experimental relative intensities and those tabulated for 126Sb alone. Looking at the decay scheme (Fig. 1), some gamma transitions (in bold in Table 6), depopulate levels in 126Te, in particular in the upper part of the scheme, which are only feed by beta transitions coming from the 126Sb ground state level. Therefore, from the relative intensities measured for Sn–Sb being at equilibrium and the assumption that the evaluated intensity values given in Nucle´ide, based on several measurements of
13 13
Measured-to-table ratio
Relative uncertainty (%) in ratio
0.19
21.4
0.18
8.6
0.12 0.22
82.1 7.0
0.23 0.13 1.00 0.22 1.00 0.15 0.19 0.18
12.2 60.2 0.1 24.3 1.2 20.0 4.7 5.5
0.13 0.16 0.18
33.6 21.5 10.0
0.19
15.6
Table 7 Gamma-ray emission intensities, I(E), and gamma-ray transition probabilities, P(g + ce), for gamma-ray transitions populated the 1361 keV level in 126Te. Energy (keV)
I(E) rel
uc
I(E) abs
856 1034 1476 414
3.08 1.75 0.6 92.52
0.062 3.07 0.032 1.74 0.078 0.60 1.295 92.17
uc
1 + at
uc
0.061 1.000832 0.000012 3.07 0.031 1.74 0.078 0.60 1.29 1.01408 0.00002 93.47 Total
126
P(g + ce) uc
98.9
0.06 0.031 0.078 1.31 1.3
Sb alone, are correct, then their ratio gives the proportion of Snm decaying by isomeric transition to 126Sb. This ratio varies from 0.12 to 0.23 and the simple mean is 0.18 with a relative standard deviation of 18%. The weighted mean is 0.186 with a relative standard deviation of 2.9%. The measured value of (18,6 (6)) % obtained here for the isomeric branching ratio of 126 Snm can be compared with a value of 14 (4) % deduced from the work of Orth et al. (1971). Moreover, the 928.2 keV gamma transition starts from the 2703.6 keV level provided by 126Sbm alone. Therefore, its absolute photon emission intensity is 1.59 (9) 0.99623 (6) =1.58 (9) %, and it is consistent with the NDS value (Katakura and Kitao, 2002): 1.3 (3)%, providing an improved uncertainty. Another check can be established at the 1361 keV level: since this level is fed only by gamma transitions, their sum should be 100%. These data are given in Table 7, where the sum of transitions gets up to (99 (1)) %; taking account of the associated uncertainties, this validates the probability balance at this level. 126
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5. Conclusion The 126Sn activity of about 200 Bq has been determined in two Sn/126Sb solutions with a relative standard uncertainty of 1.3% in order to derive the 126Sn half-life value. In addition: 126
126 Sn were measured with improved accuracy; the relative emission intensities of the gamma rays in decay of 126 Sn/126Sb equilibrium composition were derived and the 126 Sbm isomeric branching ratio was deduced being 18.6 (6) %; it was confirmed the presence of a gamma transition with energy 928.2 keV in decay of 126Sbm and the population of the 2703 keV level in 126Te; Kb/Ka intensity ratio of Sb X-rays has also been determined being 0.226 (11).
the absolute photon emission intensities in decay of
References Band, I.M., Trzhaskovskaya, M.B., Nestor Jr., C.W., Tikkanen, P.O., Raman, S., 2002. Dirac-fock internal conversion coefficients. Atomic Data and Nuclear Data Tables 81, 1–334. Be´, M.-M., Browne, E., Chechev, V., Chiste´, V., Dersch, R., Dulieu, C., Helmer, R.G., ¨ Kuzmenco, N., Nichols, A.L., Schonfeld, E., 2004. NUCLE´IDE. Table of Radionuclides on CD-ROM, Version 2-2004, CEA/BNM-LNHB, 91191 Gif-sur-Yvette, France. /http://www.nucleide.orgS. Bienvenu, P., Ferreux, L., Andreoletti, G., Arnal, N., Le´py, M.-C., Comte, J., Be´, M.-M., 2009. Determination of 126Sn half-life from ICP-MS and gamma spectrometry measurements. Radiochimica Acta, Accepted for publication in Radiochimica Acta. Close, D.A., Bearse, R.C., Malanify, J.J., Umbarger, C.J., 1973. Yield Ratios, K a/K b, L a/L b, L a/L g, and L a/L l, for X-rays Produced by Protons of 1.0 to 3.7 MeV. Physical Review A 8, 1873–1879. Katakura, J., Kitao, K., 2002. Nuclear Data Sheetsdata sheets for A= 126. Nuclear Data Sheets 97 (3-–4), 795–926. Kibe´di, T., Burrows, T.W., Trzhaskovskaya, M.B., Davidson, P.M., Nestor Jr., C.W., 2008. Evaluation of theoretical conversion coefficients using BrIcc, Nucl. Instr.Nucl. Instrum. and Meth.ods A 589, 202–229. Morel, J., Valle´e, M., 1996. De´termination de la re´ponse en efficacite´ des de´tecteurs gamma-X par lissage de points expe´rimentaux: code EFFIGIE. Private communication, LNHB. Murty, K.S.N., Pathak, B.P., Gujrathi, S.C., 1970. Decay of 126Sbm and the energy ¨ Physik A Hadrons and Nuclei 237, 313–319. levels of 126Te. Zeitschrift fur ORTEC, 2008. MAESTROs-32, Software Version 6, /http://www.ortec-online.com/ index.htmlS. Orth, C.J., Dropesky, B.J., Freeman, N.J., 1971. Decay of 126Sn and of the 19-min and 12.4-days isomers of 126Sb. Physical Review C 3 (6), 2402–2411.
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Piton, F., Le´py, M-C., Be´, M-M., Plagnard, J., 2000. Efficiency transfer and coincidence summing corrections for gamma-ray spectrometry. Applied Radiation and Isotopes 52, 791–795. Ruellan, H., Le´py, 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 to 200 keV energy range. Nucl. Instrum. Methods in Phys. Res. A 369, 651–656. Scofield, J.H., 1974. Exchange corrections of K X-ray emission rates. Exchange corrections of K X-ray emission rates, Physical Review A 9, 1041–1049. Smith Jr., H.A., Bunker, M.E., Starner, J.W., Orth, C.J., 1976. States in 126Sb populated in the beta decay of 105-yr 126Sn. Physical Review C 13 (1), 387–398.
Discussion: ˜ o): Please show again the slide describing the Q(E. Garcı´a-Toran measurement of the end point energy with scintillation counting? Is that a log scale? Then you need some form of energy calibration to obtain that, how was it done? A(P. Cassette): Yes, it is a log scale. It was calibrated from the end point energy of the nuclides present on the spectrum. The question was not in fact to have a precise determination of the end point but just to check if there is anything in the spectrum over 2 MeV and the answer is no. Comments(J. Hardy): I have a comment—not meant to be a negative to this particular measurement—about beta-decay endpoint energies measured with scintillation detectors. Throughout history these have frequently been wrong, and they have been systematically wrong. This occurred because people assumed, when they measured beta–gamma coincidences by gating on a particular gamma ray, they had isolated a single beta transition. Especially for nuclei far from stability, this is rarely the case. The gamma feeding from above can involve a large number of gamma rays, many of them unobservably weak, fed by other beta transitions which carry, in total, a significant intensity. This ‘‘Pandemonium effect’’ was first pointed out some years ago in papers by Gregers Hanson, Bjorn Jonson and myself; but many spectroscopists simply ignored it. So, in other words, they got results for beta-decay energies that were consistently too low. If you look in the Audi tables and watch as new Penning-trap mass measurements come in, most of the Audi values that are based on beta-decay measurements turn out to be wrong in the same direction. In short, do not rely too much on mass values based on beta-decay measurements.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Decay scheme study of
126
Sn and
126
Sb
L. Ferreux a,, M.-C. Le´py a, M.-M. Be´ a, P. Cassette a, P. Bienvenu b, G. Andreoletti c a b c
CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), F-91191 Gif-sur-Yvette, France CEA Cadarache DEN/DEC/SA3C/LARC, 13115 St Paul Lez Durance, France AREVA Cogema SL/UP2-800, 50444 Beaumont Hague, France
a r t i c l e in fo
Keyword: 126 Sn 126 Sb Photon emission intensities Kb/Ka intensity ratio Gamma spectrometry
abstract To study the decay scheme of 126Sn, two samples of a purified solution were measured by gamma-ray spectrometry and the relative photon emission intensities were determined. The 126Sbm isomeric branching ratio was derived to be 18.6 (6) %. The maximum beta energy of the 126Sn decay was checked by liquid scintillation. The Kb/Ka intensity ratio of Sb was determined being 0.226 (11). These new experimental results were used to re-examine the whole decay scheme of 126Sn and its daughters. & 2010 Published by Elsevier Ltd.
1. Introduction Sn-126 is a long-lived radionuclide which decays through beta transitions. Its half-life is in the order of 2 105 years with associated uncertainty about 10%. There are two short-lived daughter products (126Sbm (19.15 (8) min) and 126Sb (12.4 (1) days), and Sn–Sb equilibrium is assumed to be reached 75 days after separation. The decay scheme of the tin/antimony-126 ensemble is poorly understood. The decay scheme obtained from two references (Katakura and Kitao, 2002; Be´ et al., 2004) is given in Fig. 1. It is characterized by the presence of a metastable excited level (126Sbm) with a half-life of 19.15 min which decays, for a part, by beta minus transitions directly to 126Te excited levels and, for another part, to 126Sb ground state (12.4 days) which in turn disintegrates via beta minus decay to excited levels of 126Te. This scheme shows that the ground state of 126Te is finally reached by a single gamma transition. Therefore, measurements of the associated photon emission intensities and the knowledge of the internal conversion coefficients enable to deduce the activity of 126Sn. The initial aim of this work was to determine the half-life of 126 Sn by the dual measurements of mass and activity. This was the objective of a cooperative investigation with AREVA NC and CEA LARC who ensured purification of the solution and mass measurement, respectively. LNHB carried out the activity measurement using gamma spectrometry on the 666.1 and 695.0 keV spectral lines common to the two daughter products (126Sb and 126 Sbm). This resulted in a half-life value of 1.98(6) 105 a (Bienvenu et al., 2009). In a second step, the relative photon Corresponding author. Tel.: + 33 1 69 08 56 08; fax: +33 1 69 08 26 19.
E-mail address: [email protected] (L. Ferreux). 0969-8043/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.apradiso.2009.11.055
emission intensities were measured and these new experimental data were used to re-examine the whole decay scheme of 126Sn and its daughters.
2. Experimental protocol Photon emission intensities were measured by gamma spectrometry. On account of the low 126Sn specific activity, the measurements were made directly on purified solutions in suitable containers. Since the detectors were calibrated for point sources, for exploiting the measurements it was necessary to calculate a transfer efficiency factor to get from the calibration geometry to the geometrical conditions of the measurement. 2.1. Sources Two samples (A and B) were provided, along with an inert matrix corresponding to the nitric medium used. They were repackaged in small cylindrical sealed Plexiglas containers of 38 mm in external diameter, with internal dimensions 14 mm diameter and 10 mm height, containing solution with masses of 1.7446(6) and 1.7618(6) g, respectively. The inert matrix was set up in the same way as samples to record a background in the same configuration as the sample measurements. 2.2. Detector Samples were measured using a high-purity germanium (HPGe) detector at 10.35 cm from its beryllium window. The germanium crystal has a diameter of 48 mm and is 52.7 mm thick; it is equipped with 500 mm beryllium window. The pulses were processed using conventional analogue electronics and the
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Fig. 1.
126
Sn decay scheme (Katakura and Kitao, 2002; Be´ et al., 2004).
resulting full widths at half maximum at 122 and 1332 keV were 0.77 and 1.73 keV, respectively. The efficiency calibration of the detector was carried out using point sources corresponding to various radionuclides whose activity was measured using primary methods, with relative standard uncertainties ranging between 0.2 and 0.5%. This enabled experimental calibration points (energy-efficiency pair) to be obtained with relative standard uncertainties generally between 0.4 and 2% over the energy range from 50 keV to 2 MeV. The overall calibration curve R(E) was obtained by fitting a polynomial function to the experimental points on a logarithmic scale using the EFFIGIE software (Morel and Valle´e, 1996). The final relative standard uncertainty in the calculated efficiency was about 0.5%.
2.3. Activity measurement Each container was measured over a total acquisition time of 650 000 s. Fig. 2 shows the spectrum obtained for solution A for region 200–1800 keV that is characterised by a significant number of peaks. The activity was determined from the two peaks at 666.1 and 695.0 keV. The peak corresponding to an energy of 666.1 keV is nicely isolated, and its area is calculated by the MAESTROs software (ORTEC, 2008). On the other hand, photon emission at 695.0 keV suffers from interference with the emission at 697.0 keV. For this doublet, the COLEGRAM software (Ruellan et al., 1996) was used to separate the contributions from the two components as shown in Fig. 2.
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1573
126Sn_region 200-1800 keV
Counts per channel
Counts per channel
100000
10000
10000 6000 1000
1000
690
695 Energy (keV)
700
100
10 200
700
1200
1700
Energy (keV) Fig. 2. Spectrum of
Table 1 Results of
126
126
Sn in the 200–1800 keV region and deconvolution of the 695–697 keV doublet.
Sb activity measurement for samples A and B.
Energy (keV)
N(E)
u(N(E))
u(N(E)) (%)
e(E)
u(e(E)) (%)
I(E) (%)
u(I) (%)
CR
u(CR) (%)
666.1 695.0
204611 197826
486 495
0.24 0.25
1.87 10 3 1.80 10 3
0.30 0.30
99.623 99.661
0.006 0.005
0.90 0.90
666.1 695.0
191903 184254
463 474
0.24 0.26
1.87 10 3 1.80 10 3
0.30 0.30
99.623 99.661
0.006 0.005
0.90 0.90
1.14 1.03 0.33 1.13 1.03 0.33 Mean activity for sample A 1.14 1.03 0.33 1.13 1.03 0.33 Mean activity for sample B
Cc
u(Cc) (%)
A (Bq)
uc(A)
uc(A) (%)
193.69 194.73 194.21 181.66 181.37 181.51
2.16 2.16 2.72 2.03 2.01 2.54
1.11 1.11 1.4 1.12 1.11 1.4
Columns 2 to 4 show the net count of peaks and corresponding absolute and relative uncertainties. Columns 5 and 6 contain the efficiency values of the detector for a point source and associated relative uncertainty. Photon emission intensities and their relative uncertainties appear in columns 7 and 8. Columns 9 and 10 on the one hand, and columns 11 and 12 on the other, contain the correction factors and associated relative uncertainties for efficiency and coincidence correction respectively. Activities calculated for each peak are given in column 13, with columns 14 and 15 showing the associated absolute and relative uncertainties, respectively.
The 126Sn activity in the measured solution, provided by the full-energy peak (FEP) with energy E is calculated as: A¼
NðEÞ C ðEÞ CP CTM CC ðEÞ eðEÞ IðEÞ t R
ð1Þ
A (Bq) is the 126Sn activity in the solution at the reference date, N(E) is the number of pulses recorded in the FEP with energy E, e(E) is the FEP efficiency of the detector used for the same energy, I(E) is the corresponding emission intensity, t (s) is the acquisition time (live time). The various correction factors are:
CR(E): geometrical transfer correction factor for efficiency. This
depends on energy and is calculated using the ETNA software (Piton et al., 2000). For the two neighbouring energies being considered, the correction factor is 0.90. CP: decay correction to reconcile the pulse count with the reference date: Dt ð2Þ CP ¼ exp -lnð2Þ T
Dt is the time between the reference date and the measurement starting, and T is the half-life of 126Sn. CTM: decay correction during measurement, tR T ð3Þ CTM ¼ 1-expð-lnð2Þ Þ T lnð2Þ tR tR is the total measurement time (real time)
Taking account of the half-life of 126Sn, these last two corrections are negligible. Coincidence corrections, CC(E), for the different peaks were calculated using ETNA (Piton et al., 2000). For the present measurement conditions, they are of the order of 3%.
The activity was measured for each of samples A and B using the two peaks at 666.1 and 695.0 keV. Since no b transition is expected to the first excited level and the ground state level of 126 Te, the absolute emission intensity I(E) is deduced from the decay scheme by the relation: IðEÞ ð1 þTÞ ¼ 100
ð4Þ
where aT is the internal conversion coefficient (ICC) for the gamma transition being considered. The ICCs were interpolated from the tables of Band et al., 2002, with the BrIcc program (Kibe´di et al., 2008) in the ‘‘Frozen Orbital’’ approximation that give a666 = 0.00378(6) and a695 =0.00340(5), respectively. The absolute emission intensities (I(E) 100) of the two gamma rays used here are thus 99.623(6) and 99.6612(50), respectively. The raw data and parameters used with associated uncertainties and final results are presented in Table 1. Relative combined standard uncertainties are calculated according to the variance propagation law. The relative standard uncertainty associated with the counting process is the same for the two peaks, and is in the order of 0.25%. In particular case, the factor dominating the uncertainty is related to transfer efficiency, and it is
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approximately 1.1%. The coincidence corrections have a relative standard uncertainty of 0.3%. The 126Sn activity in each sample was obtained from the mean of the activities calculated for each of the peaks; the relative uncertainty corresponds to the relative uncertainty of the individual values. Finally, the mass activity for each sample is obtained by dividing the measured activity by the mass of the corresponding solution. Hence, mass activity for samples A and B are 111.3(1.2) Bq g 1 and 103.0 (1.1) Bq g 1, respectively. These results were used for the determination of the 126Sn half-life that is described in details by Bienvenu et al. (2009).
3. Improving understanding of the
126
Sn decay scheme
Following on from measurement of the activity of 126Sn, the spectra were also processed to calculate relative photon emission intensities for this radionuclide in an attempt to improve understanding of its decay scheme, and in particular, the branching ratio to 126Te and 126Sb from the metastable level of 19.15 min. The reference values of 126Sb gamma transitions characteristics are taken from the evaluation by Lagoutine presented in NUCLE´IDE (Be´ et al., 2004). This evaluation is based on publications from 1975 and before, as well as the Nuclear Data Sheets assessment (Katakura and Kitao, 2002). The total sum of beta transitions probabilities is 104.3% in the NDS evaluation while the same sum is 102% in the Lagoutine’s evaluation. This emphasises a need in improving the 126Sn/126Sb decay scheme. This determination of photon emission intensities was completed by controlling impurities and quantifying the 121Sbm activity. A specific liquid beta scintillation measurement also enabled the maximum energy of beta disintegrations to be determined. 3.1. Determination of
121
Sbm concentration
In gamma spectrometry, the presence of 121Sbm is indicated in particular by the 37 keV photon emission. This peak is clearly
present and its area enables to predict the activity contained in each of the samples. The activities determined for samples A and B are 14.7 and 13.6 kBq, respectively, to within roughly 10%. We may note that the ratio of impurities in solutions A and B is of the same order of magnitude. 3.2. Verification of maximum beta decay energy In the publication of Murty et al. (1970) a beta emission with an end-point energy of 2.5 MeV was reported. In order to verify the existence of such a transition which, if it exists, would populate the 1361 keV level, a liquid scintillation beta spectrometry was carried out. The spectral resolution of this type of measurement, plus various artefacts appearing at high energies, means that a usable beta spectrum cannot be obtained to deduce a Kurie line. However, quantitative indications as to the maximum beta emission can be derived. In this particular case, the information sought is the presence or otherwise of beta emission above 2 MeV. 3.2.1. Preparation of scintillation sources A scintillating source was prepared by mixing 270 mg of a 126 Sn solution with 10 ml of Ultima Gold AB scintillator in a 20 ml glass vial. A blank was also prepared using an inactive tin solution of similar chemical composition to that of the active solution. To calibrate the energy scale of the spectrometer, we prepared two calibration sources with 89Sr and 90Sr-90Y using the inactive tin solution so that the chemical composition of these sources was as close as possible to that of the 126Sn solution. 3.2.2. Spectral analysis On the spectrum obtained with a Wallac 1414 spectrometer, a distribution related to the presence of 121Snm in the sample is mainly observed in the central portion, followed by a distribution resulting from the presence of 126Sn in the high energy portion. The maximum energy of the 121Snm spectrum is at 359 keV and
Table 2 Relative photon emission intensities obtained for each sample, mean value and standard uncertainty (normalization at 100 photons per 100 disintegrations for the 666.1 keV line). Sample A
Sample B
Energy(keV)
Relative emission intensities
Rel. uncert. (%)
Relative emission intensities
Rel. uncert. (%)
MEAN
uc (%)
666.1 695.0 222.45 278.5( +279.5 ?) 296.6 + 296.8 414.4 + 414.9 555.2 573.5 591.8( +593.4?) 620.5 + 620.9? 639.4 656.5 673.5( +674.8?) 697 720.5 856.4 928.12 953.3( +954.3?) 959.6 989.1 1 034.9(+ 1 035.4?) 1 062.9 1 212.3 1 477.3
100.00 100.58 0.28 0.17 0.95 92.80 0.38 1.24 1.63 1.95 0.21 0.39 0.77 4.58 10.10 3.06 1.52 0.19 0.08 1.33 1.76 0.50 0.39 0.54
1.22 1.22 3.31 3.76 2.60 1.29 3.61 2.27 2.16 2.06 4.54 3.43 2.72 1.06 1.51 1.81 2.24 4.45 7.09 2.63 2.33 4.37 3.80 3.16
100.00 99.97 0.23 0.14 0.98 92.25 4.34 0.33 1.60 1.88 0.22 0.17 0.89 5.03 10.32 3.11 1.66 0.18 Not detected 1.16 1.74 0.64 0.47 0.65
1.22 1.22 3.54 4.03 2.60 1.30 2.05 3.90 2.21 2.12 4.52 4.34 2.61 1.06 1.52 1.91 2.40 4.68
100.00 100.27 0.26 0.15 0.96 92.52 2.36 0.79 1.62 1.91 0.21 0.28 0.83 4.81 10.21 3.08 1.59 0.19 0.08 1.25 1.75 0.57 0.43 0.60
0.4 16 14 2.0 0.4 119 81.9 1.1 2.6 5 55 11 7 1.5 1.1 6 2.9 7.09 9 0.8 17 13 13
2.88 2.39 3.81 4.26 3.24
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provides a calibrating point in the spectrum which is completed by the locations corresponding to the maximum energies of 89Sr (1495 keV) and 90Y (2280 keV). The location of the channel corresponding to the maximum beta energy of 121Snm, 89Sr and 90Y is estimated graphically by extrapolating the high energy front, and corresponds to the 140, 600 and 975 channels, respectively. This leads to an equation for the linear calibration curve: E ¼ 35:79 þ 2:33 c
ð5Þ
where E is the energy in keV and c is the channel number. Using this calibration, the spectrum attributed to 126Sn comes to a halt at an energy of (1800 7100) keV. The uncertainty in this energy is mainly the result of calibration uncertainty, but also of the uncertainty in the location of the line corresponding to the 126 Sn maximum beta energy. Enlargement of the part of the energy spectrum greater than 2 MeV shows no electron emission in this region. In conclusion, the 126Sn beta spectrum reveals no emission of energy greater than 2 MeV, which enables to conclude that a 2.5 MeV maximum beta transition energy in the decay scheme of 126 Sn is absent. 3.3. Relative photon emission intensities in decay of equilibrium in the energy range more than 100 keV
126
Sn/126Sb in
Within this range of energies we find essentially those lines corresponding to photon emissions from the daughter products of 126 Sn, viz. 126Sb and 126Sbm. To provide relative values we make use of the 666.1 keV line as a reference, and its photon emission intensity is therefore normalized to 100. Relative intensities have been calculated as ratios (Table 2); the radionuclide activity is therefore not involved, and the stated relative uncertainty is taken as equal to the uncertainty associated with the corrected counting rates. Four of the lines reported in NUCLE´IDE (Be´ et al., 2004) were not detected: these correspond to energies 149.3, 208.6, 296.8, and Table 3 Energies and relative emission intensities of additional detected peaks. Energy (keV)
1063.9 keV. The presence of the 928.12 keV line from the 2703.6 keV level was confirmed with a relative intensity of 1.59 (9), this level is populated only from the 126Sbm decay. By contrast, several low intensity lines measured in the two spectra and not given in NUCLE´IDE (Be´ et al., 2004) are listed in Table 3; such peaks are of low intensity and the energies have an absolute uncertainty of around 1 keV. Lines at 1081, 1110 and 1362 keV correspond to the summed peaks. Lines at 684.7 and 934 keV might correspond to transitions from the 3450.3 keV level to levels of 2764.6 and 2514.3 keV, respectively. 3.4. Photon emission analysis in the energy range less than 100 keV 3.4.1. Absolute emission intensities in decay of 126Sn Photon emissions following the disintegration of 126Sn are in the energy range between 18 and 88 keV. This region is also characterised by intense Sb and Te KX-ray peaks which are strongly distorted to the left by scattering effects. Moreover, the region includes background peaks (X-rays from Pb, etc.) which are identified by comparison with a spectrum acquired using a container filled with a neutral matrix. For the doublet at 86.9 and 87.6 keV the spectrum was processed with COLEGRAM (Ruellan et al., 1996) to enable efficiency and coincidence corrections associated with each of the peaks to be applied. However, the deconvolution associated with these two peaks was quite simple. The absolute intensity values were obtained using the activity value of 194.2 (2.5) Bq. Details of their determination are given in Table 4 and the results are compared in Table 5 with the values of the NDS evaluation (Katakura and Kitao, 2002). These latter values are based only on one measurement result (Smith et al,. 1976). The results obtained in the current study are in accordance and with better accuracy. 3.4.2. Antimony XKb/XKa intensity ratio Within the spectra we observe mainly the Sb X-ray emissions: for these we find a Kb/Ka ratio of 0.238 and 0.228 using two different detectors. The uncertainty is of the order of 3 to 4% (in particular on account of the peak area which is somewhat fragile, Table 5 126 Sn absolute photon emission intensities, I(E), and associated uncertainties.
Relative emission intensities
684.7 726 730.7 934 1 081 1 110 1 191 1 290 1 362 1 589
1575
Sample A
Sample B
0.19 0.07 0.12 0.17 0.44 0.37 0.25 0.19 0.32 0.06
0.14 0.03 0.14 0.10 0.47 0.41 0.28 0.27 0.16 0.11
NDS value
Measured value
E (keV)
I(E) (%)
uc(I(E))
I(E) (%)
uc(I(E))
17.7 21.6 22.6 23.3 42.7 64.3 86.9 87.6
0.0000455 1.26 0.1 6.4 0.5 9.6 8.9 37
0.0000049 0.18 0.015 0.9 0.07 1.5 1.3 4
0.47 7.80 8.59 38.4
0.01 0.18 0.19 0.9
(3) (4) (4) (4) (4) (4) (4) (4)
Table 4 Details of determination of absolute photon emission intensities in the low-energy range (decay of
126
Sn).
Energy (keV)
N(E)
u(N(E)) (%)
e(E)
ue(E)) (%)
CR
u(CR) (%)
Cc
u(Cc) (%)
I(E) (%)
u(I(E)) (%)
42.7 64.3 86.9 87.6
4521 80023 88334 394296
1.49 0.35 0.34 0.16
9.413E–03 9.685E–03 9.594E–03 9.585E–03
1 1 1 1
0.8141 0.8392 0.8492 0.8494
1.9 1.6 1.5 1.5
1.004 1.001 1.001 0.999
0.04 0.01 0.01 0.01
0.47 7.80 8.59 38.36
2.9 2.3 2.3 2.2
Columns 2 and 3 show the net count of peaks and relevant relative uncertainties. Columns 4 and 5 contain the efficiency values of the detector for a point source and associated relative uncertainty. Columns 6 and 7 on the one hand, and columns 8 and 9 on the other, contain the correction factors and associated relative uncertainties for efficiency and coincidence correction respectively. Photon emission intensities and their relative standard uncertainties are given in the last two columns.
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Table 6 Comparison of measured and evaluated relative emission intensities of
149.3 208.6 223.3 278.6 296.6 296.8 414.4 414.9 555 573.8 593 620.1 639.7 656.3 666.1 675 695 697 720.3 856.7 928.2 954 959.6 989.3 1034.9 1061.6 1063.9 1213 1476.2
Relative emission intensities (%) NUCLEIDE
Relative uncertainty (%)
0.40 0.50 1.40 2.21 4.92 0.50 83.87 1.00 1.81 6.72 7.52 0.90 0.90 2.21 100.00 3.71 100.00 32.10 53.97 17.56
50.00 40.00 14.29 9.09 8.16 40.00 2.51 30.00 11.11 4.48 6.67 11.11 11.11 4.55 0.07 21.62 0.07 18.75 4.46 5.14
1.50 0.50 6.82 1.00 0.90
33.33 20.00 4.41 5.00 0.05 6.67 8.70 16.67
2.31 0.30
126
Sn.
Measured relative emission intensities (%) (Mean)
0.26 0.15 0.96
16 14 2.6
92.52
1.3
2.36 0.79 1.62 1.91 0.21 0.28 100.00 0.83 100.27 4.81 10.21 3.08 1.59 0.19 0.08 1.25 1.75 0.57
120 82.0 2.2 2.6 5 60
3.5. Discussion 3.5.1. 126Sb metastable level Following 126Sn disintegration, the first excited level of antimony 126 is metastable, with a half-life of 19.15 min. The branching ratio between the gamma transition to the ground state of 126Sb and the beta transition to the excited levels of 126Te is poorly understood. The results shown in Table 6 enable to comment certain assumptions on the overall scheme of the two nuclides in equilibrium. The data reported in Table 6 are as follows:
Columns 2 and 3 are the relative intensities and associated
11 1.2 7 1.6 2.0 6 4.5 8 9 1.8 17
0.43 0.60
and efficiency correction factors). Hence these results are mutually compatible and we obtain a final ratio of 0.233(5), which may be compared with the only measured value of 0.226 (11) (Close et al. 1973) and the theoretical value of 0.2266 from a Hartree–Fock relativistic calculation (Scofield, 1974).
Uncertainty (%)
uncertainties for 126Sb alone, i.e. for 100 126Sb disintegrations as deduced from Nucle´ide (Be´ et al, 2004); Columns 4 and 5 list the relative intensities and uncertainties for 126Sn and 126Sb in equilibrium measured in the present work; Columns 6 and 7 show the ratio between the experimental relative intensities and those tabulated for 126Sb alone. Looking at the decay scheme (Fig. 1), some gamma transitions (in bold in Table 6), depopulate levels in 126Te, in particular in the upper part of the scheme, which are only feed by beta transitions coming from the 126Sb ground state level. Therefore, from the relative intensities measured for Sn–Sb being at equilibrium and the assumption that the evaluated intensity values given in Nucle´ide, based on several measurements of
13 13
Measured-to-table ratio
Relative uncertainty (%) in ratio
0.19
21.4
0.18
8.6
0.12 0.22
82.1 7.0
0.23 0.13 1.00 0.22 1.00 0.15 0.19 0.18
12.2 60.2 0.1 24.3 1.2 20.0 4.7 5.5
0.13 0.16 0.18
33.6 21.5 10.0
0.19
15.6
Table 7 Gamma-ray emission intensities, I(E), and gamma-ray transition probabilities, P(g + ce), for gamma-ray transitions populated the 1361 keV level in 126Te. Energy (keV)
I(E) rel
uc
I(E) abs
856 1034 1476 414
3.08 1.75 0.6 92.52
0.062 3.07 0.032 1.74 0.078 0.60 1.295 92.17
uc
1 + at
uc
0.061 1.000832 0.000012 3.07 0.031 1.74 0.078 0.60 1.29 1.01408 0.00002 93.47 Total
126
P(g + ce) uc
98.9
0.06 0.031 0.078 1.31 1.3
Sb alone, are correct, then their ratio gives the proportion of Snm decaying by isomeric transition to 126Sb. This ratio varies from 0.12 to 0.23 and the simple mean is 0.18 with a relative standard deviation of 18%. The weighted mean is 0.186 with a relative standard deviation of 2.9%. The measured value of (18,6 (6)) % obtained here for the isomeric branching ratio of 126 Snm can be compared with a value of 14 (4) % deduced from the work of Orth et al. (1971). Moreover, the 928.2 keV gamma transition starts from the 2703.6 keV level provided by 126Sbm alone. Therefore, its absolute photon emission intensity is 1.59 (9) 0.99623 (6) =1.58 (9) %, and it is consistent with the NDS value (Katakura and Kitao, 2002): 1.3 (3)%, providing an improved uncertainty. Another check can be established at the 1361 keV level: since this level is fed only by gamma transitions, their sum should be 100%. These data are given in Table 7, where the sum of transitions gets up to (99 (1)) %; taking account of the associated uncertainties, this validates the probability balance at this level. 126
ARTICLE IN PRESS L. Ferreux et al. / Applied Radiation and Isotopes 68 (2010) 1571–1577
5. Conclusion The 126Sn activity of about 200 Bq has been determined in two Sn/126Sb solutions with a relative standard uncertainty of 1.3% in order to derive the 126Sn half-life value. In addition: 126
126 Sn were measured with improved accuracy; the relative emission intensities of the gamma rays in decay of 126 Sn/126Sb equilibrium composition were derived and the 126 Sbm isomeric branching ratio was deduced being 18.6 (6) %; it was confirmed the presence of a gamma transition with energy 928.2 keV in decay of 126Sbm and the population of the 2703 keV level in 126Te; Kb/Ka intensity ratio of Sb X-rays has also been determined being 0.226 (11).
the absolute photon emission intensities in decay of
References Band, I.M., Trzhaskovskaya, M.B., Nestor Jr., C.W., Tikkanen, P.O., Raman, S., 2002. Dirac-fock internal conversion coefficients. Atomic Data and Nuclear Data Tables 81, 1–334. Be´, M.-M., Browne, E., Chechev, V., Chiste´, V., Dersch, R., Dulieu, C., Helmer, R.G., ¨ Kuzmenco, N., Nichols, A.L., Schonfeld, E., 2004. NUCLE´IDE. Table of Radionuclides on CD-ROM, Version 2-2004, CEA/BNM-LNHB, 91191 Gif-sur-Yvette, France. /http://www.nucleide.orgS. Bienvenu, P., Ferreux, L., Andreoletti, G., Arnal, N., Le´py, M.-C., Comte, J., Be´, M.-M., 2009. Determination of 126Sn half-life from ICP-MS and gamma spectrometry measurements. Radiochimica Acta, Accepted for publication in Radiochimica Acta. Close, D.A., Bearse, R.C., Malanify, J.J., Umbarger, C.J., 1973. Yield Ratios, K a/K b, L a/L b, L a/L g, and L a/L l, for X-rays Produced by Protons of 1.0 to 3.7 MeV. Physical Review A 8, 1873–1879. Katakura, J., Kitao, K., 2002. Nuclear Data Sheetsdata sheets for A= 126. Nuclear Data Sheets 97 (3-–4), 795–926. Kibe´di, T., Burrows, T.W., Trzhaskovskaya, M.B., Davidson, P.M., Nestor Jr., C.W., 2008. Evaluation of theoretical conversion coefficients using BrIcc, Nucl. Instr.Nucl. Instrum. and Meth.ods A 589, 202–229. Morel, J., Valle´e, M., 1996. De´termination de la re´ponse en efficacite´ des de´tecteurs gamma-X par lissage de points expe´rimentaux: code EFFIGIE. Private communication, LNHB. Murty, K.S.N., Pathak, B.P., Gujrathi, S.C., 1970. Decay of 126Sbm and the energy ¨ Physik A Hadrons and Nuclei 237, 313–319. levels of 126Te. Zeitschrift fur ORTEC, 2008. MAESTROs-32, Software Version 6, /http://www.ortec-online.com/ index.htmlS. Orth, C.J., Dropesky, B.J., Freeman, N.J., 1971. Decay of 126Sn and of the 19-min and 12.4-days isomers of 126Sb. Physical Review C 3 (6), 2402–2411.
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Discussion: ˜ o): Please show again the slide describing the Q(E. Garcı´a-Toran measurement of the end point energy with scintillation counting? Is that a log scale? Then you need some form of energy calibration to obtain that, how was it done? A(P. Cassette): Yes, it is a log scale. It was calibrated from the end point energy of the nuclides present on the spectrum. The question was not in fact to have a precise determination of the end point but just to check if there is anything in the spectrum over 2 MeV and the answer is no. Comments(J. Hardy): I have a comment—not meant to be a negative to this particular measurement—about beta-decay endpoint energies measured with scintillation detectors. Throughout history these have frequently been wrong, and they have been systematically wrong. This occurred because people assumed, when they measured beta–gamma coincidences by gating on a particular gamma ray, they had isolated a single beta transition. Especially for nuclei far from stability, this is rarely the case. The gamma feeding from above can involve a large number of gamma rays, many of them unobservably weak, fed by other beta transitions which carry, in total, a significant intensity. This ‘‘Pandemonium effect’’ was first pointed out some years ago in papers by Gregers Hanson, Bjorn Jonson and myself; but many spectroscopists simply ignored it. So, in other words, they got results for beta-decay energies that were consistently too low. If you look in the Audi tables and watch as new Penning-trap mass measurements come in, most of the Audi values that are based on beta-decay measurements turn out to be wrong in the same direction. In short, do not rely too much on mass values based on beta-decay measurements.