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Primary radioactivity standardization and gamma intensities determination of 124Sb
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 602 (2009) 450–456
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Primary radioactivity standardization and gamma intensities determination of 124Sb A. Iwahara , J.U. Delgado, R. Poledna, C.J. da Silva, M.C.M. de Almeida, R.L. da Silva ´rio Nacional de Metrologia das Radiac- ˜ ˜o Nacional de Energia Nuclear (CNEN), Laborato oes Ionizantes (LNMRI)/Instituto de Radioprotec- ˜ ao e Dosimetria (IRD)/Comissa Av. Salvador Allende, s/no, Recreio, CEP 22780-160, Rio de Janeiro, Brazil
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 July 2008 Received in revised form 15 December 2008 Accepted 5 January 2009 Available online 20 January 2009
A solution containing 124Sb was primarily standardized by the 4pbg coincidence and anticoincidence extrapolation methods in the frame of the EUROMET 907 international comparison organized by Laboratoire National Henri Bequerel (LNHB)/France, in 2007. The main purposes of this exercise are the improvement in the uncertainties on the gamma-ray emission intensities and they clarify the discrepancies verified among the intensity values for many weak gamma rays reported in the literature. In this work the results of the activity obtained were used to determine the absolute and relative gamma-ray intensities using a planar and coaxial HPGe detectors calibrated by 152Eu and 116mHo multigamma standard sources covering the energy range from 20 to 1408 keV. Additionally the half-life of 124 Sb was determined following the decay of a solution of 124Sb contained in a glass ampoule over a period of three half-lives using two 4pg ionization chambers. & 2009 Elsevier B.V. All rights reserved.
Keywords: 124 Sb Coincidence Anticoincidence Radioactive decay Photon intensities Half-life
1. Introduction The accurate knowledge of gamma-ray emission probabilities Pg plays important role both in dosimetry and radioactivity applications. In dosimetry the dose estimates for radiopharmaceuticals are based on Monte Carlo radiation transport codes where the nuclear decay scheme and atomic data must be accurately determined in order to have the correct input data spectrum for the theoretical calculations. Incorrect values of Pg adopted for efficiency calibration of detectors could give inconsistent and discrepant outcomes in gamma spectrometry. Multigamma sources such as 152Eu, 154Eu, 56Co, 166Hom and 192Ir are used for efficiency calibration of germanium detectors in energy range above 100 keV. Correction factors for loss counting due to coincidence sum-peaks, pulses pile-up and energy attenuation should be properly applied for these kinds of multi-gamma sources. 133Ba could be used in energy range below 100 keV where the efficiency curve is not linear, but the number of gamma lines with high intensity is very low. In this case several sources should be used, which is time consuming. 124Sb decays by b emission to excited levels of 124Te emitting tens of gamma rays ranging from 148 to 2800 keV besides the emission of KX-rays from 27 to 32 keV as presented in Fig. 1 [1]. Among these gamma rays the main emission is of 602.730 keV with almost 98% and 12 lines else can
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E-mail address: [email protected] (A. Iwahara). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.021
also be found with intensities higher than 1%. The remainder lines present intensities lower than 1% requiring too much time of measurements in order to minimize the counting uncertainty. Two methods were used for activity determination of the 124Sb solution distributed by LNHB to the participants: the 4pb(PC)–g(NaI(Tl)) coincidence counting method and the 4pb(LS)–g(NaI(Tl)) anticoincidence counting with live time and extending dead-time method. The gamma intensities were determined using two high purity germanium detectors depending on the energy ranges: a planar covering the 15–300 keV range and the coaxial covering the 50–3000 keV range. The half-life was determined using two 4pg-ionization chambers: Centronic IG11 and IG12 models.
2. Experimental procedures 2.1. Preparation of sources The ampoule numbered 7 by the supplier containing 5.17 g of Sb solution was received at our laboratory in March 2007. The chemical composition was SbCl3 in HCl 2 M. The advice sent by organizing laboratory, in adding a drop with equivalent mass of 2.5 mol L1 NaOH on each deposit of radioactive solution of 124Sb in order to avoid volatility, did not work in our laboratory because the beta efficiencies counting in the 4p proportional counter of these sources were quite below of 90%. Thus, ten solid sources for 124
ARTICLE IN PRESS A. Iwahara et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 450–456
3
60.20 days
124 51Sb73
3-
2693.675
3-
2293.72
2+
2039.295
4+
1957.86
2+
1325.515
4+
1248.585
2+
602.730
β
0
0 124 52Te72
Fig. 1. Simplified decay scheme of
124
451
Cox–Isham formulae [6] according to the algorithm developed by Smith [7]. The resolving time of 1.21970.017 ms was fixed in the coincidence unit and determined by using the accidental coincidence method [8]. The b and g dead times, determined by modified source-generator method [9], were fixed to 4.37170.059 and 3.80070.046 ms, respectively, in beta and gamma channels. The 4p counter was filled with a mixture of 90% Ar and 10% CH4 under a pressure of 0.1 MPa. The energy window of the g-channel was set around the 602.7 keV photopeak. The b-efficiency was reduced by adding gold-coated VYNS (polyvinylchloride–polyvinylacetate copolymer) films and aluminized Mylar films on both sides of the source. The maximum b-efficiency obtained was about 94% and decreased down to about 75% in order to get the extrapolation curve. The anticoincidence method was originally developed by Bryant [10,11] in the particular case of nuclides that present meta-stable levels in the decay scheme. To the initial idea of Bryant it was incorporated in the concept of shared dead time for beta and gamma channels by de Carlos and Granados [12]. In 1976, Baerg [13] introduced the use of live time to the anticoincidence system that eliminates the correction of dead time using an extending dead-time device. In 2000, Bouchard, from LNHB/France, presented a device where the concepts of live time and extended dead time were incorporated in a unique electronic module entitled MTR2 [14,15]. The anticoincidence system set up at our laboratory consists of two MTR2 modules that were kindly donated by LNHB and tested its performance in this work. For the generation of the live time a quartz oscillator of 1 MHz is used as reference time. When the system is able to count, a pulse in the entrance of the MTR2 generates a minimum dead time and simultaneously sends a pulse to the counter in order to be registered (C). The dead time is triggered by each arrival of new pulses in the entrance of the MTR2. In this way, the time Ta that the system is free can be described by
Sb.
T a ¼ ðF=F ref Þ C d 4pb(PC)-g coincidence counting were prepared by dispensing 10–20 mg of radioactive solution on conducting VYNS films (gold coated on both sides with a total surface density of E35 mg cm2) stretched on stainless steel rings with 16 and 30 mm inner and outer diameter and 0.2 mm thickness. A drop of TWEEN-20 (1:1000) wetting agent was added to get thin and homogeneous deposits. Eight liquid sources for 4pb(LS)-g anticoincidence counting using OptiPhase ‘HiSafe’ 3, Insta-Gel Plus and Ultima Gold AB cocktail scintillations contained in 20 mL Perkin Elmer vials were prepared for anticoincidence measurements. The mass of radioactive solution varied from 20 to 30 mg and the volume of the cocktail from 3 to 20 mL. Ten point sources as described in Ref. [2] were prepared for gamma intensity measurements using germanium detectors.
2.2. Activity measurements The 4pb(PC)-g coincidence extrapolation method is largely used for radionuclide activity measurements and can be easily found extensively in the literature (for instance Refs. [3–5]). Basically the coincidence extrapolation method consists of plotting (NbNg)/Nc versus (1Nc/Ng)/Nc/Ng in order to obtain the activity No when Nc/NgEeb-0, where eb is the beta counting efficiency and Nb, Ng and Nc are b, g and coincidence count rates, respectively. Nb and Ng were corrected for background, dead time and decay in the usual way, while Nc was corrected by using the
(1)
where Fref is the reference frequency, F is frequency in the counting channel and it represents the time interval during which the system is not paralyzed by dead time, C is the count rate in the counting interval, d is the width of signal pulse entering MTR2. In this context, C d is the correction due to the width of the pulse generated for the MTR2. To reduce such correction the width of the pulse generated for the MTR2 is fixed in 5 ns. In such a way the real count rate will be represented by N ¼ C=T a
(2)
The activity of a radionuclide is determined using Eq. (3), which is the classic equation of the coincidence method differing only in that Nc, the coincidence count that is determined for one given gamma window, is the difference between the gamma rate and uncorrelated gamma rate represented by
A¼
Nb Nw g w
Ng i Nw g
(3)
where Nb is the count rate in the beta channel, N w g is the count rate in the gamma window, i Nw g is the uncorrelated gamma count rate. The same extrapolation technique of coincidence counting is used and the electronic discrimination for beta efficiency
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decreases has been adopted instead of the foil absorption used in the coincidence method.
3. Results and discussion 3.1. Activity
2.4. Half-life measurements One glass ampoule containing 2.70 g of the original 124Sb solution has been prepared for the half-life measurements. Its radioactive decay was carefully followed by measuring the ionization current obtained by the Centronic IG11 and IG12 ionization chambers for over 2 and 3 half-lives, respectively. The logarithm of the experimental ionization current measurements as a function of the time was linearly fitted by least-square method where the slope is the decay constant, l.
3.2. Intensities of gamma rays with Ge spectrometers Two sets of essays for ten point source samples were carried out in July 2007 and November 2007 in order to obtain the X- and g-ray emission intensities related to the decay scheme of 124Sb. Each spectrum accumulated a counting time that varied from 80 000 to 300 000 s. The weaker lines were analyzed in a higher counting period. All data acquisitions were made in the same operational conditions of geometry defined by the efficiency calibration conditions. Measurements of the absolute emission intensities of 124Sb X- and g-rays for lines below 1000 keV were carried out with the two detectors. Both results obtained were averaged. The uncertainty activity of 0.20% was only considered at
Activity concentration (kBq g-1)
Gamma-ray emission probabilities Pg following the decay of Sb were determined at LNMRI using the expression Pg ¼ c/(eX,g, A) where c is the count rate in the photopeak (s1), eX,g is the fullenergy peak efficiency for the X- and g-ray emission and A is the activity of the measured source (Bq) at reference date, traceable to the primary measurement system. Two semiconductor detectors were used: HPGe coaxial-p-type ranging from 50 to 3000 keV, with resolutions of 1.03 and 1.86 keV at 122 and 1332 keV, respectively, and relative efficiency of 50%; Ge planar-ntype has 52 mm2 of active area, with a resolution of 0.64 at 122 keV and 20% of relative efficiency. Both detectors were coupled to a pre-amplifier with fast rise times, a conventional amplifier with a shaping-time of 6 ms, a multichannel buffer analyzer and software for spectral acquisition and data processing. The planar semiconductor is commonly used for the low energy range (15–300 keV). But, in order to enlarge its energy range once that several energies of 124Sb are over 300 keV, a device was built based on a voltage divider. This device was inserted between the pre-amplifier output and the amplifier input [16]. Then, the spectrometric operation range was enlarged up to 1000 keV. The particular photon counter–source distance adopted here allowed to neglect the counting losses due to pileup, dead time as well as g–g and X–g summing coincidence effects. Some counting corrections such as decay, sample positioning and weighting were taken into account. Each detector was calibrated on efficiency for source-to-detector distance of 20 cm using gamma rays standardized point sources of 152Eu and 166m Ho, whose total relative standard uncertainty in the 100–1000 keV region varied from 0.5% to 1.5% (k ¼ 1), for both detectors. The activities of these sources have been measured by primary methods at LNMRI. The nuclear data for these multi-gamma standards were taken from Bernardes et al. for 166m Ho [2], Silva et al. for 152Eu [17] and Be´ et al. [18]. The experimental points of the efficiency curves in function of energy between 20 and 1500 keV of germanium spectrometers have been fitted to the experimental data through a polynomial function where the coefficients are determined by the least-square procedure. The better degree was third degree and evaluated by w2 statistical test. But the efficiency responses (e) corresponding energies (E) above 1500 keV for coaxial semiconductor were obtained for extrapolation and adjusted by means of the potential curve: e ¼ aEb which could be transformed to the general linear form (linear log–log extrapolation method discussed by Debertin and Helmer in Ref. [19]). The accuracy of this method could be further improved if a large number of data points are used to define the efficiency curve once its slope is almost constant between 200 and 3000 keV. An uncertainty of the efficiency in this energy range has been estimated to vary from 2.5% to 5%, provided that the last six data calibration points are sufficiently accurate. 124
Figs. 2 and 3 show the efficiency extrapolation curves for 124Sb from the coincidence and anticoincidence measurements in which the bars denote one standard uncertainty (k ¼ 1). The correlation coefficients of 0.95 and 0.99 for coincidence and anticoincidence fits, respectively, can be related to the linearity of the measurement systems. The activity results as well as the uncertainty budgets are shown in Tables 1 and 2, respectively. Both results were in agreement within the limits of the evaluated uncertainties showing the coherence of the coincidence and anticoincidence activity measurements for 124Sb. The lower uncertainty for anticoincidence result was expected because some uncertainty components present in coincidence measurements such as dead time, resolving time and delay mismatch are removed. The final result of activity concentration (1710.8 kBq g1) on the reference date of February 15, 2007, 12h00 UTC was evaluated as the weighted mean of the two results listed in Table 1. The external and internal uncertainty was calculated as uext ¼ 1.1 kBq g1 (0.06%) and uint ¼ 3.3 kBq g1 (0.20%), respectively, and the latter was adopted as the overall standard uncertainty of the final activity value.
1725 1720 1715 1710 0.00
0.10
0.20 (1- εβ) /εβ
Fig. 2. Efficiency extrapolation curve of
Activity concentration (kBq g-1)
2.3. Gamma intensities measurements
0.30
0.40
124
Sb for coincidence counting.
1780 1760 1740 1720 1700 1680 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 (1− εβ) / εβ
Fig. 3. Efficiency extrapolation curve of
124
Sb for anticoincidence counting.
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Table 1 Activity concentration of Measurement method
Coincidence Anticoincidence
453
124
Sb solution. Activity concentration (kBq g1)
1712.6 1710.2
Table 2 Uncertainty components, in % of the activity concentration. Contributions due to
Coincidence Anticoincidence
Counting statistics Weighing Background Dead time Resolving time Delay mismatch between the beta and gamma pulses Half-life Extrapolation of efficiency curve Live time Relative combined standard uncertainty
0.24 0.05 0.05 0.012 0.05 0.24
0.11 0.05 0.06 – – –
0.16 0.02 – 0.38
0.02 0.18 0.01 0.24
the end of the calculation of the weighted average. The resulting absolute and relative photon emission probabilities for X- and g-rays emitted by 124Sb are given in Table 3 in order of energy. Relative intensities were converted from the absolute probability, whose measured value is 97.5270.72%, normalized to 100% for 602.7 keV photopeak. All these results were evaluated with a relative uncertainty estimated at the 1s confidence level that consisted in type A component due to counting statistics in peak area determination after background correction (that varied from 0.1% for the most intense peaks up to 50% for the weaker peaks); and type B components that originate from efficiency curve (0.5–2.5%) and source standardization by coincidence methods. Others small components due to correction factors were considered in the reported uncertainty. Here the values reported recently by some authors were also included. Twelve weak gamma lines of energy that appear in the evaluated data and compiled previously by other studies in the literature were not seen in the present work, according to Table 3. In this table, all the g-rays quoted in the literature are listed, filled or not in the corresponding line. With regard to the X-rays of low intensities, our data can only be compared with Patil et al. [20] and Goswamy et al. [21], which are in good agreement and with smaller uncertainties than these authors. The comparison of the stronger g-ray (higher than 1% for intensities and below 1500 keV) appears to be consistent and confirms the values published, which are satisfactory, having an uncertainty between 0.7% and 2.8%. The results measured for strong and weak peaks in a coaxial and a planar detector for energies below 1000 keV showed consistent intensities. The values for strong lines situated above 1500 keV exhibit deviation of several percent among the most recent measurements, except for results measured by NDS 109 compilation [22] and Patil et al. [20] and this work, where there is a good agreement. However, it can be seen in Table 3 high divergences for many weak gamma rays reported in different publications mainly those lines above 2283.3 keV. The lines situated from 2 MeV to 2283.3 keV present consistencies within the same order of magnitude. Regarding to the 2015.7 keV line the NDS 109 compiled data presents a value 10 times higher than the other authors. There should be a mistake in
Relative uncertainty (%) Type A (uA)
Type B (uB)
Standard uncertainty (u)
0.24 0.10
0.30 0.22
0.38 0.24
this compiled data. The origin of this inconsistency for higher energies (above 2283 keV) can be attributed to the process of obtaining detector efficiency response by extrapolation method in regions where there are a few reference energies. The response curve in this region, although continues to decrease, is perhaps found to follow a different pattern due to the dominant pair production interactions in the presence of escape peaks. Otherwise, it is known that it is hazardous to extrapolate the Ge gamma efficiency curve above the highest energy used in the calibration and, thus, the high energy gamma intensities measured can be uncertain to an unknown degree. According to the recent publication of Ala-Heikkila¨ [23], the linear log–log extrapolation method is considered adequate for this situation. In any case efficiency extrapolation outside of data above 1500 keV is prone to errors and should be used with care. The most common highest energy calibration point is the 1836 keV peak of 88Y, but this nuclide was not available in our laboratory. 3.3. Half-life The ion current originated by the 124Sb sample solution contained in a glass ampoule was measured during 120 days by IG11 ionization chamber and 200 days by IG12 chamber. The halflife was obtained from a linear least-squares fit, as physically required, of the logarithm of the activity (which is proportional to the ion current originated) as a function of the time. Fig. 4 displays, for illustration, the plot of minus natural logarithm of the ionization current Ln(I) measured in the IG12 ionization chamber versus decay time while Fig. 5 shows the respective residuals of the experimental points in comparison with the fitted values. The correlation coefficients showed the goodness of linear fit and terms higher than linear do not improve the fit. The value of the half-life employing the slope of the linear fit was determined as 60.39 and 59.93 d, from IG11 and IG12, respectively. The type A components come from the fitting procedure and are given by T1/2(Dl/l) where T1/2 is the half-life and l is the slope. The type B component results from the instability of the ionization chambers and was estimated as 0.03% and 0.05%, respectively, for IG11 and IG12, by measuring a long-lived source of 226Ra during the experiment. The statistical parameters from linear least-squares fit are shown in Table 4. No g-emitting impurities have been detected by spectrometric check using germanium detector and also corroborated by good linearity of the fitting. The final value of T1/2 was calculated as weighted mean of these two values giving 60.0770.21 d which agrees with the value of 60.2070.03 d [18] recommended in this comparison exercise.
4. Conclusions The coherence between the coincidence and anticoincidence methods for the determination of activity concentration of 124Sb has been demonstrated in this work. Both results agree within the evaluated standard uncertainties and the anticoincidence result
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Table 3 Gamma emission probabilities of Energy (keV)
Sb per 100 decays.
Absolute photon emission probabilities This work
27.3 30.9–31.8 148.2 158.9 189.6 210.3 254.3 291.4 335.8 346.1 370.4 385.9 400.0 443.9 468.6 476.9 481.1 498.4 525.4 530.3 553.8 571.6 602.7 632.3 645.8 662.4 709.3 713.7 722.7 735.7 765.8 775.2 790.7 816.8 856.9 899.6 937.9 968.1 976.2 997.7 1014.5 1045.1 1053.8 1086.3 1097.0 1163.2 1198.0 1205.5 1235.0 1253.4 1263.1 1269.0 1301.3 1325.5 1355.1 1368.1 1376.1 1385.1 1418 1428.7 1436.5 1445.0 1453.2 1488.8 1505.6 1528.1 1557.0 1565.8 1579.7 1622.4 1657.0 1690.9 1720.3
124
0.35(7) 0.087(17) 0.012(5) 0.005(1) 0.002(1) 0.0086(12) 0.0089(14) 0.0045(8) 0.077(7) 0.0033(16) 0.033(10) 0.04(2) 0.124(8) 0.186(4) 0.052(9) 0.045(16) 0.023(7) 0.037(14) 0.139(4) 0.042(5) 0.019(5) 0.020(4) 97.52(70) 0.098(7) 7.39(6) 0.040(3) 1.325(18) 2.21(3) 10.54(8) 0.134(5) 0.011(3) 0.010 (2) 0.737(7) 0.079(6) 0.0198(12) 0.022(9) 0.020(5) 1.86(5) 0.082(5) 0.024(7) – 1.84(3) – 0.040(4) 0.034(8) – – – 0.027(6) 0.041(9) 0.042(4) – 0.031(4) 1.60(3) 1.04(3) 2.58(4) 0.51(2) 0.070(6) – 0.048(7) 1.22(2) 0.328(7) 0.031(7) 0.67(2) 0.051(16) 0.410(18) – – 0.43(2) 0.039(3) – 45.56(11) 0.095(5)
Goswamy [21] 0.357(17) 0.082(10) 0.0037(7) – 0.0065(13) 0.0054(10) 0.0159(10) 0.0086(10) 0.073(3) 0.0059(13) 0.033(10) –
Relative intensity Iwata [24]
NDS 109 [19]
Jianming [25]
– – – – – – – – – –
– –
– – 0.0040(8)
– 0.0065(11) 0.0056(9) 0.0165(10) 0.0089(8) 0.076(3) – 0.051(9)
– 0.121(3) 0.188(5) 0.046(3)
– 0.024(2) – 0.137(3) 0.042(2) – 0.0190(13) 97.8(17) 0.105(2) 7.38(10) 0.031(2) 1.31(3) 2.22(4) 10.5(2) 0.126(3) 0.0121(5) 0.0091(18) 0.74(2) 0.072(3) 0.024(1) 0.0171(14) – 1.88(3) 0.083(2) – – 1.83(3) 0.005(2) 0.037(2) – – – – – – 0.041(2) – 0.034(2) 1.57(3) 1.03(2) 2.58(4) 0.48(1) 0.06(2) – – 1.22(2) 0.327(7) – 0.671(14) – 0.412(8) – 0.015(4) 0.424(8) 0.041(2) – 48.2(7) 0.094(3)
0.129(16) 0.205(10) 0.058(8) – – – 0.117(12) – – – 100.0(4) 0.114(6) 7.61(3) 0.016(5) 1.399(11) 2.338(12) 11.02(4) 0.133(9) – – 0.758(9) 0.079(6) 0.029(7) 0.016(9) – 1.919(13) 0.088(8) – – 1.864(17) – 0.039(9) – – – – – – – – 0.041(15) 1.584(22) 1.042(27) 2.67(3) 0.501(20) 0.061(26) – – 1.225(24) 0.358(17) – 0.679(19) – 0.410(24) – – – 0.035(12) – 48.58(16) 0.097(7)
0.3681(55) 0.0852(15) 0.0061(20)
– –
–
0.142(7) 0.1932(20) 0.051(3)
– 0.0257(15) – 0.155(13) 0.204(10) 0.053(3)
– 0.0242(19)
–
0.002(36) 0.0147(5) 0.0137(6) 0.0070(6) 0.0760(18)
0.0356(61) –
–
– –
0.0062(28) 0.0214(41) 0.0122(61) 0.086(6)
0.039(5) –
Patil [20]
0.1249(66) 0.1930(10) 0.0364(23) –
0.029(8) –
0.141(4) 0.0431(20)
0.0205(10) –
0.165(10) 0.047(11)
0.1429(75) 0.0421(12)
–
–
–
0.0194(13) 100.00(23) 0.107(1) 7.588(24) 0.030(4) 1.384(12) 2.327(17) 11.00(4) 0.130(7)s 0.0124(2) 0.0096(17) 0.756(5) 0.0745(18) 0.0243(10) 0.0176(14) – 1.925(9) 0.0851(16) – – 1.874(11) 0.005(2) 0.0387(18) – – – – – – 0.0422(18) – 0.0351(10) 1.616(15) 1.061(13) 2.683(13) 0.494(5) 0.064(3) – – 1.244(8) 0.337(4) – 0.687(6) – 0.418(5) – 0.014(3) 0.39(5) 0.0418(10) – 48.64(15) 0.0972(17)
0.025(10) 100.0(10) 0.101(6) 7.55(10) 0.035(11) 1.38(4) 2.29(5) 10.99(15) 0.145(21) 0.0092(41) 0.0112(41) 0.753(11) 0.074(7) – 0.02(6) – 1.945(23) 0.088(5) – – 1.897(24) – 0.043(5) – – – – – – 0.043(5) – 0.039(5) 1.645(23) 1.103(18) 2.696(31) 0.496(10) 0.071(16) – – 1.236(17) 0.346(10) – 0.709(54) – 0.434(9) – 0.013(4) 0.419(41) 0.04(4) – 48.73(61) 0.102(4)
0.0184(10) 100(1) 0.0990(9) 7.685(52) 0.0148(10) 1.394(16) 2.288(24) 10.877(118) 0.1399(19) 0.0039(3) 0.0119(12) 0.7664(89) – 0.0216(11) 0.0200(12) – 2.1051(225) 0.0841(10) – – 2.0257(95) – 0.0358(16) – – – – – – 0.0482(14) – 0.0256(13) 1.707(19) 1.093(13) 2.700(21) 0.543(5) 0.064(2) 0.005(2) 0.005(2) 1.270(12) 0.335(32) – 0.692(5) 0.008(1) 0.451(4) – – 0.460(4) 0.0477(12) – 46.627(45) 0.097(18)
This work 0.36(8) 0.089(18) 0.012(5) 0.005(1) 0.002(1) 0.0090(10) 0.0090(10) 0.005(1) 0.079(7) 0.0030(13) 0.034(9) 0.038(18) 0.127(8) 0.190(4) 0.053(8) 0.046(14) 0.024(7) 0.038(13) 0.143(4) 0.043(5) 0.019(5) 0.021(4) 100.0(6) 0.100(7) 7.57(6) 0.041(3) 1.358(18) 2.26(3) 10.81(8) 0.137(5) 0.012(3) 0.0101(16) 0.756(7) 0.081(6) 0.020(1) 0.022(8) 0.021(5) 1.91(5) 0.084(5) 0.025(7) – 1.88(3) – 0.041(4) 0.034(7) – – – 0.028(6) 0.042(8) 0.043(4) – 0.032(4) 1.64(3) 1.07(3) 2.65(4) 0.52(2) 0.072(6) – 0.049(7) 1.25(2) 0.336(7) 0.032(7) 0.69(2) 0.052(14) 0.421(15) – – 0.44(2) 0.041(3) – 46.72(9) 0.098(5)
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455
Table 3 (continued ) Energy (keV)
Absolute photon emission probabilities This work
1757.9 1851.5 1918.8 1950.4 2015.7 2039.3 2078.6 2090.9 2099.1 2108.0 2151.5 2172.1 2182.6 2203.0 2224.8 2253.9 2274.0 2283.3 2293.7 2323.1 2454.4 2682.0 2693.6 2808.0 2871.0
Goswamy [21]
– –
0.005(2) 0.0061(9) 0.054(2)
0.055(4) –
–
0.0126(16) 0.062(4) 0.028(3) 5.15(19) 0.045(3) 0.051(3) – – 0.042(3) 0.030(2) 0.019(1) – – 0.023(14) 0.080(7) 0.010(4) 0.009(3) 0.007(3) 0.005(2) 0.007(3) –
Relative intensity Iwata [24]
NDS 109 [19]
Jianming [25]
Patil [20]
This work
– –
–
–
–
– –
0.0066(13) 0.0557(16)
0.052(4) –
0.011(1) 0.065(2) 0.026(2) 5.62(10) 0.046(1) 0.044(2) – –
– 0.0093(26) 0.059(3) 0.0157(14) 5.59(2) 0.045(6) 0.044(3)
– – 0.043(2)
– – – –
–
– 0.097(5) 0.0656(19) 0.021(4) 5.61(3) 0.0467(9) 0.0443(13)
– 0.040(2)
0.0041(13) 0.031(10)
0.0022 0.045(2)
– – – 0.0027(19) – –
– – – – 0.0082(12) 0.0327(10) 0.00249(16) 0.0015(4) 0.00169(17) 0.0031(5) 0.0015(2)
–
0.056(4)
–
–
– – – –
0.0026(10) 0.058(16)
0.0124(7) 0.068(2) 0.0163(25) 5.69(9) 0.046(2) 0.044(2)
0.0021(4) 0.0434(10)
– – – – 0.0099(8) 0.057(3) 0.0026(3) 0.0018(2) 0.0020(4) 0.0046(7) 0.0015(2)
0.0112(31) 0.06(3)
– 0.0090(9) 0.0661(19) 0.0741(18) 5.397(52) 0.0572(12) 0.0501(7)
0.0131(15) 0.063(4) 0.029(3) 5.28(16) 0.046(3) 0.052(3)
– –
– – 0.036(7) 0.0310(6)
0.043(3) 0.031(2) 0.021(1)
– 0.0006(2)
– –
– 0.0076(14) 0.031(5) 0.0025(7) 0.0016(6) 0.0018(6) 0.0026(16) 0.0020(8)
–
0.0422(9) 0.056(23) 0.0060(3) 0.0092(1)
0.024(13) 0.082(7) 0.010(3) 0.009(3) 0.007(3) 0.005(2) 0.007(3)
– 0.003(1) 0.0093(4) –
–
s: Sum of 735.7+735.9 keV lines.
6.00
Table 4 Least-square fitting parameters obtained from Ionization Slope (l) chamber
2.00
IG11 IG12
Ln (I)
4.00
Range of relative residuals variation (%)
0.011472(11) 0.6 to 0.2 0.011579(5) 0.10 to 0.10
124
Sb decay.
Coefficient of Reduced Degree of correlation (w2) freedom
0.9999 0.9999
2.489 0.706
23 40
0.00 0
50
100 150 Decay time (d)
200
250
Fig. 4. Linear least-square fit of 124Sb data of IG12 ionization current (I) as function of time.
Relative residuals
0.12 0.08 0.04 0.00 -0.04
availability of radionuclides with high gamma energies such as Na, 88Y, 26Al and 56Co would improve the accuracy of Pg measurements avoiding the extrapolation technique used in this work for Pg above 1500 keV. These results also suggest that there is a need to conduct an energy determination project, in addition to the activity and emission probability measurement project, for 124Sb standardization. The value of the half-life determined by the ionization chambers agrees within the quoted uncertainty with that recommended for this comparison (60.2070.03 d).
24
References
-0.08 -0.12 0
50
100
150
200
250
Time (d) Fig. 5. Residuals of the experimental points and the fitted values for IG12 measurements.
gave smaller uncertainty as expected. The measured emission probabilities and its associated uncertainties for gamma and X-rays in this work are found, in general, to be consistent with those reported in literature, unless for weaker ones as expected which are not suitable for using in the calibration of detectors. The
[1] F. Lagoutine, N. Coursol, J. Legrand, Table des Radionucle´ides, LMRI, France, 1985. [2] E.M.O. Bernardes, J.U. Delgado, L. Tauhata, C.J. da Silva, A. Iwahara, R. Poledna, A.S. Paschoa, Appl. Radiat. Isot. 56 (2002) 157. [3] W.B. Mann, A. Rytz, A. Spernol, Radioactivity Measurements Principles and Practice, Pergamon Press, Oxford, 1988. [4] A Handbook of Radioactivity Measurements Procedures, Report no. 58, NCRP Publications, Bethesda, 1985. [5] L. Grigorescu, Nucl. Instr. and Meth. A 112 (1973) 151. [6] D.R. Cox, V. Isham, Proc. Roy. Soc. A 356 (1977) 149. [7] D.L. Smith, Nucl. Instr. and Meth. A 152 (1978) 505. [8] P.J. Campion, Int. J. Appl. Radiat. Isot. 4 (1959) 232. [9] E. Scho¨nfeld, H. Janssen, Nucl. Instr. and Meth. A 339 (1994) 137. [10] J. Bryant, Appl. Radiat. Isot. 13 (1962) 273. [11] J. Bryant, IAEA Symposium on Standardization of Radionuclides SM-79/21, 1967, p. 129.
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J.E. de Carlos, C.E. Granados, Nucl. Instr. and Meth. A 112 (1973) 209. A.P. Baerg, K. Munzenmayer, G.C. Bowes, Metrologia 12 (1976) 77. J. Bouchard, Appl. Radiat. Isot. 52 (2000) 441. J. Bouchard, Appl. Radiat. Isot. 56 (2002) 269. R.L. da Silva, Metrological aspects of isotopic ratios and abundance of nuclear material by photon spectrometry, M.S. Degree Thesis, 2007 (in Portuguese). [17] C.J. da Silva, J.U. Delgado, A. Iwahara, L. Tauhata, R. Poledna, R.N. Alves, Appl. Radiat. Isot. 56 (2002) 169. [18] M.M. Be´, V. Chiste´, C. Dulieu, E. Browne, V. Chechev, N. Kuzmenko, R. Helmer, A. Nichols, E. Scho¨nfeld, R. Dersch, Table of Radionuclides, BIPM, Se´vres, France, 2004.
[19] K. Debertin, R.G. Helmer, Gamma- and X-ray Spectrometry with Semiconductor Detectors, North-Holland, Amsterdam, 1988. [20] A. Patil, D. Santhosh, K.V. Sai, M. Sainath, K. Venkataramaniah, Appl. Radiat. Isot. 64 (2006) 693. [21] J. Goswami, B. Chand, D. Mehta, N. Singh, P.N. Trehan, Appl. Radiat. Isot. 44 (3) (1993) 546. [22] J. Katakura, Z.D. Wu, Nucl. Data Sheets 109 (2008) 1655. [23] J.J. Ala-Heikkila¨, Analysis methods for airborne radioactivity, TKK Doctoral Dissertations 129, Helsinky University of Technology, 2008. [24] Y. Iwata, Y. Massayuki, K. Maeda, Y. Yoshizawa, Nucl. Instr. and Meth. A 219 (1984) 123. [25] Y. Jianming, L. Yunzuo, H. Dailing, Z. Phys. A 331 (1988) 391.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Primary radioactivity standardization and gamma intensities determination of 124Sb A. Iwahara , J.U. Delgado, R. Poledna, C.J. da Silva, M.C.M. de Almeida, R.L. da Silva ´rio Nacional de Metrologia das Radiac- ˜ ˜o Nacional de Energia Nuclear (CNEN), Laborato oes Ionizantes (LNMRI)/Instituto de Radioprotec- ˜ ao e Dosimetria (IRD)/Comissa Av. Salvador Allende, s/no, Recreio, CEP 22780-160, Rio de Janeiro, Brazil
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 July 2008 Received in revised form 15 December 2008 Accepted 5 January 2009 Available online 20 January 2009
A solution containing 124Sb was primarily standardized by the 4pbg coincidence and anticoincidence extrapolation methods in the frame of the EUROMET 907 international comparison organized by Laboratoire National Henri Bequerel (LNHB)/France, in 2007. The main purposes of this exercise are the improvement in the uncertainties on the gamma-ray emission intensities and they clarify the discrepancies verified among the intensity values for many weak gamma rays reported in the literature. In this work the results of the activity obtained were used to determine the absolute and relative gamma-ray intensities using a planar and coaxial HPGe detectors calibrated by 152Eu and 116mHo multigamma standard sources covering the energy range from 20 to 1408 keV. Additionally the half-life of 124 Sb was determined following the decay of a solution of 124Sb contained in a glass ampoule over a period of three half-lives using two 4pg ionization chambers. & 2009 Elsevier B.V. All rights reserved.
Keywords: 124 Sb Coincidence Anticoincidence Radioactive decay Photon intensities Half-life
1. Introduction The accurate knowledge of gamma-ray emission probabilities Pg plays important role both in dosimetry and radioactivity applications. In dosimetry the dose estimates for radiopharmaceuticals are based on Monte Carlo radiation transport codes where the nuclear decay scheme and atomic data must be accurately determined in order to have the correct input data spectrum for the theoretical calculations. Incorrect values of Pg adopted for efficiency calibration of detectors could give inconsistent and discrepant outcomes in gamma spectrometry. Multigamma sources such as 152Eu, 154Eu, 56Co, 166Hom and 192Ir are used for efficiency calibration of germanium detectors in energy range above 100 keV. Correction factors for loss counting due to coincidence sum-peaks, pulses pile-up and energy attenuation should be properly applied for these kinds of multi-gamma sources. 133Ba could be used in energy range below 100 keV where the efficiency curve is not linear, but the number of gamma lines with high intensity is very low. In this case several sources should be used, which is time consuming. 124Sb decays by b emission to excited levels of 124Te emitting tens of gamma rays ranging from 148 to 2800 keV besides the emission of KX-rays from 27 to 32 keV as presented in Fig. 1 [1]. Among these gamma rays the main emission is of 602.730 keV with almost 98% and 12 lines else can
Corresponding author. Tel.: +55 21 2173 2879; fax: +55 21 2442 1605.
E-mail address: [email protected] (A. Iwahara). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.021
also be found with intensities higher than 1%. The remainder lines present intensities lower than 1% requiring too much time of measurements in order to minimize the counting uncertainty. Two methods were used for activity determination of the 124Sb solution distributed by LNHB to the participants: the 4pb(PC)–g(NaI(Tl)) coincidence counting method and the 4pb(LS)–g(NaI(Tl)) anticoincidence counting with live time and extending dead-time method. The gamma intensities were determined using two high purity germanium detectors depending on the energy ranges: a planar covering the 15–300 keV range and the coaxial covering the 50–3000 keV range. The half-life was determined using two 4pg-ionization chambers: Centronic IG11 and IG12 models.
2. Experimental procedures 2.1. Preparation of sources The ampoule numbered 7 by the supplier containing 5.17 g of Sb solution was received at our laboratory in March 2007. The chemical composition was SbCl3 in HCl 2 M. The advice sent by organizing laboratory, in adding a drop with equivalent mass of 2.5 mol L1 NaOH on each deposit of radioactive solution of 124Sb in order to avoid volatility, did not work in our laboratory because the beta efficiencies counting in the 4p proportional counter of these sources were quite below of 90%. Thus, ten solid sources for 124
ARTICLE IN PRESS A. Iwahara et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 450–456
3
60.20 days
124 51Sb73
3-
2693.675
3-
2293.72
2+
2039.295
4+
1957.86
2+
1325.515
4+
1248.585
2+
602.730
β
0
0 124 52Te72
Fig. 1. Simplified decay scheme of
124
451
Cox–Isham formulae [6] according to the algorithm developed by Smith [7]. The resolving time of 1.21970.017 ms was fixed in the coincidence unit and determined by using the accidental coincidence method [8]. The b and g dead times, determined by modified source-generator method [9], were fixed to 4.37170.059 and 3.80070.046 ms, respectively, in beta and gamma channels. The 4p counter was filled with a mixture of 90% Ar and 10% CH4 under a pressure of 0.1 MPa. The energy window of the g-channel was set around the 602.7 keV photopeak. The b-efficiency was reduced by adding gold-coated VYNS (polyvinylchloride–polyvinylacetate copolymer) films and aluminized Mylar films on both sides of the source. The maximum b-efficiency obtained was about 94% and decreased down to about 75% in order to get the extrapolation curve. The anticoincidence method was originally developed by Bryant [10,11] in the particular case of nuclides that present meta-stable levels in the decay scheme. To the initial idea of Bryant it was incorporated in the concept of shared dead time for beta and gamma channels by de Carlos and Granados [12]. In 1976, Baerg [13] introduced the use of live time to the anticoincidence system that eliminates the correction of dead time using an extending dead-time device. In 2000, Bouchard, from LNHB/France, presented a device where the concepts of live time and extended dead time were incorporated in a unique electronic module entitled MTR2 [14,15]. The anticoincidence system set up at our laboratory consists of two MTR2 modules that were kindly donated by LNHB and tested its performance in this work. For the generation of the live time a quartz oscillator of 1 MHz is used as reference time. When the system is able to count, a pulse in the entrance of the MTR2 generates a minimum dead time and simultaneously sends a pulse to the counter in order to be registered (C). The dead time is triggered by each arrival of new pulses in the entrance of the MTR2. In this way, the time Ta that the system is free can be described by
Sb.
T a ¼ ðF=F ref Þ C d 4pb(PC)-g coincidence counting were prepared by dispensing 10–20 mg of radioactive solution on conducting VYNS films (gold coated on both sides with a total surface density of E35 mg cm2) stretched on stainless steel rings with 16 and 30 mm inner and outer diameter and 0.2 mm thickness. A drop of TWEEN-20 (1:1000) wetting agent was added to get thin and homogeneous deposits. Eight liquid sources for 4pb(LS)-g anticoincidence counting using OptiPhase ‘HiSafe’ 3, Insta-Gel Plus and Ultima Gold AB cocktail scintillations contained in 20 mL Perkin Elmer vials were prepared for anticoincidence measurements. The mass of radioactive solution varied from 20 to 30 mg and the volume of the cocktail from 3 to 20 mL. Ten point sources as described in Ref. [2] were prepared for gamma intensity measurements using germanium detectors.
2.2. Activity measurements The 4pb(PC)-g coincidence extrapolation method is largely used for radionuclide activity measurements and can be easily found extensively in the literature (for instance Refs. [3–5]). Basically the coincidence extrapolation method consists of plotting (NbNg)/Nc versus (1Nc/Ng)/Nc/Ng in order to obtain the activity No when Nc/NgEeb-0, where eb is the beta counting efficiency and Nb, Ng and Nc are b, g and coincidence count rates, respectively. Nb and Ng were corrected for background, dead time and decay in the usual way, while Nc was corrected by using the
(1)
where Fref is the reference frequency, F is frequency in the counting channel and it represents the time interval during which the system is not paralyzed by dead time, C is the count rate in the counting interval, d is the width of signal pulse entering MTR2. In this context, C d is the correction due to the width of the pulse generated for the MTR2. To reduce such correction the width of the pulse generated for the MTR2 is fixed in 5 ns. In such a way the real count rate will be represented by N ¼ C=T a
(2)
The activity of a radionuclide is determined using Eq. (3), which is the classic equation of the coincidence method differing only in that Nc, the coincidence count that is determined for one given gamma window, is the difference between the gamma rate and uncorrelated gamma rate represented by
A¼
Nb Nw g w
Ng i Nw g
(3)
where Nb is the count rate in the beta channel, N w g is the count rate in the gamma window, i Nw g is the uncorrelated gamma count rate. The same extrapolation technique of coincidence counting is used and the electronic discrimination for beta efficiency
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decreases has been adopted instead of the foil absorption used in the coincidence method.
3. Results and discussion 3.1. Activity
2.4. Half-life measurements One glass ampoule containing 2.70 g of the original 124Sb solution has been prepared for the half-life measurements. Its radioactive decay was carefully followed by measuring the ionization current obtained by the Centronic IG11 and IG12 ionization chambers for over 2 and 3 half-lives, respectively. The logarithm of the experimental ionization current measurements as a function of the time was linearly fitted by least-square method where the slope is the decay constant, l.
3.2. Intensities of gamma rays with Ge spectrometers Two sets of essays for ten point source samples were carried out in July 2007 and November 2007 in order to obtain the X- and g-ray emission intensities related to the decay scheme of 124Sb. Each spectrum accumulated a counting time that varied from 80 000 to 300 000 s. The weaker lines were analyzed in a higher counting period. All data acquisitions were made in the same operational conditions of geometry defined by the efficiency calibration conditions. Measurements of the absolute emission intensities of 124Sb X- and g-rays for lines below 1000 keV were carried out with the two detectors. Both results obtained were averaged. The uncertainty activity of 0.20% was only considered at
Activity concentration (kBq g-1)
Gamma-ray emission probabilities Pg following the decay of Sb were determined at LNMRI using the expression Pg ¼ c/(eX,g, A) where c is the count rate in the photopeak (s1), eX,g is the fullenergy peak efficiency for the X- and g-ray emission and A is the activity of the measured source (Bq) at reference date, traceable to the primary measurement system. Two semiconductor detectors were used: HPGe coaxial-p-type ranging from 50 to 3000 keV, with resolutions of 1.03 and 1.86 keV at 122 and 1332 keV, respectively, and relative efficiency of 50%; Ge planar-ntype has 52 mm2 of active area, with a resolution of 0.64 at 122 keV and 20% of relative efficiency. Both detectors were coupled to a pre-amplifier with fast rise times, a conventional amplifier with a shaping-time of 6 ms, a multichannel buffer analyzer and software for spectral acquisition and data processing. The planar semiconductor is commonly used for the low energy range (15–300 keV). But, in order to enlarge its energy range once that several energies of 124Sb are over 300 keV, a device was built based on a voltage divider. This device was inserted between the pre-amplifier output and the amplifier input [16]. Then, the spectrometric operation range was enlarged up to 1000 keV. The particular photon counter–source distance adopted here allowed to neglect the counting losses due to pileup, dead time as well as g–g and X–g summing coincidence effects. Some counting corrections such as decay, sample positioning and weighting were taken into account. Each detector was calibrated on efficiency for source-to-detector distance of 20 cm using gamma rays standardized point sources of 152Eu and 166m Ho, whose total relative standard uncertainty in the 100–1000 keV region varied from 0.5% to 1.5% (k ¼ 1), for both detectors. The activities of these sources have been measured by primary methods at LNMRI. The nuclear data for these multi-gamma standards were taken from Bernardes et al. for 166m Ho [2], Silva et al. for 152Eu [17] and Be´ et al. [18]. The experimental points of the efficiency curves in function of energy between 20 and 1500 keV of germanium spectrometers have been fitted to the experimental data through a polynomial function where the coefficients are determined by the least-square procedure. The better degree was third degree and evaluated by w2 statistical test. But the efficiency responses (e) corresponding energies (E) above 1500 keV for coaxial semiconductor were obtained for extrapolation and adjusted by means of the potential curve: e ¼ aEb which could be transformed to the general linear form (linear log–log extrapolation method discussed by Debertin and Helmer in Ref. [19]). The accuracy of this method could be further improved if a large number of data points are used to define the efficiency curve once its slope is almost constant between 200 and 3000 keV. An uncertainty of the efficiency in this energy range has been estimated to vary from 2.5% to 5%, provided that the last six data calibration points are sufficiently accurate. 124
Figs. 2 and 3 show the efficiency extrapolation curves for 124Sb from the coincidence and anticoincidence measurements in which the bars denote one standard uncertainty (k ¼ 1). The correlation coefficients of 0.95 and 0.99 for coincidence and anticoincidence fits, respectively, can be related to the linearity of the measurement systems. The activity results as well as the uncertainty budgets are shown in Tables 1 and 2, respectively. Both results were in agreement within the limits of the evaluated uncertainties showing the coherence of the coincidence and anticoincidence activity measurements for 124Sb. The lower uncertainty for anticoincidence result was expected because some uncertainty components present in coincidence measurements such as dead time, resolving time and delay mismatch are removed. The final result of activity concentration (1710.8 kBq g1) on the reference date of February 15, 2007, 12h00 UTC was evaluated as the weighted mean of the two results listed in Table 1. The external and internal uncertainty was calculated as uext ¼ 1.1 kBq g1 (0.06%) and uint ¼ 3.3 kBq g1 (0.20%), respectively, and the latter was adopted as the overall standard uncertainty of the final activity value.
1725 1720 1715 1710 0.00
0.10
0.20 (1- εβ) /εβ
Fig. 2. Efficiency extrapolation curve of
Activity concentration (kBq g-1)
2.3. Gamma intensities measurements
0.30
0.40
124
Sb for coincidence counting.
1780 1760 1740 1720 1700 1680 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 (1− εβ) / εβ
Fig. 3. Efficiency extrapolation curve of
124
Sb for anticoincidence counting.
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Table 1 Activity concentration of Measurement method
Coincidence Anticoincidence
453
124
Sb solution. Activity concentration (kBq g1)
1712.6 1710.2
Table 2 Uncertainty components, in % of the activity concentration. Contributions due to
Coincidence Anticoincidence
Counting statistics Weighing Background Dead time Resolving time Delay mismatch between the beta and gamma pulses Half-life Extrapolation of efficiency curve Live time Relative combined standard uncertainty
0.24 0.05 0.05 0.012 0.05 0.24
0.11 0.05 0.06 – – –
0.16 0.02 – 0.38
0.02 0.18 0.01 0.24
the end of the calculation of the weighted average. The resulting absolute and relative photon emission probabilities for X- and g-rays emitted by 124Sb are given in Table 3 in order of energy. Relative intensities were converted from the absolute probability, whose measured value is 97.5270.72%, normalized to 100% for 602.7 keV photopeak. All these results were evaluated with a relative uncertainty estimated at the 1s confidence level that consisted in type A component due to counting statistics in peak area determination after background correction (that varied from 0.1% for the most intense peaks up to 50% for the weaker peaks); and type B components that originate from efficiency curve (0.5–2.5%) and source standardization by coincidence methods. Others small components due to correction factors were considered in the reported uncertainty. Here the values reported recently by some authors were also included. Twelve weak gamma lines of energy that appear in the evaluated data and compiled previously by other studies in the literature were not seen in the present work, according to Table 3. In this table, all the g-rays quoted in the literature are listed, filled or not in the corresponding line. With regard to the X-rays of low intensities, our data can only be compared with Patil et al. [20] and Goswamy et al. [21], which are in good agreement and with smaller uncertainties than these authors. The comparison of the stronger g-ray (higher than 1% for intensities and below 1500 keV) appears to be consistent and confirms the values published, which are satisfactory, having an uncertainty between 0.7% and 2.8%. The results measured for strong and weak peaks in a coaxial and a planar detector for energies below 1000 keV showed consistent intensities. The values for strong lines situated above 1500 keV exhibit deviation of several percent among the most recent measurements, except for results measured by NDS 109 compilation [22] and Patil et al. [20] and this work, where there is a good agreement. However, it can be seen in Table 3 high divergences for many weak gamma rays reported in different publications mainly those lines above 2283.3 keV. The lines situated from 2 MeV to 2283.3 keV present consistencies within the same order of magnitude. Regarding to the 2015.7 keV line the NDS 109 compiled data presents a value 10 times higher than the other authors. There should be a mistake in
Relative uncertainty (%) Type A (uA)
Type B (uB)
Standard uncertainty (u)
0.24 0.10
0.30 0.22
0.38 0.24
this compiled data. The origin of this inconsistency for higher energies (above 2283 keV) can be attributed to the process of obtaining detector efficiency response by extrapolation method in regions where there are a few reference energies. The response curve in this region, although continues to decrease, is perhaps found to follow a different pattern due to the dominant pair production interactions in the presence of escape peaks. Otherwise, it is known that it is hazardous to extrapolate the Ge gamma efficiency curve above the highest energy used in the calibration and, thus, the high energy gamma intensities measured can be uncertain to an unknown degree. According to the recent publication of Ala-Heikkila¨ [23], the linear log–log extrapolation method is considered adequate for this situation. In any case efficiency extrapolation outside of data above 1500 keV is prone to errors and should be used with care. The most common highest energy calibration point is the 1836 keV peak of 88Y, but this nuclide was not available in our laboratory. 3.3. Half-life The ion current originated by the 124Sb sample solution contained in a glass ampoule was measured during 120 days by IG11 ionization chamber and 200 days by IG12 chamber. The halflife was obtained from a linear least-squares fit, as physically required, of the logarithm of the activity (which is proportional to the ion current originated) as a function of the time. Fig. 4 displays, for illustration, the plot of minus natural logarithm of the ionization current Ln(I) measured in the IG12 ionization chamber versus decay time while Fig. 5 shows the respective residuals of the experimental points in comparison with the fitted values. The correlation coefficients showed the goodness of linear fit and terms higher than linear do not improve the fit. The value of the half-life employing the slope of the linear fit was determined as 60.39 and 59.93 d, from IG11 and IG12, respectively. The type A components come from the fitting procedure and are given by T1/2(Dl/l) where T1/2 is the half-life and l is the slope. The type B component results from the instability of the ionization chambers and was estimated as 0.03% and 0.05%, respectively, for IG11 and IG12, by measuring a long-lived source of 226Ra during the experiment. The statistical parameters from linear least-squares fit are shown in Table 4. No g-emitting impurities have been detected by spectrometric check using germanium detector and also corroborated by good linearity of the fitting. The final value of T1/2 was calculated as weighted mean of these two values giving 60.0770.21 d which agrees with the value of 60.2070.03 d [18] recommended in this comparison exercise.
4. Conclusions The coherence between the coincidence and anticoincidence methods for the determination of activity concentration of 124Sb has been demonstrated in this work. Both results agree within the evaluated standard uncertainties and the anticoincidence result
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Table 3 Gamma emission probabilities of Energy (keV)
Sb per 100 decays.
Absolute photon emission probabilities This work
27.3 30.9–31.8 148.2 158.9 189.6 210.3 254.3 291.4 335.8 346.1 370.4 385.9 400.0 443.9 468.6 476.9 481.1 498.4 525.4 530.3 553.8 571.6 602.7 632.3 645.8 662.4 709.3 713.7 722.7 735.7 765.8 775.2 790.7 816.8 856.9 899.6 937.9 968.1 976.2 997.7 1014.5 1045.1 1053.8 1086.3 1097.0 1163.2 1198.0 1205.5 1235.0 1253.4 1263.1 1269.0 1301.3 1325.5 1355.1 1368.1 1376.1 1385.1 1418 1428.7 1436.5 1445.0 1453.2 1488.8 1505.6 1528.1 1557.0 1565.8 1579.7 1622.4 1657.0 1690.9 1720.3
124
0.35(7) 0.087(17) 0.012(5) 0.005(1) 0.002(1) 0.0086(12) 0.0089(14) 0.0045(8) 0.077(7) 0.0033(16) 0.033(10) 0.04(2) 0.124(8) 0.186(4) 0.052(9) 0.045(16) 0.023(7) 0.037(14) 0.139(4) 0.042(5) 0.019(5) 0.020(4) 97.52(70) 0.098(7) 7.39(6) 0.040(3) 1.325(18) 2.21(3) 10.54(8) 0.134(5) 0.011(3) 0.010 (2) 0.737(7) 0.079(6) 0.0198(12) 0.022(9) 0.020(5) 1.86(5) 0.082(5) 0.024(7) – 1.84(3) – 0.040(4) 0.034(8) – – – 0.027(6) 0.041(9) 0.042(4) – 0.031(4) 1.60(3) 1.04(3) 2.58(4) 0.51(2) 0.070(6) – 0.048(7) 1.22(2) 0.328(7) 0.031(7) 0.67(2) 0.051(16) 0.410(18) – – 0.43(2) 0.039(3) – 45.56(11) 0.095(5)
Goswamy [21] 0.357(17) 0.082(10) 0.0037(7) – 0.0065(13) 0.0054(10) 0.0159(10) 0.0086(10) 0.073(3) 0.0059(13) 0.033(10) –
Relative intensity Iwata [24]
NDS 109 [19]
Jianming [25]
– – – – – – – – – –
– –
– – 0.0040(8)
– 0.0065(11) 0.0056(9) 0.0165(10) 0.0089(8) 0.076(3) – 0.051(9)
– 0.121(3) 0.188(5) 0.046(3)
– 0.024(2) – 0.137(3) 0.042(2) – 0.0190(13) 97.8(17) 0.105(2) 7.38(10) 0.031(2) 1.31(3) 2.22(4) 10.5(2) 0.126(3) 0.0121(5) 0.0091(18) 0.74(2) 0.072(3) 0.024(1) 0.0171(14) – 1.88(3) 0.083(2) – – 1.83(3) 0.005(2) 0.037(2) – – – – – – 0.041(2) – 0.034(2) 1.57(3) 1.03(2) 2.58(4) 0.48(1) 0.06(2) – – 1.22(2) 0.327(7) – 0.671(14) – 0.412(8) – 0.015(4) 0.424(8) 0.041(2) – 48.2(7) 0.094(3)
0.129(16) 0.205(10) 0.058(8) – – – 0.117(12) – – – 100.0(4) 0.114(6) 7.61(3) 0.016(5) 1.399(11) 2.338(12) 11.02(4) 0.133(9) – – 0.758(9) 0.079(6) 0.029(7) 0.016(9) – 1.919(13) 0.088(8) – – 1.864(17) – 0.039(9) – – – – – – – – 0.041(15) 1.584(22) 1.042(27) 2.67(3) 0.501(20) 0.061(26) – – 1.225(24) 0.358(17) – 0.679(19) – 0.410(24) – – – 0.035(12) – 48.58(16) 0.097(7)
0.3681(55) 0.0852(15) 0.0061(20)
– –
–
0.142(7) 0.1932(20) 0.051(3)
– 0.0257(15) – 0.155(13) 0.204(10) 0.053(3)
– 0.0242(19)
–
0.002(36) 0.0147(5) 0.0137(6) 0.0070(6) 0.0760(18)
0.0356(61) –
–
– –
0.0062(28) 0.0214(41) 0.0122(61) 0.086(6)
0.039(5) –
Patil [20]
0.1249(66) 0.1930(10) 0.0364(23) –
0.029(8) –
0.141(4) 0.0431(20)
0.0205(10) –
0.165(10) 0.047(11)
0.1429(75) 0.0421(12)
–
–
–
0.0194(13) 100.00(23) 0.107(1) 7.588(24) 0.030(4) 1.384(12) 2.327(17) 11.00(4) 0.130(7)s 0.0124(2) 0.0096(17) 0.756(5) 0.0745(18) 0.0243(10) 0.0176(14) – 1.925(9) 0.0851(16) – – 1.874(11) 0.005(2) 0.0387(18) – – – – – – 0.0422(18) – 0.0351(10) 1.616(15) 1.061(13) 2.683(13) 0.494(5) 0.064(3) – – 1.244(8) 0.337(4) – 0.687(6) – 0.418(5) – 0.014(3) 0.39(5) 0.0418(10) – 48.64(15) 0.0972(17)
0.025(10) 100.0(10) 0.101(6) 7.55(10) 0.035(11) 1.38(4) 2.29(5) 10.99(15) 0.145(21) 0.0092(41) 0.0112(41) 0.753(11) 0.074(7) – 0.02(6) – 1.945(23) 0.088(5) – – 1.897(24) – 0.043(5) – – – – – – 0.043(5) – 0.039(5) 1.645(23) 1.103(18) 2.696(31) 0.496(10) 0.071(16) – – 1.236(17) 0.346(10) – 0.709(54) – 0.434(9) – 0.013(4) 0.419(41) 0.04(4) – 48.73(61) 0.102(4)
0.0184(10) 100(1) 0.0990(9) 7.685(52) 0.0148(10) 1.394(16) 2.288(24) 10.877(118) 0.1399(19) 0.0039(3) 0.0119(12) 0.7664(89) – 0.0216(11) 0.0200(12) – 2.1051(225) 0.0841(10) – – 2.0257(95) – 0.0358(16) – – – – – – 0.0482(14) – 0.0256(13) 1.707(19) 1.093(13) 2.700(21) 0.543(5) 0.064(2) 0.005(2) 0.005(2) 1.270(12) 0.335(32) – 0.692(5) 0.008(1) 0.451(4) – – 0.460(4) 0.0477(12) – 46.627(45) 0.097(18)
This work 0.36(8) 0.089(18) 0.012(5) 0.005(1) 0.002(1) 0.0090(10) 0.0090(10) 0.005(1) 0.079(7) 0.0030(13) 0.034(9) 0.038(18) 0.127(8) 0.190(4) 0.053(8) 0.046(14) 0.024(7) 0.038(13) 0.143(4) 0.043(5) 0.019(5) 0.021(4) 100.0(6) 0.100(7) 7.57(6) 0.041(3) 1.358(18) 2.26(3) 10.81(8) 0.137(5) 0.012(3) 0.0101(16) 0.756(7) 0.081(6) 0.020(1) 0.022(8) 0.021(5) 1.91(5) 0.084(5) 0.025(7) – 1.88(3) – 0.041(4) 0.034(7) – – – 0.028(6) 0.042(8) 0.043(4) – 0.032(4) 1.64(3) 1.07(3) 2.65(4) 0.52(2) 0.072(6) – 0.049(7) 1.25(2) 0.336(7) 0.032(7) 0.69(2) 0.052(14) 0.421(15) – – 0.44(2) 0.041(3) – 46.72(9) 0.098(5)
ARTICLE IN PRESS A. Iwahara et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 450–456
455
Table 3 (continued ) Energy (keV)
Absolute photon emission probabilities This work
1757.9 1851.5 1918.8 1950.4 2015.7 2039.3 2078.6 2090.9 2099.1 2108.0 2151.5 2172.1 2182.6 2203.0 2224.8 2253.9 2274.0 2283.3 2293.7 2323.1 2454.4 2682.0 2693.6 2808.0 2871.0
Goswamy [21]
– –
0.005(2) 0.0061(9) 0.054(2)
0.055(4) –
–
0.0126(16) 0.062(4) 0.028(3) 5.15(19) 0.045(3) 0.051(3) – – 0.042(3) 0.030(2) 0.019(1) – – 0.023(14) 0.080(7) 0.010(4) 0.009(3) 0.007(3) 0.005(2) 0.007(3) –
Relative intensity Iwata [24]
NDS 109 [19]
Jianming [25]
Patil [20]
This work
– –
–
–
–
– –
0.0066(13) 0.0557(16)
0.052(4) –
0.011(1) 0.065(2) 0.026(2) 5.62(10) 0.046(1) 0.044(2) – –
– 0.0093(26) 0.059(3) 0.0157(14) 5.59(2) 0.045(6) 0.044(3)
– – 0.043(2)
– – – –
–
– 0.097(5) 0.0656(19) 0.021(4) 5.61(3) 0.0467(9) 0.0443(13)
– 0.040(2)
0.0041(13) 0.031(10)
0.0022 0.045(2)
– – – 0.0027(19) – –
– – – – 0.0082(12) 0.0327(10) 0.00249(16) 0.0015(4) 0.00169(17) 0.0031(5) 0.0015(2)
–
0.056(4)
–
–
– – – –
0.0026(10) 0.058(16)
0.0124(7) 0.068(2) 0.0163(25) 5.69(9) 0.046(2) 0.044(2)
0.0021(4) 0.0434(10)
– – – – 0.0099(8) 0.057(3) 0.0026(3) 0.0018(2) 0.0020(4) 0.0046(7) 0.0015(2)
0.0112(31) 0.06(3)
– 0.0090(9) 0.0661(19) 0.0741(18) 5.397(52) 0.0572(12) 0.0501(7)
0.0131(15) 0.063(4) 0.029(3) 5.28(16) 0.046(3) 0.052(3)
– –
– – 0.036(7) 0.0310(6)
0.043(3) 0.031(2) 0.021(1)
– 0.0006(2)
– –
– 0.0076(14) 0.031(5) 0.0025(7) 0.0016(6) 0.0018(6) 0.0026(16) 0.0020(8)
–
0.0422(9) 0.056(23) 0.0060(3) 0.0092(1)
0.024(13) 0.082(7) 0.010(3) 0.009(3) 0.007(3) 0.005(2) 0.007(3)
– 0.003(1) 0.0093(4) –
–
s: Sum of 735.7+735.9 keV lines.
6.00
Table 4 Least-square fitting parameters obtained from Ionization Slope (l) chamber
2.00
IG11 IG12
Ln (I)
4.00
Range of relative residuals variation (%)
0.011472(11) 0.6 to 0.2 0.011579(5) 0.10 to 0.10
124
Sb decay.
Coefficient of Reduced Degree of correlation (w2) freedom
0.9999 0.9999
2.489 0.706
23 40
0.00 0
50
100 150 Decay time (d)
200
250
Fig. 4. Linear least-square fit of 124Sb data of IG12 ionization current (I) as function of time.
Relative residuals
0.12 0.08 0.04 0.00 -0.04
availability of radionuclides with high gamma energies such as Na, 88Y, 26Al and 56Co would improve the accuracy of Pg measurements avoiding the extrapolation technique used in this work for Pg above 1500 keV. These results also suggest that there is a need to conduct an energy determination project, in addition to the activity and emission probability measurement project, for 124Sb standardization. The value of the half-life determined by the ionization chambers agrees within the quoted uncertainty with that recommended for this comparison (60.2070.03 d).
24
References
-0.08 -0.12 0
50
100
150
200
250
Time (d) Fig. 5. Residuals of the experimental points and the fitted values for IG12 measurements.
gave smaller uncertainty as expected. The measured emission probabilities and its associated uncertainties for gamma and X-rays in this work are found, in general, to be consistent with those reported in literature, unless for weaker ones as expected which are not suitable for using in the calibration of detectors. The
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