Precision electron–gamma spectroscopic measurements in the decay of 177Lu

Applied Radiation and Isotopes 69 (2011) 869–874

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Precision electron–gamma spectroscopic measurements in the decay of 177Lu S. Deepa n, K. Vijay Sai, R. Gowrishankar, Dwarakarani Rao, K. Venkataramaniah Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam 515 134, India

a r t i c l e i n f o

abstract

Article history: Received 11 April 2009 Received in revised form 14 January 2010 Accepted 4 February 2011 Available online 26 February 2011

Complete precision electron and gamma spectroscopy measurements were undertaken for the 6.647 day decay of 177Lu. Precision measurements of the K, L, and M internal conversion coefficients (ICCs) were made using a high transmission Mini Orange Electron Spectrometer coupled to a Si(Li) detector. We also report energies and intensities of the 6 gamma rays measured with a large volume 60 cc HPGe detector and 9 X-rays with better precision. The energies and intensities of the beta transitions from 177 Lu leading to the levels in the daughter 177Hf were calculated by transition intensity balance at the levels. & 2011 Elsevier Ltd. All rights reserved.

Keywords: 177 Lu decay Eg Ig Eb Ib ICC

1. Introduction The 6.647 day decay of the beta emitter 177Lu (ground state: 7/2 + , Nilsson configuration 7/2 + [404]) to levels in 177Hf has been extensively studied (El-Nesr and Bashandy, 1962; Alexander et al., 1964; Agnihotry et al., 1974; Hnatowicz, 1981; Mehta et al., 1987; ¨ Schotzig et al., 2001). The decay with a Q-value of about 498.3 keV primarily goes to the ground state of 177Hf. The ground state of 177 Hf has Jp ¼7/2  (Speck and Jenkins, 1956) with the Nilsson configuration 7/2  [514] assignment. Three excited levels of the daughter nucleus are populated, but to a relatively smaller extent. The excited levels at 112.95 keV (Jp ¼9/2  ) and 249.67 keV (Jp ¼11/2  ) are the excited members of the ground state band. An additional level at 321.32 keV (Jp ¼9/2 + ), band head with Nilsson configuration 9/2 + [624] is the highest level fed by this beta decay. The relatively simple decay scheme is shown in Fig. 1. Though extensive studies were undertaken by various researchers, the experimental data available from previous measurements on intensities, conversion coefficients, etc. show some discrepancies with theoretical values and are in general, not in good agreement with each other. There is also some ambiguity in the literature regarding the direct b  -decay feeding to the (Jp ¼11/2  ) level at 249.67 keV. Haverfield et al. (1967) and Alexander et al. (1964) had reported a low intensity beta feeding ¨ to this level but in the more recent measurement of Schotzig et al.

n

Corresponding author. E-mail address: [email protected] (S. Deepa).

0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.02.012

(2001) there is no report of a direct feeding. Our present work was undertaken to resolve these ambiguities. For this, we tried to precisely determine the energies and intensities of the gammas and internal conversion electrons emitted in the depopulation of excited levels in 177Hf. Further, with intense and high energy beta emission, 177Lu can readily serve as a useful tool for radiotherapeutic applications. Also, since the conversion spectrum of 177 Hf has well spaced, distinct and intense peaks it can be used for the calibration of electron detectors in the low energy region.

2. Experiments 1 ml of carrier-free radioisotope 177Lu, produced by the neutron irradiation 176Lu(n,g) reaction of enriched 176Lu in the form of Lutetium Chloride (LuCl3) in HCl solution, was obtained from the Board of Radiation and Isotope Technology, BARC, Mumbai, India, with an activity of 345.2 MBq. The source had a contaminant 177mLu (half life 160.9 days) with activity 7.2(5)  10  5 times that of 177Lu on the reference date and was corrected for. The gamma spectra of the 177Lu samples were examined for other radioisotopic impurities produced in the irradiation process or intrinsically present in the original enriched 176Lu sample. No impurities were detected in the sample. For the electron spectroscopy, it is imperative to make the source extremely thin in order to avoid back scattering and self-absorption of electrons in the source. Accordingly, very thin uncovered sources with count rates between 500 and 1000 counts were prepared by drying the source solution on aluminized Mylar backing, supported on an aluminum ring of diameter 1.0 cm.

870

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Decay Scheme 7/2+

0.0

Intensities: I (γ+ce) per 100 parent decays

6.647 d

177 Lu106 71

Q = 498.3

Iβ–

8

Log ft

11.61 <0.012

6.083 >9.21u

9/2+ 11/2–

9. 0

7. 273

9/2–

79. 4

6. 697

7/2–

32 201.315 718.36 9 E 24 .64162 E1+M 9 8 1 2 13 .674 E1++M2 0.22 6.7 2 E M 1 5 11 2 2.9 45 2 0. 20. 1 49 33 2 M 8M 0 1+ 29 E2 1+ 0 E2 . 1 01 20 .2

%β– = 100 –

321.3163 249.6745

0.663 ns 105 ps

112.9500

0.537 ns

0.0

stable

177 Hf105 72

Fig. 1. Adopted decay scheme of 177 Hf (NDS 2003).

177

Lu with Qb  ¼498.3(8) keV featuring the branching ratios, log ft values and the gamma energies, intensities, and level energies of

The Mini orange spectrometer used for the beta spectrum and conversion electron measurements, comprises of a mini orange filter of 9 thin wedge shaped permanent magnets fixed in a circular array in a brass frame of diameter 16.2 cm, with a lead central absorber that shields the detector from gamma rays emitted by the source. A windowless Si(Li) detector (surface area¼ 78 mm3, sensitive depth¼5.3 mm, and FWHM¼1 keV at 115 eV and 2.3 keV at 624.5 keV) was coupled to the mini orange electron transporter to complete the magnetic spectrometer. A clean vacuum of about 10–7 mbar was maintained within the non-magnetic stainless steel (304 L) casing that houses the filter and the detector. Standard sources were used to obtain the transmission curves of the spectrometer for electron energies up to 500 keV. The optimized transmission of electrons obtained for source to magnet distance of 7.5 cm and magnet to detector distance of 4.5 cm was employed for calculating the relative conversion electron intensities. The gamma energy and intensity measurements were made with a large volume 60 cc HPGe detector (FWHM¼600 eV at 5.9 keV of 55Fe and 1.8 keV at 1.33 MeV of 60Co, relative efficiency¼10% at 1.33 MeV of 60Co) coupled to a PC based 8 K MCA. The detector was calibrated and optimized for relative photopeak efficiency and energy linearity with standard sources obtained from IAEA.

3. Gamma spectra The Gamma singles spectra were acquired at a source-detector distance of 25 cm for counting periods lasting up to 8.96  105 s during which a total of 1.2  109 counts were registered. The intensities and energies were obtained from the counts in the full energy peaks and the relative efficiencies of the detector by making use of the computer codes GAMMAVISIONs (EG&G, ORTEC, 1998) and FIT (Petkov and Bakaltchev, 1990). The energies of the six well known gammas ranging from 71 to 321 keV were measured with relatively better precision. The most intense 208 keV line was used for normalization. The uncertainties in the relative intensities of the gammas detected with our gamma spectrometer system result from errors in the peak area determination, absorber corrections, summing corrections and efficiency errors arising from interpolation and emission rates of calibration sources. Presently, the relative intensities were determined with less than 1% error on an average. Table 1(a) and 1(b) compare

Table 1(a) Gamma energies obtained in the present study in comparison with the earlier standard measurements. Gamma energies, Ec (keV) Maier (1965)

Hnatowicz (1981)

Matsui et al. (1989)

Present

71.646(2) 112.952(2) 136.730(6) 208.359(10) 249.868(25) 321.330(40)

71.646 112.95(2) 136.72(2) 208.35(2) 249.7(5) 321.27(5)

71.6418(6) 112.9498(4) 136.7245(5) 208.3662(4) 249.6742(6) 321.3159(6)

71.6441(13) 112.9468(2) 136.7242(37) 208.3661(3) 249.673(8) 321.327(1)

Table 1(b) Comparison of our gamma intensities with Agnihotry et al. (1974), Mehta et al. ¨ (1987), and Schotzig et al. (2001). Present measurements show an improvement in precision. Ec (keV)

71.6441(13) 112.9468(2) 136.7242(37) 208.3661(3) 249.673(8) 321.327(1)

Gamma intensity, Ic Agnihotry et al. (1974)

Mehta et al. (1987)

¨ Schotzig et al. (2001)

Present

1.50(10) 60(5) 0.52(5) 100 1.90(20) 2.00(20)

1.71(5) 59.6(11) 0.457(8) 100 2.00(3) 2.17(4)

1.674(21) 59.6(6) 0.448(8) 100 1.918(17) 2.002(19)

1.780(7) 62.15(5) 0.543(6) 100.0(8) 1.982(4) 2.470(7)

energies and intensities, respectively, of our results with earlier measurements.

4. X-ray spectra The intense K and L X-ray lines were acquired with a high performance, thermoelectrically cooled Si-PIN photodiode X-ray detector (size¼ 5 mm2  680 mm) of FWHM¼ 186 eV at 5.9 keV of 55 Fe for a total duration of 5.7  105 s with 6.1  107 counts. The energy and relative efficiency calibration were carried out with standard IAEA sources. The partial spectra showing the prominent K and L X-rays from the experiment are given in Figs. 2(a) and (b). The X-rays were also obtained as multiplets in the gamma

S. Deepa et al. / Applied Radiation and Isotopes 69 (2011) 869–874

871

spectrum taken with the HPGe detector. Our X-ray intensities when normalized with respect to the intensity 2.79(4) of the Ka1 line agree very well with previous results. They also have much smaller uncertainties. The values of energies and intensities of the

9 X-ray lines (5 L-shell and 4 K-shell) are reported in Table 2. The ¨ results of an earlier measurement by Schotzig et al. (2001) and the adopted values from Nuclear Data Sheets for A¼177 (Kondev, 2003) are also shown.

Fig. 2. (a) and (b) X-ray spectra taken with the Si-PIN photodiode X-ray detector (size¼ 5 mm2  680 mm) showing the K X-rays and some of the intense L X-rays of 177Hf.

Fig. 3. (a) and (b) Partial conversion electron spectra obtained in our experiment. Prominent conversion lines have been labeled.

Table 2 Values of energies and intensities of the 5 L-shell and 4 K-shell X-ray lines. X-ray energies in the first column are from differences between atomic-shell binding energies. ¨ Results are compared with those of Schotzig et al. (2001). X-ray energies (keV)

6.96 (L1) 7.96 (La, LZ) 9.03 (Lb1,L b3, Lb4, Lb6) 9.34 (Lb215) 10.62 (Lg1, Lg6) 10.86 (Lg2, Lg3) 54.612 (Ka2) 55.791 (Ka1) 63.243 (Kb1) 64.942(Kb2)

X-ray energies (keV)

Intensity

¨ Schotzig et al. (2001)

Present

¨ Schotzig et al. (2001)

NDS 2003

Present

7.0 7.9 9.0 9.4 10.5 11.0 54.6 55.8 63.2 65.2

7.06(1) 7.956(1) 9.033(1) 9.345(1) 10.462(2) 10.763(13) 54.609(13) 55.801(8) 63.187(2) 65.048(6)

0.0734(24) 1.505(27) 1.333(24) 0.273(6) 0.231(6) 0.0223(14) 1.551(25) 2.72(5) 0.883(12) 0.238(4)

0.076(5) 1.518(25) 1.333(24) 0.273(6) 0.231(6) 0.0223(14) 1.60(5) 2.79(5) 0.896(17) 0.245(7)

0.073(1) 1.518(11) 1.373(6) 0.294(6) 0.238(4) 0.030(5) 1.60(4) 2.79(4) 0.895(4) 0.205(4)

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S. Deepa et al. / Applied Radiation and Isotopes 69 (2011) 869–874

Table 3 Conversion electron intensities and ICCs for the six transitions in first time. Ec (keV)

71.644(1)

112.947(1)

136.724(4)

208.366(1)

Ic

Ice

1.780 (7)

62.15 (5)

0.543 (6)

100.0 (8)

177

Hf. All the K, L, and M ICCs have been determined, out of which five have been reported for the

ICC

Multi-polarity

Agnihotry et al. (1974)

West et al. (1961)

Jeltema and Bernthal (1974)

Present

40(9) 3(1) 1.2(6)

K 0.90 (11)  

  

  

K 0.98 (22) L 0.081 (28) M 0.029 (14)

E1+ M2

916(32) 1541(130) 464(34)

K 0.78 (5)  

K 0.85 (11)  

K 0.63 (10)  

K 0.634 (22) L 1.06 (9) M 0.321 (23)

M1+E2

7(2) 6(1) 3(1)

K 0.49 (8)  

K 0.71 (29) L 0.67 (27) 

K 0.42 (4) L 0.39 (3) 

K 0.58 (15) L 0.44 (11) M 0.25 (7)

M1+E2

100(4) 15(2) 3.71(28)

K 0.046(4)  

K 0.043 (2) L 0.0071 (3) M 0.0020(1)

K 0.043 (4) L 0.0070 (6) 

K 0.043 (2) L 0.0064 (9) M 0.0016 (1)

E1+ M2

249.673(8)

1.982 (4)

7(2) 3(2) 2.01(28)

K 0.101(9)  

K 0.148 (23) L 0.057 (9) M 0.018 (3)

K 0.090 L 0.035 (3) 

K 0.16 (5) L 0.08 (3) M 0.043 (6)

E2

321.327(1)

2.470 (7)

7.43(98) 2.29(85) 0.27(8)

K 0.102(13)  

K 0.080 (6) L 0.0176(16) M 0.0042 (9)

K 0.078 (7) L 0.017 (2) 

K 0.13 (2) L 0.040 (15) M 0.0047(13)

E1+ M2

Table 4 Mixing ratios for the two M1+ E2 transitions in

6. Calculation of beta intensities 177

Hf.

Eg (keV)

9d9

Present

Krane et al. (1974)

Agnihotry et al. (1974)

Present

112.9468(2) 136.7242(37)

4.7(2) 3.0(7)

3.99(25) –

4.08(14) 2.60(67)

5. Electron spectra The beta and conversion electron spectra were acquired for a period of 4.14  105 s with a total of 1.8  108 counts, with the Mini Orange Electron spectrometer system described before. Partial Electron Spectra featuring the prominent conversion lines are shown in Fig. 3(a) and (b). The ICCs were determined by the Normalized Peak to Gamma (NPG) method (Hamilton, 1975). The K conversion coefficient of the 208.37 keV gamma with aK ¼0.043(2) was used for normalization. The 208 keV transition has a mixed character (E1+M2) and thus may be considered not ideal for normalization. However, from previous works on 177Lu, the multipolarities and mixing ratios of this intense transition are well known, and therefore, the theoretical BRICC values from the tables of Band et al. (2002) were used. Besides, this choice was made to enable comparisons of our results with those previously reported. A total of eighteen ICCs of the K, L,- and M shells of all the six de-exciting transitions in 177Hf were determined with good precision, out of which five are reported for the first time. The values of conversion electron intensities and ICCs from the present study are listed in Table 3 along with the corresponding values obtained by other researchers. The uncertainties in the reported values are due to peak area calculations of the nonGaussian peaks, errors of interpolation of the transmission curve of the spectrometer, absorption corrections and errors in the emission rates of standard sources. The overall calculated uncertainties were found to be comparatively higher for the weaker transitions. Table 4 shows mixing ratios for the M1+E2 transitions calculated from the presently measured conversion coefficients.

The beta decay branching ratios from the ground state of 177Lu to levels in 177Hf were calculated from the total transition intensity balance at each level of the daughter. The transition intensity balance calculations were performed using the computer code GTOL (2007) from the total gamma and conversion electron intensities feeding and depopulating each level in the decay scheme. The total gamma intensities were taken from the present study. The total theoretical conversion coefficient values were taken from the Nuclear Data Sheets (Kondev, 2003) with a relative uncertainty of 3% assigned to those values that were given without any uncertainty. A second independent calculation, using the gamma intensities and the total conversion coefficient values (aT ¼ aK + aL + aM) with their respective uncertainties, determined in the present measurements was also carried out. The resulting branching ratios are summarized in Table 5. These are in good agreement with those of Haverfield et al. (1967) and Alexander et al. (1964). In particular, the above groups had reported a higher value of beta feeding to the 249 keV level of 177 ¨ Hf. More recent work in literature (Schotzig et al., 2001), however, had calculated the transition probability to this level to be zero within uncertainties and reported the beta transition intensities only to the other three levels. In the Nuclear Data Sheets for A¼177 (Kondev, 2003), branching to this 249 keV level with Jp ¼11/2  has been evaluated to be o0.012(8). This agrees with the present values obtained from our calculations, thereby confirming a low intensity beta feeding to this level. The revised decay scheme, constructed with our gamma intensities and theoretical total conversion coefficients from Nuclear Data Sheets (Kondev, 2003) by the using the computer code ENSDAT (2006), is given in Fig. 4. The corresponding Logft values for these transitions calculated by the NNDC Log ft program are listed in Table 6. For comparison, similar results from earlier standard works are included.

7. Discussion of results Complete and precise electron and gamma spectroscopy measurements have been undertaken for the 6.647 day decay of 177Lu.

S. Deepa et al. / Applied Radiation and Isotopes 69 (2011) 869–874

Table 5 Beta intensities to the ground state and the excited states of Reference

177

Hf.

Ib from ground state of

Present calculation with aT from NDS Present calculation with (aK + aL + aM)exp ¨ Schotzig et al. (2001) Haverfield et al. (1967) Alexander et al. (1964)

873

177

Lu to levels of

177

Hf with

Jp ¼ 7/2 

Jp ¼9/2 

Jp ¼ 11/2 

Jp ¼ 9/2 +

78.4(3) 80.06(24) 79.3(5) 87.2(11) 86.3(13)

9.9(3) 8.362(7) 9.1(5) 6.0(8) 7(1)

0.002(7) 0.005(8) – 0.07(2) 0.03(3)

11.68(14) 11.58(13) 11.58(12) 6.7(3) 6.7(3)

Decay Scheme 7/2+

0.0 177 71

Intensities: I (γ +ce) per 100 parent decays

6.647 d

Lu106 8

Q = 498.3

Iβ–

Log ft

11.68 0.002

6.322 8.14

9/2+ 11/2–

9.9

7.75

9/2–

6.642

7/2–

78.4

32 201.32 718.367 E1 24 .64461 E+M2 9 13 .67 1 E11+M 0.2 6.7 3 E +M 2 74 11 1 24 2 0 2.9 2 .2 2 0. 1.05 46 8 M M1+E343 354 20 1+ E2 .12 21 2 .1

%β– = 100 –

321.3147 249.6708

0.663 ns 105 ps

112.9474

0.537 ns

0.0

stable

177 Hf105 72

Fig. 4. Revised decay scheme, constructed using our gamma intensities and theoretical conversion coefficients from Nuclear Data Sheets (Kondev, 2003) by the computer code ENSDAT (2006).

Table 6 Log ft values for the transitions calculated by the NNDC LOGFT program. End point energy (Eb  ) (keV)

Intensity

NDS 2003

Present

498.3(8) 385.4(8) 248.6(8) 177.0(8)

498.3(8) 385.4(8) 248.6(8) 177.0(8)

78.4(3) 9.9(3) 0.002(7) 11.68(14)

In the decay scheme of 177Lu, the first forbidden unique beta transition from its ground state to the Jp ¼11/2  , 249 keV level of 177 Hf has been confirmed in this study with a beta branching ratio of 0.002(7)%. The intensities of the other three beta groups from 177 Lu leading to the levels in the daughter 177Hf have been carefully calculated. The beta emission probabilities determined by our intensity balance calculations have relatively smaller uncertainties than those given in the data evaluations for the earlier measurements. Precise measurements of the K, L, and M internal conversion coefficients (ICCs) have been made using a high transmission Mini Orange Electron Spectrometer coupled to a Si(Li) detector. In all, 6 K-shell, 6 L-shell and 6 M-shell conversion coefficients have been reported out of which 5 are reported for the first time. We have also reported energies and intensities of the 6 gamma rays measured with a large volume 60 cc HPGe detector and 4 K and 5 L X-rays measured using a Si-PIN detector with better precision.

Log ft El-Nesr and Bashandy (1962)

NDS 2003

Present

6.6 7.7 8.1 6.3

6.697 7.273 49.2 6.083

6.642(4) 7.75(3) 8.14(9) 6.322(15)

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