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Photoexcitation of isomers by bremsstrahlung of 4 MeV electrons
Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 435-436, 1995 Copyright © 1995 Elsevier Science Ltd 0969-8043(95)00041-0 Printed in Great Britain. All rights reserved 0969-8043/95 $9.50 + 0.00
Pergamon
PHOTOEXCITATION OF ISOMERS BY BREMSSTRAHLUNG OF 4 MeV ELECTRONS
L. Lakosi, N.X. Khanh, N.C. Tam, J, S~far and I. Pavlicsek Institute of Isotopes of the Hungarian Academy of Sciences P.O.Box 77. H-1525 Budapest, Hungary
KEYWORDS (),,),') reactions; integrated and effective isomer excitation cross sections.
A series of isomers were excited by ()',)") reaction, using bremsstrahlung from a 0.9 mm Pt converter placed against the electron beam of a 4 MeV, 25 #A linear accelerator type LPR4. Induced radioactivities were determined by Ge and well-type NaI(TI) ),-spectrometers. A pneumatic transport system was used for successive repetition of irradiation and measurement cycles in the study of shortlived (order of seconds) isomers. Thick-target bremsstrahlung spectrum shape was calculated assuming that the distribution of the photon number was linearly decreasing with the energy, while the amplitude was fitted to total flux measurements carried out by A1203 TL and chemical dosemeters. The irradiation periods varied from 2 to 6.5 h for most of the long-lived nuclides except 1~7Sn, 193Ir and 195pt isotopes, which were irradiated for about a day. Measurements lasted for around 2 h in most cases, but Lu and Os samples were counted for 5-6 h, while UTSn, 19air and ~95Pt for about 2-3 d. Targets of natural isotopic abundance, ranging from 0.2 to 4 g mass, were used. In Table 1, the results are summarized. Effective cross sections by relating the measured isomer activity to the flux of the bremsstrahlung spectrum were evaluated. (Because the major part of the isomer yield was due to higher-lying activation levels, spectral distribution above 1 MeV was only considered.) Furthermore, isomer activation cross sections integrated up to 4 MeV were also determined, based on the Brink-Axel hypothesis, using the Lorentzian extrapolation of the photoabsorption cross sections for a reference, given for giant dipole resonance (Dietrich et al, 1988). The results show a large variety in the magnitude of the cross sections. Large cross sections have been obtained for l°3Rh, ~67Er, t89Os, where the spin difference AI between ground and isomeric states is relatively small (3), though a large strength is also displayed for 176Lu, in spite of AI=6. Particularly strong excitations for deformed nuclei (see also Collins et al, 1992) are striking. Extremely low cross sections may be due to low level densities (half-magic 89y and 117Sn) and high-lying isomeric level (89Y,Ia7Ba). Strong excitations are expected where the isomer is accessible from a level populated by E1 transition from the ground state. In this respect, almost no indications exist. For ~67Er, t79I-If, 191Ir, 197Au, only M1 +E2 upward transitions were reported up to 3.5 MeV (Johnson et al, 1970). Making a comparison with earlier experiments using high activity ~°Co, 142Pr and ~4Na sources of primary energies up to 2.75 MeV (Abrams and Lakosi, 1969; Veres et al, 1973) high jumps are observed in the cross sections, mostly for Er, Lu, Hf, Ir, Au isotopes. The results demonstrate that isomer excitation is a multiparametric process. The large differences occurring in the magnitude of cross section cannot be explained on the basis of a single quantity, e.g. deformation parameter (Collins et al, 1992) or spin difference (Mazur et al, 1993), but rather as a result of an interplay among various factors in a large ensemble. 435
436
L. Lakosi et al.
Table 1. Effective and integrated isomer excitation cross sections for 4 MeV endpoint energy bremsstrahlung Nuclide (Isomer half-life) 77Se (17.45s)
Effective cross section (for flux > 1 MeV) (/xb)
Imegrated cross section Ozb MeV) 37 5.9
Spin difference AI 3 3 4 4
mCd (48.6m) 1lain (1.658h) 11Sin (4.486h)
5.1 0.66 1.3 1.8 0.03 9.8 36 4.0 3.0 5.5 6.5
"7Sn (13.61d) 129Xe (8.89d) lalXe (11.9d) 135Ba (1.20d) 137Ba (2.552m) 1°TEr (2.28s) 176Lu (3.68h) 179Hf (18.7s) 183W (5.15s)
0.83 14 22 5.3 0.9 510 58 89 0.39
36 6.2 3560 430 610 2.7
(5)
52 57 32 17 58
380 400 230 130 440
3 4 4 6 4
79Br (4.86s) ~Kr(1.86h) 'TSr (2.80h) 89y (16.06s) ~ c (6.006h) l°3Rh (56.1 lm) t°7,a°gAg (44s, 40s)
lSgOs (5.8h) 19qr (4.94s) 193Ir (10.6d) ~95pt (4.02d) 197Au (7.8s)
lO
0.2 70 260 34 21 43 48 6
4 4
3 3 5 4 4 5 5 4 4 4 3 6 4
References Dietrich, S. S., B. L. Berman (1988). At. Data Nucl. Data Tables 38, 199 Collins, C. B. et al (1992). Phys. Rev. C 46, 952 Johnson, W. T. K., B. T. Chertok (1970). C. E. Dick, Phys. Rev. Lett. 25, 599. Abrams, I. A., L. Lakosi (1969). Latv. PSR Zinat. Akad. Vestis (Riga), Fiz. Tehn. Zinat. Ser. 1969, No6, 3 Veres, A., I. Pavlicsek, M. CsfirSs, L. Lakosi (1973). Acta Phys. Hung. 34, 97 Mazur, V. M., I. V. Sokolyuk, Z. M. Bigan, I. Yu. Kobal (1993). Yad. Fiz. 56, 1, 20
Pergamon
PHOTOEXCITATION OF ISOMERS BY BREMSSTRAHLUNG OF 4 MeV ELECTRONS
L. Lakosi, N.X. Khanh, N.C. Tam, J, S~far and I. Pavlicsek Institute of Isotopes of the Hungarian Academy of Sciences P.O.Box 77. H-1525 Budapest, Hungary
KEYWORDS (),,),') reactions; integrated and effective isomer excitation cross sections.
A series of isomers were excited by ()',)") reaction, using bremsstrahlung from a 0.9 mm Pt converter placed against the electron beam of a 4 MeV, 25 #A linear accelerator type LPR4. Induced radioactivities were determined by Ge and well-type NaI(TI) ),-spectrometers. A pneumatic transport system was used for successive repetition of irradiation and measurement cycles in the study of shortlived (order of seconds) isomers. Thick-target bremsstrahlung spectrum shape was calculated assuming that the distribution of the photon number was linearly decreasing with the energy, while the amplitude was fitted to total flux measurements carried out by A1203 TL and chemical dosemeters. The irradiation periods varied from 2 to 6.5 h for most of the long-lived nuclides except 1~7Sn, 193Ir and 195pt isotopes, which were irradiated for about a day. Measurements lasted for around 2 h in most cases, but Lu and Os samples were counted for 5-6 h, while UTSn, 19air and ~95Pt for about 2-3 d. Targets of natural isotopic abundance, ranging from 0.2 to 4 g mass, were used. In Table 1, the results are summarized. Effective cross sections by relating the measured isomer activity to the flux of the bremsstrahlung spectrum were evaluated. (Because the major part of the isomer yield was due to higher-lying activation levels, spectral distribution above 1 MeV was only considered.) Furthermore, isomer activation cross sections integrated up to 4 MeV were also determined, based on the Brink-Axel hypothesis, using the Lorentzian extrapolation of the photoabsorption cross sections for a reference, given for giant dipole resonance (Dietrich et al, 1988). The results show a large variety in the magnitude of the cross sections. Large cross sections have been obtained for l°3Rh, ~67Er, t89Os, where the spin difference AI between ground and isomeric states is relatively small (3), though a large strength is also displayed for 176Lu, in spite of AI=6. Particularly strong excitations for deformed nuclei (see also Collins et al, 1992) are striking. Extremely low cross sections may be due to low level densities (half-magic 89y and 117Sn) and high-lying isomeric level (89Y,Ia7Ba). Strong excitations are expected where the isomer is accessible from a level populated by E1 transition from the ground state. In this respect, almost no indications exist. For ~67Er, t79I-If, 191Ir, 197Au, only M1 +E2 upward transitions were reported up to 3.5 MeV (Johnson et al, 1970). Making a comparison with earlier experiments using high activity ~°Co, 142Pr and ~4Na sources of primary energies up to 2.75 MeV (Abrams and Lakosi, 1969; Veres et al, 1973) high jumps are observed in the cross sections, mostly for Er, Lu, Hf, Ir, Au isotopes. The results demonstrate that isomer excitation is a multiparametric process. The large differences occurring in the magnitude of cross section cannot be explained on the basis of a single quantity, e.g. deformation parameter (Collins et al, 1992) or spin difference (Mazur et al, 1993), but rather as a result of an interplay among various factors in a large ensemble. 435
436
L. Lakosi et al.
Table 1. Effective and integrated isomer excitation cross sections for 4 MeV endpoint energy bremsstrahlung Nuclide (Isomer half-life) 77Se (17.45s)
Effective cross section (for flux > 1 MeV) (/xb)
Imegrated cross section Ozb MeV) 37 5.9
Spin difference AI 3 3 4 4
mCd (48.6m) 1lain (1.658h) 11Sin (4.486h)
5.1 0.66 1.3 1.8 0.03 9.8 36 4.0 3.0 5.5 6.5
"7Sn (13.61d) 129Xe (8.89d) lalXe (11.9d) 135Ba (1.20d) 137Ba (2.552m) 1°TEr (2.28s) 176Lu (3.68h) 179Hf (18.7s) 183W (5.15s)
0.83 14 22 5.3 0.9 510 58 89 0.39
36 6.2 3560 430 610 2.7
(5)
52 57 32 17 58
380 400 230 130 440
3 4 4 6 4
79Br (4.86s) ~Kr(1.86h) 'TSr (2.80h) 89y (16.06s) ~ c (6.006h) l°3Rh (56.1 lm) t°7,a°gAg (44s, 40s)
lSgOs (5.8h) 19qr (4.94s) 193Ir (10.6d) ~95pt (4.02d) 197Au (7.8s)
lO
0.2 70 260 34 21 43 48 6
4 4
3 3 5 4 4 5 5 4 4 4 3 6 4
References Dietrich, S. S., B. L. Berman (1988). At. Data Nucl. Data Tables 38, 199 Collins, C. B. et al (1992). Phys. Rev. C 46, 952 Johnson, W. T. K., B. T. Chertok (1970). C. E. Dick, Phys. Rev. Lett. 25, 599. Abrams, I. A., L. Lakosi (1969). Latv. PSR Zinat. Akad. Vestis (Riga), Fiz. Tehn. Zinat. Ser. 1969, No6, 3 Veres, A., I. Pavlicsek, M. CsfirSs, L. Lakosi (1973). Acta Phys. Hung. 34, 97 Mazur, V. M., I. V. Sokolyuk, Z. M. Bigan, I. Yu. Kobal (1993). Yad. Fiz. 56, 1, 20