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High spin states above the 8+ isomer in 92Mo
Nuclear Physics A305 (1978) 163-166 ; © North-HoAand Publishing Co., Amstsrdam
Not to be reproduced by photoprint or microfilm without written permission from the publish
HIGH SPIN STATES ABOVE THE 8* ISdMER IN 92Mo T . NUMAO, H . NAKAYAMA, T. A. SHIBATA and Y . KUNG Department of Physics, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
Received 3 February 1978 (Revised 10 April 1978)
Abstract : High spin states above the 8* isomer in 92 Mo have been studied using the 9°Zr(a, 2Ry)92Mo reaction . The 10* (5118 keV) and 12* (5858 keV) states and several negative-parity states have been proposed in the present work and compared with a shell model calculation.
E
NUCLEAR REACTION ' ° Zr(a, 2ny), E = 28 MeV ; measured E~, h, y(0), prompt and delayed yy-coin . 92 Mo deduced levels, J, n. Enriched target, Ge(Li) detectors .
I . Introdutaioa
Positive-parity low spin states and negative-parity states in 9ZMo have been wt;ll studied' -4) and explained by shell model calculations s- ') in terms of (mgt, p~) a configurations outside an inert BaSr core . Jaklevic et al. t) studied the nucleus using (a, 2ny) reactions and assigned negative-parity states up to 11 - . They also placed a 4597 keV level with a spin of 12 on the 11 - state . Papadopoulos et al. 2 ), using the (p, p'y) reaction, assigned low spin levels and measured their lifetimes. On the other hand, the study of positiv~parity states with spins higher than 8+ is blocked by the presence of the 8 + isomer (T~ = 188 ns), since these levels cannot be observed by conventional coincidence techniques . A candidate for such a level with a spin of 12* was porposed at 4597 keV [ref. t)]. The excitation energy, however, is too low compared with the shell model prediction for the 12 * stateas the (mgt )a configuration. The transition property from this level also seems inconsistent . Since the transitions between positive- and negative-parity states are hindered by the 1-selection rule, the 12* state should decay to the 10* state with the (ngt)° configuration rather than to. the 11 - state . For example, the 6* state at 2613 keV decays more strongly to the 4* state than to the 5- state. The aim of the present work is to look for the 10* and 12+ states to test shell model calculations . 2. Experlmeat and results
We have studied y-rays following the 9°Zr(a, 2n)9ZMo reaction by means of the conventional in-beam y-ray spectroscopy method and a delayed y-y coincidence 163
164
T. NUMAO et al.
0
1000
-
keV
Fig. 1 . Coincidence spectra gated by the delayed lines at 329 .7 and 773 .2 keV . T~a~.c 1 A summary of the present work Er
ly
A2
85 .5 111 .2 148 .0 234 .9 244 .5
4 .5(0 .1) 2 .5(0 .1) 27 .1(0 .1) 21 .4(0 .1) 50.9(0 .2)
329 .7 480 .0 626 .8 650 .9 665 .3 740 .3 773 .2
37 .7(0 .2) 2 .5(0 .1) 28 .2(0 .2) 1 .5(0 .3) 2 .6(0 .2) 3 .3(0 .2) 96 .1(0 .4)
0 .033(0.117) 0 .322(0.085) 0 .365(0.010)
1097 .7 1509 .4
29 .7(0 .3) 100 .0(0 .6)
0 .369(0.015) 0 .252(0.007)
2085 .4 2357 .2
3 .4(0 .2) 7 .0(0 .4)
0.018(0.122) 0 .329(0.061)
Coincident y-rays (235),245,627,(651),773,(109ß),I509 86, 330, 773, 1509, 740d, 2357d 111, 235, 245, 627, 651, 665, 773, 1098, 1509 111, 235, 480, 538, 627, (651), 665, 773, 1098, 1509 148, 773, 1509, 740d, 2357d 245, 773, 1509 111 ;235,245,(651),665,773,1098,1509 1~ 1, 235, 245, 627, 1098, 1509 235, 245, 627, (773), 1098, 1509 2357, 14ßd, 3304, 773d, 1509d (86), 111, 148, 235, 245, 330, (480), (538) 627, 665, 1098, 1509, 740d, 2357d 111, 235, 245, 627, 665, 773, 1509 (86), (111), 148, 235, 245, 330, 480, 538 627, (665), 773, 1098, 740d, (2357d) (111), 235, 245, (627), (773), (1098), 1509 740, 148d, 3304, 773d, (1509d)
Delayed y-y wincdence relations are indicated by the letter d. Parentheses indicate a probable coincidence.
technique. A 2 mg/cmZ thick target of enriched metallic 9 °Zr foil was bombarded with 28 MeV a-particles from the cyclotron at the Institute of Medical Science, University of Tokyo. Singles spectra and angular distributions of y-rays at four backward angles to the incident beam were observed using 40 cm s Ge(Li) detectors with an energy resolution of 2.4 keV FWHM at 1332 keV. In angular distribution measurements, both deetectors were placed 30 cm from the target . One of them was fixed at 90° to provide a normalization for each run. The angular distributions were fitted by the equation y(~ = 1 +AZ P Z (cos B)+A 4 P4 (cos 8).
HIGH SPIN STATES
Fig. 2. Level scheme of 92 Mo . Energies are given in keV.
165
Fig. 3. Comparison between the experimental and theoretical levels.
Coincidence spectra with prompt as well as delayed y-rays were measured event by event in a 2048(E) x 2048(E~ x 1024(T) mode with a CAMAC-MBD1 I-PDP11/40 system . Coincidence spectra gated by the 329.7 keV and 773.2 keV lines are shown in fig. 1, where chance coincidence events have been subtracted. The spectrum gated by the 773.2 keV line includes y-rays between the levels above the 4487 keV level (T~ = 8.8 ns) ; i .e. the 111 .2, 665 .3 and 2085.4 keV y-rays . Table 1 shows the present results (y-ray energies, relative intensities at B = 55°, angular distribution coefficients A Z and coincidence relations) . The accuracies of the energy determinations are within 0.2 keV for the y-rays below 1510 keV and 2 keV for higher energy y-rays . The intensity errors include only statistical deviations. The present data are consistent with ref. '), except for the relative intensity of the 111 .2 keV line . This inconsistency comes from the difference in target form . Since oxide targets were used in ref. t), the y-ray spectra were affected by the 110 keV y-rays from the t 60(a, py) t 9F reaction . A summary of the present work is shown in the level scheme of fig. 2. 3. Discussion It was observed that the 740.3 and 2357.2 keV transitions were in coincidence with the delayed y-rays(the 8+-6+-4+-2+-0+ cascade)..These .y-rays were also in coincidence with each other. Thus, 5118 and 5858 keV levels were established from the intensity balance. The spin assignments of 10+ and 12+ to the 5118 and 5858 keV levels, respectively, were proposed from the stretched E2 characters of both lines (A 2 x 0.3).
166
T. NUMAO et al.
The 5152 keV level was placed from the coincidence relation of the 665.3 keV y-rays . The small anisotropy AZ x 0.03 allows only spins 10 -, 11 - and 12- for the state. The 2085.4 keV line was in coincidence with the 111 .2, 234 .9, 626.8 keV, . . . lines . The intensity balance suggests that, instead of the 111 .2 keV transition, the 2085.4 keV transition directly feeds the 11 - 4487 keV level . Therefore, we placed new levels at 6572 and 6683 keV and removed the 4597 keV level which was considered to decay to the 11 - state by the 110 keV transition t ). The existence of a level at 7334 keV is also suggested from the coincidence relations and intensity balance. The experimental and calculated levels ') of 92Mo are shown in fig. 3. The effective interactions were taken from ref. S). In this calculation the 10 + and 12+ states are described as the (ngt)4 configuration. The agreement between the experimental and calculated levels is quite good . The level energies up to 3.5 MeV are reproduced within errors of 100 keV. The 5152 keV level is a candidate for the shell model 10 state, since a calculated 564b keV state is left without corresponding experimental levels . No corresponding shell model states for the 6572, 6683 and 7334 keV levels exist in the (p~, g})a space. These levels, therefore, should be considered to be coreexcited states . We wish to thank Prof. T. Yamazaki andProf. K. Nakai for reading the manuscript and valuable discussions. We are grateful to Dr. K. Ogawa for the shell model calculation and discussions. W+r wish to thank Dr. M . Sekiguchi for the use of the target . References 1) J. M . Jaklevic, C. M. Lederer and J. M. Hollander, Phys. Lett. 29B (1969) 179 ; C. M. Lederer, J. M. Jaklevic and J. M. Hollander, Nucl. Phys . A169 (1971) 449 2) C. T. Papadopoulos, A. G. Hartas, P. A. Assimakopoulos, G. Andritsopoulos and N. H. Gangas, Nucl . Pbys. A254 (19757 93 3) M. Ishihara, H. Kawalcami, N. Yoshikawa, M. Sakai and K. Ishü, Phys. Lett . 33B (1971) 398 4) C. Gil, J. Phys. Soc. Jap. 34 (1973) 575 5) J. B. Ball, J. B. McGrory, R. L. Auble and K. H. Bhatt, Phys. Lett . 29B (1969) 182; J. B. Ball, J. B. McGrory and J. S. Larsen, Pbys . Lett . 41B (1972) 581 6) D. H. Gloeckner and F. J. D. Serduke, Nucl . Phys. A220 (1974) 477 7) K. Ogawa, private communication
Not to be reproduced by photoprint or microfilm without written permission from the publish
HIGH SPIN STATES ABOVE THE 8* ISdMER IN 92Mo T . NUMAO, H . NAKAYAMA, T. A. SHIBATA and Y . KUNG Department of Physics, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
Received 3 February 1978 (Revised 10 April 1978)
Abstract : High spin states above the 8* isomer in 92 Mo have been studied using the 9°Zr(a, 2Ry)92Mo reaction . The 10* (5118 keV) and 12* (5858 keV) states and several negative-parity states have been proposed in the present work and compared with a shell model calculation.
E
NUCLEAR REACTION ' ° Zr(a, 2ny), E = 28 MeV ; measured E~, h, y(0), prompt and delayed yy-coin . 92 Mo deduced levels, J, n. Enriched target, Ge(Li) detectors .
I . Introdutaioa
Positive-parity low spin states and negative-parity states in 9ZMo have been wt;ll studied' -4) and explained by shell model calculations s- ') in terms of (mgt, p~) a configurations outside an inert BaSr core . Jaklevic et al. t) studied the nucleus using (a, 2ny) reactions and assigned negative-parity states up to 11 - . They also placed a 4597 keV level with a spin of 12 on the 11 - state . Papadopoulos et al. 2 ), using the (p, p'y) reaction, assigned low spin levels and measured their lifetimes. On the other hand, the study of positiv~parity states with spins higher than 8+ is blocked by the presence of the 8 + isomer (T~ = 188 ns), since these levels cannot be observed by conventional coincidence techniques . A candidate for such a level with a spin of 12* was porposed at 4597 keV [ref. t)]. The excitation energy, however, is too low compared with the shell model prediction for the 12 * stateas the (mgt )a configuration. The transition property from this level also seems inconsistent . Since the transitions between positive- and negative-parity states are hindered by the 1-selection rule, the 12* state should decay to the 10* state with the (ngt)° configuration rather than to. the 11 - state . For example, the 6* state at 2613 keV decays more strongly to the 4* state than to the 5- state. The aim of the present work is to look for the 10* and 12+ states to test shell model calculations . 2. Experlmeat and results
We have studied y-rays following the 9°Zr(a, 2n)9ZMo reaction by means of the conventional in-beam y-ray spectroscopy method and a delayed y-y coincidence 163
164
T. NUMAO et al.
0
1000
-
keV
Fig. 1 . Coincidence spectra gated by the delayed lines at 329 .7 and 773 .2 keV . T~a~.c 1 A summary of the present work Er
ly
A2
85 .5 111 .2 148 .0 234 .9 244 .5
4 .5(0 .1) 2 .5(0 .1) 27 .1(0 .1) 21 .4(0 .1) 50.9(0 .2)
329 .7 480 .0 626 .8 650 .9 665 .3 740 .3 773 .2
37 .7(0 .2) 2 .5(0 .1) 28 .2(0 .2) 1 .5(0 .3) 2 .6(0 .2) 3 .3(0 .2) 96 .1(0 .4)
0 .033(0.117) 0 .322(0.085) 0 .365(0.010)
1097 .7 1509 .4
29 .7(0 .3) 100 .0(0 .6)
0 .369(0.015) 0 .252(0.007)
2085 .4 2357 .2
3 .4(0 .2) 7 .0(0 .4)
0.018(0.122) 0 .329(0.061)
Coincident y-rays (235),245,627,(651),773,(109ß),I509 86, 330, 773, 1509, 740d, 2357d 111, 235, 245, 627, 651, 665, 773, 1098, 1509 111, 235, 480, 538, 627, (651), 665, 773, 1098, 1509 148, 773, 1509, 740d, 2357d 245, 773, 1509 111 ;235,245,(651),665,773,1098,1509 1~ 1, 235, 245, 627, 1098, 1509 235, 245, 627, (773), 1098, 1509 2357, 14ßd, 3304, 773d, 1509d (86), 111, 148, 235, 245, 330, (480), (538) 627, 665, 1098, 1509, 740d, 2357d 111, 235, 245, 627, 665, 773, 1509 (86), (111), 148, 235, 245, 330, 480, 538 627, (665), 773, 1098, 740d, (2357d) (111), 235, 245, (627), (773), (1098), 1509 740, 148d, 3304, 773d, (1509d)
Delayed y-y wincdence relations are indicated by the letter d. Parentheses indicate a probable coincidence.
technique. A 2 mg/cmZ thick target of enriched metallic 9 °Zr foil was bombarded with 28 MeV a-particles from the cyclotron at the Institute of Medical Science, University of Tokyo. Singles spectra and angular distributions of y-rays at four backward angles to the incident beam were observed using 40 cm s Ge(Li) detectors with an energy resolution of 2.4 keV FWHM at 1332 keV. In angular distribution measurements, both deetectors were placed 30 cm from the target . One of them was fixed at 90° to provide a normalization for each run. The angular distributions were fitted by the equation y(~ = 1 +AZ P Z (cos B)+A 4 P4 (cos 8).
HIGH SPIN STATES
Fig. 2. Level scheme of 92 Mo . Energies are given in keV.
165
Fig. 3. Comparison between the experimental and theoretical levels.
Coincidence spectra with prompt as well as delayed y-rays were measured event by event in a 2048(E) x 2048(E~ x 1024(T) mode with a CAMAC-MBD1 I-PDP11/40 system . Coincidence spectra gated by the 329.7 keV and 773.2 keV lines are shown in fig. 1, where chance coincidence events have been subtracted. The spectrum gated by the 773.2 keV line includes y-rays between the levels above the 4487 keV level (T~ = 8.8 ns) ; i .e. the 111 .2, 665 .3 and 2085.4 keV y-rays . Table 1 shows the present results (y-ray energies, relative intensities at B = 55°, angular distribution coefficients A Z and coincidence relations) . The accuracies of the energy determinations are within 0.2 keV for the y-rays below 1510 keV and 2 keV for higher energy y-rays . The intensity errors include only statistical deviations. The present data are consistent with ref. '), except for the relative intensity of the 111 .2 keV line . This inconsistency comes from the difference in target form . Since oxide targets were used in ref. t), the y-ray spectra were affected by the 110 keV y-rays from the t 60(a, py) t 9F reaction . A summary of the present work is shown in the level scheme of fig. 2. 3. Discussion It was observed that the 740.3 and 2357.2 keV transitions were in coincidence with the delayed y-rays(the 8+-6+-4+-2+-0+ cascade)..These .y-rays were also in coincidence with each other. Thus, 5118 and 5858 keV levels were established from the intensity balance. The spin assignments of 10+ and 12+ to the 5118 and 5858 keV levels, respectively, were proposed from the stretched E2 characters of both lines (A 2 x 0.3).
166
T. NUMAO et al.
The 5152 keV level was placed from the coincidence relation of the 665.3 keV y-rays . The small anisotropy AZ x 0.03 allows only spins 10 -, 11 - and 12- for the state. The 2085.4 keV line was in coincidence with the 111 .2, 234 .9, 626.8 keV, . . . lines . The intensity balance suggests that, instead of the 111 .2 keV transition, the 2085.4 keV transition directly feeds the 11 - 4487 keV level . Therefore, we placed new levels at 6572 and 6683 keV and removed the 4597 keV level which was considered to decay to the 11 - state by the 110 keV transition t ). The existence of a level at 7334 keV is also suggested from the coincidence relations and intensity balance. The experimental and calculated levels ') of 92Mo are shown in fig. 3. The effective interactions were taken from ref. S). In this calculation the 10 + and 12+ states are described as the (ngt)4 configuration. The agreement between the experimental and calculated levels is quite good . The level energies up to 3.5 MeV are reproduced within errors of 100 keV. The 5152 keV level is a candidate for the shell model 10 state, since a calculated 564b keV state is left without corresponding experimental levels . No corresponding shell model states for the 6572, 6683 and 7334 keV levels exist in the (p~, g})a space. These levels, therefore, should be considered to be coreexcited states . We wish to thank Prof. T. Yamazaki andProf. K. Nakai for reading the manuscript and valuable discussions. We are grateful to Dr. K. Ogawa for the shell model calculation and discussions. W+r wish to thank Dr. M . Sekiguchi for the use of the target . References 1) J. M . Jaklevic, C. M. Lederer and J. M. Hollander, Phys. Lett. 29B (1969) 179 ; C. M. Lederer, J. M. Jaklevic and J. M. Hollander, Nucl. Phys . A169 (1971) 449 2) C. T. Papadopoulos, A. G. Hartas, P. A. Assimakopoulos, G. Andritsopoulos and N. H. Gangas, Nucl . Pbys. A254 (19757 93 3) M. Ishihara, H. Kawalcami, N. Yoshikawa, M. Sakai and K. Ishü, Phys. Lett . 33B (1971) 398 4) C. Gil, J. Phys. Soc. Jap. 34 (1973) 575 5) J. B. Ball, J. B. McGrory, R. L. Auble and K. H. Bhatt, Phys. Lett . 29B (1969) 182; J. B. Ball, J. B. McGrory and J. S. Larsen, Pbys . Lett . 41B (1972) 581 6) D. H. Gloeckner and F. J. D. Serduke, Nucl . Phys. A220 (1974) 477 7) K. Ogawa, private communication