- Email: [email protected]
Three-quasiparticle states in 177Ta
Volume 55B, number 5
PHYSICS LETTERS
17 March 1975
T H R E E - Q U A S I P A R T I C L E STATES IN 177Ta D. BARNI~OUD, S. ANDRI~ and C. FOIN Institut des Sciences Nucl~aires, BP 257, 38044 Grenoble-Cedex, France
Reeeived 22 November 1974 The feedingof the 21/2-, 1355 keV isomeric state in 177Tain the 175Lu (a, 2n) t 77Tareaction has been studied by meansof the delayed coincidencetechnique. Three other three-quasiparticiestates have been observedand their configurationis proposed.
Recently, high-spin levels in 177Ta have been studied in detail [1-4]. A three-quasiparticle isomer has been identified at 1355 keV [1-4] and its spin and parity determined as 21/2- [3, 4]. Such states have also been observed in 175Ta and 179Ta [5, 6], but nothing is known about their feeding. In 177Ta, excited by means of the 175 Lu(t~, 2n)177Ta reaction, the intensities of the 311 and 550 keV transitions, depopulating the 21/2- isomer, are too strong to be explained by direct feeding of this state from photons of the continuum. Such a high intensity suggests feeding from other three-quasiparticle states. Theoretically, such states are expected to occur at an excitation energy approximately equal to twice the energy gap (2A), observed in even-even neighbouring nuclei; in the case of 177Ta, Soloviev and Fedotov [7] predict several three-quasiparticle states at about 1500 keV. A search for these levels has been made at the Grenoble isochronous cyclotron by in-beam 3'-spectroscopy using large coaxial Ge(Li) detectors (45 and 75 cm3). A beam of 30 MeV a-particles on a natural lutetium target (10 mg/cm 2) was used to form the 177Ta levels. The photons feeding the 1355 keV isomeric state, T1/2 = 5.0/as [3], have been identified using the 3'-3' delayed coincidence technique [8, 9], (stop signal from the 311 keV "/-ray). Using the natural structure of the beam (75 ns between the bursts), the time gate was opened from 50 ns to 650 ns after the prompt peak (in order to keep the random coincidence rate negligible). Energies and intensities of the photons feeding and depopulating the 21/2- isomeric state are given in table 1. Prompt 3'~ coincidences between these photons were simultaneously recorded [9].
In previous experiments using 28 MeV a-particles
[1,2], most of these photons had been detected, but with small intensities. With 30 MeV a-particles, because of the spin dependence of the excitation function, these 3,-rays are strong enough to allow measurements of their angular distributions with acceptable precision (table 1). The analysis of prompt coincidences shows two strong cascades: (270.9 - 294.7) keV and (343.4 136.1 - 202.0 - 234.2)keV. The intensity balance and the quantitative coincidence study assign the order of the photons in each sequence. A complete analysis gives, without any ambiguity, the energies of nine new levels (fig. 1). With the reaction used, the only strongly excited three-quasiparticle states must be those with high Kvalue and low excitation energy, i.e. those near the yrast line. These states may be considered as obtained by coupling the 9/2- [514] or 7•2 + [404] proton states (states of high K value and close to the Fermi level in 177Ta) with the high K two-quasiparticle states of the core (176Hf). For this last nucleus, a very intensive experimental study has been performed by Khoo etal. [10]. These authors locate five two-quasiparticle states between 1330 and 1860 keV. Two 6 + states have a mixed configuration {7/2 + [404] + 5/2 + [402] p}and { 7/2- [514] n + 5/2- [312] n}, the three others are identified with practically pure configurations: 7-{9/2 + [624] n + 5/2- [512] n}, 8? {7/2+ [404] p + 9 / 2 - [514] p} and 8~- {7/2- [514] n +9/2+ [624] n}
(r~. 2). Taking these findings into account, we had identified in 177Ta the new high spin levels either as threequasiparticle states, or as members of rotational bands built on them (fig. 1). 443
Volume 55B, number 5
PHYSICS LETTERS
17 March 1975
Table 1 Summary of data of the 3`-ray feeding and depopulating the three-quasiparficle levels Angular distribution coefficients
E? (keV)
13, (tel. int.)
136.3 169.3 202.4 229.6 234.2 249.0 ~257 270.9 294.7 311.2 329.5 343.4 400.2 550.0 555.4
28 ± 5 40 ± 10 75 ± 15 < 15 23 ± 5 27 ± 5 weak 49 ± 5 14 ± 3 146± 10 10 ± 3 100 ± 7 11 ± 3 15 ± 5 5± 2
A2/Ao
A4/A 0
Nature
0.14 ± 0.07
M1 +E2
-0.07 ± 0.19
M1 + E2
-0.02 ± 0.16
M1 + E2 M1 +E2
0.15 ± 0.10 0.22 ± 0.17 0.03 ± 0.06
0.02 ± 0.12
M1 + E2
0.08 ± 0.08
M1
-0.22 ± 0.10 -0.36 ± 0.20
-0.03 ± 0.10
0.10 ± 0.10 0.09 ± 0.15
E1 (or pure M1) E1 (or pure M1)
+
To construct 2 1 / 2 - states one has the possibilities { 9 / 2 - [514] p + 7/2 + [404] p + 5/2 + [402]p} or { 9 / 2 - [514] p + 7 / 2 - [514] n + 5 / 2 - [512~n}. Indeed these two configurations are probably mixed and lead to the two states in 177Ta at 1355 and 1696 keV. Starting from the even-even 176Hf core, the three possibilities to form them are 9 / 2 - [514] p + 6~, 9 / 2 - [514] p + 65, or 5/2 + [402] p + 8i-. Energetically the isomeric level at 1355 keV appears to originate
2826.9 - - ] - - 3 3 / 2
*
from the coupling of the 9 / 2 - [514] orbit to the 61 state in 176Hf. Two rotational levels ( 2 3 / 2 - and 2 5 / 2 - ) have been identified on this isomer with reasonable 7~2/2J values (see table 3). The second 2 1 / 2 - state at 1696 keV may also be formed by coupling of 5/2 + [402] orbit to the 8 i state or b y coupling the 9 / 2 - [514] orbit to the 65 state, leading to the same configuration. The identification of spin and parity of the other levels and consequently their configurations was achieved using the multipolarities of transitions, the undelayed character of the transitions implying the
+
2271.5---~ 29/2 * I 23,4.2 23/2" 2098"8 " - - ~ ' - - - - ' ~ 3 7 3 @ 27/2+ - 1945----~ .... 1920.8 ~ 2 5 / 2 \ -264".-437 202.`4 2~, o I `4do.2 1 . 3 , . j ~ , , 2~ + ' 21/2" 294.7 136.3 .1696....J~:.~m,._1626.l @ 1 ~ : 8 . 6 ~ ,t" 23/2"
2~"
mCk.v)
25/2-
-3:*!"
2769 3`4~
1355.2 ~ 1 / 2 19/2- 311.2/ 1~t2" 1550.0
T~: 5.0p.
~-[56`41 •
Fig. 1. Partial level scheme of 177Ta showing the high-K three-quasiparticle states and their associated rotational bands. The transitions marked with an asterisk are observed in coincidence measurements only. 444
1~t+14 - ~ , ~ L I ~ . -7_,. ~ I,+~2+1+.,.~"15121.I--<,.'+°"
- ~ -- 25'2*
19oo
... ........
I
21/2"
1700
l+a~m..,+-m,l~l'" 1500
?"2"[51'4]"566'2"151:~I")\ \ 6 +" r m H l [,., ,+]
_ - 2'I/2" rrl To
1300
Fig. 2. Relative positions of two- and three-quasiparticle levels in 176Hi"and 177Ta, respectively.
Volume 55B, number 5
PHYSICS LETTERS
17 March 1975
Table 2 Proposed configurations of observed intrinsic three--quasiparticle states Energy level (keV)
K rr
1355.2
Components • .+[7/2- [514] n + 5/2- [5121nl
21/2-
(1696)
9/2-[514]p ÷ o 17/2+[4041P + 5/2+[4021pJ
(21/2-)
1698.6 1834.9 2098.8
9/2-[514]p
23/2 + 25/2 + 25•2
+ 6+/7/2-[514]n + 5/2-[5121 n'~ ~7/2+ [404] p + 5/2+ [402] p j
9/2- [5141p + 7- ~9/2+ [624] n + 5/2- [512] n) 9/2- [5141p + 8-~9/2+ [6241 n + 7/2-[5141 n) 9/2- [514] p + 8+ ~9/2+ [624] n + 7/2+[633] n)
Table 3 Rotational parameters in 176Hf and 177Ta, calculated using the formula E(I) = Eo + A[ + B[ 2 with [ = I(I+ 1) - K 2 176Hf
177Ta
[h2/2J] exp (keV)
Bex p (eV)
0+
14.7
-11
6~" 6*2 8]
12.5 12.0 8.2
- 6.3 - 6.0 11
K
[fi2/2J ] exp (keY)
[~2/2J] talc (keV)
9/221/2- (1355) 21/2- (1646) 25/2 +
13.5 11.8 10.8 7.2
11.6 11.1 7.8
change of one orbit only, and the preferential feeding of levels near the yrast line. These reasons lead us to rule out the configurations including the 7/2 + [404] proton orbit. We have thus identified the 1699 and 1835 keV levels as 23/2 + and 25/2 + obtained by coupling the 9 / 2 - [514] proton orbit to the 7 - and 8~- two-quasiparticle states, respectively (table 2). Moreover, the experimental inertia parameter (h2/2J)ex p found for the rotational band built on this 25/2 + state and the B value are consistent with the proposed configuration. The (h2/2J)exp values are compared to the calculated value using the relations Jcalc = Jever~even + + A J9/2- and AJg/2- = J9/2- -J0+(even-even). For all the bands observed good agreement is obtained (table
3). Lastly, the 2099 keV level which feeds the 23/2 + level through a 400 keV E 1 transition has probably a spin and parity 25•2- with the configuration 9 [ 2 - [514] p + 8+(7/2 + [633] n + 9/2+ [624] n)" This
Bexp (eV)
-6.5
0.2 7.8
agrees well with the fact that the same energy separation is expected between this 8 + and the 7 - ~9/2+ [624]n + 5 / 2 - [512]n ) state in 176Hf [10]. In conclusion, we have located for each two-quasiparticle level of high K-value observed in 176Hf the corresponding three-quasiparticle level in 177Ta, which can be obtained by adding the proton orbital 9 / 2 - [514]. We thank Dr. R. Piepenbring for continuing and valuable discussions.
References
[1] B. Sk~nberg, S.A. Hjorth and H. Ryde, Nucl. Phys. A154 (1970) 641. [2] D. Barn~oud et al., Nucl. Phys. A154 (1970) 653. [3] H. Hiibel, R.A. Naumann and E.H. Spejewski, Phys. Rev. C4 (1971) 2272. [4] S. Allam and K.-H. Kaun, Rossendorf annual report 1971, p. 68. 445
Volume 55B, number 5
PHYSICS LETTERS
[5] C. Foin, Th. Lindblad, B. Sk~nberg and H. Ryde, Nucl. Phys. A195 (1972) 465. [6] P. Kemnitz et al., 12th Conf. on Nuclear structure and nuclear spectroscopy, Dubna (1971) p. 153. [7] V.G. Soloviev and S.I. Fedotov, Izv. Akad. Nauk, Set. Fiz. 36 t1972) 706.
446
17 March 1975
[8] C. Foin, S. Andr6 and S.A. Hjorth, Nucl. Phys. A219 (1974) 347. [9] S. Andr6 and G. Margotton, I.S.N. Grenoble, annual report 1973, p. 89. [10] T.L. Khoo et al., Phys. Rev. Lett. 28 (1972) 1717; T.L. Khoo, J.C. Waddington and M.W. Johns, Can. J. Phys. 51 (1973) 153 and 2307.
PHYSICS LETTERS
17 March 1975
T H R E E - Q U A S I P A R T I C L E STATES IN 177Ta D. BARNI~OUD, S. ANDRI~ and C. FOIN Institut des Sciences Nucl~aires, BP 257, 38044 Grenoble-Cedex, France
Reeeived 22 November 1974 The feedingof the 21/2-, 1355 keV isomeric state in 177Tain the 175Lu (a, 2n) t 77Tareaction has been studied by meansof the delayed coincidencetechnique. Three other three-quasiparticiestates have been observedand their configurationis proposed.
Recently, high-spin levels in 177Ta have been studied in detail [1-4]. A three-quasiparticle isomer has been identified at 1355 keV [1-4] and its spin and parity determined as 21/2- [3, 4]. Such states have also been observed in 175Ta and 179Ta [5, 6], but nothing is known about their feeding. In 177Ta, excited by means of the 175 Lu(t~, 2n)177Ta reaction, the intensities of the 311 and 550 keV transitions, depopulating the 21/2- isomer, are too strong to be explained by direct feeding of this state from photons of the continuum. Such a high intensity suggests feeding from other three-quasiparticle states. Theoretically, such states are expected to occur at an excitation energy approximately equal to twice the energy gap (2A), observed in even-even neighbouring nuclei; in the case of 177Ta, Soloviev and Fedotov [7] predict several three-quasiparticle states at about 1500 keV. A search for these levels has been made at the Grenoble isochronous cyclotron by in-beam 3'-spectroscopy using large coaxial Ge(Li) detectors (45 and 75 cm3). A beam of 30 MeV a-particles on a natural lutetium target (10 mg/cm 2) was used to form the 177Ta levels. The photons feeding the 1355 keV isomeric state, T1/2 = 5.0/as [3], have been identified using the 3'-3' delayed coincidence technique [8, 9], (stop signal from the 311 keV "/-ray). Using the natural structure of the beam (75 ns between the bursts), the time gate was opened from 50 ns to 650 ns after the prompt peak (in order to keep the random coincidence rate negligible). Energies and intensities of the photons feeding and depopulating the 21/2- isomeric state are given in table 1. Prompt 3'~ coincidences between these photons were simultaneously recorded [9].
In previous experiments using 28 MeV a-particles
[1,2], most of these photons had been detected, but with small intensities. With 30 MeV a-particles, because of the spin dependence of the excitation function, these 3,-rays are strong enough to allow measurements of their angular distributions with acceptable precision (table 1). The analysis of prompt coincidences shows two strong cascades: (270.9 - 294.7) keV and (343.4 136.1 - 202.0 - 234.2)keV. The intensity balance and the quantitative coincidence study assign the order of the photons in each sequence. A complete analysis gives, without any ambiguity, the energies of nine new levels (fig. 1). With the reaction used, the only strongly excited three-quasiparticle states must be those with high Kvalue and low excitation energy, i.e. those near the yrast line. These states may be considered as obtained by coupling the 9/2- [514] or 7•2 + [404] proton states (states of high K value and close to the Fermi level in 177Ta) with the high K two-quasiparticle states of the core (176Hf). For this last nucleus, a very intensive experimental study has been performed by Khoo etal. [10]. These authors locate five two-quasiparticle states between 1330 and 1860 keV. Two 6 + states have a mixed configuration {7/2 + [404] + 5/2 + [402] p}and { 7/2- [514] n + 5/2- [312] n}, the three others are identified with practically pure configurations: 7-{9/2 + [624] n + 5/2- [512] n}, 8? {7/2+ [404] p + 9 / 2 - [514] p} and 8~- {7/2- [514] n +9/2+ [624] n}
(r~. 2). Taking these findings into account, we had identified in 177Ta the new high spin levels either as threequasiparticle states, or as members of rotational bands built on them (fig. 1). 443
Volume 55B, number 5
PHYSICS LETTERS
17 March 1975
Table 1 Summary of data of the 3`-ray feeding and depopulating the three-quasiparficle levels Angular distribution coefficients
E? (keV)
13, (tel. int.)
136.3 169.3 202.4 229.6 234.2 249.0 ~257 270.9 294.7 311.2 329.5 343.4 400.2 550.0 555.4
28 ± 5 40 ± 10 75 ± 15 < 15 23 ± 5 27 ± 5 weak 49 ± 5 14 ± 3 146± 10 10 ± 3 100 ± 7 11 ± 3 15 ± 5 5± 2
A2/Ao
A4/A 0
Nature
0.14 ± 0.07
M1 +E2
-0.07 ± 0.19
M1 + E2
-0.02 ± 0.16
M1 + E2 M1 +E2
0.15 ± 0.10 0.22 ± 0.17 0.03 ± 0.06
0.02 ± 0.12
M1 + E2
0.08 ± 0.08
M1
-0.22 ± 0.10 -0.36 ± 0.20
-0.03 ± 0.10
0.10 ± 0.10 0.09 ± 0.15
E1 (or pure M1) E1 (or pure M1)
+
To construct 2 1 / 2 - states one has the possibilities { 9 / 2 - [514] p + 7/2 + [404] p + 5/2 + [402]p} or { 9 / 2 - [514] p + 7 / 2 - [514] n + 5 / 2 - [512~n}. Indeed these two configurations are probably mixed and lead to the two states in 177Ta at 1355 and 1696 keV. Starting from the even-even 176Hf core, the three possibilities to form them are 9 / 2 - [514] p + 6~, 9 / 2 - [514] p + 65, or 5/2 + [402] p + 8i-. Energetically the isomeric level at 1355 keV appears to originate
2826.9 - - ] - - 3 3 / 2
*
from the coupling of the 9 / 2 - [514] orbit to the 61 state in 176Hf. Two rotational levels ( 2 3 / 2 - and 2 5 / 2 - ) have been identified on this isomer with reasonable 7~2/2J values (see table 3). The second 2 1 / 2 - state at 1696 keV may also be formed by coupling of 5/2 + [402] orbit to the 8 i state or b y coupling the 9 / 2 - [514] orbit to the 65 state, leading to the same configuration. The identification of spin and parity of the other levels and consequently their configurations was achieved using the multipolarities of transitions, the undelayed character of the transitions implying the
+
2271.5---~ 29/2 * I 23,4.2 23/2" 2098"8 " - - ~ ' - - - - ' ~ 3 7 3 @ 27/2+ - 1945----~ .... 1920.8 ~ 2 5 / 2 \ -264".-437 202.`4 2~, o I `4do.2 1 . 3 , . j ~ , , 2~ + ' 21/2" 294.7 136.3 .1696....J~:.~m,._1626.l @ 1 ~ : 8 . 6 ~ ,t" 23/2"
2~"
mCk.v)
25/2-
-3:*!"
2769 3`4~
1355.2 ~ 1 / 2 19/2- 311.2/ 1~t2" 1550.0
T~: 5.0p.
~-[56`41 •
Fig. 1. Partial level scheme of 177Ta showing the high-K three-quasiparticle states and their associated rotational bands. The transitions marked with an asterisk are observed in coincidence measurements only. 444
1~t+14 - ~ , ~ L I ~ . -7_,. ~ I,+~2+1+.,.~"15121.I--<,.'+°"
- ~ -- 25'2*
19oo
... ........
I
21/2"
1700
l+a~m..,+-m,l~l'" 1500
?"2"[51'4]"566'2"151:~I")\ \ 6 +" r m H l [,., ,+]
_ - 2'I/2" rrl To
1300
Fig. 2. Relative positions of two- and three-quasiparticle levels in 176Hi"and 177Ta, respectively.
Volume 55B, number 5
PHYSICS LETTERS
17 March 1975
Table 2 Proposed configurations of observed intrinsic three--quasiparticle states Energy level (keV)
K rr
1355.2
Components • .+[7/2- [514] n + 5/2- [5121nl
21/2-
(1696)
9/2-[514]p ÷ o 17/2+[4041P + 5/2+[4021pJ
(21/2-)
1698.6 1834.9 2098.8
9/2-[514]p
23/2 + 25/2 + 25•2
+ 6+/7/2-[514]n + 5/2-[5121 n'~ ~7/2+ [404] p + 5/2+ [402] p j
9/2- [5141p + 7- ~9/2+ [624] n + 5/2- [512] n) 9/2- [5141p + 8-~9/2+ [6241 n + 7/2-[5141 n) 9/2- [514] p + 8+ ~9/2+ [624] n + 7/2+[633] n)
Table 3 Rotational parameters in 176Hf and 177Ta, calculated using the formula E(I) = Eo + A[ + B[ 2 with [ = I(I+ 1) - K 2 176Hf
177Ta
[h2/2J] exp (keV)
Bex p (eV)
0+
14.7
-11
6~" 6*2 8]
12.5 12.0 8.2
- 6.3 - 6.0 11
K
[fi2/2J ] exp (keY)
[~2/2J] talc (keV)
9/221/2- (1355) 21/2- (1646) 25/2 +
13.5 11.8 10.8 7.2
11.6 11.1 7.8
change of one orbit only, and the preferential feeding of levels near the yrast line. These reasons lead us to rule out the configurations including the 7/2 + [404] proton orbit. We have thus identified the 1699 and 1835 keV levels as 23/2 + and 25/2 + obtained by coupling the 9 / 2 - [514] proton orbit to the 7 - and 8~- two-quasiparticle states, respectively (table 2). Moreover, the experimental inertia parameter (h2/2J)ex p found for the rotational band built on this 25/2 + state and the B value are consistent with the proposed configuration. The (h2/2J)exp values are compared to the calculated value using the relations Jcalc = Jever~even + + A J9/2- and AJg/2- = J9/2- -J0+(even-even). For all the bands observed good agreement is obtained (table
3). Lastly, the 2099 keV level which feeds the 23/2 + level through a 400 keV E 1 transition has probably a spin and parity 25•2- with the configuration 9 [ 2 - [514] p + 8+(7/2 + [633] n + 9/2+ [624] n)" This
Bexp (eV)
-6.5
0.2 7.8
agrees well with the fact that the same energy separation is expected between this 8 + and the 7 - ~9/2+ [624]n + 5 / 2 - [512]n ) state in 176Hf [10]. In conclusion, we have located for each two-quasiparticle level of high K-value observed in 176Hf the corresponding three-quasiparticle level in 177Ta, which can be obtained by adding the proton orbital 9 / 2 - [514]. We thank Dr. R. Piepenbring for continuing and valuable discussions.
References
[1] B. Sk~nberg, S.A. Hjorth and H. Ryde, Nucl. Phys. A154 (1970) 641. [2] D. Barn~oud et al., Nucl. Phys. A154 (1970) 653. [3] H. Hiibel, R.A. Naumann and E.H. Spejewski, Phys. Rev. C4 (1971) 2272. [4] S. Allam and K.-H. Kaun, Rossendorf annual report 1971, p. 68. 445
Volume 55B, number 5
PHYSICS LETTERS
[5] C. Foin, Th. Lindblad, B. Sk~nberg and H. Ryde, Nucl. Phys. A195 (1972) 465. [6] P. Kemnitz et al., 12th Conf. on Nuclear structure and nuclear spectroscopy, Dubna (1971) p. 153. [7] V.G. Soloviev and S.I. Fedotov, Izv. Akad. Nauk, Set. Fiz. 36 t1972) 706.
446
17 March 1975
[8] C. Foin, S. Andr6 and S.A. Hjorth, Nucl. Phys. A219 (1974) 347. [9] S. Andr6 and G. Margotton, I.S.N. Grenoble, annual report 1973, p. 89. [10] T.L. Khoo et al., Phys. Rev. Lett. 28 (1972) 1717; T.L. Khoo, J.C. Waddington and M.W. Johns, Can. J. Phys. 51 (1973) 153 and 2307.