Level scheme of
65Ga
P. BANERJEE, B. SETHI, M. B. CHATTERJEEand R. GOSWAMI
Saha Institute of Nuclear Physics - 1/AF, Bidhannagar, Calcutta 700 064, India
(ricevuto il 9 Settembre 1996; revisionato il 7 Agosto 1997; approvato il 19 Dicembre 1997)
Summary. — The level scheme of65Ga has been studied up to an excitation energy of 7035.3 keV and spin-parity values of (27/21). Gamma-ray singles, gg-coincidences, angular distributions and Doppler shift attenuation have been measured in the reaction 63Cu(a, 2ng) at 30 MeV. The spin and parity (Jp) assignments for several
states belonging to the previously known bands are confirmed. The mean life and the parity of the 2814.7 keV state, hitherto unknown, are reported. The Jp for the
3733.2 keV state is proposed to be 15/21on the basis of angular distribution data and its decay characteristics to the lower-lying states. Experimental evidence is presented in favour of the 3733.2 keV state of being a bandhead for a rotation-like DI 42 positive-parity band with the excited members at 4547.5, 5643.5 and 7035.3 keV. The deduced B(E2 ) results indicate the presence of appreciable collectiv-ity in the other positive-parcollectiv-ity band built on the 9/21, 2037.3 keV state. These bands in65Ga are discussed and compared with similar bands in the neighbouring odd-A nucleus67Ga.
PACS 21.10 – Properties of nuclei; nuclear energy levels. PACS 27.50 – 59 GAG89.
1. – Introduction
Most of the excited states of the light odd-mass gallium isotopes (Z 431)65 -69Ga can
be grouped into several bands arising from the weak coupling of a quasi-particle occupying the pf5 /2, pp3 /2and pg9 /2orbitals to the states of the neighbouring even-even
cores. These bands generally exhibit vibrational collectivity due to the vibrational nature of the core states, and is reflected in the observed enhancements of the B(E2 ) rates over the single-particle estimates by factors of 5–20 for the intra-band transitions. In addition, there is a clear evidence [1] for the existence of a positive-parity quasi-rotational band in the nucleus67Ga, built on a 15/21, 3578 keV state, which
extends up to 27/21. The energy spacings of this DI 42 band are observed to be in good
agreement with the I(I 11) rotational energy rule. Zhu et al. [2] have recently reported that in65Ga also, a similar cascade of several E2 transitions terminates on a
3733 keV state. The decay pattern of this state suggests that this is a new bandhead involving a change in structure with respect to the structure of the other bands in this
P.BANERJEE,B.SETHI,M.B.CHATTERJEEandR.GOSWAMI 1366
nucleus. However, the spins and parities of the 3733 keV state and the higher-lying states are uncertain. While tentative spin assignments of J 4 (13/2, 15/2) and ( 17 /2 , 19 /2 ) have been made for the 3733 and the next higher state at 4547 keV, respectively, negative parity has been tentatively assigned to the other states which decay down to the 3733 keV level [2, 3]. Also, there is no lifetime data to supplement the understanding of these levels.
The present work is undertaken to study the characteristics of the 3733 keV state and the higher-energy states which terminate on this level in an attempt to understand whether this sequence of states in65Ga constitutes a band similar to the 15/21band in 67Ga or not. Information for some lower-lying negative-parity states, relevant to the
study of the 3733.2 keV level and some results for the positive-parity band built on the 2037.3 keV level obtained in this work are also presented.
2. – Experimental method
In the present work, the level scheme of 65Ga has been studied up to an excitation energy of 7035.3 keV using the reaction 63Cu(a, 2ng) at 30 MeV. The a-beam was
provided by the Cyclotron at the Variable Energy Cyclotron Centre, Calcutta. A target of 99% enriched63Cu with thickness of 10.3 mg/cm2was used for all experiments except
for the Doppler shift attenuation (DSA) measurements, for which a 8 mg/cm2 natural
Cu foil was used. This was done to avoid the uncertainties associated with the estimation of the stopping time (required in the analysis of DSA data) of the recoils in the enriched target, which was prepared by deposition. Three large-volume HPGe detectors, one with an anti-Compton shield, with in-beam energy resolutions in the range 2.5–3.0 keV at a g-ray energy of A1 MeV, were used. Conventional fast NIM electronics was employed. Measurements of the g-ray intensities, angular distribu-tions, DSA and gg-coincidences were made. The angular distributions were measured at five angles between 907 and 1457 with respect to the beam direction. In the gg-coincidence experiments, a total of approximately 3.5 3 107 events were registered
from which the g-ray coincidence relationships were studied.
Gated spectra were generated by setting appropriate digital gates on the energy and time spectra and subtracting the contributions due to random and the Compton events. The level lifetimes were estimated following the method outlined in ref. [4] and the other references cited therein. The angular distribution coefficients (Ak/A0,
k 42, 4) were determined from a least-squares fit of the normalised g-ray yields to W(u) 4A01 A2P2( cos u) 1A4P4( cos u). The spin and parity assignments to the levels
were made from the g-ray angular distribution data on the basis of the x2(s, d) analyses
(s is the width of the assumed Gaussian magnetic substate distribution and d is the g-ray multipole mixing ratio) performed as described earlier [4, 5] and considerations of the deduced transition rates. The fitted value of s shows a high degree of alignment. A spin and multipole mixing ratio result is rejected if the corresponding minimum x2(s , d) exceeds the 0.1% confidence limit.
3. – Results and discussion
The singles g-ray spectrum obtained in the reaction 63Cu(a , 2 ng) at 30 MeV is
shown in fig. 1. Only the g-ray lines belonging to 65Ga are marked in the figure. A partial level scheme of65Ga is shown in fig. 2. The energies, intensities and the spin and
Fig. 1. – Partial g-ray singles spectrum observed at 907 to the beam direction in the reaction 63Cu(a , 2 ng) at 30 MeV. Only the g-rays belonging to65Ga are indicated by their energies in keV. Most of the other strong g-ray lines belong to65Zn.
parities (Jp) shown in the figure are from the present work. The Jp values for levels
below 2 MeV are from earlier work [3, 6]. Previously, Kawakami et al. [6] had reported the level scheme up to 4547.5 keV. Higher-lying states, up to 8613 keV, have been reported in the literature only by Zhu et al. [2]. The present gg-coincidence data support the placement of all transitions in the level scheme reported in ref. [2, 6], with the exception that the 5643.5 keV level appears to be fed by a 1391.8 keV g-ray, leading to a level at 7035.3 keV. Zhu et al. [2] had reported a level at 7040 keV decaying to the 5643.5 keV state by a 1396.1 keV transition. Although both 1391.8 and 1396.1 keV g-rays are observed in the singles (fig. 1) and the all-gated spectra, only the 1391.8 keV transition appears in coincidence with the other g-rays in the cascade. The other difference in the present decay scheme with respect to the reported level scheme [3]
P.BANERJEE,B.SETHI,M.B.CHATTERJEEandR.GOSWAMI 1368
Fig. 2. – The level scheme of65Ga based on the present and previous work [3,6]. The energies are given in keV and the relative intensities are shown within parentheses.
relates to the intensity of the 918.5 keV transition connecting the 3733.2 and 2814.7 keV states. This is discussed subsequently in the text. States above 7035.3 keV, if any, are not observed in the present work, possibly due to limitations in the input angular momentum.
Relative intensities are measured from the singles spectrum, recorded at 557 to the beam axis, for all transitions except for a few cases where there is either a contaminant line in the spectrum or a g-ray has been doubly placed. Based on the measured relative intensities of the 814.3 and 1391.8 keV transitions from the singles data, the 1096.0 keV transition (doubly placed) from the 5643.5 keV state has been tentatively assigned a relative intensity of A5 with respect to 100 for the 190.8 keV g-ray. These results along with the g-ray angular distribution data are presented in table I. The lifetime results are summarised in table II.
TABLE I. – Gamma-ray energies, intensities, angular distribution coefficients, multipolarities
and spin and parity (Jp) assignments for65Ga. The Jpassignments are discussed in the text.
Level energy Ex( keV ) g-ray energy Eg( keV ) Relative intensity Angular distribution coefficients Mixing ratio d Multipolarity Jp i K Jfp A2/A0 A4/A0 190.8 190.8 100 65.0 — — — — 1075.3 884.5 10.2 6 0.7 — — — — 1075.3 14.9 6 2.0 — — — — 1287.1 1096.3 A76 — — — — 2037.3 750.2 (a) A 28(b) — — — — 962.0 12.1 6 2.0 20.10 6 0.07 20.03 6 0.08 E1 91/2 K72/2 1846.5 1.6 6 0.5 — — — — 2789.2 1502.1 16.0 6 1.3 0.356 0.09 20.00 6 0.11 E2 132/2K92/2 2814.7 1527.6 7.3 6 0.8 20.29 6 0.12 — 23.060.8 M1 /E2 112/2K92/2 3064.3 1027.0 24.4 6 1.3 0.36 6 0.06 0.01 6 0.07 E2 131/2K91/2 3733.2 668.9 3.6 6 0.4 — — — — 918.5 E 1 — — — — 944.0 9.5 6 0.7 20.29 6 0.16 0.05 6 0.21 20.0560.06; E1 151/2K132/2 4122.1 1057.8 11.5 6 0.7 0.27 6 0.14 20.05 6 0.19 E2 171/2K131/2 4330.5 1266.2 (a) — — — — — 4433.2 310.1 1.5 6 0.5 — — — — 4547.5 814.3 6.5 6 0.8 0.31 6 0.14 20.05 6 0.18 E2 191/2K151/2 424 E 1 — — — — 5022.1 900.0 5.2 6 0.6 0.23 6 0.09 — E2 211/2K171/2 5643.5 1096.0 (a) A 5 — — E2c (231/2)K(191/2) 7035.3 1391.8 4.9 6 0.6 0.26 6 0.09 — E2 (271/2)K(231/2) (a) Energy determined from coincidence data.
(b) Intensity determined from coincidence data. (c) From ref. [2].
P.BANERJEE,B.SETHI,M.B.CHATTERJEEandR.GOSWAMI 1370
TABLEII. – Results on level lifetimes and transition rates in65Ga. Level energy Ex( keV ) g-ray energy Eg( keV )
JipK Jfp Mean life t (ps) Transition rate
Present Previous ref. [3] 2789.2 1502.1 132/2K92/2 G 1.0 0.9 6 0.4 B(E2 ) F6.9 W.u. 2814.7 1527.6 112/2K92/2 0.8 6 0.3 — B(E2 ) 47.114.8 22.3W.u. B(M1)41.012.0 20.6mW.u. 4122.1 1057.8 171/2K131/2 G 0.9 — B(E2 ) F44 W.u. 4547.5 814.3 191/2K151/2 F 3 — B(E2 ) G30 W.u.
5022.1 900.0 211/2K171/2 A 2 — B(E2 ) A45 W.u.
The 3733.2 keV state was first reported by Kawakami et al. [6], decaying to the 2789.2, 2814.7 and 3064.3 keV states with decay branches of 62% (944 keV), 15% (919 keV) and 23% (669 keV), respectively. They also proposed a tentative Jp
assignment of 13/2(15/22) for the 3733.2 keV state. No parity assignment is shown for
this state in ref. [3]. Besides, the previous spin and parity assignments for the three states to which the 3733.2 keV state decays are ambiguous. In the present work the properties of all the four states have been studied in order to understand clearly the decay characteristics of the 3733.2 keV level. The results for the 2789.2, 2814.7 and 3064.3 keV states are presented in subsect. 3.1, followed by the discussion on the 3733.2 keV state and the higher-energy states which terminate on this state, in subsect. 3.2. Finally, subsect. 3.3 briefly summarises some results obtained in this work for the other positive-parity band in this nucleus built on the 2037.3 keV level.
3.1. The 2789.2, 2814.7 and 3064.3 keV states. – The 1502.1 keV transition from the 2789.2 keV state is observed to be pure quadrupole in nature from the present angular distribution data (table I). Since the 1287.1 keV level to which the 2789.2 keV level decays has been previously [3] assigned Jp4 9 /22, the latter level is proposed to have
a spin and parity of 13/22. The DSA measurement for this state leads to a B(E2 ) F
6.9 W.u. for the inband 1502.1 keV transition and supports the proposed Jpassignment.
For the 2814.7 keV level, possibly belonging to the same negative-parity band as the 2789.2 keV state, probable spin values of 9/2 and 11/2 were previously proposed, based on g(u) studies, with no information on parity [3]. In this work, the g(u) and x2analyses
for the 1527.6 keV transition from the 2814.7 keV state favour J 411/2 and a mixing ratio of d 42 3.0 6 0.8. The d-value is in significant disagreement with the result reported in ref. [6]. The mean lifetime for the 2814.7 keV state, hitherto unreported, is found to be 0.8 6 0.3 ps from DSA data, with the error arising mainly from the uncertainty in the feeding time of the state from the continuum. The cascade feeding from higher-lying states is insignificant. The meanlife and the multipole mixing ratio lead to a reduced transition probability B(E2 ) 47.114.8
22.3W.u. for the 1527.6 keV g-ray
with the assumption that the 2814.7 keV state has a negative parity. A positive parity for the state would result in an unrealistically large enhancement of the M2 rate (376 W.u.). Hence, the 2814.7 keV level is unambiguously assigned Jp
4 11 /22. The angular distribution results for the 1027.0 keV g-ray depopulating the 3064.3 keV level (table I) clearly indicate that this transition is pure E2 in nature and confirm the
hitherto uncertain Jp assignment [3] of 13/21 for the level, which is a member of the
positive-parity band built on the 2037.3 keV, 9/21state.
3.2. The 3733.2 keV state and the sequence of states terminating on the 3733.2 keV state. – The Jp assignments to the three states at 2789.2, 2814.7 and 3064.3 keV
obtained in this work now permit an evaluation of the properties and decay characteristics of the higher-lying state at 3733.2 keV, which is a candidate for a band with different intrinsic structure compared to the other known bands in this nucleus. As stated earlier, the Jp for the 3733.2 keV state was proposed to be 13/2(15/22) by
Kawakami et al. [6]. In the present work, the angular distribution and the x2(s , d)
analyses for the 944.0 keV transition (fig. 3), de-exciting the 3733.2 keV state (to the 2789.2 keV, 13/22 state), rule out the spin J 413/2 for this state. The present result
leads to a most probable spin sequence of 15 /2 K13/22for this g-ray with d 42 0.05 6
0.06, indicating that the transition is almost pure dipole in nature and that J 415/2 for the 3733.2 keV level. The small value of the multipole mixing ratio for the 944.0 keV transition obtained in this work disagrees significantly with the result reported in ref. [6]. In contrast to the result of ref. [6], the present d-value does not rule out the assignment of E1 multipolarity to the 944.0 keV transition and hence a positive-parity assignment to the 3733.2 keV state. In addition, the intensity of the 918.5 keV transition from the 3733.2 keV state to the 2814.7 keV state is observed to have a vanishingly small intensity in this work, in contrast to the 15% branching for the same transition reported in ref. [6]. The assignment of positive parity to the 3733.2 keV state, with Jpfor the 2814.7 keV state being established to be 11 /22in this work, implies an
M2 multipolarity for the 918.5 keV transition. This is consistent with the poor intensity observed for the 918.5 keV g-ray in this work. Furthermore, the decay of the 3733.2 keV level to the 3064.3 keV, 13/21state, which belongs to the band built on the
2037.3 keV, 9/21 state with the configuration of a proton in the g
9 /2 orbital weakly
coupled to the64Zn core, is observed to account for 23% of the branching. This indicates that there is a large overlap in the configurations of the pg9 /2 band and the 3733.2 keV
Fig. 3. – Results of the g-ray angular distribution study for the 944.0 keV transition. In a) are shown the normalised g-ray yields at five angles with respect to the beam direction. The continuous line represents the fit of the angular distribution function W(u) to the experimental data. The results of the x2(s , d) analysis for three spin sequences are shown in b).
P.BANERJEE,B.SETHI,M.B.CHATTERJEEandR.GOSWAMI 1372
level. Also, the assignment of spin J 415/2 for the 3733.2 keV level would imply that the dominant configuration for the level should involve an odd-proton in the g9 /2orbital,
with the two other protons outside Z 428 closed shell distributed in the lower fp-shells. Such a configuration also would be consistent with the assignment of positive parity to the 3733.2 keV state. Hence, the 3733.2 keV state in65Ga is proposed to have Jp
4 15 /21 in this work.
Three other states at 4547.5, 5643.5 and 7035.3 keV, decaying by the 814.3, 1096.0 and 1391.8 keV g-rays and terminating on the 3733.2 keV level, have been observed in this work. They apparently form a band with the 3733.2 keV state as the bandhead. In the previously reported work [2, 3], while no parity assignment is shown for the 3733.2 and 4547.5 keV levels, the other two states were tentatively proposed to have negative parity. No clear basis is evident for such assignment. With the positive-parity assignment for the bandhead as discussed above and the cascade of transitions terminating on this state behaving like stretched E2 transitions (discussed below), all states in this sequence are proposed to have positive parity.
The angular distribution for the 814.3 keV g-ray yields Ak/A0 values which are
consistent with those reported by Kawakami et al. [6] and are typical for E2 transitions. This result along with the present x2(s , d) analysis leads to a Jpassignment of 19/21
for the 4547.5 keV state. This state has been previously assigned J 4 (17/2, 19/2) in the literature [3] with no information on the parity. No measurement could be made for the 1096.0 keV g-ray from the 5643.5 keV level due to the strong interference from the 1096.3 keV g-ray depopulating the 1287.1 keV level. Assuming that the 1096.0 keV transition is stretched E2 in nature, as suggested by Zhu et al. [2] and as also observed from systematics, the 5643.5 keV level is tentatively assigned Jp4 ( 23 /21). The g(u)
for the 1391.8 keV g-ray, which has not been reported previously, leads to A2/A04
0.26 6 0.09 and is consistent with a spin-parity of (27/21) for the 7035.3 keV state.
A lower limit of mean life of t F3 ps is estimated for the 4547.5 keV level on the basis of the observation of small Doppler shift in the 814.3 keV g-ray. This would lead to a B(E2 ) G30 W.u. for the 814.3 keV g-ray. Although conclusive evidence is not provided by this result, it nevertheless indicates that collectivity may exist in the structure of the 4547.5 keV state. The sequence of states at 3733.2, 4547.5, 5643.5 and 7035.3 keV, connected by E2 transitions, thus appears to form a positive-parity band with energy spacings in fairly good agreement with the I(I 11) rotational energy rule.
A comparison of the bands based on the 15/21states in 65Ga and 67Ga is shown in
fig. 4. It is noticed that both 65Ga and 67Ga have a positive-parity band built on 15/21
states at similar excitation energies of 3733.2 and 3577.8 keV, respectively. No theoretical calculations are available for these states in65Ga. For 67Ga, Zobel et al. [1]
have proposed that the configuration for the 15/21 state and the band built on this
could be associated with a broken particle pair in the pf5 /2 orbital coupled to a g9 /2
proton, leading to a stable deformation on which a quasi-rotational band is built. The configuration is supported by Alaga model calculations [1] in which a three-proton cluster in the f5 /2 and g9 /2 shells was coupled to the quadrupole vibrations of the 64Ni
core. Zobel et al. [7] have subsequently also reported that according to the results of Alaga model calculations, the configuration [( f5 /2)24, g9 /2]15 /2 can give rise to 15/21
bandheads in both65 , 67Ga. It is to be noted, however, that the 15/21bandhead in65Ga
has strong decay branches to both the negative as well as the positive-parity bands, while in 67Ga, the bandhead is known to decay only to the 13/21 state. Detailed
theoretical calculations would be necessary to explain the observed differences in the decay properties of the 15/21bandheads in the two nuclei.
Fig. 4. – Bands built on 15/21states in65Ga and67Ga. The Jp assignments in65Ga are from the present work. The level scheme of67Ga is from ref. [1].
3.3. The positive-parity band built on the 2037.3 keV state. – The Jp for the
2037.3 keV level is established from earlier work to be 9/21[3, 6]. The present g(u) and
the x2(s , d) analyses for the 1027.0 (already discussed in subsect. 3.1), 1057.8 and
900.0 keV g-rays, which de-excite the higher members of this band at 3064.3, 4122.1 and 5022.1 keV, respectively, clearly show that these are stretched E2 transitions (table I). Therefore, the 3064.3, 4122.1 and 5022.1 keV levels are assigned Jp values of 13/21,
17/21and 21/21, respectively. The 900.0 keV transition from the 5022.1 keV level is
observed to have a small Doppler shift which, following corrections arising from direct feeding, corresponds to a meanlife of A2 ps. The 1057.8 keV g-ray which depopulates the 4122.1 keV state is found to have a Doppler shift attenuation coefficient F(t) 40.14. Corrections due to cascade feeding from the 5022.1 keV state by the 900.0 keV g-ray, and side feeding from the continuum, with its associated uncertainties, lead to a mean life of 0.510.4
20.2ps for the 4122.1 keV state. However, in view of the large errors in this
result, only an upper limit of mean life of 0.9 ps is assigned to the 4122.1 keV state (table II). The deduced transition rates of B(E2 ) F44 W.u. for the 1057.8 keV transition, obtained from the upper limit of mean life derived for the 4122.1 keV level and B(E2 ) A45 W.u. for the 900.0 keV transition from the 5022.1 keV state lend additional support to the Jpassignments for these levels. Also, the large enhancements
of the B(E2 ) values with respect to the single-particle estimate are indicative of the presence of appreciable collectivity in this band.
Although bands built on the 9/21state are observed in both65 , 67Ga, their structures
appear to be somewhat different. While the members of this band in 67Ga clearly fit
with an interpretation in terms of the weak coupling of the g9 /2 proton to collective
excitations of the 66Zn core [1], where even the change in nuclear structure of the 81
core state is reflected in the odd-A spectrum, a similar interpretation for this band in
65Ga does not appear to be strictly valid. The strongly compressed nature of the
spectrum of states in65Ga compared to64Zn, and the larger enhancements of the B(E2 )
rates for the 21/21
K 17 /21and 17/21
K 13 /21transitions in comparison with those for the 61
1 K 411 and 411 K 211 core transitions [8], reflect the inadequacy of the weak
coupling model for explaining this band in 65Ga and possibly indicate appreciable polarisation of the core by the odd proton.
P.BANERJEE,B.SETHI,M.B.CHATTERJEEandR.GOSWAMI 1374
* * *
The authors thank the cyclotron operating staff and the members of the target preparation laboratory at the Variable Energy Cyclotron Centre, Calcutta for their help in running the accelerator and preparation of the targets, respectively. The help rendered by several research students and technical personnel during the experiments is also thankfully acknowledged.
R E F E R E N C E S
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