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Study of excited levels in 149Pm
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A 609 (1996) 201-217
Study of excited levels in 149pm M.A. Jones a, W. Urban a, J.L. Durell a, M. Leddy a, W.R. Phillips a, B.J. Varley a, p.j. Dagnall a, A.G. Smith a, D.M. Thompson a, Ch. Vieu b, J.S. Dionisio b, C. Schuck b, M. P a u t r a t c a Department of Physics and Astronomy, University of Manchester, Manchester MI3 9PL, UK b Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse, IN2P3-CNRS, 91405 Orsay, France c Institut de Physique Nucleaire, IN2P3-CNRS, 91405 Orsay, France
Received 17 July 1996
Abstract
Excited states in 149pm have been studied through the 15°Nd(d, 3n) reaction, y - y and electrony coincidence data have enabled partial level and decay schemes to be extended up to spin (27/2). The spins and parities of excited levels have been deduced from measurements of internal conversion coefficients, decay schemes, and from comparison to the N = 88 isotones 15~Eu and 153Tb. The data are discussed in terms of the strength of octupole correlations. PACS: 21.10.Re; 21.30.Pc; 23.20.Lv; 27.60.+j Keywords: Nuclear reactions 15°Nd(d, 3n), E = 18 MeV; Measured E~,, yy, ey, ICC, 149pmdeduced levels,
J, ~r, y-ray intensity, branchings; Multi-particle configurations,octupole deformation
1. Introduction
Octupole correlation effects in nuclei have recently been of much interest both experimentally and theoretically [ 1 ]. These effects are largest when, for both protons and neutrons, a pair of opposite-parity, single-particle orbitals with Al = Aj = 3 (where Al and Aj are respectively the differences in single-particle orbital and total angular momentum quantum numbers) lie close to each other in energy. Sequences of levels of alternating parity connected by electric dipole transitions, which may be strong, are characteristic features of nuclei in which strong octupole correlations play a significant role. The nucleus 149pm lies in the region of transitional lanthanides where the valence neutrons and protons occupy orbitals which satisfy the above conditions. Alternatingparity bands have been found in the N = 86 even-even isotones 146Nd [2] and 148Sm [3] 0375-9474/96/$15.00 Copyright (~) 1996 Published by Elsevier Science B.V. All rights reserved PII S0375-9474(96) 00308-9
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M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
and interpreted in terms of octupole deformation induced by nuclear rotation. The structure of 147pm, with N = 86, has been investigated [4] and evidence for strong octupole correlations has also been found. The study of the N = 86 nucleus 149Eu [5] revealed an alternating-parity structure at low spin values which was not found to persist to higher spins. Sizes of intrinsic electric dipole moments, determined within the rotational model from electric dipole transition rates, suggest that the correlations are weaker in 149Eu than in 146Nd, 1485m and 147pm. Features appropriate to octupole deformation at medium spins have also been found in the even-even N = 88 isotones 148Nd [6] and 15°Sm [7]. The study of the N = 88 nucleus, 151Eu [8,9], has revealed quasi-rotational level sequences which have been interpreted in terms of bands arising from an octupole phonon and decoupled 7rh11/2 states. Sizes of intrinsic dipole moments in this nucleus are larger than in I49Eu and increase with increasing spin, in contrast to 149Eu, reaching values similar to those observed in 148Nd and 15°Sm. Results obtained for the N = 90 isotopes, 151pm [ 10,1 1 ] and 153Eu [ 12], show some characteristics of octupole deformation, although the magnetic properties of the observed opposite-parity bands support reflection-symmetric, rather than asymmetric descriptions of these nuclei. This reflection symmetry may result from quadrupole correlations dominating over weaker octupole correlations at N = 90. Systematics of the sizes of intrinsic electric dipole moments in this lanthanide region of nuclei [ 10] suggest that octupole correlations are stronger at N = 88 than at N = 86 or 90, and one might therefore expect to see pronounced effects in 149pm as are seen in the N = 88 nuclei already examined. In this work, we report on the structure of the nucleus 149pro produced in the tS°Nd(d, 3n) reaction. The spins and parities of excited levels are assigned on the basis of internal conversion coefficient measurements, decay patterns, and from comparison with the N = 88 isotones 151Eu and 153Tb. An extended level scheme is presented which is discussed from the point of view of the strength of octupole correlations.
2. Experimental procedure The experiment was performed at the MP tandem accelerator at Orsay using the JS°Nd(d, 3n) reaction at a beam energy of 18 MeV. The target used was a thin layer of 15°Nd (surface density 125/xg cm -z) deposited on a carbon backing (30/zg cm-2). Inbeam, prompt y - y and electron-y coincidences were measured using the Orsay electron spectrometer [ 13] to detect electrons and four Compton-suppressed Ge detectors to observe y rays. The y - y and electron-y coincidence data obtained were sorted into two-dimensional y - y and e - y coincidence matrices which contained approximately 6.4 × 106 and 6.8 × 106 double coincidence events, respectively. The energy ranges on the y and electron axes of the matrices were approximately 1.5 MeV and 500 keV, respectively. Using coincidence data allowed the production of clean, gated y-ray and electron spectra, thus avoiding
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
203
23/2 + (2122.3)
(1885.3) (21/2-)
21/2 + (1739.0)
/
355.8 (1591.1) (19/2-)
19/2 ÷ (1549.5)
I
(1406.7) 17/'2-
32O.0
r
17/2 + (1229.5)
(1145.6) 15/2-
1 J/illl-
15/2 + (1008.4)
(1006.5) 13/2-
-
13/2+ (779.3)
-
(791.5) 11/2-
281~ 502.9 11/2 +
9/2 +
7/2 +
(0.0)
-
-
tl 149~ 61
h'ma8
Fig. 1. Partial decay scheme of 149pm as obtained in the present work. The two opposite-parity bands based on the ground state.
problems caused by contamination peaks and a high density of lines encountered when analysing singles spectra.
3. Data analysis and results Prior to this work, 149pm has been studied in the t-decay of 149Nd [14-16] from the 15°Nd(p, 2ny) reaction [ 17] and from the 15°Sm(t, at) reaction [18]. From these studies, levels up to 1.7 MeV in excitation energy and up to spin (15/2) have been reported. Identification and assignment of new y rays to 149pm was achieved by gating in the 3/-y matrix on known transitions reported from the previous studies. Partial level schemes of 149pm resulting from analysis of the present data are shown in Figs. 1, 2 and 3. The quality of the data in the Y-Y matrix is illustrated in Fig. 4, which shows three spectra obtained by gating on previously known transitions at 497.9 (shown in Fig. 1 ), 1 14.3 (Fig. 2) and 269.9 (Fig. 3) keV. Information on the multipolarities and parities of transitions was obtained by measuring internal conversion coefficients (ICC's). By gating on the total-energy peak of a particular ),-ray transition in the 7-'7' and e - 7 matrices, respectively, spectra of y
204
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217 (~1.1~) 11'2 ÷
11/2- (791J)
(6513) 7/2 "~
I
1/T
71i" 149~
611Jm88
Fig. 2. Partial decay scheme of 149pm as obtained in the present work. Low-spinexcitations. rays and electrons in coincidence with that particular 3/ray are obtained. Numbers may then be extracted for coincident decays from a level populated by the gating 3/ ray: the number of coincident 3/rays, N(3/0), and numbers of coincident electrons resulting from the internal conversion of 3/0 from different electron shells. Exactly the same gate must be set in both matrices. If N ( e o ) is the number of electrons resulting from internal conversion from the K shell, the K-shell ICC, a x , for the transition 3/0 is given by _
ax(3/o)
N(eo)
"qz,(3/o) C,
(1)
rle(eo~) N(3/o)
where r/~(e0) is the relative efficiency for electron detection at the K-electron energy, and r/r(3/0) the relative efficiency for 3/-ray detection at the energy of 3/0. These were obtained by using electron and 3/ radiation from the decay of the 31 year half-life isomer in 178Hf. Additional 3/-ray relative efficiency points were obtained from 3/-ray coincidences in the experimental data. The overall normalisation constant C was obtained by using a known electric quadrupole (E2) transition produced via Coulomb excitation of the 15°Nd target. The K and L-shell ICC's were measured for the 2 + --* 0+ transition by gating on the 4+ ~ 2+ and 6 + --~ 4~- 3/rays, and then compared with interpolations of theoretical predictions [ 19]. This yielded a value of C = 0.57(4). The quality of the data used to obtain ICC's is illustrated in Fig. 5, which shows the 3/-ray and electron spectra obtained after gating on 3/-ray transitions in the 3/-3/and e-3/matrices, respectively. Fig. 6 shows K-shell ICC's obtained for transitions in 149pm. Due to low statistics, it was only possible to measure aK values, with the exception of the 114.3 keV transition, where the L-shell conversion coefficient was also determined. Table 1 shows, for transitions identified in 149pro, the 3/-ray energies obtained, their relative intensities, their measured K-shell ICC's and their proposed multipolarities. The multipolarities are proposed from previous studies, ICC measurements, decay patterns
205
M.A. Jones et aL/Nuclear Physics A 609 (1996) 201-217 f27/2 ) ( 2 1 1 2 ~) -
-
f
- ~ (i9236)(23/2+)
~11~7
(23/2)(15048)
-
/
(19/2)(9570)
15/2
(510.2)
11/2
f240 2)
....
•
(I 162.8) (I 5/2 + )
35{3) las
7g2+ (o o)
149 61
Pm88
Fig. 3. Partial decay scheme of 149pmas obtained in the present work. Excitations above the zrhll/2 isomer. and from comparison with the N = 88 isotones 151Eu and 153Tb. In the last column of Table 1, the energy of the excited level in 149pm depopulated by the corresponding ~, ray is given.
4. Level scheme and spin and parity assignments Partial level schemes of 149pm resulting from analysis of the present data are shown in Figs. 1, 2 and 3. Our data, where there is overlap, confirm the excitation scheme reported previously, and significantly extend information on levels in quasi-rotational bands based on the ground state and the 240.2 keV isomeric state.
206
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
Gate: 497.9 keV
tt~
i
'
.
~
oo.
m "~ !=1 = 10-~
q
Gate: 114.3 keV
,II
| Gate: 269.9 keV
10
200
300
400
7
500
600
700
energy ~eV]
Fig. 4. Spectra obtained by gating in the ~-~ matrix on previously known transitions at 497.9 (shown in Fig. 1), 114.3 (Fig. 2) and 269.9 (Fig. 3) keV. The main coincident ~, rays are labelled with their transition energies in keV.
4.1. The two opposite-parity bands
These two bands are shown in Fig. 1. ICC measurements for 3, rays in this part of the level scheme are shown in Table 1 and Fig. 6a. The ground-state spin and parity were previously established from atomic-beam [20] and fl-decay [14] work to be I # = 7/2 +. The 288.5, 497.9 and 779.3 keV levels have previously been observed [17] and assigned as 9/2 +, (11/2 +) and (13/2+), respectively. The ICC measurements are in agreement with these assignments. We have observed an additional five levels above the 779.3 keV state which are assigned spin and parity, as shown in Fig. 1, of 15/2 +, 17/2 +, 19/2 +, 21/2 + and 23/2 + from the ICC measurements and their decay modes. The sequence of positive-parity levels based on the ground-state approximates a rotational band. The excited state at 791.5 keV has previously been observed [ 18] and assigned as 11/2-. The measured ICC for the 502.9 keV transition is in agreement with this 3' ray being of electric dipole ( E l ) character and therefore supports the 11/2- assignment to the 791.5 keV level. The measured ICC for the 508.9 keV transition is also in agreement
207
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
(a)
:J t~
1
~
--
*
~J
(b)
,...,15"
!
× 2.5
:I0
E k
m~ m o
1 O0
200
300
400
500
600
Channel No. Fig. 5. The projected y-ray (a) and electron (b) spectra obtained after gating on the y axes in the Y-Y and e-y matrices, respectively. The main coincident y rays are labelled with their transition energies in keV, and the corresponding electrons with their K and L-conversion character. The 130.2 and 251.2 keV y rays indicated by asterisks are transitions in 15°Nd, produced in Coulomb excitation. The spectra were generated by summing individual gates on the 114.3, 155.9, 198.5 and 246.0 keV transitions (shown in Fig. 2). The vertical scale for the portion of the electron spectra to the fight of the vertical dotted line is increased by a factor of 2.5. with this 9' ray being o f E1 character and thus allows spin and parity of 1 3 / 2 - to be assigned to the 1006.5 keV level. We observe four other levels above the 1006.5 keV state which are assigned spin and parity, as shown in Fig. 1, of 1 5 / 2 - , 1 7 / 2 - , ( 1 9 / 2 - ) and ( 2 1 / 2 - ) on the basis o f their decay modes and ICC measurements for the 354.2, 366.4 and 398.4 keV transitions. The sequence of negative-parity levels based on the 791.5 keV state approximates a rotational band. The positive-parity band based on the ground state and the negative-parity band based on the 791.5 keV 1 1 / 2 - state are connected via several crossover transitions. Of these, the 366.4, 398.4, 502.9 and 508.9 keV transitions are consistent with the stretched E1 character required by the level assignments made above, as supported by the ICC measurements for these 3/ rays. ( A stretched 7 ray has a pure multipolarity L, and makes a transition from a state o f spin I to a state o f spin ( I - L ) ) . Although it was not possible to measure I C C ' s for the 293.6, 142.7, 355.8 and 237.0 keV 3' rays, they
208
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217 7x
T
0.20 0.15' 0.10' 0.05' 0.00 4'
r..; r,.)
I
I,\
x 10
(b)
3 2' 1" O
0.15"
0.10"
0.05"
0.00
0
I O0
200
300
400
500
600
? energy [keV] Fig. 6. Measured aK values for transitions in ]49pm. Values corresponding to transitions shown in Figs. 1, 2 and 3 are shown in panels (a), (b) and (c), respectively. The vertical scales for the right-hand portions of each panel have been increased by factors of 3, l0 and 3 for panels (a), (b) and (c), respectively. Solid, dotted and dashed lines represent interpolations of theoretical values [ 19] for El, E2 and M1 transitions, respectively. Filled and open circles represent those transitions assigned to be of E1 and E2 characters, respectively, diamonds representing those assigned to be of a mixed M1 +E2 character.
are inferred to be of E1 character from the spin and parity assignments made to levels above. An analogous situation is observed in 147pm [4], where two similar quasi-rotational bands are found; a positive-parity band based on the ground state and a negative-parity band based on an excited 1 1 / 2 - state. These bands are also connected via E1 transitions.
4.2. Low-spin excitations Fig. 2 shows several low-spin levels observed in the present data. ICC measurements for y rays in this part of the level scheme are shown in Table 1 and Fig. 6b. In addition to the C~K value quoted in Table 1, the L-shell ICC, aL, was measured for the 114.3 keV transition. This yielded a value of 0.11 (2). The interpolated theoretical values are [ 19] 0.005 for El, 0.014 for E2 and 0.457 for M1. The a r and CtL values for the 114.3 keV y ray are thus both consistent with M I + E 2 character.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
209
Table 1 Energies, y-ray intensities, a r conversion coefficients and proposed multipolarities for transitions in 149pm. In the last column the energy of the excited level in 149pm depopulated by the corresponding y ray shown in column 1 is given. Errors of y-ray energies are 0.3 keV for I~, < 5, I b < 15, 1~ < 10 and 0.2 keV or better for stronger transitions. E z, (keV) 58.6 74.3 114.3 139.3 142.7 155.9 188.6 198.5 199.1 208.2 209.5 211.5 215.0 221.0 229.2 232.6 237.0 240.2 245.5 246.0 250.7 254.3 254.5 256.0 261.4 261.5 265.8 269.9 270.5 273.2 275.4 281.5 282.6 288.5 293.6 310.8 314.2 320.0 354.2 355.8 360.1 366.4 391.4 398.4 401.6 419.0 439.6
t a'b'c -~, 4( 1 )c 40(3) b 1.4(2) a 1.1 (8) a 54(4) b 44(4) b 45(4) h 33(3) h 18( 1 )a 1.2(3) a 1.3 (2) a 7.3(3) a 6( 1 )b 1.5(4) a 1.0(7)': 100 b 14(2) b 5(2) c 5(l)b 4(2) c 20(3) c 0.6(2) a 5( 1)c
aK
2.4(8) 0.8(2)
0.05(2) 0.15(7) 0.17(4) 0.16(6) 0.15(5) 0.15(5) 0.13(7) 0.14(6) 0.02( 1 )
0.09(2) 0.07(3) 0.12(5) 0.06(3) 0.11 (4)
0.06( 1) 18(4) b 10(3) c 12(1) a 12(2) b 1.7(5) a 1.5(3) c 3(1) c 2.1(2) a 3.2(4) a 1.2(2) a 19(2) a 4(1) c 15.1(2) a 1.0(7) c 2(1) c 6(2) c
0.06(4) 0.06(3) 0.08(3) 0.06( 1 )
0.03( I )
0.012(5) 0.008(5)
Multipolarity
Eex (keV)
El M 1+E2 MI+E2 MI+E2 El El E2 MI+E2 MI+E2 MI+E2 MI+E2 MI+E2 MI+E2 M 1+E2 MI+E2 El (El) M2 (MI+E2 M 1+E2 MI+E2 (MI+E2) MI +E2 (E2) (M I+E2) MI+E2 (El) E2 El E2 (MI+E2) MI+E2 MI+E2 MI+E2 El (El) (E2) MI+E2 E2 (El) M 1+E2 El (El) El (E2) (El) (E2)
270.5 188.6 114.3 1145.6 1549.5 270.5 188.6 558.9 387.5 396.8 497.9 211.5 1006.5 1229.5 1008.4 791.5 2122.3 240.2 515.6 360.2 808.8 1211.2 651.3 771.5 771.5 1406.7 1477.0 510.2 270.5 387.5 515.6 779.3 396.8 288.5 791.5 1923.6 1477.0 1549.5 1145.6 1885.3 360.2 1145.6 1162.8 1406.7 1612.8 1923.6 1211.2
210
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
Table 1--continued E~, (keY)
ta'b'c .~
ar
Multipolarity
Eex (keV)
444.7 445.3 446.6 446.8 448.7 450.2 478.2 490.8 497.9 502.9 508.9 509.5 510.5 520.0 531.2 541.1 547.8 572,8 607.6 652.9 655.7 700.8
28(2) b 4.4(6) a 2.2(5) c 100c 20(2) b 22( l )a 0.9(2) a 63 ( 6 ) a 100a 3.4(6) a 10(2)a 3.0(9) a 67(3) a 8(2) c 14(4) c 11.9(8) a 37(5) c 3.2(8) a 11(3) c 6(2) c 11(2) c 13(3) c
0.016(8)
E2 (E2) (E2) (E2) E2 E2 (E2) E2 E2 El E1 E2 E2 (El) (MI+E2) E2 (E2) E2 (E2) (El) (M 1+E2) (MI+E2)
558.9 1591.1 1923.6 957.0 808.8 1229.5 1885.3 779.3 497.9 791.5 1006.5 1739.0 1008.4 1477.0 771.5 1549.5 1504.8 2122.3 2112.1 1162.8 1612.8 1211.2
0.011 (4) 0.013 ( 4 ) 0.015 (7) 0.009( 3) 0.012( 3 ) 0.004(2) 0.003(2) 0.014(5) 0.009(3)
0.009(3) 0.007(2) 0.011(5)
Relative scale where Iv (497.9 keV) is assumed to be 100. b Relativescale where I v (246.0 keV) is assumed to be 100. c Relativescale where Iv (446.8 keV) is assumed to be 100. All of the levels observed here were seen in previous studies and assigned spins and parities (see Ref. [ 17] for example). The ICC's obtained with the present data are in agreement with these proposed assignments, which are shown in Fig. 2. The 114.3, 360.2, 558.9 and 808.8 keV states form a quasi-rotational band, with the 114.3 keV level as the band head. The 558.9 keV member of this band is observed in the present data to be linked to the 791.5 keV 1 1 / 2 - state shown in Fig. 1 via a 232.6 keV transition, which is required to be of E1 character by the spin and parity assignments of the 558.9 and 791.5 keV states. The measured ICC of the 232.6 keV transition is consistent with E1 character and therefore satisfies the above requirement. Since the 232.6 keV y ray is weak, it was not possible to obtain intensities of transitions shown in Fig. 2 relative to those shown in Fig. 1. Three other E1 transitions previously observed are also seen in the present data. These transitions depopulate the 270.5 keV 7 / 2 - level and have energies of 58.6, 155.9 and 270.5 keV.
4.3. E x c i t a t i o n s a b o v e the 2 4 0 . 2 k e V i s o m e r i c level
Fig. 3 shows the excited levels observed in the present experiment above the isomeric level at 240.2 keV. ICC measurements for 3~ rays in this part of the level scheme are shown in Table 1 and Fig. 6c.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
211
The 240.2 keV isomer has been reported in previous studies. Its lifetime has been measured [21] to be 35(3) /zs. This is much longer than the overlap time used in the present experiment for the acceptance of the 7-9' and e-T coincidences, and the ICC for the 240.2 keV 9' ray could therefore not be measured. However, the magnetic quadrupole (M2) character of this transition has been confirmed previously [21], the 240.2 keV level being assigned as 1 l / 2 - . The 510.2 keV state has previously been assigned [17] to be 15/2-. The ICC obtained for the 269.9 keV transition is in agreement with this T ray being of E2 character and hence in agreement with the 15/2- assignment. ICC's for the 446.8 and 547.8 keV 3' rays suggest that these transitions have E2 character. This suggests spin and parity assignments to the 957.0 and 1504.8 keV levels as shown in Fig. 3. The 240.2, 510.2, 957.0, 1504.8 and 2122.1 keV levels may thus constitute a band with the 240.2 keV isomeric level as the band head. The 2112.1 keV level is consequently tentatively assigned to 2 7 / 2 - on the assumption that it is a member of this band. Seven other levels have been observed which feed both directly and indirectly into members of the band based on the isomeric level and into the isomeric level itself. Of these seven levels, the 515.6 and 771.5 keV states have been reported previously [ 17] with assignments of ( 9 / 2 - ) and ( 1 3 / 2 - ) , respectively. The ICC obtained for the 261.4 keV transition is in agreement with the above spin and parity assignments to the 771.5 keV level. All the data on the decay of the 1211.2 keV level are consistent with a spin and parity assignment of ( 1 7 / 2 - ) . The 515.6, 771.5, 1211.2 and 1612.8 keV levels appear to form a band with the 515.6 keV ( 9 / 2 - ) level as the band head. Based on this assumption, the 1612.8 keV level is assigned tentatively to 2 1 / 2 - . The 1162.8, 1477.0 and 1923.6 keV levels are similar in energy and have similar decay modes to states observed in the N = 88 isotone 151Eu [8,9]. The corresponding states in ~SIEu have energies of 1114.0, 1506.9 and 1947.7 keV and have been assigned spin and parity of 15/2 +, 19/2 + and 23/2 +, respectively. These similarities suggests spin and parity assignments of (15/2+), (19/2 +) and (23/2 +) as shown on Fig. 3 to the three levels in 149pm. The 515.6 keV ( 9 / 2 - ) level is additionally linked to the 270.5 keV 7 / 2 - state via a weak 245.5 keV transition, which is not shown on Fig. 3. The ICC of this transition could not be measured. However, the spin assignments to the 515.6 and 270.5 keV levels require this 3' ray to be of M1 +E2 character.
5. Discussion 5.1. The two opposite-parity bands
The ground-state spin and parity of 149pm have been determined to be 7/2 +. These values have been attributed [20] to a configuration in which the odd proton occupies the Nilsson [404] 7/2 component of the g7/2 orbital. As in 147pro [4], two opposite-parity, quasi-rotational bands are observed: a positive-
212
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
q', ~
~
Positive parity • Negativeparity <> Octupole vibrations o
101 .
'10%
I
14
'
I
16
'
I
18
'
I
20
'
I
22
'
I
24
Spin [~/21 Fig. 7. Measured B(EI)/B(E2) branching ratios for transitions in the two opposite-parity bands shown in Fig. 1. Open circles represent values for states in the positive-parityband and filled circles values for states in the negative-parityband. Also indicated by diamonds are values for the 1477.0 and 1923.6 keV states (see Fig. 3) which are interpreted as due to an octupole phonon coupled to the ~rhll/2 isomeric state.
parity band built on the ground state and a negative-parity band based on an excited 1 1 / 2 - state. These two bands are connected via several E1 transitions. Fig. 7 shows B(E1 ) / B ( E 2 ) values measured from ),-ray branching ratios for levels in these two bands. The B ( E l ) / B ( E 2 ) ratios observed are constant within their experimental errors. Assuming the levels in the two opposite-parity bands can be described by a reflection-asymmetric rotor, the absolute B ( E 1 ) values may be estimated assuming a constant intrinsic quadrupole moment of size 4.18 eb [22], and interpreted in terms of a rotating intrinsic dipole moment, Do, via the expression B ( E 1 ; I i K --* I l K ) = 3/47rD~)(IiKlOIIfK) 2. This gives a weighted mean value of the size of Do of 0.15( 1 ) efm when a value of 7 / 2 for the quantum number K is used. The error quoted is statistical only. The values of Do do not change within their experimental errors for K values ranging from K = 1/2 up to K = l 1/2. The average value of Do determined for 149pm is in agreement within experimental error with the average value reported for 147pm (in Ref. [ 4 ] ) and for 151Pro (in Ref. [ 1 0 ] ) , for the opposite-parity bands observed in those nuclei. Fig. 8 shows these measurements of the size of the moment Do for the 147pro, 149pm and iSIPm isotopes. These values suggest that octupole correlations in the N = 88 Pm nucleus are not significantly stronger than in the N = 86 and 90 Pm isotopes. Furthermore, the value of Do determined for 149pro is smaller than the values obtained for the opposite-parity bands in 14SNd and 15°Sm, suggesting that the correlations are weaker in 149pm than in the neighbouring two N = 88 isotones. It is not yet clear whether the two opposite-parity bands at medium spin approximate a parity-doublet band which could arise from a stable, reflection-asymmetric mean field. Opposite-parity bands found in 15lpm [ 10] and 153Eu [ 12] have been interpreted as originating from different orbitals in a reflection-symmetric mean field, since the bands exhibit different magnetic properties. If the positive- and negative-parity members of an
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
213
0.3
0.2"
0.1
0.0 146
,
I
,
I
147
,
I
148
,
I
149
,
I
150
'
151
152
Mass Number Fig. 8. Values of the size of the parameter Do for Pm isotopes taken from Ref. [4] (147pm), this work (149pro) and from Ref. [10] (151pro).
alternating-parity band both originate from the same reflection-asymmetric, intrinsic configuration, the magnetic properties should be similar in both. Assuming the levels in the two opposite-parity bands can be well described as members of rotational bands, values of I ( g r - g n ) / Q o l for levels in the bands may be deduced from y-ray branching ratios using the procedure described in Ref. [23]. Fig. 9 shows these values, where K = 7/2 has been adopted for the projection of the total angular momentum onto the symmetry axis. The levels in the positive-parity band do not show the constancy expected for a good rotor. However, they can be used to estimate the magnetic moment of the ground state. Taking a positive weighted mean value of 0.045(3) eb -j , a constant intrinsic quadrupole moment of size 4.18 eb [22], and a value of Z/A = 0.41 for gR, we estimate a value 0.15 o Positive parity
• Negative parity 0.10
0.05' I
0.00
I
10
,
I
12
,
I
14
,
I
16
,
I
18
,
I
20
Spin [t//2] Fig. 9. Size of the parameter (gK -- gR)/Qo obtained from cascade to crossover "y-ray branching ratios for levels in the two opposite-parity bands. Open circles indicate values for states in the positive-parity band, and the filled circle the value for the negative-parity 15/2 state.
214
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
of +2.0(1) /z/v for the ground-state magnetic moment. This has been directly measured [24] to be +3.3(5) /xN. This disagreement can be attributed to the assumption that the positive-parity levels constitute a good rotational band. It can be seen from Fig. 9 that the single measured value o f l ( g ~ - g R ) / Q o l for the negative-parity band is somewhat larger than the values for the positive-parity levels. However, there are insufficient data on the magnetic properties of the opposite-parity bands to determine whether they differ, as in 151pm and 153Eu, or whether they remain good candidates for parity doublet bands. 5.2. Low-spin excitations All of the low-spin levels shown in Fig. 2 were seen in previous studies. The 114.3, 360.2, 558.9 and 808.8 keV states are observed to form a quasi-rotational band, with the 114.3 keV level as the band head. The 114.3 keV level has previously been suggested [ 18,25] to originate from the odd proton occupying the Nilsson [402]5/2 component of the spherical d5/2 orbital. The assumption that these levels can be described by a rotational model is not strictly valid. However, values of I(gr - g R ) / Q o l for levels in the band may be deduced from y-ray branching ratios using the procedure described above for the two opposite-parity bands. Using a value of K = 5/2, values of 0.23( 1 ) and 0.13( 1 ) eb -1 are obtained for the 558.9 and 808.8 keV states, respectively. These are not constant as expected. However, using a positive weighted mean of these two values, we estimate a value of -t-2.3(1) /zN for the magnetic moment of the 114.3 keV level. This is in agreement with the previously measured value [26] of +2.1(2) #N. The 558.9 keV member of this band is linked to the 791.5 keV 11/2- state shown in Fig. 1 via a 232.6 keV E1 transition. Three other El transitions, of energies 58.6, 155.9 and 270.5 keV, are observed with the present data to depopulate the 270.5 keV 7 / 2 level. No E2 transitions are available for comparison with these E1 y rays. However, the lifetime for the 270.5 keV state has previously been measured [21] from internal conversion coincidence work to be 2.64(7) ns. This has allowed the absolute B(E1) strengths for the 58.6, 155.9 and 270.5 keV y rays to be extracted. These strengths are of the order of 10 -6 Weisskopf units, considerably smaller than the strengths of the El transitions which interlock the opposite-parity bands shown in Fig. 1. 5.3. Excitations above the 240.2 keV isomeric level The I 1 / 2 - isomeric level at 240.2 keV results from the excitation of the odd proton into the hi1~2 orbital, and is also observed in the N = 88 isotones 151Eu and 153Tb. The excited levels above the isomeric state in 149pmshow some similarities to those observed above the hluz isomers in 151Eu [8,9] and 153Tb [27]. The bands based on the 240.2 keV isomeric level and on the 515.6 ( 9 / 2 - ) keV level are similar to bands observed in 151Eu and 153Tb, which have been attributed to a decoupled rrhll/2 structure. The levels in these bands in 151Eu and 153Tb and in the corresponding two bands in 149pm are shown in Fig. 10. This, together with similar decay patterns, suggests a decoupled 7rhll/2 interpretation for the two bands observed in 149pm.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-21 7
E ~ [MeV]
215
I"
(relative to 11/2) 2155,6
2118.0
2112.1
....... t - -
27/2"
.....
1612.8
1563.9
1532.9
23/2"
1504.8
. . . . . . 1503.3
:: ~ 1495.0
21/2"
" ' \ 1040.9 957.3 .....
979.0 i~-':: ~ . 9
19/2~ 17/2
...... 5355 . . . . . . 511.4
13/2 15/2-
...... 262.9
9/2-
1211.2 m , , , .
957.0 771.5
"'"-.. 611.4 515.6 510.2 240.2
N9pm
502.2 ".
"-\ 349.8 196.2
15tEu
163.3
11/2"
153Tb
Fig. 10. Systematics of levels in bands associated with decoupled ~'hll/2 configurations in the N = 88 isotones t49pm (this work), 151Eu (Ref. [9]) and 153Tb (Ref. I27]).
The levels at 1162.8, 1477.0 and 1923.6 keV have been assigned spins and parities in analogy with levels in 151Eu which lie at roughly the same excitation energy and have similar decay patterns. As in 15~Eu, the 1162.8 keV level may result from an octupole phonon coupled to the 7rh11/2 state. This will result in a septuplet of states with spins 5/2 + to 17/2 +, with the possibility of observing a band built on each. Only yrast and near-yrast levels are expected to be observed, and we would therefore only expect to see band heads corresponding to the spin of the phonon fully or nearly fully aligned with the spin of the proton. The excitation energies of 3- levels in even-even nuclei in this region are approximately 1.0 MeV (1000.0 and 1071.6 keV in 148Nd [6] and 15°Sm [7], respectively). The band heads are therefore expected to lie at approximately 1.2 MeV in 149pm. The octupole phonon contains a significant contribution from a pair of protons in the d5/2 and hll/2 orbitals coupled to spin 3-. This component cannot fully align with the hll/2 proton due to Pauli blocking and the 17/2 + band head will therefore be raised in excitation energy relative to the other states, leaving the 15/2 + state nearest to yrast. The 1162.8 keV (15/2 +) level observed is thus a good candidate for such a state. In 151Eu, the analogous state lies at 1114.0 keV. The assignment of [ hi 1/2 @ 3 - ] 15/2+ to the 1162.8 keV band head is supported by the
216
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
decay patterns of the 1162.8, 1477.0 and 1923.6 keV levels. With the above proposed spin and parity assignments, the transitions connecting these three states with levels in the bands based on the 1 1 / 2 - 240.2 keV and ( 9 / 2 - ) 515.6 keV states are of El character. It was possible to extract B ( E 1 ) / B ( E 2 ) values from y-ray branching ratios for the 1477.0 keV level (by comparing the 265.8 and 314.2 keV transitions) and for the 1923.6 keV level (by comparing the 310.8 and 446.6 keV transitions). These are shown on Fig. 7, and are smaller than the values obtained for levels in the two opposite-parity bands (see Fig. 1). Using the same procedure as for the two opposite-parity bands, intrinsic dipole moments were extracted from these ratios using K = 15/2. The values for the 1477.0 and 1923.6 keV states are 0.05( 1 ) and 0.09( 1 ) efm, respectively, and are insensitive to changes in K. In 151Eu [8], Do values for the corresponding two states (seen at 1506.9 keV and 1947.7 keV in that nucleus) are 0.05(1) and 0.18(2) efm. The increase of Do with spin may be due to an increase in octupole correlations with increasing spin, although the increase in Do is not as pronounced in 149pIn as in 15tEu. In 147pm [4], octupole vibrations coupled to a 7rhll/2 band have also been observed with an average value of Do obtained for these excitations of 0.10(2) efm. The description of the 1477.0 and 1923.6 keV states in 149pm in terms of an octupole vibration in a reflection-symmetric potential is consistent with the observed values of Do being lower than those values obtained for levels in the two opposite-parity bands shown in Fig. 1, where the octupole correlations are interpreted as being stronger. This is also in accordance with the octupole correlations being weaker for states above the isomeric level due to blocking by the odd proton in the hi1/2 orbital.
6. Summary We have studied the transitional lanthanide nucleus 149pm populated in the 15°Nd(d, 3n) reaction. An extended level scheme has been determined. Spins and parities of levels have been assigned on the basis of internal conversion coefficient measurements, decay patterns and by comparison to the N = 88 isotones 151Eu and 153Tb. Two opposite-parity bands connected by E1 crossover transitions have been found. Interpreting the strengths of the E1 crossover transitions in terms of a rotating intrinsic dipole moment gives a moment similar in size to that observed in the neighbouring nuclei 147pm and 151pm. The size of the moment is smaller than the moments obtained for opposite-parity bands in the other N = 88 nuclei 148Nd and 15°Sm, suggesting the octupole correlations are weaker in 149pmthan in the neighbouring two N = 88 isotones. More data are needed on the magnetic properties of the two opposite-parity bands in order to establish whether they are good candidates for parity-doublet bands. Other E1 transitions are observed which depopulate a low-lying excited state and which are significantly weaker than the crossover transitions connecting the two opposite-parity bands. This is further support for the collective nature of the E1 crossover transitions. Excited levels have been observed above the "n'hll/2 isomeric state. By comparison with the N = 88 isotones 151Eu and 153Tb, these levels have been interpreted in terms of
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
217
quasi-rotational bands arising from a decoupled 7rhll/2 structure, and from an octupole phonon coupled to the 7rhll/Z state. Electric dipole transitions which depopulate states in the latter band have been used to extract values for the size of an intrinsic dipole moment. These values show an increase with spin as observed for the corresponding band in 151Eu, where this effect has been interpreted as an enhancement in octupole correlations with increasing spin. References I 1 ] I. Ahmad and P.A. Butler, Ann. Rev. Nucl. Part. Sci. 43 (1993) 71. [ 21 V.E. lacob, W. Urban, J.C. Bacelar, J. Jongman, J. Nyberg, G. Slenen and L. Trache, Nucl. Phys. A 596 (1996) 155. 131 W. Urban, R.M. Lieder, J.C. Bacelar, P.P. Singh, D. Alber, D. Balabanski, W. Gast, H. Grawe, J.R. Jongman, G. Hebbinghaus, T. Morek, R.E Noorman, T. Rzaca-Urban, H. Schnare, M. Thorns, O. Zell and W. Nazarewicz, Phys. Lett. B 258 ( 1991 ) 293. 141 W. Urban, J.L. Durell, W.R. Phillips, B.J. Varley, Ch.P. Hess, M.A. Jones, C.J. Pearson, W.J. Vermeer, Ch. Vieu, J.S. Dionisio, M. Pautrat and J.C. Bacelar, Nucl. Phys. A 587 (1995) 541. [51 W. Urban, J.C. Bacelar, J.R. Jongman, R.E Noorman, M.J.A. de Voigt, J. Nyberg, G. Sletten, M. Bergstr6m and H. Ryde, Nucl. Phys. A 578 (1994) 204; Structure Through Static, ed. H.H. Bolotin, Conf. Proc. (1987) p. 180. 16 ] W. Urban, R.M. Lieder, W. Gast, G. Hebbinghaus, A. Kr~imer-Flecken, T. Morek and T. Rzaca-Urban, Phys. Lett. B 200 (1988) 424. 171 W. Urban, R.M. Lieder, W. Gast, G. Hebbinghaus and A. Krfimer-Flecken, Phys. Lett. B 185 (1987) 331. [ 8 ] J.R. Jongman, W. Urban, J.C.S. Bacelar, J. Van Pol, J. Nyberg, G. Sletten, J.S. Dionisio, Ch. Vieu, J.M. Lagrange and M. Pautrat, Nucl. Phys. A 591 (1995) 244. 19 ] W.J. Vermeer, W. Urban, M.K. Kahn, C.J. Pearson, A.B. Wiseman, B.J. Varley, J.L. Durell, W.R. Phillips, Nucl. Phys. A 559 (1993) 422. 10] W. Urban, J.C. Bacelar, W. Gast, G. Hebbinghaus, A. Krtimer-Flecken, RM. Lieder, I". Morek and T. Rzaca-Urban, Phys. Lett. B 247 (1990) 238. I I I W.J. Vermeer, M.K. Kahn, A.S. Mowbray, J.B. Fitzgerald, J.A. Cizewski, B.J. Varley, J.L. Durell and W.R. Phillips, Phys. Rev. C 42 (1990) R1183. 121 C.J. Pearson, W.R. Phillips, J.L. Durell, B.J. Varley, W.J. Vermeer, W. Urban and M.K. Khan, Phys. Rev. C 49 (1994) R1239. 131 J.S. Dionisio, C. Vieu, J.M. Lagrange, M. Pautrat, J. Vanhorenbeck and A. Passoja, Nucl. Instr. Meth. A 282 (1989) 10. 141 R.G. Helmer and L.D. Mclsaac, Phys. Rev. 143 (1966) 923. 15 ] E.W. Schneider, M.D. Glascock, W.B. Walte~ and R.A. Meyer, Z. Phys. A 291 (1979) 77. 161 H. limura, T. Seo and S. Yamada, J. Phys. Soc. Jpn 55 (1986) 1108. 17 J M. Kortelahti, A. Pakkanen, M. Pllparinen, E. Hammar6n, T. Komppa and R. Komu, Nucl. Phys. A 332 (1979) 422. 18] O. Straume, G. L~vhcfiden, D.G. Burke, E.R. Flynn and J.W. Sunier, Z. Phys. A 293 (1979) 75. 19 ] R.S. Hager and E.C. Seltzer, Nucl. Data Tables 4 (1968) 1. 1201 A.Y. Cabezas, i. Lindgren and R. Marrus, Phys. Rev. 122 (1961) 1796. 121] A. Backlin, S.G. Malmskog and H. Solhed, Ark. Fys. 34 (1967) 495. 1221 P. M611er, J.R. Nix, W.D. Myers and W.J. Swiatecki, At. Data & Nucl. Data Tables 59 (1995) 185. 1231 M.A.C. Hotchkis, J.L. Durell, J.B. Fitzgerald, A.S. Mowbray, W.R. Phillips, I. Ahmad, M.P. Carpenter, R.V. Janssen, T.L. Khoo, E.E Moore, L.R. Morss, Ph. Benet and D. Ye, Nucl. Phys. A 530 ( 1991 ) 111. 1241 R.W. Grant and D.A. Shirley, Phys. Rev. 130 (1963) 1100. 1251 J.K. Tuli, Nucl. Data Sheets 73 (1994) 364. 126] P. Raghavan, At. Data & Nucl. Data Tables 42 (1989) 189. [27] G. Winter, J. D6ring, L. Funke, P. Kemnitz, E. Will, S. Elfstr6m, S.A. Hjorth, A. Johnson and Th. Lindblad, Nucl. Phys. A 299 (1978) 285.
Nuclear Physics A 609 (1996) 201-217
Study of excited levels in 149pm M.A. Jones a, W. Urban a, J.L. Durell a, M. Leddy a, W.R. Phillips a, B.J. Varley a, p.j. Dagnall a, A.G. Smith a, D.M. Thompson a, Ch. Vieu b, J.S. Dionisio b, C. Schuck b, M. P a u t r a t c a Department of Physics and Astronomy, University of Manchester, Manchester MI3 9PL, UK b Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse, IN2P3-CNRS, 91405 Orsay, France c Institut de Physique Nucleaire, IN2P3-CNRS, 91405 Orsay, France
Received 17 July 1996
Abstract
Excited states in 149pm have been studied through the 15°Nd(d, 3n) reaction, y - y and electrony coincidence data have enabled partial level and decay schemes to be extended up to spin (27/2). The spins and parities of excited levels have been deduced from measurements of internal conversion coefficients, decay schemes, and from comparison to the N = 88 isotones 15~Eu and 153Tb. The data are discussed in terms of the strength of octupole correlations. PACS: 21.10.Re; 21.30.Pc; 23.20.Lv; 27.60.+j Keywords: Nuclear reactions 15°Nd(d, 3n), E = 18 MeV; Measured E~,, yy, ey, ICC, 149pmdeduced levels,
J, ~r, y-ray intensity, branchings; Multi-particle configurations,octupole deformation
1. Introduction
Octupole correlation effects in nuclei have recently been of much interest both experimentally and theoretically [ 1 ]. These effects are largest when, for both protons and neutrons, a pair of opposite-parity, single-particle orbitals with Al = Aj = 3 (where Al and Aj are respectively the differences in single-particle orbital and total angular momentum quantum numbers) lie close to each other in energy. Sequences of levels of alternating parity connected by electric dipole transitions, which may be strong, are characteristic features of nuclei in which strong octupole correlations play a significant role. The nucleus 149pm lies in the region of transitional lanthanides where the valence neutrons and protons occupy orbitals which satisfy the above conditions. Alternatingparity bands have been found in the N = 86 even-even isotones 146Nd [2] and 148Sm [3] 0375-9474/96/$15.00 Copyright (~) 1996 Published by Elsevier Science B.V. All rights reserved PII S0375-9474(96) 00308-9
202
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
and interpreted in terms of octupole deformation induced by nuclear rotation. The structure of 147pm, with N = 86, has been investigated [4] and evidence for strong octupole correlations has also been found. The study of the N = 86 nucleus 149Eu [5] revealed an alternating-parity structure at low spin values which was not found to persist to higher spins. Sizes of intrinsic electric dipole moments, determined within the rotational model from electric dipole transition rates, suggest that the correlations are weaker in 149Eu than in 146Nd, 1485m and 147pm. Features appropriate to octupole deformation at medium spins have also been found in the even-even N = 88 isotones 148Nd [6] and 15°Sm [7]. The study of the N = 88 nucleus, 151Eu [8,9], has revealed quasi-rotational level sequences which have been interpreted in terms of bands arising from an octupole phonon and decoupled 7rh11/2 states. Sizes of intrinsic dipole moments in this nucleus are larger than in I49Eu and increase with increasing spin, in contrast to 149Eu, reaching values similar to those observed in 148Nd and 15°Sm. Results obtained for the N = 90 isotopes, 151pm [ 10,1 1 ] and 153Eu [ 12], show some characteristics of octupole deformation, although the magnetic properties of the observed opposite-parity bands support reflection-symmetric, rather than asymmetric descriptions of these nuclei. This reflection symmetry may result from quadrupole correlations dominating over weaker octupole correlations at N = 90. Systematics of the sizes of intrinsic electric dipole moments in this lanthanide region of nuclei [ 10] suggest that octupole correlations are stronger at N = 88 than at N = 86 or 90, and one might therefore expect to see pronounced effects in 149pm as are seen in the N = 88 nuclei already examined. In this work, we report on the structure of the nucleus 149pro produced in the tS°Nd(d, 3n) reaction. The spins and parities of excited levels are assigned on the basis of internal conversion coefficient measurements, decay patterns, and from comparison with the N = 88 isotones 151Eu and 153Tb. An extended level scheme is presented which is discussed from the point of view of the strength of octupole correlations.
2. Experimental procedure The experiment was performed at the MP tandem accelerator at Orsay using the JS°Nd(d, 3n) reaction at a beam energy of 18 MeV. The target used was a thin layer of 15°Nd (surface density 125/xg cm -z) deposited on a carbon backing (30/zg cm-2). Inbeam, prompt y - y and electron-y coincidences were measured using the Orsay electron spectrometer [ 13] to detect electrons and four Compton-suppressed Ge detectors to observe y rays. The y - y and electron-y coincidence data obtained were sorted into two-dimensional y - y and e - y coincidence matrices which contained approximately 6.4 × 106 and 6.8 × 106 double coincidence events, respectively. The energy ranges on the y and electron axes of the matrices were approximately 1.5 MeV and 500 keV, respectively. Using coincidence data allowed the production of clean, gated y-ray and electron spectra, thus avoiding
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
203
23/2 + (2122.3)
(1885.3) (21/2-)
21/2 + (1739.0)
/
355.8 (1591.1) (19/2-)
19/2 ÷ (1549.5)
I
(1406.7) 17/'2-
32O.0
r
17/2 + (1229.5)
(1145.6) 15/2-
1 J/illl-
15/2 + (1008.4)
(1006.5) 13/2-
-
13/2+ (779.3)
-
(791.5) 11/2-
281~ 502.9 11/2 +
9/2 +
7/2 +
(0.0)
-
-
tl 149~ 61
h'ma8
Fig. 1. Partial decay scheme of 149pm as obtained in the present work. The two opposite-parity bands based on the ground state.
problems caused by contamination peaks and a high density of lines encountered when analysing singles spectra.
3. Data analysis and results Prior to this work, 149pm has been studied in the t-decay of 149Nd [14-16] from the 15°Nd(p, 2ny) reaction [ 17] and from the 15°Sm(t, at) reaction [18]. From these studies, levels up to 1.7 MeV in excitation energy and up to spin (15/2) have been reported. Identification and assignment of new y rays to 149pm was achieved by gating in the 3/-y matrix on known transitions reported from the previous studies. Partial level schemes of 149pm resulting from analysis of the present data are shown in Figs. 1, 2 and 3. The quality of the data in the Y-Y matrix is illustrated in Fig. 4, which shows three spectra obtained by gating on previously known transitions at 497.9 (shown in Fig. 1 ), 1 14.3 (Fig. 2) and 269.9 (Fig. 3) keV. Information on the multipolarities and parities of transitions was obtained by measuring internal conversion coefficients (ICC's). By gating on the total-energy peak of a particular ),-ray transition in the 7-'7' and e - 7 matrices, respectively, spectra of y
204
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217 (~1.1~) 11'2 ÷
11/2- (791J)
(6513) 7/2 "~
I
1/T
71i" 149~
611Jm88
Fig. 2. Partial decay scheme of 149pm as obtained in the present work. Low-spinexcitations. rays and electrons in coincidence with that particular 3/ray are obtained. Numbers may then be extracted for coincident decays from a level populated by the gating 3/ ray: the number of coincident 3/rays, N(3/0), and numbers of coincident electrons resulting from the internal conversion of 3/0 from different electron shells. Exactly the same gate must be set in both matrices. If N ( e o ) is the number of electrons resulting from internal conversion from the K shell, the K-shell ICC, a x , for the transition 3/0 is given by _
ax(3/o)
N(eo)
"qz,(3/o) C,
(1)
rle(eo~) N(3/o)
where r/~(e0) is the relative efficiency for electron detection at the K-electron energy, and r/r(3/0) the relative efficiency for 3/-ray detection at the energy of 3/0. These were obtained by using electron and 3/ radiation from the decay of the 31 year half-life isomer in 178Hf. Additional 3/-ray relative efficiency points were obtained from 3/-ray coincidences in the experimental data. The overall normalisation constant C was obtained by using a known electric quadrupole (E2) transition produced via Coulomb excitation of the 15°Nd target. The K and L-shell ICC's were measured for the 2 + --* 0+ transition by gating on the 4+ ~ 2+ and 6 + --~ 4~- 3/rays, and then compared with interpolations of theoretical predictions [ 19]. This yielded a value of C = 0.57(4). The quality of the data used to obtain ICC's is illustrated in Fig. 5, which shows the 3/-ray and electron spectra obtained after gating on 3/-ray transitions in the 3/-3/and e-3/matrices, respectively. Fig. 6 shows K-shell ICC's obtained for transitions in 149pm. Due to low statistics, it was only possible to measure aK values, with the exception of the 114.3 keV transition, where the L-shell conversion coefficient was also determined. Table 1 shows, for transitions identified in 149pro, the 3/-ray energies obtained, their relative intensities, their measured K-shell ICC's and their proposed multipolarities. The multipolarities are proposed from previous studies, ICC measurements, decay patterns
205
M.A. Jones et aL/Nuclear Physics A 609 (1996) 201-217 f27/2 ) ( 2 1 1 2 ~) -
-
f
- ~ (i9236)(23/2+)
~11~7
(23/2)(15048)
-
/
(19/2)(9570)
15/2
(510.2)
11/2
f240 2)
....
•
(I 162.8) (I 5/2 + )
35{3) las
7g2+ (o o)
149 61
Pm88
Fig. 3. Partial decay scheme of 149pmas obtained in the present work. Excitations above the zrhll/2 isomer. and from comparison with the N = 88 isotones 151Eu and 153Tb. In the last column of Table 1, the energy of the excited level in 149pm depopulated by the corresponding ~, ray is given.
4. Level scheme and spin and parity assignments Partial level schemes of 149pm resulting from analysis of the present data are shown in Figs. 1, 2 and 3. Our data, where there is overlap, confirm the excitation scheme reported previously, and significantly extend information on levels in quasi-rotational bands based on the ground state and the 240.2 keV isomeric state.
206
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
Gate: 497.9 keV
tt~
i
'
.
~
oo.
m "~ !=1 = 10-~
q
Gate: 114.3 keV
,II
| Gate: 269.9 keV
10
200
300
400
7
500
600
700
energy ~eV]
Fig. 4. Spectra obtained by gating in the ~-~ matrix on previously known transitions at 497.9 (shown in Fig. 1), 114.3 (Fig. 2) and 269.9 (Fig. 3) keV. The main coincident ~, rays are labelled with their transition energies in keV.
4.1. The two opposite-parity bands
These two bands are shown in Fig. 1. ICC measurements for 3, rays in this part of the level scheme are shown in Table 1 and Fig. 6a. The ground-state spin and parity were previously established from atomic-beam [20] and fl-decay [14] work to be I # = 7/2 +. The 288.5, 497.9 and 779.3 keV levels have previously been observed [17] and assigned as 9/2 +, (11/2 +) and (13/2+), respectively. The ICC measurements are in agreement with these assignments. We have observed an additional five levels above the 779.3 keV state which are assigned spin and parity, as shown in Fig. 1, of 15/2 +, 17/2 +, 19/2 +, 21/2 + and 23/2 + from the ICC measurements and their decay modes. The sequence of positive-parity levels based on the ground-state approximates a rotational band. The excited state at 791.5 keV has previously been observed [ 18] and assigned as 11/2-. The measured ICC for the 502.9 keV transition is in agreement with this 3' ray being of electric dipole ( E l ) character and therefore supports the 11/2- assignment to the 791.5 keV level. The measured ICC for the 508.9 keV transition is also in agreement
207
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
(a)
:J t~
1
~
--
*
~J
(b)
,...,15"
!
× 2.5
:I0
E k
m~ m o
1 O0
200
300
400
500
600
Channel No. Fig. 5. The projected y-ray (a) and electron (b) spectra obtained after gating on the y axes in the Y-Y and e-y matrices, respectively. The main coincident y rays are labelled with their transition energies in keV, and the corresponding electrons with their K and L-conversion character. The 130.2 and 251.2 keV y rays indicated by asterisks are transitions in 15°Nd, produced in Coulomb excitation. The spectra were generated by summing individual gates on the 114.3, 155.9, 198.5 and 246.0 keV transitions (shown in Fig. 2). The vertical scale for the portion of the electron spectra to the fight of the vertical dotted line is increased by a factor of 2.5. with this 9' ray being o f E1 character and thus allows spin and parity of 1 3 / 2 - to be assigned to the 1006.5 keV level. We observe four other levels above the 1006.5 keV state which are assigned spin and parity, as shown in Fig. 1, of 1 5 / 2 - , 1 7 / 2 - , ( 1 9 / 2 - ) and ( 2 1 / 2 - ) on the basis o f their decay modes and ICC measurements for the 354.2, 366.4 and 398.4 keV transitions. The sequence of negative-parity levels based on the 791.5 keV state approximates a rotational band. The positive-parity band based on the ground state and the negative-parity band based on the 791.5 keV 1 1 / 2 - state are connected via several crossover transitions. Of these, the 366.4, 398.4, 502.9 and 508.9 keV transitions are consistent with the stretched E1 character required by the level assignments made above, as supported by the ICC measurements for these 3/ rays. ( A stretched 7 ray has a pure multipolarity L, and makes a transition from a state o f spin I to a state o f spin ( I - L ) ) . Although it was not possible to measure I C C ' s for the 293.6, 142.7, 355.8 and 237.0 keV 3' rays, they
208
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217 7x
T
0.20 0.15' 0.10' 0.05' 0.00 4'
r..; r,.)
I
I,\
x 10
(b)
3 2' 1" O
0.15"
0.10"
0.05"
0.00
0
I O0
200
300
400
500
600
? energy [keV] Fig. 6. Measured aK values for transitions in ]49pm. Values corresponding to transitions shown in Figs. 1, 2 and 3 are shown in panels (a), (b) and (c), respectively. The vertical scales for the right-hand portions of each panel have been increased by factors of 3, l0 and 3 for panels (a), (b) and (c), respectively. Solid, dotted and dashed lines represent interpolations of theoretical values [ 19] for El, E2 and M1 transitions, respectively. Filled and open circles represent those transitions assigned to be of E1 and E2 characters, respectively, diamonds representing those assigned to be of a mixed M1 +E2 character.
are inferred to be of E1 character from the spin and parity assignments made to levels above. An analogous situation is observed in 147pm [4], where two similar quasi-rotational bands are found; a positive-parity band based on the ground state and a negative-parity band based on an excited 1 1 / 2 - state. These bands are also connected via E1 transitions.
4.2. Low-spin excitations Fig. 2 shows several low-spin levels observed in the present data. ICC measurements for y rays in this part of the level scheme are shown in Table 1 and Fig. 6b. In addition to the C~K value quoted in Table 1, the L-shell ICC, aL, was measured for the 114.3 keV transition. This yielded a value of 0.11 (2). The interpolated theoretical values are [ 19] 0.005 for El, 0.014 for E2 and 0.457 for M1. The a r and CtL values for the 114.3 keV y ray are thus both consistent with M I + E 2 character.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
209
Table 1 Energies, y-ray intensities, a r conversion coefficients and proposed multipolarities for transitions in 149pm. In the last column the energy of the excited level in 149pm depopulated by the corresponding y ray shown in column 1 is given. Errors of y-ray energies are 0.3 keV for I~, < 5, I b < 15, 1~ < 10 and 0.2 keV or better for stronger transitions. E z, (keV) 58.6 74.3 114.3 139.3 142.7 155.9 188.6 198.5 199.1 208.2 209.5 211.5 215.0 221.0 229.2 232.6 237.0 240.2 245.5 246.0 250.7 254.3 254.5 256.0 261.4 261.5 265.8 269.9 270.5 273.2 275.4 281.5 282.6 288.5 293.6 310.8 314.2 320.0 354.2 355.8 360.1 366.4 391.4 398.4 401.6 419.0 439.6
t a'b'c -~, 4( 1 )c 40(3) b 1.4(2) a 1.1 (8) a 54(4) b 44(4) b 45(4) h 33(3) h 18( 1 )a 1.2(3) a 1.3 (2) a 7.3(3) a 6( 1 )b 1.5(4) a 1.0(7)': 100 b 14(2) b 5(2) c 5(l)b 4(2) c 20(3) c 0.6(2) a 5( 1)c
aK
2.4(8) 0.8(2)
0.05(2) 0.15(7) 0.17(4) 0.16(6) 0.15(5) 0.15(5) 0.13(7) 0.14(6) 0.02( 1 )
0.09(2) 0.07(3) 0.12(5) 0.06(3) 0.11 (4)
0.06( 1) 18(4) b 10(3) c 12(1) a 12(2) b 1.7(5) a 1.5(3) c 3(1) c 2.1(2) a 3.2(4) a 1.2(2) a 19(2) a 4(1) c 15.1(2) a 1.0(7) c 2(1) c 6(2) c
0.06(4) 0.06(3) 0.08(3) 0.06( 1 )
0.03( I )
0.012(5) 0.008(5)
Multipolarity
Eex (keV)
El M 1+E2 MI+E2 MI+E2 El El E2 MI+E2 MI+E2 MI+E2 MI+E2 MI+E2 MI+E2 M 1+E2 MI+E2 El (El) M2 (MI+E2 M 1+E2 MI+E2 (MI+E2) MI +E2 (E2) (M I+E2) MI+E2 (El) E2 El E2 (MI+E2) MI+E2 MI+E2 MI+E2 El (El) (E2) MI+E2 E2 (El) M 1+E2 El (El) El (E2) (El) (E2)
270.5 188.6 114.3 1145.6 1549.5 270.5 188.6 558.9 387.5 396.8 497.9 211.5 1006.5 1229.5 1008.4 791.5 2122.3 240.2 515.6 360.2 808.8 1211.2 651.3 771.5 771.5 1406.7 1477.0 510.2 270.5 387.5 515.6 779.3 396.8 288.5 791.5 1923.6 1477.0 1549.5 1145.6 1885.3 360.2 1145.6 1162.8 1406.7 1612.8 1923.6 1211.2
210
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
Table 1--continued E~, (keY)
ta'b'c .~
ar
Multipolarity
Eex (keV)
444.7 445.3 446.6 446.8 448.7 450.2 478.2 490.8 497.9 502.9 508.9 509.5 510.5 520.0 531.2 541.1 547.8 572,8 607.6 652.9 655.7 700.8
28(2) b 4.4(6) a 2.2(5) c 100c 20(2) b 22( l )a 0.9(2) a 63 ( 6 ) a 100a 3.4(6) a 10(2)a 3.0(9) a 67(3) a 8(2) c 14(4) c 11.9(8) a 37(5) c 3.2(8) a 11(3) c 6(2) c 11(2) c 13(3) c
0.016(8)
E2 (E2) (E2) (E2) E2 E2 (E2) E2 E2 El E1 E2 E2 (El) (MI+E2) E2 (E2) E2 (E2) (El) (M 1+E2) (MI+E2)
558.9 1591.1 1923.6 957.0 808.8 1229.5 1885.3 779.3 497.9 791.5 1006.5 1739.0 1008.4 1477.0 771.5 1549.5 1504.8 2122.3 2112.1 1162.8 1612.8 1211.2
0.011 (4) 0.013 ( 4 ) 0.015 (7) 0.009( 3) 0.012( 3 ) 0.004(2) 0.003(2) 0.014(5) 0.009(3)
0.009(3) 0.007(2) 0.011(5)
Relative scale where Iv (497.9 keV) is assumed to be 100. b Relativescale where I v (246.0 keV) is assumed to be 100. c Relativescale where Iv (446.8 keV) is assumed to be 100. All of the levels observed here were seen in previous studies and assigned spins and parities (see Ref. [ 17] for example). The ICC's obtained with the present data are in agreement with these proposed assignments, which are shown in Fig. 2. The 114.3, 360.2, 558.9 and 808.8 keV states form a quasi-rotational band, with the 114.3 keV level as the band head. The 558.9 keV member of this band is observed in the present data to be linked to the 791.5 keV 1 1 / 2 - state shown in Fig. 1 via a 232.6 keV transition, which is required to be of E1 character by the spin and parity assignments of the 558.9 and 791.5 keV states. The measured ICC of the 232.6 keV transition is consistent with E1 character and therefore satisfies the above requirement. Since the 232.6 keV y ray is weak, it was not possible to obtain intensities of transitions shown in Fig. 2 relative to those shown in Fig. 1. Three other E1 transitions previously observed are also seen in the present data. These transitions depopulate the 270.5 keV 7 / 2 - level and have energies of 58.6, 155.9 and 270.5 keV.
4.3. E x c i t a t i o n s a b o v e the 2 4 0 . 2 k e V i s o m e r i c level
Fig. 3 shows the excited levels observed in the present experiment above the isomeric level at 240.2 keV. ICC measurements for 3~ rays in this part of the level scheme are shown in Table 1 and Fig. 6c.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
211
The 240.2 keV isomer has been reported in previous studies. Its lifetime has been measured [21] to be 35(3) /zs. This is much longer than the overlap time used in the present experiment for the acceptance of the 7-9' and e-T coincidences, and the ICC for the 240.2 keV 9' ray could therefore not be measured. However, the magnetic quadrupole (M2) character of this transition has been confirmed previously [21], the 240.2 keV level being assigned as 1 l / 2 - . The 510.2 keV state has previously been assigned [17] to be 15/2-. The ICC obtained for the 269.9 keV transition is in agreement with this T ray being of E2 character and hence in agreement with the 15/2- assignment. ICC's for the 446.8 and 547.8 keV 3' rays suggest that these transitions have E2 character. This suggests spin and parity assignments to the 957.0 and 1504.8 keV levels as shown in Fig. 3. The 240.2, 510.2, 957.0, 1504.8 and 2122.1 keV levels may thus constitute a band with the 240.2 keV isomeric level as the band head. The 2112.1 keV level is consequently tentatively assigned to 2 7 / 2 - on the assumption that it is a member of this band. Seven other levels have been observed which feed both directly and indirectly into members of the band based on the isomeric level and into the isomeric level itself. Of these seven levels, the 515.6 and 771.5 keV states have been reported previously [ 17] with assignments of ( 9 / 2 - ) and ( 1 3 / 2 - ) , respectively. The ICC obtained for the 261.4 keV transition is in agreement with the above spin and parity assignments to the 771.5 keV level. All the data on the decay of the 1211.2 keV level are consistent with a spin and parity assignment of ( 1 7 / 2 - ) . The 515.6, 771.5, 1211.2 and 1612.8 keV levels appear to form a band with the 515.6 keV ( 9 / 2 - ) level as the band head. Based on this assumption, the 1612.8 keV level is assigned tentatively to 2 1 / 2 - . The 1162.8, 1477.0 and 1923.6 keV levels are similar in energy and have similar decay modes to states observed in the N = 88 isotone 151Eu [8,9]. The corresponding states in ~SIEu have energies of 1114.0, 1506.9 and 1947.7 keV and have been assigned spin and parity of 15/2 +, 19/2 + and 23/2 +, respectively. These similarities suggests spin and parity assignments of (15/2+), (19/2 +) and (23/2 +) as shown on Fig. 3 to the three levels in 149pm. The 515.6 keV ( 9 / 2 - ) level is additionally linked to the 270.5 keV 7 / 2 - state via a weak 245.5 keV transition, which is not shown on Fig. 3. The ICC of this transition could not be measured. However, the spin assignments to the 515.6 and 270.5 keV levels require this 3' ray to be of M1 +E2 character.
5. Discussion 5.1. The two opposite-parity bands
The ground-state spin and parity of 149pm have been determined to be 7/2 +. These values have been attributed [20] to a configuration in which the odd proton occupies the Nilsson [404] 7/2 component of the g7/2 orbital. As in 147pro [4], two opposite-parity, quasi-rotational bands are observed: a positive-
212
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
q', ~
~
Positive parity • Negativeparity <> Octupole vibrations o
101 .
'10%
I
14
'
I
16
'
I
18
'
I
20
'
I
22
'
I
24
Spin [~/21 Fig. 7. Measured B(EI)/B(E2) branching ratios for transitions in the two opposite-parity bands shown in Fig. 1. Open circles represent values for states in the positive-parityband and filled circles values for states in the negative-parityband. Also indicated by diamonds are values for the 1477.0 and 1923.6 keV states (see Fig. 3) which are interpreted as due to an octupole phonon coupled to the ~rhll/2 isomeric state.
parity band built on the ground state and a negative-parity band based on an excited 1 1 / 2 - state. These two bands are connected via several E1 transitions. Fig. 7 shows B(E1 ) / B ( E 2 ) values measured from ),-ray branching ratios for levels in these two bands. The B ( E l ) / B ( E 2 ) ratios observed are constant within their experimental errors. Assuming the levels in the two opposite-parity bands can be described by a reflection-asymmetric rotor, the absolute B ( E 1 ) values may be estimated assuming a constant intrinsic quadrupole moment of size 4.18 eb [22], and interpreted in terms of a rotating intrinsic dipole moment, Do, via the expression B ( E 1 ; I i K --* I l K ) = 3/47rD~)(IiKlOIIfK) 2. This gives a weighted mean value of the size of Do of 0.15( 1 ) efm when a value of 7 / 2 for the quantum number K is used. The error quoted is statistical only. The values of Do do not change within their experimental errors for K values ranging from K = 1/2 up to K = l 1/2. The average value of Do determined for 149pm is in agreement within experimental error with the average value reported for 147pm (in Ref. [ 4 ] ) and for 151Pro (in Ref. [ 1 0 ] ) , for the opposite-parity bands observed in those nuclei. Fig. 8 shows these measurements of the size of the moment Do for the 147pro, 149pm and iSIPm isotopes. These values suggest that octupole correlations in the N = 88 Pm nucleus are not significantly stronger than in the N = 86 and 90 Pm isotopes. Furthermore, the value of Do determined for 149pro is smaller than the values obtained for the opposite-parity bands in 14SNd and 15°Sm, suggesting that the correlations are weaker in 149pm than in the neighbouring two N = 88 isotones. It is not yet clear whether the two opposite-parity bands at medium spin approximate a parity-doublet band which could arise from a stable, reflection-asymmetric mean field. Opposite-parity bands found in 15lpm [ 10] and 153Eu [ 12] have been interpreted as originating from different orbitals in a reflection-symmetric mean field, since the bands exhibit different magnetic properties. If the positive- and negative-parity members of an
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
213
0.3
0.2"
0.1
0.0 146
,
I
,
I
147
,
I
148
,
I
149
,
I
150
'
151
152
Mass Number Fig. 8. Values of the size of the parameter Do for Pm isotopes taken from Ref. [4] (147pm), this work (149pro) and from Ref. [10] (151pro).
alternating-parity band both originate from the same reflection-asymmetric, intrinsic configuration, the magnetic properties should be similar in both. Assuming the levels in the two opposite-parity bands can be well described as members of rotational bands, values of I ( g r - g n ) / Q o l for levels in the bands may be deduced from y-ray branching ratios using the procedure described in Ref. [23]. Fig. 9 shows these values, where K = 7/2 has been adopted for the projection of the total angular momentum onto the symmetry axis. The levels in the positive-parity band do not show the constancy expected for a good rotor. However, they can be used to estimate the magnetic moment of the ground state. Taking a positive weighted mean value of 0.045(3) eb -j , a constant intrinsic quadrupole moment of size 4.18 eb [22], and a value of Z/A = 0.41 for gR, we estimate a value 0.15 o Positive parity
• Negative parity 0.10
0.05' I
0.00
I
10
,
I
12
,
I
14
,
I
16
,
I
18
,
I
20
Spin [t//2] Fig. 9. Size of the parameter (gK -- gR)/Qo obtained from cascade to crossover "y-ray branching ratios for levels in the two opposite-parity bands. Open circles indicate values for states in the positive-parity band, and the filled circle the value for the negative-parity 15/2 state.
214
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
of +2.0(1) /z/v for the ground-state magnetic moment. This has been directly measured [24] to be +3.3(5) /xN. This disagreement can be attributed to the assumption that the positive-parity levels constitute a good rotational band. It can be seen from Fig. 9 that the single measured value o f l ( g ~ - g R ) / Q o l for the negative-parity band is somewhat larger than the values for the positive-parity levels. However, there are insufficient data on the magnetic properties of the opposite-parity bands to determine whether they differ, as in 151pm and 153Eu, or whether they remain good candidates for parity doublet bands. 5.2. Low-spin excitations All of the low-spin levels shown in Fig. 2 were seen in previous studies. The 114.3, 360.2, 558.9 and 808.8 keV states are observed to form a quasi-rotational band, with the 114.3 keV level as the band head. The 114.3 keV level has previously been suggested [ 18,25] to originate from the odd proton occupying the Nilsson [402]5/2 component of the spherical d5/2 orbital. The assumption that these levels can be described by a rotational model is not strictly valid. However, values of I(gr - g R ) / Q o l for levels in the band may be deduced from y-ray branching ratios using the procedure described above for the two opposite-parity bands. Using a value of K = 5/2, values of 0.23( 1 ) and 0.13( 1 ) eb -1 are obtained for the 558.9 and 808.8 keV states, respectively. These are not constant as expected. However, using a positive weighted mean of these two values, we estimate a value of -t-2.3(1) /zN for the magnetic moment of the 114.3 keV level. This is in agreement with the previously measured value [26] of +2.1(2) #N. The 558.9 keV member of this band is linked to the 791.5 keV 11/2- state shown in Fig. 1 via a 232.6 keV E1 transition. Three other El transitions, of energies 58.6, 155.9 and 270.5 keV, are observed with the present data to depopulate the 270.5 keV 7 / 2 level. No E2 transitions are available for comparison with these E1 y rays. However, the lifetime for the 270.5 keV state has previously been measured [21] from internal conversion coincidence work to be 2.64(7) ns. This has allowed the absolute B(E1) strengths for the 58.6, 155.9 and 270.5 keV y rays to be extracted. These strengths are of the order of 10 -6 Weisskopf units, considerably smaller than the strengths of the El transitions which interlock the opposite-parity bands shown in Fig. 1. 5.3. Excitations above the 240.2 keV isomeric level The I 1 / 2 - isomeric level at 240.2 keV results from the excitation of the odd proton into the hi1~2 orbital, and is also observed in the N = 88 isotones 151Eu and 153Tb. The excited levels above the isomeric state in 149pmshow some similarities to those observed above the hluz isomers in 151Eu [8,9] and 153Tb [27]. The bands based on the 240.2 keV isomeric level and on the 515.6 ( 9 / 2 - ) keV level are similar to bands observed in 151Eu and 153Tb, which have been attributed to a decoupled rrhll/2 structure. The levels in these bands in 151Eu and 153Tb and in the corresponding two bands in 149pm are shown in Fig. 10. This, together with similar decay patterns, suggests a decoupled 7rhll/2 interpretation for the two bands observed in 149pm.
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-21 7
E ~ [MeV]
215
I"
(relative to 11/2) 2155,6
2118.0
2112.1
....... t - -
27/2"
.....
1612.8
1563.9
1532.9
23/2"
1504.8
. . . . . . 1503.3
:: ~ 1495.0
21/2"
" ' \ 1040.9 957.3 .....
979.0 i~-':: ~ . 9
19/2~ 17/2
...... 5355 . . . . . . 511.4
13/2 15/2-
...... 262.9
9/2-
1211.2 m , , , .
957.0 771.5
"'"-.. 611.4 515.6 510.2 240.2
N9pm
502.2 ".
"-\ 349.8 196.2
15tEu
163.3
11/2"
153Tb
Fig. 10. Systematics of levels in bands associated with decoupled ~'hll/2 configurations in the N = 88 isotones t49pm (this work), 151Eu (Ref. [9]) and 153Tb (Ref. I27]).
The levels at 1162.8, 1477.0 and 1923.6 keV have been assigned spins and parities in analogy with levels in 151Eu which lie at roughly the same excitation energy and have similar decay patterns. As in 15~Eu, the 1162.8 keV level may result from an octupole phonon coupled to the 7rh11/2 state. This will result in a septuplet of states with spins 5/2 + to 17/2 +, with the possibility of observing a band built on each. Only yrast and near-yrast levels are expected to be observed, and we would therefore only expect to see band heads corresponding to the spin of the phonon fully or nearly fully aligned with the spin of the proton. The excitation energies of 3- levels in even-even nuclei in this region are approximately 1.0 MeV (1000.0 and 1071.6 keV in 148Nd [6] and 15°Sm [7], respectively). The band heads are therefore expected to lie at approximately 1.2 MeV in 149pm. The octupole phonon contains a significant contribution from a pair of protons in the d5/2 and hll/2 orbitals coupled to spin 3-. This component cannot fully align with the hll/2 proton due to Pauli blocking and the 17/2 + band head will therefore be raised in excitation energy relative to the other states, leaving the 15/2 + state nearest to yrast. The 1162.8 keV (15/2 +) level observed is thus a good candidate for such a state. In 151Eu, the analogous state lies at 1114.0 keV. The assignment of [ hi 1/2 @ 3 - ] 15/2+ to the 1162.8 keV band head is supported by the
216
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
decay patterns of the 1162.8, 1477.0 and 1923.6 keV levels. With the above proposed spin and parity assignments, the transitions connecting these three states with levels in the bands based on the 1 1 / 2 - 240.2 keV and ( 9 / 2 - ) 515.6 keV states are of El character. It was possible to extract B ( E 1 ) / B ( E 2 ) values from y-ray branching ratios for the 1477.0 keV level (by comparing the 265.8 and 314.2 keV transitions) and for the 1923.6 keV level (by comparing the 310.8 and 446.6 keV transitions). These are shown on Fig. 7, and are smaller than the values obtained for levels in the two opposite-parity bands (see Fig. 1). Using the same procedure as for the two opposite-parity bands, intrinsic dipole moments were extracted from these ratios using K = 15/2. The values for the 1477.0 and 1923.6 keV states are 0.05( 1 ) and 0.09( 1 ) efm, respectively, and are insensitive to changes in K. In 151Eu [8], Do values for the corresponding two states (seen at 1506.9 keV and 1947.7 keV in that nucleus) are 0.05(1) and 0.18(2) efm. The increase of Do with spin may be due to an increase in octupole correlations with increasing spin, although the increase in Do is not as pronounced in 149pIn as in 15tEu. In 147pm [4], octupole vibrations coupled to a 7rhll/2 band have also been observed with an average value of Do obtained for these excitations of 0.10(2) efm. The description of the 1477.0 and 1923.6 keV states in 149pm in terms of an octupole vibration in a reflection-symmetric potential is consistent with the observed values of Do being lower than those values obtained for levels in the two opposite-parity bands shown in Fig. 1, where the octupole correlations are interpreted as being stronger. This is also in accordance with the octupole correlations being weaker for states above the isomeric level due to blocking by the odd proton in the hi1/2 orbital.
6. Summary We have studied the transitional lanthanide nucleus 149pm populated in the 15°Nd(d, 3n) reaction. An extended level scheme has been determined. Spins and parities of levels have been assigned on the basis of internal conversion coefficient measurements, decay patterns and by comparison to the N = 88 isotones 151Eu and 153Tb. Two opposite-parity bands connected by E1 crossover transitions have been found. Interpreting the strengths of the E1 crossover transitions in terms of a rotating intrinsic dipole moment gives a moment similar in size to that observed in the neighbouring nuclei 147pm and 151pm. The size of the moment is smaller than the moments obtained for opposite-parity bands in the other N = 88 nuclei 148Nd and 15°Sm, suggesting the octupole correlations are weaker in 149pmthan in the neighbouring two N = 88 isotones. More data are needed on the magnetic properties of the two opposite-parity bands in order to establish whether they are good candidates for parity-doublet bands. Other E1 transitions are observed which depopulate a low-lying excited state and which are significantly weaker than the crossover transitions connecting the two opposite-parity bands. This is further support for the collective nature of the E1 crossover transitions. Excited levels have been observed above the "n'hll/2 isomeric state. By comparison with the N = 88 isotones 151Eu and 153Tb, these levels have been interpreted in terms of
M.A. Jones et al./Nuclear Physics A 609 (1996) 201-217
217
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