YRAST spectroscopy and g-factor measurements in 199Pb, 201Pb and 203Pb

Nuclear Physics A482 (1988) 573-600 North-Holland. Amsterdam

YRAST SPECTROSCOPY MEASUREMENTS

AND g-FACTOR

IN “‘Pb, “‘Pb

AND ‘03Pb*

U. ROSENCARD Physics Department

I, Row1

P. CARL&

Instirufe of Technolog?: S-10044 Stockholm, qf Physics, S-10405, Siockholm, Sweden

A. KALLBERG,

Research

Institute

L.O. NORLIN

of Physics, S-10405

and

Sweden and Research

K.-G.

Stockholm,

Institute

RENSFELT

Sweden

H.C. JAIN

Tata Institute

of Fundamenral

B. FANT Department

Research,

and

c$ Ph_vsicy, lJniver.Gt)s

Received

Colaha,

Bombay-400005,

India

T. WECKSTRoM

qf Heltinki,

28 December

00170 Helsinki,

Finland

1987

using The half-lives and g-factors of high-spin states in ““),2”‘.Z”iPb were studied “‘x,“‘r’~z”zHg(n, 3n) reactions at E,, =41 and 53 MeV. In-beam y-ray spectroscopy including yyfcoincidence measurements were performed in order to interpret the level schemes. The measured half-lives are T,,? = 7.5 (3) ns, 10.0 (2) ps and 55 (5) ns for the previously known %, $!’ and p’ states, respectively, in “‘Pb and T,,:= 508 (5) ns and 63 (3) ns for the F- and the $ states, respectively, in ““Pb. A new isomer at 4559.2+ fi keV excitation energy with T,,Z = 43 (3) ns and J” = F-’ is observed in ““Pb and a z:m isomer at 2922.7+ -1 keV exitation energy with T,,? = 122 (4) ns in ““Pb. The g-factors of the isomeric states w’ere determined using the time differential perturbed angular distribution technique and the results are g($- ) = -0.0742 (2) and g(T!) = -0.0697 (4) for the $ States in ‘“‘Pb and ““Pb, respectively, indicating a predominantly threeneutron-hole configuration i-’,J,lf5jJ for these states. The g-factor ofthe y’ state in ““Pb is obtained The g-factor of the 7’ state in ‘“‘Pb as g = -0.145 (9) which agrees with a v(i;2,,:2 ) configuration. g(‘i’) = -0.18 (4) indicates a five-quasiparticle configuration. The probable configuration for the $- isomer has also been suggested on the basis of the measured value of the g-factor g(*:-) = -0.063 (3) in ““Pb and g($-) = -0.059 (3) in ““Pb and the B( E2) values for the $! + ? and the 2% 1 +%’ transitions in the odd Pb isotopes

Abstract:

E

‘yX.Z”‘Hg(,, 2n), (u, 3n), E = 41 MeV, ““‘.““Hg(a, 3n), (a, 4n), NUCLEAR REACTIONS E-53 MeV, measured E,, I,, y(e), ~(0, B, r), yy(l). ‘yy.2”‘.‘i”Pb deduced levels, J, v, T,,2r g-factors, B(E2), configurations. Enriched thick targets, Ge, Ge( Li) detectors. Shell model. NUCLEAR

STRUCTURE

‘uu.z”‘.2”‘Pb; calculated model.

1.

by NOAC,

deduced

configurations.

Shell

Introduction

The excited states of the neutron-deficient four-quasineutron configurations involving * This work was supported

levels;

the Nordic

0375-9474/88/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)

even lead isotopes consist of two- and the ~p,,~, vf,,,, vpJI1, vi,,,, and vf,,? Committee B.V

for Accelerator-Based

Research

574

orbitals.

U. Rosen&d

The observed

et crl. / Yrast

spe~t~(~s~~p~

yrast states in lead isotopes

down to ““Pb [refs. I.‘)] are thus

dominated by these configurations. A small deformation is expected to occur at high excitation and high angular momentum in nuclei with A < 200. The observed I.‘) yrast 1 I- state in ‘V4.‘yhPb has been interpreted as a 2p2h proton intruder state with a small oblate deformation. The high-spin states in the odd lead isotopes are expected to have predominantly three-quasiparticle configurations. These states may also be described as arising from a weak coupling of the i,j,7 neutron hole to the states of the adjacent even-A core nuclei ‘). Thus the observed ‘.‘) ‘:-, q- and -$’ isomeric states in the light odd lead isotopes with A = 199 to 203 can be understood ‘) as arising from the coupling of the 9 , 7.. and 5.. two-quasiparticle states in the even core nuclei to an ilJIZ neutron hole. An isomeric ‘?’ state has also been observed ‘.x,y,‘0) in ““,‘Y7.“‘4Pb and in ““Pb I’). This isomeric state has been identified as the vi ,37/2three-quasiparticle state. The main purpose of the present investigation was to establish the configuration of the observed isomeric T+, $- and ??‘_states in “‘)*70’Pb and to search for new high-spin states - in particular the p(i;l);11)33,Z+ state in 20’.2*1Pb and the yrast ‘:state in 201Pb.

2. Experimentai

procedure

through l%?~t(‘.r\o~Hg(cy, 3’.‘) The high-spin states of ‘Y%=‘,?(tPb were popu,ated reactions using a-particles from the 225 cm cyclotron in Stockholm and enriched metallic mercury targets. The enriched liquid mercury was sandwiched between two mica foils of thicknesses of =5 mg/cm’ to a mercury thickness of about 200 mg/cm’. In-beam y-ray spectroscopy including yyf-coincidence, y-r.f. timing, g-factor and angular distribution measurements were performed in order to establish the level schemes of ““Pb and 203Pb and to obtain the magnetic moments of the isomeric states in “‘)-103Pb. For the y-ray detection, both intrinsic Ge and Ge(Li) detectors of 60-100 cm’ active volumes were used. The g-factor measurements were performed through the in-beam time differential perturbed angular distribution (TDPAD) technique in an external transverse magnetic field. The ns-isomers of IqqPb were studied using the IgXHg(cr, 3n) reaction at a bombarding energy of 41 MeV and a magnetic field of 2.028 (10) T. The g-factors of the y- and $- states in ‘“‘Pb were determined at identical conditions utilizing the ‘““Hg(cu, 3n) reaction. Three detectors placed at angles +135”and 45” with respect to the beam direction were used for detection of the y-rays. The time distributions of the y-rays from the precessing nuclei were studied in a time interval of 1.7 ys between the beam bursts. The time interval was obtained by selecting every thirteenth pulse from the cyclotron with the help of a fast pulsing system. The data was collected in a two parameter, energy versus time (E - t) mode. The decay of the isomeric 7.. level in ‘““Pb with a half-life T,;,= 11 f_~s[refs. ‘,“)I was studied in a magnetic field of 0.5543 (IO) T using a time interval of about 40 ps between the

U. Roscngo”rd

e/

ai ,’

YraAl

vprc'troscop~

575

beam bursts, which means that every five hundredth beam pulse was selected. ‘“‘Pb was also studied using the ((Y,3n) reaction at a bombarding energy of 53 MeV and a magnetic field of 2.088 (10) T. The isomers in ‘“-?Pb were studied using the ‘“‘Hgiq 3n) reaction at 53 MeV bombarding energy and a 2.088 ( 10) T. Typical time spectra obtained with the fast pulsing for some in ““I% are shown in fig. 1. Time spectra for the 372.4 keV obtained with two different pulsing systems are shown in fig.

magnetic

field

of

relevant

transitions in “‘“Pb 2. Since the decay transition

curves contain contributions from two or more isomeric states, the TDPAD spectra were fitted with an expression given by Lutter et al. ‘I) to account for the Larmor precession in each isomeric state. The fits were obtained by varying six parameters, i.e. the intensity at zero time, half-life, phase angle at zero time, the angular distribution coefficients and the g-factor for each state. It was possible to obtain a convergence and reasonable fits in some cases even when all the twelve parameters for the coupled decay of two isomeric states were varied. In some cases the Larmor frequency v,ras also obtained from the ratio R which is defined as R = 2[N(+135, N(t135,

B, t) - N(-135, R, r>+ N(-135,

B, t)]

I?, t)

’

where N(*135, B, t) are the counting rates for the y-ray transition as a function of time in the presence of the magnetic field B and with the detectors positioned at +135” with respect to the beam direction. The angular distribution measurements were pcrformcd at fi~c angles hctween 90” and 159” with respect to the beam direction. These measurements were performed in order to obtain additional spectroscopic int’ormation on ““.““Pb.

3. Results 3.1.

LbVELS

IN

“‘Pb

The level scheme of “‘Pb is presented in fig. 3. The ground-state spin of ““Pb has in earlier works “.“‘) been assigned as : but calculations and level systematics indicate If’) that it could be ;. Recent laser spectroscopic measurements “.I’) reveal a negative magnetic moment for the ground state of “‘qPb which is in agreement with the expected value for a pi:: neutron, The J n value of the ground state in ‘*‘Pb is thus most probably i The 425 keV M4 transition which is observed “,lh) to depopulate the isomeric y’ state will thus populate an excited i state which is expected close to the ; state. Richel er al. 13) determined the energy spacing between in states to 19.S8 keV, but with the reversed level order. Since no other the z- and > levels are expected “.“‘) in this energy region we suggest that the observed energy spacing is the energy spacing between the ;- ground state and the $- first excited state in ‘““Pb. This change of the level order will cause some minor conflicts in the

576 1 m5

cn 104 + z

2 0

1[a3

k k 1m2

!E 2

10'

10ID

20

40

60

60

I00

120

140

160

180

200

1[05

m

1[a4

k

5

53103 k

i

“w 1m2

F

z 10'

i 50m

lmm

20

40

60

20

40

68

80

100

80

100

NS

’

II

120

140

160

180

200

120

140

160

180

200

1 m6 1m5 m

5 3 104

0” & 1[a3

ET g 1m2 2 10' 10"

CHANNEL

NUMBER

Fig. 1. Time spectra for the 600.3, 354.3 and 913.2 keV transitions in L ‘“‘Pb in the presence magnetic field B = 2.028 T at 8 = 135” with respect to the beam direction.

of a transverse

U. Roseng&d

e! al. / Ymst spectroscop_y

577

1105

m

104

Ls 3

z 103 k 6

1102

a

5 =

10'

100 1105

100

120

140

160

180

~“““““““““““ 372 keV

0

0

10'

50

100

CHANNEL Fig. 2. Time spectra

for the 372.4 keV y-ray

150

200

NUMBER

in ‘““Pb obtained magnetic fields.

using two different

pulsing

systems

and

578

et al. / Yrast

U. Rosenghd

spectroscope

level scheme of ‘99Pb proposed by Richel et al. “), however it has no influence on the deduced yrast structure above the yrast y state. The level order and the energies of the first k, 5-mand y+ excited states of 19’Pb suggested in fig. 3 are the most logical and probable ones in view of the existing experimental data. The high-spin structure of 19’Pb has previously been studied by several groups I,‘,‘). In these studies isomeric ?;+, $- and y- states were observed. The decay of the isomeric high-spin states (see fig. 3) was studied in the present investigation. The half-life of the T- isomeric state was found to be T,,, = 10.0 (2) t.~s using the decay curves of several transitions (see table 1). The isomeric transition which is expected to be a low-energy E2 transition was not observed. The time spectra also show the presence of two shorter half-lives with Tliz= 7.5 (3) ns and 55 (5) ns, respectively. The half-life for the pm state reported earlier is 33 ns, ref. “) and 11 ns, ref. ‘). Our value of 7.5 (3) ns is thus close to the value reported in ref. ‘). The 7.5 (3) ns half-life 33/Z+ 2912+

55ns E

3349.7+a’ 3319 7+&

I

32L.l

u

5693

4397

.2

1388 6

1512’ 17/2+\

LlL22

lL57.3 3

1012 8

_

13/2*

122mln

UL.5

512. 27

,.5hAF

lg9Pb The I” value of the Fig. 3. The level scheme of “‘Pb according to refs. ‘.‘,7,‘3) and this investigation. ground state is from refs. 14.15) and the energy for the $- state from ref. 13). The level energies and the half-lives for the high-spin states are from this investigation, except for the 3.6 ns half-life of the y- state.

L’. Rotrnga”rd

et al. / Yrart spwtrmcop,

is also seen in the 324.1 keV transition

(see table

l), which

579

indicates

a 48.3 keV

transition between the isomeric ‘:- state and the y- state at 2471.3 keV. The 55 (5) ns half-life observed in the yrast transitions in ““Pb may be assigned to an isomeric state above the ;,“- state. This half-life is also seen in the 830.1 keV transition

(see fig. 4). The presence

of a 55 ns half-life

in the time spectra

of the

transitions depopulating the my;i-state indicates a prompt connection between the ‘,‘- state and the 830.1 keV transition. A feeding of the 10.0 ys state by the 830.1 keV transition would smear out the 55 ns half-life in the lower part of the cascade. The 55 ns isomeric transition is not observed, but the measured g-factor and the level systematics (see the discussion in sect. 4) indicate that this half-life belongs to an isomeric ‘$’ state, which decays through a low-energy E2 transition to a Y-t state of the same main configuration. The 830.1 keV transition cannot connect the y’ and $- states since it is a dipole according to the angular distribution “). No other transitions than the 830.1 keV transition are observed in the 55 ns isomeric decay above the ‘$ state, which indicates that the missing transition(s) must be low-energetic. A ‘:- state is observed “) in “)‘Pb close to, but above, the yrast ‘,‘- state. A y’ state is also expected y’ -2,’ dipole just below the ‘,“’ state. It is suggested “) that the high-energy transition is more probable than the low-energy 9’+ ‘,‘I transition, which is expected to proceed not as an Ml transition but as an E2 transition between similar configurations. We propose that the main decay of the y’ state goes through the 830.1 keV $- -+ ‘2i transition to a ‘,‘- state and that the unobserved transition is a low-energetic Ml transition marked 3’ in fig. 3. The results of the g-factor and half-life measurements of the ‘,‘-, y- and ‘5” isomeric states are presented in table 1. The fitting procedure of the TDPAD spectra described in sect. 2 could be checked through a fit to the precession of the known 12’ state in ““‘Pb [ref. ‘“)I. The fits to the observed precession for the 777.9 keV transition de-exciting the 12’ state in ““‘Pb gave (after correction for Knight shift “‘) and diamagnetic shielding “)) a value of g = -0.1541 (10) for the g-factor and T,,? = 202 (5) ns for the half-life of the 12’ isomer. These data are in reasonable agreement with the values reported by Stenzel et al. “). The half-life and g-factor of the 372.4, 301.5, of the ‘,“- state were extracted from the observed precession 423.2 and 977.8 keV y-transitions in an external transverse magnetic field B = 0.5543 (10) T (see table 1). These measurements gave an average value for the half-life T,,, = 10.0 (2) ~LSand the g-factor g,,,,, = -0.07499 (15), including the error in the magnetic field. The value of the g-factor after correction for Knight shift and diamagnetic shielding is g,,,., = -0.0742 (2). The g-factor of the 55 (5) ns isomeric state was obtained as g,,,,= -0.147 (9) from the R-function of the 830.1 keV transition (see fig. 4). After correction for Knight shift and diamagnetic shielding a value g,,,,= -0.145 (9) is obtained. This value is in agreement with the previously obtained ‘) value g = -0.152 (3) for this isomeric state.

580

830 keV

2410

26rn

0

31218

28rn

CHRNNEL m.8m

l-l

I”“1

32m

34m

36rn

38rn

NUMBER

"'l""l""I""l""l""l""l""l"'~l""1""l"~

‘I

830 keV m.4m fz L

m.mm

5 IL A! -10.410

-m.88

3im

32m

33m

34m

CHANNEL Fig. 4. The decay curve of the 830.1 keV transition formed from the spectra obtained in detectors

35m

360

37m

38rn

NUMBER

in “‘Pb and the R-function for the same transition at two angles *135” with respect to the beam.

U. Rosengh-d

et al. / Yrast .spectroscop~

581

TABLE 1

Half-lives and g-factors in ‘WPb determined with a pulse interval of 1.7 ps for the ns half-lives and 40 I_LS for the (J-Shalf-life. (The errors of the experimental values are taken from the fitting procedure.)

E, (keV)

830.1 372.4 324.1 301.5 388.6 423.2 977.8 1012.1 average

+? state

‘$+ state “)

T,,, (ns)

TIIZ (ns)

8.4 5.8 7.2 7.3 6.3 7.4 7.3 h,

(2) (1.1) (2) (2) (4) (2) (4)

7.5 (3)

57 (4) 85 (9) 56 (3) 100 (23) 49 (2) 7s (7) 57(12) 5s (5)

“) The g-factor g,,,, = -0.147 (9) is obtained using the R-function “) The given errors are the root-mean-square errors. ‘) The error in the magnetic field is not included.

3.2. LEVELS

p- state T,,z (w)

10.0 10.0 10.2 10.0 10.9 9.8 ll(2)

(2) (2) (3) (4) (5) (2)

10.0 (2)

gul,

-0.0750

(2)

-0.0753 -0.0751 -0.07495 -0.0750 -0.0752

(3) (4) (8) (2) (3)

-0.0749

(7) ‘)

of the 830.1 keV transition

IN ‘“‘Pb

The level scheme of ‘“‘Pb has earlier been studied by several groups ‘,6,7). Our results (see fig. 5) for the decay of the isomeric y+, $? and y- states are in agreement with those reported previously. Helppi et al. “) concluded from their measurements that the T- state decays through an unobserved E2 transition to the F- state at 2718.9 keV and reported half-lives of 540 (40) ns and 55 ns for the y- and T- states, respectively. In the present measurements the time spectra of the 222.3, 350.3, 354.3, 600.3, 913.2 and 917.1 keV transitions de-exciting levels in ““Pb (see tables 2 and 3) gave a value of the half-life T,,, = 508 (5) ns for the ‘,“- state at 2718.9+3 keV and 63 (3) ns for the $- state at 2718.9 keV excitation energy. In addition to the decay of the T- and :zm isomers, the decay of some non-yrast states was also observed through the yy-coincidence measurements. The 166.6 keV transition feeds the $” state at 1902.7 keV and the 707.9 keV transition feeds the 9’ state at 1896.3 keV. The positive A, coefficient (see table 2) favours a non-stretched Ml multipolarity for the 166.6 keV transition and a $’ spin for the state at 2069.3 keV. The negative A2 coefficient of the 707.9 keV transition indicates a $‘-+ ‘;’ transition, but a $ spin cannot be ruled out due to the complex nature of the y-line. In fact, a y state is expected in this region from level systematics (see the discussion). The time spectrum for the 707.9 keV transition shows that it is fed from the ‘; isomer. The feeding transition(s) were not observed. A weaker 708 keV transition is placed above the isomeric F- state from the coincidence relations. The low-spin (i-), z-, ‘$- and y+ excited states “) at 879.6, 1014.4, 1186.3 and 1447.1 keV excitation energy, respectively, were also observed through the 879.6, 1014.4, 171.9 and 818.3 keV transitions. The $- and i- states observed Ii) at 538.7 and 936.2 keV excitation energy,

7906

x

8256

L _--_m+-_mm-_

$2223/2

26042 21966

7079 20693 19027

Fig. 5. The level scheme

of ““Pb

as obtained

in the present

investigation.

respectively, are probably populated in the present experiment, but the y-lines from the 368.8 (G--f-), 847.7 ($- +;-) and 936.2 keV (g- + s-) transitions de-exciting these states are all complex and, therefore, their transition properties cannot be determined. The transitions above the 2zmisomer have been obtained from coincidence spectra with ‘before’ gates as shown in fig. 6. The 825.6, 287.0 and 727.7 keV transitions form a cascade feeding the $- isomer as these are found to be in coincidence with each other. The strongest of these, i.e. the 825.6 keV transition, feeds the ?;- isomer directly. The angular distribution measurements indicate E2 multipolarity for the 825.6 keV transition and Ml for the 287.0 keV transition. The 727.7 keV transition

U.

RosengBrd

et al. / TAHL.L

Properties

166.6 171.9 222.3 287.0 293.8 298.3 350.3 354.3 360 360.7 387.5 422.5 446.9 573.1 594.0 600.3 627.2 628.8 667.5 707.9 708 727.7 785.4 790.6 818.3 825.6 879.6 913.2 917.1 919.3 1014.4

of y-rays

assigned

4.3 0.9 44 7.7 3.8 1.2 32 47 =l 17 5 0.7 2.1 3.4 74 =I 194 10 =I 10 s.9 5.7 4.5 17 0.8 100 41 8.8

Yrmt

.spectroscopy

583

2

to ‘“‘Pb in the reaction

2’i’)Hg(a, 3n) at E,, = 53 MeV

0.36 (5)

0.01 (5)

0.22 (5) -0.51 (S) -0.28 (4)

0.02 (6) 0.02 (8) 0.07 (6)

0.25 (5) 0.70 (5)

-0.05 (6) 0.04 (6)

>o

-0.87 -0.06 -0.21

(5) (5) (4)

0.14 (8) 0.08 (6) -0.02 (6)

-0.10

(4)

-0.02

(6)

-0.13

(5)

-0.02

(7)

-1.06 (3) -0.21 (S) 0.28 (5)

-0.01 (5) 0.01 (7) -0.03(6)

0.32 -0.51 -0.45 0.07

-0.01 0.15 0.31 0.01

(5) (4) (4) (4)

(6) (6) (7) (6)

is also found to be a dipole transition from the angular distribution measurements. The theoretical considerations discussed below indicate J” =-!,+ for the state at

4559.2+ A keV and El multipolarity for the 727.7 keV transition. The observed multipolarities lead to J” values of y- and F for the intermediate states at 3544.5 + A and 383 1.5 + A keV, respectively. A half-life of 43 (3) ns is observed in the 727.7, 287.0 and 825.6 keV transitions (see fig. 7). This half-life cannot belong to any of the levels discussed above as they decay through high-energy El, Ml or E2 transitions. It, therefore, belongs to an isomeric level lying just above the 4559.2 + A keV level and which probably decays through an unobserved E2 transition. Theoretically, a 2’ state is expected in this region (see the discussion) providing the possibility of a y”-p’ E2 transition.

584

U. Rosengird

et al. / Yrasi spectroscopy

TABLE 3 Summary

of the g-factor and half-life measurements for they”, 6- and F- isomers in ‘“‘Pb. (The errors of the experimental values are taken from the Iitting procedure) sm z state

$+ state

B- state

E, (kev)

222.3 287.0 350.3 354.3 600.3 127.1 825.6 913.2 917.1 average h,

g,,,,

Tljz (ns)

-0.18 (4)

43 (3)

R”llC -0.0711

(5)

T,,> (ns) 507 (10)

g”“, -0.062

(5)

TIjz (ns) 64 (5)

-0.0702

(7)

515 (15)

-0.068

(5)

65 (10)

-0.0704 -0.0698

(4) (8)

503 (10) 497 (10)

-0.063 -0.064

(2) (13)

60 (6) 55 (13)

-0.0708 (10) -0.0702 (3)

528 (10) 504 (20)

-0.067 -0.066

(3) (3)

65 (5) 65 (6)

-0.0704

508 (5)

-0.0647

43 (3) 52 (2) a

-0.18

(4)

43 (3)

(2) ‘)

“) This value is not included into the mean value due to a complex h, The given errors are the root-mean-square errors. ‘) The error in the magnetic field is not included.

(13) ‘)

63 (3)

y-peak.

Thus, a combination of theoretical considerations and experimental observations determine a positive parity for the 9 state at 4559.2 keV and an isomeric state with J” = 9’ and T,,2 = 43 ns at an energy 4559.2+ 6 keV. The isomeric q- state is also populated by the 422.5, 790.6 and 293.8, 919.3 keV y-ray cascades, thus establishing a level at 3932.0 + il keV. This state is also connected to the y- state at 3544.5 + A keV through the 387.5 keV transition. A spin J = y and a positive parity is suggested for the state at 3932.0 + A keV from theoretical considerations. The y+ state is populated by the 573.1 keV transition which is in concidence with the 667.5 keV transition. The dipole character of the 573.1 keV transition gives a J = 4 state at 4505.1+ A keV. The weak 708 keV transition feeds the 4’ state and it is in coincidence with a 360 keV transition - a doublet to the 360.7 keV q”+y’ transition. There is no half-life observed in the 573.1 keV transition, but a weak ns-component is observed in the transitions depopulating the y’ state. There is, thus, an indication of a weak 627.2 keV F’ + 9’ transition - a doublet to the 628.8 keV ground-state transition - in the present experiment. However, the half-life measurements could not be performed accurately because of the weak intensity of this transition. The ‘before’ coincidences show also that the $-’ isomeric state is populated by the 446.9 and 785.4 keV transitions. The g-factors and the half-lives for the p- and y- states in *“Pb as obtained from the fits to the observed precessions of the 222.3, 350.3, 354.3, 600.3, 913.2 and 917.1 keV transitions are listed in table 3. The measurements gave an average value -0.0704 (4) and g,,,, = -0.0697 (4) for the Fstate, g,,, = -0.0647 (13) and of g”“, = &I,, = -0.063 (3) for the p- state. The g-factors are corrected for Knight shift *O)

Fig. 6. Coincidence

_

spectrum

500

with gates on transitions

2870

NUMBER

below the 508 ns isomeric

CHANNEL

p

“before”

the transitions

gatesum

state in ‘“‘Pb showing

1000

“‘Pb

feeding

this isomeric

state

586

1(D4

)

720 keV

'-

140

150

160

170

180

, 4’

130

206

210

22m

230

104

p 1m3

5

0 0

“0 1m2

E 2

is10’ 1mm

IIII,,I,,,~lIIILLLIlIIIII~lIIIIIII 100

150

2100

250

CHRNNEL Fig.

7.

300

350

400

NUMBER

Time spectra for the 727.7, 287.0 and 825.6 keV transitions $’ isomeric state. The 825 keV y-ray is contaminated

in ‘O’Pb containing the decay of the with activity from *“ITI.

U.

and diamagnetic shielding the values of the g-factors

Rnsengdrd

et al. /

Yrast

.spectrorcop?

587

?I). The error in the magnetic field has been included in given above. The signs of the g-factors were determined

from the sense of rotation as observed through the time-differential perturbed angular distribution patterns and they were also checked by comparing the phase of the precession with the phase for the 12’ state in ‘*“Pb where the sign has been determined by Stenzel ef al. I”). The g-factor of the $+ state was measured as g,,,, = -0.18 (4) {see table 3) by observing the precession of the 287.0 keV transition. This value will not change after correction for Knight shift and diamagnetic shielding and g,,,, = -0.18 (4) is obtained for this state.

3.3. LEVELS

IN ““Pb

The level scheme of ‘03Pb is shown in fig. 8. The coincidence relations among the transitions assigned to 203Pb [refs. 22.23)] were studied through the ““Hg(a, 3n) reaction. The alpha energy was selected to 53 MeV in order to optimize the ‘“‘Hg(q 4n) reaction 14), but the yield of the (a, 3n) reaction was considerable because of the thick target (-200 mg/cm’). The results of the coincidence measurement are in agreement with those of Saha et al. “). In addition to the earlier level scheme ‘I), a 979.5 keV transition is seen to populate the yrast $?’ state at 1943.2 keV. This should be a stretched dipole transition since A2 -c0 and a J - 22’state is, therefore, obtained at 2922.7 keV excitation energy. A half-life of 122 (4) ns is seen in the 979.5 keV transition and also in the 280.0 keV transition in the same cascade (see fig. 9). The isomeric transition is not observed, but it is most probably an E2 transition according suggested

to the level systematics (see fig. 10). A negative parity for this state is from the level systematics (see the discussion). A coincidence between

the 979.5 and 258.2 keV transitions indicates the presence of a 21.8 keV y’ + T+ transition. The yrast ‘,“’ state in ‘(“Pb is also seen to populate a ‘:~’ state (see fig. 5). The corresponding ‘,” state in ““Pb was not observed in the present experiment. Some tow-spin states were also observed. These states have earlier been seen in decay studies 1h.‘5) and our results are in agreement with those. In analogy with ““Pb an yL and an y+ state are observed. The %- state decays by the 264.4 keV Ml transition to the s- state at 896.8 keV and the ‘,” state by the 816.7 keV Ml transition to the yrast ‘;1” state.

The low-spin

$-, ?,

:,-, z-, ;A and 2: states ‘6,25) are also

observed through the 126.7, 186.7, 596.0, Xi0.3, 867.0, 746.1, 271.1, 746.6 and 933.3 keV transitions (see fig. 8). We are left with some weak transitions after accounting for all the y-rays below the 0.48 s isomeric y- state. These transitions belong to ‘03Pb as indicated from the excitation functions and they are assumed to feed the $- state. There are three cascades seen, i.e. the 403.0, 1105.6, the 547, 961 and the 767.4, 740.5 keV cascades. For intensity reasons the 1105.6, 961 keV and 740.5 keV transitions are assumed to feed the q- isomeric state. The 1105.6 and 740.5 keV transitions are deduced to be

588

U. Rosengdrd

et al. / Yrast spectroscopy

5571 5L6

37/Z+

I j271

1

5296 502L7

33/z+ 030 31/233/Z31/z-

1056

$1

7L0.5 I

6773

73.6

851.3

$$L_ + l

~%-L5L11 17/2+ l_L?J

16632 i&i&6

,

8167

I

11612

Fig. 8. The level scheme of *03Pb as obtained in the present investigation. The placement of the low-spin transitions is made according to ref. “). The half-lives are from refs. 23,26).

5x9 1 [D5

1m2

5m

imm

15m

2106

25m

358

3mm

1105

lmm

i5m

250

288

380

350

10.810

m.4m El 5 =

m.mm

LL &

-m.4m

-m.f3m

24m

26m

28m

3mm

CHANNEL :. 9. Time spectra

320

348

368

388

NUMBER

for the 280.0 and 979.5 keV transitions in ““Pb in the presence magnetic field and the R-function for the 979.5 keV transition.

of a transverse

U. Rosrngcird et al. / Yrast spectroscopy

590

TABLE 4 Properties

of y-rays

assigned

to ‘“‘Pb in the reaction

‘“‘Hg(a,

3n) at E,. = 53 MeV

E, (keW 21.X 126.7 153.3 174.4 186.7 217.7 239.4 258.2 264.4 “) 271 ‘0 271.1 “) 280.0 403 .o 454.5 546 :I) 541 ‘1) 56X.3 596.0 “) 434.2 677.3 740.1 “) 140.5 ~‘) 746.6 167.4 816.3 820.3 824.9 838.3 851.3 “) 867.0 873.6 896.8 933.3 :‘) Y61 “) 979.5 1026.5 “) 1105.6

2.6 0.9 9.x 3.3 -1 16.4 71.9 4 (1 =1 13.1 5.6 =l =1 3.8 8.8 25 7.3 3.2 -2.5 4 4.1 3.7 7.5 82.8 100 =3 9.6 23.2 3.2 4 -6 6.6 6.5 4.3

“) Complex peak, double. ‘) This transition is assumed distribution measurement.

0.16 (0
(6)

0.14 (8)

(6) (6) (5)

0.01 (8) 0.04 (8) 0.11 (8)

-0.29 (5) -0.15 (5)
-0.17 (8) 0.01 (8)

0.12 (4) 0.27 (9) -0.11 (5) -0.08 (6) =O

-0.15 0.31 0.27 0.31

-0.70
(4)

0.22 (7)

(5)

0.34 (7)

(7) (5) h, (6)

-0.36 (6) -0.02 (5) 0.83 (7)

0.02 (6) -1.21

(6) (9) (8) (8)

(3)

to be isotropic

-0.54

(9)

0.03 (8) -0.12 (8) 0.05 (8) 0.15 (8)

-0.04

(7)

-0.24 (8)

and it is used for calibration

in the angular

stretched dipole transitions from angular distribution measurements. The multipolarity of the 961 keV transition could not be determined since it is masked in the y-ray spectrum by the strong 960.7 keV ground-state transition in ‘“*Pb. The 740.5 keV transition coincides in energy with another transition, namely the 740.1 kevtransition between the $- and the fP low-spin states, but this transition is relatively weak and

U. Rosen&d

the negative

A, coefficient

et al. / Yrasl.speclro.wop~

of the 740.5 keV transition

591

must be due to the high-spin

transition. Therefore this transition determines a J = .;Lstate at 3688.6 keV excitation energy. If the 403.0 keV (A, = -0.15 (5)) and 547 keV (A, = 0.12 (4)) transitions are assumed to be stretched and non-stretched dipole transitions, respectively, J = y is obtained for the state at 4456.4 keV excitation energy and J = ‘i for the intermediate state at 4053.7 keV. Finally, a negative parity for both the J = ‘i states and the y state at 3909.3 keV and a positive parity for the J = 3 state at 4456.4 keV is suggested from theoretical considerations (see the discussion). The J” = :J’ state at 4456.4 keV excitation energy is fed by the 568.3 keV transition. The positive A2 coefficient of this transition indicates an E2 transition which gives a J==';+ excited state at 5024.7 keV. Two weak transitions, namely the 547 and 271 keV transitions are seen in coincidence with the 568.3 keV transition, and they will feed the ‘2’ state. There is no delayed component observed in the transitions populating the q- state. This indicates that the 7’ state which is isomeric in ‘“‘Pb, is not isomeric in ““Pb. The half-life of 122 (4) ns seen in the 280.0 and 979.5 keV transitions (fig. 9) is most probably due to the decay of an isomeric ‘$ state situated is not above the ?i- state and also above the 2,“- state. The isomeric transition observed but the lower limit of 25.4 keV for the transition energy is determined by the F--‘$energy spacing. The g-factor g,,,, = -0.0600 (25) is obtained from the R-function agreement

for the 979.5 keV transition. This gives a value g,,,,= -0.059 (3) in with the g-factor obtained for the ‘;- state in ‘“‘Pb (see subsect. 3.2).

4. Discussion 4.1. LEVEL STATES

SYSTEMATICS 1N

AND

CONFIGURATION

ASSIGNMENTS

OF

HIGH-SPIN

I’)%3~l,mp~

The level systematics of the odd A = 195 to A = 205 lead isotopes is shown in fig. 10. The configuration assignments shown for zOsPb are from ref. I’). It is seen from the systematics that most of the three-neutron hole states in ‘OsPb are also observed in the more neutron deficient isotopes. The ‘$ state is isomeric in “‘Pb due to the main decay through a low-energy ‘?- + 9 E2 transition. This state is found to be isomeric also in 1yy,201,203 Pb according to the present study. The long half-life observed for the y- state in ‘yy~z0’~2”3Pbis most probably due to different configurations of the initial and final states. The ‘;- state in ‘“‘Pb has a half-life T,,, = 480 ms as it decays through a strongly hindered E3 transition. On the other hand, the F- state in ‘y9,20’Pb is isomeric because of its decay through a low-energy p + $- transition. The isomerism of the T’ state seems to be destroyed in ‘9s,‘97Pb due to an intervening F- state ‘,‘O). The isomeric y- state in the light lead isotopes is of a rather pure zA$2f$1 configuration as in ‘O’Pb, but the q- state is mixed and configurations other than the i&p;/z configuration have to be included (see subsect. 4.2).

592

il. Roseng&i

et al. / Yrnst spectro.w~p~

The yrast \I’ and 9’ states in ‘*‘Pb are identified ‘I) as mainly three-quasiparticle states of the main configuration vi~&f5/:p~/12 but they can also be described ‘) as ui~$,,@Z’ and vi;$ 04” excitations, respectively; the latter description may be more correct for the lighter nuclei where stronger con~guration mixings are expected. A g-factor

measurement

“) on the isomeric

?i+ state in ‘03Pb shows that this state

mainly consists of the vi& 04+ configuration, where the 4+ state contains a mixing of the v(fJTJd+ and v(fi;;l?p$‘z)4+ configurations. There are also other positive-parity and the ‘;“- state in ““Pb. The states observed, such as the 4’ state in “)‘,“‘,“‘Pb ‘$’ state in “‘Pb can be explained as a five-quasiparticle state of the ui;;‘,,f;,?2p;fZ con~guration or alternatively vi L&O 5+.. The Ti’ state observed in the lighter nuclei has similarly most probably a vi$,O6configuration. The yrast ?I’- state can be described as a vi;&, @Y excitation. As is seen in fig. 10 the systematical trend of the $- state is very similar to that of the ‘?’ and the 2’ state. The y- state has a long half-life in “)5.“)7Pb [refs. ‘.8.‘0)] as it decays through a low-energy transition. The energy of the depopuIating El transition in 201,2U3Pbis higher and the half-life is (1 ns. According to the systematics the observed excited state at 2922.7 keV in “‘Pb is most probably the yrast zi- state, which decays by the 979.5 keV transition to the yrast j;’ state. The proposed p- state in 20iPb also fits well into the systematics of the $- states. The configuration of this state will be discussed in the next subsection. The ‘$-’ state identified I’) as the v(i;&)33,2+ state shows the same systematic trend as the yi state, i.e. the excitation energies of these two states are continuously lowered as the neutron number decreases, see fig. 10. The 4’ state is observed as The g-factor of this isomer (see subsect. 4.2) is in isomeric in ‘y5~‘y7~‘yY~2”5Pb. neutron configuration. According to the systematics the 9’ agreement with an i,,,, state observed at 3932.0-t A keV in “‘Pb and at 4456.4 keV in ‘03Pb may be identified state. The decay of this state to the ?’ state of the same as the v(i;&)3jj2+ configuration is inhibited in 2”‘.203Pb because of the appearance of the negative-parity %m and v- states below the 9’ state. The decay of the ?;.’ state occurs through these negative-parity states. The y and y negative-parity states are most probably the Eve-quasipa~icle states with the main configuration ui $,,@(f, p-). The calculated is 3.5 MeV in 20’Pb and energy ‘“) for the yLI-, y- states with this configuration 3.6 MeV in 2”3Pb which is much smaller than or clearly below the calculated energy of 4.46 MeV for the three-quasiparticle ‘2’ state in *‘13Pb. In order to build up states with spins greater than $?, one has to consider five-quasiparticle states. Thus states with the three-quasiparticle p’ state coupled to the 2+ and 4+ excitations of the even cores will occur. The 4’ and F* states of this excitation mode are expected ‘“) to be the yrast states. The f state observed at 4559.2 -t A keV excitation energy in 20’Pb and at 5024.7 keV excitation energy in 203Pb has most probably the suggested five-quasiparticle configuration. Since the isomeric transition in ‘O’Pb is not observed it is expected to be a low-energy E2 transition and the y isomeric state is likely to have a v(iW3 ,3,2)33,2+04+ con~guration.

U. Roseng&rd

I I

et al. /

Yrust

,spectrcscop~~

593

594

U. R[J.~~ilg~rd et al. ,f Yrast spixtroscopy

The estimated energy “) of the five-quasiparticle v(iy&z)., states with J” = 7’ and 4’ is about 1.5 MeV higher than the energy of the observed isomeric state.

4.2. eONFIGURATION g-FACTOR

ASSIGNMENTS

IN

LW,30l,ZL13PbFROM

THE

MEASUREMENTS

The g-factors for the F- and $- states can be theoretically estimated from the dominant configurations of their wave functions using the generalized Land& formula. The F state is expected to have a predominantly three neutron-hole configuration (i;72/2),Z*f~~2 which can be demonstrated through the g-factor measurement The measured values of the g-factors for the 12’ states in ““‘Pb and “‘hPb are nearly equal ‘“) which indicates the same pi;$,“i, structure for the 12+ states in this region. A mean value of g = -0.1510 (10) is obtained “),?‘)) for the 12+ state in “‘“Pb. This value gives together with the mean value gff,‘,) = 0.301 (9) obtained 30_‘7) for the f;;2 neutron-hole state in 107Pb (see table 5), a value gC~,,(~~) = -0.073 (2) for the g-factor of the ‘,“- state. The calculated value is in reasonable agreement with the measured values in ““Pb and ‘“‘Pb and the result suggests a pure three neutron-hole (i,$1),2+fs/z configuration for the ‘:-~ state in both of these nuclei. It should be noted, however, that the g-factor of the 2: state in ‘44Pb is about 6% more negative compared to its value in ““Pb. If it is assumed that the ‘:- state in ““Pb has a pure (i ,$)l?+f5iz neutron-hole configuration, then the decrease in the g-factor of this by assuming a 4% admixture of the (i ,~J~),1~f7~J state in ““Pb can be understood neutron-hole configuration. This is possible because the energy difference between the fy,\ and f_<,rzneutron hole states is rapidly decreasing with decreasing neutron number .I’). Three angular momenta can be coupled in several ways in order to give a resultant angular momentum J. In this sense the ‘: state can also be described as arising due to the coupling of an i ,;I2 neutron hole to the 9- state of the even-core nucleus. The g-factor of the 9.. state in ““‘Pb has been measured by Lutter et al. “) as g(9-) = -0.0285 (11). If this vaIue for the g-factor of the 9- state in ‘““Pb is used a

TAIII.I 5 Experimental Neutron

orbital

single particle

x-factors

g-factor -0.1510 i 10) +0..30119) +I.1566 (14) -0.87 -0.36

(9) (3)

These values are used in the calcuialions 4.2.

obtained

from neutron-hole

states in “‘“Pb and ““Pb

State

NUClWS

Ref.

12’

2””Ph

1’J.Z’) ) XLiZ)

“” Pb

3.3 1 1J.15 ) 24.v

:

iI : 7 of g-factors

WPh

Comm.

)

for different

neutron

corrected corrected

Schmidt Schmidt

configurations

value value

in SubseCt.

il.

~[}~e~t~~~r[f ri ai./ Yruct splxwomJp~

595

value of gCCi,= -0.078 (1) is obtained for the g-factor of the 7 state in lyYPb. The obtained value is close to the present experimental value. Since the 9- state in ““‘Pb has a relatively pure i,-:,2fqy)z neutron-bole conliguration I), it also supports a ’ ) +fcjiz configuration for the ‘u-state. predomin~~ntly threw-neutron-hole (i,;::, ,? The weak coupling

calculations

by King Yen Ed al. “) show that the dominant

configurations (i, ,‘/,),,-fijiz and in the $- state are the neutron-hole values of the g-factors for these configurations (i ,I z),z+pi~~\. The calculated go,[(i~.~2),21f5,1_7]~~,? = -0.122 (4) and R~‘l,[(i,,‘/,),,lp,l?],i;, = -0.186 (2) are much larger than the experimental values obtained in the present measurement. The ii-:,neutron holes can, however, couple to give a 10’ state which then couples to the g-factor for this f-12 neutron hole to give the desired ‘1’. state. The calculated configuration is g,.,,[( i;<~.l),,,+fs:‘,],,:l = -0.061 (2) which is reasonably close to the present experimental values in _““Pb and ‘“3Pb. The y-. state both in ‘“‘Pb and “‘“Pb can, therefore, have a predominantly three-neutron-hole configuration given by components

](i,i2 z),ll*fiiJ3 : . The configuration

and the g-factor of the y state can also be explained by other orbit&. Linden et al. “) found the T state in “15Pb to have a predominantly (i,$,,),,+py)2 configuration with admixtures of the (i,;‘,),,Ifi:‘,, (i,,‘1),2.p:,> and (i,&),,,+f,;‘, conligurations. The amplitudes of the different components are likely to change when neutrons are removed from ““Pb. A larger amplitude of the con~guration is expected in the ‘: state in ““Pb because of the pljiz (i;:J),zlpS;iz neutron hole moves closer to the Fermi surface. Thus the observed g-factor for the 25 7 state in “” Pb and ““Pb can also be reproduced by a wave function ~~(i,~,~),.~p,,‘~)+Bl(i,;‘,),,~pi~~) with the condition A’+B’=l. The g-factors for the single-particle p,-jz and pl.i neutron-hole states are given in table 5. It should be pointed out that the diagonal terms contribute negative values to the g-factor, whereas the off-diagonal term gives a positive contribution. The experimental value for the magnetic moment of the y- state in ““Pb is reproduced by B’= 0.07 (3) and B’=O.52 (3). The higher value of B‘ corresponds to a stronger mixing of the (i,~;,2),zp7j\ neutron-hole configuration. According to the discussion above the higher value of B is expected in the wave function of the ‘2’ state in ‘“‘Pb. Thus the measured value of the g-factor for this state can also be described by the wave function 0.69/(i ,~~;1),~Ip,,‘I)+0.72](i;i;_1),11p3:’2). An analysis as above gives a value of B’= 0.06 for the y-- state in ‘“Pb or a much smaller

amplitude

of the (i;fj2),lap;,i:

component in the wave function. Stenzel et al. ‘1 pointed out that the p,:? orbital in high-spin configurations may come down in energy if there is a coupling to a small prolate deformation. In this picture a small component of the collective g-factor which goes as g, = Z/A can bring the g-factor of a v(i,t,,),,+p,.‘z or i,‘j2@7m configuration with 7 ~i,&p,,\ close to the experimental value for the y state in ‘“‘Pb (or 1”7Pb). We have here presented different configurations for the yrast ‘T- state in the light lead isotopes, but since the fiiz orbital is lowest in energy of the possible orbit&s

596

u. Roseng&l

e1 cd. / Yrusi speciroscopy

it should be the most dominant in the configurations determining the g-factor. We thus conclude that the main configuration of the $- state in ‘“‘Pb and ‘“‘Pb is Y(i$7),,~+f;j’z. The amplitudes of the other configurations discussed above cannot be determined from the present experimental data. The measured g-factor of g = -0.145 (9) for the isomeric y’ state in ““Pb is in agreement with the values obtained for the similar state in 1y5,‘97Pb ‘I). The g-factor is consistent with the v(i ;“$2)33,2+configuration for this state. The state is not isomeric in 20’.203Pb but it has been identified through level systematics. The g-factor of the 43(3) ns isomeric state in ““Pb was found to be g = -0.18 (4). Following the discussion in subsect. 4.1, this isomeric state may be identified as a v(i$,)33,2+@4t excitation. The g-factor of the 4’ state in z04Pb is known I’) to be 0.056 (1) and when g = -0.1510 (10) is assumed for the “$’ state, the additivity rule gives a value -0.11 (2) for the y’ state. This is smaller than the measured value indicating that the above description of the 5i’ state is not quite appropriate. It is, however, possible to understand the measured g-factor if the two-neutron-hole V(fiizpA;‘J configuration is assumed to dominate the 4’ excitation in the y’ state. In this case a g-factor g,,,, = -0.15 (2) is obtained. It may be pointed out that the v(f$) configuration dominates the 16+ state in ““Pb [ref. ‘“)I. The complex nature of this state is further stressed by the B(E2) value (see the next section).

4.2.

B(E2)

VALUES

FOR

THE

OBSERVED

TRANSITIONS

IN

ivy.zi”.zr”Ph

The experimental B(E2) values for the z: ~+y- and the -T -+2- transitions in some of the Pb isotopes are listed in table 6. It is interesting to note that the B(F2, $- + 2; ) value is drastically reduced going from ““Pb to IYYPb. If a threeneutron-hoIe con~guration (i,if,),z+fs,& is assumed for the f- state and the configuration (i,32/J),0+f$‘2 for the ‘:‘- state, then the ‘2~-+‘$- transition is essentially a two transition for which the experimental B(E2) neutron hole (ii$,2),2++ (iTi,2r2/2),,j+ value ?,?4) in 200.102 Pb is determined to 52 (2) e2 fm4. This value is much larger than the experimentai B(E2) values for the %--+$transitions both in lY”Pb and ““Pb. This means that the experimental B( E2, B- + %) value has to be explained by some _ other mechanism. However according to the g-factor experiment (cf. subsect. 4.2) the (il.3Z/l),0+fliI configuration dominates the $- state in ““Pb. If the (i;,‘;),z~pLjz neutron-hole configuration would dominate the wave function of the 3 state, then the y-+5transition could be considered as a single-neutron-hole fyi:+ pri2 transition. The B(E2) value of this transition is 71 (1) e’ fm” in ‘07Pb [ref. ‘“)I. “tP*‘O’Pb, due to half-filled shells, it is much Although this value will be reduced in larger than the B(E2) value for the $- + y- transition. This indicates that the fi/12+ p ;I- single-neutron-hole transition does not play a dominant role in the ‘:- + ‘se transition. If we assume, as discussed in subsects. 4.1 and 4.2, an il$2@9- configuration for the BP state and an i rX,z@7 -’ configuration for the T state then the ‘; + Ttransition can be looked upon as a transition between the 9- and 7- states. In this

U. Rmengbrd

et al. / Yrast

TABLE

Some known

B(E2) values,

597

spectroscopy

6

which are relevant for discussion of the B(E2) transitions io 19”.Z01.107Pb Transition energy E CkeV) 569.7 69.6 97.1 27.7 374.7 A 1‘) AY A “1 46.3 168.1 422.2 AhI 1 Y 222.2 AhI 278.1 1 “1 .I “I 245.1 462.2 90”) .I “) 372.4 Ah) 90.1 A 317.7 X8.6 49 13X (3) X6.X 227.1 47 166 (2,

Observed half-life T,,? (ns)

Partial half-life “) T;,z (~1

0.133 12) 10 7 0.130 (2) 14,s (1.2) 200(14) 1.45O(lSO) 71(3) 780 (20) 217 (5) 0.2X1(11) 265 (10) 122 (4) 107(5)‘) hI 110(S) h, 52 (3) 24.2 (3) 0.11X(l) 65.4 (2) 2.06 (2). IO-’ 1.97 (2) 43 (3) h, ,1 508 (5) ) 0.082 (4) 63 (3) h 67 (5, 1 5.5 (3). loml 5(l) I> 202 (5) , 424(10) 6.3 (2) 0.056 ( 1) 46 ( 1) 0.33 (2) 0.34 (2) 10 1 55 (5) 0.648 (59) 10.0 (2) 10’ “1 7.5 (3) x.2 (3) lo-’ 11 212 (4) ) 240 (50) 2.7 (6) 4.2 10’ 0.22 ( 1) 141 1:‘) 0.015 (10) 0.655 (59) 55 (5) 49 (2) 269 (IO) Sl (5, 0.138 (13) 9s (20, 1.2 (2) 2.3 (7) 6 (2) 10-j 392 (IO) 8X (2) IS (5) 0.02X (9)

values for the observed

E2

Experimental B(E2) (e’ fm”) 71(l) 23 (2) 45 (5) 4.5 (I) 0.27 ( 1) 89 (7) X5(12) 99 (9) 51(3) 36 (0.2) 20.4 12) 253 (26) 21(2) 13 (0.6) 162(17) 61 (4) 54 (4) 22 (0.5) 11 (0.2) 78 (5) 153 (15) 1.1 (I) 9.6 (4) Sl(4) 36(7) 2.6 (2) I1 (1) 159(14) 41 (2) XI (X) 94 (20, 156 (47) 2X(l) 161 1.54)

“) The partial half-life Tit2= T, :(1 + p I( 1 + a, ) is calculated using the observed branching ratio /3 and the estimated “) total conversion coefficient u, “) The transition energy is assumed to he in the interval R, -; E,c B, where (1 +n, )El= 5.2 (4). IO-‘(MeV)‘. ‘1 The partial half-life 127 (12) ns is obtained when the branching ratio is taken into account. ‘) The transition energy is estimated from the known energies in ““~““Pb and ““Pb. ‘) It is unclear whether the 9 + 7- transition exists ‘A”‘,” 1. Here tie have calculated the H(E3) value for the proposed ‘,J’) 9 + 8- and 8 + 7 transitions, which cannot proceed as M 1 transitions due to the observed half-liles.

picture the experimental

B(E2) value will be similar to that for the 9. -+7

since the actual configuration mixing is taken into account. The B( E2) value for the 9- + 7- transition in -““‘Pb is indeed value

for the $- -+$

dominantly

transition

indicating

via the 9 _ -$ 7 _ transition

that the T-

transition

equal to the B(E2)

state in ““Pb

decays

pre-

and that the $~ and ‘g- states can be described

as a coupling of an i’$? neutron hole to the 9- and 7- core states in ““‘Pb, respectively. However, the B( E2) value for the y- + .$- transition in “‘Pb is considerably reduced whereas it increases with decreasing neutron number for the 9-j 7 transition. This could indicate a structural change either in the y- or in the ?$- state in the lighter Pb isotopes. The B( E2, $- + $Lm’)e,.pvalue in 1”5Pb is in good agreement with the calculated value if it is a two neutron hole (i,:3Tf7),r)++(i&),‘,+ transition, i.e. the Jim state in ““Pb has a dominant (i ;&)“,~pL~z configuration “). The B(E2) value of the ‘5-- ‘,‘transition in ‘ozPb is bigger than that of the similar transition in ““Pb if the transition energy is expected to be < 80 keV. Since a similar or smaller B( E2) value is expected in ““Pb the transition energy is most probably in the energy range 100-150 keV. A weak transition in this energy region may escape observation in the energy spectrum. The large reduction in the B(E2,T + 7 -)eXp value in ““Pb and ““Pb shows that the amplitude of the (i~~:z)‘o~p;~z configuration is reduced and that the (i;$z),z*pjiz configuration probably dominates the ‘$ state. These observations indicate that the 1-5-+ :- transition in the light Pb isotopes does not proceed via the 12.’ + IO’ 1 transition. Table 6 shows that the B(E2) value for the 7- + 5- transition follows the decreasing trend of the ‘2. + ‘+ transition with decreasing neutron number. The B( E2, z: ~-, T-1 values in ““‘Pb and ““Pb are also quite close to the B(E2,77+5-) values in 202Pb and ‘Or’Pb, respectively, It, therefore, seems that the 7- + 5 _ transition In this respect, the 14 ns half-life dominates the z-5_ 2 + zi transition in 705.1’J[1”20’~“)“Pb. observed by Stenzel et al. ‘) for the 7. state in ““Pb gives a R( E2) value, 11 (1) e2 fm’, in agreement with the systematic trend. It is on the other hand suggested ‘,“) that the 4 ps half-life observed in lUXPb may belong to the 7- state. The occurrence of the 4 t..~shalf-Iife breaks the systematic trend of the B(E2) values in the energy region in ‘“‘Pb where the 9- and 7 states occur. In the present paper we stress that the 4.2 +s half-life “) belongs to an &- state situated between the 9 and 7- states. This conclusion has also been drawn earlier 1.-15.47). The weak coupling calculations of King Yen et al. ‘7) show that the V(iT&zf:i/12)7 , configurations are the main configurations in the wave (i ‘$2p$‘2)7- and v(i ;&:,,p.qj’J, functions of the 7 and 5- states and do not indicate any abrupt configuration by Stenzel ef al. ‘> show that changes near IYxPb. On the other hand the calculations the i l’~,zp~;!zconfiguration may come down in energy if the state has a small prolate deformation. If this is the case such a mixed configuration has to be taken into account in the wave function of the 7 _ state. The measured value of the g-factor “1 for the ‘,‘-- state in “j5Pb indicates a .pt predominantly VI ,3j2p ,I2 configuration. The pli2 orbital is replaced by the f5/? and

U. Rosengird

P~,~ orbitals

in ““Pb

as indicated

et al. / Yrast spectrosrop_v

in the present

g-factor

599

measurement

and also by

the calculations of King Yen et al. j’). On the other hand, the ui ,:$,zp7/: configuration transitions involve the dominates the F- state. Thus, the 7---z 5- or the ‘,‘--F’ transitions (i;h3/1& + (iY2X5-, (iL:/J”lJ,+ (i&p3/‘& + (iIl&p&, (iyi,zp7/12)5m and (iy:/zf,‘z),~ + (i;$:/2f;/12)5m in the light Pb isotopes. Krien et al. “) have calculated B(E2) values for the 7- --, 5 -transitions in ““‘Pb for pure configurations. Their calculations indicate that the smallest B(E2) values are obtained for the (i,i,2p;jz), + (i&fC/:)5m and (i,~,Zf<~z)7~+ (i;~,lp;~2)c~ transitions. These calculated B( E2) values are close to the experimental B( E2, s- + q-) value in lyqPb and the experimental B(E2, 7- + 5) value in ““Pb. Since the p3,’ neutron orbital dominates the ‘,‘- state, the calculations of Krien et al. 4y) indicate that the configuration involving the f5/‘? neutron hole plays a dominant role in the 7- state in ‘““Pb and the ‘,‘- state in ““Pb. The B( E2) value of the y” + 6’ transition is by far the largest in the list presented in table 6. According to our original argument the transition will proceed as a 4’ + 2’ transition. Indeed the transition rate for this transition increases from 20.4 e’ fmJ in ““Pb to 78 e’ fm4 in ““‘Pb but it will not reach 253 e2 fm” as for the $++ ‘2’ transition. The transition rate for the ?:’ + ?;’ transition is increased from 45 e’ fm4 in ““Pb to 153 e2 fm4 in ‘““Pb. These values show that the +l’- ‘1” transition is not a pure one and that the collectivity is increasing in the high-spin states when going to more neutron-deficient nuclei. The authors would like to thank Prof. I. Bergstrom, Prof. J. Blomqvist and Dr. R.J. Liotta for useful discussions and suggestions. One of us (H.C.J.) would also like to thank Prof. S.K. Bhattacherjee for useful discussions and three of us, (H.C.J., B.F. and T.W.) Prof. I. Bergstrom for the kind hospitality during our stay at the Research Institute of Physics in Stockholm. References I) Ch. Stenzel, 2) C. Roulet, and

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