Hyperfine Interactions 34 (1987) 77-80 77

HALF-LIVES AND g-FACTORS OF SOME MICROSECOND ISOMERS IN THE LEAD REGION P. CARLE, S. EGNELL, L.O. NORLIN, K.-G. RENSFELT and U. ROSENGARD

Research Institute of Physics, S-I0405 Stockholm, Sweden

B. FANT

University of Helsinki, Department of Physics, SF-O0170 Helsinki, Finland

H.C. JAIN

Tats Institute of Fundamental Research, Bombay 400 005, India

K. JOHANSSON

Institute of Physics, University of Uppsala, S-75121 Uppsala, Sweden

The g-factors and half-lives of the (8-) isomer in 19spb, the 29 /2 - iso- mer in 199pb and the 19 + isomer in 21~ have been remeasured by the time differential perturbed angular distribution technique. A new beam pulsing equipment was used in these experiments. The results are for lgSpb; g = -0.0471(8) and T1/2 = 4.19(10)/~s, for 199pb; g = -0.0742(9) and T1/2 : 9.8(4)/~s and for 21~ g = 0.698(7) and T1/2 = 5.9(3)/~s.

Experimental g-factors in the lead region reveal details of the shell model structure and can be done very accurately by the TDPAD-technique if the life-time of the studied state is in the order of microseconds (or longer).

The fast pulsing system of the 225 cm cyclotron allows a beam pulse separation of maximum 1.8 #s /1/ . In order to extend our studies to isomers with microsecond half-lives a square wave pulsing system was constructed. It allows the selection of one or more beam pulses with a repetition time of 10 /zs or longer. A thyratron is used to charge and a pentode to discharge the deflector plates (the same as used for the fast pulsing system). In this way sufficiently fast rise and fall times of the deflector voltage (60 us) were achieved as well as short pulse length (70 ns), but also there is no principial upper limit for the pulse length or repetition time. For long beam off times the plates are recharged at intervals of about 100 #s. In the present experiments one in five hundred beam pulses was retained giving a time interval of approximately 70/zs between two consecutive pulses.

The nuclear states in 19spb and 199pb were populated in a,4n and a,3n reactions at an alpha energy of 53 MeV on a liquid 19SHg-target of thickness ~ 200 mg/cm 2. The 19 + state in 21~ was populated in the reaction2~ 3n)21OAt with 51 MeV a-particles. The 2~ target was kept molten between two thin (4 mg /cm 2) mica windows. The overall target thickness allowed the beam to pass the target and the dominating channels were (ex,3n) and (a,4n). The results are summarized in table 1.

For 19spb our experimental results are in very good agreement with results of Stenzel et al. /2 / . As can be seen in figure 1 there is some controversy about the level structure around the 7 - and 9 - states at about 2 MeV. In the following we as-

1 1 sume the state under study to have a main quasi-particle configuration of t,i~3/2 | vf~/2. If the effective single particle g-factors are taken as g(vi~-al/2) = -0.1496(18) /6 /

and g ( u f ~ ) = 0.312(10) /7 / the calculated g-factors become g(7-) = -0.085(4),

g (8- ) : -0.048(4) and g(9-) = -0.023(3). In such a simple picture our result would favour spin 8 - for the 4.2/zs isomer and the level scheme would look as suggested by Honkanen et al. /4 / .

�9 J.C. Baltzer A.G., Scientific Publishing Company

78 P. Carld et aL, Half-lives and g-factors of isomers

5 -

4*

[2] 12_* . . . . . . . . . 211 ns 10"

)3.7ps 14ns

49 ns

2*

O*

{3] 212 .4ns

. . . . . . . _-2_8_2__0__

2772.1 540

540.65 I

| 240• 9 - ~223145 9 - 7- 90112141,3%.7. ' 7 _--9~176

31795

1976

5 6 2 3 5

1063.4E

317.9

1823.4 49•

1625,8

1 0 6 3 4 5

. . . . . . . . . . _2_40ns 2772.3+c

2231.4~ ~

. . . . 3.7ps 2141.4

1823,5 49ns

197"6 t 1625.9

5624|76o.0 I

�9 I E3 1106&5

1063.5

198pb

19" 4us 402Z7

372.4 485.3 16" E3 M2 17" 3542.4

1105.7 435.2 M1 M1

16" 3107 .2

557.6 M1

15" 0,581u s 2 5 4 ~ 6

i 644.4 E3

13" 2042.9 ~ ~ 1 9 0 5 . 2

542.0 M~

27ns 1363.2 [ m . 3 E 2 1251.9

675.5 E2

0

2;0At,2 ~

Figure 1. Level schemes of lgspb as suggested by different authors /2, 3, 4/. To the right is shown the yrast level scheme of ~ l~

I@ 4 i@ 3 ~ ' I ' ' ' ' I ' = ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I . . . . I . . . . l'_ =

- ~ 31 8KEV 1 3BPB

I I

I

" " II I i'll li Pilll I * , * * I , , , , I , , , i I , , , ~ I ~ , , i I I ,hi, ll |h l lL l l

5@ 1 H 1 5 0 2@@

CHRNNEL NUMBER

I@ s

1 @2

I@ 1

1@ a

I@ 2

I@ I

I

- l l l i l l l l l l l i l l i l i l i i l i l l l l l l l l i l l i l l l l l :

~ 21eRT 19+ , ~ i L = ~ ~ 372 KEM

UJ I pS

l l l l J i l l i l l l l l I l t T i l e f I t l i l i , i l

5 0 I n 1 5 0

CHRNNEL NUMBER 2@D

Figure 2. Left part shows time spectrum obtained with a gate on the 318 keV -/-l ine in l~ T o the right is shown a time spectrum obtained by gating on the 372 keV "/-line in 2Z0At"

R Carl~ et aL, Half-lives and g-factors of isomers 79

Table 1. Results of half-life and g-factor determinations for microsecond isomers in igsPb, lggPb, 2~ and 2mAt.

Nucleus Spin and TI/2 (/~s) guneorr g~orr Parity

Present Previous Present Previous (Present)

198pb (8-) 4.19 (10)' 3.7 (4) /2 / -0.0479(5) -0.048 (1) /2 / -0.0471 (8) tggpb 29/2- 9.8 (4) 10.6 (5) /2/ -0.0750 (4) -0.075 (4) /2/ -0.0742 (9) 2~ 29/2+ 0.946(8) 0.860 (Z0) /8/ 1.0514 (17) 1.0215 (22) /g/ 1.061 (10) 21~ 15- 0.476(6) 0.580 (50) /10/ 1.023 (4) 1.0259 (25) /10/ 1.032 (10) ~~ 19 + 5.9(3) 5.g0C18)/11/ 0.6919(7) 0.728 (23)/10/ 0.698 (7)

* The corrections for diamagnetism and Knight shift were taken as i n / 2 / f o r Pb in Hg and as i n / 1 2 / f o r At in Bi.

Our results for the 29/2- isomer in 19gPb are also in good agreement with Stenzel et al. /2/ . Assuming a main configuration of vi~2/2 | vf~/12 and the same effective g-factors as for the (8-) state in lgsPb one gets g(29/2-)ealr -- -0.0700(23). The calculated value is in relatively good agreement with the measured g-factor. However a recent measurement of the g-factor of the corresponding state in 2~ /13/ gave g(29/2-) -- -0.0693(5). The two experimental results barely overlap within three standard deviations, although they axe both in agreement with the calculated value.

The results for 2~ and 21~ were achieved by fitting standard TDPAD-func- tions to the time distributions of the gamma lines 372, 424, 435, 529, 542, 644 and 725 keV.-All are showing time curves in agreement with published level schemes/8, I0/. The deduced g-factors and half-lives agree well with previous values (See table 1).

In trans lead nuclei isomeric states with very high spins are formed by the va- lence nucleons coupled to core excitations. In 2~ (31/2-), 21~ (16 +) and 2HAt (39/2-) the main configurations are supposed to be 7r(....ilSl2)| ). However the electromagnetic properties suggest that configurations of type rr(....fT/2) @ v(ji6/2....) are mixed in to considerable amounts; 2~ (46%) /14/, 21~ and 211At (40%) /15/. The 19 + state in 21~ is supposed to have a main configuration of r(h2/21~s/2) @ u(gg/2(j-2)o+) with gca~, ~ 0.74 in agreement with the old experimental value 0.737(25) /10/. However 7r(h~/2f~/2 ) | t,,(jls/2(j-2)o+) should be mixed in with about 40% also in this case giving gr162 = 0.68. The accuracy of the previous measure- m e n t / 1 0 / d i d not allow any conclusion on this point. The presently obtained value of g(19 +) in 2t~ can be explained if the above discussed mixing is 27~.

In a more detailed calculation, Dracoulis et al. /11/ explicitly included the 3- octupole vibration in the description of core excited states in ~l~ The calculated value g(19 +) = 0.71 is in good agreement with our experimental result.

80 P. Carld et al., Half-lives and g-factors o f isomers

References

/1/ K. AbrahamRson, A. FUevich, K.-G. Rensfelt and J. Sztarkier, Nucl. Instr. and Meth. 111 (1973) 125.

/2/ C. Stenzel, H. Grawe, H. Haas, H.-E. Ma~nke and K.H. Maier, Z. Phys. A322 (1985) 83.

/3/ M. Pantrat, J.M. Lagrange, J.S. Dionisio, Ch. Vieu and J. Vanhorenbeck, Nucl. Phys. A443 (1985) 172.

/4/ K. Honkanen, C.J. Herrlander, B. Fant and T. LSnnroth, Nucl. Phys. A451 (1986) 141.

/5/ V. Rahkonen, Thesis,dept. of physics, Univ. of Jyv~skyl~, Research report No 10 (1980) p. 78.

/6/ C. Stenzel, H. Grawe, H. Haas, H.-E. Ma~hnke and K.H. Maier, Nucl. Phys. A411 (1983) 248.

/7/ Weighted average of 3 measurements of g(5/2) in 2~ See Table of Isotopes, ed. C.M. Lederer and V.S. Shirley, (John Wiley & Sons, New York, 1976), Appen- dices p. 62.

/8 / V. Rahkonen and T. LSnnroth, Z. Physik A322 (1985) 333

/9 / K.-G. Rensfelt, Thesis, Research Institute of Physics, Stockholm (1976) (unpub- lished).

/10/ V. Rahkonen, I. BergstrSm, J. Blomqvist, O. Knuuttila, K.-G. Rensfelt, J. Sztarkier and K. Westerberg, Z. Physik A284 (1978) 357.

/11/ G.D. Dracoulis, C.A. Steed, A.P. Byrne, S.J. Poletti, A.E. Stuchbery and R.A. Bark, in Proc. Int. Nucl. Phys. Conf., Harrogate U.K., 1986, Vol 1. p. B21.

/12/ L.O. Norlin, I. BergstrSm, P. Carld, A. K~Ilberg, K.-G. Rensfelt, U. Rosengs K. Johansson and B. Fant, Z. Physik A322 (1985) 463.

/13/ H.C. Jain ,P. Carld, A. K~llberg, L.O. Norlin K.-G. Rensfelt and U.Rosengs to be published.

/14/ K.-G. Rensfelt, C. Roulet and K. Westerberg, Physica Scripta 14 (1976) 95.

/15/ I. BergstrSm, J. Blomqvist, P. Carld, B. Fant, A. K~llberg, L.O. Norlin K.-G. Rens- felt and U.Rosengs Physica Scripta 31 (1985) 333.