The ground state spin of 180Ta is not 8+

(1)

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Submitted on 1 Jan 1979

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The ground state spin of 180Ta is not 8+

E. Warde, R. Seltz, G. Costa, D. Magnac, C. Gerardin

To cite this version:

(2)

The

_ground

state

_spin

of

180Ta

is

not

8+

E. Warde

_(*),

R.

_Seltz,

G.

_Costa,

D.

_Magnac

and C. Gerardin C.R.N. and U.L.P., Basses Energies, 67037 Strasbourg, France

(Reçu le 30 octobre _{1978, accepte}le 13 novembre ₁₉₇₈₎

Résumé. 2014 Des membres des bandes rotationnelles

(1+, 1)

et

_(8+,

₈₎ont été identifiés dans la _régionde faible excitation de 180Ta _parune _comparaisondes sections efficaces de transfert _{(p, d)}avec des calculs en DWBA, utilisant des fonctions d’onde de Nilsson.

Abstract. 2014

Experimental (p, d) cross-sections are _comparedwith calculations _usingNilsson wave functions and the DWBA in order to locate the members of the low

_lying

₍₁

_+,

₁₎and

(8+, 8)

rotational bands in 180Ta.

Tome 40 No 1 ler JANVIER 1979

LE JOURNAL DE

_PHYSIQUE

_LETTRES

Classification Physics Abstracts 21.10H201321.60

Until

recently

[1] only

two levels have been observed in

_{18 ova,}

both isomers. The

_long-lived

isomer,

consi-dered as the

ground

state, has

_T1/2

> 1013 _y

_[2],

_[3].

The other level with

_T1/2

= 8.15 h. has been

previously

located at 212 keV

_{[4], [5]}

and

_recently

at 32 keV

_[6].

The

_{log f t}

values for the

_decay

of the latter level to the 0+ and 2+ first excited states of 180W and

_18°Hf,

are consistent with allowed or

possibly

first forbidden

transitions. Asaro et al.

_{[7] suggested}

values of _I1t,

K =

_1-,

0 and

I1t, K

=

8 +,

8 _or

9-,

9

respectively

for the short-lived

_(SLI)

and

_long-lived

_(LLI)

isomers.

Experimental

observations

_by

_{Gallagher et}

al.

_[8]

on the SLI

decay

exclude the

(1-, 0) assignment

which

arises from the

_coupling

of

_{9/2 - [514}

_T]p

with

9/2+

[624 i]o

with

_{antiparallel spins}

_(r

=

0).

The same

experiment

is consistent with

₍₁

_{+,1) explained}

as due

to the

_coupling

of

_7/2+

_[404

_JJp

with the

_9/2+

_{[624 TI.}

with

_parallel

intrinsic

_spins

_(2~

=

1).

The _two

confi-gurations

mentioned above

_give

rise also to states with

_{(9 -, 9) 2:}

= I and

(8 +,

8) 2:

= 0. Both of these

spins

have been

_proposed

for the LLI and the expe-rimental

_study

has not solved this

_ambiguity.

The intrinsic _protonstate

_{9/2 - [514}

_i]p

of the

_{(9 -, 9)}

configuration

in 180Ta

appears in i 81 Ta at 6 keV above the

_7/2+

_[404

_~]p

_ground

state which

_corresponds

to the

_{(8 +, 8) configuration.}

Due to this _verysmall energy difference the

(9 -, 9)

as well as the

(8 +, 8)

states could be the LLI. However in all other known

odd-odd tantalum nuclei the

_ground

state

_proton

configuration

is due to the

_coupling

with

_parallel

intrinsic

_spins

of the

_7/2+

_[404

_JJp

with a neutron state.

If one

_adopts

(1

+,1)

for the

SLI,

the identification

of the

_{(8 +, 8)}

member of this

_{configuration}

with a

lower

_lying

state would violate the

Gallagher-Mosz-kowski

_empirical

rule

_{(G-M rule) [9].}

This rule states

that the member of the doublet

_{corresponding}

to

parallel

intrinsic

_spins

of the

_unpaired

nucleons has

the lowest excitation. As far as is

known,

such a

violation has been observed once in

166Ho

where

the

_ground

state is

_{(0-, 0)}

_(Zp

+

_2~

= Z =

0)

but

only

5 keV below the

_{(7-, 7)}

member

_(2:

=

1).

It must

be

_pointed

out that in this case the

long-lived

isomer

(T1,2

> 1.2 x 103

_y)

is not the

_ground

state which has

_T1/2

= 27 h.

[10].

The doublet ~ = 1 + and

K>

= 8 + from the

configuration

{ 7 j2+

[404

JJp,

9/2+

[624

i]o }

has been observed in 116 Lu and the

_{(8 +, 8)}

member lies 206 keV above the

_{( 1 +,1 ) again}

in _agreementwith the G-M

rule

_[1].

The aim of this letter is to _propose,with the

_help

of new

_experimental

_data,

an alternative to the low

lying

level scheme

of 1 s °Ta

and to _tryto

_explain

some

of the contradictions outlined above.

The 180Ta nucleus has been

_investigated

_by

the

181Ta(p,

d)18°Ta

reaction at

_Ep

= 19 MeV

using

a

_{magnetic spectrograph.}

The

experimental procedure

is described elsewhere

_[1].

Assuming

a direct transfer

mechanism,

the observed

excited states in the low _energy

_region

result from the

coupling

of the transferred neutron with the

_unpaired

proton of the _target

_ground

state e.g.

7/2+

[404

JJp.

The lowest neutron state is

_9/2+

_{[624 ~, ground}

state

configuration

of 177 Yb, 179 Hf and 18 ’W,

all isotones

of 180Ta. It is this

_{configuration}

which

_produces

the

Gallagher-Moszkowski

i doublet with K~ = I + and

(3)

L-2 JOURNAL DE _{PHYSIQUE -}LETTRES

K; = 8 +.

Among

other

possible

_{neutron states}

below 900 keV excitation the

_9/2+

_{[624 T]}

is the

_only

one with even

parity [12].

The

_expected

transfers to

the members of the two bands built with this neutron state are

_{/ == 4, 7}

=

9/2

(Cfl

=

0.126)

and I = _6,

j

=

13/2

(Cfl

=

0.98).

We exclude the 1 =

6, j

=

11/2

transfer because the

_{corresponding}

_C~

is

_negligible.

For the two first members of the 1 +

band,

the

_only

allowed transfer is I = 4 with _a

relatively

small

intensity.

The 8 +

band,

in contrast is excited

_by

1 = 4

and

_strongly

so

_by

I = 6.

Referring

_tothe energy of the

observed lowest

level,

taken to zero, we see that the

angular

distributions of the levels at 0 and 41 keV

are well fitted

_by

a _{pure 1}= 4 transfer

(Fig.

la).

Fig. 1. - Angular distributions of states in 180Ta excited by the

18’Ta(p, d)18°Ta reaction at _Ep= 19 MeV. Solid lines are

theore-tical calculations using Nilsson wave functions and DWBA :

a) Members of the (1+,1) band; b) Members of the (8+, 8) band.

The two first levels are observed with a

large

experi-mental error, but the theoretical absolute

cross-sections _represent60

_%

of the observed cross-sections.

In contrast the

_(8+,

₈₎

absolute theoretical

cross-section is more than two times too

_large

to fit either

of these levels. Furthermore the

_(3+,1)

member of the 1 + band is

_easily

identified with the level at 106 keV

by

taking

an inertia _parameter

112/2

J = 10.5 keV

[1].

Both the

_angular

distribution and absolute

cross-sections are fitted

_(Table

_{I ;}

_Fig.

_la).

In a

_previous

_paper

_{[ 1 ],}

we have shown that the

(4+,1)

member of this band is a

_part

of the

experi-mental

_multiplet

at 173 keV

_including

the

_{(8 +, 8)}

state. An additional

_proof

of the location of this member of the G-M doublet is

_{given by}

the state observed at 366 keV which is identified as the

(9+,

8).

Its

_angular

distribution is 1 = 6 _as

expected

(see

Fig.

lb).

This is the

_only

1 = 6

angular

distribution

in this _energy

_region

and a

_good

_agreementbetween

theoretical and

_experimental

cross-sections is found

(Table I).

This identification leads to the location of

the

_{(8 +, 8)}

band head at 177 keV in

_good

_agreement

with a

essentially

on

cross-section considerations

_[1].

The

_{(5 +,1 )}

state is

_expected

at 294

keV,

with an

I = 6

angular

distribution.

Experimentally

_one

obser-ves a level at 303 keV

_showing

an

angular

distribution

dominated

_by

1 = 6

(see Fig. lb).

The

major

_partof

the measured cross-section comes from the

(5 +,1 )

state.

Hence the members of the

_{(1 +,1)}

band have been identified with a

good degree

of confidence up to

(5 +,1 ).

The

_{(8 +, 8)}

member of the G-M doublet is shown to be above the

_{(1 +,1)}

level. In addition to arguments

quoted

in the introduction it has been

shown that if the lowest level observed in our

expe-riment was

assigned (8 +, 8)

it would be

impossible

to

explain

the

_energy

and transfer cross-sections of other

low

_lying

observed levels.

_Finally,

the

_proposed

level scheme leads to a G-M term of the

_Vnp

residual interaction of A = - 104 keV in

very

good

agreement Table I. -

Experimental

and theoretical cross-sections

_for

members

_of

the

₍₁

_+,

₁₎

and

_(8+,

₈₎

bands in 18 0 Ta

excited in

the

_{18 OTa(p, d) 181Ta}

reaction.

(a) Unresolved in _experiment_(seeRef. _[1]).

(b) Given by jE~, K) = E(KTt, K) +

_{~/2 ~}

_[1(1+ 1) -

K2]

with ~/2 ~- = 10.5 keV.

(4)

L-3 THE GROUND STATE SPIN OF 180Ta IS NOT 8+

with calculations of Boisson et al. for this

configu-ration [13].

There are _manyuncertainties in

_previous

works on 180Ta. It was first admitted that the SLI is located above the LLI. But from more recent data

_[14]

and from neutron

_binding

energy

systematics [15]

it cannot be excluded that the lower excited state is the SLI. The facts which _appearto be well established are that the SLI is a

(1 +,1)

state and the LLI a state with K >

8,

although experimental

results on the

decay

of the

_long-lived

isomer are scarce

_{[2], [3].}

From the

present work it is established that the

_{(1 +,1)}

state with

_{configuration {7 /2 +}

_[404

_!]p,

_9/2+

_[624

_j]o}

is

excited at lower _{energy than the}

_{(8 +, 8)}

which

_belongs

to the same G-M doublet.

If the LLI is the

_{(8 +, 8)}

state then it must be an

excited state at least at 180 keV.

The other state of

_{high spin,}

the

_{(9-, 9)}

of

confi-guration {

9/2- [514

_j]p,

9/2+

[624

j]o }

which cannot be excited in this neutron transfer

_reaction,

could be the

_long-lived

isomer and be above or below the

( 1 +,1 ).

Furthermore we cannot

_completely

exclude

another

_{configuration}

for the

_{( 1 +, 1 )}

that is

~ 9/2-

[514

j]p,

7/2- [514

1]. 1

also unobservable in a

pick-up

from 181Ta. The

_~-

and E.C.

_decay

_properties

measured

_{experimentally}

_[8]

are

_compatible

with

such a

_{configuration}

but the

corresponding

1 + state

is

_expected

above the

_{(1 +,1)}

band head identified

here.

From these results one concludes that if the

long-lived isomer is the lower excited state and if the

short-lived isomer is the

_{(1 +,1)}

identified

here,

then the

(9-, 9)

state is the

only

reasonable candidate for the

long-lived

isomer.

_{Finally, concerning}

the

_ground

state

_spin

of 180Ta

_only

two

_assignments

are

_possible,

either

_{(1 +,1)}

_{from { 7/2 +}

_[404

_~]p,

9/2+

[624

j]o }

or

(9-, 9)

from { 9/2-

[514

T]p,

9/2+

[624

T]n ~.

In _any

case the

spin

(8 +, 8)

is excluded. The excitation of 18°Ta via a

_proton

transfer

reaction, (3He,

d)

or

_{(oc, t)}

would

_help

to choose between 1 + or 9-.

Acknowledgments.

- We thank Dr. R.

Piepenbring

for his

_stimulating

interest in this work and critical

reading

of the

_manuscript.

References

[1] WARDE, E., SELTZ, R., COSTA, G., MAGNAC, D. and GERARDIN, C., J. Physique Lett. 39 (1978) L-161 and to be _published.

[2] SAKAMOTO, K., Nucl. _Phys.A 103 ₍₁₉₆₇₎A134.

[3] ARDISSON, G., Radiochem. Radioanal. Lett. 29 ₍₁₉₇₇₎7.

[4] BROWN, H. N., BENDEL, W. L., SHORE, F. J. and BECKER, R. A., Phys. Rev. 84 (1951) 292.

[5] GELLER, K. N., HALPERN, J. and MUIRHEAD, E. _{G., Phys.} Rev. 118 (1960) 1302.

[6] WAPSTRA, A. H. and GOVE, N. B., quoted in Nucl. Data Sheets

15 (1975) 577.

[7] ASARO, F., PERLMAN, I., RASMUSSEN, J. O. and THOMPSON, S. G., Phys. Rev. 120 ₍₁₉₆₀₎934.

[8] GALLAGHER, C. J. Jr., JORGENSEN, M. and SKILBREID, O.,

Nucl. Phys. 33 (1962) 285.

[9] GALLAGHER, C. J. and MOSZKOWSKI, S. A., Phys. Rev. 111

(1958) 1282.

[10] Nucl. Data Sheets 14 ₍₁₉₇₅₎514.

[11] BALODIS, M. K., TAMBERGS, J. J., ALKNIS, K. J., PROKOFJEV,

P. _T.,_VONACH,W. _{G., VONACH,}H. K., KOCH, H. R., GRUBER, U., MAIER, B. P. K. and SCHULT, O. W. B.,

Nucl. Phys. A 194 ₍₁₉₇₂₎305.

[12] BUNKER, M. E. and REICH, C. W., Rev. Mod. _Phys.43 ₍₁₉₇₁₎

348.

[13] BOISSON, J. P., PIEPENBRING, R. and OGLE, W., Phys. Rep. 26

(1976)99.

[14] LANIER, R. G., LARSEN, J. T., WHITE, D. H. and GREGORY,

M. _C.,Bull. Am. Phys. Soc. 17 (1972) 899.