Some features of odd Au nuclei revealed by half-life measurements

HAL Id: jpa-00208293

https://hal.archives-ouvertes.fr/jpa-00208293

Submitted on 1 Jan 1975

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Some features of odd Au nuclei revealed by half-life

measurements

V. Berg, C. Bourgeois, R. Foucher

To cite this version:

SOME FEATURES OF ODD

Au

NUCLEI REVEALED

BY HALF-LIFE

MEASUREMENTS

V.

_BERG,

C. BOURGEOIS

_(*)

and R. FOUCHER Institut de

_Physique

_Nucléaire,

BP

_1,

91406

_Orsay,

France

(Reçu

le 17 mars

_1975,

_accepté

le 21 mars

1975)

Résumé. 2014 L’identité de structure des

premiers niveaux _9/2- dans les noyaux d’or de nombre de

masse

_impair

_{de 187 à 193 ainsi que} cette structure elle-même, sont révélées par le mode de population

et désexcitation de ces niveaux et leurs _{vies moyennes. Ceci} montre _{l’importance} de la prise en _compte

du _paramètre

_{d’asymétrie}

03B3 dans les noyaux impairs d’or comme cela a déjà

_{été montré}

_{pour les noyaux}

de _platine. Ceci montre _également la nécessité de tenir _compte dans ces

noyaux

d’états appartenant

à la couche _majeure suivante. Abstract. 2014 The half-life

(1.2 ± 0.1 _ns) of the 789.9 keV _9/2- level in 193Au is measured. The retardation of the _deexciting M 1 and E 2 transitions _suggests that this _{9/2- state,} as well as those

found in _{187, 189,} 191 _{gold nuclei,} issues from the

_h9/2

orbit. The average shape of the Au nuclei in these states and of their even Pt cores is _interpreted with the _particle +

_asymmetric

rotor model. Classification

Physics Abstracts 4.1b2

The _great interest focused on transitional nuclei in the Os to Pb

_region

is connected with observations made

_{precedently :}

-

Signs

of a

_prolate

to oblate

_shaped

transition revealed

_by

the

_study

of level

_positions

in even Pt

nuclei

_[1,

_2].

- Establishment of

9/2-

states issued from the

h9/2

orbital in Tl nuclei

_[3].

- The

discovery

and

_{interpretation}

of

_negative

parity

states in Ir nuclei

_[4].

- The existence of

unexpectedly large

retarda-tions of transitions in Ir and Pt nuclei

_[5].

- The

_large

_isotopic

shift measured between

185Hg

and

_{18’Hg [6].}

Most of the above information was obtained at

ISOLDE,

CERN and studies of odd Au and Pt

nuclei are

_actually

continued at

_ISOCELE,

_Orsay.

Half-life measurements of excited states

_[7]

and

heavy

ion reaction

_{experiments [8, 9]}

have led to the

proposal

of two

_decoupled

bands

in 189 Au.

_They

are

based on

9/2-

and

11/2- levels, joined

as

in 1871r

[4]

by

a _very retarded M 1 transition

_{(78 keV,}

Fw -

15

_000).

The

_11/2-

band is

_supposed

to result from the

coupling

of an

_h11/2

_proton hole to the « rotation » of a

Hg

core and

_has,

in

_fact,

« rotation »

_energies

similar

to those of even

_{Hg isotopes.}

In these states the deformation of the nucleus should be oblate as in

Hg

nuclei. The band _{appears in} all odd Au

_isotopes

(*) And _University Paris VII.

and is observed to have a _very stable structure

_[7-14].

As seen in

_figure

2 the band head lowers

_slowly,

when the neutron number decreases.

The

_9/2-

state is

_proposed

to arise from the

coupl-ing

of a _proton in the

_h9/2

shell to the « rotation » of a Pt core and

_has,

_according

to this

_{interpretation,}

a

prolate shape.

States issued from the

_{high lying}

h9/2

orbital,

have

_already

been observed in Tl nuclei and their _appearance at low excitation _energy is

explained by

the

_gain

in

_pairing

_energy

_[3].

In

Tl,

and,

as shown

_below,

in Au these states lower

_rapidly

with

_decreasing

mass number.

In

187Au

the

_9/2-

state is

_placed

below the

11/2-isomer

_(Fig.

₂₎

and the

_decoupled

character of the band

_is

well

_{developed [ 13].}

The half-life of the

_{11 /2}

-state has been

_measured,

but the retardation factors for the

_11/2-

to

_9/2-

transition cannot be

_given,

as the M 1 to E 2

_mixing

ratio has not _yet been deter-mined.

_{(The experimental}

rësults on

185 Au

are now

being analysed.)

In

195 Au

the

_9/2-

level is

_proposed

as

_lying

at 1 068 keV

_[15].

More information is available in

191Au :

_{Hôglund et}

al.

_[14]

have found the

_9/2-

band at about 200 keV

_higher

excitation

energy than

in 189Au

(Fig. 2),

and measured a similar

hindrance as in the latter nucleus for the

9/2-

to

11/2-

M 1 transition

_{(Table I).}

For the identification of the

_9/2-

state in

_193Au,

the extensive studies

_by

Vieu and Dionisio

_{[ 11 give}

aid. Their table of

_analogue

states in

193 Au

and

_{195 Au,}

i.e. states with the same

spin

and

_{parity, nearly}

same excitation _energy and

identical

_depopulation

_mode,

shows that the 789.9 keV

9/2-

level

in 113 Au

has no

_equivalence

in the 195

614

TABLE 1

H alf-lives of 9/2 -

levels,

and absolute transition

_{probabilities of 9/2 -}

to

_{11/2 - (7/2 -)}

transitions in

_{189, 191,}

193 Au

_isotopes

FIG. 1. -

Time distribution obtained _{by measuring} 499.65 K - y coincidences in 193Au.

tope. The level lies about 200 keV

_higher

than the

9/2 -

state in

191 Au.

It

_decays

to the

_{11/2 -}

isomer and is fed as the

_9/2-

states in

189Au

and

191Au

_by

a _{pure E 2} and a mixed M 1 + E 2 transition

_(Fig.

_1).

However,

calculations of transition rates

_performed

within the frame of the

_Alaga

Paar 3-hole cluster model

_[16]

_(using

_parameters

_fitting

best the

_positive

parity

states and without

_octupole

_coupling),

do

not

_give

the

_large

retardation factors

_expected

for the

_deexciting

transitions

_[11].

A state with the

required properties

was

_placed

at 500 keV

_higher

energy. It therefore seemed necessary to make a

half-life measurement in order to check the character of the state. The

_experiment

was

_performed

accord-ing

to the

_delayed

coincidence

_method,

with an

FIG. 2. - Ground state,

11 /2 -, and _{9/2 -} levels in odd Au _isotopes. The two _highest levels are the first two excited states of the 9/2 - band. Their relative positions indicate that, in this band, rotation aligned coupling and prolate shape dominate in 18’Au and 189Au, strong

cou-pling and oblate _shape in the heavier _gold nuclei.

coincidence _spectrometer

_[17].

The K-line of the 499.7 keV

_transition,

which

_depopulates

the 789.9 keV

level,

was then selected

_by

one lens and the

_feeding

y-rays were detected with a

_plastic

scintillator.

Figure

1 shows the time distribution _{of the e-y} coincidences and the relevant _part of the

_decay

scheme. The

_slope

of the curve

_gives

The retardation factors of the

_depopulating

M 1

transitions

_{(Table I)}

resemble those obtained for the

correspondent

transitions in

189 Au

and

191Au.

Even

the E 2 transitions are hindered

_compared

to the

Weisskopf

estimate. The measurement

_{provides good}

support to the

_assumption

that the 789.9 keV

_9/2

-state in

193Au

has the same structure as the

9/2-levels

in 189 Au

and

191 Au.

The

_study

of even Pt

_[1,

_2]

and

_Hg

_[18]

nuclei has shown the

_necessity

of

_taking

_{into account,} not

only

the variation of the deformation

_{parameter B}

from one state to another and the fluctuations of it within each state, but also the variations and the fluctuations of the

_shape

_{asymmetry parameter y.}

In a recent

_{interpretation}

of the

_11/2-

band

_{of 195Au,}

Meyer et

al.

_[19]

assume a triaxial nuclear

_shape

and

investigate,

as a first _step, the band structure of a

single j-nucleon coupled

to a

_{rigid asymmetric}

rotor.

For y =

0°,

they

find _a

pure rotation

aligned

level order with

_strongly

_populated

_I,

_{1 + 2,}

1 + 4....

states; for y = 60° the band _structure becomes

typically

strong

coupled

with an

_l,

I +

1,

1 +

_2,

...

spin

sequence. A continuous transition from

decoupl-ed to _strong

_coupled

structure takes

_{place by}

_changing

y from 0° to 60°. The _strong

_coupling

level order dominates in the interval 30° y 60°.

The model allows a

comparison

between even Pt

nuclei and the

_9/2-

states in

_Au,

_interpreted

as formed

by

the

_coupling

of an

_h9/2

_proton to the rotation of the

_neighbouring

Pt core. The measurement of the static

_quadrupole

moment in

194Pt

has

_proved

that this nucleus has an oblate

_{shape [20],}

but a

change

to the

_prolate

form is

_expected

in

_lighter

Pt

nuclei

_{[1, 21] (around}

mass

188).

In accordance with the

_negative

deformation of

194pt

and its y parameter >

_30°,

the

_{9/2 -}

band in

195 Au

shows the _strong

_coupling

level _sequence

(Fig. 2).

This is also the case for the bands in

193 Au

and

_191Au,

thus

_suggesting

an oblate

_shape

for

192Pt

and

190Pt.

In

_189Au,

the

_13/2-

member of the

_9/2-

band is found at a lower _energy than the

11/2-

one, and the band is

decoupled,

which

corres-ponds

to y 30° and a

prolate shape

for the 1 "Pt core. The

_assumption

of

_positive

deformation of

188Pt,

is

_supported

_by

the fact that the

_{11/2 -}

isomeric state in

_1871r,

_interpreted

as a hole in the

188Pt

core,

has been shown to be

_{prolate [15].}

The

_decoupled

character

of the

_9/2-

band becomes more

pronounced

in

_18’Au,

in _agreement with the

_{prolate shape}

pro-posed

for

186Pt

_[21].

According

to this

_{simple comparison,}

the

oblate-prolate

shape

transition in even Pt

nuclei,

i.e. the

transfer of the total _{energy minimum from} the oblate

to

_prolate

_side,

should occur in the _step between the

616

minimum takes

_place

between

188pt

and 186Pt

[21],

which seems to demonstrate the influence of

_{the B}

and _y

_dependent

mass _parameters on the location of

the

_shape

transition

_{[1, 21].}

In the above

_comparisôn,

no attention was

given

to the influence of the

particle-core

_interaction,

_{which may be strong}

_enough

to

shift the

_shape

transition

_[7,

_9,

_24].

Admixtures in

the

_9/2-

state were not considered either. The most

important

contribution is

_expected

to come from

the

_{9/2 (514)}

state. It should

_give

rise to a

tendancy

for a _strong

_coupling

level _pattern in the

_lighter,

more deformed

_isotopes.

This effect is not observed.

It is

_interesting

to notice that the

_{shape transition,}

which arises as a function of neutron number N in Pt

and Au

_nuclei,

also takes

_place

as a function of Z.

In 191 to 199 Tl

_isotopes

the

_9/2-

band is found to

be oblate

_[3],

in

1871r

and

189Ir

it is

_{prolate [22].}

These observations _support the

_suggestion

that the

shape

transition of the even cores occurs near a line

joining

182Hg,

188Pt,

and

1940S

_[21].

It remains now to discuss the cause of the

_large

retardations of the interband transitions. Several

reasons can be

_suggested.

In the

_lighter

_nuclei,

a

certain hindrance _may be due to the

_change

from

prolate

to oblate

_{shape. According}

to the

_simple

rotation

_aligned

and

strong

_{coupling models,}

the

9/2-

and the

_11/2-

states are

_respectively

a _pure

particle

state

_coupled

to a Pt core and a _pure hole state

_coupled

to a

_Hg

core. A transition between them is a two _{step process. In}

1871r

_[5]

the

_11/2-

to

9/2-transition is about 30 times more retarded than in

the Au

_isotopes.

In this case, the

11/2-

state is

essen-tially

strong

coupled,

while the

_{9 J2-}

one is rotation

aligned

and their wave functions have a _{very small}

overlap.

In Au

nuclei,

the

_overlap

is

_expected

to be

larger,

as the difference in structure between the initial and final states are less

_pronounced.

The

retardation in 187Ir can also be

_{explained by}

K-for-biddeness ;

the states involved

_being

_{11/2- (505)}

and a

_{strongly perturbed 1/2- (541).}

The

_study

ôf the

_9/2-

states in odd Au nuclei has shown

_that,

to a first

_{approximation,}

the

_approach

of

_{Meyer et}

al.

_[19]

can

_explain

their characteristics. This indicates that a new _type of

_coupling

between

particle

and a collective state can _{appear in} soft

nuclei,

and that y asymmetry has to be taken into

account in odd as well as in even nuclei. It must be noticed that this successful model for

_{high angular}

momentum states

_{has, necessarily,}

the limitations of the

_Davydov

model

_[23],

which

_{identifies fl}

with

Brms

and _y with _yrms, and thus

_neglects

the

_shape

and

pairing

fluctuations shown to be

_large

in the even

nuclei of this

_{region [21].}

Our result seems also to

show the limitations of models which take into

account the

_{shape fluctuations,}

but

_neglect

states

belonging

to the next

_higher

shell.

We are indebted to M.

_Jacques

Obert for the

preparation

of the 193Hg

source and to the ISOCELE collaboration for on line measured sources.

References

[1] KUMAR, K., BARANGER, M., Nucl. _Phys. A 122 ₍₁₉₆₈₎ 273.

[2] FINGER, M., FOUCHER, R., HUSSON, J. P., JASTRZEBSKI, J.,

JOHNSON, A., ASTNER, G., ERDAL, B. R., KJELBERG, A., PATZELT, P., HÖGLUND, A., MALMSKOG, S. G., HENCK, R., Proc. Int. Conf. Leysin (1970) 1031, and Nucl. Phys.

A 188 ₍₁₉₇₂₎ 369.

[3] NEWTON, J. O., CIRILOV, S. D., STEPHENS, F. S. and DIA-MOND, R. M., Nucl. _Phys. A 148 ₍₁₉₇₀₎ 593.

[4] BÄCKLIN, A., BERG, V., MALMSKOG, S. G., Nucl. Phys. A 156

(1970) 647.

[5] SEBILLE-SCHÜCK, C., FINGER, M., FOUCHER, R., HUSSON, J. P., JASTRZEBSKI, J., BERG, V., MALMSKOG, S. _{G., ASTNER, G.,}

ERDAL, B. R., PATZELT, P., SIFFERT, P., Proc. Int. Conj.

Leysin (1970) 1045, and Nucl. _Phys. A 212 ₍₁₉₇₃₎ 45.

[6] BONN, J., HUBER, G., KLUGE, H. J., KUGLER, L., OTTEN, E. W.,

Phys. Lett. 38B (1972) 308.

[7] BERG, V., FOUCHER, R., HÖGLUND, A., Proc. Int. Conf. Amster-dam (1974) 112, and Nucl. _Phys., in _print.

[8] BERG, V., DELEPLANQUE, M. A., GERSCHEL, C., PERRIN, N.,

Proc. Int. Conf. on Nuclear Structure and _{Spectroscopy,} Amsterdam ₍₁₉₇₄₎ 113 and IPNO-Ph. N-74-12.

[9] DELEPLANQUE, M. A., GERSCHEL, C., PERRIN, N., BERG, V.,

Nucl. Phys. à _paraître.

[10] VIEU, Ch., PEGHAIRE, A. et DIONISIO, J. S., Revue Phys. Appl.

8 ₍₁₉₇₃₎ 231.

[11] VIEU, Ch., DIONISIO, J. S., Proc. _of the topical Conference on

problems of vibrational _{nuclei, Zagreb (1974).}

[12] TJÖM, P. O., MAIER, M. R., BENSON, D. Jr, STEPHENS, F. S. and DIAMOND, R. M., Nucl. Phys. A 231 (1974) 397.

[13] DELEPLANQUE, M. A., GERSCHEL, C., ISHIHARA, M., PERRIN, N., BERG, V., BOURGEOIS, C., DESTHUILLIERS, M. _G., HUS-SON, J. P., KILCHER, P., LETESSIER, Jocelyne, J. Physique

Lett. à _paraître.

[14] HÖGLUND, A., BERG, V., ERDAL, B. R., JASTRZEBSKI, J.,

KILCHER, P., MALMSKOG, S. G., Annuaire IPNO ₍₁₉₇₅₎

5, and to be published.

[15] MEYER TER _{VEHN, J.,} LBL-3416 _(1974).

[16] PAAR, V., Nucl. Phys. A 211 (1973) 29.

[17] GERHOLM, T. R. and LINDSKOG, J., Ark. _Fys. 24 (1963) 171.

[18] PROETEL, D., DIAMOND, R. M. and STEPHENS, F. S., Phys.

Lett. 48B (1974).

[19] MEYER TER _{VEHN, J., STEPHENS,} F. S. and DIAMOND, R. M.,

Phys. Rev. Lett. 32 ₍₁₉₇₄₎ 1383.

[20] PRYOR, R. J. and SALADIN, J. X., Phys. Rev. 1C (1970) 1573.

[21] CAILLIAU, M., Thesis, IPN _{Orsay (1974).}

[22] ANDRÉ, S., BOUTET, J., JASTRZEBSKI, J., KACZAROWSKI, R., LUKASIAK, J., RIVIER, J., SEBILLE-SCHÜCK, C., SUJ-KOWSKI, Z., TREHERNE, J., Nashville Conference on

Reaction Between _Complex Nuclei ₍₁₉₇₄₎ 192.