g9/2 AND h11/2 bands in121,123,125Cs

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g9/2 AND h11/2 bands in121,123,125Cs

N. Yoshikawa, J. Gizon, A. Gizon

To cite this version:

L- 102

g9/2

AND

h11/2

BANDS IN

121,123,125Cs

N. YOSHIKAWA

_(*),

J. GIZON and A. GIZON

Institut des Sciences Nucléaires

_(IN2

_P3)

53, av. des

_Martyrs,

38000 Grenoble, France

(Reçu

le

_{15 fevrier}

1978,

accepte

le 2 mars

₁₉₇₈₎

Résumé. 2014

Des cascades de transitions E2 pures dans 121,123,125Cs forment les bandes _découplées résultant de l’existence d’un _proton dans la

_plus

basse orbitale de la couche

_h11/2.

La séquence de niveaux 0394I = 1 a ses

_origines

dans un trou de _proton dans la couche _g9/2. Ces résultats

_impliquent

une forme _prolate pour ces _{noyaux de césium.}

Abstract. 2014 Cascades of stretched E2 transitions in 121,123,125Cs constitute the

decoupled

bands

generated

from a _proton in the lowest orbital of the

_h11/2

shell. A 0394I = 1 _sequence

originates

from _a

hole in the _g9/2 proton shell. The results for both the

_h11/2

and the _g9/2 bands _imply a _{prolate shape}

of these nuclei.

II t JOl R’AI m I’IINI,1011 i 1, ~" I’ Rt~ :~ ionit 39, 15 wkn 1978,

Classification Plivsics Abstracts

21.60 - 27.60

Since the

_discovery

of the band structure built on a

unique-parity

state in odd-A La

_isotopes

_[1],

many

bands based on

_11/2-

states have been established in

the same nuclear

_region

₍₅₀

Z,

N

82). They

show

_{strong-coupled}

structure

_generated

from a neu-tron-hole state in

Ba, Ce,

Nd nuclei

_[2]

and

_decoupled

one based on a _proton

_particle

state in I

_isotopes

_[3]

(for

example).

The

_{rotation-aligned}

model

_[4]

has been successful in

_explaining

such features. Further studies extended to other nuclei are

_interesting

in order to

ascertain the evolution of these

_properties

of such transitional nuclei as a function of their neutron and

proton

numbers.

Results are

_reported

here for odd-A Cs

_isotopes

which were

_{produced by}

(12C,

_p2n)

reactions.

Preli-minary

data

_[5]

were

_{published simultaneously}

with those

_[6]

of the

_Stony

Brook _group who used other reactions to

_investigate

neutron-deficient Cs nuclei. The

_experiments

were carried out with the external beam

of 12C

ions from the Grenoble variable energy

cyclotron,

at various

_energies

between 55 and 63 MeV.

Leadbacked targets of about 2

_mg/cm2

were made

with metallic tin enriched to 79.6

_%,

66.5

%

and 84.4

_%

in

_112Sn,

114Sn

and 116Sn,

respectively.

The 112Sn

target was obtained from the oxide after reduction with

_hydrogen.

Excitation

_functions,

in-beam and out-of-beam

single

spectra and the

_angular

distribution of y-rays

(6

angles

including

0° to the

_beam)

were measured. The _y-y-t coincidence

_experiments

were

_performed

with two coaxial

_Ge(Li)

detectors. Due to the fact

that t the

_{(12 C,}

_3n),

_(12C,

_p2n)

and

_(12C,

_axn)

reac-tions were

competing,

the lines in the Cs

isotopes

were identified

_by

_seeking

coincidences between the

_y-lines

and the

_{evaporated particles.}

The

_protons

and a

particles

were detected with a silicon detector

_placed

near the _target and covered with a 400

J.lg/cm2

alu-minium absorber.

Energies,

intensities and

_A2

values of the transitions

placed

in the

_partial

level schemes of

121,121,125 Cs

(Figs.

1 and

2)

are listed in table I.

From y-p and y-y coincidence spectra, two _types of

cascades have been identified. The first one consists

of stretched E2 transitions with

_energies

similar to

those of the level

_spacings

in the even-even Xe cores

as indicated in

_figure

1. This is a characteristic

_property

of

_decoupled

bands based on an

11 /2-

state

_generated

from the

_{hi 1/2}

_proton shell. As in the odd-A

lantha-num cases

_{[ 1 ],}

the Fermi surface is located on the lowest orbital of the

_{hi 1/2}

shell and the

_coupling

of a

prolate

core with the odd

_quasi-proton

_gives

rise to a

AI = 2 sequence

[7].

The

_11/2-,

_{15/2-, 19/2-...}

levels are

_{strongly populated}

in the de-excitation of

the

_compound

nucleus

_produced

with

12C ions;

the unfavored states

_{(I = j}

+

_{1, j}

+

_{3, j}

+

5,

...),

on the

_contrary,

are difficult to

_{identify. Despite}

this

difficulty,

the

_assignments

of the

_{21 /2 -}

levels in

121,123CS

and of the

_17/2-

in l2sCs

could be made.

They

are

_{supported by}

the

strongly negative

_A2

L-103

g9/2 AND h11/2 BANDS IN 121,123,125cs

TABLE I

Transitions

_placed

in the

_partial

level schemes

_{of 121,123,125CS}

(°) Relative intensities normalized to the 15/2 -~11/2 transitions. (b) Value deduced from the y-y coincidence spectra.

(~

Unresolved line in the _{singles y-ray spectrum.} The energy is obtained from y-y spectra.

FIG. 1. -

Decoupled bands in odd-A Cs _isotopes. The crosses

indicate the levels in the _ground bands of the even-even Xe

neigh-.

bouring nuclei.

coefficients of the M 1 + E2 transitions which de-excite them.

The second kind of band found has a AI = 1

rotational-like character. The

_assignment

is based

FIG. 2. - The

g9/2 bands in 121,123CS.

on three main _{arguments :}

_i)

Positive

_A2

coefficients

correspond

to

_positive

6

_(E2/Ml)

_mixing

ratios for

7+1-~7,M1+E2

transitions in bands

_generated

from

_proton-hole

_{configurations.}

_ii)

For Z = 55 and for a

_{positive #}

deformation in the _range

L-104 JOURNAL DE PHYSIQUE - LETTRES

Nilsson orbital of a shell is a _g9/2 one.

_iii)

Ekstrom

et

al.

_[8]

have observed a

9/2

isomer in

121CS

which is very

likely

the basic state of the AI = 1 bands

shown in

_figure

2. This

_band,

based on a

_Ig9/2

hole-state was

_already

known in odd-A Sb

_[9]

and I

_[4]

nuclei and observed in 121 Cs

[10].

The whole set of data relative to both

_{hi 1/2}

and

g9/2 collective excitations is

compatible

with the

triaxial-rotor-plus-particle

model

_[8].

Indeed a

pro-late-type

triaxial core

coupled

to a

rotation-aligned

hi

1/2 proton and to a 99/2

proton-hole

reproduces

the main features such as level

energies, mixing

ratios and

_branching

ratios. The

_experimental

evidence for triaxial

_shapes

has been shown

_[11]

in the similar case

of

_light

La nuclei.

Information on the behaviour of these Cs nuclei

at

_{high angular}

momentum can be extracted

by

considering

the moment of inertia and the rotational

frequency.

A

_comparison

of

_{backbending plots}

for

nuclei in the Cs

_region

,is

shown in

_figure

3. It is seen that the even-even

neighbouring

nuclei do backbend

between the 10+ and 12+ levels. In

_spite

of the small number of

_experimental

_points,

it seems that the

_g9/2

positive-parity

band in

121 CS

behaves like the _yrast band

in 122Ba

at almost the same rotational

_frequency.

On the other

hand,

the

_h11/2

_decoupled

band in

121,123CS

does not backbend. These facts are in

FIG. 3. - Backbending plots for nuclei in the Cs

region.

agreement with deductions made from La data

_[12]

which,

in the

_{rotation-aligned}

_model,

_suggest that the

_backbending

is

_mainly

caused

_by

two

_h11/2

protons.

Acknowledgment.

- The authors wish to thank

Mr.

_Margotton

for his technical

_assistance,

Mr.

Richaud for

_preparing

the

_targets

and the

_operating

crew of the

cyclotron

for

providing

the beam.

One of us

_(N.Y.)

wants to thank IN2 P3 for

sup-porting

his _stay at the Institut des Sciences Nucleaires de Grenoble.

References

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DIAMOND, R. M. and STEPHENS, F. S., Nucl. Phys. A 213 (1973) 1.

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[9] GARGALAS, A. K., SHROY, R. E., SCHATZ, G. and FOSSAN, D. B., Phys. Rev. Lett. 35 (1975) 555.

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