Investigation of the neutron-deficient isotope 123Ba

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Submitted on 1 Jan 1979

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Investigation of the neutron-deficient isotope 123Ba

N. Yoshikawa, J. Gizon, A. Gizon

To cite this version:

LE

JOURNAL

DE

_PHYSIQUE

Investigation

of the

neutron-deficient

_isotope

123Ba

N. Yoshikawa

_(*),

J. Gizon and A. Gizon

Institut des Sciences Nucléaires (USMG, IN2 P3), 53, avenue des Martyrs, 38026 Grenoble Cedex, France

(Reçu le 16 octobre 1978, accepté le 22 novembre ₁₉₇₈₎

Résumé. 2014 Des niveaux de

haut-spin dans 123Ba ont été _produits dans la réaction

_114Sn(12C,

3n) et étudiés selon les _techniques de _{spectroscopie} 03B3 en

_ligne.

La structure en bande fortement

_{peuplée qui}

a été _{identifiée repose} sur un état

_7/2-

et _peut

_{s’expliquer}

_{par le couplage} d’un trou de neutron

_h11/2

au c0153ur triaxial de _{type prolate. Elle} est

comparée à celle existant dans des

_isotopes

_plus lourds de Ba et des isotones N = 67.

Abstract. 2014

High-spin

levels in 123Ba have been _produced in the

_114Sn(12C,

_3n) reaction and studied _by in-beam

03B3-ray spectroscopic

techniques.

Only one

_strongly

_populated band structure has been identified and

_explained

as an

odd-parity

level system based on a _7/2- state _{generated by} the

_coupling

of an

_h11/2

neutron-hole to a

_prolate-type

triaxial core.

_Comparisons

with heavier odd-A Ba _isotopes and N = 67 isotones _are made.

Classification

Physics Abstracts 21.60 - 27.60

1. Introduction. - In the last few

years,

systematic

studies of excited states in nuclei of the transitional

regions

have revealed a wide

_variety

of new informa-tion.

_Though

the nuclear structure in the A z 190

region

seems now to be

_relatively

well

_understood,

especially through

studies of Ir, Pt,

Au,

Hg

and Tl

isotopes,

it _{is necessary} to extend the effort towards other transitional

_regions

where the

_experimental

data are still scarce.

Several studies

_{of high-spin}

states excited

_by

_(HI,

_xn)

reactions in

odd-A 125,133 Ba

_[1],

i2y-l3sCe

_[2]

and

133-139Nd

_[3J

_isotopes

have

_pointed

out the existence

of typical

band structure _patterns based upon

neutron-hole

_{configurations}

_{(h11/2’ g 7/2)’}

_Though

several theo-retical

_approaches

can

_reproduce

such structures, until now we have

interpreted

our results

only

within the frame of the triaxial

_{rotor-plus-particle}

model

developed by

J.

_{Meyer-ter-Vehn}

[4].

This model is

especially

able to

_explain

the level

_subsystems

and the two-dimensional _patterns observed

_{experimentally}

in the

_negative

_parity

levels associated with an

_h11/2

neutron-hole [5].

To learn more about nuclei of this

region

and

even-tually

to test the

_{applicability}

of different models when the Fermi surface

_penetrates

inside a

given

shell,

it is _necessary to obtain more

_experimental

details towards more deficient nuclei.

Though

the

_production

of very neutron-deficient

isotopes

from

_heavy-ion

_compound

nuclei is

limit-ed

_[6],

we have undertaken the

_study

of 123Ba

with the

114Sn

+ 12C reaction to search for its

_high-spin

excited states.

To

date,

the

_ground-state

_spin

of 123Ba

is unknown. Arlt et al.

_[7]

have identified seven

low-energy

_y-rays of

(2.7

+

0.4)

min. half-life associated with the

_decay

of

123Ba

_produced

in the

_{114 Sn( 12C , 3n)123Ba}

reaction.

Using

in-beam

_{spectroscopic}

_methods, Conrad and

Repnow [8]

have

_assigned

to

123Ba,

a cascade of stretched E2 transitions of 321,

522.5,

684.5 and 799 keV from the

_110Cd(160,

_3n)

reaction.

Unfor-tunately,

the

_particle-y

coincidence measurements

that we have

_performed

_[9]

and two _{other very} recent

experiments

[10], [11]

prove that this

assignment

is wrong and until

now, 123 Ba

is

quite

unknown.

2.

_Experimental

_techniques.

-

Targets.

of rolled metallic tin enriched to

_{66.5 %}

in 114Sn and backed with 25 _ym of lead were bombarded at the Grenoble variable _energy

_cyclotron

with 12C

ions _{in the energy} range from 52 MeV to 63 MeV. The maximum cross section for three

_particles

emission was found at

57 MeV.

In-beam and out-of-beam _y-ray

_singles

_spectra have been obtained with several

Ge(Li)

detectors, in

parti-cular with a 3.5

cm3

planar

having

a resolution of 0.7 keV FWHM at 122 keV and with a 30

cm3

coaxial

having

a resolution of 2.0 keV FWHM at 1.3 MeV. The

_energies

of

_y-lines

were determined

_by

_recording

simultaneously

the y-rays emitted

_by

the _target and standard calibration sources. Several measurements

210

have been done after irradiation of the _target to look for short-lived and

_{long-lived activity}

_spectra. The

angular

distributions of the y-rays were made at six different

_{angles including}

00 relative to the beam direction. The _y-y-t coincidence measurements were

performed

with two 30

cm’

coaxial

Ge(Li)

detectors.

Moreover, to

_separate

the

_(12C,

3n) reaction from the

_(12C,

_p2n),

_(12C,

_2pn)

and

(12C,

a2n) reactions,

we have carried out

_y-particles

coincidence

measure-ments. In this

_experiment,

_protons and

_a-particles

were detected with a silicon detector covered

by

a 400

kig/cm’

absorber and

_placed

near the _target. 3.

_{Expérimental}

results and

_assignments.

- Since

nothing

was known about the

_ground

and excited

states

_{of 123Ba,}

the first difficult task was to

_identify

the transitions

belonging

to this

_isotope.

For this purpose we have

_{carefully analysed}

all

_experimental

data as in-beam and out-of-beam

singles

_spectra, excitation functions and

_particle-y

coincidences

spec-tra. Series of

_singles

_spectra recorded

_successively

after bombardments of the _target were also a valuable tool to

identifÿ

the _y-rays associated with the

_decay

of the A = 123 chain

[7], [12],

[13].

The low energy

part of a

_singles

_y-spectrum recorded with a

_planar

detector is shown in

_figure

1,

as an

_example.

The

Fig. l. - In-beam

y-ray spectrum observed with a _planar detector

at 90° to the beam in the 114 Sn + 12C reaction at 57 MeV. The asterisks (open circles) indicate _y-lines in the A = 123 (124) decay

chains. The full circles _correspond to y-rays associated with target

impurities and the _triangles to y-rays in 123CS.

second _step of the identification is

_mainly

based

on the _y-y coincidences. With all these

approaches,

the excitation of several

_isotopes

has been well

iden-tified,

especially

those

_previously

studied

_by

in-beam

spectroscopy. It is the case of l2oXe excited here

_by

(12C,

a2n)

reaction and known from the

reactions

_[14].

The

_l14Sn(12C,

_p2n)123Cs

channel is

strongly

fed at 57 MeV and

_produces

the more intense lines in the

_particle-y

coincidences

_spectrum.

The

corresponding partial level

scheme

of 123Cs,

establish-ed in this

_experiment,

has been

_already

_published

with results

_{concerning l2sCs}

anci 121CS

_[9].

It was _easy to eliminate the

_isotopes

_produced

_by

(12C,

3n)

reactions on the

impurities

of

l16,118Sn

present

in the _target because

_they

have been

_previously

studied in our _group

_by

in-beam _spectroscopy

_[1].

Information on the transitions

placed

in the

partial

level scheme

_proposed

for 123Ba

is

_reported

in table I. The

_energies

of the _y-rays are

given

with a

_precision

better than 0.1

_%

for the _strong and resolved lines. The results of the coincidence measurements are summarized in table II.

Table I. - Transitions

of

123Ba

observed in the

l14Sn(12C,

3n)

reaction at 57 MeV

(Q) Relative intensities normalized to this line.

(b) Composite line.

(‘) Value deduced from the y-y coincidence spectra.

The

_partial

level scheme

_proposed

in

_figure

2 was constructed on the basis of

_experimental

data and

keeping

in mind the

_systematic

results known for odd-mass heavier Ba

_isotopes.

From

_delayed

_{y-ray spectra} recorded between the beam bursts of the

_cyclotron,

a

delayed

_component with a

_Tl/2

= 40 ns half-life has been found in the

very strong 92.7 keV transition of the

_singles

spec-trum, the other one

_being

identified with

This

_delayed

92.7 keV line is in coïncidence with a cascade made of most of the strong transitions

assign-ed

_{to l23Ba,}

in

_{particular with}

the

_{110.0, 133.3, 421.3,}

569.0,

694.7 and 780.6 keV

_prompt

_{y-rays. In} this

cascade,

the 110.0 and 133.3 keV _y-rays have

_sharp

negative A 22

coefficients characteristic of M l + E2

Table II. - Gamma

rays observed in coincidence with lines

_{of l23Ba}

in the reaction

(114Sn + 12C)

(’) Transition not placed in the level scheme. z

(6) Transition _belonging to _{neighbouring isotopes} as 124,125,127,128Ba and 121Xe.

fit.

2. - Part

(a) is the _partial level scheme of 123Ba obtained in the 114Sne 2C, 3n) reaction. The black and white components of the arrows _represent the y-intensity and the estimated internal conversion contribution, respectively. A _comparison of the _hall/2

band structure in both 121 Ba and 123Ba is made in _{part (b)} where

the energies are normalized such that the 11/2- states are all set to

zero.

421-569-695-781 keV

lines)

present

positive.

A22P

coefficients characteristic of _{pure E2}

(I

+ 2 --->

_{I )}

transitions.

By

comparison

with heavier barium

_isotopes,

this

group of transitions

belong

to the Yrast cascade

preferentially

populated by

the deexcitation of the

compound

nucleus formed in the reaction and is

assigned

as a _part of the

_odd-parity

_system

_generated

from the

_hll/2

neutron shell. The resemblance of this cascade to that

_already

identified in

l2sBa

is

_striking

and constitutes the basis of our

_{spin assignments}

in

12 ’Ba.

As in other odd-mass barium

isotopes

excited

by (HI, xn)

reactions,

the sequence of stretched E2 transitions comes to the

_11/2-

state. In the _present

case, as

in 1 zs Ba,

this

11 /2 -

level is

_successively

depo-pulated

to the

_9/2-

state

_by

the 133.3

keV,

Ml + E2 transition and to the

_7/2-

basic state of the _system,

by

the

110.0 keV,

M 1 + E2 transitions.

_Looking

at _{the y-y} coincidence _spectra, few other weak

tran-sitions have been

_placed

in this

_negative

_parity

_system.

However,

it was

_impossible

to establish the existence

of a second

_15/2-

state or to

_display

a two-dimen-sional level _pattern as observed in heavier odd-mass Ba and Ce

_isotopes.

It is more difficult now to connect this

_odd-parity

system based _upon a

7/2-

state to lower unknown

states

of 123Ba.

From _prompt and

_delayed

y-y coinci-dences _spectra and _y-rays intensities it is established

that the

_delayed

92.7 keV line

_depopulates

the

7/2-level.

This 92.7 keV transition has a L =

1-type

multi-polarity

deduced from the

_angular

distribution

212

order of 105. In case of such an El

multipolarity,

the 92.7 keV y-ray feeds a state whose

_spin

and

_parity

are very

likely

5/2+.

Indeed,

the P =

7/2+

assignment

is excluded

_by

the

_negative

_A22

_angular

distribution

coefficient and I’ =

_9/2+

_corresponds

to an almost

fully aligned

7/2+

state. This last

_possibility

_9/2+

must

also be eliminated because there is no such intrinsic

9/2+

state for N = 67 _at

moderate f3

deformations.

A _gy/2 band has been identified

_[15]

in several odd-mass Ba and Ce

isotopes

with N - 73.

Though

its excitation is smaller in l2sBa than in

_127,129Ba,

one can

_hope

nevertheless to observe it in

123Ba.

For this _{purpose, the y-y} coincidences _spectra

_gated

with _y-rays

_following

a three neutron emission in the

(114 son

+

_12C)

reaction have been

_{carefully analysed.}

Only

one _sequence made of three weak lines at _169.6,

205.3 and 238.1 keV with

_respective

intensities of

30,

28 and 14 in scale used in table 1 has been retained. Both the 205 and 238 keV transitions have a sizeable

negative anisotropy

in the

_angular

distribution

mea-surements

but,

unfortunately,

it is

_impossible

to

determine the

_anisotropy

of the 169.6 keV line which is

_composite,

one _part

_being

in the

l24Ba --+ 124CS

(Tl/2

=

11.9 min.) [16].

It appears that the 97/2 neutron-hole structure does

not

_change

_very much when one _goes from

l27Ba

to

12 ’Ba

_[15].

This is related to the small

_change

of the Fermi _{energy when} two neutrons are removed from the nucleus. This

_stability

in the _97/2 level structure

could _very

_likely

continue towards more

neutron-deficient

_isotopes,

in

_particular

for

_123Ba,

because of the

_large

number of

_{positive-parity}

orbitals

_(Sl/2’

gv/2 and

d,/2)

which exist in this

region.

A AI = 1

cascade formed with the

216, 255, 291,

323...

_keV,

MI + E2 transitions has been identified in the _97/2 band

of l2SBa

and the

_{7/2 +}

basic state of this band is de-excited

_by

a _strong 168.6 keV y-ray which

pro-bably

has an MI

_{multipolarity}

_[17].

This _suggests that the 205 and 238

keV,

Ml + E2 lines of

123Ba

could be the two first transitions of this _97/2 band and that the 169.6 keV could

_depopulate

the

_7/2+

base level towards a lower state. As mentioned above

concerning

the

_density

of neutron orbitals in this

region,

this state

_probably

has a

_positive

_parity.

4. Discussion. - 4. 1 COMPARISON

OF 123Ba

wiTH

Ba ISOTOPES. - The AI = 1

odd-parity

band in 123Ba is similar to those observed in heavier Ba nuclei

_[17]’

B and its existence in a wide _range of masses is in favor of a collective structure.

The bands found in transitional nuclei can be

interpreted

by

several theoretical models

but,

in the 50 Z, N 82

_region,

most of full calculations of

energies,

transition

_{probabilities,}

moments, half-lives have been made within the frame of the

triaxial-rotor-plus-particle

model

_[4].

The AI = 1 band structure

in the

odd-A l2S-l33Ba

nuclei is

_explained

as

_resulting

from the

_coupling

of an

_{hl li2}

neutron-hole to

_{a fi}

and _y deformed core.

_Though

this AI = 1 band in

123Ba

is

not

_specially

well

_developed

i.e. the number of known levels is not _very

_large,

it _appears reasonable to treat

it in the triaxial-core model. This is

_justified

_by

considering

the

_properties

of this band in the

123-133 Ba

_systematics.

Indeed level

_energies

and

branching

ratios _vary in a monotonous _{way when}

going

from N = 77 _to N = 67. So in the triaxial-core

model,

the

_odd-parity

levels can be considered as

originating

from an

_{hl il2}

neutron-hole

_coupled

to a

fl

= ₊ 0.29 and

y = 13.50 deformed core when the

Fermi surface is located between the

_5/2-

and

7/2-orbitals

_ofhn/2

neutron shell. Such a location of the Fermi surface

_corresponds

to 7 holes in the shell. From the

_systematics,

one deduces a smooth

lowering

of level

_energies

_(relative

to the first

_{11 /2}

-state)

towards the neutron deficient

_isotopes

which

clearly

indicates an increase of the

_{positive fi}

defor-mation. The y deformation varies in an

opposite

way and _goes down to a low value in

123Ba.

_Everything

occurs as if the characteristic

properties

of the triaxial

rotors were

_{progressively}

dimmed when

_going

towards

more neutron deficient nuclei. For

_example,

it was

impossible

to find the second

_{15 j2-}

and

_{19 j2-}

levels

in 123Ba

which _very

_likely

are

_pushed

_upwards

and are

surely weakly

fed in the

_(12C,

3n)

reaction. The level sequence tends now to become similar to the one in the

strongly

deformed nuclei.

4.2 COMPARISONS BETWEEN N = 67 _ISOTONES.

-It seems

_interesting

to consider

_systematics

as a func-tion of the _proton number Z to look for

_properties

of

high-spin

levels when

_going

_away from the Z = 50

pro-ton closed shell. Results

_actually

available relative to

negative-parity

levels

_generated

from the

_h11/2

neu-tron shell are shown in

_figure

3 where the

_energies

of the

_11/2-

state are all set to zero to facilitate the

comparisons.

Fig. 3. -

Systematics of odd-parity levels from the _hlu2 shell in

N = 67 isotones. The _energies of the first _{11 /2- state are} all _{set to}

zero. _{The open} circles _correspond to the 2+ levels in the N = 66

A AI = 1 band above this

_11/2-

state has been excited in

117 Sn

_{[Ref. [18]]}

and

l19Te

_[Ref.

_[19]]

through

the

_’l6Cd(a,

3n)

and 11’Sn(a, 2n)

reactions,

respectively.

The case

of l21Xe

which is inserted in the

systematics requires

some

_{explanations.}

A cascade of E2 transitions has been found in the

reaction and

_interpreted

as a AI = 2

decoupled

band built on a

(9/2-)

state

_[20].

In a recent

investiga-tion

_[21],

it is shown in contradiction with earlier data,

that the AI = 2 cascade of

strong stretched E2 transi-tions in the

odd-A 123-133Xe

nuclei

_always

lies on the first

_11/2-

state.

_Though

the relative

_position

of the lowest

_9/2-

and

_11/2-

levels in

129Xe

is unknown until now, we believe that the observed _{E2 sequence} is built on the

11/2-

level. _So, the

_spacings

between the

11/2-,15/2-,19/2-, 23/2-

and

_27j2-

levels

in 121Xe

fit

_perfectly

with the ones in heavier

_isotopes

_(see

figure

5 in reference

_[21]).

A continuous decrease of the level

_{energies clearly}

shows _up in

_figure

3 when

_going

from Sn

(Z

=

50)

to Ba

(Z

=

56).

However, _{one must} observe that ail the levels do not behave in the same _way and it is

interesting

to _compare them to the 2+ - 0+ transitions

in the

_{corresponding}

cores.

First,

the

_15/2-

-+

11/2-

spacing

in the odd-nuclei

is

_{roughly equal}

to the 2’ --+ 0+ transition of the core

fore Z = 50 and about twice in the barium _case

(Z

=

56). Secondly,

it

appears from the N = 67

syste-matics that the energy of the

_{13/2- --+ 11/2-}

transi-tion in the odd-A cases is close to that of the first 2+

excited state of the even-even (A -

1 or A +

1) cores.

The same features exist in N =

69,

71 and 73 isotones. All these trends can be understood in a first appro-ximation if one considers

i)

11’Sn

as a vibrational nucleus where the

13/2-and

_15/2-

states are members of the almost

degene-rated doublet

_produced

_by

the

_coupling

of an

_{hi 1J2}

neutron-hole to the 2+ core excitation.

ii)

123Ba

as a

prolate-type

triaxial nucleus with a

larges

value

_{(+ 0.29)}

and a moderate _y deformation

(- 13°)

as

_already

mentionned in subsection 4 .1.

iii)

119Te

and

’2lXe

also as transitional nuclei

which can be treated in the

triaxial-rotor-plus-particle

model

_{by coupling}

an

_h11/2

neutron-hole to the triaxial core

_{[19], [21]}

or understood in a more

_general

core-quasiparticle

model

_[22].

In

conclusion,

a band structure of

_high-spin

levels has been identified

in 123Ba

_through

_{the "’Sn( 12C ,}

3n)

reaction. Its main features and

_comparisons

with

isotopes

and isotones show that this neutron-deficient nucleus is situated in a

region

between the

mode-rately

deformed transitional nuclei and the

_strongly

deformed

_{rotational-type}

nuclei.

Acknowledgment.

- One of us

(N. Y.)

wants to

thank the Institut National de

_Physique

Nucléaire

et de

_Physique

des Particules for

_supporting

his _stay

at the Institut des Sciences Nucléaires de Grenoble.

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