The 12C(16O, 12C)16O reaction : an insight into the reaction mechanism by particle correlation measurements

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The 12C(16O, 12C)16O reaction : an insight into the

reaction mechanism by particle correlation

measurements

F. Pougheon, I.M. Turkiewicz, M. Bernas, P. Dessagne, G. Rotbard, Pierre

Roussel, J. Turkiewicz

To cite this version:

The

_12C(16O,

_12C)16O

reaction :

an

insight

into the reaction

mechanism

by

particle

correlation

measurements

F.

_Pougheon,

I. M. Turkiewicz

_(*),

M.

Bernas,

P.

_Dessagne,

G.

_Rotbard,

P. Roussel and J. Turkiewicz

_(*)

Institut de

_Physique

_Nucléaire, BP n° 1, 91406

_Orsay,

France

(Reçu le 7 juillet 1983, accepté le 14

_septembre

1983)

Résumé. - La reaction

_{12C(16O,12C)16O}

a été étudiée par des mesures de corrélations _angulaires de _particules.

Cette

_expérience

a démontré : _i) que le mécanisme de double excitation en voie de sortie est un

_phénomène

très

important,

ii) que

l’alignement

du noyau résiduel 16O est très fort.

Abstract. 2014 The

12C(16O,12C)16O

reaction was studied

_{by particle}

correlation measurements. It is shown :

i) that the double

excitation

_{mechanism is very}

_important

in the exit _channel,

_ii)

that the

_alignment

of the residual nucleus 16O _{is very strong.}

Classification

Physics Abstracts 24.70 - 25.70 - 27.30

1. Introduction.

In the _past few _years a

large

amount of

_experimental

and theoretical works have been devoted to the

_study

of

_heavy

ion induced transfer reactions. In

_particular

the

_alpha

_particle

transfer has been

extensively

studied

_using

different

_targets

and

_projectiles

and

was

_proved

to be a

good

tool to

_{populate high spin}

states in

_light

even-even nuclei

_[1].

But in most cases

it has not been

_possible

to extract

_quantitative

spec-troscopic

information for these states from

_simple

angular

distributions.

A

_part

of the reaction mechanism is not _yet well understood and not

_yet

well described

_by

reaction mechanism theories. One of the open

problems

is

the

_importance

of the

ejectile

excitation and of the double excitation

_(ejectile

+ residual

_nucleus)

mecha-nism.

A

_{deeper insight}

on these

_problems

can be reached

by

correlation measurements. In a

_previous

work on

the 160(160,12C)IONe

reaction

_[2]

we have shown

that the

_polarization

measurement of the residual nucleus was a

_stringent

test of the reaction models and the

2°Ne

_polarization

was found to be _very

strong.

In the _present

_experiment

we have measured

particle-particle

correlations in the four-nucleon

trans-fer reaction :

where the

16O*

levels are selected

by

the energy of

the associated

12C.

The aim of this work was :

-

to determine the

_alignment

of

the 160

residual nucleus in order to _compare it to

_previous

_experiments;

- to evaluate the

part

of

_ejectile

and double excitation in this

_reaction,

this

_{problem being}

still

open at this date.

2.

_Experimental

_set-up.

The

_experimental

_set-up

has been described in details in a

_previous

_paper

_[2].

We chose to detect the

12C

resulting

from

the 160

_decay

instead of the a for the

optimization

of the

_efficiency.

The

16O

7+ beam from the

_Orsay

MP Tandem is focussed

on 12C

_targets

(50

Ilg/em2)

and

_stopped

in a

Faraday

cup. The energy and the

position

of the

decay products

from the

16O*

excited nucleus are

obtained in a silicon

_position

sensitive detector

(PSD)

5 cm

long, placed

in

the

reaction chamber

7 cm _away from the _target. The

’2C

_particles

from the two

_body

reaction are

_{analysed by}

a n =

1/2

spectrometer

and are localized in two

_position

sensitive _gas counters which

_provide

the _ray

_{tracing [3]}

at the exit of the _magnet. The ion identification is

66

unambiguously

achieved with a

_AE,

E ionization chamber

_[4].

For each _event, nine

_{primary signals}

are

_generated,

namely :

Sl, S2,

S3,

S4 from the two ends of the

position

counters,

AE,

E from the ionization

_chamber,

E’ and P’ E’ the _energy and

_{position signals}

from the

_PSD,

and the TAC which

_operates

between

timing signals coming

from E’ and the first _gaz

counter. These

_signals

are

_{processed through}

a computer which

_provides

for several on line visua-lizations and

_puts

the data on _tape for an off line

treatment.

3. Results.

3.1 12C

ENERGY SPECTRA FROM

THE 12C(16O, 12C)16O

REACTION. - The

12C(160,

12C)16 0

reaction has been studied at 100 MeV incident _energy.

_Spectra

obtained for this reaction are shown in

_figure

la

and 2a. These

_spectra

exhibit a _strong

_selectivity.

The

_populated

states are members of the

Kn - 0+

_(6.05

MeV

_{0+, 6.92}

MeV

_2+,10.35

MeV

_4+,

16.3 MeV

₆₊₎

and K’ = 0-

(9.63

MeV

1-,

11.60 MeV

3 -,

14.67 MeV

_{5 - ) alpha}

cluster bands. The same states are

_{selectivity populated}

in the

(’Li,

t)

[5]

and

_{(12C, 8Be)}

_[1]

reactions.

The main

_problem

in the

_study

of the

_(16p,

_12C)

reaction is the

_{important overlap}

between the

160

ground

state wave function and

the 12C*

_(4.43

MeV

_{2 +)}

p a wave function. Thus the

_population

of this 2+

excited state is

_expected

to be

_large.

This

_peak

is

clearly

observed in the _spectra in

_spite

of the

_Q

mismatch. The

_{remaining problem}

is the contribution of this state

_{(12C* (2+))}

in the

_higher

excitation energy

peaks.

In

fact,

several excitation

energies

of the

_populated

states

_correspond

to a

possible

double

excitation. For

_example,

the 4+ state of

160

at

10.36 MeV can be mixed with the double excitation

peak coming

from 6.05 MeV

_{(0+ )}

of 160

+ 4.43 MeV

(2+)

of

12C

and so on.

3.2 CORRELATION _{MEASUREMENTS;} PROBLEM OF THE EJECTILE EXCITATION. - One

way to solve this

pro-blem is to observe the deexcitation of the residual nucleus in the

_outgoing

channel.

_Figures

lb and 2b

display

the

_outgoing

_{12C spectra}

measured with a

coincidence between

a 12C

at the exit of the _magnet and a

_{decaying particle coming}

from

the 160

residual

nucleus. The detailed

_analysis

of these _spectra

toge-ther with the bidimensional

_{energy-position}

_spectra

obtained in the PSD

_permit

to

_precise

the mode of deexcitation of each level under

study

and to evaluate the contribution of the double excitation in

_simple

spectra.

- Peak around 10.40 MeV.

Figure

3 shows in details the area in the _energy

spectrum

around 10.40 MeV. The

plot (1)

is the _energy

spectrum

gated by

a coincidence with a

_signal

in

the PSD. Plot

₍₂₎

is the one obtained with a window

put

around the recoil events

_(the

4.44 MeV

_{(2+ )}

Fig. 1. - a) 12C energy spectrum observed in the

12C(160,12C)16 0

reaction at 100 MeV _{incident energy. This}

spectrum

corresponds

to _{the first exposure of the magnet.} b) Same _spectrum but

_{gated by}

a coincidence between a 12C

event at the exit of the _magnet and an event in the

_position

sensitive detector set in the reaction chamber.

Fig.

2. - a) 12C _{energy spectrum} observed in the

12C(16O, 12C)16O

reaction at 100 MeV _{incident energy. This}

spectrum

corresponds

to the second _{exposure of the magnet.}

b) Saine spectrum but

_{gated by}

a coincidence between a 12C

Fig.

3. - Detail of the 12C energy spectrum in the area

around 10 MeV. See text to have

_explanation

for the different

plots

(1) (2) and _(3).

level of

12C

_{decaying by}

_y emission

_gives

a recoil

nucleus which is detected in the PSD and

_gives

a well

defined group of events at a

_{given angle}

and

energy).

Plot

_{(3) corresponds}

to a window

_put

on the

corre-lation

plot

16O*

10.35 MeV

_{(4 +) ---+}

a +

12C

cor-responding

to these

_three-body

kinematics. The first

_thing

to note is that

_{peak (3)}

well

_corresponds

to the difference between

_peak

₍₁₎

and

_{peak (2).}

This checks that there is no other

background.

The

_bump

observed in

₍₂₎

_corresponds

to the two

double excitation

_peaks

4.44 MeV

₍₂₊₎

1zC*

+

6.05

_{(0+ )}

MeV

_and

6.91 MeV

_{(2+ )}

160* .

These

peaks

are broadened

_by

_y

_Doppler

effect. The ratio of the double excitation cross section to the 4+

16O

state was estimated to be

- Peak around 14.7 MeV.

This broad

_peak

can be attributed to the 5 - state

of 160

at 14.67 MeV or

_(and)

to the double excitation 4.44 MeV

_{(2 +)}

12C*

+ 10.35 MeV

₍₄₊₎

160*.

One

can observe

(Fig.

2)

that the ratio of this

_peak

to

the

_peak

around 10.4 MeV is not the same in the

_single

spectrum

(a)

and in the coincidence _spectra

_(b).

This is due to the fact that the PSD was not set at

an

_angle

where the recoil nucleus

12C*

_coming

from

the double excitation could be detected.

The distribution of the

_particles

detected in the PSD

_{gated by}

the coincidence with the

_peak

around 14.7 MeV in the

_two-body

reaction

_spectrum

is shown in

_figure

4. The full line is the kinematical line

_{corresponding}

to the deexcitation

of

160*

in the two different

_sequential

reactions :

Fig.

4. - Distribution of the 12C and _a

particles

detected in the PSD and

_{gated by}

the coincidence with 12C at the exit of the _magnet,

_{corresponding}

to the

_peak

at 14.7 MeV. The full line is the kinematical line

_{corresponding}

to the deexcitation of

the 16a* in

_12q160,

12C*

_(2+»)160*

(4+) -+

a + 12C

(gs)

and

_{12C(160, 12C)160*}

_{(5-) ->} a + 12C*

(2+).

The dotted line is the

kinematical

line

corresponding

to the

deexcitation

of the 5- 160* state at 14.67 MeV.

12C(16O, 12C)16O*

(5)

68

The

_three-body

kinematics are

_exactly

the same

and the

_only

_way to

_distinguish

these two reactions would be to detect the

12C

recoil nucleus after _y emission in reaction

_(1).

Nevertheless when a

_gate

is

_put

on this correlation and the energy

spectrum

is

observed,

the obtained

peak

is broad

_{( ~}

1

_MeV).

This indicates that the reaction

_{(1) corresponding}

to the double excitation is

_largely

dominant.

The dotted line is the kinematical line

_{corresponding}

to the deexcitation of the

5 - 160

state at 14.67 MeV :

The contribution of this 5 - state was evaluated to

be

_only

30

_%

of the whole

_peak

around 14.6 MeV.

-

Peak around 16.3 MeV.

This

_peak

cannot contain _any double excitation

process. But there are two

_decay

channels for this

160(6+)

state,

namely :

These two channels were also

_{clearly distinguished}

in the correlation

_figure.

The first

_decay

channel is dominant and is evaluated to be 65

_%,

of the

_decay

mode. This result is in _agreement with that obtained

by

S. S. Sanders et al.

[1].

3. 3 CORRELATION FUNCTIONS AND ALIGNMENT OF THE

160

RESIDUAL NUCLEUS. - Correlation functions

between

12C

_(at

the exit of the

_magnet)

and

12C

(coming

from

the 160

_decay)

have been measured in the reaction

_plane

_e

=

-r )

for

a 12C

lab.

angle

of 100

2

with an _aperture of 1°. A

_sample

of correlation

func-tions obtained for some levels is

_displayed

in

_figure

5.

It is seen that for the

160

states

_decaying

to

a

+ 12C

_(gs),

rather

_regular

oscillations are observed.

Their number is

_equal

to the

_spin

of the level in the _(p range of 0° to 180°. This is not true for the 6+

_decaying

to a +

12C

(2+ ).

In this case the

shape

of the

corre-lation function is _very different with no

_regular

oscillations. This is due to the fact

_that,

_only

in the first cases, all the

particles

involved have

spin

zero

(with

the

_exception

of the residual

_nucleus).

In our

_previous

work it was shown

_{[2, 6]}

that the

population

of the different

_magnetic

substates could be extracted from the

_experimental

correlation func-tions. Two different methods can be used

[2].

In this work we used the extraction from the _average value of the correlation function.

_Only

the

_alignment,

that is to _say the

_sum,

_{I p112}

₊

_{I Pj- j 12}

can be deduced

from this method.

It is recalled

that,

for a level with a

_{given spin}

_3,

the correlation

_function,

in the reaction

_plane

_W(n/2),

Fig. 5. - 12C_12C

angular correlations measured in the

12C(160,

12C)16O* -+

a

+ 12C

at 100 MeV incident energy for

different

16O* states. The correlations are observed in

the reaction

_plane

₆

=

2

for a 12C lab. _angle of 10°.

averaged

over _(p can be written as

where WJM)

is the

contribution,

with

_weight

I pM

2

+

_{I p-M}

12

of the two

_magnetic

substates M

and - M to the _average value of the correlation function.

In table

_I,

the values of the

_partial

_average value

wjj >,

that is to _say the _average value of the

cor-relation function if

_only

the two

_magnetic

substates

M = _± J _are

populated,

_are

compared

_to the

experi-Table I. - Values

of

the calculated

_partial

_average

mental values for different

160

levels. It is seen that

the

_experimental

values are _very close to calculated maximum values. The results show the

strong 160

alignment

since the contributions of the

_magnetic

substrates

I M

I

= J _are

predominant.

One _can

esti-mate that

the 16O

alignment

is of the same order of

magnitude

that as that obtained for

20Ne

in our

previous

work,

that is to _{say an}

_alignment

of 85 ± 5

_%.

4.

_Summary.

The aim of this work was to

_study

the

_12C(16O,

12C)16O

reaction mechanism

_by

_{particle angular}

correlation measurements. The reaction exhibits a strong

selectivity.

The two

_alpha

cluster bands K = 0+ and K = 0- _are

preferentially populated.

The correlation measurements have allowed us

to obtain two results.

i)

The double excitation

_(12C*(4.43

_MeV,

₂₊₎

+

residual

_nucleus)

process is very

large

and contributes for 70

_%

to the differential cross section of the

_peaks

at 10.35 MeV and 14.7 MeV.

ii)

The

_alignment

of the

160

residual nucleus has been deduced and found to be _very

_strong,

_comparable

to that obtained for

2°Ne

in a

_previous

work

_[2].

So this

_strong

_alignment

seems to be a

_general

charac-teristic of such an a transfer reaction on

_light

_targets.

Acknowledgments.

Two of us

_(I.M.T.

and

Y.T.)

want to thank IPN for the

_hospitality

received

_during

their

_stay.

This work has been carried out in the frame of the collaboration

operated

under IN2P3 - Warsaw

_University

agree-ment.

References

[1]

SANDERS, S. J., MORTZ, L. M. and _PARKER, P. _{D., Phys.} Rev. C. 20 N. 5 ₍₁₉₇₉₎ 1743.

[2] POUGHEON, F., ROUSSEL, P., BERNAS, M., DIAF, F., FABBRO, B., NAULIN, F., PLAGNOL, E. and

ROT-BARD, G., Nucl. _Phys. A 325 ₍₁₉₇₉₎ 481.

[3] ROUSSEL, P., BERNAS, M., DIAF, F., NAULIN, F.,

POU-GHEON, F., ROTBARD, G. and ROY-STEPHAN, M.,

Nucl. Instrum. Methods 153 (1978) 111.

[4] NAULIN, F., ROY-STEPHAN, M. and _{KASHY, E.,} Nucl. Instrum. Methods 180 ₍₁₉₈₀₎ 646.

[5] BRODLOW, H. _{S., RAE,} W. _{D., FISHER,} P. _S.,

GOD-WIN, N. S., PROUDFOOT, G. and _{SINCLAIR, D.,} Nucl. _Phys. A 314 ₍₁₉₇₉₎ 207.