New experimental results supporting the hypothesis of the excitation of collective modes in the 40Ca + 40Ca reaction

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New experimental results supporting the hypothesis of

the excitation of collective modes in the 40Ca + 40Ca

reaction

H. Tricoire, P. Colombani, C. Gerschel, D. Paya, N. Perrin, L. Valentin, N.

Frascaria, J.P. Garron, C. Stéphan

To cite this version:

New

_experimental

results

_supporting

the

_hypothesis

of the

excitation

of

collective

modes in the

40Ca + 40Ca reaction

H.

_Tricoire,

P.

Colombani,

C.

_Gerschel,

D.

_Paya

_(*),

N.

Perrin,

L.

Valentin,

N.

Frascaria,

J. P. Garron and C.

_Stéphan

Institut de _{Physique Nucléaire,} B.P. n° 1, 91406 _Orsay, France (Re~u le 8 _fevrier 1979, accepte le 5 mars ₁₉₇₉₎

Résumé. 2014 Dans la réaction Ca + Ca à 400 MeV, nous avons mesuré la

_{multiplicité 03B3}

en fonction de _l’énergie des

_fragments.

Des paliers de _{multiplicité} ont été mis en évidence dans la région incomplètement relaxée en _énergie.

Leur

_dépendance

angulaire et leur _énergie sont la _signature de processus directs et _{suggèrent qu’ils pourraient}

être liés à l’excitation de modes collectifs de haute

_énergie

prévue

théoriquement.

Une discussion des valeurs de l les

_{plus probables}

_pour ces excitations est _présentée.

Abstract. 2014

Steps in the 03B3-ray

multiplicity

as function of the excitation energy have been observed in coinci-dence with the

_incompletely

relaxed part of _{the energy} spectra of fragments emitted in the reaction 40Ca + 40Ca at 400 MeV. Excitation

_energies

and _angular

_dependence

of these variations suggest that

_they

are correlated

with the excitation of _high energy collective modes

through

a _{rather direct process in the early stages} of the

reac-tion. l-values of this excitation mode are discussed.

Classification Physics Abstracts 25.70

Recent studies of

_fragments

emitted in the reac-tions

40Ca

+

40 Ca

at 284 MeV and

63CU

+

63CU

at 450 MeV

_[1]

have shown the existence of some

structures, unresolved _up to now, in the

_incompletely

relaxed _component of the _{energy spectrum.} These

structures

_(bumps)

were observed for

_fragments

of

Z and N not too different from the

_projectile.

In

the

40Ca

+

_40Ca.

_system,

_they

were found at a total excitation _energy of about 50

MeV,

and it was

shown

_that,

for a

_given

Z of the detected

_fragment,

this _energy did not

_{change appreciably}

either with the C.M.

_angle

or with the

_isotope

considered. The

angular

distribution of these structures was found

to be

_strongly

forward

_peaked,

which,

together

with the

_sharp

_{(Z, N)}

_{distribution,}

_implies

a rather

short interaction time between the ions. A tentative

explanation

for these structures seemed to be the excitation of collective modes in the

early

stage of the

reaction,

as

predicted

by Broglia,

Dasso and

Winther

_[2].

Confirmation of the structure at 50 MeV as well as some indication for another

_bump

at 80 MeV in some

_isotopes

has been obtained more

_recently

in

_Orsay

_[3]

for the same _system studied at 400 MeV.

(*) Permanent address : C.E.N. Saclay, 91190 Gif sur Yvette,

France.

In this letter we _present a

_study

of the

_{y-deexcitation}

of the emitted

_fragments

in the reaction

4°Ca

+

40Ca

at 400 MeV. At this

higher

energy, a

larger

_energy gap between the

quasi-elastic

component and the

completely

relaxed events allows a detailed

inves-tigation

of the

_incompletely

relaxed

_component.

The

y-multiplicity

of the

_fragment

deexcitation has been measured in all three

_regions

and some evidence

for sudden variations of this

_quantity

in the

incomple-tely

relaxed

_region

has been found.

The 400 MeV

4°Ca

beam was accelerated at the

Orsay

ALICE

_facility.

450

_Jlgfem2

40Ca

targets on a 10

_Jlgfcm2

12C

backing

were used. Contribution of

reactions due to the

_backing

was

_unimportant

in the

_region

of interest but was however subtracted

by comparing

the results with those obtained with

a 12C target.

Z identification of the reaction

_products

was made

_by

means of a solid state E

_{(200 Il) -}

AE

_(81l)

telescope,

which

_provided

an excellent

separation

between the different Z _up to Z = 28. Four

measu-rements were

_performed

at

9.5~,

10.5~, 13°,

25° in the

_laboratory

_(the

_{grazing angle}

is at ~

_10°).

_y-rays

were detected in coincidence with the

_heavy

_fragments

in three Nal detectors 18 cm _away from the _target. Two of them were in the reaction

_plane,

and the third one was in the

_{perpendicular}

direction. Both

single

E.AE events and four-dimensional correlated

L-182 JOURNAL DE PHYSIQUE - LETTRES

events

_{E, AE,}

_Ey,

T

_(where

T is the time difference between y-ray and

fragment detection)

were recorded

on _tapes and

analysed

off-line. The

comparison

of the time spectra at the three _y detector

_angles

show that the neutron contribution to the

_y-spectra

was

unimportant.

Energy

spectra and associated

_{y-multiplicities}

for

Z = 18 and Z = 19 _at

9.5°, 10.5°,

13° _are shown in

figures

1 and 2. For these Z

values,

as well as for

Z =

20,

the

energy spectra exhibit all three compo-nents

_{(quasi-elastic, incompletely}

relaxed,

completely

relaxed).

However,

results for Z = 20 _are less

signi-ficant since the

_huge

tail of the elastic

peak

at 9.5° and 10.5°

_spreads

_up to

_relatively

_high

excitation

energies

and is

_expected

to lower the

_multiplicity

values. On _part

_a)

of these

_figures,

the

_laboratory

kinetic _{energy spectra} at

_9.5°,

10.5° and 13° are

displayed.

The total excitation _energy, calculated

taking

in account the

_evaporation

of

_particles

_[4]

is also indicated. Since we had no A

identification,

all

isotopes

of a

given

element are summed in these spectra. Because of differences in

_Q-values

and small

differences in the

_position

of the structures for dif-ferent

_isotopes,

the

_bumps

are somewhat washed out

and do not show _up

_clearly.

_However,

the

incomple-tely

relaxed _part of the _{energy spectrum} behaves

quite differently

at the various

_{laboratory angles,}

showing

some evidence for a broad

bump

at the

most forward

_angles.

Moreover,

the

_experiment

of ref.

_[3]

leaves no doubt about their existence.

Fig. 1. - Part _a) Kinetic _{energy spectra (lab.)} of the Z = 18

fragment at 9.5°, 10.5°, 13°. The relative scaling of the number of counts is _arbitrary. Part _{b) Multiplicity} spectra at 10.5° and 13° (lab.). The arrows show the position of the structures observed in ref. _[3] in 37 Ar _{isotopes. Only} statistical uncertainties are indicated. The total _{excitation energy is also indicated.}

Fig. 2. - Same

as _figure 1 for Z = 19. The _arrow shows the

position of the structure observed in 4°K in ref. [3].

On _part

_b)

of these two

_figures,

the

_{corresponding}

multiplicities

(deduced

from the sum of events in the three

_NaI)

are shown at 10.50 and 130. Statistical uncertainties were too

_large

at 9.50 to allow an accurate

analysis.

Even at 130 statistics are not as

good

as at 10.50 but nevertheless the

_comparison

between the

two _spectra is instructive. The main features of these _spectra are the

following :

- Increase of the

multiplicity

from the

quasi-elastic to the

_completely

relaxed

_{region corresponding}

to an increase of the contact time and as a conse-quence to an increase in the transferred

_angular

momentum. -

The maximum value of the

_multiplicity

is obtained at a total excitation _energy of about 100 MeV and is not _very

_high

_{(My ~}

_6),

that is

_{My ~}

3 for each

_fragment.

Since

_only

a small number of

_y’s

is

emitted _per event

_(the

_{y energy} spectra associated with these Z values do not exhibit _any kind of _yrast

bump),

the

_angular

momentum carried away

by

y-deexcitation

is _very small

_compared

to the total transferred

_angular

momentum

_{( ~}

30 fii

_given

with

1 >

= 100 Ii

by -1 1 >,

evaluation valid _as the

system

is

_symmetric,

for both the

_rolling

and the

sticking

hypotheses).

It is clear

that,

due to the

_high

excitation _energy of the _system, a

_large

number of

particles

are emitted in this

region, carrying

away

with them most of the

_angular

momentum. We will

come back to this

_point

later.

-

Above about 100 MeV total excitation energy,

Fig. 3. -

y-ray multiplicity spectra at 10.5°, 13°, 25° (lab.) as

function of the total excitation energy, showing the variation of the centre of mass _angle with the laboratory angle and the excita-tion _energy.

be

_explained

_by

the

_increasing

number of

_evaporated

particles

and

_by

the variation of the centre of mass

angle

with the excitation _energy

_(see

_Hg.

3 at

10.50,

130,

250

_(lab.)

for Z =

18).

One should note

that,

in the

_incompletely

relaxed

_region,

the centre of

mass

_angle

is almost constant for a

_given

_laboratory

angle,

thus

_allowing

a direct

_comparison

of the

multiplicity

values.

-

The

most

_interesting

feature of the

_multiplicity

spectra

precisely

occurs in the

_incompletely

relaxed

region :

at

10.50,

the _spectra exhibit _steps centred

at a total excitation

energies

of 45 and 55 MeV for

Z = 19 and

35,

55 and 75 MeV for Z = 18.

Bet-ween each

level,

the variation of the

multiplicity

is

AM = 1.

Although

statistics _are not so

_good,

the

data at 130 seem to show that these steps

disappear

at

_{larger angles, indicating}

that

_they

may be the

signature

of a rather direct process. This process

could be the excitation of

_high

_energy states,

deexciting

in such a _way that a

large

excitation energy variation

corresponds

to an almost constant

_angular

momentum

transfer. It would be an indication that for some

correct _energy and

_angular

momentum

_matching,

high I-pole giant

resonances are excited. One should

note that some of the observed steps

approximately

correspond

to the structures observed in refs.

_[1]

and

_[3].

But whereas it is often difficult to extract

the

_bumps

from the

_{quasi-elastic}

or

_deep

inelastic

background, especially

when

_only

a Z identification

is

_performed,

the

_multiplicity

measurement allows a clean identification of

_phenomena

correlated with a

_given

value of the

_angular

momentum. Even if differences in excitation

_energies

and

_Q-values

for the different

_isotopes

smear the _{energy spectrum,}

they

are not

_likely

to wash out the

_multiplicity

spec-trum, since the I values of the excited

multipoles

are smaller than the I values associated with the

back-ground

and not

_expected

to

_depend

_very much on the A of the

_fragment.

It is

_interesting

to _get an evaluation of the l-value

of the

_possibly

excited

_multipole.

_y-rays are the ultimate _stage of the

_{deexcitation,}

_following

_particle

emission. In a _system

_symmetric

in the entrance

channel and for a

binary

_process, the centre of mass cross section should be

_symmetric

around Z = 20

if there were no

_evaporation

from the

_fragments.

Thus for _any

_given

excitation _energy, our

_experiment

allows the _{determination of the average number of}

charges

AZ emitted in the deexcitation and of the variance _(Jz of the Z distribution. For the N

distri-bution,

AN was evaluated from ref.

_[1]

as well as

from the ALICE code

predictions [6],

and included in the

_evaporation

calculations. In our

_experiment,

in the

_region

of interest for collective excitations

(40

MeV

_Ex

75

_MeV)

AZ has been measured

to _vary

_linearly

with the excitation _energy between

1.2 and

_2.4,

_{and (Jz}

between 1.5 and 3. AN has been taken as

_AZ/(2.3).

This means that the _{steps observed}

in our

_experiments

in Z =

18,

19 _are the

signature

of excitations in the Z =

19,

20 _or Z = 21 initial

fragments.

Thus,

the

_giant

_resonances, if such is the observed

_phenomenon,

would be

_mainly

excited in the

4°Ca

_itself,

with the

_possibility

of a

_subsequent

particle

transfer of one or two units

_(protons

or

neutrons).

To evaluate the total

_angular

momentum

associated with the steps,

particle

emission from both

_fragments

has to be taken in account. Since initial

_angular

momenta involved are small

_{.( ~}

5 ? for each

_fragment),

an evaluation of the

_angular

momentum removed _per emitted nucleon is between 0 and 1 ?

_[7].

_y emissions _{carry away}

_angular

momenta

between 1 x

_My

_(dipole

emission

_only)

and 1.8 x

_My

(dipole plus quadrupole emission)

[8].

With these

assumptions,

the total

_angular

momentum windows

corresponding

to the _steps in Z = 18 _{are :} 4/! _to

9 ? for the 35 MeV

_step,

5 ? to 13 /! for the 55 MeV

step,

6% to 161ï for the 75

_{MeV step.}

Let us now

consider how this total

_angular

momentum is shared between the

_fragments.

Two cases can be

distin-guished :

a)

In the direct _process,

_only

one Ca

nucleus is excited in a

_giant

resonance mode. Then

the total

_angular

momentum measured

_corresponds

to the l-value of the excited

_multipole.

The values of I obtained in this

_hypothesis

are _very

_high

but not

unreasonable. For

_example

Liu and Brown

_[9]

have calculated that the isoscalar

_hexadecapole

_giant

’resonance in

4°Ca

has a I = 4 _{component at an}

L-184 JOURNAL DE _{PHYSIQUE -} LETTRES

of the _energy

_weighted

sum

_{rule ;}

_b)

If both Ca nuclei are

excited,

then the I windows

_{corresponding}

to the _steps are 2 ?-4.5 ? for 35

MeV,

2.5 ? to 6.5 ? for 55

_MeV,

3 ? to 8 ? for 75 MeV. In this case,

the _energy of the resonances would be half the total excitation

_energies.

It seems that the l-values extracted on the

assumption

of mutual excitation of the two Ca nuclei are more reasonable when

_compared

to known

experimental

(isoscalar

quadrupole

resonance at

18 MeV

_[10])

and theoretical

(isoscalar octupole

resonance at 33 MeV

_[9])

evidence. This remains rather

_puzzling

since such mutual excitation would be

_expected

to be much less

_probable

than

_single

excitation. It is worth

_noting

however that the

extract-ed

_angular

momenta are

_{probably slightly}

overesti-mated,

since

_{they correspond}

to _average

multipli-cities

_{(including background multiplicity).}

In

_conclusion,

the variations of the

_multiplicity

observed in this

_experiment

_support

the

_hypothesis

of the excitation of

_high-energy

collective states

during

heavy-ion

collisions,

through

a rather direct

process. The measurement of the _{y -ray}

_multiplicity

proved

to be a sensitive tool for

investigating

details of the nuclear structure and variations of

_angular

momentum at a

_relatively

low excitation _energy, where little

_{particle evaporation}

takes

_place.

Limits on the I value of the excited

multipole

have been obtained. At

_higher

excitation

_energies,

where more

particles

are

emitted,

the _y-ray

multiplicity

is

clearly

not sufficient to determine the initial

_angular

momen-tum

_[11]

_]

in such a low A

region

of nuclei. As indi-cated

_above,

in the course of the same

_experiment,

more detailed information on

particle

emission has been obtained and will be

_published

later.

Acknowledgments.

- We would like

to thank A. Winther and L. G. Moretto for continuous interest in this work and _many fruitful

_discussions,

M. Leblanc and Y.

_Sugiyama

for valuable

_{help during}

the expe-riment.

References

[1] FRASCARIA, N., STÉPHAN, C., COLOMBANI, P., GARRON, J. P., JACMART, J. _{C., RIOU,} M. and TASSAN-GOT, L., Phys.

Rev. Lett. 39 (1977) 918.

[2] BROGLIA, R. A., DASSO, C. H. and WINTHER, A., Phys. Lett.

61B ₍₁₉₇₆₎ 113.

BROGLIA, R. A., Lecture to the Nuclear Study Summer School,

Dogashima, Aug. 28-Sept. 1 ₍₁₉₇₇₎ to be published. [3] FRASCARIA, N. et al., Communication to the workshop on

High Resolution _Heavy Ion _Physics at 20-100 MeV/A,

Saclay, May 31-June 2 (1978), C. Volant, editor and to be _published.

[4] TRICOIRE, H. et al., To be published.

[5] BROGLIA, R. A., DASSO, C. H., POLLAROLO, G. and

WIN-THER, A., Phys. Rev. Lett. 41 ₍₁₉₇₈₎ 25.

[6] PLASIL, F., Report ORNL/TM 6054.

[7] WILLIAMS, D. C. and THOMAS, T. _D., Nucl. Phys. A 92 (1967) 1. [8] LIOTTA, R. J. and _SORENSEN, R. _A., Nucl. _Phys. A 297 ₍₁₉₇₈₎

136.

[9] LIU, K. F. and BROWN, G. E., Nucl. _Phys. A 265 ₍₁₉₇₆₎ 385. [10] BERTRAND, F. E., Ann. Rev. Nucl. Sci. 26 ₍₁₉₇₆₎ 457.

[11] NATOWITZ, J. B., NAMBOODIRI, M. N., KASIRAJ, P., EGGERS, R., ADLER, L., GONTHIER, P., CERRUTI, C. and ALLEMAN, T.,