Collective excitations of spin-isospin modes

187~

Nuclear Physics A488 (1988) 18’7c-202~ North-Holland, Amsterdam

COLLECTIVE EXCITATIONS OF SPAN-ISOSPIN MODES Michele

ROY-STEPHAN

Institut de Physique

Nucleaire,

B.P. not, 91406 Orsay,

Cedex,

France

The response of nuclei to spin-isospin excitation which is displayed through heavy ion induced charge-exchange reactions, is concentrated in two excitation energy domains. At low excitation energy, several particle-hole states contribute : the Gamow-Teller resonance and higher multipolarity spin-flip resonances. Around 300 MeV, a nucleon from the target is excited to a a resonance. On an average, the strength in the A sector has the same order of magnitude as in the nuclear sector. In detail, the A excitation strength depends on the nuclear structure of the projectile-ejectile pair. The peak corresponding to A excitation in nuclei is energy shifted from the peak of the free A created in the reaction on hydrogen.

1. INTRODUCTION The spin-isospin dominated

interaction

by genuine

the finite

is the only nucleon-nucleon

one meson exchange

range and tensor components

and of the effective

interaction

isospin correlations

build collective

giant resonance

modes are a propitious channel,

ed. The nucleon excitation excited

of the free nucleon-nucleon

in the spin-isospin states,

nuclear

to observe

interaction

In nuclei, spin-

the Gamow-Teller

itself can be excited

is concentrated

in this channel,

pionic degrees

and nucleonic

degrees

In the

are deeply

link-

centered excited

one is the first

which has spin S = 312 and iso-

at 1232 MeV with

state which

the spin and the isospin of one quark without of the quarks.

of freedom

too. The strongest

the A resonance

it is the nucleon

spin-isospin

of freedom'.

: the nucleonic response to spin-isospin

in resonances

spin T = 3/Z and mass distribution

guration

channel.

for example

role of the 71 meson

ground

state of the nucleon,

quark formalism

is

(AL = 0, AS = 1, AT = 1)'.

In view of the crucial

spin-isospin

channel which

: one pion exchange largely determines

It may be considered

120 MeV width.

is obtained

any change

by flipping

in the spatial

as the Gamow-Teller

In

excited

confistate

of the nucleon. In nuclei Therefore

the spin-isospin

lable energy exchange

around

are coupled

carry virtual

demonstrate

into a A resonance

excitation.

A-nucleus

037L9474/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

to A-hole

A excitation.

a real A can be excited.

f GeV per nucleon

from the target

to spin-isospin

excitations

vibrations

is large enough,

reactions

one nucleon response

the U.T particle-hole

states3.

If the avai-

Results from chargethat the excitation

of

is a major

part of the nucleus

attraction

has been observedwith

M. Roy-Stephan

188c these reactions.

The collective

/ Spin-isospin

character

modes

of the spin-isospin

excitation

in the

A region will be discussed. Results program and

from the Saturne

includes

charge-exchange

experiments

program

will be presented.

on (3He,t) and (d,2He) between

1 GeV per nucleon, and heavy ion charge-exchange

nucleon

various

incident

surements,

which

of original

particles

dity spectrometer,

features

(polarized

of the Saturne

particles,

SPES 4. This spectrometer

400 MeV per

is essential

in studying

recalled. altogether

2.LIGHT

a consistent

picture

for example,

heavy ions) and a high rigi-

peripheral

We will see that these reactions

facility,

is well suited for 0 degree mea-

We will focus on heavy ion charge-exchange

reactions.

but light ion results will also be exhibit

of spin-isospin

common features

excitations

and give

in nuclei.

ION CHARGE-EXCHANGE

2.1. ("He,t) reaction

from 600 MeV to 2.3 GeV

Angular

from (3He,t) reaction

distributions

been measured.

The strength

well known matrix

elements

of selected

on H and on several

Fermi and Gamow-Teller

have been measured

It has been demonstrated

per nucleon,

the spin-isospin

nuclei

have

transitionswith

in order to check the interac-

in charge-exchange

that

part of the interaction

(fig. 1). Using 2 GeV 3He, spectra energy

between

and 1100 MeV per nucleon.

It takes advantage

tion4.

This

200 MeV per nucleon

have been recorded

reaction

above 200 MeV

is highly dominant up to 600 MeV excitation

(fig. 2).

They show that the spin-isospin excitation

energy

and L = 2 spin-isospin mechanism

is concentrated

At high excitation

quadrupole

energy,

into a R resonance This twofold

to the excitation

giant resonance,

on a nucleon

the A mass and the nucleon

around

it corresponds

: the L = 0 Gamow-Teller

exchange

of nuclei

in two

energy domains.

At low excitation states

response

resonance,

the L =

of particle-hole

1 spin-isospin

dipole

and also to a quasi-elasticcharge

of the target.

around

mass,

300 MeV, which

it corresponds

is the difference

to a nucleon

between

being excited

in the target.

spectrum

is a common

1 GeV and even higher

feature

of all charge-exchange

7,9 . It illustrates

that the nucleon

reactions and the

A are

two states of the same particle. In the reaction created. angular

on hydrogen,

This reaction distribution

above

1.5 GeV incident

is a test of our understanding

is quite well reproduced

energy,

the free

A++ is

of the mechanism.

when assuming

TI exchange8.

The

189c

M. Roy-Stephan / Spin-isospin modes

-IT

I

20 -

(3He,t) 2 GeV

.I5 -

I 200

ENERGY

I

I

400

600

_A 800

PER NUCLEON

(MeV)

TRITON

FIGURE 1 Ratio between u.~ and T interaction as measured bv (o.n) and (3He,t), with predictions based on im ulse.approximation (continuous curve) g and on a G matrix calculation (dotted curve)c.

(3He,t) has revealed charge-exchange studied

reactions

up to now

When comparing observes

an intriguing around

effect which,

1 GeV per nucleon

the energy

spectra

obtained

the position

the reaction Twoeffects

For example

with different

to explain

of the peak corresponding is created

one

to A excita-

in the reaction

on

is, withinuncertainties,

triton energy corresponding

(fig. 3), 70 MeV to the A peak from

on 12C.

this 70 MeV shift

: the projectile form factor

effects.

- Form factor effect

:

Both peaks are moved towards form factor which decreases ly with energy

to all

that have been

targets,

in (3He,t) at 2 GeV and 0 degree

on H and the reaction combine

and medium

is common

and above,

For target nuclei with A > 12, this position

shift is found for the outgoing

effect

in fact,

("he,t) at

: (p,n), (d,2He), (3He,t) and heavy ion induced ones.

a shift between

A independent'.

(GeV)

FIGURE 2 Triton energy spectra from 2 GeV on 12C and "Ca.

tion in nuclei and the peak of the free A which hydrogen.

ENERGY

transfer

lower energy

by the effect of the 3He

very rapidly with momentum

transfer

: the free A peak maximum corresponds

of 1192 MeV instead of 1232 MeV as observed case of A in nuclei,

transfer

the form factor moves

larger width due to Fermi motion.

Assuming

tion on the proton and for the reaction

in IT nucleon

and consequent-

to a mean A mass

scattering.

the peak even more because

In

the

of its

the same form factor for the reac-

on nuclei,

and taking

the same binding

M. Roy-Stephan / Spin-isospin modes

19oc

energy for the experimental

70 MeV.

Distortions advocated

A and for the target nucleon, one explains 35 MeV out of the of the form factor by the target nucleus

recently.

But it turns out that this effect

simpler model based on eikonal tion effect".

Experimentally

dependence

attributes

the invariant

tion on proton and on nuclei, transfer

approximation

have been strongly 10 . A is weak in DWBA 7 MeV to the distor-

cross-section

of the (3He,t) reac-

from C to Pb, have exactly

the same momentum

in the whole domain where we have measured

the angular

distributions'. Therefore applying

it is realistic

- Medium effects : A-nucleus attractive exchange model'*

to think that distortions

a mere normalisation

reactions predicts

may be accounted

for by

factor.

interactions

to play a role in charge-

are expected

and therefore

contribute

mean field effects

to the observed

according

to which,

shift. The n-hole

the

A in a nucleus

probed by real or virtual II should have a mass 30 MeV lower and a width 80 MeV Moreover

larger13.

calculations

of the longitudinal

spin-isospin

response

pre-

dict strong attractive correlations, which at normal density could produce a 14 (around 150 MeV). The effect of correlations should decrease very big shift at the surface of nuclei where the density From IO to 30 MeV out of the experimental of correlations

is smaller

at the surface of nuclei where

ISo state. This guarantees

tion is of (n,p) type. The mechanism actually

be measured.

an example

no (?i,$) facility

ponding

to

spin flip occurs.

of B+ strength

used in order to measure as a spin transfer

were performed,

they

This reaccan

in 54Fe is shown as

investigation

(?,$) reaction.

of giant spin-isospin

energy,

A excitation

A excitation

in nuclei

A excitation.

measured

for the outgoing

'He energy

reaction

on H and the reaction

on "C.

(z,2He)

powers,

Since the first

started at Triumf, is a very valuable

but tool

modes.

is observed.

is energy

For example

tensor analysing

(n,p) experiments

exists up to now. Therefore

At high incident

to the free

are detect-

therefore

being well under control,B+strength

The first measurement

is as informative

(d,ZHe) measurements

for detail

that

energy,

(fig. 4).

Tensor polarised dare wtich

pair. Both protons

in SPES 4. They have a very small relative

are in the singlet

density.

to the effect 11,13,15 . takes place

the reaction

2.2. (d,'He) reaction from 650 MeV to 2 GeV 11 In (d,"He) reaction , the ejectile is a proton ed together

than the central

shift may be attributed

Here too, the peakcorres-

shifted from the peak corresponding

at 2 GeV and 0 degree, corresponding

to the

65 MeV shift is

A peak from the

M. Roy-Stephan

/ Spin-isospin

191c

modes

54Fe(d,2He) Ed=650 MeV

I

1400

2000

1800

1600

FIGURE 4 0 and 2 degrees spectrum of (a,2He) on 5"Fe at 650 MeV. The resolution is 1.2 MeV.

FiGURE 3 Energy spectra of the outgoing triton from (3He,t) reaction at 2 GeV 0 dearee on H, 0 and "C.

Through

tensor analysing

power measurements

it becomes

the contribution

of the mesons which may be exchanged,

On the contrary,

cross-section

cellation

between

proceeds

through TI exchange

can be neglected. quasi-elastic repulsive

measurements

r and p occurs.

and the quasi-elastic

which

? Several

is related

3. HEAVY

i.e. the TI and the p. since a can-

data show that A excitation

measurements

show medium

region

is still a bit puzzling.

theoretical

in the

of short range

measurements

in the A

How do they reconcile

which could be responsible works are in progress

to the very fundamental

effects

The importance

shows up from the polarization

with the long range attraction regions

The polarisation

to determine

alone with a short range cut. The p contribution

Tensor analysing

which

possible

alone are ambiguous,

peak region and in the A region.

effects

(MeV)

&

TRITON ENERGY (MeV)

for the shift in both

on this important

nature of nuclear

problem

forces.

ION CHARGE-EXCHANGE

The heavy

ion charge-exchange

isospin

response

nucleon

gave.evidence

of nuclei.

appears

as a novel tool to study the spin-

Charge-exchange

of the A excitation

experiments in nuclei

around

1 GeV per

by means of heavy ions

22

.

192~

M. Roy-Stephan

At high enough

incident

on the nuclear

structure

Due to absorption

energy,

/ Spin-isospin

this excitation

of the projectile

may be very strong,

the reaction

question

channel

is very peripheral.

is probed, one may think that collective

effects will be wiped out. On the other hand IT exchange fore in the spin-isospin

depending

and the ejectile.

in target and projectile,

As the very surface of the nucleus

modes

collective

is long range. There-

modes might still be excited.

This

will be addressed.

Another

interesting

spin modes

feature

of heavy ions is the ability

in two (p,n) and (n,p) type channels,

will discuss

whether

the reactions

to excite

spin-iso-

with the same projectile.

involved with a given projectile

We

are actual-

ly isospin symmetrical. Several models

have been developed

They can be checked

DWBA and coupled

channel

show that two-step become negligible

by comparison

equation

mechanisms

140 is interpreted 22

applied

to subthreshold

to the experimental

calculations

may be important

above. This is in agreement

in 14N induced charge-exchange

energy

heavy ion charge-exchange

Some of them are further

cross-section17'18y20. duction.

to predict

presented below

21

at thisconference

120 MeV per nucleon

with our experimental

at 900 MeV per nucleon

as a proof of pure direct

TI pro-

results.

where the absence

one step mechanism

and

findings of

at this

.

3.1. Experimental

method

Heavy ion beams up to '"Ne have been available 1987 the new injector ing, together 6 IO8 "C, 23 energy .

of Saturne,

with a new EBIS source,

IO8 *'Ne and 5 IO' "Ar

Systematic

measurements

at LNS since 1984. In October 23 has started operat-

the Mimas Synchrotron

Diane. The current

pulse, i.e. every

have been performed

intensities

are

1 to 3secondsdepending

on

with '*C, I60 and *'Ne beams at

900 MeV per nucleon and with l*C beam at 1100 MeV per nucleon on the following natC targets H, D, , *'Y and natPb. The H and D spectra have been measured with CH2 and CD2 targets. The ejectile trometer

was identified

and momentum

SPES 4 at O"24. Two independent

scintillators

provided

Mass identification

ion charge

was performed

basis with 300 ps resolution. chambers25.

The resolution

The momentum

resolution

the horizontal been measured

AE signals,

identification

by using the magnetic derived

Ray-tracing

was performed

in the impact location

AZ/Z = 0.035. on a 17 m long

using two sets of drift

on each plane was 0.3 mm.

was 7 10s4, and the resolution

angle was 2 mrad. The integrated

14 mrati x 14 mrad aperture

spec-

from 1 cm thick

with resolution

by time of flight measurement

of the spectrometer

scattering with

analysed

collimator.

cross-sections

in have

M. Roy-Stephan

In this very peripheral peaked. cepted

/ Spin-isospin

the angular

process,

modes

193c

distribution

From 85 % to 100 % of the total cross-section

is sharply forward

is expected

to be inter-

in this opening.

Very preliminary

angular

distribution

measurements

have been carried

out by

using ray-tracing. The momentum complete

acceptance

spectra

was determined

of the spectrometer

could be recorded

by carbon

activation

measurements.

uncertainty

is or 20 %. The 12C monitoring

P(12C, "N)n

measurement

It is important detected

with the

to notice

the projectile

focal plane.

cannot be observed

3.2. (12C, 12N) and (l'C,l'B) 3.2.1.

Experimental

Fig. 5 shows

kinetic

transferredenergy, spectra

cross-section

by comparing

results

our

at 800 MeV per nucleon

26

.

must be bound in order to be

In particular

the A excitation

in these experimental

in

conditions.

at 900 and 1100 MeV per nucleon

results

12C(12C,12N)

On x axis is the energy the ejectile

12C(p,n)12N

the

The beam flux

The absolute

is checked

that the ejectile

in the spectrometer

is f 3.5 10S2. Therefore

in a single field setting.

and 12C(12C, 12B) spectra at 900 MeV per nucleon.

transfer

energy.

on the collimator

for the same reactions from

kinetic

On y axis is the cross-section

integrated

Fig. 7 shows spectra

w, i.e. the projectile

aperture.

energy minus

per unit of Fig. 6 shows the

at 1100 MeV per nucleon.

(12C,12N)

at 1100 MeV per nucleon

on 12C, "'Pb,

and on the proton. Like in (3He,t) and (d,ZHe),the energy

domains

strength

- At low w. that is, at low excitation corresponds

to particle-hole

ly in the projectile.

The Gamow-Teller

ed, even in the ground

(The ground-state

excitation

very rough estimate

of transverse

angular

momentum

peak in '*C(12C, 12N) reaction to 30 % to the spin dipole

- Around

transfer

is expected

from several

since the experimental

to be dominant

at

states may be mixresolution

in target and projectile

based on (3He,t) spectra

transfer corresponds

is

corresponds

to

to the Gamow-Teller

resonance

and 10

The tail of this peak on the

side is due to the quasi-elastic energy,

in the same range

tells that about 8 % of the nuclear

and spin quadrupole.

300 MeV in excitation

(A N-l) states.

the sharp peak

in the target and possib-

on "C).

For example,a

high energy

in two excitation

in the target,

resonance

but contributions

state region

u = 31 MeV in the reaction

energy

(NN-l) states excitation

the very end of the spectra,

16 MeV.

is concentrated

:

mechanism.

the broad peak corresponds

to A-hole

194c

M. Roy-Stephan

/ Spin-isospin

modes

r

E(12C)

- E(ejectile)

_c_--I

1100 MeV

E(l’C)

(GeV)

- E(ejectile)

(GeV)

FIGURE 5 FIGURE 6 0 degree cross-section integrated over Same as fig. 5 but the incident 14 x 14 mrad, versus energy transfer, gy is 1100 MeV per nucleon. for the reactions (12C,12N) and (l'C,l'B) at 900 MeV per nucleon on 12C.

Two effects

occur when the incident

energy

per nucleon

: P(~'C,~'N) and 12C(12C,12N)

the nuclear

sector and on the contrary at both incident

In (12C, “B)

from 900 to 110.0 MeV

cross-sections

increase

energies,

decrease

by 20 % in

by 35 % in the A sector.

a large shift is observed

free A++ peak to the peak for A excitation per nucleon

increases

in nuclei.

For example

the peak for A in '*C is 80 f 20 MeV lower in energy

i.e. 40 +I0

MeV for 12C and 60 * IO MeV for "*?b

nucleon.

But one should know that the experimental

position

is large. Preliminary

both channels, noticeable.

transfer w

distribution

angular

(see the following

at 1100 MeV per

measurements

distribution

discussion

on the mass

in the A0

the shift exists at every angle where A excitation

As expected

p(12C,12B)A++ 3.2.2.

angular

uncertainty

from the

at 900 MeV

than the free A peak. In (12C, 12N) the shift is smaller and depends of the target

ener-

peak

show that, in is

on form factor)

is rather flat.

Discussion

The heavy ion charge-exchange

reaction

can be described

by a coherent

mecha-

nism where the projectile excitations sections

.

and the target undergo collective spin-isospin 20,za This model explains very well the experimental cross-

in the nuclear

and in the A sector for the reaction

(12C,12N)

on the

M. Roy-Stephan

proton and on I%. agreement tivity

For A-hole

is achieved

/ Spin-isospin

and for NN'l particle-hole

with TI exchange

to some ingredients

19%

modes

excitation

the best

alone and a short range cut. The sensi-

of the model

is illustrated

by the spectra

on fig.

5to7:

- Nucleon-nucleon The decrease

interaction.

sector when the incident predictions

and 12C(12C,12Nf

of pf12C,L2N)

energy

based on nucleon

in the nuclear

agrees with impulse approximation 28

increases,

nucleon

cross-section

phase shift analysis

.

- Projectile form factor. Only one state of the l*N ejectile which corresponds Concerning ground

of 12C(p,p')12C*

The 2' state excitation

tation

in the "8

excitation

energy

regions

of the transverse corresponds

the four-~mentum is respectively

because

transfer 1.07 and

It is no of

The 2+ state excirole in the two

momentum

to q,=

transfer

: for example the

1.4 fm-l at 900 MeV per nucleon.

corresponding

momentum

transfer

to 300 MeV excitation

: for example

energy

.94 fm-l at 900 and 1100 MeV per nucleon.

the more shifted,

in form factor

damped

qualitatively

in 12C,

Around

is very steep and the L = 2 form factor

the form factor

the A peak. The difference between

for q < 0.5 fm".

play an appreciable

of the longitudinal

1 fm-' the L = 0 form factor flat. The steeper

.

:

1) At low w because

2) In the A region,

is negligible

state contribution.

may therefore

collimator

aperture

from the angular 26,27

for q > 0.9 fm-l the contribution

than the ground

ejectile

The

at 800 MeV

should have the same cross-section.

true for q > 0.5 fm-l. Moreover

the 2+ state is bigger

reactions

12B ejectile

: the

L r_:2 transition.

may be deduced

and "Cfp,n)'*N

in the

to contribute

and a 2+ level at

a spin-isospin

of the L = 0 and 1 = 2 transitions

In this domain both channels longer

through

state I+, T = I,

with L = 0.

is the 12N ground state analogue,

state which

distributions

the ground

transition

12B bound states are expected

(12C,12B)rtwo

0.95 MeV, which may be excited form factor

is bound,

to a pure Ga~w-Teller

rather

and narrow will be

explains

the difference

(12C , '*N) and f12C,12Bf spectra on fig. 5 and fig. 6. The steep rise 26 form factor when momentum transfer decreases, quantitative-

of the (12C, "Nf ly explains 12C(12C,12N)

the increase when

of A excitation

incident

energy

cr;;s-section

increases

in p(12C,12N)

and

.

- Absorption Up to now there is no marked exchange.

Of course

difference

the absorption

normalization

factor

between

cross-section

is the following

between

is stronger

plane wave calculated for different

light and heavy ion charge-

with heavy

ions. For example

and experimental

charge-exchange

the

0 degree

reactions

on12C

:

196c

M.

Roy-Srephan

/

Spin-isospin

modes

0.46 in (p,n) at 800 MeV, 0.31 in (?!,ZHe) at 2 GeV, 0.21 in (3He,t) at 2 GeV.

In (12C,12N)

900 MeV per nucleon the normalisation

at

ed cross-section

is 0.07 in Glauber

(12C + "C)

comparing

charge-exchange

theory

20

and (12C + p) experimental

occurs for grazing

cross-sections.

The heavy ion

nuclei when the impact parameter

to the sum of both radii and the overlapping central

factor for the integrat-

; it is estimated to 0.09 by

densities

around

is close

10 % of the

density.

- Target nuclear

structure.

There is no Pauli blocking

for a A in a nucleus,

form the target may participate excitation

cross-section

ed by absorption. effects

sector

on the proton and on nuclei for protons

same normalisation

(taking

energy

all the nucleons of the A

should be dominat-

nuclear

structure

peak). An empirical

the A excitation

into account

cross-sections

isospin Clebsch-Gordan

For a given target nucleus we find the

in both channels, of this method.

the target are the following

important

by comparing

and neutrons).

factor

proof of the consistency

an uncertainty

one expects

(low excitation

factor can be deduced

coefficients

therefore

The evolution

with the mass of the target nucleus

On the contrary

in the nuclear

absorption

to A excitation.

(l*C,l*N)

and (12C,12B).

The number of effective

: 2.3 in "C,

nucleons

region.

In order to see a departure sector we have calculated

(for the (p,n) channel)

from quasi-elastic

the cross-section

or per effective

shows these cross-sections

per effective

from

3.8 in 8gY and 4.3 in 2n8Pb, with

of 20 % at least. Now we turn to the low excitation

nuclear

This is a

excitation

per effective

energy in the neutron

proton

(for the (n,p) channel).Fig. 8 l/3 nucleon, versus A at 900 MeV per

nucleon. The 2+ state in I28 is responsible channels elastic

(as discussed mechanism

to the estimated in the target.

in form factor

contribution

that the 3(N-Z)

Gamow-Teller

resonance

spin quadrupole excitation "*Pb

In this picture

both

the quasi-

for the excitation

the Gamow-Teller

quenching

cross-section

the results are plausible.

will be possible

the

of the spin dipole and

for the cross-section

In the (n,p) channel

analysis

in *'Y and 208Pb

In spite of neglecting

and the excitation

which may be important,

A detailed

8 "6 of the low excitation

sum rule is exhausted.

in the target may account

be measured.

between

of A. The shaded area corresponds

based on (3He,t),i.e.

in the (p,n) channel.

effective.

difference

of the Gamow-Teller resonance is For 12C we have taken the calculated value of 3 ub which

peak. We have deduced

assuming

subsection).

is independent

cross-section

agrees with our estimate energy

for a trivial

increase

Pauli blocking when angular

Gamow-Teller from 12C to

obviously

distributions

is wi?l

M. Roy-Stephan

/ Spin-isospin

modes

197c

1100 MeV per Nucleon

It 40

20

I

Ip .3ooo

.O

- E(“N)

(GeV)

.6ooo

E(*‘C)

0

FIGURE 7 Same as fig. 5 for (l*C,l'N) on p, '*C, and *"'Pb at 1100 MeV per nucleon. Please note the scale change between P(12C, "N)n and P(~*C,~*N)A”. 3.3. (*'Ne,*'F)

(*'Ne,*"Na)

Fig. 9 and fig. IO display (*'Ne,*'Na) Around

on H,"C

I

I

P

1%

E9Y

I

*l/3

‘08 Pb

transfer

at 900 MeV per nucleon spectra

from

(*'Ne,*'F)

and

and *'*Pb.

1 GeV per nucleon,

(*'Ne,*'F)

I

FIGURE 8 Cross-se tion per effective nucleon versus A'iI3 for (l*C,'.*N) and (12C,12B) reactions at 900 MeV per nucleon.

and (160,16N) energy

t

we observe

the A excitation

is strong

in both channels.

In

70 + 10 MeV shift from the free A peak to the peak for A

in A > 12 nuclei. In ("Ne,*' Na) on heavy targets, ture. The background,

A excitation

excitation

can be isolated

we observe

that, above

'*C the position

the target

in contrast

to (*'Ne,"F).

A dependence

is actually

the dominant

if any, under the A peak is very low. Therefore with almost

no uncertainty.

In (*'Ne,*'Na)

of the A peak depends

Two explanations

spectra,

on the mass of

may be proposed

: it could be some medium effect, exhibited

fea-

the A

for this

in (n,p) but hidden

hf. Roy-Stephan

19s

.#oo

- E(“F)

E(20Ne)

(GeV)

by the tail of the low energy

- E(20Na)

(GeV)

FIGURE 20 0 degree cross-section integrated over 14 x 14 mrad, versus energy transfer, for the reactions j20Ne,2 Na) at 2:: PV per nucleon on H, "C and P.

FIGURE 9 0 degree cross-section integrated over 14 x 14 mrad, versy; energy transfer, 'OF) at 900 MeV i:F ,"c,'l~?%~~~,' N"C and zasPb.

of (n,p) reactions

modes

.D

.3ow

E(20Ne)

/ Spin-isospin

peak in the (p,n) channel

- it could be specific

and related to the isospin dependence

of the A-nucleus

interaction. For a given target, A cross-section this excess analogue

to spin-isospin

of which are unbound

At 400 MeV per nucleon However

beyond q = 0.9 fm-'

~mentum

transfer

to four states In (160,16N)

We attribute

in (20Ne,20F).

is above threshold

might be explained

for the reaction

by the form factor

: A excitation would require 1.5 to 2 fm-' four

at 400 MeV per nucleon.

This reaction

can be studied

likely proceeds

in 16N described

the energy

is 70 f: 15 MeV.

has been observed

energy

disappearing

With 160, only the (p,n) channel 16F is unbound.

in ("Ne,*'F).

in "Na.

incident

5 excitation

decrease

larger

to bound states in 'OF, the

no 5 excitation

400 MeV per nucleon

p(20Ne,20F)5'+.

is

transitions

because

via L =

on the fn,p) side

1 and L = 3 transitions

by (lp)-l (2s) and (Ip)-l

(Id) configurations.

shift from the free A peak, to the peak of A in nuclei,

M. Roy-Stephan

/ Spitl-isospin

199c

modes

4.CONCLUSION Charge-exchange excitation tile.

reactions

of collective

The nucleon

two excitation energy,

and nuclei

energy

particle

300 MeV,

1 GeV per nucleon

response

regions

in details

: the peak for A excitation

spin-isospin

energy around

ions and heavy ions

results.

These results which

shifted

experiments.

around

excitationswithheavy

freedom

from subthreshold

We see interesting

200 MeV per nucleon

ion charge-

prospects

measure~nts

in nuclear

and in the field of mesonic

71 production

form the free

have been presented.

1 GeV per nucleon should be angular distribution

studies

are

is currently

NA. A common observation

is energy

of heavy ion charge-exchange

Next step in investigating

and coincidence

Light

for both channels, NN + NN and NN +

A peak. Thedominantfeatures

: at low excitation

interaction

in nuclei

shows up in

excitation

strength

give consistent

is made

through coherent

at high excitation

spin-isospin

studied

around

to spin-isospin

in the target.

reactions

to the nucleon-nucleon

proceed

in the target and in the projec-

with comparable

are excited

induced charge-exchange

exchange

modes

hole states are excited,

A-hole states

sensitive

around

spin-isospin

up to hadronization

structure

degrees

of

process at very high

energy,

ACKNOWLEDGEMENTS The Saturne tion with

Heavy ion charge-exchange

experiments are performed in collaborax*x* ** J.L. Boyard* A. Brockstedt D. Contardo ;** ;** ;* ;* C. Ellegaard , C. Gaarde , J.Y. Grossiord , A. Guichard , *** ******T. Jorgensen , 3-C. Jourdain*, J.S. Larsen , B. Million ,

: 0. Bachelier*

R. Ekstrom'. T. Hennino*, M. Osterlund

****

i* , J.R. Pizzi , P. Radvanyi", J. Tinsley", P. Zupranski".

thank V.M. Datar and P. Dekker for their participation. ledge, J. Faure, M. Olivier, National

Saturne.

S. Gardien

We warmly

and the whole

than R. Skowron,

M. Jacquin,

for their technical

assistance.

Brown and his co-workers

especially

sions on our experimental

results

* ** *** **** 0

We gratefully

P.A. Chamouard

Soyeur

P. Courtat

for enlightening

and their theoretical

acknow-

staff of Laboratoire

We are very much indebted

Madeleine

We

work.

IPN Orsay - B.P. n"l, 91406 Orsay - France IPN Lyon- 69622 Villeurbanne - France

Niels Bohr Institute - DK 2100 Conenhaqen - Denmark LUnd University S 223 62 Lund - SwedenLaboratoire National Saturne - F 91191 Gif sur Yvette

- France

and to G.E. discus;

ZOOC

h4. Roy-Sephan

/ Spin-isospin

modes

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201c