Nuclear physics A447 (1985) 635~~642~ North-Holland, Amsterdam
EXCITATION
63%
OF THE A RESONANCE IN RELATIVISTIC
HEAVY ION
CHARGE EXCHANGE REACTIONS
M. ROY-SI'EPH4N*+D. BAMELIER* J.L. BOYARD* D. CONTARt0+, C. ELLEGA@, C. GAARDE § J.Y: GROSSIORD+ A: GUICHAPD+ T: HENNINO*, J.C. JOURDAIN , J.S. LAP.%& J.R. PIZZI+, P: RADVANYIt,J: TINSLEYt * I.P.N. Orsay F 91406 ORSAY CEDEX France + I.P.N. Lyon F 69621 VILLEURBANNE France 5 Niels Bohr InstituteDK 2100 CopenhagenDenmark t LaboratoireNational Saturne F 91191 GIF-sur-YVETTEFrance Excitationof the A resonance has been observed for the first time in a heavy ion reaction. The strength of the A signal depends on the projectile nuclear structure. In the same incident energy range about 900 MeV per nucleon, we observe - a strong A excitation in 27AR(10Ne, 20F) and 27AR(20Nel20Na), - a weak A in i2C(i4N, i4C) and - nearly no A excitation in (12C 'B).and (i2CI i2N) whatever the target is among a sample of nuclei from "6 to "Y.
The A1232 resonance (S = 3/2, T = 3/Z) may be seen as the excited state of the nucleon obtained by spin-isospinflip of one quark. Its free mass and width are respectivelyM = 1232 MeV and r = 120 MeV. Real or virtual A excitation plays a very important role in intermediateenergy physics. In the elementary charge exchange reaction two processes appear : -1) the elastic one p+n+n+p
and
- 2) if the incident energy is large enough, the A excitation,for example : p+n+n+A+ p +p-+n+A++. Nucleus-Nucleuscharge exchange reaction is expected to proceed via these two mechanisms. In a light ion charge-exchangereaction namely (3He,t) around 2 GeV',
a
strong A excitationhas been found, but it had not until now been
observed with heavy ions. The present experiment gives evidence, for the first time, of the A resonance excitation in a nucleus by means of heavy ions.
EXPERIMENTALCONDITIONS This experimentbelongs to a research program on spin-isospinexcitationby charge exchange reactions at Intermediateenergy in LaboratoireNational Saturne 0375-9474/86/$03.50 @ElsevierSciencePublishers B.V. (North-Holland
Physics Publishing
Division)
636~
of the A resonance
M. Roy-Stephan et al. /Excitation
The heavy ion part has just available
started,
using “Ne,
“N and “C beams that are now
in Saturne through the Cryebis source and the WQ preinjector.
*‘Ne,
14N and r2C induced charge exchange reactions
have been studied at incident
energies
(‘“Ne,
also
around 900 MeV per nucleon.
A single
been performed at 400 MeV per nucleon.
The heavy ion charge exchange reaction
is expected to be very peripheral
sharply peaked at 0”. The spectrometer
and therefore
ted for this program. It consists It
‘OF) measurement has
of 5 dipoles,
is very long (33 meters from the target
extremely
advantageous to eliminate
The spectrometer
is divided
half way. This intermediate ming coincidences lators
sitting
SPES IV is very well sui-
6 quadrupoles and 2 sextupole 3.
to the final
focal
background especially
plane),
in two parts with an intermediate
focusing
allows good event
between corresponding
at the intermediate
which is
at 0”. focal
triggering
by perfor-
elements of two hodoscope of scintil-
and final
focal
plane.
Particles
of rigi-
dity up to 4 GeV/c can be momentumanalysed in SPES IV, with a typical lution @
= 7.10-‘.
The momentumacceptance
TherePfore the complete spectra in a single tively
field
setting.
including
The horizontal
ABH= f 0.8” and Af+, = It 1.5”,
tracing
information
tained
in this solid
we believe angle.
is 9
“C,
impurities since
“0,
reso-
= + 3.5 10e2.
A egitation and vertical
could be recorded aperture were respec-
centered at 0”. Relying on the ray
that most of the total
Background conditions
cross
section
r2B or i2N produced in the target
in the beam. In the case of incident
is
con-
have to be commented : in
the measurements with “‘N or “C beam, some background is present. tually
plane,
It is ac-
by fragmentation
of heavier
“Ne we observe no background
there is no heavier contaminant in the beam. At last
one should notice
that some of the measurements reported here, have been performed in a very short beam time, in order to get a general view of the dominant features this process.
bbre
complete and detailed
sent stage of our analysis The spectra
measurements are planned. At the pre-
we shall not give
thus, strictly
because the ejectile
may be produced either
state provided
of the spectrometer). is not large, ( ‘+N, “C)
absolute
obtained with various projectiles
are Q value spectra, excited
will
cross
sections.
be presented now. They
speaking they are not missing mass spectra in its ground state,
it is bound (the ejectile Under the circumstances,
it goes from 0 in the (I%,
case.
of
or in an
has to be detected the uncertainty
at the end
on missing mass
r*N) case to about 8 MeV in the
84. Roy-Stephan et al. /Excitation
27AR(20Ne,
of the A resonance
63lc
*OF) AND 27AR(20Ne, *ONa) AT 950 MeV PER NUCLEON
We have studied the reactions *7AR(2"Ne, 'OF) and 27AR(20Ne, *ONa) at 0" and 950 MeV per nucleon using a 135 mg/cm* *'1u? target. The *OF and *'Na spectra are displayed on figure 1.
E=950
MeV/A
27A1(20Ne,
0’
b)
20Na)
20
150
E=950
MeV/A
0”
F 100 z 50
0 600
FIGURE 1 20F and *'Na spectra from 27A11(20Ne, 20.F)and 27AR(20Ne, 20Na) at 950 MeV per nucleon and 0". The abscissa is the negative of the reaction Q value (- Q)in MeV, the ordinate is the counting rate dN/dQ in arbitrary unit. Both 'OF and "Na spectra are normalized to the same ntnnberof incident 20Ne. These spectra consist of two peaks : 1) The sharp peak at low excitation energy is due to particle-holestate
excitation in the target and/or the projectile nucleus. In the energy range between 200 MeV per nucleon and 1 GeV per nucleon, we are concernedmainly with spin-isospinexcitation. 2) The broad peak at about Q = - 300 MeV is the expression of a very strong internal excitationof a nucleon from the target into a A resonance. Beyond the A peak, the cross section is very small.
638~
M. Roy-Stephan et al. /Excitation
of the A resonance
This twofold structure illustratesthe statement that the nucleon and the A are two states of the same particle connected by spin isospin excitation. The A excitation in a heavy ion induced reaction has been observed here for the first time. The A excitation is very strong in both (p,n) and (n,p) like reactions.The ratio between the A excitationand the nuclear particlehole excitation cross sections in both reactions is approximatelythe same : A/N = 1.6 f 0.2. On the other hand, the ratio between the A excitation cross sections in 27A!?,(20Ne, 'OF) and 27AR(20Ne,'ONa) is 2.7. 27AR not being isospin symmetrical,a more relevant quantity is the ratio between the A excitation cross section in these (p,n) and (n,p) like reactions on a target nucleus with an equal number of neutrons and protons (2 = N = 13). We have calculated this number to be 2.8, taking into account the isospin Clebsh Cordan coefficients for the elementaryprocesses,namely : p+n+n+A
p+p+n+A
+ and ++
n+p+p+A"
n+n+p+A-.
We will comment on both ratios (A/N = 1.6 and A"F/A*'Na = 2.8) in the last section, in connectionwith projectile nuclear structure effects. We have also performed a measurementat 400 MeV per nucleon with a *'Ne beam on a '*C target. The *OF spectrum shows no A excitation at this incident energy. In fact 400 MeV per nucleon is below threshold for the nucleon nucleon process N + N + N + A i.e. 630 MeV per nucleon, but it is above threshold for the coherent process *'Ne + nucleon + *OF + A i.e. 315 MeV per nucleon. A's and IT'Scould be formed at these energies*,but the correspondingmomentum transfer is rather large. Thus the ejectileswould more likely be unbound, thereforewe would not see them in this experiment.
'*C(l*N,“C)
AMI
'*C(14N,"0) AT 880 MeV PER NUCLEON
We have looked for 14C and 140 from both (p,n) like and (n,p) like reactions induced by 'liNat 880 MeV/nucleonon a "C target. We have observed 14C from (14N, 14C) but no I40 nuclei from (14N, 140) have appeared above the background The absence of 'l*Oproduction is a straightforwardconsequenceof the hindrance
* For the observationof subthresholdpion production by heavy ions see Ref. 7.
M. Roy-Stephan et al. /Excitation
of "C -+14N or “0
of the A resonance
639c
+ 14N ground state to ground state transition,which is
well known from B decay. If it is a direct reaction (14N, 140) can only proceed via g.s. + g.s. transition since 140 ground state is the only bound state of this nucleus. Non observationof I40 from (i'N, '"01, thus confirms that heavy ion charge exchange at relativisticenergy is a direct one step process. 100
E ~880 MeV/A
0”
FIGURE 2 14C spectnnn from "C(14N, '"C) at 880 MeV per nucleon, 0" on a 50 mg/cm' i2C target. The backgroundwhich is visible above the kinematicallimit is due to fragmentationof heavier impurities in thebeam.
Gn the other hand, in the "C case there exist bound excited states. Some of them correspondto spin isospin transitions,at least a strong Gamow-Teller transition to a 2+ state at 7.01 MeV and perhaps a spin dipole transitionto a 3- state at 6.73 MeV, the strength of which is still debated3. In the l*C(r'N, r4C) spectrum (Fig. 21, A excitation is sizeable but less strong than in the ('"Ne, *OF) or ("Ne, *ONa) reactions at a similar incident energy per nucleon.
(12C, '*B) AND (I?, "N) AT 900 MeV PER NUCLEON We have studied both reactions on several targets H, "C, 4oCa and *'Y
E 1900 MeV/A 600FIGURE 3 12N syectrm from lZC(l c, 12N) at 900 MeV per nucleon 0" on a 200 mg/an* '*C target. Same origin for background as in Fig. 2.
g
0"
12C(12C,12N)
LOO-
z‘ = 200-
LOO
200
0
-"fQ (MeV)
640~
M. RoyStephan
et al. /Excitation
of the A resonance
H cross sectionswere obtained by subtractionmeasurementson CH2 and C targets. A striking feature is the almost total absence of A excitation in both P2c, '*N) and (I?, 12B) reactiorson 12C, "Ca and *'Y,(see for example Fig. 3).H(i2C, 12B) A ++ is the only case where A excitation is observed. This difference stresses the dominant role of the projectile absorption by the target nucleus. H(i'C, 12N) A0 cross section is weak. It should be so, since the isospin factor for A0 is l/3 of A++ one. On the other hand we observe a large cross section for p(12C, 12N)n.
DISCUSSIONAND CONCLUSION In this section, the sensitivityto projectile nucleon structure is explained by momentum matching considerations.The crucial point is the variationwith momentum transfer of the overlap of projectile and ejectile wave functions.Let us call the square of this overlap the projectile form factor. To first order, the cross section depends linearly on this form factor. The momentum transfer-q varies with reaction Q value : q = 0 at Q = 0 and four-momentumq g 0.9 fin-lat Q = - 300 MeV where A excitation is expected. Concerningthe projectile form factor behaviour and strength for various projectile* ejectile transitions 6 we refer to data from (p,n)4, (TI;Y)573and to calculations . In short,GamowTeller form factorshavemaxima at q = 0 and decrease rapidly with q. In contrast spin dipole form factorshave maxima in the q region corresponding to A excitation.In (12C, i2N) and (i2C, 12B) cases the only strong transition is of Gamow Teller type. The form factor is very small in the A region, in contrast to the ("Ne, 20F) and ('ONe, 'ONa) cases, where the Gamow Teller transition is weak5'6 and the dominant transitionsare spin dipole. An extra spin dipole transitionwhich is bound in 20F and unbound in 20Na could be responsible of cross section excess in (20Ne, 2oF) versus (20Ne, 'ONa). In (l'N,'$C) we have already quoted the two possible transitions : a strong Gamow Teller and a spin dipole, the strength of which is not known. The small value of A/N that we observe indicates that this transition should be weak. In summary : - A excitationhas been observed for the first time in a heavy ion induced reaction - Relativisticheavy ion charge exchange is a direct process. - The momentum matching requirementcan explain why the strength of A excitation in the target is very sensitive to nuclear structure of the projectile.
M. Roy-Stephan et al. /Excitation
of the A resonance
641~
ACKNOWLEDGFMENT We are grateful to J. Faure, M. Olivier, R. Vienet and to the LNS staff for very efficient support. We thank S. Gardien and M. Jacquin for invaluabletechnical assistance,and J. Hufner for very stimulatingdiscussions.
REFERENCES 1) C. Ellegaard et al. Phys. Rev. Lett. 50(1983)1745.
2) E. Grorud et al. N.I.M. 188(1981) 549. 3) H.W. Baer et al. Phys. Rev. Cl2 (1975) 921.
H.R. Kissener et al. Nucl. Phys. A312 (1978) 394 4) R.P. De Vito et al. IUCF Report 1982 p. 32 J. Rapaport et al. Phys. Rev. CZ4 (1981) 335 51 C.J. Martoff et al. Phys. Rev. Lett. 46 (1981) 891
6) B.H. Wildenthal and W. Chung, The (p,n) reaction and the nucleon-nucleon force Ed. C. Goodman (Plenum Press New York 1980). 7) E. Grosse talk at this conference V. Bernard et al. Nucl. Phys. A 423(1984) 511 W. Benenson et al. Phys. Rev. Lett. 43(1979) 683.