Measurement of the GMR in the Unstable 56Ni Nucleus using the Active Target Maya

Abstract

The measurement of the Isoscalar Giant Monopole Resonance(GMR) in unstable nuclei remains a major experimental challenge due to low radioactive beam intensities and unfavourable conditions in reverse kinematics. At GANIL, we have tested a new experimental method based on the unique capabilities of the active target Maya to probe the GMR by the inelastic scattering reaction 56Ni(d,d') at 50 AMeV. The preliminary excitation energy spectrum of 56Ni presents a bump between 12 and 25 MeV where isoscalar resonances are expected.

Full text

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Measurement of the GMR in the Unstable56Ni Nucleus using the Active
Target Maya
C. Monrozeaua, E. Khana, Y. Blumenfelda, W. Mittigb, D. Beaumela, M. Caama˜ noc,
D. Cortina-Gilc, C.E. Demonchyd, N. Frascariaa, U. Garge, M. Gelinb, A. Gillibertf,
D. Guptaa ∗, F. Mar´ echalg, A. Obertellif, P. Roussel-Chomazband J-A.Scarpacia
aInstitut de Physique Nucl´ eaire (IN2P3/CNRS, Univ. Paris Sud), 91406 Orsay Cedex,
France
bGANIL (DSM/CEA, IN2P3/CNRS), BP 5027, 14076 Caen Cedex 5, France
cUniv. Santiago de Compostela, E-15706 Santiago de Compostela, Spain
dUniv. of Liverpool, Dep. of Physics, Olivier Lodge Lab., Liverpool L69 7ZE, U.K.
eUniv. of Notre-Dame, Dep. of Physics, Notre Dame, IN 46556 USA
fCEA/DSM/DAPNIA/SPhN, Saclay, 91191 Gif-sur-Yvette Cedex, France
gInstitut de Recherches Subatomiques (IN2P3/CNRS), BP 28, 67037 Strasbourg, France
The measurement of the Isoscalar Giant Monopole Resonance(GMR) in unstable nuclei
remains a major experimental challenge due to low radioactive beam intensities and unfa-
vourable conditions in reverse kinematics. At GANIL, we have tested a new experimental
method based on the unique capabilities of the active target Maya to probe the GMR by
the inelastic scattering reaction56Ni(d,d’) at 50 AMeV. The preliminary excitation energy
spectrum of56Ni presents a bump between 12 and 25 MeV where isoscalar resonances are
expected.
1. INTRODUCTION
The GMR in finite nuclei is of major importance because its properties are related to
the nuclear matter incompressibility K∞. The interest in determining K∞stems from its
impact on the nuclear matter equation of state. Moreover the incompressibility of asym-
metric matter is a basic parameter in calculations describing neutron stars or supernovae.
Some significant progress in our understanding of how K∞ can be constrained have
been achieved in recent times. Namely the procedure generally adopted to extract the
incompressibility from the measurement of the GMR energy is the so called microscopic
approach [ 1]. It is based on energy functionals which allow one to calculate finite nuclei
and nuclear matter on the same footing. The first step consists in constraining the energy
∗Present address : Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700064, India
Nuclear Physics A 788 (2007) 182c–187c
0375-9474/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.nuclphysa.2007.01.080

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functional in order to reproduce the experimental value of the monopole energy in a gi-
ven nucleus. Theoretically the best way to extract the monopole energy is by means of
constrained Hartree-Fock calculations [ 2, 3]. Then using this constrained energy functio-
nal the value of K∞is extracted.
Within the non-relativistic framework K∞can be fixed with an accuracy of 10% [ 3] and
no discrepancy between the use of Skyrme and Gogny forces is observed. Nethertheless the
extraction of K∞remains in some way model dependent : non-relativistic and relativistic
mean field models predict values for K∞which differ significantly, namely K∞≈ 220−235
MeV and K∞≈ 250 − 270 MeV respectively when experimental data for the GMR in
208Pb are used. Part of the remaining uncertainty may well be due to our poor knowledge
of the symmetry energy [ 3]. Therefore the investigation of the GMR along an isotopic
chain and particularly in neutron rich nuclei should be relevant to probe the role played
by the symmetry energy in the determination of the nuclear matter incompressibility.
Isoscalar resonances, and in particular the GMR, have been extensively studied in stable
nuclei. The best probes for their investigation are isoscalar probes such as deuterons or
alpha particles at energies between a few tens and a few hundreds AMeV. Essentially no
work has been done up to now for unstable nuclei, due to the very unfavourable condi-
tions in reverse kinematics. Indeed, the GMR cross section peaks at 0 ˚in the centre of
mass frame which gives rise to very low recoil velocities for the light probe. To measure
the excitation energy range between 0 to 30 MeV in reverse kinematics, it is necessary
to detect the recoiling particle (d or α) with energies ranging from 100 keV to 2 MeV at
angles from 0 to 40 ˚in the laboratory frame. A standard set-up with a recoiling particle
telescope such as MUST [ 4] would necessitate a very thin target (∼ 100 µg/cm2) to
minimize straggling and thus require an intensity of over 107pps which is prohibitive for
current radioactive beam facilities.
With respect to these experimental constraints an active target such as Maya [ 5] could
be the key to measuring the GMR in unstable nuclei. An active target is a gasous detector
in which the detector gas also acts as target. Such a set-up has in principle an angular
coverage close to 4π, a low energy threshold and a large effective target thickness. In order
to test the method, we have performed inelastic deuteron scattering on the unstable56Ni
nuclei at the GANIL facility. Alpha particle scattering could not be tested because the
detector, like any detector target, sparked when filled with pure He.
In this paper we first present the experimental set-up. Then preliminary results such
as excitation energy spectra are shown.
2. EXPERIMENTAL TECHNIQUE
2.1. The Maya active target
Active targets such as bubble chambers were developped since a long time in high
energy physics. In the domain of secondary beams the archetype is the detector IKAR [
6] which can be typically used at GSI energies. For the domain of lower energies a new
detector called Maya was developed [ 7] at GANIL. The active target Maya is shown
schematically in figure 1.
Maya can be characterised as a Time and Charge Projection Chamber. The electrons
from the ionisation of the gas by particles drift down the electric field to amplifying wires
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set parallel to the beam. For a two body reaction scattered and recoiling particles are in a
plane which can be determined by the drift time to wires. The amplified signal is induced
on the anode, a matrix of 35 by 34 pads connected to Gassiplex chips. The Gassiplex [ 8]
is a 16 channel analogical multiplexed ASIC developed at CERN. A hexagonal structure
was chosen for these pads in order to have the best conditions for the reconstruction of the
projected recoil trajectory, independant of the direction. A typical event read-out matrix
is shown in figure 2.
Figure 1. Schematic diagrams of the active target Maya. The secondary beam is incident
from the left. Plates which are not shown in these schematic drawings prevent the beam
(in dashed line in the left figure) from inducing charges in Maya. Recoiling particles
produce electrons by ionising the gas. The reaction plane is determined by the drift times
of electrons. From the signal induced after amplification in the anode, the 2D trajectory of
the recoil is reconstructed. Ancillary Si detectors outside the active volume detect escaping
recoils.
2.2. Experimental Set-Up
The secondary56Ni beam at 50 AMeV was produced by fragmentation of58Ni at 75
AMeV on a 70.5 mg/cm2C target using the SISSI device and sent to the SPEG area.
Maya was placed on the focal plane of the SPEG spectrometer [ 9] which was used to
purify the beam. The main contaminant of the beam,54Co is a N=Z nucleus and cannot be
separated from56Ni by the time of flight . A 1 mg/cm2Au foil was placed at the entrance
of SPEG to induce charge exchange.54Co26+and56Ni27+were transported through SPEG
with slightly different trajectories. Just before Maya a plate stopped the54Co so that only
56Ni entered Maya through a circular window made of Mylar. We added two plates above
and below the beam trajectory into Maya to prevent the highly ionising beam particles
from inducing charges in Maya. However, due to the angular spread of the beam, some
non-interacting56Ni ions were detected. This limited the beam intensity that could be
used to 5.104pps, while 106pps of56Ni was available.
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Figure 2. Typical event matrix as read out for the Maya detector. The calibrated amplitude
from the Gassiplex matrix is printed out for 32 columns, and the first 20 rows. The beam
particle is coming from the left but does not induce charges in Maya. Only the deuteron
from inelastic scattering is detected. Times on wires are printed out on the right.
The Maya active target filled with deuterium gas, at a pressure of 1050 mbar, forms
a pure deuterium target of 1.6 mg/cm2that is equivalent to a target of 6.3 mg/cm2of
CD2. In that case deuterons of 700 keV, which is our effective threshold, have 5 cm long
trajectories. As pointed out above, reverse kinematics generate recoil particles in a large
energy domain. High energy light particles (E ≥ 2 MeV) such as deuterons could not
be stopped in the Maya gas volume. For such escaping particles, we added ancillary 500
µm Si detectors outside the active volume covering about 30 degrees around the beam
direction for events in the midle of the detector. Data from these detectors have not yet
been analysed and only particles stopping in Maya are considered in the following. For
normalization purposes the beam was counted in a diamond detector after traversing
Maya.
3. PRELIMINARY RESULTS
For each event two physical observables have to be determined in order to reconstruct
the kinematics of the reaction : the energy and the angle of the recoil. The 2D recoiling
angle in the plane (x,y) is reconstructed by linear least square fitting of the positions
of the pads touched with a weight proportional to the amount of charge deposited. The
intersection of this line with the beam axis gives the vertex of the interaction. The recoil
stopping point is determined by looking for the Bragg peak in the charge distribution
along the trajectory. From the drift times the reaction plane is reconstructed and it is
possible to determine, for each event, the recoil angle and the range in the deuterium gas.
The energy of the recoil is related to its range in the gas through tables [ 10]. The 3D-
reconstruction of scattering trajectories in MAYA provides the kinematics of the56Ni(d,d’)
reaction which is shown in figure 3.
The data are then transformed into the center of mass frame using two-body kinematics.
The preliminary excitation energy spectrum of56Ni is shown in figure 4. This spectrum
presents a peak centered at 0 MeV corresponding to elastic and inelastic scattering. The
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FWHM gives an energy resolution of 3 MeV. At energies ranging from 12 to 25 MeV
a bump in the excitation energy spectrum points out the presence of isoscalar giant
resonances. According to a recent study on58Ni [ 12] the E0 and E2 strength are located
respectively at 18.4 MeV and 16.6 MeV. The spectrum stops at 28 MeV because of
experimental cuts. Higher excitation energies of56Ni will be reconstructed by the analysis
of the Si detectors.
Since the experimental set-up was designed to detect very low deuteron energies, the
method usually applied with Maya to disentangle proton from deuterons is not applicable
[ 7]. Consequently some proton contamintation may be expected in the excitation energy
spectrum. Protons can be produced mainly by two mechanisms : the deuteron break-
up and one neutron transfer (d,p) reactions. The contribution of the deuteron break-up
was estimated from cross-sections measured for58Ni at 50 AMeV [ 11]. These data are
transformed in the reverse kinematics frame, convoluted by the geometrical acceptance
of Maya and normalised to experimental data. It appears that the contribution of the
deuteron break-up to the giant resonance bump is small. Concerning the (d,p) reaction,
the lowest energy protons are scattered at backward angles. They can not interfere with
giant resonances events.
Figure 3. Experimental kinematics of the reaction56Ni(d,d’) at 50A.MeV : scattered-plot
of the energy of the deuteron versus angle in the laboratory frame. Lines are calcula-
ted kinematics. The solid line corresponds to elastic scattering. Isoscalar resonances are
expected in the region limited by the dashed lines, between 12 and 25 MeV excitation
energy.
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