AREA 2. - EXPERIMENTAL NUCLEAR PHYSICS
Activity 2.1 - Fission and evaporation of highly excited systems
A.Brondi, G.La Rana, R.Moro, A.Ordine, M.Trotta, E.Vardaci
During 2004 different experiments were carried out at Laboratory Nazionali di
Legnaro (LNL) and the GANIL Laboratory in Caen (France). The main research
topics were:
1) Fission dynamics
2) Isospin effects on the nuclear properties
3) Hyper-deformation and nuclear properties of nuclei far the stability valley
1) Fission dynamics
We are carrying out a detailed inquiry on fission dynamics for systems of
intermediate fissility, looking at the maximum number of observables, to obtain
realistic estimation of fission time scale. In this framework we have undertaken a
campaign of measurements of particle multiplicity in fission and evaporation
residue channels. Neutron, proton and alpha pre-scission multiplicity
measurements are usually utilized to extract an estimation of the fission time
scale by a comparison with statistical model calculations in which a delay time
on the fission process is introduced. The study of both fission and evaporation
residue channels by also measuring cross-sections will allow to better define the
statistical model parameters and give more confidence on the measured fission
time.
Along this line, we carried out an experiment at Laboratori Nazionali di Legnaro
by the apparatus LP on the reaction 18O + 150Sm at 122 MeV beam energy. By
the analysis of the coincidences between fission fragments and -particles we
obtained -multiplicity spectra for many correlation angles and a fairly good
statistics for each angle in spite of very low particle multiplicity of fission events.
By comparison of experimental data and statistical model simulations we
extracted an -pre-scission multiplicity value of 0.04. This value agrees with
predictions of the code PACE for a fission delay range between 0 and 30 10-21 s.
To obtain a more precise determination of fission time scale for this system the
analysis of the proton- pre-scission emission and charged particle emission in the
evaporative residue channel is underway.
Furthermore, we measured residue and fission fragment cross sections at the
Laboratori Nazionali di Legnaro by the MAIALE electrostatic separator coupled
to the CORSET system to detect fission fragments. Analysis is in progress.
2) Isospin effects on the nuclear properties
Nuclear level density is a fundamental quantity in many aspects of nuclear
reactions and in particular in nuclear astrophysics. At present, isospin effects are
usually neglected, as they are expected to be relatively small for nuclei close to
the stability valley, as those produced by stable beams. Recently, a reduction
factor of the level density based on the distance from the valley of stability has
been introduced which produces a better fit to the experimental energies of lowlying states. These effects should also show-up in the evaporative decay of
compound nuclei as the level density plays a crucial role in the particle emission.
In this framework, we have undertaken the study of the nuclear level density of
nuclei produced in the particle decay of the 139Eu nucleus excited at an energy of
90 MeV by the 180 MeV 32S + 107Ag reaction. Experimental energy spectra and
in-plane and out-of-plane multiplicity angular distributions were measured for
alpha particles and compared with the simulations obtained by the Statistical
model using a MonteCarlo code without taking into account the isospin
dependence and with the prescription recently proposed. From a preliminary
analysis a good reproduction of alpha particle emission without inclusion of
isospin dependence has been found. This finding brings to the conclusion that
isospin effects are no present or are negligible in the decay of this system at this
excitation energy.
3) Hyper-deformation and nuclear properties of nuclei far the stability valley
An interesting subject of investigation in the last few years has been the search
for exotic shapes, hyper-deformation (axis ratio 3/1) and Jacobi transitions (from
oblate to prolate shape).
The analysis of the huge amount of data collected in a very long experiment
carried out in December 2003 in Strasbourg is producing interesting results. A
beam of 64Ni was impinging on a target of 64Ni and gamma rays in coincidence
with light charged particles were measured.
The analysis of high energy gamma rays de-exciting the giant resonances
indicates that the system undergoes the Jacobi transition in the route toward
fission.
The high statistics data allowed a detailed spectroscopic study of Barium nuclei.
New Electric Dipole transitions have been identified in 125Ba which represent the
fingerprint of octupole correlations. In addition, a newly identified 3- level in
124
Ba has an energy well reproduced by octupole-including microscopic
calculations.
Analysis of data collected in a second experiment at GANIL using the
radioactive beam 76Kr is underway. Spectroscopy of very neutron-deficient rareearth nuclei will be investigated.
Program for 2005
1) Fission dynamics
2) Isospin effects on the nuclear properties
3) Hyper-deformation and nuclear properties of nuclei far the stability valley
For 2005 maintenance and significant implementations of the LP apparatus are
scheduled. A system based on the measure of the time of flight will be
developed, in collaboration with researchers of the Dubna Laboratory (Russia),
to measure mass and total kinetic energy of the fission fragments. In addition, a
new circular Parallel Plate Avalanche Counter will provide greater efficiency for
the detection of the evaporation residues.
The apparatus in the new configuration will allow to study the characteristics of
the fission process for different mass ratio of the two fragments and improve
significantly the measurements of evaporation residues for the study of the
isospin effects.
Regarding fission, experiments are also scheduled at University of Jyväskylä
(Sweden), where a fruitful collaboration has been established.
Regarding the isospin effects on the nuclear properties, a new experiment has
been proposed. Significant implementations in the statistical codes will be
performed for comparing simulations to the new very detailed experimental data.
Measurements using radioactive beams both to study isospin effects and nuclear
spectroscopy of exotic nuclei are in program.
Activity 2.2 - reaction mechanisms of light exotic nuclei at coulomb barrier
energies
A. De Rosa, G. Inglima, M. La Commara, B. Martin, D. Pierroutsakou, M. Romoli, M. Sandol
The commissioning of the EXOTIC beam line for the production of light exotic
nuclei through inverse kinematics reactions induced by the LNL Tandem beams
has been completed. The first experiment aimed to the break up cross section
measurement of the 17F on 208Pb has been approved by the PAC of the LNL
Laboratory. At the RIKEN Ring Cyclotron Lab. (Japan) the fusion cross section
at energies close to the barrier was measured for the 9,10,11Be+209Bi systems.
The results of these experiments show that the fusion cross section of the three
reactions investigated has the same behaviour around the Coulomb barrier while
a larger cross section was expected for the exotic 11Be nucleus. At the NCSR
Demokritos (Greece) the exclusive break up cross section for the 6Li+28Si
reaction has been measured. The results of the 7Li+28Si elastic scattering have
been published. The angular distribution was analysed in the double folding
framework for the potential and the behaviour of the imaginary potential has
been compared to the real potential one. The potentials has been found different
for the two isotopes 6Li and 7Li.
Program for 2005
The investigation of the reaction mechanisms of light exotic nuclei will be
continued. The measurements with the 17F beam will be completed at the LNL. A
new beam, 8B, will be developed by means of the EXOTIC apparatus and, after
its commissioning, some experiments will be submitted to the PAC approval.
Activity 2.3 - Isospin effects in the gdr excitation in fusion reactions between
heavy ions
A. De Rosa, G. Inglima, M. La Commara, B. Martin, D. Pierroutsakou, M. Romoli, M. Sandoli
The data concerning the pre-equilibrium emission of rays from the GDR
excited in the 40,36Ar+92,96Zr reactions have been analysed. The dependence of the
pre-equilibrium ray emission on both the isospin of the ions in the entrance
channel and the colliding energy was investigated. The presence of a colliding
energy interval where the GDR pre-equilibrium ray emission shows a
maximum points out the possible role of such a mechanism in the cooling of the
compound nucleus formed in fusion reactions.
Program for 2005
The GDR pre-equilibrium ray emission will be investigated in fusion reaction
between heavier ions to collect more information on the possible cooling effect
of the compound nucleus. Such a research is interesting for the formation of
superheavy nuclei.
Activity 2.4 - Nucleus-Nucleus Collisions
E.Burattini, L.Campajola, M.De Gregorio, R.Prevete, E.Rosato, G.Spadaccini, O.Talamo and
M.Vigilante
The group was engaged with leading responsibilities in several international
collaborations. The activity took place both in Italy and abroad. The group
participated in the measurement campaign of the 4π CHIMERA multidetector in
its complete configuration (1192 Si-CsI(Tl) telescopes covering the  = 1°176°,  = 0°-180° angular range with a /4 = 0.94 geometrical efficiency).
The beams were delivered by the Van de Graaff tandem accelerator and the
superconducting cyclotron of the Laboratori Nazionali del Sud (LNS) in Catania.
Meanwhile the R&D activity has been pursued on pulse shape analysis. The main
idea is to deeply investigate the signals produced by ionizing particles passing
through silicon detectors and deconvolute the contributions to the current due to
holes and electrons aiming at isotopically identify the impinging particles. To
this end a special charge– and current-sensitive preamplifier (PACI) well suited
for pulse shape discrimination techniques was developed and successfully tested.
Several measurements were performed at the LNS, the Grand Accélérateur
National d’Ions Lourds (GANIL) of Caen (France) and the TTT-3 tandem
accelerator of Naples by the CHIMERAPS collaboration and the Italian-French
AZ4π group, respectively, (the Naples group participates in both of them). A
dedicated apparatus has been especially designed and installed at the Laboratorio
dell’Acceleratore of Naples to study the doping homogeneity of nTD silicon
detectors taking advantage of the new PACI preamplifier. It is a part of a biennial
joint research project in collaboration with the Universities of Catania and Milan
that was partly funded for 2003 and 2004 by the Italian Ministry of Education,
University and Research. The objective is to characterise the nTD’s and to
measure charge and mass of particles stopped inside the silicon crystals.
In the meantime the analysis was continued of data collected by INDRA and
CHIMERA multidetectors on nucleus-nucleus collisions around the Fermi
energy. The research concerned collision dynamics, multifragmentation and the
liquid-gas phase transition of hot nuclear matter. The last phenomenon is
expected to occur due to the analogy of nuclear interaction with the van der
Waals forces but it is still controversial. A major goal was to find convincing
signals of such a phase transition. In this respect a number of experimental
correlated signatures were observed:
 negative heat capacity inferred from kinetic energy fluctuations that, for
finite systems, suggest the occurrence of a I order phase transition;
 enhanced production of equal-size fragments testifying that nuclear matter
has entered the spinodal (negative compressibility) region of liquid-vapour
coexistence;
 bimodality (i.e. two peak distribution) of selected observables when sorted
according to an order parameter (for example, the distribution of the two
biggest fragments as a function of the excitation energy);
 universal scaling (-scaling) of fluctuations of an order parameter (for
example, the probability distribution of the biggest fragment in each event)
as expected in a mixed phase regime or close to criticality.
It has to be stressed that the first two results are model dependent, while the last
two are robust and do not require preliminary assumptions. Furthermore a crossanalysis of the simultaneous occurrence of the various signals has been started all
over the large INDRA database.
Program for 2005
The activity will continue according to the schedule:
 calibration and analysis of data collected by INDRA and CHIMERA
multidetectors;
 measurement campaign of CHIMERA at LNS;
 experiments at GANIL and at the Institut de Physique Nucléaire d’Orsay
for the AZ4 group;
 measurements at the Laboratorio dell’Acceleratore in Naples for the JRP
“Study and characterisation of nTD silicon detectors for isotopic identification of light charged particles (LCP) and intermediate mass fragments
(IMF)”;
 summary report of the AZ4π working group and provisional design of a
new 4π detector for both charged products and neutrons.
Activity 2.5 - Nuclear Astrophysics
N.De Cesare*, A. D'Onofrio*, L. Gialanella*, G .Imbriani*, B.N. Limata, C. Lubritto*, A.
Ordine*, V. Roca, M. Romano, C. Sabbarese*, F. Terrasi*
The research activity in Nuclear Astrophysics focussed on the study of important
nuclear reactions of astrophysical interest: 14N(p,)15O, 25Mg(p,)26Al (LUNA
experiment) and 12C(,)16O and 3He(4He,)7Be (ERNA experiment). Moreover,
again in the framework of the LUNA collaboration, the electron screening in
metals has been studied at the 100 kV accelerator in Bochum.
The experiment ERNA (European Recoil separator for Nuclear Astrophysics) is a
collaboration between the Physics Department and the INFN Section in Naples,
the Institut fuer Experimentalphysik III in Bochum, Germany. Aim of the
experiment is the study of 12C()16O, a key reaction in stellar evolution and
nucleosynthesis.
During 2004 the total cross section of 12C()16O was investigated: 76 data
points have been measured in the energy range Ecm=1.9-4.9 MeV. The typical
statistical error was below 5% with a normalization error of 6.5%. A new
resonance J=0+has been found at Ecm=4.888 MeV
Also, the investigation of the role of 12C()16O in stellar evolution and
nucleosynthesis was continued, with special attention to the interplay between
this reaction rate and the convection in stellar models. Finally, a new experiment
was started extending the collaboration to the ATOMKI Institut, Debrecen,
Hungary, in order to study 3He(4He,)7Be in the energy range 700<Ecm<2500
keV using ERNA. The acceptance of the separator turned out to be sufficient to
measure the total cross section of this reaction and test runs have been done.
Besides the data taking, many efforts were devoted to the design of several
improvements to the experimental setup, including a new jet gas target, a new ray detection system based on NaI and HPGe detectors
During 2004, in the framework of the LUNA (Laboratory for Underground
Nuclear Astrophysics) collaboration, the Naples group gave a major contribution
to the completion of the analysis of the data collected in a first experiment, where
the 14N(p,)15O reaction was studied at the 400 kV accelerator of the Gran Sasso
National Laboratory in the energy range Ecm=130-400 keV using a solid target
and a HPGe detector. The careful evaluation of these data suggested to extend
the measurements toward higher energies, which is required in order to get more
precise and accurate extrapolation of the cross section to the relevant
astrophysical energy. Therefore, a new series of measurements at the 500 kV and
4MV tandem accelerators at the Dynamitron Tandem Laboratory (DTL) of the
Ruhr University Bochum, Germany has been done, whereas at these energies the
reaction rate is high enough and the natural shield of the underground laboratory
is not necessary. These measurements allowed to obtain a complete picture of the
reaction rates at 0 energy, by means of a R-matrix analysis.
The extrapolation to the relevant astrophysical energy provided a new estimate of
the CNO rate, which is about one half of the previous estimate. This has in turn,
as a consequence, that the expected neutrino flux from the CNO cycle in the Sun
is also reduced of about a factor of 2. In spite of the fact that this component
represents only 1% of the total flux, it plays an important role in some solar
neutrino experiments, such as BOREXINO, where it accounts for about 10% of
the expected detected neutrinos. More important, the new determination of the
CNO rate has an important influence on the estimate of the age of the oldest
globular clusters, which, in turn, provides a lower limit of the age of the universe.
On the basis of these new results, the age of globular clusters was increased by
about 1 Gy.
During 2004 a second experiment has been finished, using a windowless gas
target in combination with a high efficiency BGO detector, whose efficiency
allowed to investigate the reaction down to energies Ecm=70 keV. The results
show a good agreement, on overlapping energies, with the solid target data.
Finally, as regards the electron screening in metals, temperature effects on the
screening potential have been studied, confirming that the Debye plasma model
applied to the quasi free electrons in metal is able to describe the observed
electron screening in low energy nuclear reactions in metals.
Also, an experiment started, where the possibility of applying the Debye model
to the free electrons in metals will be exploited to change the half-life of 7Be,
which is expected to be influenced by the electron density at the nucleus. A pure
7
Be beam has been implanted in W, Ta, Zr, and Pd targets and the measurement
of the 7Be half life has been started in three different laboratories (Naples,
Bochum, Debrecen).
Program for 2005
As regards the ERNA experiment, the measurements of the total cross section of
12
C()16O will be continued. Also, a study of the different amplitudes
contributing to the total cross section will be started by means of the analysis of
the spectra and angular distribution of gamma rays detected in coincidence with
the recoils. Finally the stellar rate based on these measurements will be
calculated. As regards 3He(4He,)7Be, a recirculating 3He windowless gas target
will be developed, allowing to study the reaction in inverted kinematics, and first
measurements will be performed at Ecm=1.5-2.5 MeV
LUNA plans in 2005 the beginning of the measurements of the 25Mg(p,)26Al
reaction at the 500 kV and the 4MV tandem in Bochum, in order to optimize the
experimental setup before the installation of the apparatus at LNGS, which is
planned for the beginning of 2006. This reaction is crucial for the understanding
of the MgAl cycle and the production of 26Al, whose delayed -decay was
observed by means of the COMPTEL satellite.
Also, the setup for the study of 3He(,)7Be reaction will be completed.
Finally, the first data on the 7Be half life in metals should be available.