Letter of intent for the proposal of an experiment by sig51858



       Letter of intent for the proposal of an experiment at LNS entitled

    “Study of the isospin dependence of the nuclear equation of state via
               two-particle and neutron correlation functions”

We propose a series of experiments to study the isospin dependence of the nuclear equation of
state through two-particle correlation functions measured at Laboratori Nazionali del Sud, LNS,
Catania. In particular, we want to focus on correlations involving neutrons.

Introduction and physics goal
The isospin dependence of the nuclear equation of state (EOS) is probably the most uncertain
property of neutron-rich matter. This property is essential for the understanding of extremely
asymmetric nuclei and nuclear matter as it may occur in the r-process of nucleosynthesis and in
neutron stars. In order to study the isospin-dependent EOS, heavy ion collisions with isotope
separated beam and/or target nuclei can be utilized. In these collisions, where excited systems
are created with varying degree of protons-neutron asymmetry, a noticeable isospin dependence
of the decay mechanism has been predicted [1]. To properly understand the decay mechanisms
and their origins, it is important to measure neutrons in addition to charged particles1.
The two-nucleon correlation function (in particular the neutron-proton) is one of the theoretically
suggested probes for the density dependence of the nuclear symmetry energy [2]. Theoretical
calculations indicate that a stiff EOS causes high momentum neutrons and protons to be emitted
almost simultaneously, thereby leading to strong correlations, while a soft EOS delays proton
emission, which weakens the neutron-proton correlation function [2]. The first experimental
evidence of an isospin dependence of the two-nucleon correlation function has been provided by
our group in the 61 MeV/nucleon 36Ar + 112-124Sn reaction, measured at KVI [3].
With this letter of intent we propose a new experiment to study the symmetry interaction using
small-angle correlation between light particles (including neutrons). The difference in the
emission time distributions of light particles emitted from systems similar in size but with
different isospin content, can be used to extract information on the nuclear interaction and the
symmetry energy

Past experience
An experimental investigation of two-nucleon correlation functions was performed by our group
at LNS in 1998. The aim was to study the space-time evolution of the reaction zone in heavy ion
collisions, by simultaneously measuring proton-proton, proton-neutron and neutron-neutron
correlation functions in coincidence with forward-emitted fragments. Energy spectra as well as
total and gated correlation functions from 45 MeV/nucleon Ni + Al, Ni, Au collisions, were

    Most 4π detector systems, such as Chimera and Indra, measure only charged particles.


measured and compared to theoretical calculations. The values of Gaussian radii, emission
lifetimes and initial temperatures were extracted. Thus, our previous experiment at LNS was
unusually successful in that it produced a lot of interesting and well recognized results, including
five publications in refereed journals [4-8] and several conference reports. The latest result from
the LNS experiment on the emission time chronology of neutrons and protons [8] triggered
indeed the community, and was the main motivation for a new interferometry experiment, that
we performed at KVI, Groningen, in 2002.
In the KVI experiment, we studied the full emission time chronology of neutrons, protons and
deuterons for 61 MeV/nucleon Ar + Al collisions [9], thereby consolidating the technique and
the findings of the LNS experiment. The KVI data for Ar +112-124Sn collisions, instead, led us to
a new direction, namely the investigation of the isospin dependence of the nuclear equation of
state. With the KVI data we have, for the first time demonstrated that an isospin signal appears in
the two-nucleon correlation functions measured for systems similar in size, but with a different
isospin content [3]. By comparing this information with the predictions of isospin dependent
theoretical models, information on the symmetry energy of the nuclear interaction can be
inferred. While the analysis of the KVI data is still in working progress, the KVI experiment has
already yielded three publications in refereed journals [9-11], three conference proceedings [12-
14], one publication submitted [3] and two manuscripts in preparation [15, 16].

Proposed experiment
With this letter of intent we propose a new interferometry experiment to be performed at the
superconducting cyclotron of LNS. We suggest to measure several reactions, covering various
degrees of isospin asymmetry, from neutron poor to neutron rich, as well as covering different
kinematical regions. The proposed reactions are:
1.             Sn + 58,64Ni collisions at 35 MeV/nucleon.
2.           Ni + 112,124Sn collisions at 35 and 50 MeV/nucleon.
3.           Ar + 112,124Sn collisions at 35 and 50 MeV/nucleon.
These systems ensure a larger isospin asymmetry than was achieved at KVI. We propose to
study the Sn+Ni systems both in reverse and in direct kinematics to ensure that all sources
created in the reactions are covered by our measurements and that effects, seen in the
observables because some source may be enhanced relative to another, can be minimised.
Furthermore we suggest to measure the same reaction as was done at KVI (Ar+Sn), although at
lower bombarding energies. In this way, direct comparisons can be made with the KVI results, as
well as with the new systems with larger isospin asymmetries (Ni+Sn). The direct reactions are
suggested to be measured at two different bombarding energies, to gain further insight into the
isospin dependence of the measured observables.
We suggest that these reactions are studied in two (or more) different experimental
measurements, separated in time. In this way, experience and results obtained from the first
measurements can be utilised to optimise the later measurements. Furthermore, making
measurements separated in time will allow to keep the beam-time request in each measurement
reasonably short.


Relation to the CHIMERA experiment
The suggested systems are also being investigated by the REVERSE experiment [17] of the
CHIMERA collaboration, within the project ISOSPIN, that aims to study the influence of the
isospin degree of freedom in multifragmentation, with special regard to the mid-rapidity source
formed in semiperipheral collisions. The major limitation of the CHIMERA multidetector
system is that neutrons are not measured by this apparatus. Thus, the proposed experiment will
give complementary information to that of the CHIMERA group.

Dates for the proposed experiment
The experiment should be carried out during year 2005, due to detector availability and previous
commitments of the proponents. Spring 2005 would be the first possible date for the experiment.

Short description of the experimental procedure
We propose to measure small angle correlations between neutrons, protons, deuterons and tritons
by means of liquid and CsI scintillators in the angular range 30 – 120 degrees. Event
classification, made through projectile fragments identified in a forward wall, will help sorting
out equilibrium and pre-equilibrium components.
We suggest to utilise the same experimental area (MEDEA hall) as in the LNS experiment of
19982, and a small scattering chamber. The beam should be provided in burst suppression mode,
for determination of the neutron time of flight.
The proposed detector setup for the experiment will be the result of a compromise between
position and energy resolution, population of low relative momentum region, minimization of
the cross-talk between neutron detectors and mechanical/technical constraints.

Experimental setup
•     EDEN neutron detector system (borrowed from KVI Groningen/IPN Orsay). It consists of 40
    NE213 cylindrical liquid scintillator cells of 5 cm thickness and 20 cm diameter. The entrance
    face of the cell is made of a 0.2 mm stainless steel sheet. The side is a stainless steel cylinder 1
    mm thick, surrounded by a 2 mm thick iron cylinder. The EDEN modules will be provided
    with passive (lead) absorbers for charged particles. For the mechanical support there are 5
    modules of 9 positions each. The angle between 2 detectors is 9o. The EDEN detectors should
    be placed at least 3 m from the target position.

•     ARGOS forward wall, fragment detector system (INFN Catania, Italy). ARGOS consists of
    112 phoswich detectors, made of a thin plastic scintillator optically coupled to 5 or 10 cm thick
    BaF2 crystals (25 cm2 face). ARGOS will be mounted inside the vacuum chamber at polar
    angles less than 10o. The purpose of utilizing the forward wall is that by gating on the
    multiplicity of detected fragments, the centrality of the collision can be roughly selected.

  Alternatively, we could utilise the new beamline and the new neutron measurement point near the MAGNEX
experimental hall, would it be ready at the beginning of 2005.


•     EMRIC light charged particles detector system (Lund university, Sweden). EMRIC consists
    of 16 CsI(Tl) scintillator crystals with face of 4x4 cm2 and 10 cm thickness. The light response
    of each CsI(Tl) crystal has been studied and the procedures for the particle identification and
    energy calibration are described in Ref. [11]. The detectors will to be positioned at various
    polar angles outside a thin window in the scattering chamber, in positions matching the empty
    cells of the corresponding neutron detector clusters, thus allowing for the measurement of
    neutron-proton correlations.

•    Acquisition system: local system at LNS.

Participating persons

Person                         Institute              Responsibility
Roberta Ghetti                 Lund University        Spokesperson; analysis
Gaetano Lanzano’               Catania Univ.          Vicespokesperson; local support
Enrico de Filippo              Catania Univ.          Acquisition system; ARGOS
Nicola Colonna                 INFN Bari             Trigger electronics
Giuseppe Tagliente             INFN Bari              Trigger electronics; acquisitions system
Sytze Brandenburg              KVI Groningen         EDEN
Bo Jakobsson                   Lund University       EMRIC; EDEN
Pavel Golubev                  Lund University        EMRIC; EDEN
Vladimir Avdeichikov           Dubna                 EMRIC; EDEN
Johan Helgesson                Malmö University       Analysis; theory
Mats Berg                      Malmö University       Electronics; mechanics

[1] See “Isospin physics in Heavy-Ion Collisions at intermediate energies”,
edited by Bao-An Li and W.U. Schröder (Nova Science Publisher, Inc., New
York, 2001), and references therein; V. Baran, M. Colonna, M. Di Toro, et
al., “Isospin effects in nuclear fragmentation”, Nucl. Phys. A 703 (2002)603.
[2] L.W. Chen, et al., Phys. Rev. Lett. 90 (2003) 162701; L.W. Chen, et al.,
Phys. Rev. C 68 (2003) 014605.
[3] R. Ghetti, et al., ”Isospin effects on two-particle correlation functions
in E/A = 61 MeV 36Ar + 112,124Sn reactions”, nucl-ex/0310012.
[4] R. Ghetti, et al, Nucl. Phys. A 660 (1999) 20.
[5] R. Ghetti, et al, Nucl. Phys. A 674 (2000) 277.
[6] R. Ghetti, et al, Phys. Rev. C 62 (2000) 037603.
[7] R. Ghetti, et al, Phys. Rev. C 64 (2001) 017602.
[8] R. Ghetti, et al, Phys. Rev. Lett. 87 (2001) 102701.
[9] R. Ghetti, et al, Phys. Rev. Lett. 91 (2003) 092701-1.
[10] R. Ghetti, et al, Nucl. Inst. Meth. A 516 (2004) 492.


[11]   V.   Avdeichikov, et al, Nucl. Inst. Meth. A 501 (2003) 499.
[12]   R.   Ghetti, et al, Proceedings Int. Winter Meeting, Bormio 2003.
[13]   R.   Ghetti, et al., Proceedings NN2003, Moskow 2003.
[14]   R.   Ghetti, et al., Proceedings Erice Heavy Ion School, 2003.
[15]   R.   Ghetti, et al., to be submitted to Phys. Rev. C.
[16]   V.   Avdeichikov, et al., to be submitted to Phys. Rev. C.
[17]   A.   Pagano, REVERSE collaboration, LNS Activity Report 2002, p.54.


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