Direct binary projectile fragmentation is being investigated for the case where a 400 MeV 12C projectile breaks up into an particle and a 8Be fragment in the interaction with a thin 93Nb and 197Au target. While the 8Be fragments were measured at 9 , the correlated particles were detected in an angular range between 16 and 30 on the opposite side of the beam. From the preliminary results presented here one may obtain information on the amount of quasi-elastic fragmentation (both fragments do not suffer any further interactions after they are produced). These experimental results indicate that the quasi-elastic break-up process is the dominant contribution to the measured correlation spectra. As was also observed in earlier work, the most forward quasi-elastically emitted particles have energies exceeding the beam velocity

Bassini, R

Buthelezi, E Z

Cerutti, F

Connell, S H

Cowley, A A

Fujita, H

Förtsch, S V

Gadioli, E

Mabiala, J

Mairani, A

Mira, J

Neveling, R

Papka, P

Smit, F D

Steyn, G F

CERN Document Server

 545 Binary projectile fragmentation of 12C at an incident energy                         of 33.3 MeV/nucleon S.V. Förtsch1, G.F. Steyn1, E.Gadioli2, R. Bassini2, E.Z. Buthelezi1, F. Cerutti4, S.H. Connell3,         A.A. Cowley1,6,  H. Fujita5, J. Mabiala6, A. Mairani7,8, J Mira1, R. Neveling1, P. Papka6, F.D. Smit1 1 iThemba LABS, Somerset West, South Africa. 2 Dipartimento di Fisica, Università di Milano and INFN, Sezione di Milano, Italy 3 Department of Physics, University of Johannesburg, Johannesburg, South Africa 4 CERN, Geneva, Switzerland 5 RCNP, Osaka, Japan 6 Department of Physics, University of Stellenbosch, Stellenbosch, South Africa 7 Heidelberg Ion Therapy Center, Heidelberg, Germany 8 INFN, Section of Milan, Milan, Italy Abstract Direct binary projectile fragmentation is being investigated for the case where a 400 MeV 12C projectile breaks up into an  particle and a 8Be fragment in the interaction with a thin 93Nb and 197Au target.  While the 8Be fragments were measured at 9 , the correlated  particles were detected in an angular range between 16  and 30  on the opposite side of the beam. From the preliminary results presented here one may obtain information on the amount of quasi-elastic fragmentation (both fragments do not suffer any further interactions after they are produced).  These experimental results indicate that the quasi-elastic break-up process is the dominant contribution to the measured correlation spectra. As was also observed in earlier work, the most forward quasi-elastically emitted  particles have energies exceeding the beam velocity.  1 Introduction At energies of a few tens of MeV/nucleon, both experimental and theoretical efforts are still underway to fully understand the reaction mechanisms involving heavy ion interactions of light ion projectiles with both heavy and light target nuclei. Several experiments were performed by this collaboration during the past few years in which the energy spectra of projectile-like and intermediate mass fragments produced in the interaction of 12C and 16O at incident energies of 100 MeV to 400 MeV with different nuclei were measured. These target nuclei ranged from 12C to 197Au. All these spectra were characterized by two main features which could be reproduced successfully with theoretical calculations by applying the following two unrelated mechanisms: projectile fragmentation (which was evaluated within the local plane wave approximation [1]) and  nucleon coalescence, a statistical process (which was described by means of a set of Boltzmann Master equations [2]) occurring during the N-N interaction cascade by means of which the nuclei, produced in the partial or complete fusion of the projectile and the target, reach a state of thermal equilibrium. In contrast to pure break-up, first introduced by Serber [3] to describe the break-up of the deuteron, binary fragmentation of heavier nuclei lead to fragments with a slightly lower average energy as well as a broader energy distribution than predicted by the Serber formalism (see e.g. Ref. [4]). This implies either an initial state interaction between the projectile and the target and/or a final state interaction of the observed fragment with the residual nucleus. Based on the satisfactory description of the 8Be spectra studied in the break-up of 12C, it was concluded [5] that the initial state  546 interaction plays an important role in reproducing the break-up contribution of these spectra. This interpretation was further supported in subsequent studies of intermediate mass fragments (IMF) produced in the interaction of 16O with 59Co, 93Nb and 197Au [6]. However, in all these experiments inclusive spectra were measured, i.e. essentially summing up the contributions of all processes in which the observed fragment is produced. The motivation for this study, whereby a coincidence experiment is employed to measure the energies of the two fragments following projectile break-up, is twofold. Firstly, to test and further improve the theoretical framework in order to disentangle the direct binary fragmentation process from the competing sequential break-up mechanism as observed e.g. in similar studies [7,8]. Secondly, to ascertain the amount of quasi-elastic fragmentation (in which case both fragments do not suffer any further interactions after they are produced).  In this paper we present first experimental data of an experiment, carried out at iThemba LABS, South Africa, in which the correlation between  particles and 8Be fragments were measured following the interaction of 12C with 93Nb and 197Au targets at an incident energy of 400 MeV. The experimental set-up and technique is described in Section 2. As the data analysis is ongoing a selection of preliminary results of the experimental data, representing quasi-elastic break-up, is presented in Section 3. Hence only a qualitative discussion of these results is given in Section 4. 2 Experiment The experiment made use of a 400 MeV 12C beam supplied by the cyclotron facility of iThemba LABS, Somerset West, South Africa.  The beam was focused on the target to a spot of less than 3 mm in diameter, mounted at the centre of a 1.5 m diameter scattering chamber. Beam intensities were kept at levels such that the electronic dead time never exceeded 2%. Targets of 12C, 93Nb and 197Au with a thickness of 1.05 mg/cm2, 1.05 mg/cm2 and 0.9 mg/cm2, respectively, were used.  The self-supporting target foils were mounted on aluminium frames with 20 mm diameter apertures.  The target frames were mounted onto an aluminium ladder, which fits into a target driving mechanism at the centre of the scattering chamber.  The target thicknesses were confirmed to an accuracy of 5% by means of the energy loss of -particles emitted from a 228Th source.  Two detector telescopes, referred to as telescopes 1 and 2, respectively, were mounted inside the scattering chamber on rotatable arms on opposite sides of the beam direction in the same reaction plane. The  particles were measured with a standard ΔE-E detector telescope, labeled 1, which consisted of two silicon surface-barrier ( E) detectors of thickness 20 m and 500 m, respectively and a NaI (E) stopping detector. This detector configuration allowed for a low -energy threshold of about 6 MeV. A 50 mm thick brass collimator equipped with a brass insert with an aperture of 14 mm in diameter defined the solid angle of 2.0 msr subtended by telescope 1.  A resonant particle spectrometer, referred to as telescope 2, made up of a silicon strip detector (SSD), used as E detector, in conjunction with a NaI (E) stopping detector was employed to detect and measure the energy spectra of the unbound 8Be fragments in their ground state.  Since the 8Be nucleus consists of two correlated  particles, a 50 mm x 50 mm SSD with 16 strips was used to measure the two  particles in coincidence. The thickness of the SSD was 250 μm which resulted in a low energy threshold of about 50 MeV. By using a Monte Carlo simulation, the effective solid angle of telescope 2 was calculated varying between 1 msr and 4.2 msr in the energy range of the 8Be fragments detected in their ground state. Further details on the application of the SSD can be found in [9]. The stopping E detectors each were a NaI(Tl) crystal of 3 inches in diameter and 5 inches in thickness.  It has a HAVAR entrance window of 2.5 inches in diameter and 7 m thickness.     547 Energy calibrations of the Si surface-barrier detectors were performed with characteristic -particles emitted from a 228Th source, while the calibration parameters of the NaI detector were taken from a previous experiment as described in Ref. [9].  Energy loss calculations of  particles traversing the SSD were used to calibrate the SSD by overlaying the calculated energies onto the experimental -particle locus. The energy uncertainty in each detector telescope did not exceed 2 MeV. Standard electronics were used to perform the particle identification, making use of the ΔE-E technique. 4He events could be well separated from 3He events while a technique described in [9] made it possible to distinguish between 8Be events in the 0+ and 2+ states and to select the dominant 8Be events in the 0+ ground state. Apart from recording data for 8Be-alpha coincidence events, the electronics were also set up to process pre-scaled singles events in each of the two detector telescopes.  The pre-scaling factor was chosen to accept 5% of all the events in each telescope. Energy spectra of pre-scaled singles events were extracted for both  particles and 8Begs fragments. As a consistency check of the detector efficiencies, these spectra were compared to similar spectra measured in earlier studies. In Fig. 1 such a comparison is shown for the double differential cross sections of 8Begs emitted at 9  in the interaction of 400 MeV 12C with 197Au.    Coincidence measurements were carried out with telescope 2 fixed at a laboratory angle of Be = 9  while the angle of telescope 1 was positioned at an angle  on the opposite side of the beam.  A summary of the angle pairs at which the coincidence measurements were carried out, is provided in Table 1. The overall systematic uncertainty of the absolute cross sections is estimated to be less than 15 . Table 1: Summary of angle pairs ( Be / ) at which the coincidence measurements were carried out. Target Nucleus Be  93Nb +9  -16 , -18 , -24 , -26 , -30  197Au +9  -16 , -18 , -20 , -24 , -26     Fig.1: Double differential cross sections of 8Begs emitted in the interaction of 400 MeV 12C with 197Au at a laboratory angle of 9 . The symbols are renormalized pre-scaled singles data obtained in the present experiment while the histogram represents data from Ref. [5].  548 3 Experimental Results In this section, preliminary results are shown of the experimental data for the interaction of 12C with 93Nb and 197Au, respectively, at an incident energy of 400 MeV. Since this experiment was  specifically set up to investigate the binary breakup of 12C into its constituent  particle and 8Be partner, the first step of the data analysis was to obtain coincidence cross sections as a function of the sum of the kinetic energies of the  particles and 8Begs fragments. A subset of these coincidence spectra are shown in Fig. 2 for the interaction of 12C with 93Nb and 197Au, respectively, at 400 MeV incident energy. These spectra are characterized by a sharp peak at the energy corresponding to the beam velocity. This peak is referred to in the following discussion as the quasi-elastic break-up peak. Its yield decreases with increasing  particle emission angle. At about 20 MeV lower down in energy a second peak can be observed for  particle emission angles between 16  and 24 . The nature of this peak is most likely linked to the presence of hydrogen on the targets. Furthermore, the spectra reveal tails which extend from the quasi-elastic peak to lower energies and cover almost the entire energy range. It is assumed that these tails contain mainly events which originate from an inelastic break-up process whereby the  particle undergoes final state interactions with the target nucleus leaving the 8Be partner intact in its ground state.  In order to gauge the amount of hydrogen in the targets equivalent measurements were done with a (CH2)n target. As part of the further analysis it is intended to complement these by measuring the elastic 1H(p,p)1H yields, using a proton beam together with the high resolving power of a magnetic spectrometer. With these additional data at our disposal will we be able to consistently correct the contribution due to scattering off hydrogen in the coincidence spectra. At this stage only very preliminary data are presented here and only the quasi-elastic break-up component of the coincidence spectra can be interpreted at this stage.  In order to extract information relating to the quasi-elastic break-up contribution to the coincidence spectra, two dimensional energy spectra were generated of which an example is shown in Fig. 3 for 400 MeV 12C on 93Nb at the emission angle pair of Be /  = 9  / -16 .  Also shown in Fig. 3 is the gate defining the quasi-elastic break-up events. The event locus thought to be originating from the emission off hydrogen is also clearly visible in Fig. 3 at about 20 MeV below the quasi-elastic locus.  Energy integrated cross sections were extracted for the binary quasi-elastic break-up of 12C scattering off 93Nb and 197Au which are presented as function of the  particle emission angle in Fig. 4. These cross sections decrease with increasing angle and seem to be fairly similar in trend with respect to the target nucleus. The roles which both the  particles and the 8Be fragments play in a quasi-elastic break-up process is closer examined by investigating the energy distributions of the  particles and the corresponding 8Be fragments. These distributions are shown for the events defined by the gate in Fig. 3 and are presented as function of the respective energies and coincidence angle pairs as shown in Fig. 5. The arrows in Fig. 5 indicate the respective values of the beam velocity. While the energy distributions displayed at the larger -particle angle of  = -24  follow the expected trend, the result at the more forward emission angle of  = -16  is surprising. The -particle energy distribution is found to peak at an energy which exceeds the corresponding beam velocity by about 15%. Although this result is puzzling, it is consistent with earlier observations made for similar reactions of a binary break-up process whereby the  particle is emitted at a forward angle in coincidence with a heavier partner [7,10]. To our knowledge, no consistent explanation could thus far be given for this phenomenon and remains one of the unresolved issues in this field [10].    549  Fig. 2: Coincidence cross sections as a function of the sum of the kinetic energies of the 8Be fragments detected in the ground state at 9  and the  particles detected at the angles as indicated.  Fig. 3: An example of a two dimensional coincidence energy spectrum. Quasi-elastic events are selected by the gate. Also visible is the second locus most probably due to interaction with hydrogen on the target, as well as other events originating from inelastic break-up.     550  Fig. 4: Angular distributions of energy-integrated quasi-elastic coincidence cross sections. Open and solid symbols represent data from the 93Nb and 197Au target, respectively. Statistical errors are indicated by the bars.  Fig. 5: Coincidence cross sections for quasi-elastic events as defined by the gate in Fig. 3 shown as function of the -particle energy and 8Begs energy, respectively. The solid and dotted histograms represent data of the angle pairs ( Be /  = 9  / -16 ) and ( Be /  = 9  / -24 ), respectively. The arrows indicate the corresponding energies with respect to the beam velocity.  4 Summary and Outlook A coincidence experiment was performed to measure the correlation of  particles and 8Be fragments produced in the direct binary break-up of 12C in the interaction with 93Nb and 197Au at an incident energy of 400 MeV. Coincidence energy spectra were obtained for a forward fixed 8Be emission angle of +9  and -particle angles ranging from -16  to -30 . The two detector telescopes employed in these measurements allowed for a clean particle separation between 3He and 4He events as well as to distinguish between the 0+ ground state of interest and the first excited 2+ state of 8Be. The coincidence spectra as a function of the sum of the energies reveal two distinct regions, namely a quasi-elastic peak at around the beam energy and a tail consisting of inelastic break-up events. Since it is unlikely that the 8Be fragments would survive any final state interaction with the residual target nucleus, such interactions are believed to be mainly inelastic processes between the  particle and the target nucleus. A second peak at about 20 MeV below the quasi-elastic peak which is observed for  particle angles between 16  and 22 , could originate from interactions between 12C and hydrogen in the target. Analyses and measurements are underway to determine and correct for these contributions to the  551 inelastic parts of the spectra. First and preliminary conclusions can therefore be drawn only from the quasi-elastic break-up of 12C. Energy-integrated angular distributions would indicate that this break-up process seems to be dominant in the measured correlation spectra. The individual role of the  particles and 8Be fragments in the binary quasi-elastic break-up process was investigated by extracting their respective energy distributions at the different  particle emission angles. As was previously found in similar studies, the -particle distribution, at the most forward emission angle, is found to peak at an energy which is considerably higher than the corresponding beam velocity. Attempting to understand this phenomenon together with a consistent description of the inelastic break-up processes will be at the center of the further experimental and theoretical analyses of this work.  Acknowledgement This conference contribution was funded by the South African National Research Foundation (NRF) under UID No. 69618. The financial support is gratefully acknowledged. References [1] K. W. McVoy and M. Carolina Nemes, Z. Phys. A295 (1980) 177. [2] M. Cavinato et al., Nucl. Phys. A 679 (2001) 753 and references therein. [3] R. Serber, Phys. Rev. 72 (1947) 1008. [4] E. Gadioli et al., Eur. Phys. J. A  8 (2000) 373. [5] E. Gadioli et al., Eur. Phys. J. A 11 (2001) 161. [6] E. Gadioli et al., Eur. Phys. J. A 17 (2003) 195. [7] P. L. Gonthier et al., Phys Rev. C 35 (1987) 1946. [8] A. Magiera et al., Phys. Rev. C 57 (1998) 749. [9] G. F. Steyn et al., Nucl. Inst. Meth. A 538 (2005) 559. [10] H. Fuchs and K. Möhring, Rep. Prog. Phys. 57 (1994) 231.                       

Binary projectile fragmentation of 12C at an incident energy of 33.3 MeV/nucleon

Binary projectile fragmentation of 12C at an incident energy of 33.3 MeV/nucleon

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