- Email: [email protected]
nucleon
Nuclear Physics A 734 (2004) E100–E103 www.elsevier.com/locate/npe
Mid-rapidity emission in MeV/nucleon
124
Sn,
124
Xe +
124
Sn,
112
Sn reactions at 28
D.V. Shettya , A. Keksisa , E. Martina , A. Ruangmaa , G. A. Souliotisa, M. Veselskya , E.M. Winchestera , S.J. Yennelloa , K. Hagela , Y.G. Maa , A. Makeeva , N. Mariea , M. Murraya , J.B. Natowitza , L. Qina, P. Smitha, R. Wadaa , J. Wanga , M. Cinauserob , E. Fiorettob, G. Preteb , D. Fabrisc , M. Lunardonc , G. Nebbiac , V. Rizzic , G. Viestic , J. Cibord, Z. Majkae , P. Staszele , R. Alfarof , A. Martinez-Davalosf , A. Menchaca-Rochaf , Y. El Masrig and T. Keutgeng a
Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA
b c
INFN Laboratori Nazionali di Legnaro, I-35020 Legnaro, Italy
INFN and Dipartimento di Fisica dell Universita di Padova, I-35131 Padova, Italy
d
Institute of Nuclear Physics, PL-31342, Krakow, Poland
e
Jagellonian University, M. Smoluchowski Institute of Physics, PL-30059, Krakow, Poland
f
Instituto de Fisica, Universidad National Autonoma de Mexico, Mexico City, Mexico
g
IPN, FNRS, Universite Catholique de Louvain, B-1348, Louvain-la-Neuve, Belgium
Light charged particle, (Z = 2), and intermediate-mass fragments, (2 < Z < 8), emitted in the mid-rapidity region were studied as a function of collision violence in 124 Sn, 124 Xe + 124 Sn, 112 Sn reactions at 28 MeV/nucleon. With increasing centrality, the heavier 6 He isotope is found to be emitted closer to mid-rapidity than the lighter 3 He isotope. The emission becomes increasingly favorable for particles with higher charge, Z, indicating mid-rapidity region to be not only neutron rich, but also a rich source of heavy fragment (cluster) formation. 1. INTRODUCTION Heavy ion reactions at low energies (E ≤ 15 MeV/nucleon), for noncentral collisions, are essentially binary in nature. The energy dissipation and mass transfer at these energies are well explained by nucleon exchange models. At relativistic energies (E ≥ 300 MeV/nucleon), where the overlap region (participant) between the projectile and the target decouples from the spectators, the collisions are explained within the participantspectator model. In the intermediate energy region, a transition between the two energy regimes in the form of intense emission of light particles and heavy fragments has been observed [1,3]. These emissions, often referred to as midvelocity or mid-rapidity emissions, 0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.03.030
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
E101
take place on a very short time scale with parallel velocities intermediate between those of the projectile and the target. The intensity of emission is found to be unexpected on the basis of the statistical emission from the hot, fully accelerated main fragments, and depends strongly on the dynamics of the collision. A variety of mechanisms, such as, fast pre-equilibrium emissions, neck emitted particles and fragments, as well as light fission fragments preferentially aligned in between the two main reaction partners, have been proposed [4,5]. It has also been shown that the mid-rapidity emission source is neutron rich relative to the projectile-like and target-like source. 2. EXPERIMENT The present measurements were carried out at the Cyclotron Institute of Texas A&M University using 124 Sn and 124 Xe beams, at 28 MeV/nucleon, on self-supporting 124 Sn and 112 Sn targets. Light charged particles (Z ≤ 2) and intermediate mass fragments (2 < Z ≤ 8) were measured using NIMROD (Neutron Ion Multidetector for Reaction Oriented Dynamics), a 4π charged particle detection system [2]. The data presented here are mainly from the Si-Si-CsI detectors of the detection system. 3. RESULTS AND DISCUSSION In order to study the mid-rapidity emission, it is important to characterize the emission pattern of the observed light charged particles and intermediate mass fragments as a function of collision violence. Fig. 1 shows the charged particle multiplicity distribution used to characterize the collision violence. The multiplicity distribution is divided into four different bins shown by the dotted lines. The lowest bin (bin 1) with low charged particle multiplicity corresponds to peripheral collisions, and the highest bin (bin 4) with large multiplicity to central collisions. The inclusive measurement of the parallel velocity distribution for the alpha particle at various emission angles is shown in Fig. 2 (top panel). The middle and the bottom panels show the exclusive results, for the four charged particle multiplicity bins, for emission at an intermediate angle (θ lab = 13◦ ) and back angle (θlab = 32◦ ), respectively. For the lowest multiplicity bin (bin 1), a two-peak structure is observed in the intermediate angle rapidity distribution. The higher component of the two peaks originates from a projectile-like source and the lower component from a targetlike source in binary type collisions. With increasing centrality, the two peaks merge into a single broad peak localized close to the mid-rapidity region, indicating a more violent collision and increasing contribution from an intermediate-velocity source. The rapidity distribution at θ lab = 32◦ , however, shows little difference for different multiplicity bins and is mainly dominated by target-like emission. The intermediate-angle rapidity distribution thus appears to cover a broad range of emission from various sources and therefore the most suitable for probing the origin of the emitted light charged particles and heavy fragments. In the following, we compare the rapidity distributions for various He isotopes and the heavier Li and B fragments obtained at this angle. Fig. 3 shows the rapidity distribution for the 3 He, 4 He and 6 He isotopes. The different panels in the figure (from top to bottom for each isotope) correspond to the rapidity distribution for the four different charged particle multiplicity bins. The dotted lines in each panel indicate the mid-rapidity velocity. One observes that, at low charged-particle
E102
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
multiplicity (peripheral collisions), the projectile and target-like sources are the dominant contributors for all three isotopes. With increasing multiplicity, the two peaks in the rapidity distribution come closer, indicating the dissipative nature of the collision, until
Figure 1. Charged-particle multiplicity distribution for the reaction 124 Sn + 124 Sn. The dotted lines show the four different region of the multiplicity distribution as discussed in the text.
Figure 2. Parallel velocity distributions of 4 He from 124 Sn + 124 Sn reaction. (Top) inclusive distribution for various laboratory angles. (middle) velocity distribution at θlab = 13◦ for different charged particle multiplicity bins. (bottom) for θ lab = 32◦ .
they merge into a single peak for all the three isotopes. At the highest multiplicity bin, the peak positions in the rapidity distribution, however, vary for the different He isotopes. While the 3 He isotope is observed to be centered closer to the projectile-like source velocity, the distributions for the 4 He and 6 He appear to evolve slowly toward midrapidity. A similar observation was made by Dempsey et al. [3] for the reaction of 124 Xe + 124 Sn at 55 MeV/nucleon. The 6 He cross section distribution in this work was observed to peak at the center of mass, stretched along the beam direction, and with almost no indication of contribution from a projectile-like source. It thus appears that the heavier isotopes of He have a different origin compared to their lighter counterparts. A similar evolution of the rapidity distribution with the multiplicity bin can also be seen for heavier fragments by comparing the 4 He with 7 Li and 11 B fragments (Fig. 4). However, the difference here is that, for the highest multiplicity bin, the fragments tend to localize closer to the mid-rapidity region with increasing fragment charge, Z. This suggests that
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
E103
the mid-rapidity region becomes increasingly favorable for the production of fragments with higher charge, Z. The relative increase in the production of heavy fragments with increasing centrality was also observed in the yield ratios for fragments (Z = 3 - 7) emitted in the forward and backward angles as a function of multiplicity. The heavy fragments show a steep rise in their yields compared to a gradual increase for the lighter fragments as the centrality of the collision increases. The mid-rapidity region thus seems not only a neutron-rich source but also a rich source for heavy-fragment formation.
Figure 3. Rapidity distribution for the 3 He (left), 4 He (center) and 6 He (right) isotopes in 124 Sn + 124 Sn reaction at a laboratory angle of 13◦ . Each row (from top to bottom) corresponds to a different charged particle multiplicity bin. The dotted line indicates the mid-rapidity velocity.
Figure 4. Rapidity distribution for the He (left), Li (center) and B (right) fragments in 124 Sn + 124 Sn reaction at a laboratory angle of 13◦ . Each row (from top to bottom) corresponds to a different charged particle multiplicity bin. The dotted line indicates the mid-rapidity velocity.
REFERENCES 1. 2. 3. 4. 5.
J. Toke et al., Phys. Rev. Lett. 75 (1995) 2920. R. Wada et al., Progress in Research, Cyclotron Institute, TAMU, V28 (2001). J. Dempsey et al., Phys. Rev. C 54 (1996) 1710. C. Montoya et al., Phys. Rev. Lett. 73 (1994) 3070. L. Gingras et al., Phys. Rev. C 65 (2002) 061604.
Mid-rapidity emission in MeV/nucleon
124
Sn,
124
Xe +
124
Sn,
112
Sn reactions at 28
D.V. Shettya , A. Keksisa , E. Martina , A. Ruangmaa , G. A. Souliotisa, M. Veselskya , E.M. Winchestera , S.J. Yennelloa , K. Hagela , Y.G. Maa , A. Makeeva , N. Mariea , M. Murraya , J.B. Natowitza , L. Qina, P. Smitha, R. Wadaa , J. Wanga , M. Cinauserob , E. Fiorettob, G. Preteb , D. Fabrisc , M. Lunardonc , G. Nebbiac , V. Rizzic , G. Viestic , J. Cibord, Z. Majkae , P. Staszele , R. Alfarof , A. Martinez-Davalosf , A. Menchaca-Rochaf , Y. El Masrig and T. Keutgeng a
Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA
b c
INFN Laboratori Nazionali di Legnaro, I-35020 Legnaro, Italy
INFN and Dipartimento di Fisica dell Universita di Padova, I-35131 Padova, Italy
d
Institute of Nuclear Physics, PL-31342, Krakow, Poland
e
Jagellonian University, M. Smoluchowski Institute of Physics, PL-30059, Krakow, Poland
f
Instituto de Fisica, Universidad National Autonoma de Mexico, Mexico City, Mexico
g
IPN, FNRS, Universite Catholique de Louvain, B-1348, Louvain-la-Neuve, Belgium
Light charged particle, (Z = 2), and intermediate-mass fragments, (2 < Z < 8), emitted in the mid-rapidity region were studied as a function of collision violence in 124 Sn, 124 Xe + 124 Sn, 112 Sn reactions at 28 MeV/nucleon. With increasing centrality, the heavier 6 He isotope is found to be emitted closer to mid-rapidity than the lighter 3 He isotope. The emission becomes increasingly favorable for particles with higher charge, Z, indicating mid-rapidity region to be not only neutron rich, but also a rich source of heavy fragment (cluster) formation. 1. INTRODUCTION Heavy ion reactions at low energies (E ≤ 15 MeV/nucleon), for noncentral collisions, are essentially binary in nature. The energy dissipation and mass transfer at these energies are well explained by nucleon exchange models. At relativistic energies (E ≥ 300 MeV/nucleon), where the overlap region (participant) between the projectile and the target decouples from the spectators, the collisions are explained within the participantspectator model. In the intermediate energy region, a transition between the two energy regimes in the form of intense emission of light particles and heavy fragments has been observed [1,3]. These emissions, often referred to as midvelocity or mid-rapidity emissions, 0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.03.030
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
E101
take place on a very short time scale with parallel velocities intermediate between those of the projectile and the target. The intensity of emission is found to be unexpected on the basis of the statistical emission from the hot, fully accelerated main fragments, and depends strongly on the dynamics of the collision. A variety of mechanisms, such as, fast pre-equilibrium emissions, neck emitted particles and fragments, as well as light fission fragments preferentially aligned in between the two main reaction partners, have been proposed [4,5]. It has also been shown that the mid-rapidity emission source is neutron rich relative to the projectile-like and target-like source. 2. EXPERIMENT The present measurements were carried out at the Cyclotron Institute of Texas A&M University using 124 Sn and 124 Xe beams, at 28 MeV/nucleon, on self-supporting 124 Sn and 112 Sn targets. Light charged particles (Z ≤ 2) and intermediate mass fragments (2 < Z ≤ 8) were measured using NIMROD (Neutron Ion Multidetector for Reaction Oriented Dynamics), a 4π charged particle detection system [2]. The data presented here are mainly from the Si-Si-CsI detectors of the detection system. 3. RESULTS AND DISCUSSION In order to study the mid-rapidity emission, it is important to characterize the emission pattern of the observed light charged particles and intermediate mass fragments as a function of collision violence. Fig. 1 shows the charged particle multiplicity distribution used to characterize the collision violence. The multiplicity distribution is divided into four different bins shown by the dotted lines. The lowest bin (bin 1) with low charged particle multiplicity corresponds to peripheral collisions, and the highest bin (bin 4) with large multiplicity to central collisions. The inclusive measurement of the parallel velocity distribution for the alpha particle at various emission angles is shown in Fig. 2 (top panel). The middle and the bottom panels show the exclusive results, for the four charged particle multiplicity bins, for emission at an intermediate angle (θ lab = 13◦ ) and back angle (θlab = 32◦ ), respectively. For the lowest multiplicity bin (bin 1), a two-peak structure is observed in the intermediate angle rapidity distribution. The higher component of the two peaks originates from a projectile-like source and the lower component from a targetlike source in binary type collisions. With increasing centrality, the two peaks merge into a single broad peak localized close to the mid-rapidity region, indicating a more violent collision and increasing contribution from an intermediate-velocity source. The rapidity distribution at θ lab = 32◦ , however, shows little difference for different multiplicity bins and is mainly dominated by target-like emission. The intermediate-angle rapidity distribution thus appears to cover a broad range of emission from various sources and therefore the most suitable for probing the origin of the emitted light charged particles and heavy fragments. In the following, we compare the rapidity distributions for various He isotopes and the heavier Li and B fragments obtained at this angle. Fig. 3 shows the rapidity distribution for the 3 He, 4 He and 6 He isotopes. The different panels in the figure (from top to bottom for each isotope) correspond to the rapidity distribution for the four different charged particle multiplicity bins. The dotted lines in each panel indicate the mid-rapidity velocity. One observes that, at low charged-particle
E102
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
multiplicity (peripheral collisions), the projectile and target-like sources are the dominant contributors for all three isotopes. With increasing multiplicity, the two peaks in the rapidity distribution come closer, indicating the dissipative nature of the collision, until
Figure 1. Charged-particle multiplicity distribution for the reaction 124 Sn + 124 Sn. The dotted lines show the four different region of the multiplicity distribution as discussed in the text.
Figure 2. Parallel velocity distributions of 4 He from 124 Sn + 124 Sn reaction. (Top) inclusive distribution for various laboratory angles. (middle) velocity distribution at θlab = 13◦ for different charged particle multiplicity bins. (bottom) for θ lab = 32◦ .
they merge into a single peak for all the three isotopes. At the highest multiplicity bin, the peak positions in the rapidity distribution, however, vary for the different He isotopes. While the 3 He isotope is observed to be centered closer to the projectile-like source velocity, the distributions for the 4 He and 6 He appear to evolve slowly toward midrapidity. A similar observation was made by Dempsey et al. [3] for the reaction of 124 Xe + 124 Sn at 55 MeV/nucleon. The 6 He cross section distribution in this work was observed to peak at the center of mass, stretched along the beam direction, and with almost no indication of contribution from a projectile-like source. It thus appears that the heavier isotopes of He have a different origin compared to their lighter counterparts. A similar evolution of the rapidity distribution with the multiplicity bin can also be seen for heavier fragments by comparing the 4 He with 7 Li and 11 B fragments (Fig. 4). However, the difference here is that, for the highest multiplicity bin, the fragments tend to localize closer to the mid-rapidity region with increasing fragment charge, Z. This suggests that
D.V. Shetty et al. / Nuclear Physics A 734 (2004) E100–E103
E103
the mid-rapidity region becomes increasingly favorable for the production of fragments with higher charge, Z. The relative increase in the production of heavy fragments with increasing centrality was also observed in the yield ratios for fragments (Z = 3 - 7) emitted in the forward and backward angles as a function of multiplicity. The heavy fragments show a steep rise in their yields compared to a gradual increase for the lighter fragments as the centrality of the collision increases. The mid-rapidity region thus seems not only a neutron-rich source but also a rich source for heavy-fragment formation.
Figure 3. Rapidity distribution for the 3 He (left), 4 He (center) and 6 He (right) isotopes in 124 Sn + 124 Sn reaction at a laboratory angle of 13◦ . Each row (from top to bottom) corresponds to a different charged particle multiplicity bin. The dotted line indicates the mid-rapidity velocity.
Figure 4. Rapidity distribution for the He (left), Li (center) and B (right) fragments in 124 Sn + 124 Sn reaction at a laboratory angle of 13◦ . Each row (from top to bottom) corresponds to a different charged particle multiplicity bin. The dotted line indicates the mid-rapidity velocity.
REFERENCES 1. 2. 3. 4. 5.
J. Toke et al., Phys. Rev. Lett. 75 (1995) 2920. R. Wada et al., Progress in Research, Cyclotron Institute, TAMU, V28 (2001). J. Dempsey et al., Phys. Rev. C 54 (1996) 1710. C. Montoya et al., Phys. Rev. Lett. 73 (1994) 3070. L. Gingras et al., Phys. Rev. C 65 (2002) 061604.