physics.fsu.edu | Experimental Nuclear Physics

The Mechanism of Intermediate Energy Fragmentation Reactions

A. Obertelli, K. Yoneda, A. Gade, D. Bazin, B.A. Brown, C.M. Campbell, J.M. Cook, A.D. Davies, D.-C. Dinca, T. Glasmacher, P.G. Hansen, T. Hoagland, J.-L. Lecouey, W.F. Mueller, J.R. Terry and H. Zwahlen
NSCL/MSU

P.D. Cottle, K.W. Kemper, R.R. Reynolds and B.T. Roeder
FSU

J.A. Tostevin
University of Surrey

Intermediate energy one-nucleon knockout reactions have been developed into important spectroscopic tools for exotic isotopes [1]. Two-proton knockout reactions have also been used with notable success in neutron-rich nuclei, including the near-dripline nucleus ⁴²Si [2-4]. The work summarized here aimed to examine the reaction mechanism at work in two-neutron knockout reactions in proton-rich nuclei and to more generally characterize whether fragmentation reactions with varying numbers of removed nucleons proceed via direct or statistical mechanisms. These experiments were performed at the National Superconducting Cyclotron Laboratory using the SeGA g-ray array and the S800 spectrograph, and the results have been published in two papers in Physical Review C [5,6].

The experiments were performed with proton-rich isotopes in the sd shell because the wavefunctions for these nuclei are relatively well-understood. For the two-neutron knockout reactions, two mechanisms – direct one-step and multistep – can compete, in principle. However, the multistep process was expected to be suppressed because the multistep process would necessarily involve the evaporation of single neutrons. In these proton-rich nuclei, evaporation of single protons would be favored instead.

The two-neutron knockout reactions ²⁶Si®²⁴Si+2n, ³⁰S®²⁸S+2n, and ³⁴Ar®³²Ar+2n were measured [6]. The inclusive cross sections were 1.01(10) mb, 0.73(8) mb and 0.48(6) mb, respectively. In all three cases, most of the cross section went to the ground state. These cross sections are comparable to those measured for the two-proton knockout reactions in neutron-rich nuclei by Bazin et al. [2]. The observed cross sections – particularly the preference for populating the 0⁺ ground states - confirmed the direct one-step model for these reactions [6].

The data also provided the opportunity to study whether the reaction mechanism varied with the number of removed nucleons [5]. The residual nuclei ¹⁸Ne, ²¹Na and ²⁴Si were populated with a variety of fragmentation reactions in which the numbers of nucleons removed ranged from two to sixteen. The patterns of populating the ground and excited states in the residual nuclei confirmed that two-nucleon removal reactions are direct and non-statistical, and that reactions where more than five nucleons are removed are purely statistical. Both mechanisms participate in reactions where three, four or five nucleons are removed [5].

[1] P.G. Hansen and J.A. Tostevin, Annu. Rev. Nucl. Part. Sci. 53, 219 (2003).

[2] D. Bazin et al., Phys. Rev. Lett. 91, 012501 (2003).

[3] J. Fridmann et al., Nature 435, 922 (2005).

[4] J. Fridmann et al., Phys. Rev. C 74, 034313 (2006).

[5] A. Obertelli et al., Phys. Rev. C 73, 044605 (2006).

[6] K. Yoneda et al., Phys. Rev. C 74, 021303(R) (2006).




	FSU Experimental Nuclear Physics Main			*The Mechanism of Intermediate Energy Fragmentation Reactions* A. Obertelli, K. Yoneda, A. Gade, D. Bazin, B.A. Brown, C.M. Campbell, J.M. Cook, A.D. Davies, D.-C. Dinca, T. Glasmacher, P.G. Hansen, T. Hoagland, J.-L. Lecouey, W.F. Mueller, J.R. Terry and H. Zwahlen NSCL/MSU P.D. Cottle, K.W. Kemper, R.R. Reynolds and B.T. Roeder FSU *J.A. Tostevin* *University* *of Surrey* Intermediate energy one-nucleon knockout reactions have been developed into important spectroscopic tools for exotic isotopes [1]. Two-proton knockout reactions have also been used with notable success in neutron-rich nuclei, including the near-dripline nucleus ⁴²Si [2-4]. The work summarized here aimed to examine the reaction mechanism at work in two-neutron knockout reactions in proton-rich nuclei and to more generally characterize whether fragmentation reactions with varying numbers of removed nucleons proceed via direct or statistical mechanisms. These experiments were performed at the National Superconducting Cyclotron Laboratory using the SeGA g-ray array and the S800 spectrograph, and the results have been published in two papers in Physical Review C [5,6]. The experiments were performed with proton-rich isotopes in the sd shell because the wavefunctions for these nuclei are relatively well-understood. For the two-neutron knockout reactions, two mechanisms – direct one-step and multistep – can compete, in principle. However, the multistep process was expected to be suppressed because the multistep process would necessarily involve the evaporation of single neutrons. In these proton-rich nuclei, evaporation of single protons would be favored instead. The two-neutron knockout reactions ²⁶Si®²⁴Si+2n, ³⁰S®²⁸S+2n, and ³⁴Ar®³²Ar+2n were measured [6]. The inclusive cross sections were 1.01(10) mb, 0.73(8) mb and 0.48(6) mb, respectively. In all three cases, most of the cross section went to the ground state. These cross sections are comparable to those measured for the two-proton knockout reactions in neutron-rich nuclei by Bazin et al. [2]. The observed cross sections – particularly the preference for populating the 0⁺ ground states - confirmed the direct one-step model for these reactions [6]. The data also provided the opportunity to study whether the reaction mechanism varied with the number of removed nucleons [5]. The residual nuclei ¹⁸Ne, ²¹Na and ²⁴Si were populated with a variety of fragmentation reactions in which the numbers of nucleons removed ranged from two to sixteen. The patterns of populating the ground and excited states in the residual nuclei confirmed that two-nucleon removal reactions are direct and non-statistical, and that reactions where more than five nucleons are removed are purely statistical. Both mechanisms participate in reactions where three, four or five nucleons are removed [5]. [1] P.G. Hansen and J.A. Tostevin, Annu. Rev. Nucl. Part. Sci. 53, 219 (2003). [2] D. Bazin et al., Phys. Rev. Lett. 91, 012501 (2003). [3] J. Fridmann et al., Nature 435, 922 (2005). [4] J. Fridmann et al., Phys. Rev. C 74, 034313 (2006). [5] A. Obertelli et al., Phys. Rev. C 73, 044605 (2006). [6] K. Yoneda et al., Phys. Rev. C 74, 021303(R) (2006). Back to Dr. Cottle's Research Menu ^{Back to Dr. Kemper's Research Menu}