This chapter presents the process of beta-delayed emission of a single neutron, beginning with some of the main theoretical ideas that enter into the interpretation of neutron spectra such as the beta strength function and the concept of Porter-Thomas fluctuations. It addresses the question of the experimental tools and their inherent limits. The most basic limit is clearly the one at which prompt instability to particle emission sets in, but in many cases the experimental production cross-sections go to zero long before this limit is reached. The main reactions used for producing neutron-rich products are deep-inelastic and two-body transfer reactions during peripheral collisions at relatively low energy and fragmentation reactions for ions at relativistic and intermediate energies. Neutron emission from excited states populated in beta decay is the process unique to neutron-rich nuclei, and in the following we give examples for both light and heavy systems.

Results are presented from a one-neutron knockout experiment at relativistic energies of \( \approx 420 A\) MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the \( \nu p_{1/2}\) , \( \nu p_{3/2}\) , (L = 1 and \( \nu f_{7/2}\) , \( \nu f_{5/2}\) (L = 3 neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the \( \nu f_{7/2}\) to the \( \nu p_{3/2}\) orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions \( V_{low k}\) obtained by evolving a chiral N3LO nucleon-nucleon potential.

The gamma-ray spectroscopy of Si-25 and S-29 has been performed using single neutron knockout reactions with intermediate energy beams of the exotic isotopes Si-26 and S-30. Two gamma rays have been observed in Si-25 and three in S-29. These are the first gamma rays observed in these two isotopes. These two nuclei appear to be well deformed, and possible future intermediate-energy Coulomb excitation measurements would confirm their rotational nature.

The one-proton knockout reaction Be-9(Ti-54, Sc-53 + gamma) X at 72 MeV/nucleon has been measured. The location of the first 3/2(-) state at 2110(3) keV was confirmed, and new gamma-ray transitions were observed at 1111(2), 1273(2), 1539(4), and 2495(5) keV. Large spectroscopic strength to excited states in 53Sc was found and attributed to the knockout of sd-shell protons.

Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV nu p3/2-hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1/2- state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N=34 gap in 54Ca.

Both one-proton and one-neutron knockout reactions were performed with fast beams of two asymmetric, neutron-deficient rare isotopes produced by projectile fragmentation. The reactions are used to probe the nucleon spectroscopic strengths at both the weakly and strongly bound nucleon Fermi surfaces. The one-proton knockout reactions Be-9(S-28, P-27)X and Be-9(Si-24, Al-23)X probe the weakly bound valence proton states and the one-neutron knockout reactions and Be-9(S-28, S-27)X and Be-9(Si-24, Si-23)X the strongly bound neutron states in the two systems. The spectroscopic strengths are extracted from the measured cross sections by comparisons with an eikonal reaction theory. The reduction of the experimentally deduced spectroscopic strengths, relative to the predictions of shell-model calculations, is of order 0.8-0.9 in the removal of weakly bound protons and 0.3-0.4 in the knockout of the strongly bound neutrons. These results support previous studies at the extremes of nuclear binding and provide further evidence that in asymmetric nuclear systems the nucleons of the deficient species, at the more-bound Fermi surface are more strongly correlated than those of the more weakly bound excess species.

The nuclei Ca-47 and Ti-55 were populated in one-neutron knock-out reactions at relativistic energies. Momentum distributions of the residual nuclei as well as gamma-ray spectra were measured at the GSI fragment separator (FRS). Preliminary results of the ongoing analysis including cross sections and spin/parity assignments are presented.

K. Yoneda, ∗ A. Obertelli, A. Gade, 2 D. Bazin, B. A. Brown, 2 C. M. Campbell, 2 J.M. Cook, 2 P. D. Cottle, A. D. Davies, 2 D.-C. Dinca, 2, † T. Glasmacher, 2 P.G. Hansen, 2 T. Hoagland, K. W. Kemper, J.-L. Lecouey, ‡ W. F. Mueller, R. R. Reynolds, B. T. Roeder, J.R. Terry, 2 J. A. Tostevin, and H. Zwahlen 2 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 Department of Physics, Florida State University, Tallahassee, FL 32306 Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom (Dated: February 8, 2008)

The breakdown of the N=20 magic number in the so-called island of inversion around Mg-32 is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of Mg-30 and Mg-32, with the latter consisting nearly purely of 2p-2h N=20 cross-shell configurations. Single-neutron knockout experiments on Mg-30,Mg-32 projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

K. L. Yurkewicz,1,2,∗ D. Bazin,1 B. A. Brown,1,2 J. Enders,1,† A. Gade,1,2 T. Glasmacher,1,2,‡ P. G. Hansen,1,2 V. Maddalena,1,2,§ A. Navin,1,‖ B. M. Sherrill,1,2 and J. A. Tostevin3 1National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA 2Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA 3Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom (Received 20 March 2006; published 16 August 2006)

Measurements of the N=28 isotones 42Si, 43P and 44S using one- and two-proton knockout reactions from the radioactive beam nuclei 44S and 46Ar are reported. The knockout reaction cross sections for populating 42Si and 43P and a 184 keV gamma-ray observed in 43P establish that the d_{3/2} and s_{1/2} proton orbits are nearly degenerate in these nuclei and that there is a substantial Z=14 subshell closure separating these two orbits from the d_{5/2} orbit. The increase in the inclusive two-proton knockout cross section from 42Si to 44S demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the d_{5/2} proton strength in 43P via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure.

We report on direct experimental evidence of the population of the 3/2− intruder state in 27Ne in the knockout of a single neutron from the ground state of 28Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z≪N and N≈20. Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N=20 where normal and intruder configurations coexist at low excitation energy. This observation demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.

The two-proton knockout reaction Be-9(Ti-54,Ca-52+gamma) has been studied at 72 MeV/nucleon. Besides the strong feeding of the Ca-52 ground state, the only other sizeable cross section proceeds to a 3(-) level at 3.9 MeV. There is no measurable direct yield to the first excited 2(+) state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of Ca-52.

K. L. Yurkewicz, 2, ∗ D. Bazin, B. A. Brown, 2 J. Enders, † A. Gade, 2 T. Glasmacher, 2, ‡ P. G. Hansen, 2 V. Maddalena, 2, § A. Navin, ¶ B. M. Sherrill, 2 and J.A. Tostevin National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 Department of Physics, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom (Dated: June 26, 2006)

The single-particle structure of Ni-57 and level structure of Ni-56 were investigated with the Be-9 (Ni-57,Ni-56+gamma)X reaction at 73 MeV/nucleon. An inclusive cross section of 41.4(12) mb was obtained for the reaction, compared to a theoretical prediction of 85.4 mb, hence only 48(2)% of the theoretical cross section is exhausted. This reduction in the observed spectroscopic strength is consistent with that found for lighter well-bound nuclei. One-neutron removal spectroscopic factors of 0.58(11) to the ground state and 3.7(2) to all excited states of Ni-56 were deduced.

Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient Ar-34, S-30, and Si-26. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the Ar-32, S-28 and Si-24 knockout residues, have been determined using the combination of particle and gamma-ray spectroscopy. Similar to the two-proton knockout mechanism on the neutron-rich side of the nuclear chart, these two-neutron removal reactions from already neutron-deficient nuclei are also shown to be consistent with a direct reaction mechanism.

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process whereas the removal of more than five nucleons appears to be mainly statistical.