Engineering topics
Christian, G.
Publications and source records attributed to Christian, G..
Three-body correlations in the ground-state decay of O 26
Background: Theoretical calculations have shown that the energy and angular correlations in the three-body decay of the two-neutron unbound 26 O can provide information on the ground-state wave function, which has been predicted to have a dineutron configuration and 2n halo structure. Purpose: To use the experimentally measured three-body correlations to gain insight into the properties of 26 O, including the decay mechanism and ground-state resonance energy. Method: 26 O was produced in a one-proton knockout reaction from 27 F and the 24 O + n + n decay products were measured using the MoNA-Sweeper setup. Additionally, the three-body correlations from the 26 O ground-state resonance decay were extracted. The experimental results were compared to Monte Carlo simulations in which the resonance energy and decay mechanism were varied. Results: The measured three-body correlations were well reproduced by the Monte Carlo simulations but were not sensitive to the decay mechanism due to the experimental resolutions. However, the three-body correlations were found to be sensitive to the resonance energy of 26 O. A 1σ upper limit of 53 keV was extracted for the ground-state resonance energy of 26 O. Conclusions: Lastly, future attempts to measure the three-body correlations from the ground-state decay of 26 O will be very challenging due to the need for a precise measurement of the 24 O momentum at the reaction point in the target.
Search for unbound Be 15 states in the 3 n + Be 12 channel
Background: Be-15 is expected to have low-lying 3/2(+) and 5/2(+) states. A first search did not find the 3/2(+) [A. Spyrou et al., Phys. Rev. C 84, 044309 (2011)]; however, a resonance in Be-15 was populated in a second attempt and determined to be unbound with respect to Be-14 by 1.8(1) MeV with a tentative spin-parity assignment of 5/2(+) [J. Snyder et al., Phys. Rev. C 88, 031303(R) (2013)]. Purpose: Search for the predicted Be-15 3/2(+) state in the three-neutron decay channel. Method: A two-proton removal reaction from a 55 MeV/u C-17 beam was used to populate neutron-unbound states in Be-15. The two-, three-, and four-body decay energies of the Be-12+ neutron(s) detected in coincidence were reconstructed using invariant mass spectroscopy. Monte Carlo simulations were performed to extract the resonance and decay properties from the observed spectra. Results: The low-energy regions of the decay energy spectra can be described with the first excited unbound state of Be-14 (E-x = 1.54 MeV, E-r = 0.28 MeV). Including a state in Be-15 that decays through the first excited Be-14 state slightly improves the fit at higher energies though the cross section is small. Conclusions: A Be-15 component is not needed to describe the data. If the 3/2(+) state in Be-15 is populated, the decay by three-neutron emission through Be-14 is weak,
Structure and decay correlations of two-neutron systems beyond the dripline
The two-neutron unbound systems of 16 Be, 13 Li, 10 He, and 26 O have been measured using the Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet setup. The correlations of the 3-body decay for the 16 Be and 13 Li were extracted and demonstrated a strong correlated enhancement between the two neutrons. The measurement of the 10 He ground state resonance from a 14 Be(-2p2n) reaction provided insight into previous predictions that wavefunction of the entrance channel, projectile, can influence the observed decay energy spectrum for the unbound system. Lastly, the decay-in-target (DiT) technique was utilized to extract the lifetime of the 26 O ground state. The measured lifetime of 4.5$+1.1\atop{-1.5}$ (stat.)±3(sys.) ps provides the first indication of two-neutron radioactivity.
First observation of 15 Be
The neutron-unbound nucleus 15 Be was observed for the first time. It was populated using neutron transfer from a deuterated polyethylene target with a 59 MeV/u 14 Be beam. Neutrons were measured in coincidence with outgoing 14 Be particles and the reconstructed decay energy spectrum exhibits a resonance at 1.8(1) MeV. This corresponds to 15 Be being unbound by 0.45 MeV more then 16 Be thus significantly hindering the sequential two-neutron decay of 16 Be to 14 Be through this state.
First observation of the 13 Li ground state
The ground state of neutron-rich unbound 13Li was observed for the first time in a one-proton removal reaction from 14Be at a beam energy of 53.6 MeV/u. The 13Li ground state was reconstructed from 11Li and two neutrons giving a resonance energy of 120+60 -80 keV. All events involving single- and double-neutron interactions in the Modular Neutron Array (MoNA) were analyzed, simulated, and fitted self-consistently. The three-body ( 11Li + n + n) correlations within Jacobi coordinates showed strong dineutron characteristics. The decay energy spectrum of the intermediate 12Li system (11Li + n) was described with an s-wave scattering length of greater than -4 fm, which is a smaller absolute value than reported in a previous measurement.
Modeling interactions of intermediate-energy neutrons in a plastic scintillator array with Geant4
Here, a Monte Carlo simulation of a large-area neutron time-of-flight detector, built on the Geant4 framework, has been compared with an experimental measurement of the 16 B → 15 B + n decay produced from a 55 MeV/ u 17 C beam. The ability of the Monte Carlo simulation to reproduce the intermediate-energy neutron interactions within the detector has been explored using both the stock Geant4 physics processes and a custom neutron interaction model, menate_r. The stock Geant4 physics processes were unable to reproduce the experimental observables, while excellent agreement was obtained through the inclusion of the menate_r model within Geant4. The differences between the two approaches are shown to be related to the modeling of the neutron–carbon inelastic reactions. Additionally, the use of menate_r provided accurate reproduction of experimental signals associated with neutron scattering within the detector. These results provide validation of the Monte Carlo simulation for modeling measurements of multiple neutrons where the identification and removal of false neutron signals, due to multiple neutron scattering, are required.
ATF2 Commissioning
ATF2 is a final-focus test beam line that attempts to focus the low-emittance beam from the ATF damping ring to a beam size of about 37 nm, and at the same time to demonstrate nm beam stability, using numerous advanced beam diagnostics and feedback tools. The construction is well advanced and beam commissioning of ATF2 has started in the second half of 2008. ATF2 is constructed and commissioned by ATF international collaborations with strong US, Asian and European participation.