4. Recent HRIBF Research - Resonant Scattering of ¹⁰Be on ⁴He
[M. Freer (University of Birmingham, UK), spokesperson]

The structure of the beryllium isotopes is rather unusual, but highlights the spectrum of possible structural characteristics of light nuclei. The nucleus ⁸Be is well-known to posses a structure which can be described in terms of two alpha-particle clusters. From an experimental perspective the ground state rotational band may be described by a moment of inertia commensurate with two touching alpha-particles. Such a structure is also found in the Greens Function Monte Carlo calculations [1] in which no cluster structure is apriori assumed. The addition of neutrons to the ⁸Be nucleus to form isotopes such as ^9,10Be produces some rather interesting characteristics. Rather than destroying the symmetries responsible for the clustering in ⁸Be, the two alpha-particle cluster structure remains and the valence neutrons are exchanged between the two alpha-particle cores in a covalent, molecular, manner [2]. The orbitals of the neutrons have either σ or π character, just as electrons exchanged in atomic molecules. The nature of the bonds depends on the orientation of the p-orbitals which are occupied by the neutrons at each alpha-particle centre. When the orbitals are aligned perpendicular to the separation axis then π-bonding results, whereas if the alignment is parallel the bonding is of σ-character (see Fig. 4-1) [2].

Figure 4-1: The formation of the molecular orbitals from the linear combination of p-orbitals built around two alpha particles.

In the ground-state of ¹⁰Be the two neutrons, asymptotically, have π-behaviour. Such systems are nuclear dimers. The question then arises as to if it is possible to form trimers, i.e. three-alpha-particles and valance neutrons. From a theoretical perspective the answer appears to be yes, though the precise geometric arrangement of the alpha-particles is unclear (linear or triangular) [3,4]. The experimental situation is a little more unclear. Reference [5] summarises the latest situation. A great number of resonant states have been observed that exist above the alpha-decay threshold (12 MeV), but their spins are largely unknown and this prevents any systematics being determined.

The present study was of resonances in ¹⁴C populated in the ¹⁰Be+⁴He resonant scattering reaction. The ¹⁰Be beam used had an intensity of ~10⁷ pps and the contamination from ¹⁰B was ~1%. Nine beam energies ranging from 25 to 46 MeV were used to perform the measurements. The experimental arrangement is shown schematically in Fig. 4-2. The SIDA chamber was positioned in front of the Daresbury Recoil Separator and was sealed with respect to the beam line with a 5 μm thick havar window. Helium gas was held at a pressure of 900 mb.

Figure 4-2: The arrangement of the two LAMP configurations used to coincidently detect the 10Be and 4He products. The detectors were placed within the gas volume.

Within the gas volume were a number of silicon detectors. At zero degrees a three-element silicon telescope formed from three, 150-μm thick detectors was screened from the beam by mylar foils. These foils allowed the resonantly scattered alpha particles to pass through to the telescope behind. This provided a measurement of the zero-degree yield for the alpha-particles. In addition two LAMP arrays of YY1 type silicon detectors (Micron Semiconductors Ltd) were located within the helium gas volume. These were used to detect the ⁴He+¹⁰Be nuclei in coincidence as illustrated in Fig. 4-2. These detectors allowed the elastic and inelastic channels to be identified, the angular distributions of the reaction products to be calculated, and finally the location (distance from the window) of the interaction to be calculated. The data from these latter detectors permit the angular distributions of the resonances observed in the zero-degree detector to be calculated and thus the resonance spins to be deduced. The zero-degree excitation function for the 25-MeV beam energy is shown in Fig. 4-3 and the energy angle systematics for the ⁴He particles in Fig. 4-4. The analysis of data is ongoing.

Figure 4-3: The ¹⁴C excitation energy spectrum populated in ⁴He+¹⁰Be resonant scattering for the beam energy of 25 MeV.

Figure 4-4: Energy versus angle systematics (measured with respect to the chamber window) for the two LAMP arrays for E_beam=25 MeV.

References:

[1] R. B. Wiringa, S. C. Pieper, J. Carlson, and V. R. Pandharipande, Phys. Rev. C 62, 014001 (2000).
[2] M. Freer, Rep. Prog. Phys. 70, 2149 (2007).
[3] N. Itagaki, et al., Phys. Rev. C 64, 014301 (2001).
[4] W. von Oertzen, et al. Euro. Phys. J A 21, 193 (2004).
[5] P. J. Haigh, et al., Phys.Rev. C 78, 014319 (2008).