New measurement of the 10B(p,α0)7Be reaction cross section at low energies and the structure of 11C

We report preliminary results of a new measurement of the B(p,α0)Be cross section in the 0.6 1.0 MeV bombarding energy domain. In this region very few data have been reported in the literature. Excitation functions at both forward and backward angles have been obtained. Angular distributions testify the contribution due to several excited states in the C compound nucleus. The experimental S-factor is about 30% lower than the one reported in the literature and based on activation methods.


Introduction
The 10 B(p,α) 7 Be reaction is important in various fields of Nuclear Physics.In fact, (1) it allows to study the spectroscopy of the non self-conjugated 11 C compound nucleus, with particular regard to the appearance of α-cluster states (see, e.g.[1][2][3]); (2) it is important to accurately describe the destruction of the 10 B isotope in stars [5][6][7]; (3) it is involved in radio-protection issues in Nuclear Fusion technology [4].
For example, from the Nuclear Structure point of view, it is important to have available angular distributions and excitation functions of several reaction channels leading to the population (or coming from the decay) of 11 C excited states.In this case, a R-matrix analysis of experimental data allows to determine the partial width and the branching ratios of the various channels.In particular, the analysis of states near and above the α emission threshold is an useful tool to unveil the possible existence of molecular rotational bands [2,3] in 11 C, that can constitute a fingerprint of α-clustering in non self-conjugated nuclei.
10 B(p,α) reaction is important also for Applied Physics purposes, and in particular in Nuclear Fusion technology [4].In fact, the 11 B(p,α)αα reaction has been proposed as a possible candidate for laser-induced fusion reactions without neutron production (aneutronic fusion) [8,9].In this way, it would be possible to overcome radio-protection hazards due to the large neutron flux emitted by the usual d + d or d + t fusion reactions.Unfortunately, the natural boron is composed both of 11 B (80.1% natural abundance) and 10 B (19.9% natural abundance).Reactions induced by protons on 10 B contaminants can lead to the emission of α particles and of long-lived 7 Be radioactive nuclei (τ 52.7 days) that can implant on the reactor vessel, leading to radiation safety issues.For this reason, it is necessary to isotopically enrich the 11 B target, but a correct estimate of the enrichment level needed to avoid radioactive hazard necessarily relies on an accurate knowledge of the 10 B(p,α) 7 Be cross section at low energies (< 0.3 MeV) [10].
Unfortunately, direct data reported in the literature at low bombarding energies, especially in the E ≈ 1 − 2 MeV region, are few [11][12][13], being often characterized by large error bars.Recent measurements performed with the Trojan Horse method allowed to explore the S-factor down to zero energy [5].These data are usually obtained in arbitrary units, and therefore they need to be normalized to higher energy direct data.Considering all these reasons, we decided to perform a new experiment aimed at measuring the 10 B(p,α) 7 Be reaction in the E p =1.028-0.630MeV bombarding energy domain.Some experimental details and preliminary results of this analysis are reported in the following sections.

Experimental methods
Proton beams at bombarding energies E p =0.630-1.028MeV were delivered by the ttt3 tandem accelerator of the Laboratorio dell'Acceleratore (LdA) at Federico II University of Naples [14].The energy step here used was about 40 keV.To reduce pile-up effects, in this experiment the beam intensity did not overcome 1 nA.The beam energy calibration was determined by analysing the γ yield of the 19 F(p,αγ) 16 O reaction around the 0.872 MeV resonance and by investigating several elastic scattering resonances induced by proton and α-particles on several light targets.The maximum beam energy spread was of the order of 0.2%, and the diameter of the beam on target was smaller than 2 mm.The beam intensity was measured by means of a Faraday-cup.The vacuum in the scattering chamber was of the order of 10 −6 mbar.
A boron foil (38 μg/cm 2 thick) was used as reaction target.Because of its extreme fragility, a small amount of polyvinyl formal (C 3 H 6 O 2 , some μg/cm 2 thick) has been used to strengthen it.The target surface was orthogonal to the beam direction.Because of the manufacturing process, the target was affected by the presence of several contaminants.They have been characterized with elastic backscattering analysis, with proton beams of about 1 MeV.The main observed contaminants were Li, C, O, Al, Cl, Cu, Ba.Because of kinematic reasons, peaks due to the heavier of these contaminants are expected to overlap with the α 0 signal at backward angles, leading to strong difficulties in the extraction of α 0 yields from the energy spectrum of ejectiles.
For this reason, to measure the α 0 yields at backward angles (θ lab = 120 • − 160 • ) we used a method based on the simultaneous measurements of two ejectile spectra at the same angle (in opposite directions with respect to the beam axis), the first one taken with an unshielded silicon detector, the second one taken by a silicon detector covered by a thin Al foil (3μm thick) that is able to stop (or highly slow down) the α 0 particles, allowing the elastically scattered protons to punch through.Then, the α 0 yields have been derived by subtraction, taking into account the energy loss and straggling of protons in the aluminium foil.Details on this experimental technique will be described in a forthcoming paper.
sections by internal normalization to the 10 B(p,p 0 ) elastic scattering signal, taking into account the 10 B(p,p 0 ) elastic scattering differential cross sections reported in Refs.[15,16].

Preliminary experimental results
An example of preliminary angular distribution obtained at E p =0.747 MeV is reported in Figure 1.The red line represents its fit by using Legendre Polynomials up to the 2 nd degree, corresponding to max =1 in the p+ 10 B channel.This angular distribution might be slightly anisotropic, testifying the possible contribution given by = 0 partial waves.The preliminary analysis of experimental data shows a continuous evolution of angular distribution shapes as a function of energy.In particular, in the E p = 0.630-0.785MeV energy range, angular distributions show quite small anisotropies, similar to the one reported in Figure 1, while at higher energies (E p = 0.827-1.028MeV) the angular distributions are more anisotropic.R-matrix calculations of angular distributions are currently in progress; they will help us to improve the spectroscopy of 11 C states in an excitation energy region E x ≈9-11 MeV,) where several ambiguities in the J π assignments exist [17].A preliminary estimate of the angle integrated reaction cross section has been obtained from the present angular distributions; consequently, our preliminary 10 B(p,α 0 ) 7 Be S-factor is shown in Figure 2, as green dots.Vertical bars represent statistical errors, while the grey band indicates non statistical ones, derived from the internal normalization procedure.We compared our experimental data with results taken from the nacre and nacre2 compilations [12,13], and derived from a differential cross section reported by Brown et al [18] at θ lab =137.8 • (blue stars) and from an activation experiment reported by Roughton et al [11] (red triangles; these measurements refer to the α 0 + α 1 channels; the α 1 channel is expected to have cross section well lower than 10% of the α 0 channel at energies E p <1 MeV [19]).Our data are ≈30% lower than the estimates based on Roughton et al data, even if they agree within the error bars.Similar conclusions can be drawn by considering the Brown et al data.
The next step in the data analysis will be the related to the definitive estimate of the S-factor from the present experimental data.This analysis is currently in progress.

Figure 1 :Figure 2 :
Figure 1: (color online) Angular distribution of the 10 B(p,α 0 ) 7 Be reaction at E p =0.747 MeV.The red line indicates Legendre polynomial fit of data (truncated to the 2 nd order).