Beta-delayed Alpha-particle Emission Research Articles

How well do we know the beta-decay of and oxygen formation in helium burning?

We review the status of the reaction rate, of importance for stellar processes in a progenitor star prior to a supernova collapse. Several attempts to constrain the p-wave S-factor of the reaction at Helium burning temperatures (200 MK) using the beta-delayed alpha-particle emission of have been made, and it is claimed that this S-factor is known, as quoted by the TRIUMF collaboration. In contrast, reanalyses (by GM Hale) of all thus far available data (including the data) does not rule out a small S-factor solution. Furthermore, we improved our previous Yale-UConn study of the beta-delayed alpha-particle emission of by improving our statistical sample (by more than a factor of 5), improving the energy resolution of the experiment (by 20%), and in understanding our line shape, deduced from measured quantities. Our newly measured spectrum of the beta-delayed alpha-particle emission of is not consistent with the TRIUMF (1994) data, but is consistent with the Seattle (1995) data, as well as the earlier (unaltered!) data of Mainz (1971). The implication of these discrepancies for the extracted astrophysical p-wave S-factor is briefly discussed.

Further measurement of the beta-delayed alpha-particle emission of 16N

The 12 C( α, γ) 16 O reaction is a key (but still unknown) reaction in helium burning. Several attempts to constrain the S-factor at Helium burning temperatures (200 MK) using the beta-delayed alpha-particle emission of 16 N have been made. However, some discrepancy exists between the spectra measured at Seattle and that of TRIUMF. We have improved our previous study of the beta-delayed alpha-particle emission of 16 N by improving our statistical sample (by more than a factor of 5), improving the energy resolution of the experiment (by 20%), and in understanding our line shape. The observed spectrum of the beta-delayed alpha-particle emission of 16 N is consistent with the Seattle('95) data, as well as an earlier experiment performed at Mainz('71) and is not consistant with the TRIUMF('94) data.

Nuclear astrophysics with secondary (radioactive) beams

Some problems in nuclear astrophysics are discussed with emphasize on problems central to the field which were not solved over the last two decades, including helium burning in massive stars and the 12 C( α, γ) 16 O reaction, as well as the 8 B solar neutrino flux and the 7 Be( p, γ) 8 B reaction. It will be demonstrated that these problems could be solved by measuring the time reversed process(es), such as the beta-delayed alpha-particle emission of 16 N and the Coulomb Dissociation of 8 B, using radioactive beams (of 8B and 16N ), that allow for amplification of the sought for small cross section, of relevance for stellar processes.

Study of the beta-delayed alpha-particle emission ofN16

The beta-delayed alpha-particle emission of $^{16}\mathrm{N}$ has been studied, with $^{16}\mathrm{N}$ nuclei produced using 80 MeV/nucleon $^{18}\mathrm{O}$ beams on $^{9}\mathrm{Be}$ targets. The $^{16}\mathrm{N}$ secondary nuclei were mass analyzed and separated from the reaction products using the Michigan State University A1200 isotope separator. A detector array, including four thin surface barrier detectors, a p-i-n diode, a Ge gamma-ray detector, and a two-dimensional position sensitive parallel plate avalanche counter, was used for implantation and study of the separated nuclei. A beta-decay branching ratio of (1.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ to the ${1}^{\mathrm{\ensuremath{-}}}$ state at 9.6 MeV and a centroid of 2.35\ifmmode\pm\else\textpm\fi{}0.05 MeV for the beta-delayed alpha-particle emission were measured. These results are essential for the analysis of a high sensitivity measurement at Yale University of the low-energy beta-delayed alpha-particle emission of $^{16}\mathrm{N}$, and for understanding the $^{12}\mathrm{C}$(\ensuremath{\alpha},\ensuremath{\gamma}${)}^{16}$O reaction in the helium burning process in massive stars.

Astrophysical S factor of 12C( alpha, gamma )16O from the beta-delayed alpha-particle emission of 16N.

The spectrum of low energy beta-delayed alpha-particle emission of $^{16}\mathrm{N}$ was measured with high sensitivity; e.g., at ${\mathit{E}}_{\mathrm{\ensuremath{\alpha}}}^{\mathit{L}}$\ensuremath{\approxeq}1 MeV, a sensitivity for a beta-decay branching ratio in the range of ${10}^{\mathrm{\ensuremath{-}}9}$ (to ${10}^{\mathrm{\ensuremath{-}}10}$) was achieved. We used previous theoretical work of Ji, Filippone, Humblet, and Koonin to extract from these data the astrophysical p-wave S factor for the $^{12}\mathrm{C}$ (\ensuremath{\alpha},\ensuremath{\gamma}${)}^{16}$O capture reaction, and find ${\mathit{S}}_{\mathit{E}1}$(300)=95\ifmmode\pm\else\textpm\fi{}6(stat)\ifmmode\pm\else\textpm\fi{}28(syst) keV b. This result is crucial, for example, for understanding the last stages of a massive star before a supernova.

Beta decay of 18N to alpha particle emitting states in 18O and a proposed search for parity violation in 18O.

The beta-delayed alpha-particle emission from $^{18}\mathrm{N}$ has been studied and the absolute beta decay branching ratios of $^{18}\mathrm{N}$ to the ${1}_{3}^{\mathrm{\ensuremath{-}}}$ and ${1}_{4}^{\mathrm{\ensuremath{-}}}$ states in $^{18}\mathrm{O}$ measured. The $^{18}\mathrm{N}$ nuclei were produced using a 35-MeV/nucleon $^{22}\mathrm{Ne}$ beam and a thick Ta target. Reaction products were separated (from the beam) using a reaction products mass separator. A silicon telescope including five detectors and a two-dimensional, position-sensitive, gas counter was used in the focal plane of the mass separator. We obtain beta decay branching ratios of 6.8\ifmmode\pm\else\textpm\fi{}0.5% and 1.8\ifmmode\pm\else\textpm\fi{}0.2 % to the ${1}_{3}^{\mathrm{\ensuremath{-}}}$ and ${1}_{4}^{\mathrm{\ensuremath{-}}}$ states, respectively, assuming 100% alpha-particle decay branching ratios for these states. Shell-model predictions are compared with these experimental results. The relevance of these data to a new proposed search for parity violation in $^{18}\mathrm{O}$ is discussed.

Beta-delayed alpha-particle emission from 118, 120 Cs

The isotopes 118 Cs(T 1 2 = 16 s) and 120 Cs (58 s) were found to have unresolved, bell-shaped α-particle spectra extending from 7 to 10 MeV and from 6 to 9 MeV, respectively. The branching ratios are (2.4 ± 0.4) × 10 −5 and (2.0 ± 0.4) × 10 −7. The experiment confirms 120Cs to be a delayed-proton emitter with a proton branch of (7±3) × 10 −8.

Beta-delayed alpha-particle emission in the decay of 3.6s 181Hg

The emission of α-particles with energies between 10 and 12 MeV has been observed in the decay of 181Hg in an intensity of (9 ± 3) × 10 −8. These events are taken to represent β-delayed α-particle emission from levels at 4–6 MeV excitation energy in 181Au and provide an estimate of the α-strength function for this excitation energy. The results are discussed in terms of an α-particle cluster model. As a by-product the experiment gives new values for the α-particle branching ratios of 181Hg and 181Au: 0.26 ± 0.04 and 0.011 ± 0.0025 .