Richard N. Boyd

Black hole formation in core-collapse supernovae and time-of-flight measurements of the neutrino masses

In large stars that have exhausted their nuclear fuel, the stellar core collapses to a hot and dense proto-neutron star that cools by the radiation of neutrinos and antineutrinos of all flavors. Depending on its final mass, this may become either a neutron star or a black hole. Black hole formation may be triggered by mass accretion or a change in the high-density equation of state. We consider the possibility that black hole formation happens when the flux of neutrinos is still measurably high. If this occurs, then the neutrino signal from the supernova will be terminated abruptly (the transition takes ≲0.5 ms). The properties and duration of the signal before the cutoff are important measures of both the physics and astrophysics of the cooling proto-neutron star. For the event rates expected in present and proposed detectors, the cutoff will generally appear sharp, thus allowing model-independent time-of-flight mass tests for the neutrinos after the cutoff. If black hole formation occurs relatively early, within a few (∼1) seconds after core collapse, then the expected luminosities are of order LBH=1052 erg/s per flavor. In this case, the neutrino mass sensitivity can be extraordinary. For a supernova at a distance D=10 kpc, SuperKamiokande can detect a νe mass down to 1.8 eV by comparing the arrival times of the high-energy and low-energy neutrinos in νe+p→e++n. This test will also measure the cutoff time, and will thus allow a mass test of νμ and ντ relative to νe. Assuming that νμ and ντ are nearly degenerate, as suggested by the atmospheric neutrino results, masses down to about 6 eV can be probed with a proposed lead detector of mass MD=4 kton (OMNIS). Remarkably, the neutrino mass sensitivity scales as (D/LBHMD)1/2. Therefore, direct sensitivity to all three neutrino masses in the interesting few-eV range is realistically possible; there are no other known techniques that have this capability.

Can Be-9 provide a test of cosmological theories?

The Li-7(H-3,n)Be-9 reaction, previously neglected in nucleosynthesis determinations, is found to greatly increase the predicted abundance of Be-9 resulting from the big bang. For a nonuniform density universe, a primordial Be-9/H-1 number density ratio of 10 to the -13th is obtained. It is noted that recent Be-9/H-1 measurements in Population II stars approach the nonuniform density universe value. 22 references.

Information about the 12C(α,γ)16O reaction from the β-delayed proton decay of 17Ne

We are studying the β-delayed proton decay of 17Ne with the goal of determining the E2 part of the 12C(α,γ)16O cross section at energies relevant to helium burning in stars. We have determined branching ratios for proton and α-decay for states in 17F from 8.08 to 11.193 MeV. In addition, we have observed the break-up of the isobaric analogue state (IAS) at 11.193 MeV into three particles via two channels: proton decay to the 9.59 MeV state in 16O which breaks up into an α-particle plus 12C; and α-decay to the 2.365 MeV state in 13N which breaks up into a proton plus 12C. This is the first reported observation of the decay of the IAS to the 1− state in 16O at 9.59 MeV.

Technique for Direct eV-Scale Measurements of the Mu and Tau Neutrino Masses Using Supernova Neutrinos

Early black hole formation in a core-collapse supernova will abruptly truncate the neutrino fluxes. The sharp cutoff can be used to make model-independent time-of-flight neutrino mass tests. Assuming a neutrino luminosity of 10(52) erg/s per flavor at cutoff and a distance of 10 kpc, Super-Kamiokande can detect an electron neutrino mass as small as 1.8 eV, and the proposed OMNIS detector can detect mu and tau neutrino masses as small as 6 eV. We present the first technique with direct sensitivity to eV-scale mu and tau neutrino masses.

The 8Li(α, n)11B reaction and primordial nucleosynthesis

Abstract The cross section for the 8 Li(α, n) 11 B reaction, of importance to primordial nucleosynthesis in the inhomogeneous models, has been measured using a 8 Li radioactive beam. The center-of-mass energy range studied was 0.64–2.2 MeV. The cross section to all 11 B states is found to exceed that for 8 Li(α, n) 11 B(g.s.) by a factor of at least five over the entire energy region studied.

Geometrical Effects of Baryon Density Inhomogeneities on Primordial Nucleosynthesis

We discuss effects of fluctuation geometry on primordial nucleosynthesis. For the first time we consider condensed cylinder and cylindrical-shell fluctuation geometries in addition to condensed spheres and spherical shells. We find that a cylindrical shell geometry might allow for an appreciably higher baryonic fraction of the closure density (? -->b h -->2500.2) than that allowed in spherical inhomogeneous or standard homogeneous big bang models. This result, which is contrary to those of some other recent studies, is due to both geometry and recently revised estimates of the uncertainties in the observationally inferred primordial light-element abundances. We also find that inhomogeneous primordial nucleosynthesis in the cylindrical shell geometry can lead to significant Be and B production. In particular, this geometry produces a primordial beryllium abundance as high as [Be] = 12 + log (Be/H) ? -3 while still satisfying all of the light-element abundance constraints.

EXPLAINING THE Sr AND Ba SCATTER IN EXTREMELY METAL-POOR STARS

Compilations of abundances of strontium and barium in extremely metal-poor stars show that an apparent cutoff is observed for [Sr/Ba] at [Fe/H] –3.6 with a clear upper bound depending on metallicity. We study the factors that place upper limits on the logarithmic ratio [Sr/Ba]. A model is developed in which the collapses of type II supernovae are found to reproduce many of the features seen in the data. This model is consistent with galactic chemical evolution constraints of light-element enrichment in metal-poor stars. Effects of turbulence in an explosive site have also been simulated, and are found to be important in explaining the large scatter observed in the [Sr/Ba] data.

Measurements of proton radiative capture cross sections relevant to the astrophysical rp- and γ-processes

Measurements have been made of the Zr-96(p,gamma)Nb-97, Sn-112(p,gamma)Sb-113, and Sn-119(p,gamma) Sb-120 cross section excitation functions, Incident proton energies ranged from 2.8 MeV to 8.5 MeV. These reactions are relevant to several processes of stellar nucleosynthesis. The resulting astrophysical S-factors are compared to those from theoretical statistical model calculations using the SMOKER, and the more recent NON-SMOKER, codes to judge their applicability to these reactions

Study of the β-delayed neutron emission of 19C

Abstract The β-delayed neutron emission of 19 C has been studied with the hope of observing states in 19 N close enough to the 18 N+n threshold to be of astrophysical significance. Although transitions to three-neutron unbound states in 19 N have been observed, no states in the energy region of astrophysical relevance were found. The 19 C β-decay branching ratios to the observed states have been measured. All states show log( ft ) values typical of allowed transitions. The results are compared with shell-model predictions carried out in the psd model space.

The Observatory for Multiflavor NeutrInos from Supernovae

Abstract OMNIS, the Observatory for Multiflavor NeutrInos from Supernovae, is being designed to detect supernova neutrinos via their interactions with lead nuclei. The information obtained will impact our understanding of the stellar collapse process, neutrino mass and oscillations, nucleosynthesis, black hole astrophysics, and nucleon decay.