P. D. Parker

Solar fusion cross-sections

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

Solar fusion cross sections II: the pp chain and CNO cycles

The available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production are summarized and critically evaluated. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for {sup 8}B solar neutrinos. Opportunities for further increasing the precision of key rates are also discussed, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to 1998, Rev. Mod. Phys. 70, 1265.

10Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behavior

Abstract The concentration profiles of 9 Be, 10 Be, 230 Th, 232 Th, 231 Pa (via 227 Th) and 238 U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the 10 Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ☐ cores raised from the manganese nodule field in the North Pacific have been measured. The 10 Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator. The concentrations of 10 Be, 230 Th exc and 231 Pa exc and ratios of 10 Be/ 9 Be and 230 Th exc / 232 Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (∼ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on 230 Th exc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on 230 Th exc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the 10 Be based rate is less than the 230 Th exc rate and in R/V Vitiaz the 10 Be based rate is greater than the 230 Th exc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from 230 Th exc and 10 Be profiles document changes in nodule growth rate with time rather than mixing effects on 230 Th exc profiles. The 10 Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface 10 Be/ 9 Be ratio of North Pacific nodules predicts a 9 Be concentration within the limits of measured values. The inventory of 10 Be and 230 Th exc in the nodules is only ∼ 10% of the total, the remaining being in sediments. The 10 Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both 10 Be and 230 Th based on 230 Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

The energy calibration of tandem accelerators

Abstract The beam energy homogeneity, stability and reproducibility of a model MP tandem Van de Graaff accelerator has been investigated. The 90° analyzing magnet system was calibrated using a number of (p,n) thresholds whose proton energies are known absolutely in the range from 4 to 10 MeV. The linearity of the beam momentum analysis system was then checked for higher magnetic rigidities by observing the 2 H( 16 O, n) 17 F neutron threshold for several different 16 O ionic charge states. The magnet calibration constant, obtained from a least-squares different reactions, was reapplied to the data to obtain a new set of self-consistent energies for the calibration reactions themselves. This procedure, which relies on the assumption that the data are all measures of the same calibration constant, results in smaller statistical errors for the calibration energies than those previously cited. Measurements were then extended using the linearity of the analysis system as established by the 2 H( 16 O,n) 17 F measurements to include the 12 C(p,n) 12 N, 24 Mg(p,n) 24 Al, 28 Si(p,n) 28 P, 32 S(p,n) 32 Cl and 40 Ca(p,n) 40 Sc neutron thresholds at energies of 19.658 ± 0.005 MeV, 15.286 ± 0.003 MeV, 15.669 ± 0.004 MeV, 13.899 ± 0.014 MeV, and 15.491 ± 0.003 MeV, respectively. With the exception of the 32 S(p,n) 32 Cl reaction, for which there are target difficulties, these thresholds provide the first series of calibration points in the energy range from 10 to 20 MeV.

Low-energy behavior of the 3He(α, γ)7Be cross section

Cross sections for the ^3He(α, γ)^7Be reaction have been measured at several energies from E_(c.m.) = 165 to 1169 keV by counting prompt γ-rays from a windowless, differentially pumped, recirculating, ^3He gas target. The cross-section factor S_(34)(E_(c.m.)) and branching ratio γ_1/γ_0 were determined at each energy. Cross sections were also measured at E_(c.m.) = 947 and 1255 keV by counting the γ-rays from the ^7Be produced in a ^3He gas cell with a Ni entrance foil. Combining the results of these two independent experiments yields a zero-energy intercept for the cross-section factor of S_(34)(0) = 0.53 ± 0.03 keV · b. The relationship between these measurements and several theoretical calculations, and the import of the extrapolated cross section for the solar-neutrino problem are discussed.

Nuclear structure relevant to neutrinoless double beta decay: 76Ge and 76Se.

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the effective neutrino mass if the nuclear matrix element were known. Theoretical calculations are uncertain, and measurements of the occupations of valence orbits by nucleons active in the decay can be important. The occupation of valence neutron orbits in the ground states of 76Ge (a candidate for such decay) and 76Se (the daughter nucleus) were determined by precisely measuring cross sections for both neutron-adding and removing transfer reactions. Our results indicate that the Fermi surface is much more diffuse than in theoretical calculations. We find that the populations of at least three orbits change significantly between these two ground states while in the calculations, the changes are confined primarily to one orbit.

Measurement of 15O(α,γ)19Ne resonance strengths

Abstract States in 19 Ne above the 15 O + α threshold were populated by means of the 19 F( 3 He,t) 19 Ne ∗ reaction, and their alpha-particle decays to the 15 O ground state were measured. The branching ratios Γ α /Γ total for the E c . m . = 850-, 1020-, 1971-, 1183- and 1563- keV resonances in 19 O + α were determined. This information together with alpha-particle and/or gamma-ray partial widths (determined from knowledge of these quantities for the mirror states in 19 F) determines the strengths of these 15 O ( α , γ ) 19 Ne resonances and the 15 O ( α , γ ) 19 Ne reaction rate for temperatures between 7 × 10 8 and3 × 10 9 K.

A high-resolution study of the 20Ne(3He, t)20Na reaction and the 19Ne(p, γ)20Na reaction rate

A high-precision measurement of the 20Ne(3He,t)20Na reaction has been made using implanted 20Ne transmission targets to obtain pertinent information on the low-energy resonances in the 19Ne(p, γ)20Na reaction. Resonance energies (447±5, 658±5, 787±5, and 857±5 keV) and upper limits on total intrinsic widths (<10, <6, <10, and <16keV) have been measured for four excited states above the 2.199 MeV proton threshold in 20Na. The stellar 19Ne(p, γ)20Na reaction rate is calculated for temperatures between 1×108 and 1×109 K. When combined with a recent study of the 15O(α, γ)19Ne reaction, a new estimate is made of the conditions required for breakout from the Hot CNO cycle to the rapid proton capture process.

Nucleosynthesis of /sup 26/Al at low stellar temperatures

Recent nuclear reaction studies on /sup 26/Al have established the existence of a previously unobserved s-wave resonance in the /sup 25/Mg+p system at E/sub cm/ = 37.2 keV and a possible resonance at E/sub cm/ = = 94.0 keV. The /sup 25/Mg(p, ..gamma..)/sup 26/Al reaction rate has been calculated for temperatures characteristic of both nonexplosive and explosive hydrogen burning, and the astrophysical consequences of the 37.2 keV and 94.0 keV resonances are discussed; it is demonstrated that the contributions of these resonances increase the reaction rate by 10 orders of magnitude at stellar temperatures near T/sub 9/ = 0.04.