H.W. Becker

Low-energy fusion cross sections of D + D and D + 3He reactions

Abstract The D(d, n) 3 He and D(d, p)T reactions have been investigated at E c.m = 2.98 to 162.5 keV and the 3 He(d, p) 4 He reaction at E c.m. = 6.95 to 141.8 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems of the extended and quasi-point supersonic jet type. The measurement of absolute cross sections, angular distributions and excitation functions is reported. The data extend into the thermal energy region of future fusion reactors. The results for the D(d, n) 3 He and D(d, p)T reactions are in good agreement with previous work, while substantial differences are found for 3 He(d, p) 4 He.

Stellar reaction rate of 14N(p, γ)15O and hydrogen burning in massive stars

Abstract The capture reaction 14 N(p, γ) 15 O has been investigated in the energy range E p = 0.2 to 3.6 MeV with the use of windowless gas targets as well as implanted 14 N solid targets of high isotopic purity. The measurement of absolute cross sections, γ-ray angular distributions and excitation functions is reported. The data provide information on the capture amplitudes involved in the transitions to all bound states of 15 O. The astrophysical S -factor at stellar energies has been determined by means of theoretical fits. The result of S (0) = 3.20 keV · b is in good agreement with the value incorporated in the compilations. Also discussed are the nuclear physics aspects of the data.

Astrophysical S(E) factor of 3He(3He, 2p)4He at solar energies

Abstract The 3 He( 3 He, 2p) 4 He reaction has been investigated in the energy range E c.m. = 17.9 to 342.5 keV. The studies involved high-current accelerators with well-known beam characteristics and windowless gas target systems. At low energies the studies required the measurement of coincidences between the two protons in the exit channel as well as precautions for contributions from cosmic background and electronic noise. The data extend into the thermal energy region of the sun, e.g., σ = 7 ± 2 pb at E c.m. = 24.5 keV, and upper limits for the reaction yield have been obtained down to E c.m. ∼- 17.9 keV. No evidence for a suggested low-energy resonance has been found. The astrophysical S ( E ) factor at zero energy is S (0) = 5.57 ± 0.32 MeV · b. The implications of the data for the solar-neutrino problem are discussed.

The3He(α,γ)7Be reaction and the solar neutrino problem

The capture reaction3He(α,γ)7Be has been investigated in the energy range ofEc.m. =107 to 1,266 keV. The4He or3He beams of up to 300 μA particle current were incident on3He or4He gas targets, respectively. The gas target systems were all of the windowless and recirculating type. Excitation functions have been obtained with the use of an extended-static gas target, while the measurements ofγ-ray angular distributions involved a quasi-point supersonic jet system. The determination of absolute cross sections has been carried out with both types of gas target systems. Theγ-ray yields in the3He(α,γ)7Be reaction were detected using 80 cm3 Ge(Li) detectors. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)=0.30±0.03 keV-b. This result reduces the calculated solar neutrino rate by a factor of 1.76.

The4He(12C, γ)16O reaction at stellar energies

The capture reaction4He(12C, γ)16O (Ec.m.= 1.34–3.38 MeV) as well as the elastic scattering process4He(12C,12C)4He (Ec.m.=1.44–3.38 MeV) have been investigated with the use of an intense12C beam and a windowless and4He recirculating gas target system. The measurements involved two large NaI(T1) crystals in close geometry to an extended gas target, whereby angle-integrated γ-ray yields were obtained. A large area plastic detector was used for the suppression of time-independent background. A search for cascade γ-ray transitions was carried out by coincidence techniques. The measurement of absolute cross sections is also reported. Theoretical fits of the excitation function for the groundstate γ-ray transition requireE1 as well asE2 capture amplitudes, which are of equal importance at stellar energies. This result increases significantly the stellar burning rate of4He(12C, γ)16O and leads to16O as the dominant product at the end of helium burning in massive stars. The observed capture yield to the 6.92 MeV state is dominated by the direct capture mechanism and plays a small role at stellar energies.

The 12C(α, γ)16O cross section at stellar energies☆

Abstract The capture reaction 12 C(α, γ) 16 O has been investigated at E = 0.94 to 2.84 MeV with the use of an intense α beam and implanted 12 C targets of high isotopic purity. The studies involved NaI(Tl) crystals and, for the first time, germanium detectors. The measurement of absolute cross sections, γ-ray angular distributions and excitation functions is reported. A cross section of 48 pb is found at E = 0.94 MeV. The data provide information on the E1 and E2 capture amplitudes involved in the transition to the ground state as well as to excited states. The S -factor at stellar energies has been determined by means of theoretical fits. The results verify the previous report of a substantial higher S -value compared to the value recommended in 1975. The present uncertainty in the S -value as well as possible improvements are discussed. This S -value is of crucial importance to nuclear astrophysics.

The scattering of alpha particles from 12C and the 12C(α, γ)16O stellar reaction rate

Abstract The elastic scattering of alpha particles from 12 C has been investigated for 35 angles in the range θ lab = 22° to 163° and for 51 energies at E α = 1.0 to 6.6 MeV. The extracted phase shifts for l = 0 to 6 partial waves have been parametrized in terms of the multilevel R -matrix formalism. Information on the deduced parameters of states in 16 O is reported. The data reveal reduced α-particle widths for the 6.92 and 7.12 MeV subthreshold states consistent with recent work. The implications for the stellar reaction rate of 12 C (α, γ) 16 O are discussed.

Nuclear and astrophysical aspects of 18O(p, γ)19F☆

Abstract The capture reaction 18O(p, γ)19F has been investigated in the energy range Ep = 80–2200 keV. The seven known resonances have been studied in detail and twelve new resonances have been found. The resonances at ER = 680, 977 and 1670 keV correspond to new states in 19F. The known resonance at ER = 631 keV is observed to consist of a doublet (ΔEp = 7 keV). Information on resonance energies, total and partial widths, branching and mixing ratios and ωγ values is reported. Transition strength arguments as well as analyses of γ-ray angular distribution data combined with results from previous work resulted in Jπ assignments for some of the resonances and low-lying states in 19F. The assignment of several states in 19F as T = 3 2 analogue states of 19O is discussed. A direct capture process to several final states in 19F up to Ex = 8.8 MeV has been observed revealing information on the orbital momenta of the captured protons in the final states, their spectroscopic factors and Jπ assignments for interfering resonances. Special efforts were made to detect this process to states near the proton threshold, which are of importance to stellar hydrogen burning of 18O. The results are compared with corresponding information from other reactions. The investigated energy range of the 18O(p, γ)19F reaction corresponds to the important stellar temperature range of T = 0.01 to 5 × 109 K. The energy-averaged astrophysical reaction rates determined from the present data are compared with previous estimates for this reaction. The data permit reliable conclusions to be drawn concerning the final termination of the CNO tri-cycle.

High-Z electron screening: the cases 50V(p,n)50Cr and 176Lu(p,n)176Hf

The electron screening in 50V(p,n)50Cr has been studied at Ep = 0.75 to 1.55 MeV for different environments: VO2 insulator, V metal and PdV10% alloy. Relative to the insulator we find for the metal and alloy a large screening energy UD = 27 ± 9 and 34 ± 11 keV, respectively. We also studied 176Lu(p,n)176Hf at similar proton energies for a Lu2O3 insulator, a Lu metal and a PdLu10% alloy; here we observed a narrow resonance at Epr = 0.81 MeV exhibiting a prominent Lewis peak and a shift in proton resonance energy of UD = 32 ± 2 and 33 ± 2 keV for the metal and alloy, respectively, relative to the insulator. The data demonstrate that the electron screening occurs across the periodic table and is not restricted to reactions among light nuclides studied so far. The two reactions with neutrons in the exit channel demonstrate furthermore that the electron screening is an effect in the entrance channel of the reaction and is not influenced by the ejectiles of the exit channel, i.e. by the charged particles of the exit channel studied so far. The UD values can be explained by the plasma model of Debye applied to the quasi-free electrons in the metallic samples. The data together with previous studies of the fusion reactions d+d, Li+p and Be+p in metals verify the Debye model prediction, i.e. a scaling of UD with the nuclear charge of the target atoms. Finally, we discuss the possibility for an enhanced α-decay rate of transuranic waste in metals.

Cross section of the C-13(alpha, n)O-16 reaction: A Background for the measurement of geo-neutrinos

The absolute cross section of the