Search for low-energy resonances in25Mg(p,γ)26Al

The reaction²⁵Mg(p,γ)²⁶Al has been investigated atE _p (lab)=80-350keV. The existence of the resonance atE _p =317keV has been verified and new resonances have been found atE _p =198, 255 and 304 keV. TheE _p = 304keV resonance represents a new compound state in²⁶A1. Information on branching ratios,Ωγ values, total widths andJ ^π assignments for the observed resonances as well as upper limits on themy strengths for the expected resonances atE _p =97 and 134 keV are given. The astrophysical aspects of the data are discussed in the light of the renewed interest in the precise location of²⁶A1 nucleosynthesis.     

Stellar reaction rate of26Mg(p, γ)27Al

The reaction²⁶Mg(p, γ)²⁷Al has been investigated atE _p (lab)=80–355 keV. The existence of the resonances atE _p =292 and 338 keV has been verified and new resonances were found atE _p =227 and 275 keV. Information on branching and mixing ratios,ωγ values, total widths, andJ ^π assignments for the observed resonances is given. Upper limits on theωγ strengths for expected resonances in the energy range covered are presented. The astrophysical aspects of the data are discussed in the light of the²⁶Mg(²⁶Al) isotopic anomaly.     

The 25Mg(p, γ)26Al and 25Mg(p,p′) resonances for Ep = 0.31–1.84 MeV

Thin target yield curves with good statistics have been measured for the ²⁵Mg(p, γ)²⁶Al and ²⁵Mg(p, p′)²⁵Mg reactions in the E_p = 310–1845 keV region. Altogether 76 resonances have been observed, of which 45 only in (p, γ), 30 in both (p, γ) and (p, p′), and one only in (p, p′). Several of these resonances had not been observed previously in either of the two reactions, and several of the previously observed resonances turned out to be doublets. All 39 ²⁵Mg(p, p₀)²⁵Mg resonances found previously in the E_p = 700–1845 keV region resonate in (p, γ) and many also in (p, p′). Resonance strengths have been derived relative to that of the E_p = 593 keV resonance.The analysis of weak (p, p₁) and (p, p₂) resonance yields has been supported by a calculation of the yields in the continuum (in between resonances) due to the Breit-Wigner tails of strong and broad resonances in the neighbourhood (with interference between resonances with the same J^π; T taken into account), and to Coulomb excitation.     

Gamma-rays from the24Mg(p,γ)25Al reaction for proton energiesE p=800 keV toE p=1,830 keV

The yield curve and resonance strengths of resonances in the²⁴Mg(p, γ)²⁵Al reaction for the proton energy rangeE _p=800 keV toE _p=1,830 keV, have been determined and the existence of a resonance atE _p=1.184 MeV confirmed. Single energy spectra as well as decay schemes and branching ratios are given for these resonances. An upper level of 2.0×10^?15 sec for the lifetime of the 3.079 MeV level in²⁵Al results from a Doppler shift attenuation measurement carried out on the 2.624 MeV gamma line originating in the 3.070 MeV→0.455 MeV transition in²⁵Al.     

Low-energy resonance strengths for proton capture on Mg and Al nuclei

Absolute strengths of selected low-energy resonances for proton captures on the nuclei ²⁴Mg, ²⁵Mg, ²⁶Mg and ²⁷Al have been determined. The experiments were carried out by measuring the number of resonant γ-rays, integrated over the yield curve, simultaneously with the number of Rutherford scattered protons. The method applied in the present work is independent of target stoichiometries and uniformities, stopping powers, beam straggling and current integration. The ratio of charged-particle and γ-ray detection efficiencies was measured by using the ¹⁹F(p,α₂γ)¹⁶O reaction at E_R=340 keV. A new set of reliable resonance strength standards at low bombarding energies is presented and our results are compared with previous work.     

Resonance strength measurements in the 24Mg(p, γ) 25Al reaction

The strength ωγ of the E_p = 823 keV resonance in ²⁴Mg(p, γ) ²⁵Al was determined to be ωγ = 490 ± 70 me V, by comparison with the Rutherford cross section measured in the energy region of E_p = 600–800 keV. The strength of other low-lying resonances at E_p = 224, 419, 1483, 1623, 1654 and 2010 keV were determined relative to this new value.     

(p, γ) Resonance strengths in the s-d shell

The strengths of selected resonances in the range E_p = 0.5–2.0 MeV in the (p, γ) reactions on Mg, ³⁰Si, ³⁴S, ³⁷C1, ³⁹K and ⁴⁰Ca have been found relative to the E_p = 632 and 992 keV resonances in ²⁷Al(p, γ)²⁸Si by relative yield measurements. Targets were made from mixtures or chemical compounds such that each contained at least two of the isotopes of interest and their chemical composition was determined by Rutherford back-scattering of α-particles. Absolute measurements were conducted on the selected resonances in ²⁷A1(p, γ)²⁸Si and ³⁰Si(p, γ)³¹P by semi-thick target and thin target techniques with the target thickness, needed for the latter technique, found by Rutherford back-scattering of protons. Absolute strengths for all of the resonances treated, together with one from each of ²³Na, ³¹P and ³⁵Cl, reported in a previous paper, were deduced by normalizing to the absolute measurements on the Al(p, γ)²⁸Si resonances.     

Strengths of (p, γ) resonances in 33Cl, 35Cl and 28Si

Absolute strength measurements have been performed for the E_p = 580 and 588 keV ³²S(p,γ)³³Cl, E_p = 1214keV³⁴S(p,γ)³⁵Cl and E_p = 633 and 744 keV²⁷Al(p,γ)²⁸Si resonances with a Ge(Li) detector. Results are discussed with regard to the decay of isobaric analog resonances in ³⁵Cl and ³⁷Cl.     

Investigation of 28Si levels with the (α, γ) and (p, γ) reactions

Particle energies have been measured for resonances in the ²⁷Al(p, γ)²⁸Si and ²⁴Mg(α, γ)²⁸Si reactions with an accuracy of 0.5 × 10^?4 and 1 × 10^?4, respectively. The E_p = 991.88 ± 0.04 keV²⁷Al(p, γ)²⁸Si resonance served as calibration point. From these data the Q-value of the reaction ²⁷Al(p, α)²⁴Mg has been determined as 1600.14 ± 0.21 keV. Excitation energies of ²⁸Si levels have been measured with the ²⁷Al(p, γ)²⁸Si reaction; the reaction energy is Q = 11584.5 ± 0.4 keV.Of 33 resonances observed in the ²⁴Mg(α, γ)²⁸Si reaction (E_α = 1.5–3.8 MeV), energies, strengths and γ-ray decay have been measured; five of these resonances had not been reported previously. The γ-ray angular distribution measurements at three resonances yield the resonance J^π values and the mixing ratios of the strongest transitions involved in the decay. The 10.38 MeV level has J^π = 3⁺, T = 1. The arguments on which T-assignments can be based are critically reviewed. These arguments are used to assign T = 1 character to ¹⁹ states of ²⁸Si.     

Étude des états excités du noyau 34Cl: (I). Courbe d'excitation de la réaction 33S(p, γ)34Cl, énergies,rapports d'embranchement et vies moyennes des niveaux du 34Cl

Resonance energies and strengths are reported for forty-five ³³S(p, γ)³⁴Cl resonances in the range E_p = 1.0–2.0 MeV. Analysis of the γ-ray spectra by means of two Ge(Li) detectors leads to precise excitation energies of bound states and branching ratios of resonant levels and bound states. New bound levels are found. The reaction Q-value is 5140.3 ± 1.5 keV. Mean lives (or limits) of twelve bound states of ³⁴Cl have been obtained from DSA measurements at seven of the studied ³³S(p, γ)³⁴Cl resonances.     

T=1 states in26Al

The resonance reaction²⁵Mg(p, γ)²⁶Al in the energy rangeE _p =300–400 keV was used to populate high-lying bound states in the self-conjugate nucleus²⁶Al. The existence of three resonances atE _p =304, 316 and 388 keV was verified, the spins of the ones at 316 and 388 keV were found to be 3^? and 2⁺, respectively. The spin of the bound level at 4,547 keV level was fixed to beJ ^π=2⁺. Several strong isovector Ml transitions were observed, which led toT=1 assignments for the levels atE _x =4,191, 4,547, 4,599, 5,141 and 5,542 keV. The results for excitation energies and values ofJ ^π,T together with previous experimental and theoretical data on²⁶Al and the neighboring |T _z |=1 nuclei²⁶Mg and²⁶Si are discussed in the framework of the isospin model.     

The structure of27Al

Gamma decay modes and spin assignments of levels in²⁷Al have been investigated by proton-γ-ray angular correlation measurements in the²⁴Mg(α, pγ) reaction at 14.2, 15 and 15.6 MeV bombarding energy and byγ-ray angular distribution measurements on the E_p=1820, 2114, 2293 and 2574 keV resonances of the²⁶Mg(p, γ) reaction. Unique spin assignments were obtained as follows: I=3/2 for the 5827 keV state, I=5/2 for the 6115, 6465, 6765, 7577, 7721, 8097, 8136, 8324 keV states, I=7/2 for the 5433, 6533, 7413, 8037, 8442, 8586 keV states, I=9/2 for the 5418, 6512, 6713, 7997 keV states, I=11/2 for the 5500, 6948, 7400, 8396 keV states. The level scheme and electromagnetic properties of levels are compared with the results of shell model calculations which use the complete configuration space of the 0d_5/2-1s_1/2-0d_3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent and extends into the region of high level density above 7 MeV excitation energy. The total B(M1↑) below 8.5 MeV excitation energy is evaluated, using published resonance fluorescence and (e, e′) data, and quenching relative to the free-nucleon predictions is reexamined. Evidence in favour of a prolate ground state deformation of²⁷Al and implications of this work for the astrophysically interesting²⁶Al(p, γ) reaction are discussed.     

Threshold states in 26Al revisited

Threshold states in ²⁶Al have been re-examined using the ²⁷Al(³He, α)²⁶Al and ²⁵Mg(³He, dγ)²⁶Al reactions in order to resolve apparent ambiguities in some of the previously reported properties of these states. In particular, the s-wave resonance strength reported at E_c.m. = 37 keV is now found to be located at E_c.m. = 57.54 keV, and the proton width for the 374 keV resonance has been revised to Γ_p = 0.82 eV. These results have been used to calculate a new resonance strength of ωγ = 1.6 × 10⁻¹³ eV for the 57.54 keV resonance. As a result, the stellar production rate for ²⁶Al is increased by a factor ∼- 3–38 for temperatures T₉ = 0.05–0.1.     

Investigation of 38Ar levels by 37Cl+p reactions: (I). Yield curves,excitation energies and branching ratios

Yield measurements have been performed on the reactions ³⁷Cl(p, p₀)³⁷Cl, ³⁷Cl(p, α₀)³⁴S (E_p = 1.17–1.70 MeV) and ³⁷Cl(p, n₀)³⁷Ar, ³⁷Cl(p.γ)³⁸Ar (E_p = 0.64–2.92 MeV); detailed investigations are restricted to resonances below E_p = 1.79 MeV. Resonance energies, strengths and (some) widths are reported for 12, 26 and 247 resonances in the reactions (p, p₀), (p, n₀) and (p, γ), respectively.Simultaneous (p, α₀) and (p, γ) yield measurements over selected E_p intervals in the E_p = 0.88–1.64 MeV range established the existence of 42 resonances decaying by both γ and α₀ emission. Analysis of the γ-ray spectra of 100 ³⁷Cl(p, γ)³⁸Ar resonances leads to precision excitation energies (ΔE_x = 0.1–2 keV) of 57 bound states (E_x < 8 MeV) and branching ratios of the resonances and of 50 bound states.     

High-spin states in27Al

The²⁴Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the²⁷Al levels atE _x =5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theE _x =7399 keV level and a limitτ<20 fs for the levels atE _x =7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in²⁷Al were obtained by a study of the²⁶Mg(p, γ) reaction at the E_p=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theE _x =5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atE _x =6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the²⁴Mg(α, p) and²⁶Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.     

Helium burning of22Ne

The²²Ne(α, γ)²⁶Mg and²²Ne(α, n)²⁵Mg reactions were investigated forE _α(lab) from 0.71 to 2.25 MeV. Neon gas enriched to 99% in²²Ne was recirculated in a differentially pumped gas target system of the extended type. Theγ-ray transitions were observed with Ge(Li) detectors and the neutrons with³He ionization chambers. A previously known resonance at E_R(lab)=2.05 MeV was verified and 15 new resonances were found in the energy range covered, with the lowest at E_R(lab)=0.83 MeV. Information on resonance energies, widths, strengths,γ-ray branching ratios, as well asJ ^π assignments, is reported. The energy range investigated corresponds to the important temperature range ofT ₉ from 0.3 to 1.4 (10⁹ K), for which the astrophysical rates were determined for both reactions. The results show that the ratios of the rates for²²Ne(α, n)²⁵Mg and²²Ne(α, γ)²⁶Mg are significantly smaller than the previously adopted values, e.g., by at least a factor of 60 nearT ₉=0.65. Thus, the²²Ne(α, n)²⁵Mg reaction will likely play a smaller role as a neutron source fors-process nucleosynthesis, than has frequently been assumed.     

Internal electron-positron pair production from electric monopole transitions

The yield curve of the reaction⁵⁶Fe(p,γ)⁵⁷Co has been measured over the energy rangeE _p =1,300–1,900 keV and the decay of nine resonances has been investigated. For twelve of the resonances the strengths have been determined. The angular distributions of the gamma rays have been recorded for resonances atE _p =1,599, 1,623, 1,643 and 1,649 keV, giving spin-parity assignmentsJ ^π=3/2^? for all four resonances. The resonances atE _p =1,623, 1,643 and 1,649 keV have been identified as the split analogue resonances of the 367 (J ^π=3/2^?) states in⁵⁷Fe. TheM1 transition strengths to the corresponding antianalogue states have been measured and compared with theoretical predictions.     

Voltage calibration of low-energy accelerators

The¹²C(p, γ)¹³N reaction has been used as a method to determine the absolute proton beam energy in the region ofE _p =150–350 keV with a precision of the order of 0.4 keV. The method makes use of the resulting captureγ-ray transition, whose intensity varies smoothly with beam energy and whose varying energy can be determined to high accuracy by comparison with precisely known energies ofγ-rays from radioactive sources. The energy calibration atE _p =80–150 keV has been carried out with the use of the nonresonant capture reaction D(p, γ)³He. The results have been applied to determine the absolute energies of proton-induced resonances on several light target nuclei.     

A study of the 35Cl(p, γ)36Ar reaction and the energy levels of 36Ar

Proton energies and strengths of (p,γ) and (p, p₁) resonances of the ³⁵Cl + p reaction were determined for E_p = 0.4?3.0 MeV and 1.9?3.0 MeV, respectively. The γ-decay of 84 resonances was studied with a 40 cm³ Ge(Li) detector. The branching ratios and excitation energies of 38 bound levels were determined. A new bound level was observed at E_x = 8472.0 ± 1.0 keV. Doppler-shift attenuation experiments yielded lifetimes of 20 bound levels. Spins and/or parities for bound levels and resonances were derived on the basis of observed transition strengths.     

Proton threshold states in 26Al

The energy levels of ²⁶Al between E_x = 6.3 and 6.5 MeV, corresponding to proton threshold energies in the ²⁵Mg + p reaction from $\text{E}{}_{\mathrm{<mtext>p</mtext>}}\text{= 0}\text{to}\text{200}\text{keV}\text{, have been investigated using the reactions}{}^{27}\text{Al(su3}$He, α)²⁶Al and ²⁴Mg(³He, p)²⁶Al. Despite early work reporting a doublet at E_x = 6346 keV and E_x = 6362 keV, most subsequent work reported a single state with conflicting spin and parity assignments. Our work clearly establishes the presence of the doublet and resolves the conflicts. We find that there are six states in this excitation energy region, including a new state at E_x = 6410 ± 5 keV. The l-values leading to possible spin and parity assignments for all these states have been made using DWBA. We conclude however that only three of these six states may contribute to the production of ²⁶Al by the ²⁵Mg(p, γ) reaction in the MgAl cycle and the new structure of ²⁶Al reported here can substantially increase the production rate of ²⁶Al in stellar reactions.