A.J. Howard

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.

Threshold states in 26Al: (II). Extraction of resonance strengths

Abstract The total width of the Ec.m. = 376 keV resonance in the 25Mg + p system has been measured using the 25Mg(p, γ)26Al reaction and is found to be 460 ± 70 eV. From this information, a resonance strength ωγ = 5.7 × 10−16 eV is obtained for the astrophysically important 37.2 keV resonance in 26Al through an R-matrix parameterization of the relative energy dependence of the resonance width. In a similar manner, an upper limit ωγ ≦ 10 × 10−11 eV is deduced for a possible resonance at Ec.m. = 94.0 keV.

Threshold states in 26Al revisited

Abstract Threshold states in 26 Al have been re-examined using the 27 Al( 3 He, α) 26 Al and 25 Mg( 3 He, dγ) 26 Al 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 26 Al is increased by a factor ∼- 3–38 for temperatures T 9 = 0.05–0.1.

Studies on the 21Ne(p, d)20Ne and 21Ne(d, p)22Ne reactions

Angular distribution measurements have been performed on the 21Ne(p, d)20Ne and 21Ne(d, p)22Ne reactions at Ep = 20 MeV and Ed = 10.2 MeV, respectively. In the 21Ne(p, d) 20Ne reaction, the prolific formation of the Jπ = 2+, 1.63 MeV state was characterized by ln = 2 pickup, and the distribution associated with the 44, 4.25 MeV state was suggestive of a weak ln = 2 pickup. All of the observed ln = 1 pickup strength is associated with formation of the 2−, 4.97 MeV 20Ne level. The 21Ne(d, p)22Ne results indicate that ln = 2 transfer is involved in the formation of the 1.28, 3.36, 5.52, 5.63 and 6.65 MeV 22Ne states. The angular distribution observed for the 2+, 4.46 MeV state and also the unresolved 5.33, 5.36 MeV composite of states required both ln = 0 and ln = 2 components in the associated distorted-wave Born approximation fits. The spectroscopic factors extracted from the present results are compared with those predicted by the Nilsson model without mixing: Applications of the angular momentum projection rule to the 21Ne(d, p)22Ne reaction are considered.

Constraining 26Al+p resonances using 26Al(3He,d)27Si.

The {sup 26}Al({sup 3}He,{ital d}){sup 27}Si reaction was measured from 0{degree}{le}{theta}{sub c.m.}{le}35{degree} at {ital E}({sup 3}He)=20 MeV using a quadrupole-dipole-dipole-dipole magnetic spectrometer. States in {sup 27}Si were observed above the background at 7652 and 7741 keV and upper limits were set for the state at 7592 keV. Implications for the {sup 26}Al({ital p},{gamma}){sup 27}Si stellar reaction rate are discussed. {copyright} {ital 1996 The American Physical Society.}

Studies of 37Ar via the 36Ar(d, pγ)37 Ar reaction

Abstract The 37 Ar states below an excitation energy of 3.7 MeV have been studied via the 36 Ar(d, pγ) 37 Ar reaction at deuteron bombardment energies in the vicinity of 3 MeV. Direct γ-ray spectra were recorded in Ge(Li) spectrometers at θ γ = 0° and 90° with gas target pressures in the range 25 to 3000 Torr. The following excitation energies were observed for the low-lying levels of 37 Ar: E x = 1408.8 ± 1.5, 1611.2 ± 0.8, 2218.2 ± 1.5, 2490.6 ± 1.3, 2796.3 ± 1.8, 3171.3 ± 1.7, 3186.4 ± 1.7, 3273.4 ± 1.6, 3516.1 ± 1.8, 3526.1 ± 2.1 and 3607.7 ± 2.9 keV. An upper limit on the mean lifetime of τ ≦ 30 ps was deduced from the Doppler shift for all of the above states except the relatively long-lived 1611 keV state. Proton-gamma angular correlation studies were performed in a collinear geometry at E d = 2.800, 2.975 and 3.100 MeV . The following spin restrictions resulted therefrom: E x (J) = 1611 ( 5 2 , 7 2 ), 2218 ( 5 2 , 7 2 ), 2491 ( 3 2 , 5 2 ), 2796 (≦ 7 2 ), 3171 ( 3 2 , 5 2 ), 3273 ( 3 2 , 5 2 ), 3516 ( 3 2 ), 3526 ( 7 2 ), and 3608 ( 3 2 , 5 2 ) . The results of the present and previous studies on the low-lying levels of the 37Ar structure are synthesized and compared with existing shellmodel calculations.

The identification of mirror state structure in 21Na and 21Ne

Abstract Angular correlation studies with the 24 Mg ( p ,α) 21 Na (P O ) 20 Ne reaction establish J = 5 2 and 1 2 spin assignments for the respective 3540 and 2804 keV 21Na states. Mirror pair identification between the 21Ne 2790 keV and 21Na 2804 keV states indicates a J π = 1 2 − assignment thereto.

Proton threshold states in 27Si and the destruction of 26Al at low stellar temperatures

Abstract Using the 27Al(3He, t)27Si reaction, we have located and measured accurate excitation energies for seven states in 27Si between the 26Al + p threshold and the lowest observed 26 Al ( p , γ) 27 Si resonance. By examining the mirror states in 27Al via the 27Mg(3He, d)27Al and 26 Mg (α, t ) 27 Al reactions, we find that in the 26AI + p channel there can be at most only one s-wave resonance that lies below the lowest directly observed resonance; this possible l = 0 resonance most probably corresponds to either the 7589, 7557 or 7533 keV state in 27Si. The remaining six states are most probably formed via l ⩾ 2 proton capture. Upper limits for the reduced proton widths of these states have been used to estimate an upper limit for the 26 Al ( p , γ) 27 Si thermonuclear reaction rate.

A versatile data acquisition system for general gamma ray spectroscopy

Abstract A description is given of the associated hardware and instrumentation which have been designed and adapted for use in general gamma-ray spectroscopy studies involving nuclear reactions where high bombarding energies, typically available from tandem electrostatic accelerators, are employed. The function of the system is illustrated by experimental data taken from recent particle-gamma-ray angular correlation and Doppler shift lifetime measurements involving utilization of the laboratory on-line IBM 360/44 data acquisition system.