Direct Proton Capture Research Articles

Development of a NiO target for the production of 11C at ISAC/TRIUMF

High intensity 11C beams are necessary for the investigation of the formation of 12C via the nuclear reaction 11C(p, γ)12N→12C+e++ν. The production of intense carbon beams on-line is quite challenging due to the thermodynamic properties and chemical reactivity of carbon at high temperatures. A previous attempt, using a medical isotope cyclotron production method in batch mode, was not conclusive. The intensity obtained was at least one order of magnitude too low for a direct proton capture experiment using the DRAGON facility at ISAC/TRIUMF. Producing a 11C beams using the ISOL method requires a target capable of efficiently releasing the carbon isotopes. NiO has been selected as a target material because most of the nickel carbides are not stable at high temperature. The development of carbon beams using a composite NiO/Ni target on-line is described.

Theoretical studies of proton capture reactions in A ∼ 25 proton-rich nuclei

The direct proton capture and resonance proton capture properties of stellar reactions 22Mg(p,γ)23Al and 26Si(p,γ)27P are studied by employing a mean-field potential obtained from the Skyrme-Hartree-Fock (SHF) model. Calculations with the SHF potential reproduce well the loosely-bound structure of the ground states as well as the widths of the resonant states in these nuclei. With the obtained potential we estimate the reaction rates of direct proton capture and resonance proton capture to nuclei 23Al and 27P. The effect of the 27P loosely-bound structure on the S factor of the direct proton capture is also discussed.

PROTON RESONANCE PROPERTIES IN LIGHT NUCLEI WITH MEAN-FIELD TYPE POTENTIALS

The properties of proton resonances in the p and sd shells are studied with a single-particle potential generated by self-consistent-mean field calculations. Besides, we also propose to calculate the proton-nucleus interaction by a double-folding procedure with the density of the daughter nucleus represented by the result of the same mean-field model. Without raising any adjustable parameter, these calculations provide reasonable estimates for the widths of observed proton resonances in light nuclei. The mean-field-based calculation is particularly useful for the predications of the properties of resonances in light exotic nuclei and the direct proton capture reactions of astrophysics.

Asymptotic normalization coefficients for mirror virtual nucleon decays in a microscopic cluster model

It has been suggested recently (Phys. Rev. Lett. 91, 232501 (2003)) that charge symmetry of nucleon-nucleon interactions relates the Asymptotic Normalization Coefficients (ANCs) of proton and neutron virtual decays of mirror nuclei. This relation is given by a simple analytical formula which involves proton and neutron separation energies, charges of residual nuclei and the range of their strong interaction with the last nucleon. Relation between mirror ANCs, if understood properly, can be used to predict astrophysically relevant direct proton capture cross sections using neutron ANCs measured with stable beams. In this work, we calculate one-nucleon ANCs for several light mirror pairs, using microscopic two-, three- and four-cluster models, and compare the ratio of mirror ANCs to the predictions of the simple analytic formula. We also investigate mirror symmetry between other characteristics of mirror one-nucleon overlap integrals, namely, spectroscopic factors and single-particle ANCs.

Spectroscopic factors from direct proton capture

Spectroscopic factors derived from direct capture studies are systematically compared to those obtained from transfer reaction experiments and from shell model calculations for the $\mathrm{A}=16--32$ target mass region. The direct proton capture and proton transfer spectroscopic factors are obtained in the present work from a reanalysis of literature data by using the same bound state potential parameters. Our direct capture spectroscopic factors differ significantly from the values originally reported in the literature. The sensitivity of the direct capture cross section to different choices for the scattering potential is explored. We find evidence that spectroscopic factors obtained from direct capture studies are as reliable as those extracted from transfer measurements if the (direct capture) radial scattering wave function is calculated with a zero nuclear scattering potential instead of the common choice of a hard-sphere potential.

Spectroscopic factors from direct proton capture

Spectroscopic factors from direct proton capture

Determination of the24Mg(p, γ)25Al Reaction Rate at Low Stellar Temperatures

Standard stellar evolution theory predicts for low-mass stars in globular clusters only minor changes in surface composition during the red giant phase. However, observations over the past two decades have shown a large variation in the abundances of C, N, O, Na, Mg, and Al among globular cluster red giant stars (see R. P. Kraft, C. Sneden, G. H. Smith, M. D. Shetrone, G. E. Langer, & C. A. Pilachowski, AJ, 115, 1500 [1998]). Moreover, it has been found that the abundances of O and Na are anticorrelated in a number of globular clusters, as are the abundances of Mg and Al. These correlations are thought to be the result of deep mixing of material from the surface of the star into regions where the temperature is high enough to allow proton captures to take place. Additional observations of M13 red giant branch stars show an anticorrelation between 24Mg and Al and that the abundance sum 25Mg]26Mg is approximately constant for large variations in Al (M. D. Shetrone, AJ, 112, 2639 [1996]). Production of Al and destruction of 24Mg depend on the 24Mg(p, c)25Al reaction rate at stellar temperatures 0.04 (where T9D T9 109 K). However, the commonly used rate (G. R. Caughlan & W. A. Fowler, At. Data Nucl. Data Tables, 40, 283 [1998]) is too low to account for the observations within the present mixing models. In the stellar temperature range of the T9 0.02E0.055, 24Mg]p reaction rate is determined by contributions from the lowest lying resonance at keV and the direct E R 223 capture process into Ðnal bound states of 25Al. Direct proton capture on 24Mg has been studied extensively by H. P. Trautvetter, & C. Rolfs (Nucl. Phys., A242, 519 [1975]). The strength of the keV resonance has E R 223 also been measured previously. Based on this experimental information, the reaction rate at low stellar temperatures has been calculated by Caughlan & Fowler (1988). It was pointed out recently by C. S. Zaidins & G. E. Langer (PASP, 109, 252 [1997]) that at the temperatures important for globular cluster red giant stars, the Gamow window is located far below the energy of the keV resoE R 223 nance. Consequently, the contribution from the low-energy wing of this resonance has to be taken into account. Accurate calculation of the wing contribution requires additional information regarding resonance parameters (such as the proton partial width c-ray partial width and the p , !c, total width !). However, only an experimental upper limit of ! 32 eV has been reported in the published literature (M. Uhrmacher, K. Pampus, F. J. Bergmeister, D. Purschke, & K. P. Lieb, Nucl. Instrum. Meth., B9, 234 [1985]). It was estimated by Zaidins & Langer (1997) that the 24Mg]p reaction rate at can increase by a factor of 32 due T9 0.04 to the contribution of the resonance wing if the experimental upper limit for is adopted. The main goal of this thesis was to accurately determine the properties of the E R 223 keV resonance in order to improve the reaction rate estimate for 24Mg(p, c)25Al. Resonance strengths uc, c-ray branching ratios and !c/!, mean lifetimes were measured for the keV resoE R 223 nance in 24Mg(p, c)25Al at the Triangle Universities Nuclear Laboratory. The measured resonance strength of uc 12.7^ 0.9 meV and branching ratio of !c/! 0.91 ^ 0.04 allow for the determination of and !. The !p, !c, measurement of the mean lifetime of fs via the q ~2.4 `2.9 Doppler shift attenuation method also allows for an independent determination of (by using q +/!) and therefore provides a test of internal consistency. The present experimental results establish the rate for the 24Mg(p, c)25Al reaction for temperatures of T9 0.02E2. The reaction rate deviates from the previous estimates (Caughlan & Fowler 1988) by 18%E45% with statistical uncertainties of 5%E21%, and therefore the suggestion of Zaidins & Langer (1997) that the total width of the E R 223 keV resonance may have a signiÐcant inNuence on the total 24Mg]p reaction rate for can be ruled out. T9B 0.04 P. A. Denissenkov, G. S. Da Costa, J. E. Norris, & A. Weiss (A&A, 333, 926 [1998]) suggested that the existence of an undetected low-energy resonance could also enhance the 24Mg(p, c)25Al reaction rate. Based on our current experimental and theoretical understanding of the 25Al level structure, this possibility is extremely unlikely. Our results suggest that hydrogen burning of 24Mg at temperatures below cannot account for the anticorrelation of T9 0.055

Some practical applications of the observation of the 16O-direct proton capture reaction with a Ge(Li) detector

The direct proton capture reaction in 16O is shown to provide a tool for the determination of oxygen distribution in metals, beam energy and energy spread in favourable circumstances.

(d, n) thresholds and direct proton capture

An examination is made of the correlation between the occurence of intense neutron thresholds in (d, n) reactions and the observation of direct radiative capture to the corresponding levels in (p, γ) reactions on the same target nuclei. It is concluded that all experimental evidence favours a one-to-one correspondence between these effects, indicating large proton reduced widths for all levels observed as intense (d, n) thresholds. It is argued that direct radiative capture should be observable in the B 10(p, γ)C 11 reaction and to a lesser extent in certain other reactions.

Lifetimes of the First Excited States ofF17andO17

Using the gamma-gamma coincidence method and an electronic time-to-amplitude converter, the lifetimes of the first excited states in the mirror pair ${\mathrm{O}}^{17}$(0.87 Mev) and ${\mathrm{F}}^{17}$(0.50 Mev) have been measured. The reactions initiating the gamma cascades were ${\mathrm{O}}^{16}(d, p){\mathrm{O}}^{17*}$ and direct proton capture in ${\mathrm{O}}^{16}$. The time resolution of the coincidence circuit was good enough to allow the lifetime measurements to be made by direct observation of ${\mathrm{O}}^{17*}$ is (2.55\ifmmode\pm\else\textpm\fi{}0.13)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec; the measured mean life of ${\mathrm{F}}^{17*}$ is (4.45\ifmmode\pm\else\textpm\fi{}0.22)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. Both of these results are in reasonable agreement with earlier, less accurate values.