The contribution of the {sup 18}F(p,{gamma}) reaction to the production of {sup 19}Ne, which is an important isotope in connection with the breakout from the hot CNO cycle, has been investigated in experiments with {sup 18}F beams. Measurements of the cross sections for the {sup 18}F(p,{alpha}){sup 15}O and {sup 18}F(p,{gamma}){sup 19 }Ne reactions indicate that the contribution of the {sup 18}F(p, {gamma}) route to the formation or {sup 19}Ne is small

Blumenthal, D.

Davids, C. N.

DeCrock, P.

Fischer, S. M.

Gehring, J.

Henderson, D.

Jiang, C. L.

Lister, C. J.

Nickles, Robert J.

Nolen, J.

Pardo, R. C.

Paul, M.

Rehm, K. E.

Roberts, Andrew D.

Schiffer, J. P.

Segel, R. E.

Seweryniak, D.

English

Institutional Repository for Minnesota State University, Mankato

DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privateiy owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. . # d -- The submitted manuscript has been authored by a contractor of the U. S. Government under contract No, W-31-109ENG-38. Accordingly. the U. S Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U. S. Government purposes. 1 The Influence of 18F Induced Reactions in the Hot CNO Cycle -. K. E. Rehm", M. Pad', A. D. Roberts', C. L. Jiang", D. Blumenthal", C. N. Davids", P. DeCrocka, S .  M. FischerO, J. Gehringa, D. Hendersona, C. J. Lister", J. Nickles", J. Nolen", R. C. Pardo", J. P. ScMer" R. E. Segeld and D. Seweryniak" "Argonne National Laboratory, Argonne, IL 'Hebrew University, Jerusalem 'University of Wisconsin, Madison, VI dNort hwest em University, Evanst on, IL The contribution of the I8F(p,7) reaction to the production of "Ne, which is an im- portant isotope in connection with the breakout from the hot CNO cycle, has been in- vestigated in experiments with 18F beams. Measurements of the cross sections for the 18F(p,~)150 and '*F(p,~)l~Ne reactions indicate that the contribution of the I8F(p,7) route to the formation of "Ne is small. 1. INTRODUCTION The synthesis of heavier elements in explosive nucleosynthesis in a proton-rich environ- ment is believed to proceed through the nuclide "Ne which is is produced either directly via the 150(a,7)19Ne reaction or via the 140(a,p)17F reaction followed by the sequence 17F(p,7)18Ne(P+) 18F(p,-y)19Ne[l]. 19Ne is then the starting point for the rp-process where, in a series of radiative capture reactions followed by p+ decays, nuclei up to 56Ni and be- yond are producedf21. Since 18F can also be destroyed via the 18F(p,(r)150 reaction the 'breakout' from the hot CNO cycle via '*F is controlled by the ratio of the reaction rates R[18F(p,a)]/R[18F(p,7)]. In a series of experiments using radioactive 18F beams we have therefore studied the astrophysical reaction rates for the (p,a) and the (p,7) reactions on I8F. 2. EXPERIMENTAL DETAILS The experiments were performed at the ATLAS accelerator system of Argonne National Laboratory using a two-accelerator method for generating the 18F ion beam. The 18F material (Tl/z=llO min) was produced at the medical cyclotron of the University of Wis- consin via the '80(p,n)18F reaction with 11 MeV protons bombarding an enriched ["O] water target. After chemical separations the material was flown to Argonne National o.-- Laboratory and installed in the negative ion sputter source of the tandem accelerator which is one of the two injectors of the superco S UNLIMITED DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. . 2 more detailed description of the production method can be found in Ref. [3]. The tandem accelerator produced 18F ions in their 4+ state with energies between 11.7- 15.1 MeV. For a typical run ef 2h the average beam current on target was ~ 5 x 1 0 ~  I8F/sec. The resulting beam is a mixture of radioactive 18F4+ and 1 8 0 4 +  ions from the production target with the l8O being 500-2000 times more intense than the 18F. For the (pia) reaction, particle identification was obtained using the gas-wed mag- net method which gives clean mass and Z identification even for particles with energies of about 500 keV/nucleon. Details of this technique are given in Ref. [4]. To improve background snppression the a particles from the P ( ' ~ F , ~ ~ O ) C ~  reaction were detected in kinematic coincidence with a large area Si detector, mounted at the appropriate scattering angle. The (p,7) measurements were performed with the Fragment Mass Analyzer (FMA) [5] which has a high (- 30 7%) efficiency for radiative capture measurements. In the focal plane of the FMA the incoming particles were identified in a position-sensitive pardel- grid avalanche counter according to their m/q ratio. This detector was followed by a large-volume ionization chamber for Z identification. With this arrangement a suppres- sion ratio of for (p,7) reaction products relative to the incident beam has been achieved. In both experiments thin stretched polypropylene foils (-J 60-100 pg/cm2) were used as targets. The detection efficiencies of the two experimental setups were determined by measuring the excitation functions for the I80-induced reactions ~ ( ' ~ 0 , ' ~ N ) a  and P ( ~ ~ O , ' ' F ) ~ ,  respectively. 3. EXPERIMENTAL RESULTS Figure 1 shows the cross sections for the 18F(p,a)150 reaction measured in the energy region Em=550-800 keV. The resonance corresponds to a state in I9Ne at an excitation energy E,= 7.063 MeV. The horizontal bars represent the energy range due to the en- ergy loss in the target. The solid line represents a Lorentzian averaged over an energy range of 55 keV with parameters obtained from a least-squares fit to the energy-averaged yields as described below. The energy range of 55 keV represents an average value for the different target thicknesses used in the experiments. From a comparison of the mea- sured proton width l?, with its Wigner limit and the results of (3He,d) measurements[6] populating states in the mirror nucleus 19F, a spin value of 3/2+ for this state has been derived[7]. This assignment agrees with the results from a thick target measurement for the 18F(p,a)'50 reaction in Ref. 181. From a least squares fit to the data, values of wy=2. l f0 .7  keV, rP=5f1.6 keV, r,=8.6-+2.5 keV, and I',=13.6&4.6 keV were obtained. Five runs were carried out with 18F beams to study the 18F(p,7)19Ne reaction at a bom- barding energy of E,,= 670 keV, Le. slightly above the s-wave resonance found in the . 3 ANL.P-21,948b 401 I I I I I I ANL-P.21.798 I I I I j  I I I 1  18F (p , C Z ) ' ~ ~  I ec,,,,= 110+5" 30 lOO.4 / !  0.6 0.8 1.0 1.4 2.0 j T9 _ -  I Figure 1. Excitation function for the Figure 2. Ratio of the reaction rates be- 18F(p,a)150 reaction. The horizontal er- tween the 18F(p,a)150 and the 18F(p,?)19Ne ror bars represent the various target thick- reactions. The two solid lines represent the nesses. The solid line represents the cross upper and lower limits for this ratio. section from a resonance with parameters given in the text averaged over an energy interval of 55 keV. 18F(p,a)150 reaction. The 18F beam intensity was monitored by collecting elastically scat- tered 18F particles on a circular aperture which covered the angular range Orab=3.6-100. The pf activity of each collimator was measured off-line after each run with a calibrated Ge detector. From the integrated charge associated with the 18F beam (2.8 pnC) and the total detection efficiency of the FMA an upper limit for the 18F(p,7)19Ne reaction at E,= 670 keV of 42 pb has been deduced. Assuming the widths for the 3/2+ resonance as given above, upper limits for the resonance strength UT= 740 meV and the gamma width rr= 3 eV have been calculated. 4. DISCUSSION As shown in Ref. [9], the 3/2+ s-wave resonance in "Ne has a strong influence on the astrophysical reaction rate for the "F(p,a) reaction. At temperatures above Tg=0.5 the reaction rate is dominated by the new 3/2+ state at 7.063 MeV. Only at temperatures Tg <0.5 do contributions from other states start to be significant, with the 3/2- level at 6.742 MeV excitation energy being the most important. From the limit for the resonance strength for the 18F(p,7)19Ne reaction given above, an upper limit for the astrophysical reaction rate can be calculated which is smaller than the corresponding (p,a) value by at least three orders of magnitude. A lower limit for the reaction rate is provided by the cross section for direct proton capture as calculated in Ref. [lo]. The ratio of the reaction rates R[18F(p,a)]/R[18F(p,~)] which is the controlling factor for the breakout from the hot CNO cycle to the rp-process is plotted in Fig. 2. The two solid lines represent the lower and upper limits for this ratio caused by the limits 4 of the (p,7) reaction rate mentioned above. Due to the large value of the 18F(p,a)150 cross section, the ratio of the reaction rates is larger than - lo3 over the whole range of temperatures. This means that for the production of IgNe the "F(p,7) route is smaU, and the dominant mechanism for generating this isotope must be the 150(a,7) reaction. 5. SUMMARY This work provides the first experimental limit for the ratio of the astrophysical re- action rates between the 18F(p,a)250 and the 18F(p,7)19Ne reactions. The large cross section for the first reaction makes the (p,7) route a small branch for the production of "Ne which is more effectively produced via the "0(a,7) reaction. The gas-filled magnet technique allowed a clean mass and Z identification for reaction products with energies below 1 MeV/u. The use of the Fragment Mass Analyzer for the measurement of radiative capture reactions results is a considerable improvement over gamma detection techniques, especially when unstable reaction products have to be detected. Improvements in beam intensity, which should be possible for less chemically reactive unstable isotopes, should allow the use of thinner targets and thus the measurement of excitation functions in finer steps than was done in these first experiments with radioactive ion beams. This work was supported by the U.S. Department of Energy, Nuclear Physics Division, under contract W-31-109-ENG-38 and the National Science Foundation. REFERENCES 1. A. E. Champagne and M. Wiescher, Ann. Rev. Nucl. Part. Sci., 42 ,  39 (1992) 2 . .  R. K. Wallace and S. E. Woosley, Astrophys. J. Suppl. 45, 389 (1981) 3. A. Roberts et al., Nucl. Instrum. Methods B103,  523 (1995) 4. K. E. Rehm et al., Nucl. Instrum. Methods A370,438 (1996) 5. C. N. Davids et al., Nucl. Instrum. Methods B70,358 (1992) 6. C. Schmidt and H. H. Duhm, Nucl. Phys. A155,644 (1970) 7. K. E. Rehm et al., Phys. Rev. C52, R460 (1995) 8. R. Coszach et al., Phys. Lett. B353 ,  184(1995) 9. K. E. Rehm et al., Phys. Rev. C53, 1950 (1996) 10. M. Wiescher and K. U. Kettner, Astrophys. Journ. 263, 891 (1982) 

The Influence of {sup 18} F Induced Reactions in the Hot CNO Cycle

https://cornerstone.lib.mnsu.edu/cgi/viewcontent.cgi?article=1139&amp;context=phys_ast_fac_pubs

The Influence of {sup 18} F Induced Reactions in the Hot CNO Cycle

Abstract

Similar works

Full text

Available Versions

Institutional Repository for Minnesota State University, Mankato