Download
1 / 26
260 likes | 370 Views
The evaluation of a new method to extract spectroscopic factors using asymptotic normalization coefficients and the astrophysical 14 C( n , γ ) 15 C reaction rate. Matthew McCleskey. Neutron capture on unstable nuclei.
E N D
The evaluation of a new method to extract spectroscopic factors using asymptotic normalization coefficients and the astrophysical 14C(n,γ)15C reaction rate Matthew McCleskey
Neutron capture on unstable nuclei • Neutron direct capture reaction cross sections on unstable nuclei are needed for nuclear astrophysics (BBN, s- and r-processes), stockpile stewardship and for new reactor designs. • Because no neutron target exists, and many of the nuclei of interest are short-lived indirect methods using inverse kinematics at laboratory energies need to be developed. • Unlike proton direct capture, which is peripheral and where the cross section can be determined using the ANC, neutron capture is not as simple, may have a significant contribution from the interior • most n-capture is s-wave → Must use SF • some cases may be dominated by p-wave capture → Use ANC
New method A new method to extract SFs has been proposed* that utilizes the ANC to fix experimentally the SPANC and thus determine the SF. • Need peripheral reaction to determine ANC • Need non-peripheral reaction to get SF 15C↔14C+n system is being used as a test case for this method. Will also use the ANC found to calculate the 14C(n,γ)15C reaction rate *AM Mukhamedzhanov and FM Nunes Phys Rev C 017602 (2005)
The matrix element can be split into external and internal parts: One can then define a function the experimental counterpart of which is Comparing these two functions experimentally fixes the SPANC therefore giving the correct SF: New method
MDM spectrometer (D.M. Pringle et al. NIM A245 (1986) pg. 230-247)
Oxford detector • ionization chamber filled with ~50 torr isobutane • anode plates to measure energy loss • plastic scintillator to measure residual energy • 4 resistive wires (avalanche counters) to give position
½ + 15C (ground state) 5/2+ 15C 0.74 MeV 2+ 12C and 5/2+ 15C 5.17 MeV 48Ti/56Fe (imp.) 27Al/28Si (imp.) 16O (imp.) Elastic g.s. ½+ 13C 5/2+ / 3/2- 13C 2+ 12C (imp.) Focal plane position Reconstructed target angle Reconstructed target angle Particle ID: 14C+13C 15C 14C ∆E Focal plane position Eres
Finding an OMP • Grid search in V • Use OMP of WS form: • Fit other 5 parameters for each V, pick several values of V for further fitting
Finding an OMP • Grid search in V • Use OMP of WS form: • Fit other 5 parameters for each V, pick several values of V for further fitting • Double folding calculation • Semi-microscopic approach • Double folding calculation using JLM effective interaction • Only 2 parameters (normalizations) to fit
Transfer: 13C(14C,15C)12C DWBA calculations performed using PTOLEMY, using different potentials ←using OMPs from grid search ←using OMP from double folding
ANC results from HI Uncertainties: 4% target thickness, 3% normalization to the number of incident particles, 5% data extraction and disentanglement from the 1st excited state of 15C, 6% statistical uncertainty and 10% systematic uncertainty in the calculations. This gives overall uncertainty of 14% for the ANC2 1st excited state had lower statistical uncertainty (~1%) giving an overall uncertainty for that ANC2 of 13%
d(14C,p)15C TECSA (Texas A&M-Edinburgh-Catania Silicon Array)
TECSA target TECSA silicon ring array MARS Radioactive beam from MARS Distance to target determines angular range TECSA : d(14C,p)15C TECSA target is CD2 ~250μg/cm2 thick For 14C beam, no primary (production) target in MARS is used.
TECSA: d(14C,p)15C ADWA calculation using FRESCO with CH89 nucleon potentials (Adiabatic Distorted Wave Approximation)
Results from d(14C,p)15C ANC for ground state: fm-1 ANC for 1st excited state: fm-1 Uncertainties: 2% due to target thickness, 2% incident beam normalization, 4% for the analysis and < 2% for statistics. This combined with a 10% systematic uncertainty gives an overall error in C2 of 12%.
Deuterons Protons 14C(d,p)15C • 60 MeV deuteron beam impinges on thin, enriched 14C target • Higher energy and light projectile means that this reaction is expected to be not peripheral, so we can extract the spectroscopic factor using the previously determined ANC • Used MDM spectrometer and Oxford detector- Same setup as for HI, but with more gas pressure and a much thicker scintillator to stop protons • Particle ID in scintillator:
14C(d,p)15C counts Position in focal plane (mm)
14C(d,p)15C Angular distributions and ADWA calculations performed using FRESCO
Rexp vs RDW Recall: GS Rexp vs. Rth 1st exc. Rexp vs. Rth This figure shows an upper limit of r0 of ~1.15 fm, which corresponds to b2 = 4.01∙10-3fm-1. From the relation Weak dependence indicates a peripheral reaction, so even at 60 MeV deuteron energy we can get the ANC… but no information about the SF one obtains a lower limit of SF=1.05
Astrophysical 14C(n,γ)15C rate • Important for: • Inhomogeneous BBN • Depletion of CNO isotopes in AGB stars • Effect on seed nuclei for r-process in core-collapse SN • Dominated by p-wave capture → peripheral reaction, can use ANC • Calculate rate using the code RADCAP • Include capture to GS and 1st exc state Black squares are the direct measurement (Reifarth et al. PRC 77 015804 (2009)), blue is calculation using the ANC, red lines show uncertainty in the calculation due to the uncertainty in the ANC
Acknowledgements Collaborators: R. Tribble, L. Trache, A. Mukhamedzhanov, F. Carstoiu, A. Alharby, A. Banu, V. Goldberg, Y.-W. Lui, B. Roeder, E. Simmons, A. Spiridon Special thanks: N. Nguyen Work funded by: NNSA-SSAA, DOE
