1. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 This work was supported by the U.S. DOE Office of Science under Grant No. DEFG02-88ER-40404 Indiana University: T. Steinbach, M.J. Rudolph, Z.Q. Gosser, K. Brown ✼,V. Singh, J. Vadas, B. Wiggins, S. Hudan, RdS Florida State: I. Wiedenhover, L. Baby, S. Kuvin, V. Tripathi Grappling with in nuclear fusion to overcome barriers

2. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 This work was supported by the U.S. DOE Office of Science under Grant No. DEFG02-88ER-40404 Indiana University: T. Steinbach, M.J. Rudolph, Z.Q. Gosser, K. Brown ✼,V. Singh, J. Vadas, B. Wiggins, S. Hudan, RdS Florida State: I. Wiedenhover, L. Baby, S. Kuvin, V. Tripathi Grappling with in nuclear fusion to overcome barriers Look! I know exactly how to get out of this damn valley! FRIB

3. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Motivation Fusion reactions in the outer crust result in X-ray bursts and superbursts Problem: At the temperature of the crust, the Coulomb barrier is too high for thermonuclear fusion of carbon – another heat source is needed. 1)Nuclear astrophysics: Nuclear reactions in outer crust of a neutron star 2) Nuclear science: dynamics of fusing two neutron-rich nuclei Horowitz et al. proposed that 24 O + 24 O or 28 Ne + 28 Ne might be the heat source. More recently mid-mass nuclei have been suggested. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Energy output of a single burst equal our sun’s solar output for a decade!

4. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 R.T. deSouza, et al Phys. Rev. C88 014602 (2013) Density constrained TDHF calculations  If valence neutrons are loosely coupled to the core, then polarization can result and fusion enhancement will occur 16 O Core valence neutrons Horowitz et al., Phys. Rev. C 77, 045807 (2008) Umar et al., Phys. Rev. C 85, 055801 (2012)

5. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 What do we want to measure? Evaporation residuesEvaporated particles Target Beam Evap. Residue  As the excited nucleus decays, emission of evaporated particles kicks evaporation residues away from zero degrees  To measure the fusion cross-section we need to count the number of evaporation residues relative to the number of incident O nuclei

6. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Experimental Setup: Florida State Tandem  Incident Beam: 18 O + 12 C @ 1-2 MeV/A  Intensity of 18 O: 3x10 5 pps  Target: 100 µg/cm 2 carbon foil  T2: θ Lab = 3.5 - 10.8°; T3: θ Lab = 11.3 - 21.8°  Time-of-Flight (TOF) between target-MCP and Si (T2, T3) T.K. Steinbach et al., PRC90, 041603 (2014).

7. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Identifying Evaporation Residues T.K. Steinbach et al., Phys. Rev. C 90, 041603(R)(2014)  Evaporation residues are well separated from elastic and slit scattered beam particles  Alpha particles are also cleanly resolved

8. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 18 O + 12 C Fusion Excitation Function  Well described by Wong formalism (parabolic barrier) Rc = 7.34 ± 0.07 fm V = 7.62 ± 0.04 MeV h  /2  = 2.86 ± 0.09 MeV  Measured the cross section for E CM ~ 5.3 – 14 MeV matches existing data  Extends cross-section measurement down to ~800 µb level (~30 times lower than previously measured)

9. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Comparison of 18 O + 12 C Fusion Excitation Function with DC-TDHF  Above the barrier, DC-TDHF with pairing predicts a larger cross-section  In this energy regime the quantity  expt. /  DC-TDHF is roughly constant at 0.8  Below the barrier the experimental cross-section falls less steeply with decreasing E c.m. than the DC-TDHF predictions.  Below the barrier the quantity  expt. /  DC-TDHF increases from 0.8 to approximately 15 at the lowest energy measured.

10. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Energy and angular distribution of ERs in 18 O + 12 C Energy and angular distributions of evaporation residues (ERs) reveal a clear component associated with alpha emission. This component is under- predicted by the statistical model codes PACE4 and evapOR.

11. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Decay channels following fusion The large angle part of the ER angular distribution is populated almost exclusively by  and  +n decay with the largest angles populated by single  emission only. The small angle region of the distribution is dominated by nucleon emission, principally 2n and n+p In order to explain the observed angular distribution it is necessary to increase the single  emission by a factor of 11 and the n+  by a factor of 1.9; Due to the increase in the  emission channels, it is necessary to reduce the nucleon emission channels by 0.825 to reproduce the small angle portion of the spectrum

12. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Direct measurement of the  cross-section The increased  emission indicated by the energy and angular distribution of ERS is directly confirmed by the extraction of the  cross-section. The statistical model codes significantly under-predict the measured  cross-section. At the lowest energies the measured  cross-section is under-predicted by a factor of three. A stronger energy dependence is exhibited by the experimental data as compared to the model predictions.

13. Romualdo deSouza, CKG Symposium, Oct. 8, 2015  19 O produced by: 18 O(d,p) @ ~68 MeV  Intensity of 19 O: 1-2x10 4 pps  Beam tagging by  E-TOF  Target: 100 µg/cm 2 carbon foil  T2: θ Lab = 3.5 - 10.8°; T3: θ Lab = 11.3 - 21.8°  Time-of-Flight (TOF) between target-MCP and Si (T2, T3) Radioactive beams at Florida State : The RESOLUT facility 18 O 7+ 19 O 7+ 18 O 6+ Ion chamber

14. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 19 O + 12 C Fusion Excitation Function  Wong formalism (parabolic barrier) Fusion excitation functions for 19 O + 12 C and 18 O + 12 C were simultaneously measured The excitation function for 18 O matches the results of the high resolution measurement previously performed within the statistical uncertainties. At all energies 19 O + 12 C is associated with a significantly large cross-section. 18 O 19 O Rc7.34 ± 0.07 fm8.00 ± 0.40 fm V7.62 ± 0.04 MeV7.64 ± 0.61 MeV h  /2  2.86 ± 0.09 MeV6.47 ± 2.50 MeV

15. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Fusion Enhancement in 19 O + 12 C  Above the barrier, the fusion cross-section for 19 O is roughly 20% larger than that for 18 O  Just above the barrier at ~9 MeV the fusion cross-section for 19 O increases dramatically as compared to 18 O.  At the lowest energy measured the cross- section for 19 O exceeds that for 18 O by approximately a factor of three.

16. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Conclusions Developed an efficient method to measure the fusion excitation function for low intensity radioactive beams at energies near and below the barrier. For 18 O + 12 C:  We have measured the fusion cross-section down to the 800  b level a factor of ~30 lower than previously measured.  In the sub-barrier regime the cross-section is substantially larger than that predicted by the DC-TDHF model suggesting a narrower barrier.  Alpha emission is substantially enhanced over the predictions of the statistical model codes. For 19 O + 12 C: First measurement of fusion in this system indicates a significant enhancement of fusion due to the presence of a single extra neutron as one approaches the barrier.

17. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Conclusions Developed an efficient method to measure the fusion excitation function for low intensity radioactive beams at energies near and below the barrier. For 18 O + 12 C:  We have measured the fusion cross-section down to the 800  b level a factor of ~30 lower than previously measured.  In the sub-barrier regime the cross-section is substantially larger than that predicted by the DC-TDHF model suggesting a narrower barrier.  Alpha emission is substantially enhanced over the predictions of the statistical model codes. For 19 O + 12 C: First measurement of fusion in this system indicates a significant enhancement of fusion due to the presence of a single extra neutron as one approaches the barrier. … just like a single valence neutron, a single individual can make a big difference!

18. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Grappling with Konrad Grappling hook (aka valence neutron) Clueless postdoc (RdS)

19. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Additional slides

20. Gridless MCP Detector  Minimize extraneous material in the beam path  Crossed electric and magnetic field transports electrons from secondary emission foil to the microchannel plate (MCP)  20 neodymium permanent magnets produce magnetic field (~85 gauss)  6 grid plates produce electric field (~101,000 V/m)  C foil frame biased to -1000 V  MCP with 18 mm diameter  Time resolution (MCP-MCP) ~ 350 ps Beam B E Bowman et al., Nucl. Inst. and Meth. 148, 503 (1978) Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) Romualdo deSouza, TAMU, Dec. 2, 2014

21. 18 O + 12 C Measurement at Florida State U. 21 Tracy K. Steinbach April 5, 2014  Time-of-flight of beam measured between US and Tgt gridless MCP detector  Elastically scattered beam particles and evaporation residues:  Time of flight measured between Tgt MCP and Si detectors  Energy measured in annular Si detectors (T2, T3) 18 O BEAM US MCP Detector Tgt MCP Detector T2 ~ 130 cm ~ 13 cm PMT T3 LCP Det. Array  7 CsI(Tl)/photodiode detectors used to measure light charged particles  PMT (coupled to plastic scintillator) measures zero degree beam particles

22. Si Detector  New design (S5) from Micron Semiconductor  Single crystal of n-type Si ~ 300 μm thick  Segmented to provide angular resolution  Used to give both energy and time information S5 (T2) Si Design Pies16 Rings 6 24 ring segments Inter-strip width50 μm Entrance widow thickness 0.1-0.2 μm www.micronsemiconductor.co.uk Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) deSouza et. al., Nucl. Inst. and Meth. A 632, 133 (2011)  Fast timing electronics gives timing resolution of ~ 450 ps (Need ~ 1 ns time resolution) Romualdo deSouza, ACS, Indianapolis 2013

23. Romualdo deSouza, CKG Symposium, Oct. 8, 2015 Is fusion of neutron-rich light nuclei enhanced relative to  -stable nuclei? R.T. deSouza, S. Hudan, V.E. Oberacker, S. Umar Phys. Rev. C88 014602 (2013) Density constrained TDHF calculations TDHF provides good foundation for describing large amplitude collective motion 3D cartesian lattice without symmetry restrictions Skyrme effective nucleon-nucleon interaction (SLy4) Ion-ion potential calculated as: V(R) = E DC (R) – E A1 – E A2 Romualdo deSouza, TAMU, Dec. 2, 2014

24. Romualdo deSouza, CKG Symposium, Oct. 8, 2015

25. Comparison of  emission in similar systems