Accreting Neutron Stars

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1 Tracy K. Steinbach Indiana University Accreting Neutron Stars ² The outer crust of an accreting neutron star is an unique environment for nuclear reactions ² Identified as the origin of energetic X-ray superbursts (~ 1042 ergs per burst) ² X-ray superbursts thought to be fueled by 12C + 12C fusion in the outer crust ² Temperature of the outer crust is too low (~ 3x106 K) for 12C fusion Tracy K. Steinbach 1 Horowitz et al., Phys. Rev. C 77, (2008) Haensel et al., Neutron Stars 1, 2007 Cumming et al., Astrophys. J. Lett. 559, L127 (2001) Strohmayer et al., Astrophys. J. 566, 1045 (2002)

2 Fusion of Neutron-rich Light Nuclei ² One potential heat source, proposed to heat the crust of neutron stars and allow 12 C fusion, is the fusion of neutron-rich light nuclei (ex. 24 O + 24 O) 24 O + 24 O Fusion: ² If valence neutrons are loosely coupled to the core, then polarization can result and fusion enhancement will occur 16 O Core valence neutrons 16 O Core ² 24 O is currently inaccessible for reaction studies ² Instead study other neutron rich isotopes of oxygen (18,19,20) O on 12 C ( 19,20 O are radioactive) Horowitz et al., Phys. Rev. C 77, (2008) Umar et al., Phys. Rev. C 85, (2012) Tracy K. Steinbach 2

3 Density Constrained Time Dependent Hartree Fock Calculations ² State of the art theoretical calculations predict fusion dynamics for neutron-rich light systems ² Experimental measurements of the fusion cross-section provides a test of fusion models Umar et. al. Phys. Rev. C 74, (2006) desouza et al., Phys. Rev. C 88, (2013) Cross Section (mb) DC-TDHF 16 O + 18 O + 20 O + 12 C 12 C 12 C E CM (MeV) Tracy K. Steinbach 3

4 What do we want to measure? Beam Target Evap. Residue 18 O+ 12 C 30 Si (E 35 MeV) ² Excited nucleus decays: 30 Si 28 Si + 2n Evaporation residues 28 Al + p + n 25 Mg + α +n Evaporated particles ² To measure the fusion cross-section we need to count the number of evaporation residues relative to the number of incident O nuclei ² Emission of evaporated particles kicks evaporation residues off of zero degrees Tracy K. Steinbach 4

5 Method for Identifying Evaporation Residues Beam Start time Residue ² To distinguish fusion residues from beam particles, one needs to measure: v Energy of the particle v Time of flight of the particle ² 18 O beam was provided by the Tandem van de Graaff accelerator at Florida State University (Feb. 2014) ² 18 E lab = MeV I Beam ~ 1-4.5x10 5 p/s Stop time Energy E = 1 2 mv2 m Et 2 Wiedenhover et al., (5 th Int. Conf. on Fission & Prop. of Neutron-rich Nuclei, 2012) Tracy K. Steinbach 5

6 18O + 12C Measurement at Florida State U. ~ 130 cm 18O US MCP Detector ~ 13 cm BEAM Tgt MCP T2 LCP Det. Detector Array T3 PMT ² Time of flight of beam measured between US and Tgt gridless MCP detector ² Elastically scattered beam particles and evaporation residues: v Time of flight measured between Tgt MCP and Si detectors v Energy measured in annular Si detectors (T2, T3) ² 7 CsI(Tl)/photodiode detectors used to measure light charged particles ² PMT (coupled to plastic scintillator) measures zero degree beam particles Tracy K. Steinbach 6

7 Gridless MCP Detector ² Minimize extraneous material in the beam path ² Crossed electric and magnetic field transports electrons from secondary emission foil to the microchannel plate (MCP) ² 20 neodymium permanent magnets produce magnetic field (~85 gauss) B ² 6 grid plates produce electric field (~101,000 V/m) E ² C foil frame biased to V Beam ² MCP with 18 mm diameter Bowman et al., Nucl. Inst. and Meth. 148, 503 (1978) Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) Tracy K. Steinbach 7

8 Si Detector ² New design (S5) from Micron Semiconductor Ltd. ² Single crystal of n-type Si ~ 300 µm thick ² Segmented to provide angular resolution ² Used to give both energy and time information S5 (T2) Si Design Pies 16 Rings 6 24 ring segments Inter-strip width 50 µm Entrance widow thickness µm ² Fast timing electronics gives timing resolution of ~ 450 ps (Need ~ 1 ns time resolution) Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) desouza et. al., Nucl. Inst. and Meth. A 632, 133 (2011) Tracy K. Steinbach 8

9 Identifying Evaporation Residues 18 O + 12 E Lab = 35 MeV Elastic scattering peak Fusion residues Slit Scattering PRELIMINARY Tracy K. Steinbach 9

10 Identifying Evaporation Residues 18 O + 12 E Lab = 35 MeV Elastic scattering peak 20 O + 12 E Lab = 41 GANIL (France) Fusion residues Atomic scattering Slit Scattering PRELIMINARY Rudolph et al., Phys. Rev. C 85, (2012) Rudolph, Master s Thesis, IU, 2012 Tracy K. Steinbach 9

11 18 O + 12 C Fusion Excitation Function ² Measured the cross section for E CM ~ 6 14 MeV ² Capable of measuring to approx. the 10 mb level ² Next measurement: 19 O + 12 FSU Tracy K. Steinbach 10

12 Acknowledgements ² Indiana University Nuclear Chemistry: R.T. desouza, S. Hudan, M. Rudolph, Z. Gosser, J. Vadas, C. Haycraft, K. Brown, J. Schmidt, A. Liao ² Florida State University: I. Wiedenhover, L. Baby, J. Baker, J. Baron, J. Belarge, R. Dungan, S. Kuvin, D. McPherson, J. Parker, N. Rijal, P.L. Tai, K. Villafana, J. VonMoss ² Indiana University Department of Chemistry: Mechanical Instrument Services and Electronic Instrument Services ² DOE under Grant No. DEFG02-88ER Tracy K. Steinbach 11

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14 Si Detector Design ² Reduced segmentation and inter-strip width ² Charge trapping reduced from a 20-30% effect to a 1-2% effect ² Reduced entrance window thickness ² Beneficial for low energy heavy residues New Si Design Old Si Design Pies Rings 6 24 ring segments Inter-strip width 50 µm 100 µm Entrance widow thickness µm ~ 0.7 µm Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) Tracy K. Steinbach 13