Accreting neutron stars provide a unique environment for nuclear reactions

Size: px

Start display at page:

Download "Accreting neutron stars provide a unique environment for nuclear reactions"

Beverly Shaw
5 years ago
Views:

, Tracy Steinbach, Jon Schmidt, Varinderjit Singh, Sylvie Hudan, Romualdo de Souza, Lagy Baby, Sean Kuvin, Ingo Wiedenhover Accreting neutron stars provide a unique environment for nuclear reactions

1 , Tracy Steinbach, Jon Schmidt, Varinderjit Singh, Sylvie Hudan, Romualdo de Souza, Lagy Baby, Sean Kuvin, Ingo Wiedenhover Accreting neutron stars provide a unique environment for nuclear reactions High density (~10 14 g/cm 3 ), relatively low temperatures (~10 6 K), large neutron abundance (90% of the mass) Identified as the origin of X-ray superbursts Releases more energy in a few hours than our sun does in a decade X-ray superbursts thought to be fueled by 12 C+ 12 C fusion in the outer crust Temperature of the outer crust is too low (~ K) relative to the Coulomb barrier for 12 C fusion A potential heat source is the fusion of neutron-rich light nuclei (eg. 24 O + 24 O)

Density Constrained Time-Dependent Hartree-Fock calculations Fully microscopic many-body theory No free parameters Only input is the nuclear effective interaction 20 O + 12 C DC-TDHF

2 Density Constrained Time-Dependent Hartree-Fock calculations Fully microscopic many-body theory No free parameters Only input is the nuclear effective interaction 20 O + 12 C DC-TDHF calculations predict a fusion enhancement for neutron-rich systems Experimental measurements of the fusion cross-section provide a test of the fusion models Umar et al., Phys. Rev. C 85, (2012) 2

3 18 OO + 12 CC 30 SSSS 28 SSSS + 2nn 28 AAAA + pp + nn 25 MMMM + αα + nn Evaporation residues Excited compound nucleus decays by emitting neutrons, protons, and α particles EE = EE cc.mm. + QQ QQ = MMMMMM Evaporated particles The resulting heavy nucleus is known as an evaporation residue Beam Residue Stop Time Energy Emission of these light particles impart transverse momentum on the residue, kicking them off zero degrees and allowing for direct measurement of the residues and light particles Start Time Light Particle EE = 1 2 mmmm2 mm EEEE 2 3

4 ~130 cm ~16 cm 18 O beam US MCP Tgt MCP T3 T2 E lab = MeV Intensity of ~ pps Beam count measured between US and Tgt MCP detectors Energy measured in segmented annular silicon detectors (T2,T3) Fusion product time-of-flight measured between target MCP and Si detectors Reaction products distinguished by energy and time-of-flight 4

5 Beam E B E x B fields transport electrons from foil to MCP E field produced by biasing array of ring plates B field produced by NdFeB permanent magnets Timing resolution ~300 ps Single crystal Si(IP) detectors Subtend 4.5 θ lab 21 Segmented to provide angular resolution and reduce detector capacitance Timing resolution ~450 ps 5

6 Crosssection σσ = NN IIII Number of residues Beam count Target thickness T.K. Steinbach et al., PRC 90, (R) (2014) 6 Intense peak corresponds to elastically scattered beam particles Points in the band originating from this peak are slit scattered beam particles Evaporation residues are found in the island with longer TOF values than the beam scatter line α particles correspond to the band with very short TOF values

7 Data fit to functional form that describes the penetration of an inverted parabolic barrier Comparison with TDHF reveals a different shape, emphasized by the ratio between the experimental and theoretical excitation functions Increase in the ratio can be interpreted as a larger tunneling probability, which can be associated with a narrower barrier Deviation increases dramatically at energies below 7 MeV, highlighting the importance of subbarrier measurements 7

8 σ Measured the fusion cross-section down to the ~650 µb level New data consistent with previous measurements Deviation from DC-TDHF predictions continues to increase as E cm decreases 8

9 While the statistical model calculations describe the small angle component relatively well, they underpredict the large angle component 9 The alpha angular distribution has a negative slope because of the forward momentum in the lab frame

10 Recoil considerations suggest that the low energy and large angle components of the residue distributions could be residues resulting from α emission 10 <E> of the low energy component of the total distribution represented here as dashed line Coincident measurement of evaporation residues and α particles demonstrates that the low energy component is associated with α channels

11 TT EE aa aa = Level density parameter < EE cc.mm. αα > = VV BBBBBBBBBBBBBB + 2TT The peaks of the distributions shift lower as incident energy decreases 11

12 TT EE aa aa = Level density parameter < EE cc.mm. αα > = VV BBBBBBBBBBBBBB + 2TT The peaks of the distributions shift lower as incident energy decreases Experimental α energy distribution reasonably described by evapor statistical model calculation 11

13 <E α > increases linearly with increasing E* PACE4 band shown for a = A/12 A/8 evapor shown with a = A/8 Energy distribution widths sensitive to T Since the widths represented by the models are approximately the same as the measured widths, T is approximately the same <E c.m. (α)> = V Barrier + 2T α emission is reasonably described by statistical decay 12

14 As E* increases, α emission becomes an increasingly important channel in the de-excitation process With increasing E c.m., the measured σ α increasingly deviates from statistical model predictions 13

15 As E* increases, α emission becomes an increasingly important channel in the de-excitation process With increasing E c.m., the measured σ α increasingly deviates from statistical model predictions Similar systems also exhibit the same features of σ α The n + 28 Si 29 Si* system suggests this enhancement is an entrance channel effect 13

16 Measured the fusion cross-section of 18 O + 12 C down to the submillibarn level Different shape of the experimental excitation function compared to DC-TDHF calculations suggests a different barrier The increasing deviation from DC-TDHF predictions with decreasing incident energy highlights the importance of measuring the cross section below the barrier A large σ α with energy and angular distributions described by statistical decay, along with an increase in the σ α /σ fusion with increasing E c.m. could signal the survival of the α cluster structure of the projectile/target through the fusion process 14

17 Extract mass distributions of the residues Analyze data from the 19 O + 12 C 31 Si* experiment currently being conducted Prepare to measure the fusion cross-section and de-excitation of 39,47 K + 28 Si 67,75 As* using NSCL s ReA3 reaccelerator facility Enrich an oxide target to measure the fusion cross-section and de-excitation of 16,17,18 O + 18 O 30,31,32 S* 15

18 Indiana University Romualdo de Souza, Sylvie Hudan, Tracy Steinbach, Jon Schmidt, Varinderjit Singh Florida State University Lagy Baby, Ingo Wiedenhover DOE under Grant No. DE-FG02-88ER

19 17

As the neutron star accretes material from its companion star, H and He burning occurs The ashes are then pushed further down as the neutron star accretes more material Density increases, allowing

20 As the neutron star accretes material from its companion star, H and He burning occurs The ashes are then pushed further down as the neutron star accretes more material Density increases, allowing further reactions such as neutron capture, electron capture, and fusion, creating a mass-stratified outer crust Statistical decay of compound nuclei formed through fusion in the outer crust increases He concentration in mid-mass regions, potentially enabling asymmetric reactions Outer Crust of an Accreting Neutron Star Atmosphere: Accreted H/He Ocean: Carbon + Heavy Elements Heavy Elements Crust Density Depth ~10 5 g/cm 3 5 m ~10 9 g/cm 3 30 m ~10 10 g/cm m 18

At low excitation, the compound nucleus de-excites by statistical emission of light particles (n,p,α)

At low excitation, the compound nucleus de-excites by statistical emission of light particles (n,p,α) Does the α cluster structure in light nuclei persist through the fusion process? Justin Vadas, T.K. Steinbach, J. Schmidt, V. Singh, S. Hudan, R.T. de Souza; Indiana University L. Baby, S. Kuvin, I. Wiedenhover;