This work was supported by the U.S. DOE Office of Science u - PowerPoint Presentation

Slide1

This work was supported by the U.S. DOE Office of Science under Grant No. DEFG02-88ER-40404

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016

Indiana

University:

T. Steinbach

, J. Vadas, V. Singh, B. Wiggins, J. Huston, S. Hudan, RdSFlorida State: I. Wiedenhover, L. Baby, S. Kuvin, V. TripathiVanderbilt University: S. Umar, V. Oberacker

Forging elements in a flash

Only elements Z=1-4 produced in the Big Bang



Fundamentals of supernova explosions are not understood!

 Synthesis of the heavy elements is not understood Limits of nuclear stability (superheavy elements, N/Z exotic) poorly known

Chemical diversitySlide2

The Chart of the Nuclides and Terra IncognitaRadioactive beam facilities allow one to produce neutron-rich nuclei along the r-process path. Neutron stars represent an extreme point on this diagram

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide3

The two nuclei have to overcome the Coulomb repulsion (barrier) in order to experience the attraction of the strong nuclear force which is short range.

Overcoming barriers

Particle can tunnel (exist in classically forbidden region) and emerge on other side. SENSITIVE TO WIDTH OF BARRIER

Effect of barrier is observed even if E > UMeasuring the fusion cross-section is therefore intrinsically related to measuring the detailed shape of the fusion barrier (a dynamic quantity as the nuclei approach)

(Z

1,A1)(Z2,A2)

(Z1+Z2,A1+A2)

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide4

Fusion reactions in the outer crust

are responsible for the

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.

X-ray bursters and SuperburstersRossi Explorer satellite1995-2012

Energy output of a single burst equal our sun’s solar output for a decade!Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide5

The crust of an accreting neutron star is a unique environment for nuclear reactions

Atmosphere:

Accreted H/He

Ocean:

heavy elements

CrustCarbon + heavy elementsDensitydepth~105 g/cm3

5 m~109 g/cm330 m~1010 g/cm3100 mOuter crust of an accreting neutron starStructure of an accreting neutron star crust

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide6

Fusion of Neutron-rich Light Nuclei One potential heat source, proposed (Horowitz et al.) to heat the crust of neutron stars and allow 12C fusion, is the fusion of neutron-rich light nuclei (ex. 24O + 24O) -- More recently mid-mass nuclei have been suggested.

24O +

24

O Fusion:If valence neutrons are loosely coupled to the core, then polarization can result and fusion enhancement will occur24O is currently inaccessible for reaction studiesInstead study other neutron rich isotopes of

oxygen (18,19,20, 21)O on

12C (19,20O are radioactive)

16O Core16O Corevalence neutronsHorowitz et al., Phys. Rev. C 77, 045807 (2008)Umar et al., Phys. Rev. C 85, 055801 (2012)

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide7

R.T.

deSouza, S.

Hudan

, V.E. Oberacker, S. Umar Phys. Rev. C88 014602 (2013)Density constrained TDHF calculations

Damped dipole oscillation and presence of surface waves clearly visible.

Fusion events and deeply inelastic events dominate at these near barrier energiesFusion is distinguished from deeply inelastic collisions by the existence of a single heavy nucleus after the collisionQuantum mechanical calculations can also be performed to investigate the fusion process

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide8

Nuclear astrophysics: Nuclear reactions in outer crust of a neutron star Nuclear physics: dynamics of fusing two neutron-rich nuclei

Motivation

24

O + 24O not possibleMeasure fusion in 16,18,19,20O + 12

C

19O and 20O are radioactive beams!Challenge: Radioactive beams are/will be available at intensities of ~103 – 105 ions/sec– a million times less intensity than previously used in fusion studies.

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide9

When comparing 18O + 12C to 16O + 12

C DC-TDHF predicts a larger increase as compared to the experimental data.

What happens (in reality) to the fusion cross-section as the oxygen nucleus becomes increasingly neutron-rich?

Systematic fusion data to address this question is necessary!

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide10

What do we want to measure?Excited nucleus decays:To measure the fusion cross-section we need to count the number of evaporation residues relative to the number of incident O nucleiEmission of evaporated particles kicks evaporation residues off of zero degrees

Target

Beam

Evap. Residue

Evaporation residues

Evaporated particles

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide11

A “simple” counting experimentMeasure the number of beam particles by counting them individuallyCount the number of residues

Reciprocal of target thickness

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide12

Method for Identifying Evaporation ResiduesStop timeEnergyStart timeBeam

Residue

To distinguish fusion residues from beam particles, one needs to measure:

Energy of the particle Time of flight of the particle

18O beam was provided by the Tandem van de

Graaf accelerator at Florida State University (Feb. 2014)18O @ Elab = 16 – 36 MeV IBeam ~ 1 - 4.5x105 p/sWiedenhover et al., (5

th Int. Conf. on Fission & Prop. of Neutron-rich Nuclei, 2012)www.physics.fsu.edu/Nuclear/Brochures/SuperconductingLinearAcceleratorLaboratory/default.htmRomualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide13

Gridless MCP DetectorMinimize extraneous material in the beam pathCrossed 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 diameterTime resolution (MCP-MCP) ~ 350 ps

Beam

BEBowman et al., Nucl. Inst. and Meth. 148, 503 (1978)

Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014)Slide14

Si DetectorNew design (S5) from Micron Semiconductor Single crystal of n-type Si ~ 300 μm thick Detector acts as a reverse biased diodeSegmented to provide angular resolutionUsed to give both energy and time information

S5 (T2)

Si DesignPies16Rings624 ring segmentsInter-strip width50 μm

Entrance widow thickness

0.1-0.2 μmwww.micronsemiconductor.co.ukSteinbach 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, Univ. of Kentucky, Feb. 4, 2016Slide15

18O + 12C Measurement at Florida State U.Time-of-flight of beam measured between US and Tgt gridless MCP detectorElastically scattered beam particles and evaporation residues: Time of flight measured between Tgt

MCP and Si detectors

Energy measured

in annular Si detectors (T2, T3)18O BEAM

US MCP

DetectorTgt MCPDetectorT2~ 130 cm~ 13 cm

PMTT3LCP Det.Array7 CsI(Tl)/photodiode detectors used to measure light charged particlesPMT (coupled to plastic scintillator) measures zero degree beam particles

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide16

Identifying Evaporation Residues18O + 12C @ ELab = 36 MeVRudolph et al., Phys. Rev. C 85, 024605 (2012)Rudolph, Master’s Thesis, IU, 2012

Evaporation residues are clearly identified from Elastic scattering and beam scatter particles

Alpha particles are also clearly distinguished

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide17

18

O +

12

C Fusion Excitation FunctionMeasured the cross section for ECM ~ 5.3 – 14 MeV matches existing data

Extends cross-section measurement down to ~800 µb level (~30 times lower than previously measured)

Fit experimental data with penetration of an inverted parabolic barrier (Wong formalism)Slide18

18O + 12C : Comparison with DC-TDHFEven with inclusion of pairing the theoretical excitation function has the wrong SHAPE!In the sub-barrier region DC-TDHF significantly under-predicts the cross-section

Above the barrier the

quantity

expt./DC-TDHF is roughly constant at 0.8Below the barrier the quantity expt./DC-TDHF increases from 0.8 to approximately

15.

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide19

What can we learn from the emitted particles (protons, alphas, and neutrons)?The velocity (energy) distribution of the particles is described by a Maxwell-Boltzmann distributionIf the gas is in equilibrium with the liquid then measuring the velocity (energy) distribution of gas particle teaches us about the temperature of the liquidAs the nucleus can be viewed as a charged droplet, measuring the energies of the emitted particles can provide information on the temperature of the nuclear system.

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide20

Yield of α particles decreases as incident energy decreases.Distributions can be characterized by a first and second moment (average value and width).

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide21

<E(α)> increases roughly linearly with increasing bombarding energy.(α) first increases linearly then appears to saturate with increasing bombarding energy.A statistical model (PACE4) does a reasonable job of describing the widths but under-predicts the average energy of the α particles.

Turning to the average energy and width of the distribution…

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide22

The yield of  particles…The experimental α cross-section decreases as the bombarding energy decreasesAt Ec.m

. = 14 MeV,



α is close to fusionAs the bombarding energy decreases, alpha emission becomes a smaller and smaller fraction of the fusion cross-section.A statistical model, evapOR, that describes the de-excitation of excited nuclei does a poor job of describing the dependence of α on bombarding energy.

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide23

Energy and angular distribution of ERs in

18

O +

12CEnergy 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.

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide24

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+pIn 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

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide25

19

O produced by:

18

O(d,p) @ ~68 MeVProduction target of D2 gas 300 torr at 77K19O filtered from beam using RESOLUT facilityIntensity of 19O: 1-2x104 pps

Beam tagging by E-TOF

Target: 100 µg/cm2 carbon foil T2: θLab = 3.5 - 10.8°; T3: θLab = 11.3 - 21.8°TOF between target-MCP and Si (T2, T3)Radioactive beams at Florida State : The RESOLUT facilityIon chamber

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide26

Radioactive beams at Florida State : The RESOLUT facility18O7+19

O7+

18

O6+19O clearly resolved from 18O over 3m flight path by TOFSimultaneous measurement of 19O + 12C and 18O + 12C

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide27

Identification of ERs for

19

O +

12C

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide28

Fusion enhancement for

19

O +

12CThe excitation function for 18O matches the results of the high resolution measurement previously performed within the statistical uncertainties.At all energies 19O + 12C is associated with a significantly large cross-section.

Wong formalism (parabolic barrier)

18O19ORc7.34 ± 0.07 fm8.10 ± 0.40 fmV

7.62 ± 0.04 MeV7.73 ± 0.61 MeVh/2 2.86 ± 0.09 MeV 6.38 ± 2.50 MeV Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide29

Fusion enhancement for

19

O +

12CAbove the barrier, the fusion cross-section for 19O is roughly 20% larger than that for 18OJust above the barrier at ~9 MeV the fusion cross-section for 19O increases dramatically as compared to 18O.

At the lowest energy measured the cross-section for 19O exceeds that for

18O by approximately a factor of three.

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide30

Conclusions

Developed an efficient method to measure the fusion excitation function for low intensity radioactive beams at energies near and below the barrier.

For

18O + 12C: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 19O +

12C: 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.

Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016Slide31

Tracy Steinbach, Jon Schmidt, and Dr. Sylvie HudanAt Florida State UniversityJustin Vadas

Dr. V. SinghBlake

Wiggins

http://nuchem.iucf.Indiana.edu

Romualdo

deSouza, Univ. of Kentucky, Feb. 4, 2016Slide32

Additional slidesSlide33

Is fusion of neutron-rich light nuclei enhanced relative to

-stable nuclei?

Density constrained TDHF calculations

TDHF provides good foundation for describing large amplitude collective motion3D cartesian lattice without symmetry restrictionsSkyrme effective nucleon-nucleon interaction (SLy4)Ion-ion potential calculated as:V(R) = E

DC (R) – EA1 – EA2Slide34
Slide35

Comparison of

 emission in similar systemsSlide36

100 feetSPIRAL@GANIL

Good cheese, cider, and an

a

ccelerator facilitywith excellent technical supportISOL techniquePrimary beam: ArProduction target: C(graphite)CIME reacceleration: ~10 MHz20O2+ t1/2 : 13 s