Justin Vadas, Tracy Steinbach, Jon Schmidt, Varinderjit...

Justin Vadas, Tracy Steinbach, Jon Schmidt, Varinderjit Singh, Sylvie Hudan, Romualdo de Souza, Lagy Baby, Sean Kuvin, Ingo Wiedenhover

April 14, 2015

Studying the structure and reactions of neutron-rich nuclei is of particular interest with the use of modern radioactive beam facilities

Reactions such as near-barrier fusion produce nuclei at relatively low excitation energy ( E* ~ 10-40 MeV)

Limited data exists for the de-excitation of light compound nuclei formed at low excitation energy

Justin Vadas

Elab = 16.25 – 36 MeV

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

April 14, 2015

US MCP Tgt MCP

T3T2

~130 cm ~16 cm

18O beam

𝐸∗ = 𝐸𝑐.𝑚. + 𝑄 𝑄 = 23.649 𝑀𝑒𝑉

2Justin Vadas

T.K. Steinbach et al., PRC 90, 041603(R) (2014)

While the statistical model calculations describe the small angle component relatively well, they underpredict the large angle component

April 14, 2015

Recoil considerations suggest that the low energy and large angle components of these distributions could be residues resulting from α emission

3Justin Vadas

<E> of the low energy component of the total distribution represented here as dashed red line

Coincident measurement of evaporation residues and α particles demonstrates that the low energy component is associated with α channels

April 14, 20154Justin Vadas

𝑇 ∝𝐸∗

𝑎𝑎 = Level density parameter

< 𝐸𝑐.𝑚. 𝛼 > = 𝑉𝐵𝑎𝑟𝑟𝑖𝑒𝑟 + 2𝑇

The peaks of the distributions shift lower as incident energy decreases

Justin Vadas 5 April 14, 2015


𝑇 ∝𝐸∗

𝑎𝑎 = Level density parameter

< 𝐸𝑐.𝑚. 𝛼 > = 𝑉𝐵𝑎𝑟𝑟𝑖𝑒𝑟 + 2𝑇

The peaks of the distributions shift lower as incident energy decreases

Experimental α energy distribution reasonably described by evapOR statistical model calculation


<Eα> 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

<Ec.m.(α)> = VBarrier + 2T

Lower <Ec.m.(α)> suggests that the αemission barrier is lower than predicted

As E* increases, α emission becomes an increasingly important channel in the de-excitation process

The higher measured σα as compared to the statistical model calculations indicates a lower α emission barrier


SummaryMeasured α energy distributions and σα provide insight into the decay of 30Si* at low excitation energy

Lower measured <Ec.m.(α)> and higher σα suggest a lower barrier for αemission as compared to statistical model calculations

Future PlansDetermine the barrier for α emission

Extract the mass distributions of the residues

Investigate the decay of 31Si* produced by 19O + 12C


Indiana University

Romualdo de Souza, Sylvie Hudan, Tracy Steinbach, Jon Schmidt, Varinderjit Singh

Florida State University

Lagy Baby, Ingo Wiedenhover, Sean Kuvin

DOE under Grant No. DE-FG02-88ER-40404


Accreting neutron star crusts provide a unique environment for nuclear reactions to occur

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

April 14, 2015

Atmosphere: Accreted H/He

Ocean: Carbon + Heavy Elements

Heavy Elements

Crust

Density

~105 g/cm3

~109 g/cm3

~1010 g/cm3

Depth

5 m

30 m

100 m

Outer Crust of an Accreting Neutron Star

11Justin Vadas

Single crystal Si(IP) detectors

Subtend 4.5° ≤ θlab ≤ 21°

Segmented to provide angular resolution and reduce detector capacitance

Timing resolution ~450 ps


Beam

B

EE 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

6 mm thick CsI(Tl) crystals

0.5” Plexiglas light guides

2 cm x 2 cm Hamamatsu photodiodes

7 detectors subtend 36.3° ≤ θlab ≤ 62.0°

Two component decay


CSA

Shaping Amp

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