Post on 07-Mar-2018
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
April 14, 20155Justin Vadas
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
Justin Vadas 6 April 14, 2015
<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
April 14, 20157Justin Vadas
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
April 14, 20158Justin Vadas
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
Justin Vadas 9 April 14, 2015
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
Justin Vadas 12 April 14, 2015
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