Precision Measurement of Isospin DiffusionJ. R. Winkelbauer, for the HiRA Collaboration112Sn

‘B’

124Sn

‘A’

124Sn

‘A’

n

112Sn

‘B’

112Sn

‘B’

124Sn

‘A’

p

Symmetry Energy in the Nuclear Equation of State

NSCL Experiment 07038

Probing the Symmetry Energy through Isospin Diffusion

112,118,124Sn+ 112,118,124Sn Collisions at 70 MeV/u

Isospin Diffusion: Protons “diffuse” from a proton-rich nucleus to a proton-deficient nucleus in intermediate energy collisions.

- Measures light fragment (Z<6) distributions with the Large Area Silicon Strip Array (LASSA)
- Measures heavy fragment (Z=30-45) distributions using the S800 Spectrometer
- Measures total charged particle multiplicity using the Miniball-Miniwall array (extract centrality)

LASSA

- Isospin diffusion occurs only in asymmetric systems A+B
- No isospin diffusion between symmetric systems A+A, B+B
- Non-isospin effects:
- -Same for A in A+B and A+A
- -Same for B in A+B and B+B

The Symmetry Energy is the difference between energies of pure neutron matter and symmetric nuclear matter

“stiff”

Ri = 1

Ri = -1

NIM A473 3 302-318 (2001)

“soft”

Isospin Transport Ratio Ri: Attempts to remove “non-isospin” effects from an observable X

Figure: Betty Tsang

Miniball

S800 Spectrometer

NIM B204 629-633 (2003)

Different forms for the symmetry energy result in a different amount of isospin equilibration (left). Isospin Diffusion using light fragment yields has been used to constrain the symmetry energy.

Statistics (Millions of Events)

NIM A295 109 (1990)

Symmetry Energy in Nuclear Astrophysics

- Affects Neutron Stars
- Masses and Radii
- Composition
- Cooling Rates

BUU

- Affects Supernovae
- Densities from ~1 ρ0 -10ρ0
- δ from ~.2 to ~.8
- Important for Nucleosynthesis

imQMD

Tsang, Phys. Rev. Lett. 92, 062701 (2004)

Z=28

Models predict that measuring isospin diffusion using heavy residue yields provides a different sensitivity to the symmetry energy (right).

N-Z=3

Data analysis is in progress

http://en.wikipedia.org/wiki/Neutron_star

Zhang, PRC 85, 024602 (2012)