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Heat Capacities of 56 Fe and 57 Fe

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Heat Capacities of 56Fe and 57Fe

Emel Algin

Eskisehir Osmangazi University

Workshop on Level Density and Gamma Strength in Continuum

May 21-24, 2007

- Apply Oslo method to lighter mass region
- SMMC calculations predict pairing phase transition
- Astrophysical interest

- 28 NaI(Tl) detectors
- 2 Ge(HP) detectors
- 8 Si(Li) ∆E-E particle detectors (thicknesses: 140μm and 3000 μm)
at 45° with respect to the beam direction

- 45 MeV 3He beam
- ~95% enriched, 3.38mg/cm2, self supporting 57Fe target
- Relevant reactions:
57Fe(3He,αγ) 56Fe

57Fe(3He, 3He’γ) 57Fe

- Measured γ rays in coincidence with particles
- Measured γ rays in singles

- Particle energy → initial excitation energy (from known Q value and reaction kinematics)
- Particle-γ coincidences → Ex vs. Eγ matrix
- Unfolding γ spectra with NaI detector response function
- Obtained primary γ spectra by squential subtraction method → P(Ex, Eγ) matrix

→ Radiative Strength Function

Least method → ρ(E) and T(Eγ)

Transformation through equations:

Common procedure for normalization:

- Low-lying discrete states
- Neutron resonance spacings
- Average total radiative widths of neutron resonances

●LD obtained from Oslo method

O LD obtained from

55Mn(d,n)56Fe reaction

discrete levels

BSFG LD with von Egidy and

Bucurescu parameterization

Normalization:

● LD obtained from SMMC

◊ LD obtained from Oslo method

* Discrete level counting

--- LD of Lu et al. (Nucl. Phys. 190,

229 (1972).

●LD obtained from Oslo method

discrete levels

BSFG LD with von Egidy and

Bucurescu parameterization

data point obtained from

58Fe(3He,α)57Fe reaction

(A. Voinov, private communication)

Normalization:

Isotope a(MeV-1) E1(MeV) σηρ(MeV-1) at Bn

56Fe 6.196 0.942 4.049 0.64 2700±600

57Fe 6.581 -0.523 3.834 0.38 610±130

BSFG is used for the extrapolation of the level density

in order to extract the thermodynamic quantities.

In microcanonical ensemble entropy S is given by

→ multiplicity of accessible states at a given E

One drawback:

We have level density not state density

Spin distribution usually assumed to be Gaussian

with a mean of

σ: spin cut-off parameter

In the case of an energy independent spin

distribution, two entropies are equal besides an

additive constant.

Here we define “pseudo” entropy based on

level density:

Third law of thermodynamics:

Entropy of even-even nuclei at ground state

energies becomes zero:

ρo=1 MeV-1

Strong increase in entropy at

Ex=2.8 MeV for 56Fe

Ex=1.8 MeV for 57Fe

Breaking of first Cooper pair

Linear entropies at high Ex

Slope: dS/dE=1/T

Constant T least-square fit gives

T=1.5 MeV for 56Fe

T=1.2 MeV for 57Fe

Critical T for pair breaking

Entropy excess ∆S=S(57Fe)-S(56Fe)

Relatively constant ∆S above Ex~ 4 MeV: ∆S=0.82 kB.

In canonical ensemble

where

- - - - 56Fe

57Fe

n: # of thermal particles

not coupled in Cooper

pairs

Typical energy cost for creating a quasiparticle is

-∆ which is equal to the chemical potential:

at T=Tc

Tc= 1 – 1.6 MeV

The probability that a system at fixed

temperature has an excitation energy E

where Z(T) is canonical partition function:

Recall critical temperatures:

T=1.5 MeV for 56Fe

T=1.2 MeV for 57Fe

- A unique technique to extract both ρ(E) and fXL experimentally
- Extend ρ(E) data above Ex=3 MeV (where tabulated levels are incomplete)
- Step structures in ρ(E) indicate breaking of nucleon Cooper pairs
- Experimental ρ(E) → thermodynamical properties
- Entropy carried by valence neutron particle in 57Fe is ∆S=0.82kB.
- Several termodynamical quantities can be studied in canonical ensemble
- S shape of the heat capacities is a fingerprint for pairing transition
- More to come from comparison of experimental and SMMC heat capacities

U. Agvaanluvsan, Y. Alhassid,

M. Guttormsen, G.E. Mitchell,

J. Rekstad, A. Schiller, S. Siem,

A. Voinov

Thank you for listening…