1. Nuclear photonics: Learning from the nuclear response to real photons
γ + (N,Z)
Giornate di Studio su IRIDE
March 14th – 15th, 2013
LNF
G. Colò

2. ELI – NP Experiments
Measuring detailed doorway states by means of (γ,p), (γ,n) … reactions
Test of chaos in nuclei (spectra fluctuations vs. random matrix theory)
Fine structure of photo-response above particle threshold: (γ,α), (γ,p) and (γ,n)
Nuclear resonance fluorescence experiments on rare isotopes and isomers …
Management of radioactive waste and isotope-specific identification
Medical applications: producing new medical radioisotopes via (γ,n)
New brilliant neutron source produced via (γ,n)
From: NuPECC meeting, Milano, March 7th-8th, 2012 (A. Bracco)
Cf. also ELI-NP Workshop, Milano, May 14th-16th, 2012 (L. Serafini, A. Bracco)

3. Outline (mainly physics cases)
Introduction: nuclear resonance excitation by photons and typical experiment(s)
Case 1 : Symmetry energy and its impact on astrophysics
Case 2 : Is the nucleus (an)harmonic ?
Other cases: nuclear scales, details of nuclear w.f. …
Simulations by Milano group  numbers !

4. Elastic photon scattering
These amplitudes are forward peaked:
AR : Rayleigh scattering (excitation and decay of bound electrons).
AD : Delbrück scattering (e+e- pair creation and annihilation).
The other two have a specific angulat dependence (1 + cos2θ or …):
ANT : The nucleus acts as structureless charged particle and performs oscillations as a whole. It is nearly E-independent.
ANR : goes through the excitation of a nuclear (e.g. dipole) resonance and we assume it can be distinguished using the above arguments of angle and energy dependence.
CONCLUSION : RESONANCE CONTRIBUTION CAN BE SINGLED OUT.
S. Kahane and R. Moreh, PRC 9, 2384 (1974)

5. A typical experiment
Below separation energy: discrete levels with large branching ratio B = Γ0/Γ to the g.s.
Above separation energy: resonances with small gamma-decay branch Γγ << Γn .

6. Region ≈10-30 MeV: Giant Resonances
Isovector probes excite in the nucleus vibrational modes in which neutrons and protons oscillate in opposition of phase.
GDR
Nuclear excitation suffers from uncertainties due to the incomplete knowledge of the effective nucleon-nucleon (NN) interaction, and its energy dependence.
Moreover, not always a good energy resolution can be achieved with nuclear probes.
Wavelength >> nuclear dimension

7. Classification / Motivation to study
IVGDR
IVGMR
IVGQR
IVSGMR
IVSGDR
ΔL=0
ΔL=1
ΔL=2
Goal:
relate their properties to more general features of the nuclear medium, like e.g. the incompressibility.
Problems:
the nucleus is not a homogeneous system, and it has a shell structure.

8. The nuclear symmetry energy
Z N
Everybody is familiar with the symmetry coefficient in the semi-empiric (i.e., macroscopic) mass formula.
The microscopic concept associated with this, is the symmetry energy, which is the energy needed to transform a neutron into a proton (or vice-versa) when the system has a given density.
Symmetric matter EOS
Symmetry energy S
Nuclear matter EOS

9. Impact of symmetry energy on n-stars
Ultimately, the energy balance is dominated by: energy of neutron matter (more precisely, β-equilibrated matter) vs. gravitational energy. The stiffer the energy of neutron matter grows with density, the larger is the mass.
P.B. Demorest et al., Nature 467, 1081 (2010)
M/Msun = 1.97 ± 0.04

10. Nuclear structure experiments Hadronic/EM probes Milano
Main parameters that govern S:
M.B. Tsang et al., PRC 86, (2012)
J.M. Lattimer, J. Lim, arXiv:
Nuclear structure experiments
Hadronic/EM probes
Milano
O. Wieland et al., Phys. Rev. Lett. 102, (2009)
Weak probes
PREX (Roma)
S. Abrahamyan et al., Phys. Rev. Lett. 108, (2012).
Nuclear reaction experiments
LNS
Observational data

11. The isovector quadrupole resonance
S. Henshaw et al., PRL 93, (2004).
HIγS (107 γ/s, ΔE/E≈2-3%)
Scattering parallel and perpendicular to the polarization plane
High intensity polarized photon beam on 209Bi
Three-parameter fit of the IVGQR energy, width and strength

12. Extraction of the symmetry energy parameters
Analysis (rather) model independent
Not necessarily nucleus-independent

13. Overcoming energy limitations
The energy of the IVGQR (in analogy with that of other GRs) scales as
135 A-1/3
208Pb: ≈ 23 MeV
120Sn: ≈ 28 MeV
40Ca: ≈ 40 MeV
Need of higher excitation energy range as compared with existing or already planned facilities.

14. Harmonic behavior of the nucleus
Coulomb excitation data exist for the double IVGDR (136Xe and 208Pb), with low energy resolution.
For the double GQR, nuclear excitation data from HI reactions are available (strong background and large errors).
Real photon excitation can shed a new light on this question.
Ann. Rev. Nucl. Part. Sci. 48, 351 (1999)

15. In the double GQR of 40Ca the cross section has a big error: ratio with that of the single GQR is 15 ± 8.
The energy of the double IVGDR obeys the harmonic expectation, but the cross section does not.
Scanning the high energy region (20-30 MeV) for a double-GDR search with good energy resolution and without uncertainties related to the reaction process, would be very beneficial.

16. Nuclear scales
Wavelet analysis is a way to extract the CHARACTERISTIC ENERGY SCALES of the system
In the nucleus three main scale arise
Origin ?
PRL 93, (2004).

17. Feasibility of experiments (F. Camera/O. Wieland)
Typical cross section for dipole excitation: mb
Thick target 3-5 g/cm2: 1022 atoms/cm2
We assume a high intense and monochromatic beam of 109 γ/s

19. Conclusions
Nuclear photonics is a well established field, that aims at understanding the properties of the nuclear excitations without the uncertainties associated with nuclear excitation mechanisms.
This study has general interest (harmonic behaviour of the nucleus, order vs. chaos), and/or impacts on other fields (e.g., nuclear astrophysics).
Nuclear resonance scattering can give access to many physics cases. We are interested, among others, in those for which the energy goes below MeV. Other specific needs are discussed.

20. Acknowledgments Bracco F. Camera O. Wieland P.F. Bortignon
have contributed to the preparation of this talk

21. Backup slides

22. Hydrostatic equilibrium of a n-star
Classical gravity (Newton)
General relativity corrections (TOV)
In either case, one has to input P(ρ) from the nuclear EOS.

23. Bohr-Mottelson model ⇒ extension to S
Schematic RPA:
Bohr-Mottelson formula:
We assume: (i) simple density profile;
(ii) relationship with S
pot
shell gap

24. Neutron skin from the total dipole polarizability
“core”
excess neutrons
There is a certain correlation between the neutron skin of a nucleus and the dipole polarizability, defined as
In order to measure it, the dipole response must be scanned with high precision, especially at low energy.

25. The γ-decay of the GDR The GDR is fragmented in several states
The decay to the 2+1 state is different for each state
Old measurement of the n-decay showed E-dependence

26. Theory works at the eV level !
There are several quenching mechanisms acting, since the typical s.p. p-h transition has a width of ≈ 103 eV.

27. γ-decay of the GQR in 208Pb to g.s. and 3-1:
M. Brenna, G.C., P.F. Bortignon, PRC 85, (2012).