Nuclear photonics Learning from the nuclear response to

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

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: Nu. PECC meeting, Milano, March 7 th-8 th, 2012 (A. Bracco) Cf. also ELI-NP Workshop, Milano, May 14 th-16 th, 2012 (L. Serafini, A. Bracco)

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 !

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 + cos 2θ 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

A typical experiment • Below separation energy: discrete levels with large branching ratio B = Γ 0/Γ to the g. s. • Above separation energy: resonances with small gamma-decay branch Γγ << Γn.

Region ≈10 -30 Me. V: 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. Wavelength >> nuclear dimension Moreover, not always a good energy resolution can be achieved with nuclear probes.

Classification / Motivation to study IVGMR IVSGMR ΔL=0 IVGDR IVSGDR IVGQR IVSGDR 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.

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 viceversa) when the system has a given density. Nuclear matter EOS Symmetric matter EOS Symmetry energy S

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

Main parameters that govern S: Nuclear structure experiments • Hadronic/EM probes Milano O. Wieland et al. , Phys. Rev. Lett. 102, 092502 (2009) • Weak probes PREX (Roma) S. Abrahamyan et al. , Phys. Rev. Lett. 108, 112502 (2012). Nuclear reaction experiments LNS Observational data M. B. Tsang et al. , PRC 86, 015803 (2012) J. M. Lattimer, J. Lim, ar. Xiv: 1203. 4286

The isovector quadrupole resonance S. Henshaw et al. , PRL 93, 122501 (2004). HIγS (107 γ/s, ΔE/E≈2 -3%) High intensity polarized photon beam on 209 Bi Scattering parallel and perpendicular to the polarization plane Three-parameter fit of the IVGQR energy, width and strength

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

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

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

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

Nuclear scales PRL 93, 122501 (2004). • Wavelet analysis is a way to extract the CHARACTERISTIC ENERGY SCALES of the system • In the nucleus three main scale arise • Origin ?

Feasibility of experiments (F. Camera/O. Wieland) • Typical cross section for dipole excitation: 10 -300 mb • Thick target 3 -5 g/cm 2: 1022 atoms/cm 2 • We assume a high intense and monochromatic beam of 109 γ/s

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 20 -25 Me. V. Other specific needs are discussed.

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

Backup slides

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.

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

Neutron skin from the total dipole polarizability excess neutrons “core” 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.

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

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

γ-decay of the GQR in 208 Pb to g. s. and 3 -1: M. Brenna, G. C. , P. F. Bortignon, PRC 85, 014305 (2012).