1. Ohio University: A.V. Voinov, S.M. Grimes, C.R.Brune, T. Massey, B.M. Oginni, A.Schiller, Oslo University: M. Guttormsen, A.C. Larsen, S.Siem, N.U.H. Syed Experimental study of level density and gamma-strength functions at Edwards Laboratory, Ohio University

2. Excitation energy Level density Bn Level density is known for most of the stable nuclei Level density is unknown for most of the nuclei Traditionally, for most of the nuclei, the level density is estimated on the basis of experimental information from low-lying discrete levels and neutron resonance spacing E a, δ -parameters σ = f(a, δ) Nuclear level density

3. The level density from particle spectra of compound nuclear reactions The concept: The problem : Make sure that the compound reaction mechanism dominates. Possible solutions: 1.Select appropriate reactions (beam species, energies, targets). 2.Measure the outgoing particles at backward angles 3.Compare reactions with different targets and incoming species leading to the same final nuclei

4. Reactions with deuterons and He-3 3 He 58 Fe 59 Co d ++ 61 Ni n p α 60 Ni 60 Co 57 Fe

5. beam Target Si 2m flight path Scheme of experimental set-up for charge-particle spectra measurements Edward’s Accelerator Lab, Ohio University

6. d, 3 He neutrons NE213 Flight path 8m target Swinger facility at Edwards Lab. Ohio University

7. Experimental level densities measured at Edwards Lab. of Ohio University Testing the technique with 27 Al(d,n) 28 Si Excitation energy, MeV Level density, 1/MeV

8. 55 Mn(d,n) 56 Fe, E d =7.5 MeV 56 Fe Points are from our experiment, line – Fermi-gas model with parameters from systematics T.von Egidy, D.Bucurescu, Phys.Rev. C 72, 044311 (2005);

9. Points are from our experiment, red line is the Fermi-gas model with parameters found from the fit to discrete levels and neutron resonance spacing, black line is the Fermi-gas model fit to experimental points

10. 65 Cu(d,n) 66 Zn, E d =7.5 MeV 56 Ni, 60 Ni, 60 Cu, 60 Co, 56 Fe, 57 Fe, 60 Zn, 64 Zn, 66 Zn

11. Fermi-gas or constant temperature ?

12. 6 Li 55 Mn 59 Co d ++ 61 Ni n p α 60 Ni 60 Co 57 Fe E ex =33 MeVE ex =23 MeV 15 MeV 7.5 MeV

14. Main conclusions from experimental results : 1.The functional form of the level density especially at low excitation energies is more complicated than Fermi-gas simple model. 2. The constant temperature model seems to be valid for excitation energies far beyond the particle separation energy (at least for some nuclei)

15. 0 γ – strength function in continuum i Excitation energy γ- Energy (MeV) From (γ,n) reactions Particle separation threshold

16. γ-strength function of iron isotopes Low energy upbend phenomenon E γ (MeV) - 56 Fe - 57 Fe Phys.Rev.Lett. 93, 142504 (2004)

17. Method of two-step γ – cascades from neutron capture reactions Ground state BnBn E1E1 E2E2 E 1 +E 2 EγEγ Problem: level density is needed !!! Intensity

18. Measurement of gamma-strength function at Edwards Lab. Of Ohio University (p,2γ) (d,n) Produce the same product nucleus 1. We obtain a level density from neutron evaporation spectra. 2. We obtain a γ-strength function from 2γ- spectra Idea The first candidate is 59 Co(p,2γ) 60 Ni reaction at E p =1.9 MeV The level density of 60 Ni has already been measured from 59 Co(d,n) 60 Ni reaction:

19. First results from 59 Co(p,2γ)

21. Conclusions Level density: The total level density exhibit step structures new region of discrete levels. The interpolation between the density of discrete levels and neutron resonance spacing with simple models do not provide a good accuracy. The constant temperature model of level density is valid above the particle separation threshold, at least for some nuclei. Gamma strength function Gamma strength function can be studied with combination of (p,2g) and (d,n) reactions. The low energy enhancement is supported by results of these experiments for the 60 Ni.