Level Densities, Decay Probabilities and Cross sections in the Actinide Region

1. Level Densities, Decay Probabilities and Cross sections in the Actinide Region J.N. Wilson Institut de Physique Nucléaire, Orsay

2. NLD Measurements in the Actinides: Goals & Motivations Fundamentalphysics – NLD and GSF fine structure, statisticalproperties of the hot compound nucleus Indirect cross section measurements via surrogatereactions Leveldensityknowledgehelps cross section calculationswhere direct measurements are difficult or impossible

3. 1. Fundamental Physics Extrapolation Oslo method Resonance spacings at the neutron binding energy Spectroscopy of lowlying states

4. 2. Indirect Measurements

5. 2. 233Pa Capture ExperimentResults S. Boyer et al. Nucl.Phys. A775, 175 (2006)

6. 3. x-section calculations

7. Part I First Experimental Data

8. Oslo Cyclotron Experiments Si Ring Cactus NaIarray

9. The Silicon Ring • High efficiency/high angular resolutiondetection of the outgoing particles (<2°) • E/ΔE collimators not necessary

10. 2. The Thorium Cycle FISSION FISSION 232U 233U 234U 235U 236U a (n, 2n) (n, g) (n, g) (n, g) b- b- b- 231Pa 232Pa 233Pa 234Pa (n, g) (n, 2n) (n, g) a b- b- 230Th 231Th 232Th 233Th (n, g) (n, 2n)

11. Experiment232Th: 01/12/09 – 13/12/09 232Th(d,x) @ 12 MeV 232Th(3He,x) @ 24 MeV 351 M events 135 M events

13. Ring 0 12C (d,d’) 16O (d,d’) 12C(d,p)13C ex2 12C(d,p)13C ex1 16O(d,p) 17O ex1 12C(d,p) 13C

15. Part II Extracting the LevelDensity

16. The Brink Axel hypothesis Leveldensity Transmission Coefficient Gamma strengthfunction Assuming dominance of dipole radiation (E1 and M1)

18. 3.6 MeV 3.9 MeV 3.1 MeV 2.15 MeV ??? 0.87 MeV

20. MCNP simulations of Cactus detector response

23. Cactus ResponseMatrix

24. Matrix *response; Spec *after=before->SubtractComptons(response);

25. 233Th* - After Compton Subtraction

26. 233Th* -Primary gamma rays

27. Part III The surrogatemethod

28. Decay probability measurement Efficiency of detecting a cascade, Ec of m gammas is: If εissmall, then: ε = k Eγ Suppose detector has thisproperty: Theneffeciency of detecting a cascade becomesproportional to cascade energy and independent of cascade path!!

29. Minimisation with the Downhill simplex method « With four parameters, I could fit an elephant, with five I couldmakehimwigglehistrunk » W(E) = a + bE + cE2 + d E3 + eE4 + f E5

30. WeightingFunctionMatrix

32. 17O

35. n,n' correction to measured decayprobability Primary gamma rays (New Technique) n,gamma spectral extrapolation below n,n’threshold

36. Results

37. TALYS calculations Compare resultswith TALYS calculations: Since 232Th(n,g) has been measureddirectlyatnTOFwecanperform a direct test of the optical model and the validity of the surrogatemethod.

38. Conclusions Leveldensitymeasurements possible in: 233Th - d,p (0- 5 MeV) 232 Th - d,d’ (0- 3.5 MeV) 231Th - d,t (0 – 3 MeV) Surrogatereaction & decayprobabilitymeasurements possible in: 233Th* - d,p 230Ac* - d,a 234Pa* - 3He,p 232Pa* - 3He,t 231Th - 3He,4He

39. The Future • 233U, 231Pa, 234U targets • Fission detectors

40. Oslo Cyclotron S. Siem S. Rose A. Bürger M. Guttormsen A-C. Larsen T. Wiborg H. Toft IPN Orsay J.N. Wilson B. Leniau CEA Saclay F. Gunsing A. Georgen Thankyou Takk Merci Lawrence LivermooreLab M. Weideking L. Bernstein

41. 147Sm 146Sm

42. Hauser-FeshbachFormalism a + A B + b Differential cross section depends on transmission coefficients, Γ and level densities of the residual nuclei, ρ

43. Level Densities Theory and Experiment • Leveldensity changes due to collectivity, deformation etc. • Large effectscanoccur over a smallnumber of nucleons

44. Nuclei where direct measurements are difficult 163 d 242Cm 243Cm 244Cm 245Cm 246Cm 247Cm Target activity > 109 Bq/mg a (m) 244Am 241Am 243Am 242Am 88 y 242Pu 243Pu 239Pu 240Pu 241Pu 238Pu 69 y a 237Np 238Np 239Np 26 h 232U 233U 234U 235U 237U 239U 236U 238U a b- 233Pa 231Pa 5.0 h a 6.8 d 232Th 233Th 229Th 230Th 231Th (n, g) a (n, 2n) 22 m 26 h