Analysis of Mg Isotopes Neutron Capture Reactions

1. Session 3: Measurements (long) before 2010 Results on 24,25,26Mg(n,) with C6D6 Cristian Massimi and others Collaboration annual meeting 2010, Sevilla - Spain, 15-17 December 2010

2. Outline • Introduction and Motivation • Experiment • Data analysis • Resonance shape analysis 24,26Mg • Problem with 25Mg sample • Results and Conclusions

3. Introduction The s-process and Mg stable isotopes • 22Ne(,n)25Mg is the main neutron source in massive stars (M > 10 – 12Msun) • 22Ne(,n)25Mg is an important neutron source in AGB stars, 1.2Msun < M < 3Msun Other important reactions:22Ne(,g)26Mg, 25Mg(n,g)26Mg, 26Al (n,p)26Mg

4. Motivations The s-process and Mg stable isotopes • (24),25,26Mg are the most important neutron poisons due to neutron capture on Mg stable isotopes in competition with neutron capture on 56Fe that is the basic s-process seed for the production of heavy isotopes. • Several attempts to determine therate for the reaction 22Ne(,n)25Mg either through direct 22Ne(,n)25Mg measurement or indirectly, via 26Mg(,n)25Mg or charged particle transfer reactions. In both cases the cross-section is very small in the energy range of interest. • The main uncertainty of the reaction rate comes from the poorly known property of the states in 26Mg. Information can come from neutron measurements (knowledge of J for the 26Mg states).

5. Motivations Mg data in literature • Charged particle reaction • Denhard et al., Phys. Rev. 160, 964 (1967) • Hardy et al., Academic • Photo-neutron (g,n) • Baglan et al. Phys. Rev. C 3, 672 (1971) • Same experiment, Phys. Rev. Lett. 24, 319 (1970) • Neutron induced reactions A) Transmission • Singh et al. Phys. Rev. C 10 2150 (1976) • Newson, Block et al., Ann. Phys.8, 211 (1959) B) Capture • Macklin et al., Rev. Mod. Phys. 37, 166 (1965) • Bergquist et al., Phys Rev. 158, 1049 () • Block et al., RPI technical report • Nystrom et al., Phys. Scr. 4, 95 (1971) • Activation (thermal) • Walkiewicz and Ranan Phys. Rev. C 45, 4 (1992) • Activation (MACS) • Mohr et. al., Phys. Rev. C 58, 2 (1998) Transmission + Capture: Weigmann, Macklin and Harvey, Phys. Rev. C 14, 4 (1976) + Koehler, Phys. Rev. C 66, 055805 (2002) ORELA

6. Motivations Mg data in literature • Nutron induced reactions The available experimental data for 24,25,26Mg are essentially based on a time-of-flight measurement performed at ORELA: • very high-resolution transmission measurement (200-m flight path, metallic natMg sample, plastic scintillator) • high-resolution capture measurement (40-m flight path, 97.9% 25Mg enriched sample, fluorocarbon scintillators) Capture data on 24,26Mg from ORELA are missing

7. Motivations Mg data evaluation • Evaluation does not consider: • - the photoneutron cross-section measurement 26Mg(,n)25Mg by Berman et al.(PRL 1969),  J • - a recent work by P. E. Koehler (PRC, 2002)  R-matrix analysis of existing measurements • Existing evaluations are based on JENDL3.3 (by T. Asami) • JENDL 4 (2010) isnot updated • Resonance parameters in resolve resonance region are taken from BNL (Mughabghab) and negative resonances were added to reproduce the measured thermal cross-section

8. Motivations New measurement at n_TOF Improve existing data: • Detector with very low neutron sensitivity; • Enriched samples isotopic (n,g) data; Resonance Shape Analysis (simultaneous: Capture data from n_TOF + Transmission data from ORELA); Determination of the Maxwellian-averaged capture cross-section.

9. Experiment Mg samples 2.2-cm in diameter. Oxide sample sealed in an aluminum canning. Al canning is 0.35 g (1.127E-03 at/b). First TOF measurement on an enriched 26Mg sample

10. Experiment Mg samples 2.2-cm in diameter. Oxide sample sealed in an aluminum canning. Mg isotopes Gold Lead Carbon same diameter

11. Experiment n + 25Mg

12. Data analysis • Detector energy calibration: g-ray sources • Weighting Function (F. Gunsing) • Normalization: C6D6 and SiMon calibrated to 3% accuracy by saturating the 4.9-eV resonance in Gold with a 0.25-mm thick sample. • Time-energy calibration: • TOF = Tn - T0 • Where T0 = Tγ – L / C • L = 185.07 m, from resonances in Au • En = mc2 (1 – γ) • Where:

13. Data analysis

14. Resonance shape analysis Evaluation ≈ ORELA n + 26Mg Assumed reaction width ORELA n_TOF When data are not available, EVALUATION uses average reaction width  Gg = 3 eV This resonance was doubtful, thought to belong to 24Mg.

15. Resonance shape analysis Evaluation ≈ ORELA n + 26Mg n_TOF n_TOF Huge discrepancy between Weigmann [TOF at ORELA] and Mohr [ACTIVATION]. “Resonance strength should be a factor 2.8 lower than the result from ORELA”.

16. 26Mg MACS n + 26Mg MACS at 30 keV from present measurement: 0.106 ± 0.01 mb Preliminary calculations (A. Mengoni): Direct capture component to the MACS < 1 mb Negative resonance changed to reproduce the thermal neutron cross section  New value for the Radius

17. Resonance shape analysis n + 24Mg Evaluation ≈ ORELA Assumed reaction width ORELA n_TOF Doubtful resonance at 68 keV, assigned to both 24Mg and 26Mg

18. Resonance shape analysis n + 24Mg Evaluation ≈ ORELA n_TOF ORELA

19. Resonance shape analysis n + 24Mg Evaluation ≈ ORELA n_TOF ORELA

20. 24Mg MACS n + 24Mg MACS at 30 keV from present measurement: 4.16 ± 0.2 mb Preliminary calculations (A. Mengoni): Direct capture component to the MACS < 1 mb Negative resonance changed to reproduce the thermal neutron cross section  New value for the Radius

21. Resonance shape analysis n + 25Mg Previous data ≈ ORELA

22. Resonance shape analysis n + 25Mg Evaluation ≈ ORELA n_TOF n_TOF

23. 25Mg MACS n + 25Mg MACS at 30 keV from present measurement: 3.65 ± 0.2 mb Preliminary calculations (A. Mengoni): Direct capture component to the MACS < 1 mb Negative resonance changed to reproduce the thermal neutron cross section  New value for the Radius

24. 25Mg MACS n + 25Mg MACS at 30 keV from present measurement: 3.65 ± 0.2 mb … but it is wrong !

25. Problem with 25Mg n + 25Mg, the large s-wave resonance at 20 keV In capture, when Gn >> Gg the RSA is sensitive to ngGg

26. Problem with 25Mg n + 25Mg

27. Problem with 25Mg n + 25Mg

28. Problem with 25Mg n + 25Mg

29. 25Mg MACS, n scaled n + 25Mg MACS’ at 30 keV from present measurement: 6.5 ± 0.4 mb Scale factor, from natMg sample: n’ = 0.56 n ! ! !  uncertainty on scaled areal density (very high): 10-15%

30. Results • Resonance parameters of the 24Mg(n,) and 26(?)Mg(n,) reaction cross-sections have been determined. • Sizable differences have been found respect to the existing evaluation, JENDL4 (2010). • Smaller differences respect to ORELA data. • Direct capture component is an important mechanism, upper limit: 1 mb to the MACS.

31. Conclusions • The present (n,) measurements improve the cross section data on 24,26(?)Mg isotopes. New value for the MACS are available. • From this analysis we find that the MACS of 25Mg isotope is lower than reported previously because of a problem with the sample. • The contribution of the direct capture mechanism should be investigated experimentally. • Is 26Mg sample thickness correct? • Is it worth to repeat the measurement on 25(,26)Mg?

32. Cristian Massimi Dipartimento di Fisica massimi@bo.infn.it www.unibo.it

33. Capture

34. Elastic

35. 26Mg

36. 25Mg

37. 25Mg and 26Mg

38. Amplitude spectra Amplitude spectra

39. Without WF Without WF

40. With WF With WF

41. WF Weigthing Functions

42. WF

43. MACS 26Mg ORELA (PK)

44. MACS 26Mg n_TOF, no 68 keV

45. Capture Area 24Mg n_TOF / ORELA Neutron energy (eV)

46. Capture Area 24Mg n_TOF / ORELA Neutron width (meV)

47. Motivations The reaction 22Ne(,n)25Mg Only natural-parity (0+, 1-, 2+) states in 26Mg can participate in the 22Ne(,n)25Mg reaction, so only a subset of 26Mg states in the relevant energy range observed via neutron reactions can contribute to the reaction rate