Title

1. PHOTONEUTRON REACTION CROSS SECTIONS IN THE REGION OF GIANT DIPOLE RESONANCE – ANALYSIS AND EVALUATION USING PHYSICAL CRITERIA B.S.Ishkhanov, A.I.Davydov, N.N.Peskov, V.N.Orlin, M.E.Stepanov, V.V.Varlamov Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University Title

2. Focus The talk is focused on the problem as old and well-known as modern and actual – the problem of reliability of cross sections data for partial photonuclear (primarily, photoneutron) reactions – (, 1n), (, 2n), (, 3n), ... Those data are widely used in many fields of research and applications: - competition of direct and statistical processes in GDR decays; - GDR configurational and isospin splitting effects; - sum rule exhaustion and many others; - astrophysics problems.

3. Subject Data were obtained many years ago (~ 1960 – 2000). Practically we have no modern data. Data are included into various reviews, atlases and international “NUCLEAR REACTION DATABASE (EXFOR)”: - IAEA Nuclear Data Section, https://www-nds.iaea.org/exfor/exfor.htm; - USA National Nuclear Data Center, http://www.nndc.bnl.gov/exfor/exfor.htm; - MSU SINP Centre for Photonuclear Experiments Data (CDFE), http://cdfe.sinp.msu.ru/exfor/index.php.

4. MSU SINP CDFE USA National Nuclear Data Center International Atomic Energy Agency Nuclear Data Section Various versions of international “NUCLEAR REACTION DATABASE (EXFOR)” IAEA, NNDC, CDFE

5. Many data on total and partialphotoneutron reaction cross sections were obtained in experiments of different types, primarily in the following: • - experiments using beams of quasimonoenergetic photons obtained by the annihilation in flight of relativistic positrons; the majority of partial reaction cross sections were obtained using quasimonoenergetic annihilation photons in two laboratories – Livermore (USA) and Saclay (France) and some others. • experiments using beams of bremsstrahlung with continuous spectrum of photons; the majority of data were obtained in Russia (Moscow and Saratov State Universities, Institute of Nuclear Research of Academy of Science), Australia (Melbourne University) and some others. Methods

6. Q-mon. • For quasimonoenergetic annihilation photons – special subtraction procedure: • measuringof the yield Ye+(Ej) of reaction with incident photons from both annihilation and • bremsstrahlung of positrons; • 2) measuring of theYe-(Ej) of reaction with incident photons from electron bremsstrahlung; • 3)  subtraction Ye+(Ej) - Ye-(Ej) = Y(Ej) (k).

7. Brems. For continuous -spectra ofbremsstrahlung – the solving of inverse task of unfolding of reaction cross section from the measured reaction yield: where  (k) is the cross section at energy E of reaction with threshold Eth; W(Ejm,k) is the spectrum of bremsstrahlung photons; using special methods.

8. Quasimonoenergetic photons: partial reactions  total reactions. Using quasimonoenergetic annihilation photon beams firstly by various methods of neutron multiplicity sorting the partial reaction cross sections (, 1n), (, 2n), (, 3n),…, are obtained and used for total photoneutron (, Sn) and neutron yield (, xn) reaction cross sections determination by correspondent summing. Bremsstrahlung: total reactions  partial reactions. Using beams of bremsstrahlung the cross section of the neutron yield reaction (, xn) = (, 1n) + 2(,2n) + … , is measured firstly from which using special corrections based on statistical theory the total photoneutron reaction cross section (, Sn) = (, 1n) + (,2n) + …, is obtained and used for determination of partial reaction cross sections (, 1n), (, 2n), …,by correspondent subtraction procedures, for example (,2n) = (, xn) - (, Sn). Ways

9. Because of using noticeably different methods for determination partial reactions cross sections significant systematic disagreements between the results of various kinds and the same kinds experiments exist. Disagreements

10. The most well-known are systematic disagreements between partial photoneutron reaction cross sections obtained using quasimonoenergetic annihilation photons at Livermore (USA) and Saclay (France): as a rule values of (, 1n) are larger at Saclay but those for (, 2n) – vice versa at Livermore. Disagreements

11. 133Cs 159Tb 296 mb 321 mb 331 mb 344 mb 4 % 8 % (, xn) (, xn) 3 % 8 % 296 mb 321 mb 259 mb 266 mb (, 1n) (, 1n) 21 % 44 % 145 mb 101 mb 75 mb 62 mb (,2n) (,2n) (, 3n) (, 3n) Saclay Livermore S.S.Dietrich and B.L.Berman. Atom. Data and Nucl. Data Tables, 38 (1988) 199 xn-sn-n-2n-3n

12. n-2n The main problem – for 20 nuclei investigated in both labs data are systematically disagree: (, 1n)are noticeably (100%) larger at Saclay but(, 2n) – at Livermore. very large disagreements Integrated cross section ratios“S/L” small disagreements 159Tb Squares - - ratios for (, 1n) reactions – are larger than 1.0: <R> ~ 1.07. (, xn) 1.07 (, 1n) 0.84 Triangles -  - ratios for (, 2n) reactions – are smaller than 1.0: <R> ~ 0.84. (, 2n) 159Tb

13. Methods Possible reasons for clear systematic disagreements The same neutron multiplicity sorting method based on neutron kinetic energy measurement was used in both Labs based on supposition that one neutron from (, 1n) reaction has energy larger than both neutrons from reaction (,2n). But experimental methods for neutron energy measurements were different: - at Saclay the large Gd-loaded liquid scintillator was used (“suffered from a high background rate, made up largely of 1n-events, which introduced larger uncertainties in the background subtraction and pile-up corrections” – citation from B.L.Berman and S.C.Fultz, Rev.Mod.Phys., 47, 713 (1975)); - at Livermore so-called “ring-ratio” method was used (concentric rings of counters in paraffin moderator): low-energy neutrons (from reaction (,2n)) should have enough time for moderation in the way to inner ring but high-energy neutrons (from reaction (, 1n)) should go to the outer ring passing inner ring (due to multiple scattering high energy-neutron could return to inner ring).

14. Significant systematic disagreements between results of various experiments are the reasons of search for objective physical criteria of data reliability not dependent on the methods of their obtaining. In the Centre for Photonuclear Experiments Data of the Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics the program of investigation of reliability of partial photoneutron reactions (, 1n), (,2n), (,3n) cross section data was realized during several years (financial support from the Russia RFBR – 13-02-00124). Criteria

15. F2 Main objective criterion for data reliability (,2n) F2=________________________ < 0.50 (!) (, 1n) + 2(,2n) + 3(,3n) +… The natural and physically reliable energy dependence of F2 should be following: – Below the (, 2n) reaction threshold B2n only the (, 1n) reaction is possible: F2 = 0; – Above B2n both (, 1n) and (, 2n) reactions are possible, F2 increases due to competition between decreasing (, 1n) and increasing (, 2n), going to the theoretical limit of 0.50, but never reach it because of a high–energy part in (, 1n); – Above the B3n threshold the (, 3n) reaction is also possible, F2decreases due to a 3 (, 3n). F2 F B3n F-functions obtained for data calculated in frame of combined model of nuclear reactions. even-evenSn Theoretically calculated data. B2n

16. Model • Model • B.S.Ishkhanov, V.N.Orlin. Physics of Particles and Nuclei, 38, 232 (2007), • Physics of Atomic Nuclei, 71, 493 (2008): • semiclassical exitonpreequilibrium model of photonuclear reaction based on the Fermi gas densities; • effects of nucleus deformation; • effects of Giant Dipole Resonance isospin splitting. Model is well-tested for description of many total photoneutron reaction cross sections for medium and heavy nuclei.

17. Criteria Very simple and convenient for using objective physical criteria of data reliability not dependent on the methods of their obtaining were proposed: (, 2n) F2=____________________________________ (, 1n) + 2(,2n) + 3(,3n) +… In accordance with definition: F1< 1.00; F2< 0.50;F3< 0.33;F4< 0.25, …; The most interesting is F2– effective tool for investigation of competition between three partial photoneutron reactions under discussion - (, 1n), (, 2n) and (, 3n).

18. Tb-Sn 159Tb Not reliable negative cross section values 116Sn 2.0! Significant disagreements: F2 > 0.6 –not reliable sorting of neutrons with multiplicity «1» and «2» Significant fall below B3n: not reliable sorting of neutrons with multiplicity «1» and «3» Dramatic extra disagreements: F2 = 1.5 – 2.0 (!) –not reliable sorting of neutrons with multiplicity «2» и «3». (, 2n) =________________________ < 0.50 (, 1n) + 2(, 2n) + 3(, 3n) +… F2= (, 2n)/(, xn)

19. Zoo Not reliable data on neutron sorting between the channels with multiplicity «1» - «2», «1» - «3» and «2» - «2»from the experiments carried out using quasimonoenergetic annihilation photons. F2 > 0.50 F2 > 0.50 F2 > 0.50 > 0.33 > 0.33 > 0.33 Physically forbidden negative cross section values 91Zr 94Zr 188Os 189Os

20. Bremsstrahlung Not reliable data on neutron sorting between the channels with multiplicity «1» - «2» and «1» - «3» from the experiments carried out using bremsstrahlung and statistical theory corrections (Yu.I.Sorokin, B.A.Yur’ev. Sov.J.Nucl.Phys. 20, 123 (1975). Bull.Acad.Sci.USSR, Phys.Ser. 39, 98 (1975) ). 112Sn 114Sn 119Sn E, MeV Lines - model

21. Nuclei Investigations carried out before for many nuclei (63,65Cu, 80Se, 91,94Zr, 115In,133Cs,138Ba,159Tb, 181Ta,186,188,189,190,192Os,112,114,116,117,118,119,120,122,124Sn, 197Au, 208Pb, 209Bi) using both quasimonoenergetic annihilation photons and bremsstrahlung show that in many cases data obtained do not satisfy proposed objective physical criteria of partial reaction cross section data reliability.

22. Experimentally-theoretical method for partial photoneutron reaction cross section evaluation: eval(γ, 1n)= F1theorexp(γ, xn), eval(γ,2n)= F2theorexp(γ, xn), eval(γ,3n)= F3theorexp(γ, xn),… • That treatment means: • the competition of partial reactions is in accordance with equations of model; • the sum of evaluated partial reaction cross sections • eval(, xn) =[(, 1n) + 2(, 2n)+ 3(, 3n)]. • is equal to the experimental exp(, xn). Evaluation method

23. Examples of significant disagreements between evaluated data and experimental cross sections obtained usingquasimonoenergetic annihilation photons and the methods of neutron multiplicity sorting. 208Pb (, 1n) 91Zr (, 1n) (, 2n) (, 2n) (, 3n) Evaluations

24. 94Zr  > +18 % < -16 % F2 grows although (, 3n) appeared! > +65 % < -21 % < -51 % 94Zr

25. 133Cs 133Cs Comparison of integrated cross sections: Livermore Saclay (, 1n) 1860.91630.71543.9 -20% -2% (, 2n) 452.6 375.1 367.9 -21% +2% (, 3n) 11.9 - new data

26. 209Bi 209Bi (Livermore) Comparison of integrated cross sections: 2230.42482.9 +11% (, 1n) 706.9 611.0 (, 2n) -16% (, 3n) 8.2 - new data

27. Conclusions In detailed investigations for many nuclei (63,65Cu, 80Se, 90,91,94Zr, 115In,133Cs,138Ba,159Tb, 181Ta, 186,188,189,190,192Os, 112,114,116,117,118,119,120,122,124Sn, 197Au, 208Pb, 209Bi) it was shown that the main reasons of disagreements under discussion are the significant systematic uncertainties of the neutron sortingbetween multiplicity channels “1n”, “2n” and “3n”. Erroneous moving noticeable number of neutrons from “1n” channel to “2n” decreases (, 1n) down to physically forbidden negative values and at the same time (, 2n)unreliablyincreases (F2 increases up to not reliable values “> 0.50”). The analogous is the situation for “2n” and “3n” channels.

28. Conclusions There is additional serious source of uncertainties in neutron multiplicity determination based on neutron energy measurement – the contribution of the reaction (, 1n1p). In experiments under discussionthe reaction denoted by (, 1n) is in fact the [(, 1n) + (, 1np)] reaction. Obviously, the distribution of excited nucleus energy between the emitted neutron and proton in the (, 1np) reaction is expected to be close to the distribution of this energy between two neutrons in the (, 2n) reaction. However, the neutron multiplicity is 1 in the (, 1np) and 2 in the (, 2n) reaction. So the neutrons from those reactions could be mixed. This kind uncertainty can be clear described based on the data for 63,65Cu, 80Se.

29. 63Cu 63Cu (Livermore) +13% -96% Amount of neutrons moved out from the channel “1n” is noticeably smaller than that added to the channel“2n” (, 1n) (, 2n)

30. 65Cu 65Cu (Livermore) +34% -64% Amount of neutrons moved out from the channel “1n” is smaller than that added to the channel“2n” (, 1n) (, 2n)

31. 80Se 80Se (Saclay) (, 1n) +15% Amount of neutrons moved out from the channel “1n” is near to that added to the channel“2n” (, 2n) -19%

32. 63Cu-65Cu-80Se Comparison of experimental and evaluated data obtained for 63,65Cu and 80Se could explain one of the important reasons of shortcomings of the method of neutron multiplicity sorting under discussion – missing the role of the reaction (, 1n1p). In both two-nucleon reactions (, 1n1p) and (, 2n) neutrons could have close energies but different multiplicities: Δint(2n) = [int-exp(, 2n) - int-eval(, 2n)] Δint(1n) = [int-eval(, 1n) - int-exp(, 1n)]

33. Conclusions (g, 1n1p) and (g, 2n) reactions competition (g, 2n) 83% 30% 4% (g, 1n1p)

34. Conclusions • Summary • Experimental data on partial photoneutron reactions cross sections • (, 1n), (, 2n) and (, 3n) formany nuclei • (63,65Cu, 80Se, 90,91,94Zr, 115In,133Cs,138Ba,159Tb, 181Ta, 186,188,189,190,192Os, 112,114,116,117,118,119,120,122,124Sn, 197Au,208Pb, 209Bi), • obtained using both the method of photoneutron multiplicity sorting • and statistical theory corrections of neutron yield reaction cross sections • do not satisfy objective physical criteria of data reliability. • 2. Two main reasons are: • significant systematic uncertainties of the processes of neutron multiplicity sortingbetween the channels “1n”, “2n” and “3n”; • ignoring of proton channels contributions into photoneutron reaction cross sections.

35. Conclusions • Conclusion • For obtaining reliable partial reaction cross section new measurements using alternative methods such as activation methods or methods with detection of produced neutrons in coincidences is needed. • - Before such alternative experiments it is useful to use as reliable data evaluated in the frame of described experimentally-theoretical method or another methods without neutron multiplicity sorting.

36. Thanks! Thanks a lot for attention! Photoneutron reactions cross sections in the region of giant dipole resonance – analysis and evaluation using physical criteria B.S.Ishkhanov, A.I.Davydov, V.N.Orlin, N.N.Peskov, M.E.Stepanov, V.V.Varlamov,

37. RTM Independent test – activity method:identification of reaction using not outgoing neutrons but final nucleus electrons bremsstrahlung photons target bremsstrahlung target Race-track microtron RTM-70 HPGE-detector

38. Integrated cross sectionsint (Eint = 17 - 31 MeV) 189Os > 118%  < 25% «1n» «3n» Comparison of differences: [ exp(, 1n) - eval(, 1n)] – circles and [exp(, 2n) - eval(, 2n)] – squares. 17 31 17 31 189Os

39. 181Ta - comparison 181Ta Decays of 181Ta(γ, 1n) and 181Ta(γ, 2n) reactions final nucleusdiffer significantly: 181Ta(γ, 1n)180Ta, T1/2 = 8.154 hour, E = 93.326 кэВ E = 103.557 кэВ 181Ta(γ, 2n)179Ta, T1/2 = 1.82 year, E = 63.0 кэВ The comparison of ratios of reaction yields Y and integrated cross sections int obtained for experimental and evaluated data for 181Ta for Eint = 65 MeV. Ratios Evaluation Saclay Activity F1,2,3

40. 138Ba 138Ba (, 1n) 1548.3 10% 1459.8 (, 2n) 564.0 15% 490.4 77% (, 3n) 7.1 4.0

41. Theory Theory Semiclassical exitonpreequilibrium model of photonuclear reaction based on the Fermi gas densities and taking into account the effects of nucleus deformation and of GDR isospin splitting. Bohr description of (,lpkn): i -one of 4 components (2 isospins - T0 and T0 + 1 and 2 directions of vibration), GDR - Lorenz lines with W - decay probabilities (recurrent): where

42. CRP-Tb Our evaluationsbased on F-functions are quite different from IAEA CRP (1996 – 1999) evaluations. , mb , mb 159Tb CRP evaluations have been done using GUNF and GNASH codes in order to model accurately Saclay (, Sn) data.

43. 197Au (, 2nx) Saclay Our evaluationsbased on F-functions: Livermore data are much more reliable but Saclay not. Livermore CRP evaluations have been done using GUNF and GNASH codes in order to model accurately Saclay (, Sn) data. CRP - Au

44. Value & shape 16O(,хn) There are disagreements both in value and shape: quasimonoenergetic data look like as smoothed bremsstrahlung ones. intBR (MSU - error bars) = 36.9 MeVmb intQMA (Saclay - squares) = 34.6 MeVmb intQMA (Livermore - crosses) = 32.1 (27.6  1.12) MeVmb

45. Value & shape Quasimonoenergetic photons Bremsstrahlung Disagreements in neutron yield reaction cross sections both in value and shape

46. Conclusions