Some ideas and questions about the reaction of fission

Transcription

1 1 Some ideas and questions about the reaction of fission G. Mouze, S. Hachem and C. Ythier University of Nice, France R.A Ricci Laboratori Nazionali di Legnaro, Università di Padova, Padova,Italy SIF XCVII Congresso Nazionale, L Aquila, Settembre 2011

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3 3 Indeed the mass spectrum of Signarbieux can be translated into a Gaussian curve centered on the discrete mass-value 104 If DA,the f.w.h.m., measures the uncertainty in the mass A of the fragment, the uncertainty in t is D t = h/(c 2 DA) = second The reaction time is shorter than the lifetime of the W +- and Z 0 bosons, and s

4 4 The isotopic mass distribution reported by Signarbieux at Z= 50 can be translated into a Gaussian curve, centered on N = 83.3, which reveals an uncertainty DN in the neutron number equal ~ 2.54 u.

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6 n = N MAX - N 6 These Gaussian curves can be viewed as distributions of N around its most probable value N n = 0, N MAx = Cm 142 Ba Zr or as distributions of n around its mean value n n = 0, N MAX = Cm 148 Ba Zr they are reminiscent of that of prompt neutrons:

7 7 The law giving the probability P(n) of emitting n neutrons per fission as a function of (n n) was established by James Terrell (PR 108 (1957) 783) From the value 1.08 of the s- parameter one gets DN = 1.08 X 2.35 = neutrons. Terrell s law is a consequence of DE.Dt = h

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9 9 Dt is close to Dt ( 235 U + n), equal to s Is Dt constant for all fissioning systems? Is the main step of fission the same for all them, e.g. the transfer of a number of nucleons, say 68, from a 208 Pb core to a cluster of mass A cl? For Flynn et al [Phys. Rev.C 5(1972)1725], A L (product) is a linear function of A F. If A F is written as A cl, the law, after correction for neutron emission, becomes A L (fragment) = A cl + 68 Mouze and Ythier NCA 103 A (1990)617.

10 Pu + n 208 Pb + 32 Mg MeV: 32 Mg is a new kind of cluster*. It differs from 14 C, 20 O, 23 F, 22,24-26 Ne, 28,30 Mg, 32,34 Si clusters emitted in cluster-radioactivity ** The clusterization energy Q cl can be converted into vibrational energy and induce fission System Q cl (MeV) A cl 235 U + n Ne 239 Pu + n Mg 252 Cf sf S 258 Fm sf Ar 265 Hs + n ~ Fe 286 (112) sf ~ Zn If Q cl is greater than B C, cluster-fission becomes possible (EPL 58(2002)362) *Ythier and Mouze J. Nucl. Materials 166 (1989)74. ** Poenaru et al PRL 107 (2011)

11 11 Fragments of mass 82 and 126 do not emit prompt neutrons. The neutron yield increases linearly above these values (J. Terrell.Phys.Rev.127(1962)880) For Mouze and Ythier (2008 Bormio Meeting) this situation means that nucleon shells are closed at A = 82 and 126. The 208 Pb core can be destructed and changed into a hard A H = 126 core and 82 nucleons. Part of them are transferred to the cluster, and form a hard A L = 82 core. The remaining 82 ( 82 A cl ) nucleons are shared between these cores.

12 U + n 208 Pb + 28 Ne (A cl = 28) 208 Pb + 28 Ne A = 126 nucleon core + 82 nucleons There are two regions of appreciable yield: A L, from A = 82 to A cl + 82; A H, from A = 126 to A cl Width = A cl u [ JMPE 17,2008]

13 13 The nucleon phase model explains the width of asymmetric mass distributions. Data from Flynn and Glendenin (1970)(without correction) for 235 U + n D A L = 28 u D A H = 28 u Asymmetric fission is strongly confined: up to 256 Fm no fragment pair has a fission energy greater than its own Coulomb barrier

14 14 Symmetric fission results from the closure of a shell made of 126 nucleons in the light fragment (Mouze and Ythier,2008 Bormio Meeting) For 286 (112)(s.f.) the width of the region of appreciable yield is equal to (A cl -44) u, i.e = 34 u

15 15 The nucleon phase model explains the symmetric fission of superheavy nuclei (M.G. Itkis, et.al., Lipari Conf.2001) Symmetric fission Bormio, A cl = 78 Cluster-fission Europhys.Lett. 58(2002) A cl = A cl = 88 This fission is barrier-free (G. Mouze,2005 Bormio Meeting)

16 16 Discussion The magic mass numbers 82 and 126 result from a spin-orbit coupling similar to that of the neutron phase of ordinary matter, as if the Coulomb potential of the protons had disappeared... Moreover, uncertainty in Z exists, but no proton is emitted. Could new physics be hidden in back of this nucleon phase model? Anyway this model consistently explains a great number of experimental facts

17 17 Conclusion Recently Mouze et al. found in Terrell s law on prompt-neutron an answer to Gönnenwein s comment to my 2009 EPN-paper: Dt (fission) = 0.17 ys. And Terrell s law, which allows the chain reaction is simply the way in which Nature warrants the uncertainty in N of any fission fragment.

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