Spontaneus pion emission during fission - a new nuclear radioactivity

a rXiv:n ucl-t h /3156v122Oct23SPONTANEOUS PION EMISSION DURING FISSION A NEW NUCLEAR RADIOACTIVITY D.B.Ion 1,Reveica Ion-Mihai 2,M.L.Ion 2and Adriana I.Sandru 11National Institute for Physics and Nuclear Engineering,IFIN-HH ,Bucharest,Romania 2University of Bucharest,Department of Atomic and Nuclear Physics ,Bucharest,Romania February 8,2008Abstract In this paper a short review of the theoretical problems of the pionic radioactivity is presented.The essential experimental results obtained in the 18years of existence of the nuclear pionic radioactivity are reviewed.Moreover,using the recent results on the spontaneous fission half lives T SF of the heavy nuclei with Z ≥100new predictions on the pionic yields in the region of superheavy elements are established.1INTRODUCTION In 1935Japanese physicist Hideki Yukawa predicted [1]the existence of a parti-cle -a meson -with a mass about 200times that of the electron,which mediated the nuclear forces.At first,the muon,discovered in 1937,looked like a good candidate for the meson,but soon it became clear that the muon’s properties did not match those predicted by Yukawa theory.Then,in 1947,Lates,Oc-chialini and Powell discovered [2]the π-meson,or pion,which did have the predicted meson properties.Today,we know that the muon is an elementary particle [3],cousin to the electron,-leptons and neutrinos,and that mesons (π,K ,η,etc.),including the pion,are combinations of quark and an antiquark.Everybody know that the discovery of -meson was an very important finding since this was in fact a fundamental step in the understanding the subatomic world.It heralded the beginning of a deep revision of the physical concepts on the structure of matter.However,who can tell us what exactly are those mesons?Also,it is well known that the pion as lightest of mesons has a finite1size,with a mean charge radius of0.66fm,and that the long range part of the nucleon-nucleon forces necessarily arises from one-pion-exchange.The traditional picture of the nucleus as a collection of neutrons and protons bound together via the strong force has proven remarkable successful in under-standing a rich variety of nuclear properties.However,it is a well grounded achievement of modern nuclear physics that not only nucleons are relevant in the study of nuclear dynamics but that pions and the baryonic resonances like ∆’s and N*play an important role too[4].So,when the nucleus is explored at short distance scales the presence of short lived subatomic particles,such as the pion and delta,are revealed as nuclear constituents.At even shorter distance scales the basic building blocks of matter,the quarks and gluons,are also re-vealed as nuclear constituents.The role of pions,deltas,quarks and gluons in the structure of nuclei is one frontier of modern nuclear physics.This modern picture of the nucleus bring us to the idea to search for new kind of natural radioactivities such as:(π,µ,∆,N∗etc.)-emission during the nuclearfission in the region of heavy and superheavy nuclei.Moreover,new mode of nuclear fission such as:deltonicfission an hyperfission,was also suggested and inves-tigated.So,in1985,D.B.Ion,M.Ivascu and R.Ion-Mihai initiated[5]the investigation of the nuclear spontaneous pion emission as a new possible nuclear radioactivity called nuclear pionic radioactivity(NPIR)with possible essential contributions to the instability of heavy and superheavy nuclei.In this paper we present a short review not only of the main theoretical problems of the NPIR but also of the essential experimental results obtained in these18years of existence of the nuclear pionic radioactivity.Moreover,in this paper by using the recent results on the spontaneousfission half lives T SF of the heavy nuclei with Z>100we present new prediction on the pionic yields in the region of superheavy elements.2FISSION-LIKE MODEL FOR PIONIC RA-DIOACTIVITYThe nuclear pionic radioactivity of a parent nucleus(A,Z)can be considered as an inclusive reaction of form:(A,Z)→π+X(1) where X denotes any configuration offinal particles(fragments,light neutral and charged particles,etc.)which accompany emission process.The inclusive NPIR is in fact a sum of all exclusive nuclear reactions allowed by the conservation laws in which a pion can be emitted by a nucleus from its ground state.The most important exclusive reactions which give the essential contribution to the inclusive NPIR(1)are the spontaneous pion emission accompanied by two body fission::A Z X→π+A1Z1X+A2Z2X(2)where2A=A1+A2Z=Z1+Z2+Zπ(3)Hence,the NPIR is an extremely complex coherent reaction in which we are dealing with a spontaneous pion emission accompanied by a rearrangement of the parent nucleus in two or manyfinal nuclei.Charged pions as well as neutral pions can be emitted during two body or many bodyfission of parent nucleus.The energy released in an exclusive nuclear reaction(1)is given byQπF=Q SF−mπ(4) where Q SF is the energy released during spontaneousfissionQ SF=m(Z,A)−m(Z1,A1)−m(Z2,A2)(5) Then the energy condition for the pionic radioactivity channel isQπF>0(6) orQ SF>mπ(7) Other interesting new natural radioactivities predicted at IFIN-Bucharest are as follows•Muonic radioactivity[25]is a nuclear process in which the lepton pair (µ±,υµ)is emitted during two or many bodyfission of the parent nucleus.For the the nuclear spontaneous emission of(µ±,υµ)in binaryfission we can writeA Z X→µ±+νµ+A1Z1X+A2Z2X(8)A=A1+A2Z=Z1+Z2+Zµ(9)•Lambdonic radioactivity[26]is just a nuclear reaction of formA Z X→Λ0+A1Z1X+A2Z2X(10)A=A1+A2Z=Z1+Z2(11)•Hyperonic radioactivities Σ−,Σ0,Σ+ are also the possible nuclear decays of formA Z X→Σ±,0+A1Z1X+A2Z2X(12) 3A=A1+A2Z=Z1+Z2+ZΣ(13)•Hyperfission or weakfission[27]is afission process in which in one of fragment is hypernucleus:A Z X→A1Z1X+A2Z2Xγ(14)A=A1+A2Z=Z1+Z2(15)•Deltonicfission[28]is also afission process in which in one of fragment contain a delta resonance:A Z X→A1Z1X+A2Z2X∆(16)A=A1+A2Z=Z1+Z2(17)Afission-like model(see Ref.[5])for the pionic radioactivity was regarded as a first stage in the development of an approximate theory of this new phenomenon that takes into account the essential degree of freedom of the system:π-fissility,π-fission barrier height,etc.Therefore,let us considerEπF C(0)=E0C−αmπ(18)EπF S(0)=E0S−(1−α)mπ(19) where E0C and E0S are the usual Coulomb energy and surface energy of the parent nucleus given byE0C=γZ2/A1/3(20)E0S=βA2/3(21) withβ=MeV andγ=0.71MeV.αis a parameter defined so thatαmπand (1−α)mπare the Coulombian and nuclear contributions to the pion mass.For α=1,the entire pion mass is obtained only from Coulomb energy of the parent nucleus.So,by analogy with binaryfission was introduced the pionicfissility XπF which is given byX(α)πF =EπF C(0)E0S−(1−α)mπ(22)4orZ2A−mπ1−(1−α)mπ/E0S(23)In the particular caseα=1we haveXπF=X SF−mπ5 2EπF S(0)−EπF C(0) =2ε2EπF S(0)λ[1+ε2P2(cosθ)+ε4P4(cosθ)+ε6P6(cosθ)+...](29)5whereε2=2.33(1−XπF)−1.23(1−XπF)2+9.50(1−XπF)3−8.05(1−XπF)4+...ε4=1.98(1−XπF)2−1.70(1−XπF)3+17.74(1−XπF)4+...ε6=−0.95(1−XπF)+...(30)R0is the spherical radius andλis a scale factor just as in binary spontaneous fission.In Fig.4and5we present the values of EπF(LD)as well as the nuclear configuration at the saddle point compared with those fromfission(F)or hy-perfission(HF).True barrier height for the pionicfission isEπF=EπF(LD)−∆EπF shell(s.p.)−∆E shell(g.s.)(31) where and∆EπF shell(s.p)and∆E shell(g.s)are correction due to shell effect at saddle point and ground state,respectively.Next,the essential ingredients used for the estimation of the relative pionic yieldsΓπF−37,5(32)AθπF= Z2γA2/3α−(1−α)E0C/E0Sfor parent nuclei with A-odd.For example in Fig.6we presented the experimental values of theτ(θ) compared with thefitted curve(37).Finally,by using(36)-(37)we obtain the following important predictionΓπT CSFδα(A,Z)(38) whereδα(A,Z)=∆θαγA2/3α−(1−α)E0C/E0SΓSF <1,Γπ0,±ΓSF calculated with(38)-(40)forα=1andα=0.75,for even-even and A-odd parent nuclei,are presented in Table2a,b.3EXPERIMENTAL SEARCH FOR NUCLEAR PIONIC RADIOACTIVITY3.1Experimental limits on NPIR-yieldThe pionic radioactivity was experimentally investigated by many authors[13]-[24].A short review of the results obtained on the spontaneous NPIR yields is presented in Table3.73.2Recent results from LIPPE-Obninsk(Russia) Recently Khryachkov et al.[24]presented a new high performance spectrometer for investigation of ternary nuclearfission.The measured characteristics of this spectrometer allow for its successful use in studies of ternaryfission with the emission ofα-particles,tritons,and protons as well as in the search for exotic nuclearfission accompanied by the emission of charged mesons(π±,µ±).This new spectrometer was tested with a reaction of spontaneous252Cf ternary fission.This choice was determined by the fact that this reaction is well studied and the available data can be employed for the performance check of this facility. So,a252Cf layer5mm in diameter with an activity of15fissions/s was placed inside the spectrometer.The measurements were carried out continuously1.5 months.A2D spectrum of the scintillator signals obtained in coincidence with fragments is shown here in Fig.8(see Fig.9in Ref.[24]).Analyzing spectrum in Fig.8has shown that,in the space between the elec-tron and proton regions there are events that cannot be explained by an external background or as effect of neutrons.A detailed analysis of the parameter SF does not allow us to assign these events to protons or electrons.We estimate these events as an indication of the possibility of existence of a ternary nuclear decay with the participation of charged mesons.Subsequently,it is suggested to study the meson emission during the spontaneousfission on this new facility which has a number of substantial advantages.4SUMMARY AND CONCLUSIONSThe experimental and theoretical results obtained on the NPIR can be summa-rized as follows:1.The best experimental limit forπ-yields has been reported for252Cf by Bellini et al.[19].They reached an upper limit of3·10−13,a value close to the theoretical prediction[8].2.The unusual background,observed by Wild et al.[33]in(∆E−E)-energy region below that characteristic for long range alpha emission from257F m was interpreted by Ion,Bucurescu and Ion-Mihai[9]as being produced by negative pions emitted spontaneously by257F m.Then,they inferred value of the pionic yield is:Γπ/ΓSF=(1.2±0.2)·10−3forπ−/fission.In a similar way,Janko and Povinec[22],obtained the yield:Γπ/ΓSF=(7±)·10−5forπ+/fission.3.The supergiant radiohalos(SGH),discovered by Grady and Walker[34] and Laemmlein[35]are interpreted[12]as being theπ−-radiohalos andπ+-radiohalos,respectively.Hence these radiohalos are experimental signatures of the integral record in time of the natural pionic radioactivity from radioactive inclusions in ancient minerals.4.A new interpretation of the experimental bimodalfission is obtained in terms of the unitarity diagrams.is obtained in Ref.[29]So,the presence of the symmetric mode in the fragment mass-distribution at transfermium nuclei can be interpreted as experimental signature of the pionic radioactivity.Hence,it8is expected that this new degree of nuclear instability can becomes dominant at SHE-nuclides.This idea is illustrated not only by the symmetrization of frag-ment mass-yields from SF of the heavy nuclei with98≤Z≤104experimentally observed in Fig.7but also by high values ofΓπ/ΓSF-yields obtained by using the theoretical prediction(38)-(39).5.The nuclides No with A=242−250are expected to be allπF-nuclides while the No-isotopes with A≥252are all SF-nuclides(see Table2a).High pionic yields are obtained for250No(6.67)as well as for258No(7.1·10−2)and 262No(1.2·10−2).6.The nuclides Lr and Rf are all predicted to be SF-nuclides(see Table 2a).7.The nuclides258Sg(1.3·107),259Sg(6.1·106),260Sg(1.2·108),261Sg (1.3·1011),263Sg(5.2·1027)as well as264Hs(1.2·102)all are predicted to be πF-nuclides.Finally,we note that dedicated experiments,using nuclei with theoretically predicted high pionic yields(e.g.258F m,259F m,258No,260No,254Rf,Sg and264Hs(1.2·102)),are desired,since the discovery of the nuclear pionic radioactivity is of fundamental importance not only in nuclear science(e.g., for clarification of high instability of SHE-nuclei)but also for geochemistry and cosmology.References[1]H.Yukawa,Proc.Phys.Math.Soc.Japan.17,48(1935).[2]tes,G.P.S.Occhiallini and C.F.Powell,Nature,100,453(1947).[3]Particle Data Group[4]Delta Cenni etc.Vezi Rep in st year...[5]D.B.Ion,R.Ion-Mihai,M.Ivascu,Ann.of Phys.(N.Y.)171,237(1986),and IPNE-preprint FT-266-1985,Bucharest1985.[6]D.B.Ion,R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.31,205,209,551(1986).[7]D.B.Ion and R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.32,299and1037(1987).[8]D.B.Ion and R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.33,1071and1075(1988).[9]D.B.Ion,D.Bucurescu and R.Ion-Mihai,Rev.Roum.Phys.34,261(1989).[10]D.B.Ion and R.Ion-Mihai,Rev.Roum.Phys.36,163and261(1991).[11]D.B.Ion,Rev.Roum.Phys.37,347(1992).9[12]D.B.Ion and R.Ion-Mihai.Phys.Lett.B338(1994)7.[13]D.Bucurescu et al.,Rev.Roum.Phys.32,849(1987).[14]M.Ivascu et al.,Rev.Roum.Phys.,32,937(1987).[15]J.R.Beene,C.M.Bemis,Jr.and M.L.Halbert,Phys.Rev.C38,569(1988).[16]S.Stanislaus,D.S.Armstrong and D.F.Measday,Phys.Rev.C39,295(1989).[17]C.Cerruti,J.M.Hisleur,J.Julien,L.Legrain,Y.Cassagnou and M.Ribrag,Z.Phys.A329,383(1989).[18]J.Julien et al.,Z.Phys.A332,473(1989).[19]V.Bellini et al.,Proceedings14-th Europhys.Conf.,Rare Nuclear Pro-cesses,Bratislava,22-26October1990,(Ed.P.Povinec),p.116.[20]J.N.Knudson,C.L.Morris,J.D.Bowman,W.C.Sailor and S.J.See-strom,Phys.Rev.C44,2869(1991).[21]Yu.V.Adamchuk,P.E.Vorotnikov,M.A.Voskanyan and G.V.Muradyan,Sov.J.Nucl.Phys.49,932(1989).[22]K.Janko and P.Povinec,Proceedings14-th Europhys.Conf.,Rare NuclearProcesses,Bratislava,22-26October1990,(Ed.Povinec),p.121.[23]H.Otsu,N.Sakamoto,H.Okamura and H.Sakai,Z.Phys.A342,(1992)483[24]Khryachkov et al.,Instr.Exp.Tech.45,615(2002).[25]D.B.Ion,R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.31,209(1986).[26]D.B.Ion and R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.33,109(1988).[27]D.B.Ion and R.Ion-Mihai and M.Ivascu,Rev.Roum.Phys.34,461(1989)..[28]D.B.Ion,A.C.D.Ion and R.Ion-Mihai,Rev.Roum.Phys.38,3(1993).[29]D.B.Ion and R.Ion-Mihai,Rom.Journ.Phys.43,179(1998).[30]D.B.Ion and R.Ion-Mihai,Rom.Journ.Phys.44,703(1999).[31]D.B.Ion,Rev.Roum.Phys.37,347(1992).[32]N.E.Holden and D.C.Hoffman,Pure Appl.Chem.,72,1525(2000).[33]J.F.Wild et al.,Phys.Rev.C32,488(1985).10[34]C.A.Grady and R.M.Walker,Proceedings Intern.Symposium on Super-heavy Elements,Lubbock,Texas,March9-14,1978,(Ed.M.A.K.Lodhi),Pergamon,New York(1978),p.191.[35]emmlein,Nature,155,724(1945).Table1:Definitions and characteristic features of(SF,πF,T)-nuclides.Type of Mass Half LivesYield Z2/A<42.5Unitarity(MD)SF-nuclideΓSF<1T SF>T C SF D SFT-nuclideΓSF=1T SF=T C SF DπF+D SFπF−nuclideΓSF>1T SF<T C SF a new phaseΓSF yields for heavy parent nucleiwith Z between Z=100and Z=108,obtained forα=1by using Eqs.(38)-(40)and T SF from Ref.[32].A T SF/T C SFΓπ0/ΓSFF m2422.259·10+003.30·10−021001.046·10−108.59·10−043.85·10−04 2453.080·10+049.32·10−14 1004.753·10−071.17·10−125.16·10−14 2481.017·10+081.48·10−18 1008.000·10−013.28·10−211.54·10−23 2521.114·10+136.12·10−23 1006.242·10−019.36·10−171.41·10−18 2562.948·10+074.84·10−11 1001.310·10+022.75·10−147.93·10−16 2581.045·10+009.46·10−01 1004.753·10−085.44·10−023.91·10−02 2601.130·10+015.95·10−02Md2457.001·10−031.58·10+12 1016.338·10−091.73·10−011.42·10−01 2558.401·10+066.27·10−14 1016.338·10−091.37·10−126.16·10−14 2594.536·10+049.04·10−08Table2a(continued)A T SF/T C SFΓπ0/ΓSFNo2507.061·10−016.67·10+001023.169·10−073.33·10−093.69·10−10 2522.542·10+043.96·10−181029.126·10−041.02·10−241.91·10−27 2561.525·10+061.64·10−161025.324·10−053.53·10−102.97·10−11 2583.389·10+007.06·10−021021.141·10−031.68·10−111.06·10−12 2602.994·10+022.08·10−051021.584·10−101.30·10−027.94·10−031034.183·10−061.39·10−252.14·10−28 2551.680·10+051.77·10−281036.274·10−051.25·10−161.96·10−18 2592.411·10+025.09·10−081037.415·10−052.85·10−111.84·10−121041.521·10−121.16·10−646.44·10−72 2546.496·10−022.96·10−691049.190·10−081.81·10−755.87·10−84 2561.751·10+012.47·10−241041.065·10−056.30·10−392.81·10−43 2583.954·10+018.74·10−141041.331·10−062.13·10−163.55·10−18 2605.648·10+017.16·10−101042.091·10−058.73·10−171.31·10−18 2625.931·10+033.27·10−15DbπF2556.223·10+014.08·10+11SgπF2588.190·10+001.27·10+041067.605·10−084.36·10+062.47·10+07πF2601.977·10+013.24·10+081068.239·10−087.76·10+101.39·10+12πF2632.100·10+015.17·10+271064.119·10−071.01·10−914.69·10−1021073.803·10−097.93·10−017.72·10−011086.338·10−111.08·10+021.84·10+02 2653.734·10−024.25·10−051083.169·10−093.84·10−013.44·10−0112Table2.b:Theoretical predictions of theΓπ0,±/ΓSF yields for heavy parentnuclei with Z between Z=100and Z=108,obtained forα=0.75by using Eqs.(38)-(40)and T SF from Ref.[32].A T SFΓπ−Γπ0Γπ+Cf2385.931·10+014.47·10−03 981.008·10−047.44·10−091.39·10−09 2424.148·10+092.03·10−10 981.800·10+031.39·10−121.20·10−13 2482.852·10+152.83·10−12 988.000·10+107.33·10−153.97·10−16 2501.515·10+153.46·10−11 988.600·10+013.74·10−096.58·10−10 2541.486·10+108.32·10−07 982.282·10−052.41·10−041.14·10−04 2531.546·10+143.18·10−11 992.600·10+035.73·10−091.04·10−09 1002.535·10−111.39·10−011.17·10−01 2449.320·10+001.67·10−02 1001.255·10−042.74·10−085.76·10−09 2464.236·10+041.15·10−07 1001.141·10−033.91·10−114.57·10−12 2507.130·10+109.94·10−13 1001.250·10+028.65·10−145.84·10−15 2545.563·10+103.47·10−10 1003.308·10−047.79·10−072.21·10−07 2573.216·10+109.03·10−09 1001.172·10−119.67·10−019.64·10−01 2591.167·10+011.76·10−01 1001.268·10−101.86·10−011.60·10−01 1012.852·10−111.02·10+074.33·10+07 2471.556·10+003.64·10−01 1013.422·10−021.84·10−083.73·10−09 2571.546·10+071.05·10−07 1011.848·10−046.98·10−052.96·10−05 1027.922·10−122.97·10+003.28·10+00 2517.779·10+011.31·10−05 1022.852·10−079.10·10−111.14·10−11 2548.134·10+071.26·10−14 1021.711·10−056.61·10−109.95·10−11 2571.307·10+043.11·10−06 1023.803·10−112.12·10−011.85·10−01 2592.800·10+055.01·10−07 1023.359·10−091.80·10−031.02·10−03 2621.412·10+017.79·10−02Table2.b(continued)A T SFΓπ−Γπ0Γπ+Lr2531.027·10+035.94·10−15 1036.845·10−041.12·10−164.17·10−18 2571.540·10+046.66·10−10 1039.823·10−075.96·10−052.49·10−05 2611.820·10+047.39·10−07Rf2533.734·10−046.18·10−37 1047.288·10−131.26·10−394.15·10−43 2552.256·10+013.04·10−43 1041.965·10−103.49·10−142.17·10−15 2572.614·10+031.50·10−22 1044.436·10−103.34·10−087.16·10−09 2593.267·10+021.02·10−09 1046.338·10−105.54·10−061.87·10−06 2615.134·10+035.64·10−10 1046.654·10−084.38·10−097.82·10−10 1052.535·10−073.41·10+061.31·10+07 2576.223·10+016.00·10+11 1053.169·10−072.93·10−205.21·10−22 2633.734·10+022.60·10−12Sg2588.190·10+001.78·10+02 1067.605·10−086.47·10+031.42·10+04 2601.977·10+014.86·10+04 1068.239·10−081.89·10+066.93·10+06 2632.100·10+012.31·10+15 1064.119·10−071.22·10−532.22·10−58 1073.803·10−098.76·10−018.66·10−01 1086.338·10−111.42·10+011.81·10+01 2653.734·10−024.15·10−03 1083.169·10−095.78·10−015.51·10−0114Table3:Experimental results on spontaneous NPIR-yieldsYear LaboratoryΓπ0/ΓSFnucleiD.B.Ion et al.259Md<10−5Romania1987IFIN-RomaniaRev.Roum.Phys.32,849M.Ivascu et al.252Cf<10−8Romania1988CEN-Saclay<10−8Z.Phys.A329,383J.R.Beene et al.252Cf1989CEN-Saclay France<10−12Z.Phys,A332,473Yu.Adamchuk et al.252Cf<5·10−8 1989IFIN-BucharestRev.Roum.Phys.34,261·10−3(π−) S.Stanislaus et al.252CfColumbia Canada1989University British<3.1·10−4 Phys.Rev.C39,295J.N.Knudson et al.252Cf1991CEN Saclay France<3·10−13 Bratislava ConferenceK.Janko et al.[22]Proc.257Fm<7·10−5Czechoslovakia1992Tokio UniversityZ.Phys.A342,483(π−)Khryachkov et al.252Cf NotRussiaFigure1:The energies Qπ=QπF calculated by using Eq.(3)and Tables of Wapstra and Audi.16Figure 2:The physical regions from the plane (A,Z)in which the parent nu-clei are able to emit pions by tunnelling pi-fission barr ˆier (++++)and without tunnelling πF −barrier (oooo)(see the text).Figure 3:Definition of πF −barrier as a function of distance between the frag-ments during the pionic fission.17Figure4:The values of the barrier height EπF(LD)for the neutral pion emission during spontaneousfissionfission.Figure5:The nuclear configuration at the saddle point for spontaneousfission (F),spontaneous hyperfission(HF)and spontaneous pionicfission(πF).18Figure6:The experimental values ofτ(θ)are compared wit the linearfit(35).Figure7:The predicted values ofΓπ0Figure8:The experimental results(see Fig.9in Ref.[27])on the light charged particle emission during the spontaneousfission of252Cf.20。