Lise Meitner, Otto Hahn Nuclear Fission Hans-Jürgen Wollersheim
Details of the 252 Cf decay α s: 96.9% SF: 3.1% T 1/2 = 2.647 a Q α = 6.217 MeV E α = 6.118 MeV α α α α
α-decay of 252 Cf Mass data: nucleardata.nuclear.lu.se/database/masses/ BE( 252 Cf) = 1881.275 MeV BE( 248 Cm) = 1859.196 MeV BE( 4 He) = 28.296 MeV Q α = 6.217 MeV momentum conservation: m v k k = m α v α v k m = m α k v α 252 248 CCff 154 CCmm 152 + α 98 96 energy conservation: Q α k α = Ekin + Ekin mk 2 α = vk + Ekin 2 2 mk mα 2 = vα + E 2 2 mk mα α α = Ekin + Ekin mk mα + mk α = Ekin m k α kin EE αα mm kk kkkkkk = QQ αα = 6.217MMMMMM 248 mm kk + mm αα 252 = 6.118MMMMMM
Binary spontaneous fission of 252 Cf α s: 96.9% SF: 3.1% T 1/2 = 2.647 a parameter experimental value < A L > 108.9 ± 0.5 u <A H > 143.1 ± 0,5 u < E L > 103.5 ± 0.5 MeV < E H > 78.3 ± 0.5 MeV < TKE > 181.8 ± 0.7 MeV < A/Z > L 2.50 < A/Z > H 2.56 VV CC RR iiiiii = 1.44 43 55 14 TTTTTT = 0.1071 ZZ 2 1/3 CC AA CC = 244 MMMMMM + 22.3 = 185 MMMMMM
Ternary spontaneous fission of 252 Cf 252 Cf source T 1/2 = 2.645 y bin. fission/α-decay = 1/31 ter. fission/α-decay = 1/8308 photographic emulsion Tsien San-Tsiang, Phys. Rev. 71 (1947), 128
Tsien San-Tsiang, Phys. Rev. 71 (1947), 128 Quaternary spontaneous fission of 252 Cf
Details of the 252 Cf source
Spontaneous fission of 252 Cf 252 Cf spin J = 0 fragment spin J = < 7-8 > < N γ > = 9.35 The origin of fragment spins and their alignment: Collective vibrational modes like bending or wriggling at the saddle-to-scission stage and subsequent Coulomb excitation. Experimental method: fragment γ-ray angular correlation measurement K. Skarsvag Phys.Rev. C22 (1980) 638
γ-ray emission from aligned nuclei dddd = 2QQ + 1 ττ ddω rrrrrrrr 4ππ QQQ JJ ii FF QQ JJ ff, LL, LL, JJ ii PP QQ ccccccθθ γγγγ QQ=0,2,4 Legendre polynomials γ-γ correlation coefficients example: ττ 20 JJ = JJ JJ + 1 2JJ 1 2JJ + 3 1 2 KK 2 3 JJ JJ + 1 1 E2 E1 JJ 3 JJ + 1 3 ττ 40 JJ = 2JJ 3 2JJ 2 2JJ 1 2JJ + 3 2JJ + 4 2JJ + 5 KK 4 35 JJ 2 JJ + 1 2 30 KK 2 5 2 1 + 3 1 JJ JJ + 1 6JJ JJ + 1 JJ JJ + 1 1/2 KK nn = KK nn γγ KK wwwwwww γγ KK = eeeeee KK2 2σσ 2 KK eeeeee KKK2 2σσ 2 KK Gaussian distribution A.L.Barabanov IAE-5670/2 (93) S.R. de Groot & H.A. Tolhoek, Beta and gamma-ray spectroscopy, ed. K. Siegbahn, p 613 (1955)
γ-ray emission from aligned nuclei WW θθ = 2QQ + 1 ττ 4ππ QQQ JJ ii FF QQ JJ ff, LL, LL, JJ ii PP QQ cccccccc QQ=0,2,4 example: ττ 20 JJ = JJ JJ + 1 2JJ 1 2JJ + 3 1 2 KK 2 3 JJ JJ + 1 1 E2 E1 K-distribution at saddle point A.L.Barabanov IAE-5670/2 (93)
Anisotropy of γ-ray in binary fission of 252 Cf Darmstadt Heidelberg Crystal Ball fragment γ-ray angular correlation measurement ΔE γ = 90 kev (no discrimination between different multipole transitions) Yu.N.Kopatch et al. PRL 82(99),303 K.Skarsvag, PR C22(80),638
4π twin ionization chamber for fission fragments measured quantities: E H A H E L A L e - drift-time ϑ segmented cathode φ methane at 570 torr cathode diameter 15cm 252 Cf source (25k f/s) T 1/2 = 2.645 y E α = 6.118 and 6.076 MeV bin. fission/α-decay = 1/31 ter. fission/α-decay = 1/8308 M.Mutterer et al. Sanibel (97)
4π twin ionization chamber for fission fragments measured quantities: E H A H E L A L e - drift-time ϑ segmented cathode φ 252 Cf source (25k f/s) T 1/2 = 2.645 y E α = 6.118 and 6.076 MeV bin. fission/α-decay = 1/31 ter. fission/α-decay = 1/8308 M.Mutterer et al. Sanibel (97)
Fission fragment mass measurement The kinetic energies of the fragments are converted into ionization energy, and the fragments stop before reaching the Frisch grids. mm 1 vv 1 + mm 2 vv 2 = 0 mm 1 EE 1 = mm 2 EE 2 mm 1 = mm 1 + mm 2 EE 2 EE 1 + EE 2 EE LL = 103.5 ± 0.5 MMMMMM AA LL = 108.9 ± 0.5 EE HH = 78.3 ± 0.5 MMMMMM AA HH = 143.1 ± 0.5 m 1 + m 2 = 252
Fission fragment mass measurement <108.9 u> <143.1 u> mass resolution σ = 3 u
Determination of the polar angles ϑ l The anode time signals are caused by the first electrons which pass the Frisch grids and are thus linear dependent on bot the lengths of the fragment tracks and the cosine of the polar angle θ. dddddddddd tttttttt: TT = ss vv dddddddddd angular resolution ss = dd l EE, AA cccccc θθ cccccc θθ = dd TT vv dddddddddd l EE, AA drift velocity: v drift = 10 cm/μs range of fragments in methan gas: l(e,a) distance cathode-anode: d = 3.8 cm ϑ σ 30 0 4.2 0 50 0 2.5 0 70 0 2.3 0
Determination of the azimuthal angle The energy signal of cathode sections azimuthal angle φ energy signals of the four sectors depend on the orientation of the fission axis VV 13 = EE SSS EE SSS + EE SSS VV 24 = EE SSS EE SSS + EE SSS ttttttφφ = VV 24 0.5 VV 13 0.5
Determination of the azimuthal angle The energy ratios for different emission angles ϑ VV 13 = EE SSS EE SSS + EE SSS VV 24 = EE SSS EE SSS + EE SSS ttttttφφ = VV 24 0.5 VV 13 0.5
Experimental set-up 4 segmented Clover detector
Spectroscopy of binary fission fragments vv cc = 3.4 4.5 % θθ γγ = 18 0 ΔEE γγ EE γγ = 1% ε ph = 2.5% ΔEE γγ EE γγ = 1%
Analysis of the particle-γ angular correlation 10 0 θθ pp 80 0 34 0 θθ γγ 34 0
Spontaneous fission process of 252 Cf excitation energy of fission fragments 35 MeV evaporation of 3 neutrons
Fission fragment γ-ray angular correlation 44 + 22 + tttttttttttttttttttt no loss of alignment! crystal ball
Ternary spontaneous fission of 252 Cf
Ternary spontaneous fission of 252 Cf Fragments E H, E L, ϑ, φ LCPs E, ΔE, ϑ, φ γ-rays E, ϑ, φ 2 rings of ΔE-E telescopes
Separation of light charged particles simulation data
Summary γ-ray spectroscopy of fission fragments open fission source, Doppler-shift correction access to short-lived γ-ray transitions angular anisotropy of γ-rays angular anisotropy of individual γ-ray transitions - spin orientation - changes in the spin population between binary and ternary fission fragment LCP correlations - isotope yields of heavier LCPs - formation of LCPs in excited states - quaternary fission (emission of 2 LCPs) Soap Bubble Experiments (M. Schuyt, Seifenblasen, die Kugeln der Götter, 1988, Köln, Du Mont)