M. Asghar

Fission-fragment energy correlation measurements for 252Cf(sf) and structures in far-out asymmetric fission

Abstract Fragment-mass and kinetic-energy distribution measurements for 252 Cf(sf) have been extended to the far-out asymmetric low-yield fission region of M H = 190 compared with the existing data that go up to M H ≈ 168. The mass yield Y ( m ) shows a shoulder for M H ≈ 163 and an important enhancement for M H ≳ 176. The 〈 E K 〉( m ), σ E K ( m ) and, to a lesser extent, Y ( m ) distributions show strong and correlated amplitude fluctuations for M H ≳ 170. These data indicate strong and rapid changes in deformation of the nascent fragments and the coexistence of more than one deformed-shell stabilized scission configuration for the same mass splits. These structures and those present in the relatively high-yield regions are discussed in terms of the static macro-microscopic potential-energy calculations.

High resolution measurements of mass, energy and nuclear charge correlations for 229Th(nth, f) with the cosi fan tutte spectrometer

Abstract The COSI FAN TUTTE spectrometer has been used to measure for the first time in one go the mass-charge-energy correlations for well-resolved light group masses for 229 Th(n th , f). About 4.8 × 10 5 events were collected in a ≈ 3 months run. The global mass distribution shows structures and the distribution for E L = (112±0.5) MeV is dominated by mass numbers A L = 86 and 96. The mean kinetic energy 〈E L 〉 shows strong fluctuations as a function of the light fragment mass A L . The mean proton odd-even effects in yields and energy are д p = (40.0±1.0)% and д E o-e TKE = (1.35±0.17) MeV , respectively. The δ p(E L ) varies by a factor of ≈ 3 for E L = 02 to 107 MeV with a plateau for E tL ≈ 96 to 103 MeV. The mean neutron odd-even effect is δ n = (1.95±0.5)%). The mean value of the conditional variance is 〈σ 2 ( Z ¦ A )〉 = (0.33±0.02) and it varies little with energy. The conditional variance seems to decrease as one goes from 252 Cf to 230 Th. These data are discussed in the context of existing results on other nuclei.

Fission-fragment energy correlation measurements for (nth, f) of 232U and 233U

Fission-fragment mass and kinetic energy distributions and their correlations have been measured for 232U and 233U. The results on these uranium isotopes and 235U are compared. The mass peak/valley ratio of 785 ± 68 for 232U is the highest of the three isotopes. The 〈EK〉(μH) distributions show significant differences. The dip ΔEK at symmetry is 16.2 ± 1.0 MeV (232U), 17.0 ± 1.0 MeV (233U) and 20.6 ± 1.1 MeV (235U). In the yields for high-kinetic-energy-selected events, the mass 134 dominates for 233U and 235U, but for 232U it is μH≈144, which dominates. This complete reversal of profiles can be understood in terms of fragment shells.

A multi-parameter study of LRA fission and binary fission of 235U with thermal neutrons

Abstract The α-particle ternary fission (LRA) and the binary fission of 235 U with thermal neutrons have been studied with a multiparametric system. The aim was to get significant results ove: the whole mass range including the symmetric and the far-out asymmetric regions, where the yields are low and the contamination due to the spurious events is high. A technique has been used to eliminate the spurious events. Results are presented on the energy and the mass distributions in LRA fission and binary fission. A difference of 2.6 ±0.5 MeV is found in the mean total fission fragment kinetic energy for these two modes of fission. The α-particle probability P α (M) is calculated under certain assumptions and compared with the prompt neutron ν(M) and the prompt γ-ray Nγ(M) yield data. Results are presented on P α (R) . There exists an anti-correlation between E¯ α and E T : ∂Ē α /∂E T = -0.105±0.01, and between E¯ T and E α : ∂Ē T /∂E α = -0.45±0.06. An effort is made to explain these various results either in a quantitative or in a qualitative way.

The 89Y(p, γ) 90Zr, 88Sr(p, γ)89Y AND 86Sr(p, γ)87Y reactions with 4 MeV protons

Abstract The reactions 89Y(p, γ) 90Zr, 88Sr(p, γ)89Y and 86Sr(p, γ)87Yhave been studied at 3930 keV proton energy. A pair spectrometer consisting of two NaI(Tl) crystals and a Ge(Li) diode was used to measure theθ-ray spectra. Six, four and five high-energy θ-ray transitions were observed respectively in 90Zr, 89Y and 87Y. The energies of these transitions, the proton separation energies Sp for each of these nuclei and proton-proton pairing energies δp for Z = 40 have been determined with high precision. The relative intensities and the values of absolute capture cross sections for various transitions were also determined. To interpret the results on 90Zr, it is necessary to assume the existence of 2p-2h configurations in the capturing 1− S = 0, L = 1; S = 1, L = 1 giant resonances. A value of 3.9 for the 2p-2h amplitude to the 1p-lh amplitude is determined. The 2+ levels at 3293 keV and 3827 keV in 90Zr are proposed to have respectively the [(2 p 3 2 ) p −2 (lg 9 2 ) p 2 ] [2+] and if[(1 f 5 2 ) p −2 (1 g 9 2 )] [2+] configurations. In 90Zr s-wave proton capture dominates, therefore it is essential to invoke the existence of both s-wave and d-wave capture for 89Y (for 87Y perhaps, also) in order to explain the results. It is suggested that this important d-wave capture may be due to the presence of the wing of a proposed proton resonance at Ep ≈ 5 MeV, which should be an analogue to the ground state of 89Sr.

A high-resolution multi-parametric study of 239Pu(nth, f) with the Cosi-Fan-Tutte spectrometer

Abstract The Cosi-Fan-Tutte spectrometer has been used to measure for the first time in one go the mass-nuclear charge-energy correlations of well-resolved light group masses for 239 Pu(n th , f). About 2 × 10 6 events were collected and analysed. The global mass distribution is consistent with the Lohengrin data. The mean proton odd-even effects in yields and energy are δ p =(11.8±0.3)% and δE K o −e=0.49±0.15 MeV respectively. The δ p ( E L ) varies from ≈7% to ≈25% for E L =90 to 116 MeV. The present analysis consists of 21 values of E L compared to only 5 values for Lohengrin. The apparent mean neutron odd-even effect δ n =(3.5±0.1)% is much lower than the δ n = (6.5±0.7)% for Lohengrin. The mean value of conditional variance 〈 σ z 2 ( Z | A )〉=(0.42±0.02); the 〈 σ z 2 ( Z | A )〉 increases from 230 Th to 250 Cf and this behaviour turns out to be important for understanding the dynamics of the fission process.

Fission fragment energy correlation measurements for the sub-barrier (nth, f) of 241Am and 243Am

Abstract Fission-fragment mass and kinetic-energy distributions and their correlations have been studied for the sub-barrier fission of the odd-odd 242 Am and 244 Am nuclei resulting from thermal neutron capture in 241 Am and 243 Am. Unwanted events were eliminated by a coherence test based on the time of flight. The 243 Am mass distribution is more asymmetric and shows structures at μ H ≈ 139 and 144, compared with that for 241 Am which is smooth and structureless. The structure at μ H ≈ 144 is caused by deformed neutron shells at N ≈ 88 in the heavy fragment and N ≧ 60 in the light fragment. While the 〈 E K 〉 at symmetry for 241 Am is ≈ 19 MeV higher than that for 235 U, it is shifted upwards only ≈ 6.5–10 MeV in the other mass regions. However, 〈 E K 〉 at symmetry for 243 Am is ≈ 6 MeV lower than for 241 Am and this decrease tapers down to ≈ 1 MeV for μ H > 135. These data show a decrease in the total fragment deformation for 241 Am at symmetry as predicted by calculations. However, the 243 Am data show a sudden change back to higher deformation.

Measurement of cold fission for 229Th(nth, f), 232U(nth, f) and 239Pu(nth, f) with the Cosi fan tutte spectrometer

Abstract The light-fragment-group mass-energy correlations for 229 Th(n th , f), 232 U(n th , f) and 239 Pu (n th , f) measured with the Cosi fan tutte, have been used to determine the cold fission probability for these fissioning systems. These are the first results on cold fission for 229 Th(n th , f) and 232 U(n th , f). For 229 Th(n th , f) cold fission is realised for the whole mass range M L ≈ 80–99 present in the mass spectrum, but the coldest fission shows up for M L ≈ 85. For 232 U(n th , f) cold fission is present for M L ≈ 84–107, with the coldest fission manifesting for M L ≈ 100–105. In the case of 239 Pu(n th , f), cold fission exists for M L ≈ 95–112 and here the coldest fission is produced for M L ≈105–110. This systematic study shows that the shells in the nascent fragments seem to play a decisive role in the realisation of cold fission. These results are discussed in terms of the existing concepts and ideas on this phenomenon.

Structures in far-out asymmetric mass distributions in low energy fission

Abstract The far-out asymmetric mass distributions for 235 U(n th , f), 239 Pu(n th , f), 243 Am(n th , f) and 252 Cf(sf) have been determined. The structures in these data and those in the existing results on 238 U, 234 Np and 235 Np have been analysed in a systematic way in terms of the static scission-point model of Wilkins et al . The model explains well all the structures seen in very asymmetric fission.

Fission-fragment energy correlation measurements for the spontaneous fission of 244Cm

Abstract Fission-fragment mass and kinetic energy distributions and their correlations have been measured for the spontaneous fission of 244 Cm. About 3.54 × 10 5 fission events were collected. The global mass distribution shows shoulders at μ H ≈ 134 and μ H ≈ 143−144. The peak/valley ratio is 86. The dip ΔE K value at symmetry is 13.4 ± 1.5 MeV. In the yields for high-kinetic-energy selected events, the masses 139 and 144 dominate. The results of 244 Cm are compared with the other isotopes of Cm and discussed in terms of the structures present in the potential energy surface of the fissioning system caused by the structures in the nascent fragments.