D. De Frenne

A decomposition study of the EPR spectrum of irradiated sucrose

In general, the EPR spectra of irradiated sugars are very complex because of their multicomponent character. In this study we applied a multivariate statistical method called MLCFA, maximum likelihood common factor analysis, and it predicted at least six components contributing to the total EPR spectrum of irradiated sucrose. Three dominant components have already been isolated in an irradiated sucrose single crystal using electron nuclear double resonance (ENDOR) and ENDOR induced EPR (EI-EPR). Results of EPR simulations based on the ENDOR data are in a reliable agreement with the experimental EPR spectra of irradiated sucrose single crystals.

Excitation energy dependence of charge odd-even effects in the fission of 238U close to the fission barrier *

Abstract Independent yields of the heavy fragments have been determined for the photofission of 238 U at excitation energies in the barrier region. Bremsstrahlung with six different endpoint energies between 6.1 and 11 MeV was used. The independent and cumulative yields of the fragments were measured using γ-spectrometry techniques. Fragment charge, isotopic mass, elemental yield distributions and proton and neutron odd-even effects are deduced from these independent yields. The experiments revealed a sizeable odd-even effect [ δ p = (29±2)%] in the element distributions, but on the other hand a small effect [ δ n = (3±2)%] in the isotonic distributions. The proton odd-even effect in the photofission of 238 U remains fairly constant for excitation energies up to about 7.85 MeV. When the excitation energy of the fissioning nucleus is raised slightly higher, the proton odd-even effect drops off quickly.

Fragment characteristics for the photofission of 238U with 6.1–13.1 MeV bremsstrahhmg☆

Abstract The photofission of 238 U has been studied experimentally for seven bremsstrahlung endpoint energies between 6.1 and 13.1 MeV. The postneutron yields of the fission products were obtained with γ-spectrometry techniques. Provisional mass and kinetic energy distributions were measured using a double energy detection technique. From the combination of both methods average neutron emission curves and preneutron mass-energy distributions were deduced. The systematic trends of the fragment characteristics have been studied as a function of the compound nucleus excitation energy. The results are compared with expectations from the energy partition model of Ruben et al. They are also discussed in the framework of the scission point model of Wilkins et al., and of the multimode fission with random neck rupture model of Brosa et al. The mass and kinetic energy data can be represented well by the superposition of two dominant mass asymmetric fission modes (standard I and standard II), and one relatively weak mass-symmetric fission mode (superlong). The standard I mode yield diminishes slightly with increasing compound-nucleus excitation energy. Up to an excitation energy of 7.9 MeV, i.e. the fission barrier plus pairing gap, the average fission-fragment total kinetic energy increases, and the average total ‘asymptotic’ excitation energy and the charge odd-even effect in the element yields are about constant ( δ P = (29 ± 2)%). These results hint to low energy dissipation. At higher compound nucleus excitation energies the fragment kinetic energy decreases, their excitation energy increases, and the proton odd-even effect decreases exponentially.

Mass and kinetic energy distributions for the photofission of 232Th with 6.44 to 13.15 MeV bremsstrahlung

Abstract Post- and preneutron-emission mass and kinetic energy distributions of the fragments emitted in the photofission of 232 Th with 6.44, 7.33, 8.35, 9.31, 11.13 and 13.15 MeV have been studied. Energy correlation and γ-spectrometric measurements were performed. Sb, Ru and Cd were separated chemically to determine postneutron yields in the symmetric mass region. The 232 Th system predominantly splits in an asymmetric way with a maximum yield for heavy fragments in the region of mass 140. An enhanced yield around heavy mass 134 is observed, becoming of increasing importance with increasing compound nucleus excitation energy. For 6.44 and 7.35 MeV bremsstrahlung induced fission no symmetric component in the mass distribution could be observed. For the higher endpoint energies symmetric fission becomes more and more evident. From the symmetric fission yields at different excitation energies, using barrier penetration calculations, the height of the symmetric fission barrier is estimated to be of the order of 7.5 to 7.7 MeV. The total fragment kinetic energy shows a minimum for symmetric splits and a maximum for splits with heavy mass in the vicinity of mass 132. It increases with increasing excitation energy of the 232 Th compound nucleus. This effect is especially pronounced in the energy region just above the barrier. It is observed for all masses, but mass splits with heavy mass in the vicinity of mass 132 show the strongest effects. The fragment mass distributions for 232 Th(γ, f) show a clear difference when compared with those for α-particle accompanied fission of 235 U. Our results are interpreted in the framework of the Brosa fission channels model and in the scission point model. They also provide information concerning the dissipation of collective energy into the intrinsic degrees of freedom during the transition from saddle to scission point.

Study of the catcherfoil technique with the aid of252Cf s.f. and235U(nth, f)

Abstract The well-known post neutron mass distributions of 235 U(n th , f ) and 252 Cf s.f. were studied with the catcherfoil technique in order to investigate the possibilities of this technique for fission studies. A set of necessary spectroscopic data of the studied fission products was selected from the available literature. In the case of 235 U(n th , f ) and 252 Cf s.f. relative yields of respectively 37 and 43 masses could be determined. The agreement between the obtained results and those given in the literature is very good. In the case of 252 Cf s.f. the relative yields of seven mass chains were determined for the first time by another method than semiconductor detector measurements. At the same time the catcherfoil method was elaborated so that in a few cases we were able to measure fractional independent chain yields. To test this, the fractional independent chain yield of 134 I and 135 Xe was measured in both fissioning systems, mentioned above. Here too our results agree very well with the independent yields given in the literature.

Measuring the (d,He-2) reaction with the focal-plane detection system of the BBS magnetic spectrometer at AGOR

At intermediate energies, the (d, He-2) charge-exchange reaction can be used to observe Gamow-Teller strength in the direction. He-2 denotes the two-proton system being in the singlet S-1(0) state. In the present experiment the two protons, which in the laboratory frame are emitted into the forward direction, have been momentum analyzed and detected in coincidence by the same spectrometer and detector. Protons from deuteron breakup processes can induce a large accidental coincidence background because of the much larger breakup cross-section as compared to the (d, He-2) cross-section. Nevertheless, background-free He-2 spectra with a resolution of 145 keV at an incident energy of 170 MeV are obtained, allowing the identification of many levels with high precision in the residual nuclei. The essential features of the detection system and the data-acquisition and analysis techniques which make our (d 2 He) experiments possible are described. Two nuclei, C-12 and Mg-24, have been used as a test case

Observation of a 1− two phonon 2+ ⊗ 3− excitation in 116Sn and 124Sn

Abstract In nuclear resonance fluorescence experiments with unpolarized and linearly polarized bremsstrahlung we have observed for the first time very strong electric dipole transitions at about 3.5 MeV in the spherical, semi magic 116 Sn and 124 Sn nuclei. These transitions can be attributed to possible two phonon 2 + ⊗ 3 − excitations. The measured transition strengths are compared to the results of a QRPA calculation.

Polarised bremsstrahlung nuclear-resonance fluorescence set-up at the 15~MeV linac in Gent.

Abstract Nuclear resonance fluorescence experiments using unpolarised as well as off-axis linearly polarised bremsstrahlung represent an outstanding tool to determine in a completely model independent way transition probabilities, multipole orders and parities of electromagnetic transitions to bound states in nuclei. A new polarised bremsstrahlung facility has been constructed at the 15 MeV linac in Gent. The experimental arrangement is discussed and first results are presented.

Nuclear data sheets update for A = 110

Abstract The 1983 evaluation on mass A=110 has been revised, taking into account all data available before November 1991. Detailed experimental information is presented from the neutron rich nucleus 110 Mo to the neutron deficient 110 Xe nucleus.

Excitation energy dependence of fragment characteristics for the photofission of 232Th

Abstract Independent and cumulative product yields were measured for the photofission of 232Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported 232Th-samples. The independent heavy fragment yields for the fission of the 232Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems 232Th, 234U and 236Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.