Cyrillus Wagemans

Fission fragment energy correlation measurements for the thermal neutron fission of 239Pu and 235U

Abstract Fission fragment mass and kinetic energy distributions and mass-versus-energy correlations have been measured for the thermal neutron induced fission of 239Pu and 235U. Unwanted events were eliminated by a coherence test based on time of flight. The global mass distribution for 239Pu shows much weaker fine structure than 235U. However, for events with high total kinetic energy this fine structure becomes more prominent and the yield of mass 135 dominates for 239Pu compared to mass 134 for 235U. Moreover, a prolate deformation for mass 102 in the 134 102 mass ratio for 235U is compatible with the data. The results show that EK(μH), hence EL and/or EH for 239Pu and 235U in the high mass yield regions have low energy tails produced in the act of fission. Furthermore, the data suggest that most of the proton pairs are broken after the effective determination of the structure of the nascent fragments and probably just before and/or in the act of scission itself.

Measurement and resonance analysis of the 33S(n, α) cross section

Abstract The 33 S(n, α ) cross section has been measured for neutron energies up to 1 MeV. The neutron flux has been determined by the 6 Li(n, α) and 235 U(n, f) reactions. A resonance analysis has been performed up to 400 keV neutron energy which yields α-widths Γ α or values of gΓ n Γ α / Γ for 16 resolved resonances. Average resonance parameters are discussed and the maxwellian averaged (n, α) cross section is obtained for the temperature range from 10 to 200 keV.

Energy distributions and yields of long range alpha particles emitted during spontaneous fission of 238,240,242,244Pu isotopes

Abstract The energy distribution and yield of Long Range Alpha (LRA) particles emitted during spontaneous fission of 238,240,242,244Pu-isotopes have been measured. Correlations between these experimental results with the total average excitation energy 〈TXE〉 the α-cluster preformation probability Sα and the fission modes have been shown. This permitted us to demonstrate that the LRA emission probability is strongly influenced by Sα and also that it is very much enhanced for the (more deformed) Standard II fission mode compared to the (more spherical) Standard I mode.

Fission fragment energy correlation measurements for the thermal neutron sub-barrier fission of 237Np and above-the-barrier fission of 235U

Fission fragment mass and kinetic energy distributions and mass-versus-energy correlations have been measured at the Grenoble high-flux reactor for the thermal neutron induced sub-barrier fission and above-the-barrier fission of 237Np and 235U, respectively. Unwanted events were eliminated by careful timing. In the case of 237Np(nth, f), the two odd particles (one proton and one neutron) are, on the average, shared out equally between the two complementary fragments leading to 〈μH〉, being 1 amu higher than the value for the neighbouring even-even fissioning nuclei. The peak mass yield for 237Np is 5.7 % compared to 6.5 % for 235U. The peak valley ratios of mass distribution are 124 and 544 for 237Np and 235U, respectively. The mass distribution for 237Np is wider than for 235U and broadens more towards more asymmetric fission than towards symmetric fission. The global mass distributions for 237Np and 235U show fine structures. For events with high total kinetic energy, the yield of mass around 136 seems to dominate for 237Np compared to mass 134 for 237U. The periods of the fine structures seem to be ≈ 2.5 amu and 5 amu for 237Np and 235U, respectively. The 〈EK〉 for 237Np is about 4 MeV less than the expected value. It is, respectively, ≈ 0, 5 and 1.5 MeV less for the mass regions μH≈ 119–124, 124–135, and 135 and beyond. It is shown that these results are qualitatively consistent with Norenbergs model. However, it is shown that this model does not explain these results quantitatively.

Energy and mass distributions for 241Pu(nth, f), 242Pu(s.f.) and 244Pu(s.f.) fragments

The energy and mass distributions and their correlations have been studied for the spontaneous fission of 240 Pu, 242 Pu and 244 Pu and for the thermal neutron induced fission of 239 Pu and.241 Pu. A comparison of the 242 Pu(s.f.) and the 241 Pu(nth,f) results shows a narrower mass distribution, a much higher peak yield and a more pronounced fine structure for the spontaneous fission than for the neutron induced fission. Also the average masses differ by 2 mass units. The average total kinetic energy is higher for 242 Pu(s.f.) than for 241 Pu(nth,f), and also the energy-mass correlations behave differently in both cases. Preliminary results seem to indicate that the fissioning system 240 Pu behaves in a very similar way as 242 Pu. Also the 244 Pu(s.f.) data are very similar to these for 242 Pu(s.f.). All these results are discussed and interpreted in the frame of the scission point model of Wilkins et al.

Determination of the 35Cl(n, p)35S reaction cross section and its astrophysical implications

Abstract The 35 Cl(n, p) 35 S reaction cross section has been measured with thermal neutrons at the High Flux Reactor of the ILL (Grenoble) and with resonance neutrons at the linear accelerator GELINA of the IRMM (Geel). For the thermal cross section, a precise value of 440 ± 10 mb was obtained. In the region up to about 100 keV neutron energy, several resonances were determined with high resolution. These data were used to calculate the maxwellian averaged cross section at various temperatures. Their impact on stellar-nucleo-synthesis calculations is discussed.

Energy distributions and absolute yields of the charged light particles emitted during the thermal neutron induced ternary fission of 235U

Abstract The energy distributions and the yields of the main charged light particles emitted during the thermal neutron induced ternary fission of 235U were studied using a very pure neutron beam of the Grenoble high-flux reactor. All the measured energy distributions except that of the α-particles are compatible with a gaussian shape. Explanations for this phenomenon are presented. For the main ternary particles following absolute yields were determined: protons 0.016 × 10−3, tritons 0.106 × 10−3, alphas 1.696 × 10−3 and 6He 0.029 × 10−3 particles per fission reaction. These results are discussed and compared with other data.

Study of some characteristics of the thermal-neutron-induced sub-barrier fission of 231Pa, 232Th and 237Np

Abstract Some characteristics of the thermal-neutron-induced fission of 231Pa, 232Th and 237Np were studied using a highly pure thermal-neutron beam of the Grenoble High Flux Reactor. For the thermal-neutron-induced-fission cross section we obtained 20 ± 4 mb for 237Np, 19 ± 4 mb for 231Pa and less than 2.5 μb for 232Th, in agreement with calculations based on a double humped fission barrier. We also determined a mean total fission fragment kinetic energy of 172.8 + 2.5 MeV for 237Np, which fits into the semi-empirical systematic of E K versus Z 2 /A case1 3 . Finally, we determined LRA/B ratios of (1.94±0.17) × 10−3 for 237Np and (1.67±0.11) × 10−3for 231Pa, in disagreement with the theoretical predictions of Feather, but consistent with the semi-empirical systematic of Halpern.

Study of neutron induced charged particle reactions on 40K: (II). Resonance neutrons

Abstract The 40 K(n, α) 37 Cl and 40 K(n, p) 40 Ar reaction cross sections were measured in the neutron energy region from 0.02 eV up to about 70 keV. The proton and α-particles were detected with a large surface barrier detector, and the neutron energy was measured by time of flight. The reaction cross sections show a pronounced resonance structure. A comparison of the density of s- and p-wave resonances (compound spins between 5 2 and 11 2 in 40 K + n to the density of spin − 5 2 compound states observed as resonances in the reaction 40 Ar + p by Keyworth et al. , allows a direct determination of the spin cut-off parameter of the level density.

Experimental study of some important characteristics of the thermal neutron induced fission of 237Np

Abstract Fission fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of 237Np. The global mass distribution is rather smooth, apart from a weak shoulder at μH = 140–141. When low excitation events are selected, fine structures associated with the charge of the fragments are observed. Furthermore, there is a sudden increase in Ek for μH > 155, which is probably due to a spherical shell N = 50 in the light fragment and the corresponding deformed (but stable) heavy fragments with masses in the rare earth region. For the average (pre-neutron emission) total fragment kinetic energy, a value of 176.4 ± 0.6 MeV has been obtained, in agreement with the systematics. Also the prompt neutron emission curve v(m ∗ ) has been calculated, which shows the well-known saw-tooth shape. Finally, the energy distribution and the emission probability of the ternary α-particles have been determined.