When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning
; this phenomenon has been a mystery in nuclear physics for over 40 years
. The internal ...generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum
. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors
, for the study of the structure of neutron-rich isotopes
, and for the synthesis and stability of super-heavy elements
.
The proton drip-line nucleus 17Ne is considered a good candidate for a Borromean two-proton halo with a 15O + p + p structure. Angular distributions of the elastic scattering and inclusive 15O ...production for a 136 MeV 17Ne beam incident on a 208Pb target were measured for the first time at the SPIRAL1 facility, GANIL. Use of the GLORIA detector array allowed high-resolution data over a wide angular range from 20∘ up to 95∘ in the laboratory frame to be obtained. The elastic scattering angular distribution shows similarities with those for both 6He and 20Ne at equivalent collision energies with respect to the corresponding Coulomb barriers, exhibiting the suppression of the Coulomb rainbow peak characteristic of strong coupling. Optical model and coupled channel fits suggest that this is due to a combination of coupling to low-lying quadrupole resonances and Coulomb dipole coupling to the low-lying continuum, although their relative importance depends on the relevant B(E2) values which remain to be firmly determined.
Following work done in the energy region above 100 keV, the high-precision calibration of a co-axial high-purity germanium detector has been continued in the energy region below 100 keV. Some of the ...previous measurements and Monte-Carlo simulations have been repeated with higher statistics and a new source measurement with 169Yb has been added. For the energy range from 40 keV to 100 keV, an absolute precision for the detection efficiency of ±0.2% has been reached, as previously obtained for energies above 100 keV. The low-energy behaviour of the germanium detector was further scrutinised by studying the germanium X-ray escape probability for the detection of low-energy photons. In addition, one experimental point, a γ ray at 2168 keV from the decay of 38K, has been included for the total-to-peak ratios agreeing well with simulations. The same γ ray was also added for the single- and double-escape probabilities. Finally, the long term stability of the efficiency of the germanium detector was investigated by regularly measuring the full-energy peak efficiency with a precisely calibrated 60Co source and found to be perfectly stable over a period of 10 years.
This contribution presents a precise measurement of the excitation energy of the lowest (0+, T=2) state in 32S. Combined with the mass excesses of the 32S ground state and of the four other members ...of the (A=32,T=2) quintet of analogue states, it allows to test the validity of the Isobaric Multiplet Mass Equation to the third order in Tz, which renders it highly sensitive to the mechanisms inducing isospin mixing in the involved sd-shell nuclei. The (0+, T=2) isobaric analogue state in 32S was resonantly populated in the 31P(p, γ) reaction at ~3.3 MeV incident energy. The measurement procedure, involving high-purity germanium detectors, is described and the preliminary result obtained with a digital data acquisition system is presented.
Commissioning of the DESIR high-resolution mass separator Michaud, J.; Alfaurt, P.; Balana, A. ...
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
08/2023, Letnik:
541
Journal Article
Recenzirano
DESIR is, together with S3-LEB, the low-energy part of the SPIRAL2 ISOL facility at GANIL. The High-Resolution mass Separator (HRS) included in DESIR is a 180° symmetric online separator with two 90° ...magnetic dipole sections arranged with electrostatic quadrupoles, sextupoles and a 48-pole electrostatic multipole on the mid plane. The HRS is now completely mounted at LP2i Bordeaux and under commissioning for the next years before its transfer to the entrance of the DESIR facility. Optical aberrations, mainly introduced by the dipoles, must be corrected up to the highest possible order to guarantee an optimal resolution of the separator. They are measured with a pepperpot-type emittance-meter, analyzed then corrected with the multipole. Up to now, 2nd order (hexapolar) and part of 3rd order (octupolar) aberrations are under control and an optimal FWHM separation has been achieved for two identical beams with a relative energy difference of ΔE/E(≡ΔM/M)=1/23400.
In this paper, we present the effects of optical aberrations on the beam and its emittance figure, as well as the effect of the associated corrections with the multipole. Finally, we will show the latest resolution measurements and associated methodology.
The article describes the commissioning and technical development of the Weak Interaction Studies with 32Ar Decay (WISArD) experiment, installed at the radioactive ion-beam facility ISOLDE/CERN. The ...experiment aims to extend the present limits on scalar and tensor currents in the weak interaction and hence search for physics beyond the Standard Model. The evaluation of these limits relies on measuring the proton energy in beta-delayed proton emission, sensitive to both the beta-neutrino angular correlation coefficient aβν and the Fierz interference term b. The method tries to improve previous studies by considering the positron-proton coincidences when determining the kinematic shift in the energy of the emitted protons. Using this coincidence technique, the aβν and b coefficients will be measured at the per mil level. Simulations were employed to optimize the ion beam transport efficiency and validate proof-of-principle results obtained in November 2018 (Nov2018). Upgrades are ongoing, and we are looking into improvements to the overall performance of the setup.
The population of isomeric states in the prompt decay of fission fragments-so-called isomeric yield ratios (IYRs)-is known to be sensitive to the angular momentum J that the fragment emerged with, ...and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete gamma rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of 134Te, strongly populated in actinide fission, from the three different fissioning systems: 232Th(n, f), 238U(n, f), at two different neutron energies, as well as 252Cf(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of 134Te are determined. The decay code TALYS is used in combination with the fission simulation code FREYA to calculate the primary fragment angular momentum from the IYR. We find that the IYR of 134Te has a slope of 0.004 +/- 0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with 134Te, it is revealed that the IYR of 134Te is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the FREYA+TALYS simulations reveal that the average angular momentum in 134Te following 238U(n, f) is 6.0 h over bar . This is not consistent with the value deduced from recent CGMF calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the gamma -ray heating problem of reactors.
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