Search

Calorimetric low temperature detectors (CLTD’s) for heavy-ion detection have been combined with the LOHENGRIN recoil separator at the ILL Grenoble for the determination of nuclear charge distributions of fission fragments produced by thermal neutron-induced fission of $$^{235}$$ U. The LOHENGRIN spectrometer separates fission fragments according to their mass-to-ionic-charge ratio and their kinetic energy, but has no selectivity with respect to nuclear charges Z. For the separation of the nuclear charges, one can exploit the nuclear charge-dependent energy loss of the fragments passing through an energy degrader foil (absorber method). This separation requires detector systems with high energy resolution and negligible pulse height defect, as well as degrader foils which are optimized with respect to thickness, homogeneity, and energy loss straggling. In the present, contribution results of test measurements at the Maier Leibnitz tandem accelerator facility in Munich with $$^{109}$$ Ag and $$^{127}$$ I beams with the aim to determine the most suitable degrader material, as well as measurements at the Institut Laue–Langevin will be presented. These include a systematic study of the quality of Z-separation of fission fragments in the mass range $$82\le A \le 132$$ and a systematic measurement of $$^{92}$$ Rb fission yields, as well as investigations of fission yields toward the symmetry region.

An array of calorimetric low temperature detectors (CLTD’s) for energy sensitive detection of heavy ions was combined with time-of-flight (TOF) detectors to obtain a detector system for high resolution mass identification of low energy heavy ions. In addition the same setup was used to prove the ability of CLTD’s to be used in electronic stopping power measurements for heavy ions in matter. Experiments with 50 MeV 63Cu and 65Cu ions at the tandem accelerator at the MPI at Heidelberg, and with 25 to 250 MeV 238U ions at the UNILAC accelerator at GSI at Darmstadt have been performed. For 63,65Cu at 50 MeV a mass resolution of Δm(FWHM)=0.9 amu, and for 238U in an energy range of 65 to 150 MeV a resolution of Δm(FWHM)=1.28 amu, was obtained. The results for stopping powers of 238U in carbon and gold are presented and compared with theoretical predictions and data from the literature.

Experiments have been performed for studying quaternary fission (QF) in spontaneous fission of 252Cf, on the one hand, and for the neutron-induced fission reactions 233, 235U(nth, f ), on the other hand. In this higher-multiplicity fission mode, by definition, four charged products appear in the final state. In other words, as a generalization of the ternary-fission process, not only one but two light charged particles (LCPs) are accompanying the splitting of an actinide nucleus into the customary pair of fission fragments. In the two sets of measurements, which have used quite different approaches, the yields of several QF reactions with α-particles and tritons as the LCPs have been determined and the corresponding kinetic-energy distributions of the α-particles measured. The QF process can appear in two basically different ways: i) the simultaneous creation of two LCPs in the act of fission (“true” QF) and ii) via a fast sequential decay of a single but particle-unstable LCP in common ternary fission (“pseudo” QF). Experimentally the two varieties of QF have been distinguished by exploiting the different patterns of angular correlations between the two outgoing LCPs. The experiments described in the present paper are the first to demonstrate that both types of reactions, true and pseudo QF, occur with quite comparable probabilities. As a new result also, the kinetic-energy distributions related to the two processes have been shown to be significantly different. For all QF reactions which could be explored, the yields for 252Cf(sf) were found to be roughly by an order of magnitude larger than the yields found in the 233U(nth, f ) and 235U(nth, f ) reactions. An interesting by-product has been the measurement of yields of excited LCPs which allows to deduce nuclear temperatures at scission by comparison to the respective yields in the ground state.

A new multiparameter experiment on ternary fission of 252Cf has recently been successfully performed by using the CODIS2 detector system and two GSI segmented Super Clover Ge γ-ray detectors. The experimental setup and the main subjects of interest are briefly described.

A pronounced fine structure (FS) in the form of distinct peaks was observed in neutron gated mass spectra from the decay of the 278110 composite system produced in the reaction 238U + 40Ar (243 MeV) at an initial excitation energy E * > 70 MeV. The FS peaks are located in the vicinity of mass numbers 70-80, 100, and 130, which correspond to those of magic nuclei (clusters). In the data there is also evidence for a new type of decay -- collinear cluster tripartition of an excited nucleus.

Fission fragment mass distributions down to super-asymmetric mass region and both pre- and post-scission neutron multiplicity for238U(p,fission) reaction atEp = 20, 35, 50, 60 MeV and for232Th(p, fission) reaction atEp = 50, 60 MeV were measured using HENDES set-up. The results indicate enhancement for super-asymmetric mass division at intermediate excitation energies.

An experiment was performed on prompt γ-ray emission in binary and α-particle accompanied spontaneous fission of 252C f using the Darmstadt-Heidelberg 4π NaI Crystal Ball spectrometer. The enhancement in γ-ray yield, denoted as the “high-energy component”, which appears between 3.5 and 8 MeV and in the region of near-symmetric fragment mass splits, was observed to be equally pronounced in both fission modes. Analyzing the fragment mass dependence of the mean γ-ray multiplicity in both fission modes clearly identifies the disintegration of equilibrated fission fragments in a narrow mass range around the double-magic 132Sn as the source of these γ-rays.

High-spin states in 22Ne have been studied using the 14C(12C,α)22Ne(α)18O reaction at E(12 C)= 44MeV. The spin assignments were obtained as results of the analysis of double (α, α) angular correlations with the residual 18O nucleus in the 0+ ground state. The obtained values of spin and parity of five levels are: 20.0 MeV (9−), 20.7 MeV (11−), 21.6 MeV (9−), 22.2 MeV (12+), 25.0 MeV (9−).

The γ-ray spectrum of 252Cf(sf) was measured in the Darmstadt-Heidelberg Crystal Ball spectrometer, with a double ionization chamber mounted inside to detect the fission fragments. The measurement was aimed at a better understanding of an unusual component found in the high-energy region between 3 and 8 MeV, with fragment mass splits near symmetry. This component was proved to be predominantly emitted by the heavier fragment, to reach its highest intensity at a fragment mass split of 132:120, and to have an almost isotropic angular distribution. Calculations with the statistical code CASCADE could reproduce the main features.