Complete characterization of the fission fragments produced in reactions induced byPb208projectiles on proton at500AMeV

Abstract

The two fragments produced in fission reactions induced by 208 Pb projectiles impinging on a liquid hydrogen target at 500A MeV have been fully identified in coincidence. The experiment was performed at GSI Darmstadt, where the combined use of the inverse kinematics technique with an efficient detection setup permitted one to detect and to fully identify the fission fragments in a range from Z = 27 to Z = 52. The corresponding isotopic cross sections and velocities of the fission fragments were measured with high accuracy. The results are compared to state-of-the-art model calculations that reproduce the measured observables. Correlations between the two fragments were used to assess the role of charge polarization and the excitation energy gained by the nascent fragments using the neutron excess of the final fragments. The analysis of the average velocities of the fission fragments allowed us to parametrize the distance between the two fission fragments at scission as a function of the size of the fissioning system.

First Direct Measurement of Isotopic Fission-Fragment Yields of U239

“…The use of inverse kinematics allowed the direct measurement of the atomic number of complete fission-fragment distributions [22]. In the last decade, the simultaneous use of surrogate reactions, inverse kinematics, and magnetic spectrometers has opened a new field of study measuring complete isotopic fission-fragment distributions [23][24][25][26], and leading to an improved understanding of the fission process [12].…”

Presaddle and postsaddle dissipative effects in fission using complete kinematics measurements

“…The combined use of the inverse kinematics technique and an efficient detection setup made it possible to identify fission reactions and determine the atomic number of the final fission fragments very accurately. The sum of the charges of both fission fragments was used as a measurement of the initial excitation energy while partial fission cross sections and the width of the charge distribution of the fission fragments were shown to be sensitive to presaddle dynamical effects [18,20,23,24]. The measurements obtained for many different fissioning systems over a broad range in fissility and temperature were compatible with a constant value of the reduced dissipation parameter at small deformations of β = 4.5 × 10 21 s −1 , corresponding to transient times (τ trans ) between 1.0 and 3.3 × 10 −21 s. These results are compatible with the ones obtained from the investigation of some fusion reactions [16,25].…”

“…The use of the inverse kinematics allowed us to separate fission from other reaction channels and also facilitated the identification of both fission fragments. Fission events were identified by using energy-loss measurements performed with a double multisampling ionization chamber (Twin MUSIC), providing the atomic number of the two fission fragments with a resolution better than 0.43 charge units full width at half maximum (FWHM) [24]. In addition, the tracking capabilities of this detector permitted us to select fission events produced at the target position [32].…”

“…In addition, the tracking capabilities of this detector permitted us to select fission events produced at the target position [32]. Mass numbers were determined from magnetic rigidity and time-of-flight measurements with an average resolution of A/A ∼ 0.63% (FWHM) [24], providing for the first time a complete and unambiguous identification in mass and atomic number of both fission fragments. Other details can be found in Ref.…”

“…The description of the range of compound nuclei produced in spallation reactions, whose characteristics cannot be accessed experimentally, has been validated describing the isotopic composition of final residual nuclei in reactions with nonfissile nuclei [33]. Moreover, the ground-to-saddle dynamics was constrained by measuring the atomic numbers of both fission fragments [16,18,23,30] or the isotopic composition of one of the two fragments [24,44,45]. We can then conclude that these calculations provide a reasonable description of the fissioning nuclei at the saddle-point deformation.…”