Fragment Atomic Number Research Articles

Insight into excitation energy and structure effects in fission from isotopic information in fission yields

The use of the inverse kinematics technique and a magnetic spectrometer permits the simultaneous measurement of proton and neutron content of full fission fragment distributions. This paper reports new measurements of five fissioning systems - U238,Np239,Pu240,Cm244, and Cf250 - produced in inelastic scattering, transfer, and fusion reactions at different excitation energies. As a result, isotopic, elemental, and isotonic fission yields are presented. The contribution of the different fission modes and the proton even-odd effect are studied. Structure effects are investigated by means of the neutron excess and the total neutron multiplicity as a function of the fragment atomic number.

CMOS active pixel sensors response to low energy light ions

Recently CMOS active pixel sensors have been used in Hadrontherapy ions fragmentation cross section measurements. Their main goal is to reconstruct tracks generated by the non interacting primary ions or by the produced fragments. In this framework the sensors, unexpectedly, demonstrated the possibility to obtain also some informations that could contribute to the ion type identification. The present analysis shows a clear dependency in charge and number of pixels per cluster (pixels with a collected amount of charge above a given threshold) with both fragment atomic number Z and energy loss in the sensor. This information, in the FIRST (F ragmentation of I ons R elevant for S pace and T herapy) experiment, has been used in the overall particle identification analysis algorithm. The aim of this paper is to present the data analysis and the obtained results. An empirical model was developed, in this paper, that reproduce the cluster size as function of the deposited energy in the sensor.

Fission in Inverse Kinematics: A path to new experimental observables

Historically, experimental fission studies were based on reactions in direct kinematics with fixed target-like fissioning systems. Besides its advantages, this technique suffers from some drawbacks such as the difficulty of producing exotic fissioning systems and the seldom measurement of the fragment atomic number. Inverse kinematic provides an alternative to ease these issues and offers a new set of experimental observables that improves our level of information about the fission process, including an unprecedented access to the scission point. In this document, we review some of the observables obtained from the experimental campaign based on inverse kinematics, performed at VAMOS/GANIL.

Characterization of the scission point from fission-fragment velocities

The isotopic-yield distributions and kinematic properties of fragments produced in transfer-induced fission of 240Pu and fusion-induced fission of 250Cf, with 9 MeV and 45 MeV of excitation energy respectively, were measured in inverse kinematics with the spectrometer VAMOS. The kinematic properties of identified fission fragments allow to derive properties of the scission configuration such as the distance between fragments, the total kinetic energy, the neutron multiplicity, the total excitation energy, and, for the first time, the proton- and neutron-number sharing during the emergence of the fragments. These properties of the scission point are studied as functions of the fragment atomic number. The correlation between these observables, gathered in one single experiment and for two different fissioning systems at different excitation energies, give valuable information for the understanding and modeling of the fission process.

PROJECTILE FRAGMENTATION OF 36,40Ar INDUCED REACTIONS

Fragment yields for Z ≥ 5 from projectile fragmentation using primary beams of 36,40Ar at 50 MeV/nucleon on 64 Ni target have been measured in RIBLL fragment separator. We compare the fragment cross sections with the predictions of the empirical EPAX parametrization of fragmentation cross-sections and Statistical Abration-Ablation model (SAA) by considering the RIBLL separator transmission rate. Isotope yield ratios between these two reactions were calculated and isoscaling parameters α and β are extracted, their dependences on fragment atomic number Z and neutron number N were presented.

Isocaling and the symmetry energy in the multifragmentation regime of heavy-ion collisions

The ratio of the symmetry energy coefficient to temperature, $a_sym/T$, in Fermi energy heavy ion collisions, has been experimentally extracted as a function of the fragment atomic number using isoscaling parameters and the variance of the isotope distributions. The extracted values have been compared to the results of calculations made with an Antisymmetrized Molecular Dynamics (AMD) model employing a statistical decay code to account for deexcitation of excited primary fragments. The experimental values are in good agreement with the values calculated but are significantly different from those characterizing the yields of the primary AMD fragments.

PROJECTILE FRAGMENTATION OF 36,40Ar INDUCED REACTIONS

Fragment yields of projectile fragmentation reactions using primary beams of 36,40Ar at 50 MeV/nucleon on 64 Ni target have been measured. Fragment yields of different isotopes were obtained for Z ≥ 5 in RIBLL fragment seperator. We compare the extracted yields to the predictions of the empirical parametrization of fragmentation cross-sections(EPAX) and Statistical Abration-Ablation model(SAA) considering the RIBLL separator transmission rate. Isotope yield ratios between these 2 reactions were calculated, isoscaling parameters α and β, their dependence on fragment atomic number Z and neutron number N were calculated.

Isotopic scaling of heavy projectile residues from the collisions of25MeV/nucleon 86Krwith124Sn,112Snand64Ni,58Ni

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is studied. Isotopically resolved yield distributions of projectile fragments in the range Z=10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7-5.3 degrees. The velocities of the residues, monotonically decreasing with Z down to Z~26-28, are employed to characterize the excitation energy. The yield ratios R21(N,Z) for each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are alpha=0.43 and beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni system. For the Kr+Sn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Z<26 and N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically, alpha decreasing with Z and beta increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Such high-resolution mass spectrometric data can provide important information on the role of isospin in peripheral and mid-peripheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.

Isospin effect of fragmentation reactions induced by intermediate energy heavy ions and its disappearance

The fragments produced in the reaction of O-18 + Be-9 at 60 MevV/nucleon were measured experimentally. The isotopic distribution of the fragmentation reaction products was well reproduced by using a modified statistical abrasion-ablation model This model predicts that the fragment isotopic distribution at a fixed atomic number Z shifts towards the neutron rich side for the neutron-rich projectile. This isospin effect will decrease with decreasing the fragment atomic number Z and disappear when (Z(proj)-Z)/Z(proj) becomes larger than 0.5. It was shown that the disappearance of the isospin effect of fragmentation reaction is induced by the geometry effect in abrasion stage and the evaporation effect later.

Experimental determination of fragment excitation energies in multifragmentation events

For 50 MeV/nucleon 129Xe+natSn multifragmentation events, we deduced, by means of correlation techniques, the multiplicities of the hydrogen and helium isotopes which were emitted by the hot primary excited fragments produced at the stage of the disassembly of an equilibrated hot source. We also derived the relative kinetic energy distributions between the primary clusters and the light charged particles that they evaporate. From the comparison between the secondary multiplicities observed experimentally and the multiplicities predicted by the GEMINI model, we concluded that the source breaks into primary fragments which are characterized by the same N/Z ratio as the combined system. Knowing the secondary light charged particle multiplicities and kinetic energies, we reconstructed the average charges of the hot fragments and we estimated their mean excitation energies. The fragment excitation energies are equal to 3.0 MeV/nucleon for the full range of intermediate mass fragment atomic number. This global constancy indicates that, on the average, thermodynamical equilibrium was achieved at the disassembly stage of the source.Received 13 March 1998DOI:https://doi.org/10.1103/PhysRevC.58.256©1998 American Physical Society

Excitation energy equilibration in damped 139La+40Ar collisions at 15 MeV per nucleon.

Energy and angular distributions of neutrons emitted in coincidence with projectile-like fragments from the damped reaction $^{139}\mathrm{La}$${+}^{40}$Ar at a bombarding energy of 600 MeV have been measured. Components corresponding to sequential emission from two fully accelerated reaction fragments as well as preequilibrium neutron emission have been separated as a function of energy loss and fragment atomic number. The deduced division of total excitation energy between the reaction partners is very well described by the one-body nucleon-exchange model for all energy losses. The data provide no evidence for correlations between the excitation energy division and the measured projectile-like fragment atomic number.

Spin polarization in 16O-induced heavy-ion reactions

The circular polarization P γ of γ-rays from 16O-induced reactions on 27Al, 48Ti, 58Ni and 62Ni at 100 MeV incident energy has been measured in coincidence with projectile-like fragments detected at angles behind the grazing angle. The dependence of P γ on the reaction Q-value, on the fragment atomic number and, for 16O+ 48Ti, on the scattering angle was investigated. The results show that deep-inelastic (DI) collisions in these fairly light heavy-ion systems are associated with predominantly negative scattering angles. The average value of P γ (DI) = 0.6 obtained for 16O + Ni at θ lab= 35° is consistent with pure negative-angle scattering. The magnitude of P γ (DI) is smaller for the two lighter systems which is interpreted as mainly arising from the increasing probability for orbiting trajectories leading to scattering angles beyond 180° Trajectory calculations within a classical friction model were performed, and the parameter dependence of the results was studied. Strong radial friction forces were found necessary to simulate the observed negative-angle cross sections.

Fragment atomic-number identification using a gas ionization chamber in fission yield measurements

A thin-windowed gridded ionization chamber was developed and used with the fission-fragment recoil mass spectrometer HIAWATHA for the direct physical measurement of nuclide yields from fission. The technique developed represents a unique method for the identification of fission fragment atomic mass and atomic number. The sensitivity of the mean energy loss to fragment atomic number and the amount of fragment energy straggling in the chamber gas were measured and gave an atomic-number resolving power of 38 for 95 MeV fragments. Fragment energy losses measured in counter gas P-5 were in good agreement with the Moak-Brown semiempirical method of predicting stopping powers although small systematic discrepancies were found. Fragment energy-loss widths measured in counter gas P-5 were in good agreement with energy loss widths for fragments traversing solid carbon foils.

The response of a surface barrier detector to mass and energy separated fission fragments

The pulse height defects for fission fragments separated according to their kinetic energies and masses were determined for a heavy ion silicon surface barrier detector. The experiment used the fission fragment mass separator ‘Lohengrin’ at ILL Grenoble. The data recording system is described. The light fragments defects plotted against energy in LSS dimensionless units fall progressively further with decreasing mass below a universal curve which has been previously used for other ions and on which all the heavy fragment data fall. The fwhm of the defect, being accountable largely to atomic collisions and being less mass dependent than the defect itself, indicates that this strong mass dependence in the defect for light fragments is due to other causes than atomic collision losses. The contribution to the defect due to charge recombination is determined by subtraction from the observed defect of calculated detector window losses and atomic collision losses. The dependence of the recombination defect on fragment atomic number, Z 1, (or mass) and electronic stopping power is compared with the predictions of various models. Fragment atomic shell effects including Z 1 oscillations in the stopping power can explain the light fragment behaviour of the defect.