Average Kinetic Energy Release Research Articles

Kinetic-energy distributions in the reactor neutron fission ofU232and thermal neutron fission ofPu241andCm245

Recoil ranges of fission products from the reactor neutron fission of /sup 232/U and thermal neutron fission of /sup 241/Pu and /sup 245/Cm were determined using radiochemical as well as ..gamma.. spectrometric techniques. Using a semiempirical range-energy equation the recoil ranges were converted into kinetic energies of the fission fragments and hence the kinetic energy distribution as a function of fragment mass from which the kinetic energy deficit and the average total kinetic energy release were obtained. An attempt is made to correlate the variation of the average total kinetic energies in low-energy fission of heavy nuclei with shell structure.

Formation of CF + 3 ions in photoionization of hexafluoroethane

A detailed study of the formation of CF+3 ions from C2F6 has been undertaken by photoelectron-photoion coincidence spectrometry. The distributions of kinetic energy released in CF+3 production have been measured over a range of internal energies of the parent ion. All the CF+3 ions observed are apparently formed by primary dissociation of the molecular ions; fission of the C—C bond in highly excited C2F+6 ions probably occurs before internal conversion to the ground electronic state.The distributions of kinetic energy released in formation of CF+3 from ground state C2F+6, and the rate of change of the average kinetic energy release with the ionization energy, are consistent with predictions made on the basis of statistical theories of reaction. The distributions of kinetic energy released in dissociation of highly excited C2F+6 ions are broader, and also change unexpectedly slowly with internal energy.

Kinetic energy distributions from the shapes of metastable peaks

A general procedure is presented for determining the distribution of translational energies released in unimolecular ionic reactions occurring in the analyser of a mass spectrometer. It is shown that the method is applicable to reactions in which the average kinetic energy release covers a wide range (< 1 meV to ca . 3 eV). The treatment avoids making approximations either in the calculation of metastable peak shapes or in the subsequent determination of the translational energy distribution. Excellent agreement is achieved between experimental and calculated metastable peak shapes and the kinetic energy distributions represent the first accurate values obtained by a generally applicable technique. The relation between the average kinetic energy released and the width at half height of the corresponding metastable peak has been explored for peaks of various shapes. For gaussian-type peaks the kinetic energy release calculated from the half height is smaller by a factor of two to three than the average value, while for dish topped peaks it approximately equals the average value.

ChemInform Abstract: ENERGY PARTITIONING BY MASS SPECTROMETRY, CHLOROALKANES AND CHLOROALKENES

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2‐elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3‐elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2‐elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four‐centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2‐elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3‐dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.

Energy partitioning by mass spectrometry: Chloroalkanes and chloroalkenes

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2-elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3-elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2-elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four-centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2-elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3-dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.

Thermochemistry and energy partitioning in the charge separation reactions of doubly charged triatomic ions

Ion kinetic energy and appearance potential measurements have been used to study the unimolecular charge separation reactions of the triatomic ions CO22+, COS2+, CS22+, N2O2+, NO22+ ·, and SO22+. The experimentally determined metastable peak shapes have been compared with calculated peak shapes including calculations that determine both the average kinetic energy release and the distribution about this average. The product ion states have been assigned from the thermochemical data, and in all the reactions total spin and angular momentum are found to be conserved. There is evidence that the metastable peaks are associated with maxima on single potential surfaces rather than with a slow predissociation. For CO2++ and COS++ almost all the available energy is partitioned into translational energy of product separation.

Threshold electron-photoion coincidence mass spectrometric study of CH4, CD4, C2H6, and C2D6

An experiment is reported which directly determines the kinetics of dissociation of a molecular ion as a function of the internal energy of that ion. The experiment involves photoionization, followed by detection of only those mass analyzed ions which are produced in coincidence with photoelectrons of zero initial kinetic energy. Under these conditions the internal energy of a parent ion is equal to the photon energy minus the adiabatic ionization potential of the molecule. With this technique, in contrast to previously used techniques, the kinetics of dissociation can be studied independently of assumptions concerning threshold laws, i.e., variation in cross section above threshold, and autoionization. Fragmentation as a function of internal energy is reported for CH4+, CD4+, C2H6+, and C2D6+. The results are compared with previous photoionization, charge exchange and photoelectron-photoion coincidence mass spectrometry, and with fragmentation calculations based on quasiequilibrium theory of unimolecular dissociation. The major discrepancies of earlier experiments with the predictions of the quasiequilibrium theory are shown to be in error. The new experimental results are in good qualitative agreement with the predictions of the theory. Although this experiment was not designed to determine the average kinetic energy release in unimolecular reactions, it is sensitive to this energy. An analysis of the instrument and the data indicate average fragmentation energies which are of the same order as those obtained by other workers. The implications of these data on the thermochemistry of the molecules studied is discussed.

Recoil properties and charge distribution in the fission of 208Pb by 450 MeV protons

Cross-sections and thick-target recoil properties have been determined for three isobaric nuclides with A = 111 as well as for several other palladium and silver nuclides formed in the fission of 208Pb by 450 MeV protons. The cross-sections have been used to construct a Gaussian charge distribution. The analysis indicates that the neutron-to-proton ratio of the most probable product, N Zp , has a value of 1·337 ± 0·008 for A = 103–111; the full width at half maximum of the distribution is 1·88 ± 0·04 Z units; the isobaric fission cross-section for A = 111 is 6·1 ± 0·4 mb. The recoil data have been used to derive, for a given mass chain, the dependence on atomic number of the average cascade deposition energy, the average total kinetic energy release, and the average separation distance between the primary fragment charge centers. The results are compared with similar data previously obtained for 238U.

Recoil study of the symmetric fission of 238U by 450 MeV protons

A thick-target recoil study of isobaric products with mass number 103 and 111 from the fission of 238U by 450 MeV protons has been performed. The results have been analyzed to yield, for a given mass chain, the dependence on atomic number, Z, of the average cascade deposition energy, the average total kinetic energy release, and the average separation distance of the primary fragment charge centers. The average deposition energy increases by about 25 MeV per Z unit whereas the other two quantities are essentially independent of Z. A correlation between the most probable value of Z for a given mass chain and the deposition energy leading to this product is derived on the basis of the unchanged charge distribution postulate.

U238-Spaltung mit 14 MeV Neutronen

A measurement on the reaction U238(n,f) with 14 MeV neutrons has been done. The energies of the two fission fragments were analyzed in coincidence using Si surface detectors. The angle between incident neutron and outgoing fragment direction was set to 0° and 90° in the laboratory system. Some 2300 events have been recorded. The resultant mass distribution is in fairly well agreement with radiochemical data. The average kinetic energy release before prompt neutron emission has been found to beĒ=(160,8±4) MeV. For symmetric fission a kinetic energy dip has been observed. The anisotropy α in the postulated angular distributionW(Θ)=1+α cos2Θ is strong dependent on mass ratio, being positive and greatest for asymmetric fission. The value of α averaged over all mass ratios, ¯α=0,33±0,07, compares well with already published data. The results are discussed in terms of new fission models.

A comparison of fission fragment measurements made by double-energy and double-velocity techniques

Abstract The energies of two coincident fission fragments emitted from spontaneous fission of Cf252 and 25.5-MeV-He4 induced fission of U233 have been measured with semiconductor detectors. The mass distributions and the average total kinetic energy release as a function of the mass ratio obtained from these measurements have been compared in detail with results obtained by the double-velocity time-of-flight technique. For Cf252 this comparison shows that the distributions obtained from double-energy measurements are affected by the emission of several prompt neutrons from the fragments, and that if the results are corrected to account for these neutron effects, good agreement can be obtained between the double-energy and double-velocity measurements. For He4-induced fission of U233, which involves a more highly excited fissioning nucleus, the neutron effects are apparently less serious, so the double-energy results require only relatively simple corrections. The good agreement between results obtained using the two techniques confirms the validity of the calibration procedure for the semiconductor detectors, and suggests that precision experiments can be performed with a semiconductor detector system if corrections are made for the effects of prompt-neutron emission.

Kinetic Energy Distributions of Fragments from the Fission of Au, Tl, Pb, and Bi

Single fragment and total kinetic energy distributions have been measured for fission induced by helium ions and deuterons in Au, Tl, Pb, and Bi targets. The average total kinetic energy release has the same proportionality with $\frac{{Z}^{2}}{{A}^{\frac{1}{3}}}$ found for fission of heavier elements. The widths for the total kinetic energy distributions are comparable to those observed for fission of heavy elements. A preliminary mass distribution curve has been obtained by measuring the energies of coincident fragments.