Total Formation Cross Sections Research Articles

Ionization and Single and Double Electron Capture in Proton-Ar Collisions.

Total cross sections for formation of H and H-, and electron production, in H+ + Ar collisions have been calculated at energies between 100 eV and 200 keV by employing two methods: for E < 10 keV, a semiclassical treatment with an expansion in a basis of electronic wave functions of the ArH+ quasimolecule and, for E > 10 keV, the switching-classical-trajectory-Monte Carlo method (s-CTMC). The semiclassical calculation involves transitions to molecular autoionizing states, calculated by applying a block-diagonalization technique. The s-CTMC method is adept to treat two-electron processes and yields total cross sections for H- formation in reasonably good agreement with the experimental data. Cross sections for electron- and H-production processes, which are dominated by one-electron transitions, are in good agreement with the experimental data.

Fragmentation of the adenine and guanine molecules induced by electron collisions.

Secondary electron emission is the most important stage in the mechanism of radiation damage to DNA biopolymers induced by primary ionizing radiation. These secondary electrons ejected by the primary electron impacts can produce further ionizations, initiating an avalanche effect, leading to genome damage through the energy transfer from the primary objects to sensitive biomolecular targets, such as nitrogenous bases, saccharides, and other DNA and peptide components. In this work, the formation of positive and negative ions of purine bases of nucleic acids (adenine and guanine molecules) under the impact of slow electrons (from 0.1 till 200 eV) is studied by the crossed electron and molecular beams technique. The method used makes it possible to measure the molecular beam intensity and determine the total cross-sections for the formation of positive and negative ions of the studied molecules, their energy dependences, and absolute values. It is found that the maximum cross section for formation of the adenine and guanine positive ions is reached at about 90 eV energy of the electron beam and their absolute values are equal to 2.8 × 10(-15) and 3.2 × 10(-15) cm(2), respectively. The total cross section for formation of the negative ions is 6.1 × 10(-18) and 7.6 × 10(-18) cm(2) at the energy of 1.1 eV for adenine and guanine, respectively. The absolute cross-section values for the molecular ions are measured and the cross-sections of dissociative ionization are determined. Quantum chemical calculations are performed for the studied molecules, ions and fragments for interpretation of the crossed beams experiments.

Formation of antihydrogen in three-body antiproton-positron collisions

A quantum-mechanical approach is proposed for the formation of antihydrogen $(\overline{H})$ in the ground and excited states $(2s,2p)$ via the mechanism of three-body recombination (TBR) inside a trapped plasma of antiprotons $(\overline{p})$ and positrons $({e}^{+})$ or in the collision between the two beams of them. Variations of the differential as well as the total formation cross sections are studied as a function of the incident energies of both the active and the spectator ${e}^{+}$'s. Significantly large cross sections are found at very low incident energies in the TBR process as compared to other processes leading to antihydrogen. The present $(\overline{H})$ formation cross section decreases with increasing positron energy (temperature) but no simple power law could be predicted for it covering the entire energy range $(\ensuremath{\sim}5--50\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, corroborating the experimental findings qualitatively, the latter being at a much lower energy regime $(\ensuremath{\sim}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The formation cross sections are found to be much higher for unequal energies of the two ${e}^{+}$'s than for equal energies, as expected physically.

Doppler Shift as a Tool for Studies of Isobaric Analog States of Neutron-Rich Nuclei: Application toHe7

We have developed a new technique to study exotic neutron-rich nuclei via their isobaric analog states (IAS). We populate high-isospin states in resonant reactions of radioactive ion beams with protons. Characteristic gamma rays emitted from excited decay products were used to identify the population of the IAS. We show that information on the differential and total cross section for formation of the IAS can be extracted from the energy spectrum of the Doppler-shifted gamma rays. This technique was applied to the study of T=3/2 states in 7Li, which are analogs of states in 7He. The analog of the 7He ground state was clearly observed, whereas the presence of the analog of a narrow 1/2(-) state at 0.6 MeV excitation in 7He reported by M. Meister et al. [Phys. Rev. Lett. 88, 102501 (2002)] was excluded at the 90% confidence level. Evidence is presented for a broad 1/2(-) state at a higher excitation energy in 7He.

Positronium formation in positron-hydrogen scattering using Schwinger’s principle

Accurate results are reported for ground-state positronium formation in positron-hydrogen collisions using 12 correlated basis functions in a multichannel Schwinger's principle in momentum space at intermediate energies 30--150 eV in an extension of our earlier work [Phys. Rev. A 59, 1913 (1999)]. The rich structure in the differential cross section is found to be present due to the existence of critical angles. Results for the total formation cross section are in excellent agreement with other variational and nonvariational calculations available in the literature. The observed data of Zhou et al. [Phys. Rev. A 55, 361 (1997)] are also in good accord with these predictions.

Dynamics of chemical and charge transfer reactions of molecular dications: beam scattering and total cross section data on CF 2D + (CF 2H +), CF 2+, and CF + formations in CF 22+ + D 2(H 2) collisions

Dynamics of formation of the chemical rearrangement product CF 2D +, the charge transfer product CF 2 +, and the dissociative products CF + and CFD + in collisions of the molecular dication CF 2 ++ with D 2 was investigated in crossed beam scattering experiments over the collision energy range 0.3–1.0 eV (center of mass). The scattering data show that coulomb repulsion between two singly charged products, CF 2 + + D + and CF 2 + + D 2 +, plays a dominant role in the nondissociative processes. A large fraction of the energy available (about 6 eV in the chemical reaction, about 4 eV in the charge transfer) goes into relative translational energy of the products. Relative total cross sections for formation of the nondissociative and dissociative products in collision of CF 2 ++ with D 2 and H 2 were determined over the collision energy range of 0.2–3.6 eV. The shape of the relative velocity dependence of the cross section for CF 2 + formation can be described by a simple model based on the Landau-Zener formalism. The data suggest that the dissociative product CF + is formed prevailingly in a subsequent dissociation of the charge transfer product CF 2 +. A potential surface model is described which accounts for competition of various processes in dication–neutral collisions.

Positronium formation cross-sections in the ground and excited states ( $\mathsf{n=2}$ levels) in an $\mathsf{e^{+}}$ -He atom collision

The positronium formation cross-sections in the ground and excited n=2 levels have been studied in an -He atom collision in the framework of eikonal approximation. Both the differential and total formation cross-sections have been investigated in the intermediate- and high-energy regime. Present eikonal results are found to differ appreciably from the corresponding first Born values even at very high incident energies. The total cross-section results have been compared with available experiments due to different groups as well as with other existing theoretical results.

Positronium formation in the ground and n=2 levels in an e+-H collision.

The positronium formation processes in the ground and excited n=2 levels have been investigated in ${e}^{+}$-H atom collision using the Glauber eikonal approximation. Both the differential and total formation cross sections have been studied in the intermediate- and high-energy regime. Present eikonal results are found to differ appreciably from the corresponding first-order Born values even at very high energies. The total-cross-section results have been compared with other existing theoretical values as well as with the corresponding Born cross sections.

Laser fluorescence study of AlO formed in the reaction Al + O2: Product state distribution, dissociation energy, and radiative lifetime

The reaction Al + O2 → AlO + O has been investigated by the method of laser−induced fluorescence. Excitation spectra are reported for AlO products formed under single−collision conditions in a ’’beam−gas’’ arrangement. The relative vibrational populations (proportional to total cross sections for formation), as well as the relative rotational populations for the v = 0 and v = 1 vibrational levels, are derived. The rotational distributions are found to differ significantly, with v = 0 having more rotational excitation than v = 1. The vibrational distribution is non−Boltzmann, the falloff with v ≳ 1 exceeding v = 1 compared to v = 0. The observed internal state distributions are compared with those calculated from phase space theory. It is concluded tentatively that the partitioning of the Al + O2 reaction energy among the product modes is not governed solely by statistical considerations. The dynamics of the Al + O2 reaction are compared with those of the exothermic Ba + O2 and endothermic K + O2 reactions. From a knowledge of the AlO internal states populated by the Al + O2 reaction and from an estimate of the thermal reactant energies, a lower bound for the ground state dissociation energy of AlO is derived. By combining the present lower bound with the upper bound previously derived from observation of the onset of an AlO absorption continuum, the value D00(AlO) = 121.5±1 kcal/mole is recommended for aluminum monoxide. Direct measurement of the fluorescence decay of the AlO B 2Σ+ state as a function of time after excitation by the short (∼5 nsec) laser pulse has allowed the determination of the radiative lifetimes τ (v′ = 0) = 100±7 nsec, τ (v′ = 1) = 102±7 nsec, and τ (v′ = 2) = 102±4 nsec, where the error estimates represent three standard deviations.

Electronic excitation in collisions of H andH+withO2

The 115-750-nm radiation produced in collisions of 1.5-25-keV ${\mathrm{H}}^{+}$ and H with ${\mathrm{O}}_{2}$ has been studied in an atomic-beam experiment under thin-target conditions. For both ion and neutral impact, the ($b^{4}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}\ensuremath{-}a^{4}\ensuremath{\Pi}_{u}$) first-negative ($1N$) and ($A^{2}\ensuremath{\Pi}_{u}\ensuremath{-}X^{2}\ensuremath{\Sigma}_{g}^{+}$) second-negative ($2N$) band systems of ${\mathrm{O}}_{2}^{+}$ are the most intense spectral features. Also observed are several emissions from excited states of O and ${\mathrm{O}}^{+}$. Relative cross sections for individual bands of the ${\mathrm{O}}_{2}^{+}$ $1N$ system, obtained by deconvolution of overlapped features in several vibrational sequences, were used to show that the vibrational population $P({v}^{\ensuremath{'}})$ for ${\mathrm{O}}_{2}^{+}$ $b^{4}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$ formed in both ${\mathrm{H}}^{+}$ and H impact is in good agreement with the predictions of a simple Franck-Condon mechanism over the 2-25-keV energy range. Total cross sections for formation of ${\mathrm{O}}_{2}^{+}$ $b^{4}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$ in ${\mathrm{H}}^{+}$ and H impact, determined from the latter results by normalization to previous proton-impact data, are in good agreement with values calculated via application of the semiempirical procedure developed recently by Edgar et al. For both projectiles the semiempirical approach gives a good prediction of the energy dependence and an adequate prediction of the magnitude of the cross sections for formation of ${\mathrm{O}}_{2}^{+}$ $A^{2}\ensuremath{\Pi}_{u}$ determined by integration of measured $2N$ band intensities.

Formation ofN2+BΣu+2andN2CΠu3in collisions ofH+and H withN2

The 200-500-nm radiation excited by collisions of beams of 1-25-keV ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, H, and D with ${\mathrm{N}}_{2}$ has been studied under thin-target conditions with a viewing geometry chosen to minimize polarization effects. For both ion and neutral impact, the $\mathrm{N}_{2}^{}{}_{}{}^{+}$ ($B^{2}\ensuremath{\Sigma}_{u}^{+}\ensuremath{-}X^{2}\ensuremath{\Sigma}_{g}^{+}$) first negative ($1n$) bands are the most intense spectral features in this wavelength range. As expected from consideration of electron-spin conservation, the probability of excitation of the ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}\ensuremath{-}B^{3}\ensuremath{\Pi}_{g}$ second positive ($2p$) bands by H or D impact greatly exceeds that for ${\mathrm{H}}^{+}$ or ${\mathrm{D}}^{+}$ bombardment. Relative emission cross sections for the (0,0) bands of the $1n$ system at 391.4 nm and the $2p$ system at 337.1 nm were determined and made absolute via normalization to measurements reported at higher energies by previous workers. The relative vibrational population $P({v}^{\ensuremath{'}})$ for ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$ formed in 3-25-keV H and D impact is in good agreement with the prediction of a simple, Franck-Condon excitation mechanism; however, for $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ formed in collisions with 4-25-keV H and D, a dramatic enhancement in $P({v}^{\ensuremath{'}})$ for ${v}^{\ensuremath{'}}&gt;0$ over the Franck-Condon values is observed at low projectile velocities. The higher-velocity onset and greater magnitude of this effect for H and D in comparison with that observed for ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ are in disagreement with the predictions of a modified Franck-Condon mechanism that allows for polarization of ${\mathrm{N}}_{2}$ by the incident projectile. The measured values of $P({v}^{\ensuremath{'}})$ were used in conjunction with the (0,0) band cross sections to derive total cross sections for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ and ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$. A maximum in the cross section for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ of 1.12\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 10 keV was found for ${\mathrm{H}}^{+}$ impact, while for H, the cross section for formation of this state rises steadily with increasing collision energy until reaching a nearly constant value of 3.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ in the 15-25-keV range. The fraction of the total $\mathrm{N}_{2}^{}{}_{}{}^{+}$ yield that is formed in the $B$ state increases from about 0.03 to 0.08 in the energy range studied. For formation of ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$ the cross section has a maximum value of 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 5 keV. At H-atom energies below 7 keV, exchange excitation of ${\mathrm{N}}_{2}$ to the $C^{3}\ensuremath{\Pi}_{u}$ state is more probable than ionization to yield $\mathrm{N}_{2}^{}{}_{}{}^{+}$ in the $B$ state while, at higher energies, ionization to yield the $B$ state is the more probable process. Comparison of our results with values calculated via the application of recently developed, semiempirical scaling relationships to electron impact cross-section data indicates that the latter approach yields a good prediction of the energy dependence and an acceptable prediction of the magnitude of the $C^{3}\ensuremath{\Pi}_{u}$ cross section for 1-100-keV H impact. The semiempirical approach also makes a good prediction of the cross section for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ in ${\mathrm{H}}^{+}$ or H impact above 10 keV; however, primarily because of the lack of experimental data at lower energies, previous semiempirical predictions have overestimated the ionization contribution to the over-all $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ cross section.

Energy levels of 14C

The 9Be ( 6Li, p) 14C reaction has been studied using 20 MeV 6Li ions and the Penn multiangle spectrograph. Proton groups are reported corresponding to thirty excited states of 14C with E x < 18.2 MeV. The total cross section for formation of the six bound excited states, whose J π are known, is proportional to 2 J f + 1. Possible new spin assignments are suggested for several unbound levels of 14C, based on the 2 J f + 1 rule and a comparison of the experimental widths and widths predicted from neutron penetration of a centrifugal barrier.

Total Cross Sections for Formation of Ions from CsBr by Collision with Ar, Xe, and NaBr(Ar)

Measurements have been made by cross beam techniques on the absolute total cross section Q for formation of ions from CsBr in collisions with Ar and Xe, and relative total cross section for NaBr with Ar, in a range of relative kinetic energies (Xe: 12–120 eV; Ar: 20–125 eV). The absolute cross sections obtained for CsBr, with estimates of accuracy, are 5.0±1.2 Å2 for Ar and 12.6±2.7 Å2 for Xe at a relative kinetic energy of 46 eV. The energy dependence of the cross sections follows closely that predicted by the simple reaction model of hard spheres. A fit of this model to the measurements leads to estimates of threshold energies. Measurements have also been made on a crude energy analysis of the ions formed. About half the ions of either sign have laboratory energies below 5 eV and the remainder is distributed continuously over larger energies up to the maximum. Negative ions have somewhat lower energies than positive ones. The results are consistent with a knockout model of collisional dissociation to ions, with a preference for dissociation by collision of the rare gas atom with the heavy (positive) end of the molecule.

Formation ofH(2p)andH(2s)in Collisions of Protons and Hydrogen Atoms with Hydrogen Molecules

Cross sections for emission of Lyman-$\ensuremath{\alpha}$ radiation owing to formation of $\mathrm{H}(2p)$ and $\mathrm{H}(2s)$ in collisions of 5-25-keV protons and hydrogen atoms with molecular hydrogen have been determined. The intensity of Lyman-$\ensuremath{\alpha}$ emitted spontaneously from $\mathrm{H}(2p)$ was measured at 54.7\ifmmode^\circ\else\textdegree\fi{} or 125.3\ifmmode^\circ\else\textdegree\fi{} with respect to the projectile beam in an essentially field-free collision chamber with an oxygen-filtered photometer calibrated by reference to previous results for ${\mathrm{H}}^{+}$ on Ar. The narrow bandwidth of the oxygen filter allowed separation of the Doppler-shifted Lyman-$\ensuremath{\alpha}$, emitted by $\mathrm{H}(2p)$ formed in electron capture by fast incident protons or collisional excitation of fast incoming hydrogen atoms, and the virtually unshifted Lyman-$\ensuremath{\alpha}$ emitted by the far slower $\mathrm{H}(2p)$ produced in dissociative excitation of ${\mathrm{H}}_{2}$. The increase in intensity of Lyman-$\ensuremath{\alpha}$ when an electric field, oriented in the direction of the beam, was applied within the collision chamber has been used to derive separate cross sections for formation of $\mathrm{H}(2s)$ owing to projectile and dissociative excitation. The experimental configuration was designed to minimize the effect of polarization of the light emitted from $\mathrm{H}(2p)$ and $\mathrm{H}(2s)$, and, after small corrections for cascade effects, the data yield cross sections for population of these states. The total cross section for formation of $\mathrm{H}(n=2)$ via projectile excitation exceeds that for dissociative excitation in either ${\mathrm{H}}^{+}$ or H impact. ${\mathrm{H}}^{+}$ is generally more efficient than H in the production of Lyman-$\ensuremath{\alpha}$ at low energies in projectile excitation and at all energies in dissociative excitation, and the cross section for formation of $\mathrm{H}(2p)$ usually exceeds that for $\mathrm{H}(2s)$. Scaling relationships from limiting high-energy-scattering theory are found to have only moderate success in relating our results to previous measurements involving excitation of H to the $n=3 \mathrm{and} 4$ states in projectile excitation and as well as in dissociative excitation by other heavy particles.

Inelastic Scattering of Protons fromB10between 5 and 16.5 MeV

Angular distributions for inelastically scattered protons leading to the 0.717-, 1.74-, 2.15-, 3.59-, 4.77-, and 6.04-MeV states in ${\mathrm{B}}^{10}$ were measured in 300-keV steps between 5 and 12 MeV and in 500-keV steps from 12 to 16 MeV. Yield curves for protons feeding these states were measured in 100-keV steps from 5 to 16 MeV at five angles. Resonance structures in the region $5\ensuremath{\le}{E}_{p}\ensuremath{\le}10$ MeV were observed in the yield curves. The observed cross section for formation of the 6.04-MeV state was five times that for any of the lower-lying levels. A weak correspondence between the total cross section for formation of these states and its radiative-decay width $\ensuremath{\Gamma}(E2)$ is noted. The experimental differential cross sections are compared with distorted-wave Born-approximation calculations. While the calculations are not in quantitative agreement with the experimental cross sections, some conclusions can be drawn from their qualitative similarities.

Cross Sections for Charge-Exchange Reactions of the TypeH++H(n1l1m1)→H(n2l2m2)+H+

We consider charge-exchange reactions of the general type ${\mathrm{H}}^{+}+\mathrm{H}({n}_{1}{l}_{1}{m}_{1})\ensuremath{\rightarrow}\mathrm{H}({n}_{2}{l}_{2}{m}_{2})+{\mathrm{H}}^{+}$, and use some recently derived sum rules to get comparatively simple closed expressions for the cross section. Calculations are performed in the Born approximation. We consider all excited target H atoms with ${n}_{1}\ensuremath{\ge}2$ and all (appropriate) ${l}_{1}\ensuremath{\le}3$; we average over ${m}_{1}$. We evaluate the total cross section for formation of final states with given principal quantum number ${n}_{2}$; i.e., we sum over ${l}_{2}$ and ${m}_{2}$. These results may be easily modified to include the case where the target atoms are H-like alkali atoms.

Excitation Functions and Nuclear Charge Dispersion in the Fission of Uranium by 0.1- to 6.2-GeV Protons

Cross sections for formation of cesium and rubidium isotopes produced by bombardment of uranium with protons ranging in energy from 0.1 to 6.2 GeV were measured both radiochemically and mass spectrometrically. Independent yields were determined for ${\mathrm{Rb}}^{84}$, ${\mathrm{Rb}}^{86}$, ${\mathrm{Cs}}^{127}$, ${\mathrm{Cs}}^{129}$, ${\mathrm{Cs}}^{130}$, ${\mathrm{Cs}}^{131}$, ${\mathrm{Cs}}^{132}$, ${\mathrm{Cs}}^{134}$, ${\mathrm{Cs}}^{136}$, and, at some energies, ${\mathrm{Rb}}^{83}$ and ${\mathrm{Cs}}^{135}$. In addition, the independent yield of ${\mathrm{Ba}}^{131}$ and the chain yields of ${\mathrm{Cs}}^{125}$, ${\mathrm{Cs}}^{127}$, ${\mathrm{Cs}}^{129}$, ${\mathrm{La}}^{131}$, ${\mathrm{Cs}}^{135}$, ${\mathrm{Cs}}^{137}$, ${\mathrm{Rb}}^{83}$, ${\mathrm{Rb}}^{87}$, and ${\mathrm{Ba}}^{140}$ were obtained. The formation cross sections of the Cs and Ba products on the neutron-excess side of stability decrease monotonically with increasing energy above 0.1 GeV, whereas the excitation functions for independent formation of the more neutron-deficient products in the Cs-Ba region and of ${\mathrm{Rb}}^{84}$ and ${\mathrm{Rb}}^{86}$ all go through maxima. The proton energies at which these maxima occur fall on a smooth curve when plotted against the neutron-proton ratio of the product, with the peaks moving to higher energies with decreasing neutron-proton ratio. Under the assumption that the mass-yield curve in the region $125lAl140$ is rather flat at each proton energy, the cross-section data in the Cs region can be used to deduce the charge dispersion in this mass range. Plots of $log\ensuremath{\sigma}$ vs $\frac{N}{Z} (or Z\ensuremath{-}{Z}_{A})$ show symmetrical bell-shaped peaks up to a bombarding energy of 0.38 GeV, with full width at half-maximum increasing from $3.3 Z$ units at 0.10 GeV to about $5 Z$ units at 0.38 GeV, and with the peak position (${Z}_{p}$) moving from ${Z}_{A}\ensuremath{-}1.44$ to ${Z}_{A}\ensuremath{-}0.85$ over the same energy range. At all higher energies, a double-peaked charge distribution was found, with a neutron-excess peak centered at $\frac{N}{Z}\ensuremath{\sim}1.515({Z}_{p}\ensuremath{\sim}{Z}_{A}\ensuremath{-}1.9)$, and having approximately constant width and height at bombarding energies greater than 1 GeV. The peak on the neutron-deficient side which first becomes noticeable at 0.68 GeV appears to become broader and shift slightly to smaller $\frac{N}{Z}$ values with increasing energy. The two peaks are of comparable height in the GeV region, and the peak-to-valley ratio is only \ensuremath{\sim}2. The total formation cross section per mass number in the Cs region decreases from \ensuremath{\sim}52 mb at 0.1 GeV to about 29 mb at 1 GeV and then stays approximately constant; the contribution of the neutron-excess peak above 1 GeV is about 12 mb. The neutron-excess peak corresponds in width and position to that obtained in fission by \ensuremath{\sim}50-MeV protons. The recoil behavior of ${\mathrm{Ba}}^{140}$ leands support to the idea that the neutron-excess products are formed in a low-deposition-energy process. The recoil behavior of ${\mathrm{Ba}}^{131}$ indicates that it is formed in a high-deposition-energy process. Post-fission neutron evaporation is required to account for the observed characteristics of the excitation functions of the rubidium isotopes and the neutron-deficient species in the Cs region. The correlation between neutron-proton ratios and positions of excitation function maxima is semiquantitatively accounted for if fission with unchanged charge distribution, followed by nucleon evaporation, is assumed.

Nitrogen-Induced Nuclear Reactions in Potassium

Excitation functions have been measured for the production of ${\mathrm{Fe}}^{53}$, ${\mathrm{Fe}}^{52}$, ${\mathrm{Mn}}^{52}$, ${\mathrm{Mn}}^{52m}$, ${\mathrm{Mn}}^{51}$, ${\mathrm{Cr}}^{49}$, ${\mathrm{V}}^{48}$, ${\mathrm{V}}^{47}$, and ${\mathrm{Ti}}^{45}$ from the nitrogen bombardment of potassium in its normal isotopic mixture. The method used was activation of thick targets of KBr followed by chemical separations and absolute beta-counting. The cross sections at 27.5 Mev range from 0.14 mb for ${\mathrm{Fe}}^{52}$ to 8.3 mb for ${\mathrm{Mn}}^{51}$. The relation of the ${\mathrm{Mn}}^{52}$: ${\mathrm{Cr}}^{49}$: ${\mathrm{Fe}}^{52}$ ratio to the odd-even effect in the nuclear level density is discussed. The total cross section for formation of the compound nucleus was estimated at several energies. The fraction of this total accounted for by one-particle emissions from the compound nucleus is surprisingly large.