Mutual Recombination Research Articles

Irradiation creep by the climb-controlled glide mechanism in pulsed fusion reactors

Abstract The effects of irradiation pulsing on the enhanced climb-glide creep strain are analyzed. A variety of pulsed systems have been studied by adytically modeling the time-dependence of the bulk point-defect concentrations given by the rate theory. Equations for the creep strain are derived for Tokamak-type fusion reactors, pulsed accelerators and inertial confinement fusion reactors, including the effects of mutual point-defect recombination. The creep in pulsed fusion reactors is compared with the creep in systems subject to steady irradiation of equal total dose. It is found that point-defect concentration cycling (due to radiation pulsing) results in enhancement of the irradiation creep strain. In particular, a very large temperature-dependent enhancement is found for inertial confinement fusion reactors. The theoretical results are also compared with experimental data on deuteron irradiated nickel. Good agreement is obtained with the parameters used.

Void-swelling in irons and ferritic steels: I. Mechanisms of swelling suppression

Point defect and dislocation behaviour in α-iron and ferritic steels relevant to the understanding of their void-swelling resistance during elevated temperature irradiation is summarized. The key role played by both interstitial and substitutional solutes in inducing enhanced mutual recombination of point defects through trapping processes and also in lowering dislocation mobility and bias for preferential self-interstitial capture in these materials, and thereby controlling their void-swelling response, is emphasized.

The effect of mutual recombination on dislocation point defect sink strengths

Abstract The diffusion equations which describe the distribution of point defects around a line dislocation have been solved by a computer iterative technique for the case of dislocations in copper. The sink strengths have been calculated in a manner consistent with the requirements of the rate theory continuum model [RTC]. When mutual recombination is negligible the appropriate analytic results are reproduced exactly. An increase in mutual recombination leads to an increase in both sink strengths and the bias parameter. An empirical relation is derived for the variation of sink strength with mutual recombination.

The effect of mutual recombination and point defect loss to fixed sinks on the void sink strength

Abstract The diffusion equations which describe the distribution of point defects around an isolated void have been solved by a computer iterative technique. The void sink strengths have been calculated in a manner consistent with the rate theory continuum model. Any competition for defects between the void and either fixed sinks or mutual recombination results in an increase in the void sink strengths. An empirical relationship between the resulting void sink strengths and the perturbing parameters is developed from the computer simulation data.

Correlation of neutron and heavy-ion damage: II. The predicted temperature shift if swelling with changes in radiation dose rate

The theoretically predicted shift in the temperature regime of void growth with changes in atomic displacement rate is investigated. The general relations from which the swelling rate may be calculated as a function of temperature and dose rate are first recalled. Based on these, simpler explicit expressions for temperature shift with dose rate are derived. Each of these expressions applies in a different limiting regime of void growth, which is determined by the dominant process of point defect loss — that is, whether by mutual recombination in the matrix or by absorption at sinks, and by the mode of point defect absorption at voids, i.e., whether surface reaction or diffusion controlled. The physical bases of differences in these expressions are described. A compact summary of this information is presented.

Recombination of Ion Radicals in Single Crystals of Cytosine Monohydrate

Single crystals of cytosine monohydrate were irradiated with ..gamma.. rays at 77 K, and the electron spin resonance (ESR) spectra were recorded while gradually increasing the temperature. The analysis of the decrease in line intensity shows that the disappearance of cation and anion radicals is due predominantly to their mutual recombination. The kinetics study clearly displayed the existence of recombination processes with several distinct groups of activation energies. It is assumed that activation energies are reduced by the Coulomb interaction of closely spaced ions. The track model for the deposition of energy during irradiation is discussed.

First mass spectrometric measurements of positive ions in the stratosphere

ROCKET and balloon-borne electrostatic probe measurements have shown that positive and negative ions are present in the stratosphere reaching number densities between 100 and 10,000 cm−3 (refs 1–5). The observed densities indicate that galactic cosmic radiation is the major ionisation source and mutual recombination is the dominant ion loss process. No experimental information exists, however, on the nature of stratospheric ions. Here we communicate the first mass spectrometric measurements of positive ions in the stratosphere.

The analysis of void swelling experiments—part one

Abstract In materials under irradiation Frenkel Pairs are produced at a rate proportional to the irradiating flux. Of these defects a fraction (1-α') suffer immediate recombination, and, under neutron or heavy ion irradiation a further fraction of the vacancies condense into vacancy dislocation loops. The effective defect production rates are therefore substantially less than those assumed in previous studies. Moreover the fraction α' is not the same for different types of irradiation. Under electron irradiation the effective bias B becomes B eff = α'electron B while under ion or neutron irradiation B eff becomes B eff = (α'hB—f) where α'electron # α'h. A comparison has been made of the respective void swelling equations with available experimental information under both election and heavy ion irradiation and it has been possible to conclude the following. 1. Using the Heald variable bias model it has been possible to “fit” the rate theory to HVM results in pure copper and 316 and to the VEC experiments in 316 and 321. 2. The ion irradiation results vindicate the subsequent modification to the Heald theory in which competition between neighbouring dislocations is considered. 3. Using the constant bias model, both Cu and 316 HVM results require a bias of about 16% well in excess of the 1–2% originally suggested. 4. Mutual recombination, as distinct from immediate recombination, appears to be important in HVM experiments on Cu and 316. Further experiments are suggested to establish the magnitude of this effect and its dependence on alloying constituents.

Chain initiation of neopentane pyrolysis and a suggested reconciliation of the thermochemically calculated and measured rate constants for the recombination of t-butyl radicals

The rates of initiation of neopentane pyrolysis over the temperature range 756–845 K have been measured at reactant pressures in the range 100–200 Torr, by processing of data relating to the formation of the termination product, ethane, using an exact algebraic procedure. Thus, the data are free of the ambiguities introduced by the empirical extrapolation procedures used in previous direct determinations of the initial rates and the derived Arrhenius parameters of neo–C5H12→ CH3+ t–C4H9. (1) The result obtained is log (k1/s–1)= 16.1 ± 1.0 –(330 ± 15 kJ mol–1)/2.303 RT. Values of k1 are in agreement with previously measured values but the Arrhenius parameters are inconsistent with the presently accepted thermochemistry of reaction (1). A modified thermochemistry for the t-butyl radical is proposed leaving the heat capacity unchanged but using the new values for 298 K, ΔHf/kJ mol–1= 39, S(1 mol cm–3)/J mol–1 K–1= 210.8. These, and the presently derived k1 values allow the calculation for 800 K of log (k–1/cm3 mol–1S–1)= 12.9 with essentially zero activation energy and hence, via the geometric mean rule, lead to log (k9/cm3 mol–1 s–1)= 11.9 for the mutual recombination reaction, 2t–C4H9→ 2,2,3,3-tetramethylbutane. (9)The new thermochemistry suggested for t-butyl reconciles most thermochemically calculated values of k9 with directly measured values over a wide temperature range. However, further experimental and theoretical justification is necessary before the new thermochemistry can be unequivocally accepted.Concurrently evaluated with k1 is the result log (k3k6/k5/cm3 mol–1 s–1)= 13.2 ± 1.5 –(127 ± 12 kJ mol–1)/2.303 RT CH3+ neo–C5H12→ CH4+ C5H11(3), CH3+ i–C4H8→ CH4+ C4H7(6), 2CH3→ C2H6(5) which is shown to be in excellent agreement with that calculated from the independent results of other workers.

Mechanism and rate parameters for the pyrolysis of 2,3-dimethylbutane

The pyrolysis of 2,3-dimethylbutane (DMB) was investigated in the ranges of 667-770K and 10-181 Torr (1.3-24kPa) at up to 3% decomposition. A Rice-Herzfeld chain terminated by recombinations of methyl and isopropyl radicals is postulated with self-inhibition due to the abstraction of hydrogen atoms from the major olefin product, 2-methyl-2-butene (2M2B), giving resonance-stabilized 2-methyl-2-butenyl radicals which participate in termination reactions. DMB---- >2i-C 3 H 7 (1a) DMB----> CH 3 + C 5 H 11 (1b) can be represented for the temperature range investigated, by k 1 = k 1a + k 1b — 10 16.7 exp ( — 322 kj mol -1 / RT ) s -1 which is in agreement with a previous measurement, with thermochemical calculation and with the results of the present investigation. The values of the ratios of rate constants for the reactions i-C 3 H 7 — -►H + C 3 H 6 (3a) i-C 3 H 7 — —> CH 3 + C 2 H 4 (3b) 2,3-dimethylbutyl —->i-C 3 H 7 + C 3 H 6 (5a) 2,3-dimethylbutyl ——> CH 3 + 3-methyl-1-butene (5b) are estimated to be k 3b / k 3a = 0.040 and k 5a / k 5a = 55.6, independent of temperature. The values of the rate constants for the reactions CH 3 + DMB----> CH 4 + C 6 H 13 (7) i-C 3 H 7 + DMB -> C 3 H 8 + C 6 H 13 (4) CH 3 + 2M2B -> CH 4 + C 5 H 9 (12) are estimated to be k 7 = 10 13.3 exp ( — 60 kJ mol -1 / RT ) cm 3 mol -1 s -1 , k 4 = 10 13.2 exp (— 89 kJ mol -1 / RT ) cm 3 mol -1 s -1 , and, k 12 = 10 13.7 exp (— 63 k J mol -1 / RT ) cm 3 mol -1 s -1 . The differential activation energy for abstraction of primary/tertiary hydrogen atoms from DMB is found to be 21 k J mol -1 . The results confirm also the value 10 11.6 cm 3 mol -1 s -1 for the rate constant of the mutual recombination of isopropyl radicals.

Steady-state point-defect diffusion profiles in solids during irradiation

Abstract The steady-state distribution of mobile point defects in a monatomic solid under irradiation has been calculated for a semi-infinite solid and for a foil. The calculation was performed for defect annihilation by mutual recombination, at internal sinks, and by diffusion to the surface. Simple approximations, accurate to within 5%, have been developed for the analytical solutions to the differential rate equations.

The ultraviolet absorption by alkylperoxy radicals and their mutual reactions

Methylperoxy radicals, CH 3O 2 · have been detected by UV light absorption in the photolysis of azomethane, oxygen and nitrogen mixtures using the technique of molecular modulation spectrometry. The radical concentration is shown to be governed by second-order kinetics. The results give the extinction coefficient, o (exponential), at the peak (λ = 240 nm) = 4.4 × 10 −18 molecule −1 cm 2 and a value for the rate constant of mutual recombination, k, which lies between 2.2 × 10 −13 and 4.4 × 10 −13 molecule −1 cm 3 sec −1. Similar results have been obtained when the azomethane was replaced by azoethane, suggesting the formation of C 2H 5O 2.

Buildup of point-defect diffusion profiles in a foil during irradiation

Abstract The distribution of interstitials and vacancies in a foil under irradiation has been calculated as a function of both distance from the surface and irradiation time by solving the diffusion equation numerically on a computer. The defects were considered to annihilate at randomly distributed sinks, by mutual recombination, and by diffusion to the surface. Defect jump frequencies appropriate to silver at 125°C and foil thicknesses of 1 pm and 300 A were used. Large “humps” in the plot of vacancy concentration versus distance were found near the surface of the 1 pm foil at short irradiation times, unless the internal sink concentration was high. These humps may be responsible for some unusual void distributions observed near grain boundaries.

The diffusion of point defects to the foil surface during irradiation damage experiments in the high voltage electron microscope

Abstract The high voltage electron microscope can be used to simultaneously produce and observe irradiation damage structures in thin foils of many materials, including in particular the formation of voidage at elevated temperatures. In such experiments some of the vacancies and interstitial atoms produced in the damage process will be lost by diffusion to the foil surface, so that the results may not be characteristic of the bulk material if the foil is too thin. The steady state point defect concentrates built up in a thin foil have been computed as a function of the foil thickness, defect production rate and vacancy jump frequency, taking into account the effect of the mutual recombination of vacancies and interstitial atoms. The coupled diffusion equations for the point defect concentrations have been solved numerically for a thin foil, assuming the foil surfaces to be perfect sinks for the point defects, and the results for any material are expressed in terms of a single dimensionless parameter. The computions show that for a foil containing few internal sinks, which is the most demanding case, defect concentrations typical of the bulk material can be achieved within an appreciable fraction of a 0.5μm thick foil, for temperatures up to about one-half the absolute melting temperature. Foils of this thickness can readily be penetrated by high voltage electrons for most materials. The loss of point defects to the foil surface is shown to be further reduced when the foil contains a high density of internal sinks (e.g. dislocations, voids, precipitate particles) and there is an additional improvement at very high temperatures due to the effect of thermal vacancies.

Electron Spin Resonance of Gamma-Irradiated Single Crystal of Cytosine Monohydrate at 77°K

Single crystals of cytosine monohydrate have been irradiated and analyzed with electron spin resonance spectroscopy at 77°K. Two types of radicals have been observed and identified as the cytosine cation and anion radicals. The hyperfine splittings of the 14N and 1H nuclei for the cation radical are found to be in fairly good agreement with the predictions of the molecular orbital theory. The anion-radical hyperfine splittings could not be resolved in all crystal orientations. Both types of radicals rapidly disappear upon heating, probably via mutual recombination.

Diffusion creep under neutron irradiation

Integration of the diffusion equation shows that in a uniform stress or a non divergent stress gradient, the instantaneous neutron flux has no effect on steady diffusion controlled creep. This statement is general; it allows the mutual recombination of vacancies and interstitials, the creation of dislocation loops and other sinks by neutron irradiation, the presence of self stresses in the specimen and the dependence of the boundary conditions on neutron flux. A significant divergence is unlikely in practical cases, it requires a strength greater than is attainable in macroscopically strong solids. The existence of a Kirkendall effect is pointed out, together with the possibility of volume expansion.

Intcractions of Macroradicals in Polyethylene‐Polypropylene Blends

AbstractTransfer reactions of primary radicals formed by decomposition of dicumyl peroxide in atactic polypropylene–polyethylene blends give rise to the corresponding macroradicals. A mutual recombination of polypropylene and of polyethylene macroradicals leads to the formation of hybrid copolymer. From the values obtained experimentally it follows that the total crosslinking efficiencyE of the blend (defined as a number of total crosslinks formed per mole of dicumyl peroxide decomposed) depends on the concentration of atactic polypropylene in the blend and not on the concentration of dicumyl peroxide. The decrease in the total crosslinking efficiency of the blend is attributed to scission reactions of polypropylene macroradicals. On the basis of kinetic analysis of the crosslinking process, relations expressing the dependence of the total and interpolymer crosslinking efficiencies of the blend (the latter defined as the number of crosslinks between polypropylene and polyethylene macromolecules per mole of dicumyl peroxide decomposed) on the concentration of polypropylene and polyethylene monomer units in the blend were derived. The calculated course of interpolymer crosslinking efficiency of blend is in good agreement with the values of the interpolymer crosslinking efficiency E' for the blends determined from the concentration of polypropylene bound in the crosslinked blends. For the ratio of the rate constants of transfer to polypropylene and polyethylene we obtained a value k3/k2 = 2.3. The influence of the microheterogeneity of blends on the relations derived is discussed briefly.

Recombination between Positive Ions of Caesium and Negative Ions of Iodine in the Afterglow†

ABSTRACT The chemical reactions between caesium and halogen vapours at pressures of the order of 10−1 mm Hg or lower have been investigated. It was found that only the reaction in caesium and iodine mixture was sufficiently moderate for its recombination coefficient to be measured. Since caesium has the lowest ionization potential among the elements and iodine has a large value of electron affinity, it is to be expected that in the afterglow following a pulsed radio-frequency discharge in such a vapour mixture, the positive carriers will be caesium positive ions and the negative carriers will be iodine molecular ions, and the de-ionization will proceed mainly by their mutual recombination. On making these assumptions the recombination coefficient of was found to be 2·3±0·7 × 10−8 cm3 sec−1 for the process, Cs+ +I 2 −=Cs+I 2, at room temperature.