Showing papers by "Chalk River Laboratories published in 1968"

Characteristics of Neutron Damage in Silicon

Abstract: The production and annealing behavior of the divacancy and the $A$ center in fission-neutron-irradiated silicon was studied by infrared absorption, using the 1.8-, 3.9-, and 12-\ensuremath{\mu} bands. The production rate of the divacancy was found to be high, about 5.7 ${\mathrm{cm}}^{\ensuremath{-}1}$, and to be enhanced by the presence of boron (\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ atoms per ${\mathrm{cm}}^{3}$), but not by the presence of oxygen (\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ atoms per ${\mathrm{cm}}^{3}$). The annealing of divacancies in neutron-irradiated Si required an activation energy of 1.25 eV, as in electron-irradiated Si, indicating that most of the divacancies were removed by diffusion to sinks. The annealing results also indicate that the local defect concentrations in the damaged regions can be as high as \ensuremath{\sim}${10}^{20}$ defects ${\mathrm{cm}}^{\ensuremath{-}3}$, in which the divacancies still retain their individual properties as far as their infrared absorption and annealing properties are concerned. The production rate of the $A$ center was found to be extremely low. The near-edge absorption band was also studied. About 95% of the near-edge band disappeared upon annealing in the same broad temperature range as did the divacancies. From these results, it was concluded that the majority of the total volume in localized damage regions produced by the fission-neutron irradiation of silicon is rich in divacancies and is still crystalline.

Lattice Dynamics of Niobium-Molybdenum Alloys

Abstract: The frequency-versus-wave-vector dispersion relations for the normal modes of vibration of a series of alloys of the transition metals niobium and molybdenum have been measured at 296\ifmmode^\circ\else\textdegree\fi{}K, and previous measurements on the pure metals have been extended, using coherent, one-phonon scattering of thermal neutrons. The phonon dispersion relations are very different for the two pure metals, suggesting that the electronic structure, acting through the electron-phonon interaction, plays a significant role in the determination of the dynamics of these materials. The observed neutron groups corresponding to the phonons in the alloys are not significantly broader than in the pure metals. The dependence of the dispersion curves on alloy composition is found to be complicated, both the general level of frequencies and the shape of the curves changing significantly. Fourier analysis indicates that the interatomic forces in the metals are oscillatory and of long range. Suspected Kohn anomalies are observed on several branches of the dispersion curves. With the assumption of a rigid-band model, the positions of several of these anomalies correlate with the calculated electron band structure for tungsten. The dimensions of the Fermi surface obtained from this correlation are in agreement with other Fermi-surface information.

Magnetic excitations in uranium dioxide.

Abstract: The spin-wave dispersion relations have been measured in antiferromagnetic uranium dioxide by inelastic neutron-scattering techniques. The triple-axis crystal spectrometer at the C5 facility of NRU was used throughout in its constant-Q mode of operation. The dispersion relations were obtained for spin waves propagating along the main symmetry directions at 9\ifmmode^\circ\else\textdegree\fi{}K, and less complete measurements were made at higher temperatures both above and below the N\'eel temperature. The theory of spin waves in U${\mathrm{O}}_{2}$ is developed and various models are used in attempts to deduce the exchange and anisotropy parameters from the experimental results. None of the models are completely satisfactory, because of the difficulties arising from the multidomain character of the specimen, and from the strong interaction between the magnons and the phonons. A theory of this interaction is also developed which gives quite reasonable agreement with experiment.

A characterization of the boundary between the cycling and resting states in ascites tumor cells

Abstract: Growth deceleration of an Ehrlich ascites tumor with increasing mass is associated with a prolongation of the cell cycle and a decline in the growth fraction. These effects are reversed upon transfer of cells from an older tumor into a new host. Studies were made to locate the stages at which a cell cycle could be suspended or resumed. Transplantation caused a prompt rise in both mitotic and flash H3TdR labeling indices. When all the cells in cycle including mitoses were prelabeled with H3TdR in older tumors, the fraction of labeled mitoses did not decline for a considerable period after transplantation into new hosts. This suggests that the early rise in mitoses is not due to a flow of resting (Go) cells from a G2 store (G2-Go transition). It appears rather to be a reflection of a lag of the mitotic process relative to other stages during the initial readjustment of the cycle. A prompt rise in flash H3TdR indices in the transplants suggested cell entry into S from either a suspended GI (G1-Go transition) or a suspended S (S-Go transition). These possibilities were examined by relating micro-spectrophotometric estimates of DNA to the cell cycle stage as revealed by H3TdR autoradiography. Since Go cells had DNA values corresponding to GI, it was concluded that decycling or recycling could occur only after mitosis and before DNA synthesis.

Nuclear Structure of Ca 40 and Elastic Scattering of 750-MeV Electrons

Abstract: It is shown that inclusion of the short-range nucleon-nucleon correlations in the usual shell-model description can provide adequate agreement with the known results of the electron elastic-scattering cross section and can make some interesting predictions

The influence of niobium in irradiation strengthening of dilute Zr-Nb alloys

Abstract: The irradiation damage configuration and yield stress response obtained in the c.p.h. α’and α phases in a Zr-2·7% Nb alloy after fast neutron irradiation have been studied as a function of pre-irradiation solute distribution. Largest yield stress increments were obtained for specimens irradiated with the solute initially in metastable solution, pre-irradiation ageing treatments reducing the yield stress response. Electron microscope studies have shown that the irradiation damage configuration was also determined by the solute super-saturation in the alloy phases, maximum strengthening being associated with a population of defect clusters or loops of < 30 A diameter. A mechanism for the defect stabilization effect is proposed.

X- and gamma-irradiation of dilute solutions of chymotrypsin: the active intermediate.

Abstract: SummaryX- and γ-irradiation of dilute aqueous solutions of chymotrypsin in 10−3 M HCl or in water produce difference spectra, over the range 210–330 mµ, qualitatively similar to that obtained on reaction of the enzyme with hydroxyl radicals from Fenton's reagent.The protection of the esterolytic properties of chymotrypsin against irradiation has been measured using BTEE as the substrate and sodium formate, acetone, glucose, ethanol and iso-propanol as protectors. The results obtained, when combined with absolute rate constants available for reactions of the hydroxyl radical, show that radical to be the predominant reactive species in the irradiation of dilute aqueous solutions of the enzyme.

Experimental Difference between First and Zero Sound in KBr

Abstract: Long-wavelength acoustic phonons have been studied for each of the $[00\ensuremath{\zeta}]T$, $[00\ensuremath{\zeta}]L$, $[0\ensuremath{\zeta}\ensuremath{\zeta}]{T}_{1}$, and $[0\ensuremath{\zeta}\ensuremath{\zeta}]L$ branches in KBr at 95 and 463\ifmmode^\circ\else\textdegree\fi{}K by means of inelastic neutron scattering. This study has shown that the elastic constants of KBr determined by inelastic neutron scattering (zero-sound regime) exhibit a different temperature dependence from those determined by ultrasonic techniques (first-sound regime). For example, the change in the zero-sound elastic constant for the $[0\ensuremath{\zeta}\ensuremath{\zeta}]{T}_{1}$ branch between 95 and 463\ifmmode^\circ\else\textdegree\fi{}K is (23\ifmmode\pm\else\textpm\fi{}4)%, while the change in the first-sound elastic constant determined by Hauss\"uhl using ultrasonic techniques is (35.1\ifmmode\pm\else\textpm\fi{}1.2)%. Cowley's theory for first and zero sound shows reasonable agreement with experiment.

Mitotic activity in the bone marrow of rats and its relation to the level of plasma calcium.

Abstract: The mitotic activity of bone marrow in rats has been correlated with the concentration of calcium circulating in the blood. Both the mitotic activity and the plasma calcium were decreased after removal of the parathyroid glands and increased following a single injection of parathyroid extract. These effects were observed both in the presence and absence of colchinine. Colchicine was found to impair the activity of the hormone preparation in normal and parathyroidectomized rats, but not in thyroparathyroidectomized animals, indicating the suppressive action of the mitotic poison was mediated through the thyroid gland. The relations obtained between plasma calcium and the mitotic activity of bone marrow suggested that calcium, and indirectly the hormones governing calcium, may play a role in the control of haematopoiesis.