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Summary The response of Achillea millefolium L. to herbicides was measured to determine the effectiveness of the current recommendations and to test alternative herbicides. Five plots at each of the three replicate sites were selected and randomly treated with one of the four herbicides: dicamba/2,4-D, glyphosate, metsulfuron-methyl and triclopyr/picloram. After 12 months, the recommended treatment, dicamba/2,4-D, did not cause a significant reduction in aerial biomass compared with untreated controls and the number of flowering stems was significantly increased in treated plots. Metsulfuron-methyl and triclopyr/picloram caused a significant reduction in A. millefolium aerial biomass but did not affect the number of flowering stems. Glyphosate produced a significant reduction in aerial biomass and was less effective. These results suggest that the current recommendation for A. millefolium might be improved. Site-specific effects such as aspect may also influence the effectiveness of herbicides on this species.

A portable cardiac mapping system is used to improve the accuracy of diagnosis of acute ischaemic injury outside hospital. Patients presenting chest pain suggestive of myocardial infarction (MI) were mapped by attendant medical personnel operating from a mobile coronary unit. These first MI maps were compared against average normal maps using QRS and ST-T isointegral values. Discriminant function analysis performed on the parameters achieved a sensitivity of 90% and a specificity of 96%.

Fluorescence excitation spectra are measured for some organic crystals using excitation wavelengths down to 2200 A. The fluorescence quantum efficiency of crystalline p-terphenyl is found to be constant with wavelength of exciting radiation but apparent variations are found for trans-stilbene and particularly for anthracene crystals. These variations are due to surface escape of fluorescence the extent of which is sensitive to the depth of penetration of the exciting light in crystals which re-absorb their molecular fluorescence. The fluorescence excitation spectrum thus provides information about the absorption spectrum of the crystal. For excitation of the 001 crystal face of anthracene absorption maxima are found at 3960 A, 3740 A, 3540 A, 3370 A, 3220 A; these wavelengths are rather longer than hitherto reported. The first fluorescence maximum lies at 3980 A hence the 0-0 absorption and fluorescence transitions effectively coincide. Measurements made with an anthracene crystal having a very thin surface quenching layer of low fluorescence efficiency show that migration of electronic excitation energy through the crystal lattice occurs initially by a short range and non-radiative process. Possible transfer mechanisms are discussed and it is concluded that the most probable process is a quantum-mechanical resonance exchange between adjacent molecules resulting in a random diffusion of the excitation energy through the lattice. This is followed by the secondary and trivial process of fluorescence emission and re-absorption. The absolute photofluorescence quantum efficiencies of p-terphenyl and trans-stilbene crystals are measured by comparison with anthracene for which the quantum efficiency is 0.80 ± 0.05. At a temperature of 290°K the efficiencies of p-terphenyl and trans-stilbene crystals are respectively 0.52 and 0.65.

Experiments with mixed crystals of anthracene in naphthalene are described. A fluorimeter technique is used to measure crystal concentrations, and it is found that considerable purification of the melt occurs during crystallization. No significant difference is observed in the relative fractional intensity of each fluorescence component for excitation with 2540 ? radiations and with alpha-particles. A simple theory is formulated and agrees well with the experimental data. The equations obtained are used to explain the decrease in radioluminescence efficiency of organic phosphors due to radiation damage, and it is estimated that for anthracene each 5 MeV alpha-particle absorbed produces approximately 2 x 104 effective non-radiative dissipative centres.

Discrimination between voltage pulses of different magnitudes is effected by simultaneously applying each pulse and its differential to the X and Y deflecting plates respectively of an oscilloscope and by scanning the screen in the X direction with a thin optical slit and a photomultiplier. The pulse height distribution is displayed on a second oscilloscope and recorded photographically. The apparatus is free from zero drift and drift of channel width. The maximum counting rate for regularly occurring pulses is 3 × 105 per sec.

Measurements of scintillation light output versus α-particle energy are made for single crystals of organic and inorganic substances excited by monoenergetic α-particles. A new method is used to deduce a relation between the differential efficiency dL/dx and the specific energy loss dE/dx, which in the case of calcium fluoride is verified experimentally using thin evaporated layers. The effect of phosphorescent decay is observed by varying the circuit time constant. The intrinsic spread in scintillation pulse magnitude due to the phosphor is studied by inserting neutral filters between phosphor and photomultiplier and by variation in particle energy. Apart from a spread due to surface irregularities sources of spread are observed in diamond probably due to alternation of fluorescent and non-fluorescent crystal domains, and in anthracene, most likely due to knock-on protons near the end of the α-particle track.

Injected space charge trapped in the space-charge-limited dielectric diode by cooling under forward voltage bias is thermally released on warming under reverse voltage bias. Measurements on cadmium sulphide diodes are presented. The temperature dependence of reverse current shows narrow peaks produced by the thermal emptying of trapping levels. A three-level model is proposed for cadmium sulphide consisting of high and nearly equal densities of shallow donor levels and deep trapping levels and a moderately shallow trapping level with a density of about 1017/cm3 at a depth of about 0.40 ev below the conduction levels.

Space-charge-limited current in solids has been studied experimentally in crystals of cadmium sulphide. Accurate compensation of trapping defects has been achieved and the fundamental Child's square law for solids has been verified experimentally over a temperature range from 77° to 400 °K. Under space-charge-limited conditions the density of charge carriers available for current is independent of temperature; current is proportional to carrier mobility and provides a means of studying this quantity directly. By observing Child's law over the whole temperature and voltage range of interest it may be ensured that true space-charge-limited conditions are existing and consequently that reliable and accurate information is obtained. An insulator crystal provided with an emitting contact and a collector contact forms a dielectric diode. The emitting contact is a space-charge cloud of mobile carriers which is experimentally reproducible and is amenable to quantitative theoretical description; accordingly, the experimental characteristics of the diode are determined by the physical nature of the collector contact which is made in the normal manner. It is therefore possible to investigate experimentally the properties of a single contact. Rectification characteristics have been measured and compared with theory for a number of diodes over ranges of current covering nine orders of magnitude. The straightforward concept of a contact as two different sets of energy levels simply butted together is remarkably well vindicated.

The fabrication of experimental solid state heterojunction triodes is described using high resistivity cadmium sulphide films evaporated in vacuo on to low resistivity single-crystal silicon. Interfacial transmissivities for electrons through the heterojunction gate as high as 90% are obtained, with excellent saturation of drain characteristics. It is expected that useful incremental operation up to microwave frequencies should ultimately be possible with large signal switching times less than 0.1 nsec.

The various causes of the distribution in magnitude of the voltage pulses from the scintillation counter are analysed. A general expression is derived for the variance of this distribution in terms of variations in the number of photons emitted by the phosphor, variations in collection efficiency for photons emitted at different points within the phosphor and photomultiplier statistics. The effects of phosphor imperfections, non-uniform response of photocathode and dynode structures, thermionic emission of electrons from the photocathode and satellite pulses are also considered. The use of the equations developed is illustrated by detailed discussion of some commonly occurring arrangements.

Single crystal heterojunctions between n-type silicon (ND = 1015 cm-3) and p+-type germanium (NA = 5 × 1018 cm-3) have been fabricated by a `continuous flow' halide reduction epitaxy process. The energy band structure of the heterojunction has been investigated using measurements of reverse-bias incremental capacitance; the conduction band discontinuity at 20°C is ΔWc = 0.26 ± 0.02 eV and the valence band discontinuity is ΔWv = 0.21 ± 0.02 eV. Current-voltage measurements are consistent with this energy band configuration. In particular, forward current is carried by electron thermoemission current and not by hole diffusion current. Some potential applications for semiconductor heterojunctions are briefly reviewed.

The absolute scintillation efficiency of anthracene crystals excited with fast electrons is measured. The experimental method used avoids the errors existing in previous measurements which are due to the anisotropic refraction, spatial distribution and re-absorption of fluorescence in these crystals. At a temperature of 290°K the intrinsic scintillation efficiency (in the absence of re-absorption of fluorescence) is found to be 0 062 ± 0.004; this is reduced to 0 050±0 005 for the crystal by self-absorption of fluorescence. Reasons for this low efficiency are discussed. A detailed description is given of excitation conditions produced in organic crystals by ionizing particles with a discussion of possible quenching mechanisms in the excitation column. For electrons these are considered to be due to internal molecular dissipation of electronic excitation energy, to mutual interaction between excited molecules and to the strong electric fields surrounding ions. For the heavy particles these processes are more pronounced and further quenching occurs due to thermal fields and to damaged molecules. The role of migration of excitation energy is also considered. This description of the scintillation process satisfactorily explains the low efficiencies and the decay shapes of fluorescence observed experimentally.

Electron energy levels in cadmium sulphide single crystals have been investigated by making Hall effect, conductivity and space-charge-limited current measurements. Data for many crystals have been combined and indicate levels at 0.16, 0.24, 0.45, 0.63 and 0.82 ev below the conduction band edge. Level densities are generally in the range 1015-1017 cm-3.

Relative quantum intensity measurements have been made of the fluorescence spectra, excited by 254 mμ radiation, of crystalline anthracene, trans-stilbene, para-terphenyl and diphenylacetylene at room temperature, using a calibrated spectrophotometer and photomultipher. Observations were made of the reflection and transmission spectra from 1 cm thick crystals of each compound, and also of the reflection spectrum from a thin microcrystalline layer of anthracene. The latter corresponds closely to the true molecular emission. Due to self-absorption, this has only a probability ke = 0.24 of direct transmission through a thick crystal. This value confirms the difference in technical and molecular photo-fluorescence decay times, reported by Birks and Little (1953). For the other materials, ke is larger, and for para-terphenyl it is practically unity. Vibrational bands have been resolved, which have been assigned to the 1-0 and 0-n (n = 0 to 3) transitions in anthracene, and to the 0-n (n = 0 to 5) transitions in para-terphenyl. The 1-0 transition in anthracene is attributed to fluorescence prior to the establishment of thermal equilibrium.

The paper describes the observation, recording, and interpretation of a pre-breakdown phenomenon occurring in the liquid dielectric hexane. The effect is seen as a region of changed refractive index, and an optical Schlieren system and a high speed streak camera have been constructed and used to investigate its nature and in particular its rates of growth and decay. From the results obtained a theory of breakdown of the point-plane electrode system is deduced and shown to explain the observed phenomena remarkably well. The region of disturbance itself is interpreted as a local heating which marks the extent of a weak plasma. This plasma forms at the point cathode, grows in size with time to span most of the electrode gap, and breakdown indicated by an arc finally ensues. From the plasma geometry it is estimated that the minimum field existing at the plasma boundary during breakdown is about five times greater than the mean applied field. Measurements of the rate of movement of the plasma boundary suggest that at these high fields electron mobilities are becoming field dependent and rising rapidly. The minimum breakdown field is estimated as about 620 kv cm-1 and the average electron mobility at this field is estimated as about 0.03 cm2 v-1 sec-1.

The fluorescence characteristics of organic crystals differ appreciably from those of the emitting molecules due to self-absorption of the molecular fluorescence. These differences are investigated and properties of the fluorescence of the crystal are related to the corresponding properties of the molecular fluorescence; the equations developed are made the basis of an experimental determination of molecular quantum efficiencies of photofluorescence for the crystalline state. This method avoids the considerable errors involved in a direct measurement which are occasioned by having to integrate numerically over the whole sphere of emission inside an optically anisotropic crystal the fluorescence intensity that is measured directly only within a certain solid angle external to the crystal. The method developed is particularly applicable to crystalline anthracene which has considerable overlap of absorption and fluorescence spectra. Experimental data for this substance are presented. These include the molecular and crystal fluorescence spectra and decay times; the latter are found to be 6 4 ± 0.2 mμ sec and 18.0 ± 0 5 mμ sec respectively. The molecular quantum efficiency of photofluorescence of crystalline anthracene at a temperature of 290°κ is found to be 0.94 ± 0 02 which is reduced, by the effects of self-absorption of fluorescence, to a quantum efficiency for the crystal of 0.80 ± 0.05; efficiencies of photoluminescence are found to be the same within the limits of experimental error.

Space charge limited current in solids ideally follows a square-law dependence on applied voltage. Deviations from this law are caused by carrier diffusion at low voltages and by field dependence of carrier mobility at high voltages. Between these extremes is a useful range of three to four decades of current available from zero to ultra-high frequencies. Experimental measurements of power in the random voltage waveform of noise fluctuations are shown and illustrate the linear relation between noise power and diode current.

Operation of joule-heated, continuous slurry-fed melters has demonstrated that off-gas aerosols are generated by entrainment of feed slurry and vaporization of volatile species from the melt. Effective off-gas stream decontamination for these aerosols can be obtained by utilizing a suitably designed and operated wet scrubber system. Results are presented for performance tests conducted with an air aspirating - type venturi scrubber processing a simulated melter off-gas aerosol. Mass overall removal efficiencies ranged from 99.5 to 99.8%. Details of the testing program and applications for melter off-gas system design are discussed.

Surface escape of fluorescence is enhanced in crystals which strongly re-absorb their own fluorescence. This is investigated quantitatively for anthracene. The apparent fluorescence efficiency, measured by viewing the intensity of fluorescence transmitted through the crystal, is found to decrease as the depth of penetration of the excitant decreases. For photo-excitation this produces a fluorescence excitation spectrum since different exciting wavelengths penetrate different distances. The minima of the excitation spectrum correspond to absorption maxima. The maxima of the first absorption band of the crystal are hence found to occur at wavelengths 3960 A, 3740 A, 3540 A, 3370 A, and 3220 A.

THE modern theory of solids predicts that an electron can travel freely through the periodic potential field of a perfect crystal lattice provided that its energy lies within certain well-defined limits, or energy bands, which are characteristic of the crystal. A practical implication of this is that, if a sufficient number of such ‘free’ electrons can be introduced from a metal cathode into the allowed bands of an insulating crystal, it should be possible to obtain a space-charge limited current flowing through the crystal. Such a current flow would be analogous to the space-charge limited current which flows across the evacuated region between the cathode and anode in a thermionic vacuum diode.