Photoelectron Production Research Articles

The pre-dawn enhancement of λ6300 Å airglow at higher midlatitudes

All-sky records of λ6300 A ̊ airglow intensity have been obtained at Hobart (geomagnetic latitude, 54°S) for the years 1968 and 1969. These observations cover geomagnetic latitudes 44°S to 64°S. A winter pre-dawn enhancement (P.D.E.) of about 50 R is observed for this line down to about 60°S. The commencement time of the phenomenon is usually inconsistent with the increase in production of photo-electrons at the magnetically conjugate point (M.C.P.). In addition ionospheric evidence points to an increase in the dissociative recombination of O 2 + as being a major cause of the enhancement. However, it is proposed that M.C.P. photo-electrons do play an important role in the P.D.E. but that the commencement of the P.D.E. is controlled by the morphology of the neutral atmosphere winds superposed on effects due to exospheric cooling.

Photoelectron escape flux observations at midlatitudes

Photoelectron production and escape flux at midlatitudes, using spectrometric measurements from ISIS-1 satellite

Delayed Coincidence Time Resolution: NaI(Tl)

For delayed coincidence experiments using NaI(Tl) scintillation counters, first photoelectron triggering (FPET) has been practicable since the advent of the RCA 8575. Using FPET, improved time resolution at low y ray energies was first demonstrated by Lynch, who also described his results by a model that fails to take into account the photomultiplier response. Using FPET, we have done systematic experiments from 6-511 keV for three different crystal sizes. We describe our results quantitatively by a simple model that takes into account the statistics of photoelectron production and the photomultiplier response, similar to El-Wahab and Kane. From the model calculations, we conclude that with present photomultipliers and NaI(Tl) scintillators, further improvements in resolution cannot be expected. The major difficulty is not the light output of NaI(Tl), but the single electron response of the photomultiplier.

A study of the daytime electron temperature

The electron density and electron and ion temperatures given by the Saint-Santin-Nançay Thomson scatter experiment allow a detailed energy balance study for thermal electrons including their energy loss to ions and neutral particles, the energy received by conduction from the magnetosphere, and the energy they gain from photoelectrons. The collision process recently pointed out by Dalgarno and Degges (1968) (concerning electronatomic oxygen collisions involving the fine structure of O) is taken into account, but it cannot explain the relatively low temperatures observed around 200 km. Nevertheless, by taking the non-local heating into account, the agreement between theory and experiment is significantly improved. The same Thomson scatter observations were used by Vasseur and Waldteufel (1968) to solve approximately the continuity equation for electron density. This solution gives a value for the production of primary photoelectrons which is compared with our independent determination of the same quantity, obtained through the thermal electron energy balance.

Variations of plasma scale height in the upper ionosphere during the sunrise period

Alouette 1 topside soundings have been used to study predawn variations of plasma scale height for different seasons and altitudes. Plots of average plasma scale height against solar zenith angle show a difference in behavior for altitudes above and below 550 km, for solar zenith angles greater than 95°. Above 550 km there is an initial increase in plasma scale height that occurs at progressively higher solar zenith angles as the season changes from summer through equinox to winter. Subsequently, as the solar zenith angle changes from 105° to 95°, the plasma scale height decreases to its lowest value of the day. Below 550 km the plasma scale height is initially constant and then steadily increases during the period between solar zenith angles 105° and 95°. For solar zenith angles less than 95°, the behavior is of two types, either a sharp peak in the plasma scale height values, which is most pronounced at high altitudes, or fairly constant scale height values with a small altitude gradient in these values. It is shown that for the midnight and midday conditions the seasonal and altitude variations in plasma scale height can be explained in terms of the values of plasma temperatures, temperature gradients, and mean ionic masses. The way in which the values of the three quantities change during the sunrise period is studied, and it is seen that these changes can account for the plasma scale height variations during this time. Seasonal variations are explained in terms of photoelectron production in the conjugate ionosphere and possibly in the protonosphere. For solar zenith angles less than 95°, the plasma scale height variations are in agreement with the temperature changes predicted by da Rosa [1966].

Some aspects of the photochemistry of coordination compounds

The results of some investigations in this Laboratory on the photochemistry of Cr III and Co III complexes are summarized. Cr III complexes undergo only photosubstitution reactions, but not necessarily the ones that predominate in their thermal reaction chemistry. Co III complexes not only show the same type of photochemistry as do the Cr III ones, but also can undergo photoredox decompositions yielding Co II and oxidized ligand. Another kind of photoredox reaction, so far only observed with certain cyano complexes, is that of photoelectron production. A given complex may show one or another type of reaction, depending on the wave-length of light used, and one can generally account for the reaction mode in terms of the kind of excited state involved. Thus irradiation of d-d absorption bands tends to produce photosubstitution, and that of charge transfer bands, to give redox decomposition or photoelectrons. Often, more than one kind of substitution reaction is possible, and in the case of Cr III complexes the reaction chemistry of the excited quartet state supposed to be involved can be described with some success by means of empirical rules. The situation is complicated by competing radiationless deactivation processes, and by the likelyhood that the various excited states may have a different geometry from, i.e. be distorted relative to the ground state.

Effect of Naphthalene on the Quenching of Liquid Scintillation Solutions

IN liquid scintillation counting the passage of β-particles through the phosphor solution results in a complex series of events, the useful ones culminating in the emission of photoelectrons at the photocathode of the photomultiplier. A number of parasitic events occur concomitantly that lead to a lowering of the yield, or total loss of the photoelectrons, that is, quenching. Possible sequences for these events, which occur during a time-interval of approximately 10−9 sec, have been outlined by several authors1–3. The addition of naphthalene to some liquid phosphors improves their counting characteristics. The enhancement in counting efficiency has been ascribed by Fust and Kallmann4 to the ability of the naphthalene to act as an intermediate solvent that is capable of transmitting the energy from the primary solvent to the scintillators, thereby by-passing the quenching agent. If this is true the series of events leading to photoelectron production could be outlined as follows: (A) emission of β-particle; (B) excitation of primary solvent molecules (toluene); (C) excitation of secondary solvent molecules (naphthalene); (D) excitation of primary solute molecules (PPO); (E) excitation of secondary solute molecules (POPOP); (F) production of photons.

Phenomenological Theory of Townsend Breakdown in Dielectrics

The Townsend model of dielectric breakdown is discussed. The transient equations describing the behavior of the electron and positive ion current densities are solved rigorously with boundary conditions appropriate to a simplifed Townsend model employing photoelectron production at the cathode as the only secondary mechanism. The correct boundary condition is found to lead to a set of integro-difference equations. Solution of the integro-difference equations allows rigorous specification of the criterion for breakdown. This criterion is found to become equivalent to the familiar Townsend criterion in the asymptotic limit of large time intervals. The results of our development may be applied to the calculation of formative time lags and to the estimation of space-charge distortion effects.

The Application of a Magnetic Lens Spectrometer to the Measurement of Gamma-Radiation fromZn65andCo60

A thin magnetic lens spectrometer for the investigation of gamma-ray spectra is described. The effect of the thickness of the radiator used for the production of photoelectrons and the influence of the earth's magnetic field are reported. Based on a calibration of the instrument by means of annihilation radiation and the F line of ThB, energy values of ${1.10}_{6}$ Mev for the gamma-ray of ${\mathrm{Zn}}^{65}$, and ${1.15}_{5}$ and ${1.31}_{7}$ Mev for the two lines of ${\mathrm{Co}}^{60}$ are obtained. The probable error is estimated as 0.5 percent.

Some remarks concerning the production and absorption of soft X-rays and secondary electrons

In the present paper an attempt is made to estimate the efficiency of the production of photoelectrons from metals by soft X-rays. When due consideration is paid to the absorption of radiation and electrons in the target the number of photoelectrons for one quantum of radiation absorbed comes out of the order unity. This result in connection with efficiency measurements on soft X-ray production, using the photoelectric method, shows that the yield of such radiation, when metals are bombarded with electrons, is extremely small. A very much larger portion of the energy of the bombarding electrons, possibly all the rest of it, reappears in smaller units as energy of secondary electrons. The question as to the probable origin of these electrons is discussed; it appears most likely that at least a great part of these are initially free conduction electrons of the substance. Before the manuscript of this paper had been completed, a paper by Prof. O. W. Richardson appeared dealing chiefly with the same problems. Some of the results presented here have already been arrived at by the latter author. Since, however, my own reasoning in deriving these results differs in several respects from that of Prof. Richardson’s, I have decided to publish the present paper in its original form, in the hope that the different treatment would in itself present some points of interest.

The Direction of Ejection of Photo-electrons Produced by X-rays

Stereoscopic photographs of photo-electrons produced by 40 kv Mo Kα x-rays, were examined by a stereoscopic comparator which gave a quantitative measure of the initial direction of ejection in space. The measurement of twenty-four distinct tracks in a series of twenty-nine pairs of photographs showed that the most probable angle of ejection was about 70° with the primary beam. This supports the view that the electric vector of the radiation plays a very important part in the production of photo-electrons. The small forward or backward component, noticed in many tracks, must come from the random momentum of the electron in its orbit before ejection.