Gravitational models have been widely used to study the effects of fields on particle motion. The advantages of such models are: (1) They are often easier or cheaper to construct and use than the system which they represent. (2) Particle motion is readily seen and may be photographed. (3) Events lasting for a fraction of a microsecond as, for example, with electrons moving in electrostatic fields, may be demonstrated in models as events which take a few seconds. The article describes the use of a tilted plane as a two-dimensional, uniform gravitational field. It has been used successfully as a sixth-form physics experiment to demonstrate the motion of projectiles fairly close to the Earth over a limited range. Thus it shows the motion of artillery shells (ignoring air resistance) but not of artificial satellites or long-range rockets, for which a simple uniform field model is not valid.

Energy distributions of electrons in beams from hollow-anode gas discharge electron guns have been determined for electron flight paths of 0.1, 0.3 and 0.6 m. Guns have been operated at potentials between 5 kV and 20 kV in hydrogen, helium, oxygen, air, nitrogen and argon at pressures in tht approximate range 10-100 N m-2. A typical spectrum obtained has a sharp peak at an energy equivalent to that achieved by electrons accelerated to the full cathode potential. A second peak of amplitude 0.05-0.5 times the primary peak amplitude occurs at an energy lower by approximately one ionization potential. This secondary peak is broader, the amplitude falling to approximately zero within an energy range of about 100 eV.

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In addition to the gamma ray highintensity dosimetry systems discussed in Nucleonics in October, 1959, several special systems have been utilized by this laboratory in studying radiation effects on materials. A few of these systems are discussed below in the belief that they offer certain advantages in cost or performance and may be of interest to other workers in the field. Since almost any material that exhibits a measureable and reproducible change in physical or chemical properties can be used as a dosimeter, the usefulness of such systems depends on a thorough knowledge of these changes. The foremost requirement, that property changes be accurately measureable, limits their usefulness to limited regions of integrated dose. The requirements for approximating the energy sensitivity, volume, and density of the test materials must also be considered when the energy spectra of the source is unknown or when self-absorption characteristics of the material sample are believed significant. In addition to using dosimetry that approximates the test materials in effective atomic number and sample self-absorption characteristics, more standard techniques such as graphite ion chambers and ferrous-ferric chemical dosimeters are used for comparison with other facility results when possible. At high dose rates or high total dose applications only a few satisfactory systems are available. The use of more than one system is of value for reliability as well as for diagnosis of dose rate and energy sensitivity effects which may influence results obtained with the test material. Integrated dose systems generally involve measurement of some chemical change in a material and are most useful in performing two functions. The first is as a monitor of the total radiation received. This is particularly valuable in sources having a rapidly changing dose rate, especially when the dosimeter system is chosen to have a spectral response similar to the test specimen. The other function is as an absolute calibration of dose or dose rate.