The investigations of high-frequency radio waves of insects have shown that when the insects are imbedded in flour, grain, or similar material no noticeable selective action occurs on the insect. That is probably due to the conduction of heat either to or away from the insect as the temperature varies between the two. It is therefore likely that even though each insect might have an optimum frequency of its own, practical considerations would require the use of the optimum frequency for the material in which it is imbedded.

The electromagnetic waves can be reflected, refracted, and diffracted. The waves can be reflected from any sharply defined gap if its dimensions are at least comparable to the wavelength and of a different dielectric Constant from that of the medium.

Infrared rays have been used for heating insects to the death point. If the insects are in grain or other such material, the material itself has to be heated to the required temperature. The infrared rays are readily absorbed by most materials so that the penetration is not so deep as with radio waves. The usual method of producing infrared rays is by means of the red incandescent bulbs that are widely used as heat lamps. In some commercial applications for heating grain to kill insects, the loose grain has been carried on belt conveyors between banks of infra- red lamps both above and below the belt. In this way the grain is quickly brought to the required temperature to kill the insects infesting it.

VISIBLE LIGHT RAYS have been used to attract insects so that they could be trapped and destroyed. No other effects have been found that could be used in insect control. They are produced by the common incandescent or fluorescent light bulbs giving a wide range of wavelengths of relatively low intensity. Light and ultraviolet rays, X-rays, and gamma rays are thought to be photons resulting from the collision of two atomic particles, the electron and the positron.

ULTRAVIOLET RAYS, which have various effects on biological organisms, usually are produced by means of the mercury arc enclosed in quartz. This produces wavelengths from about 2,400 to 4,350 angstrom units. Most of the wavelengths shorter than 3,000 angstrom units can be shielded out by using ordinary glass. The intensity of this type of radiation is given in ergs per square centimeter. The equipment used for biological studies has produced relatively low intensity.

Ultraviolet rays cause excitation of the molecules but not ionization. This excitation sometimes causes chemical changes. The absorption of ultraviolet rays seems to depend on the molecular structure of the material and on its color. The wavelength of about 2,600 angstrom units is the maximum absorption length for one group of acids often found in biological life.

Ultraviolet rays do not penetrate very deeply because of high absorption and some reflection.

G. F. MacLeod, at Cornell University, found that bean weevil adults were not killed when exposed for 20 minutes at 30 cm. to a 5-ampere, 100-volt, 60-cycle Cooper Hewitt burner, but most of the eggs were killed when exposed for 15 minutes.

J. G. Carlson and A. Hollaender, at the Oak Ridge National Laboratory, found that the mitotic ratio of grasshopper neuroblast exposed to ultraviolet rays of 2,537-angstrom wavelength was reduced from 0.97 to 0.58 as the intensity was increased from 750 to 24,000 ergs per centimeter. They also found that the length of exposure time was not important as long as the material received the same total amount of radiation.

SEVERAL FORMS of radiant energy can be projected with enough intensity to cause ionization of molecules on which they impinge. X-rays, one of the widely used forms, are usually produced by impinging high-velocity electrons (cathode rays) against platinum in a vacuum tube. Gamma rays resemble X-rays, but have shorter wavelength and are usually produced by the disintegration of radium. Various atomic particles can be projected against materials with extremely high velocities and thus cause ionization. The particles include electrons, alpha particles, deutrons, (which are the nucleus of "heavy" hydrogen), and neutrons (which can be obtained from atomic piles). The atomic particles can be accelerated to high velocity by such machines as the Betatron and the Cyclotron. These machines use magnets to rotate the charged particles in spiral paths while they are being accelerated by high voltage. The X-rays, neutrons, and gamma rays cannot be bent by magnets. The different forms of radiant energy produce ionization by various methods. The electromagnetic (X- and gamma) rays produce ionization by Compton scattering or the photoelectric effect. They are much less efficient than electrons or alpha particles, which utilize an electric charge to produce ionization. The neutron with no charge produces ionization indirectly by giving high velocity to a nucleus by inelastic collision or by disrupting a nucleus. When ionization is severe, often other side effects, such as secondary X-rays, are produced. The Geiger-Muller counter is one of the methods for determining ionization.

The principal difference in the effects of the various types of ionizing radiation is in the density of the ion clusters and the depth of penetration. X-rays and gamma rays produce very low ion densities but penetrate deeply. The shorter wavelengths penetrate more deeply than the longer wavelengths. The atomic particles produce ion densities in relation to their mass or self-energy and their electric charge. The alpha particles produce much higher ion densities than the smaller electrons, but the penetration is greatest with the small particles. The penetration would depend on the intensity of electromagnetic radiation or the velocity and electric charge of the atomic particles and on the density of the material on which they impinge. The greater the penetration the less dense the ion clusters, but the density of the ionization would not necessarily be uniform along the path of the ionizing radiatron. In the case of the electrons, the maximum ionization occurs at about one-third the depth of penetration. When the velocity of the atomic particles increases sufficiently their mass also increases, which fact agrees with the Einstein theory that mass and energy are the same thing.

The energy unit used in nuclear physics is the electron volt. It is defined as equal to the kinetic energy that a particle carrying one electronic charge acquires in falling freely through a potential drop of one volt. It is often convenient to use the million-times greater unit : million electron volt (mev).

1 mev = 3.83 X 10^-14 g. cal.=

1.07 X 10^-3 mass units=

1.60 X 10^-6 ergs= 4.45 X 10^-20 kw.-hrs.

Most of the machines used to accelerate particles or produce X-rays are classified according to their potential in volts.

The unit used to express the absorbed energy in ionizing radiation is rep ( roentgen-equivalent physical). This replaces the roentgen unit (r) which has been widely used in X-ray work and which is primarily a unit for photon energy dissipated in an arbitrary material, air, where 1 r is about 83 ergs/g. The two are somewhat similar, but the roentgen unit would vary for tissue absorption to some extent with the type of tissue and the amount of radiated energy. One rep equals 83 to 100 ergs/g. tissue.

A dose of 100,000 rep corresponds to a temperature rise in water of 0.2 C. The temperature effects caused by ionizing radiation in the absorber are negligible.

Many experiments have been conducted on the chemical and biological effects of ionizing radiation since 1900, and yet the exact nature of what takes place has yet to be explained. These experiments indicate that doses of radiation required to produce measurable chemical changes in vitro often far exceeded those required for profound biological changes in vivo.

According to F. G. Spear, of the British Strangeways Research Laboratory, ionization and not excitation has become generally regarded as the link between energy absorption and biological response there exists in the cell a specially sensitive volume within which ionizations are biologically effective; any ionization outside the sensitive volume is ineffective. It is known as the target or "Quantum hit" theory. Differences in sensitivity to radiation are explained by the chance distribution of ionization in the vital volume of the cell.

One of the methods of producing ionizing radiation has been investigated by the Bureau of Entomology and Plant Quarantine. The method uses cathode rays with ultrashort exposure time to treat food and commodities. The inventors say the ultrashort exposure time of using accelerated electrons destroys biological organisms with the minimum amount of harmful side effects to the material. The reason is that a time element is required for a chemical change but not for a biological change.

All kinds of ionizing radiation can be used under proper conditions for sterilization and preservation. The argument in favor of the electrons is that X-rays and gamma rays are not usable for practical purposes. The biological intensity of penetrating electrons is about 500,000 to 1,000,000 times greater than that obtainable with X-rays. The neutron particles that are easily obtained from the atomic pile cause a high amount of concentrated ionization that leads to greater amounts of side reactions than would be tolerable. The electrons accelerated by tensions up to io million volts would cause practically no radioactive byproducts in the irradiated products, but the contrary is true of neutrons.

According to R. D. Evans, a physicist at Massachusetts Institute of Technology, low-energy electrons, when traversing matter, result in elastic scattering by atomic electrons or nuclei. The intermediate-energy electrons result in ionization by inelastic collision with atomic electrons. The high-energy electrons (above 1-5 mev) cause inelastic collision with atomic nuclei and are deflected by the Coulomb field, so that X-radiation, together with ionization, is produced. The proportion of radiative and ionization losses depends on the type of material.

The cathode-ray machine investigated was of 3-million-volt capacity. The electrons were released from a specially designed tube through a window device about 15 cm. in diameter and of very thin aluminum. Each discharge of approximately one-millionth of a second was made by means of a spark gap and the discharges could be produced about once every second. The air scatters the rays slightly after they emerge from the window so that at a distance of I foot from the window the rays cover an area of about 1 square foot.