Understanding Radiation: Scientific Basis of Nuclear Explosions - Thermal Radiation
The observed phenomena associated with a nuclear explosion and the effects on people and materials are largely determined by the thermal radiation and its interaction with the surroundings. It is desirable, therefore, to consider the nature of these radiations somewhat further. Thermal radiations belong in the broad category of what are known as “electromagnetic radiations.”
These are a kind of wave motion resulting from oscillating electric charges and their associated magnetic fields. Ordinary visible light is the most familiar kind of electromagnetic radiation, and all such radiations travel through the air (or, more exactly, a vacuum) at the same velocity, namely, the velocity of light, 186,000 miles per second.
Electromagnetic radiations range from the very short wavelength (or very high frequency) gamma rays and X rays, through the invisible ultraviolet to the visible region, and then to the infrared and radar and radio waves of relatively long wavelength (and low frequency).
The approximate wavelength and frequency regions occupied by the different kinds of electromagnetic radiations are indicated in Fig. 1.74. The wavelengthl in centimeters and the frequencyv in hertz, i.e., in waves (or cycles) per second, are related bylv = c, where c is the velocity of light, 3.00 x 1010 cm per second. According to Planck’s theory, the energy of the corresponding “quantum” (or unit) of energy, carried by the “photon,” i.e., the postulated particle (or atom) of radiation, is given by
E (ergs) = hv = hc / l
= 1.99 x 10-16 /l (cm) (1.74.1)
where h is a universal constant equal to 6.62 x 10-27 erg-second. The energy quantum values for the various electromagnetic radiations are included in Fig. 1.74; the results are expressed either in MeV, i.e., million electron volt, in keV, i.e., kilo (or thousand) electron volt, or in eV, i.e., electron volt, units. These are obtained from equation (1.74.1) by writing it in the form
E (MeV) = 1.24 x 10-10 /l (cm) (1.74.2)
It is seen that the energy of the radiations decreases from left to right in the figure, i.e., as the wavelength increases and the frequency decreases.
The (thermal) radiation energy density for matter in temperature equilibrium is given by
E (radiation) = 7.6 x 10-15 T4 ergs/cm3,
where T is the temperature in degrees Kelvin. At the temperature of a conventional chemical explosion, e.g., 5,000K, the radiation energy density is then less than 1 erg/cm3, compared with roughly 108 ergs/cm3 for the material energy, i.e., kinetic energy and internal (electronic, vibrational, and rotational) energy. Hence, the radiation energy is a very small proportion of the total energy.
In a nuclear explosion, on the other hand, where temperatures of several tens of million degrees are reached, the radiation energy density will be of the order of 1016 ergs/cm3, whereas the material energy is in the range of 1014 to 1015 ergs/cm3. It has been estimated that in a nuclear explosion some 80 percent of the total energy may be present initially as thermal radiation energy.
Not only does the radiation energy density increase with temperature but the rate of its emission as thermal radiation increases correspondingly. For materials at temperatures of a few thousand degrees Kelvin, the energy is radiated slowly, with the greatest part in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum (Fig. 1.74). At the temperatures of a nuclear explosion, however, not only is the radiation energy emitted very rapidly, but most of this energy is in the spectral region with wavelengths shorter than the ultraviolet.
When a nuclear weapon explodes, temperature equilibrium is rapidly established in the residual material. Within about one microsecond after the explosion, some 70 to 80 percent of the explosion energy is emitted as primary thermal radiation, most of which consists of soft X rays.6
Almost all of the rest of the energy is in the form of kinetic energy of the weapon debris at this time. The interaction of the primary thermal radiation and the debris particles with the surroundings will vary with the altitude of burst and will determine the ultimate partition of energy between the thermal radiation received at a distance and shock.
When a nuclear detonation occurs in the air, where the atmospheric pressure (and density) is near to sealevel conditions, the soft X rays in the primary thermal radiation are completely absorbed within a distance of a few feet. Some of the radiations are degraded to lower energies, e.g., into the ultraviolet region, but most of the energy of the primary thermal radiation serves to heat the air immediately surrounding the nuclear burst.
It is in this manner that the fireball is formed. Part of the energy is then reradiated at a lower temperature from the fireball and the remainder is converted into shock (or blast) energy. This explains why only about 35 to 45 percent of the fission energy from an air burst is received as thermal radiation energy at a distance, although the primary thermal radiation may constitute as much as 70 to 80 percent of the total. Furthermore, because the secondary thermal radiation is emitted at a lower temperature, it lies mainly in the region of the spectrum with longer wavelengths (lower photon energies), i.e., ultraviolet, visible, and infrared.7
In the event of a burst at high altitudes, where the air density is low, the soft X rays travel long distances before they are degraded and absorbed. At this stage, the available energy is spread throughout such a large volume (and mass) that most of the atoms and molecules in the air cannot get very hot.
Although the total energy emitted as thermal radiation in a high-altitude explosion is greater than for an air burst closer to sea level, about half is reradiated so slowly by the heated air that it has no great significance as a cause of damage. The remainder, however, is radiated very much more rapidly, i.e., in a shorter time interval, than is the case at lower altitudes.
A shock wave is generated from a high-altitude burst, but at distances of normal practical interest it produces a smaller pressure increase than from an air burst of the same yield.