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A New Look at the Fission-Product Gamma-Ray Component of Nuclear Weapon Initial Radiation*

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A New Look at the Fission-Product Gamma-Ray Component of Nuclear Weapon Initial Radiation* Paper 13 A NEW LOOK AT THE FISSION-PRODUCT GAMMA-RAY COMPONENT OF NUCLEAR WEAPON INITIAL RADIATION* L. G. Mooney and R. L. French Radiation Research Associates, Inc. Fort Worth, Texas *RRA-M-7102; based on work sponsored by the Defense Nuclear Agency and performed under subcontract for the Oak Ridge National Laboratory. 251 253 ABSTRACT A new model has been developed for predicting the fission- product gamma-ray component of the initial radiation exposure during the first minute following a nuclear detonation in air. The model incorporates Monte Carlo air transport data, fission- product source spectra, cloud rise approximations, air-ground interface effects, and hydrodynamic enhanc€iment treatments from the work of a numbar of previous investigators. Fission-product gamma-ray doses calculated with the model,, when combined with secondary gamma-ray doses based on StrakerTs air-over-ground neutron transport calculations, give results that generally agree with weapons test data within 25% for low-yield weapons and 50% for high-yield weapons. Calculations performed with the model show that the fission-product component of nuclear weapon initial radiation is often much more important than is generally indicated in the weapons effects literature. 254 INTRODUCTION The fission-product gamma-ray component of the initial radiation exposure from nuclear weapons has been mentioned in a number of publi- cations dealing with initial radiation.l*z*3However, the importance of fission-product- gamma rays during the first minute or so following a detonation has generally been underestimated in both the unclassified and the classified literature. The relatively poor understanding of the fission-product gamma rays has been due largely to the fact that, in the absence of a systematic approach for calculating this component from theoretical considerations, most information has been based on data measured in weapon tests where the distinction between the fission- product and the secondary gamma-ray components is often not clear. (The integral dose from prompt fission gamma rays is negligible com- pared to the other components because they are emitted and largely absorbed by the weapon while it is still intacti) The recent availability of accurate data5 describing the pro- duction and transport of air- and ground-secondary gamma rays and techniques6 for applying these data to specific weapons has provided new incentive for calculating the transport of fission-product gamma rays from basic principles. Knowing the portion of the measured initial gamma-ray dose which may be attributed to secondary gamma rays, one can compare calculated fission-product doses with the remainder. With this opportunity in hand, a reasonably successful method has been developed essentially from basic principles which permits calculations of the fission-product dose. These doses, when combined with those resulting from secondary gamma rays, have been found to give good agreement with measured data for a wide range of weapons. The number of gamma rays emitted from fission products, their energies, and the decay rates are well known. Gamma-ray cross sections are relatively easy to handle in transport calculations. Several methods are available with which gamma-ray transport in an air or 255 air-over-ground geometry may be calculated to a high degree of accuracy. In spite of these facts, there is a dearth of reliable quantitive calculated data on the magnitude and spatial distributions of the gamma dose from the decay of fission products during the first one or two minutes after the detonation of a nuclear weapon. This situation results largely from a combination of source and media dynamics that is probably unique in transport problems. These dyna- mics include the formation and evolution of the fireball, the cloud expansion and rise, the decay of the fission products with time, and the severe pertubation of the air through which the radiation penetrates. The formation of the fireball and the expansion and rise of the cloud depends upon weapon yield, weapon design, atmospheric conditions, and other parameters. The distribution of fission products in the cloud as a function of time is not well known, and, consequently, the attenuation of fission-product gamma rays within the cloud can only be approximated. The sudden release of energy in the form of blast and shock produces an immediate increase in temperature and pressure thus pro- ducing hot, compressed gases from the weapon material. The gases ex- pand rapidly and, in so doing, initiate a pressure wave, called a "shock wave" in the surrounding medium. The characteristic of a shock wave is that there is a sudden increase of pressure at the front with a gradual decrease behind it. Associated with this pres- sure change is a much more severe change in the density of the heated air behind the shock front. It is the large reduction in the air density or optical depth between the rising source and the receiver which produces the enhancement of gamma-ray intensities from hydro- dynamic effects. These effects may last for several minutes and reach out to large distances from the detonation. The intensity and duration of hydrodynamic effects increase with weapon yield. 256 A fission-product gamma prediction model capable of con- sidering all of these factors in full detail is impossible to for- mulate with existing technology. However, there are adequate data and techniques available for treating the most important considera- tions and there are reasonable approximations available for the remainder. The principal objectives of the study upon which this paper is based were to develop the available treatments into a single comprehensive fission-product calculation model and to evaluate the model through compairsons with weapon test data. The study was performed in 1969 and was originally reported in the classi fied literature. During the preparation of the present paper, the authors learned of another formulation7 which is similar in many details to the one described herein. 257 METHODS DEVELOPMENT In developing the model to calculate the fission-product gamma exposure from weapons, the effects of cloud rise (assuming a point source of radiation), source decay with time, and hydrodynamic enhancement were studied in the order mentioned. The final model in- cludes these factors plus an accurate definition of the predetonation environment and geometry. Limited sensitivity studies indicated that the effect of cloud expansion and attenuation within the cloud and associated debris could be assumed negligible for separation distances of practical interest. Hence the cloud is treated as a point iso- tropic source.* The model is unique in that all factors considered in the dynamics of the problems with the exception of the cloud-rise formula, are based on data or methods obtained from basic considerations, either from theoretical calculation or from data generated in the laboratory. The model employs the predetonation geometry and environment (i.e., the detonation height, detector height, air density, air pressure, etc.). The gamma-ray attenuation in air following detonation is obtained from Monte Carlo transport data8 using an energy spectrum measured in the laboratory9. The formalisms giving the original source intensity and decay with time were also obtained from laboratory measurements9. The hydrodynamic enhancement is based on empirical expressions fitted to data obtained from a mathematical treatment of idealized shock front behavior. Air Transport Data Development of the model for calculating the fission-product gamma exposure starts with consideration of the transport of gamma rays from an idealized point isotropic source of gamma rays in air. Monte Carlo data giving the dose versus distance in an infinite air medium for monoenergetic gamma-ray point sources with energies of 0.5, 1, 2, 4, 6, 8, and 10 MeV have been reported by Marshall and Wells8. These data were folded with the fission-product energy spectrum in the 0.2 - 0.5 second-time interval following fission. This energy spectrum was taken * Studies reported in Reference 7 further substantiate the point source approxitaation. 258 from t.he measurements of Engle and Fishet* and is representative of all gamma rays emitted during the first minute following fission* As the next step, the resulting data were plotted in the form of R* dose per source photon from fission-product decay versus range R expressed in mass thickness or optical depth (gn»/cra2). This curve was fitted by the expression R2D(R> - Aa~R/A where A « 3.611 X 10~17 and A « 97.04 for R < 21.28 gas/cm2, and A - 5.826 X l<f17 and X * 30.56 for R > 21.28 gta/cm2. Source Strength Since R2D(R) is the dose (rad) per fission-product source photon, it must be multiplied by the source strength of fission products. 23 Assuming 1.45 X 10 fissions/KT, a fission fraction F (i.e., fraction of the total yield attributed to fission), a total yield of W(KT) and a time-dependent fission-product gamma-emission rate of G(T) photons/sec- fission, the total source strength (photons/sec) at time T after fission is S(T) « 1.45 X 1023 F W G(T) • (2) The decay rate <photons/sec~fission) is represented by a fit to the Engle and Fisher data:9 G<*> - 1 +°0887 T • <3> Application of these terms to Equation (1) and rearrangement gives the equation for the time-dependent dose (rad/sec) from a stationary point fission-product source in unperturbed homogeneous air (i.e., without 259 the hydrodynamic effect and air-ground interface effect) 1.16 x 10 J F W A -R/A D(T,R) = (4) (1 + 0.87T)R2 Three modifications of Equation (4) are required to consider the air- ground interface effect, the cloud-rise effect and the hydrodynamic effect. Air-Ground Interface Effect A systematic method for adjusting fission-product gamma-ray data for air-grcund interface effects is not yet available.
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