Automated Inspection Device for Explosive Charge in Shells - AIDECS

Certain defects in the explosive charge of an artillery shell can cause the projectile to explode prematurely in the barrel of the launcher from which it is fired. The sensitivity of the radiographic technique presently used is limited by the large influence of the steel shell casing on the transmitted radiation. A filmless radiometric technique utilizing the basic radiation principle of Compton scattering, which will detect cavities in the explosive filler with minimal interference from the steel casing, has been identified and tested. By scanning the shell with a beam of radiation and observing the Compton scattering through a unique collimating system, it has been possible to detect voids as small as 1/16 inch in cross section. The hardware consists of the source, beam collimator, detector collimator, and a large plastic scintillator detector system. The projectile is inserted into the beam path and moved through a fixed scanning pattern by a mechanical handling system. The scanning sequence is computer contra ll ed and results in a threedimensional data matrix giving a direct representation of density within the projectile. Voids are identified and classified by computer analysis, and shell acceptability decisions are automatically generated. An engineering prototype system is currently being assembled and tested. (A production prototype conceptual design is concurrently under development.) This new technique will replace an existing film radiography inspection procedure and eliminate the need for human interpretation of the defects, while providing more consistent and reliable inspections at lower costs. INTRODUCTION Experience has shown that certain defects in the explosive charge of an artillery shell can cause the projectile to explode prematurely in the barrel of the launcher from which it is fired. Since such failures are dangerous and costly, their incidence must be reduced to a minimum by ensuring that the defective shells are detected with the highest possible confidence. Therefore, a great need exists for a reliable, nondestructive inspection technique that provides a means of identifying defects inside a shell with a speed compatible with the production rates anticipated in the U.S. Army's Ammunition Base Modernization Program. Inspection programs currently in use rely primarily on radio.graphic techniques, utilizing x-ray sources and film radiographs that inspect only a limited sample of the entire output of a given production facility. With the development of new, automated production facilities under the Ammunition Base Modernization Program, the demands on nondestructive inspection programs are becoming considerably more severe, since they must ensure with a high degree of confidence that the highvolume production output is sufficiently free of defects. It is recognized that currently employed radiographic methods do not provide a sufficiently accurate and economical inspection capability for automated production of shells, as evidenced by the Army's current sponsorship of the AIDECS program to develop an engineering prototype for the automated, * Work supported by U.S. Army Materials & Mechanics Research Center, Watertown, Massachusetts, under Contract DAAG-77-C-0009. 612 filmless, high-speed inspection of 105 mm projectiles. The inspection technique embodied in this approach is based on the measurement of Comptonscattered radiation. The method was identified as the most suitable method for inspecting explosive charges for cavitation defects. It pro vi des high resolution data in a three-dimensional format which readily lends itself to a completely automated, cost-effective defect analysis. A further major advantage is that the technique is inherently less sensitive to defects in the steel projectile casing than transmission techniques. After demonstrating the feasibility of the technique in laboratory experiments, an engineering prototype which embodies the scattering technique was fabricated. This prototype is capable of performing a complete three dimensional inspection of the explosive filler charge in 105 mm, M1 projectiles. Based upon the full seale operation of the 105 mm engineering prototype system, it is projected that with some product improvements required to accommodate larger projectiles, several inspection modules will inspect artillery ammunition production on a 100% basis. The cost for this total inspection service is estimated to be significantly less than radiography costs incurred by the current sampling plan. COMPTON SCATTERING TECHNIQUE A review of the basic photon scattering physics and its relationship to artillery projectile inspection is presented below. This is followed by a brief description of an analytical model used as an aid in decisions. making design tradeoff This photon scattering inspection technique is based upon the fact that sufficiently energetic gamma radiation interacts with the material in its path by scattering a portion of the incident beam. This interaction, known as Compton scattering, is inspection volume element. The introduction of a void into the volume element means a reduction in the amount of material available to scatter garrma rays, and consequently results in a decrease in the detector response. On the other hand, the presence a higher density inclusion causes an increase in the detector response. the dominant mode of interaction between gamma rays The photon scattering technique collects this --<tnd--tal"get-mate+-i-a-~S--for-garrma-l"iiy.-efle~:i-e-s---tl&-weeft--s-C--at-t-€-1'-ed----r--a-G+a-t-'i-e-rt--eve-F-----a--Vel";ilf'9€-siJi--i-d-----angi-e---approximately 200 keV and several MeV. In this through the use of a large scintillator which views interaction mode, part of the energy of the garrma the projectile through a "focusing" collimator. ray is transferred to a target electron during a This 'is analogous to integrating the output of collision. Conservation 1 aws require that the severa 1 detectors, each monitoring the scattered photon be deflected in a particular direction as a radiation at a different angle about the incident result of the collision. A small loss of energy is beam. The focusing collimator allows only radiassociated with a small angular deflection; while ation from a small segment of the incident beam to 1 arger energy losses occur at 1 arger deflection reach the sci nt ill a tor;radiation scattered from angles. The maximum energy loss occurs when other regions of the incident beam is blocked by photons are scattered 180 degrees, directly back the focusing collimator.. The geometric design of into the incident beam. The probability for a the focusing collimator defines the inspection gamma ray to be scattered through a particular aperture, or one dimension"of the inspection volume angle is a clearly defined function of the incident element. The other two dimensions of the inbeam energy and the angle. Also, for gamma-ray spection volume are formed by the collimation energies in excess of about 100 keV the number of control of the incident beam. scattered photons is independent of the material composition and depends almost wholly on the electron density of the scattering target. In other words, the number of scattering events from a unit volume in the target depends almost entirely upon target density (number of electrons per'unit volume) in the volume element.