Detection and Characterisation of Low Dense Charges Inside Metallic Devices Used in Space Applications by Neutron Radiography

Abstract

Neutron radiography (NR) is a non-destructive evaluation method for detecting the presence of hydrogenous and other neutron absorbing compounds present inside sealed metal enclosures. Most of the explosives used in pyro devices are hydrogenous or contains boron. Detection of such compounds at its location becomes very critical when it has a specified function to perform such as cable cutting in satellites, stage separation in launch vehicles etc. Hence NR is a mandatory inspection technique for many of the pyro devices used in launch vehicles and satellites. Absence of explosive will lead to failure in the predefined action to be performed by the device. The paper concentrates on detection and the possible extent of characterisation of various neutron absorbing compounds when they are present in metal shells of variable sizes. The experiments are carried out using a low-flux deuterium–tritium (accelerator based) neutron source, a moderator-collimator set up developed indigenously to suit the low flux source and a 12 bit cooled CCD based neutron imaging camera. The study shows typical grayscale value ranges for various compounds present in same volume inside the metallic shell of variable wall thickness. Experiments carried out with specimens made of SS304 material having microscopic absorption cross-section of 13.9 barns per molecule and Aluminium of 1.62 barns per molecule shows the effect of variable microscopic cross-sections offered by the metallic cases and variable wall thickness on the grayscale value obtained. Attenuation chart for various compounds when present inside metallic casings are prepared which shows a typical grayscale value range for various thicknesses. Results also show a difficulty in distinguishing compounds having similar range of macroscopic cross-sections. From the present study, it can be established that it is possible to distinguish low dense materials such as chalk powder of macroscopic cross-section 0.13 cm−1 from compounds such as sucrose of macroscopic cross-section 1.63 cm−1 when present inside metal devices. The study focuses on the limitations of an accelerator based low flux neutron radiography setup.