Prompt gamma neutron activation analysis system

A method for analyzing an interrogation volume containing a prescribed target element, comprising the steps of:

• (a) irradiating an interrogation volume with a neutron burst characterized by an intensity and pulse width, and thereby effecting an emission of gamma radiation from said interrogation volume;
• (b) acquiring groups of gamma radiation data during a plurality of time intervals, each group being indicative of the number or intensity of gamma rays and energy levels of said gamma rays during a corresponding time interval, and a first group corresponding to a first interval during which said neutron burst is on, and at least one other group corresponding to a time interval during which said neutron burst is off;
• and
• (c) analyzing said gamma data to detect the presence and determine the amount of said prescribed target element in said interrogation volume and to determine whether said prescribed target element is present within a prescribed depth range in said interrogation volume;
• and
• (d) determining calibration data by carrying out the following sub-steps:
• (1) placing a slug of mass M of said target element at a plurality of depths, including:
• 0 inches;
• X.sub.1, where X.sub.1 is the deepest depth from which gamma rays of energy E.sub.1 can escape the interrogation volume in sufficient numbers to be detected;
• and X.sub.2, where X.sub.2 is the deepest depth from which gamma rays of energy E.sub.2 can escape the interrogation volume in sufficient numbers to be detected;
• and measuring yields Y.sub.E1 (0), Y.sub.E1 (X.sub.1) of gamma rays of energy E.sub.1 at depths of 0 inches and X.sub.1, respectively, and Y.sub.E2 (0), Y.sub.E2 (X.sub.1), Y.sub.E2 (X.sub.2) of gamma rays of energy E.sub.2 at depths of 0 inches, X.sub.1 and X.sub.2, respectively;
• (2) defining the following ratios:
• ##EQU9## performing a field measurement of yields y(E.sub.1), y(E.sub.2) of gamma rays of energies E.sub.1, E.sub.2, respectively;
• performing a depth inference calculation by determining whether y(E.sub.2)/y(E.sub.1) is greater than R.sub.21 (X.sub.1) and whether y(E.sub.1) is greater than 0, and then determining a range of depths at which said target element is located by the following formulas:
• Case 1
• if y(E.sub.2)/y(E.sub.1) is greater than R.sub.21 (X.sub.1) and y(E.sub.1) is greater than 0, then the target element is present between 0 and X.sub.1 and between X.sub.1 and X.sub.2 ;
• Case 2
• if y(E.sub.2)/y(E.sub.1) is less than R.sub.21 (X.sub.1) and y(E.sub.1) is greater than 0, then the target element is between 0 and X.sub.1 ;
• Case 3
• if y(E.sub.2) is greater than 0 and y(E.sub.1) is 0, then the target element is between X.sub.1, and X.sub.2 ;
• and then, performing an equivalent mass calculation by the following formulas:
• ##EQU10## wherein ES.sub.max and ES.sub.min represent the maximum and minimum equivalent mass within the specified ranges;
• wherein steps a, b, and c are performed with a neutron source, detector and analysis system, respectively, wherein said neutron source, detector and analysis system are maintained outside said interrogation volume and in a non-contacting relation with said target element.