Probe coil for detecting NQR-responsive materials in large volumes

a bA coil assembly comprising two coil sections (and ). Each of the sections consists of a combinat of a spiral coil having a variable coil pitch arranged in a saddle shape configuration. The saddle shape has a central port and a pair of outer coextending portions. The coil sections are located opposite each other with the central p() of the respective sections disposed in parallel spaced apart relationship and the corresponding coextending portions of respective sections disposed in coplanar alignment with each other so that the sections combine to circumscribe a scanned volume within which the target substance is to be disposed.