<title>Prototype explosives detection system based on nuclear resonance absorption in nitrogen</title>

Abstract

A laboratory protoype system has been developed for the experimental evaluation of an explosives detection technique based on nuclear resonance absorption of gamma rays in nitrogen. Major subsystems include a radiofrequency quadrupole proton accelerator and associated beam transport system, a high-power gamma-ray production target, an airline-luggage tomographic inspection station, and an image-processing/detection-alarm subsystem. The detection system performance, based on a limited experimental test, is reported.

Proof of principle of a high-spatial-resolution, resonant-response γ-ray detector for Gamma Resonance Absorptionin14N

“…The interactions via which the energy is transferred to the scintillator depend on the incident γ-ray energy. Since the special-purpose scintillator inside the capillaries is itself nitrogen rich, resonant γ-rays can undergo the nuclear resonant reaction, producing 1.5 MeV photoprotons and 13 C atoms. The proton path in the scintillator is essentially straight (apart from the extremely rare event of a nuclear collision, when large angle scattering occurs) as the momentum transfer in these ion-ion collisions is relatively small.…”

“…In an operational GRA system operating with a 5 mA proton beam, the total event rate (electrons and protons) in a 20×20×240 mm 3 resonant detector positioned at about 250 cm from the 13 C target, is expected to be about 500 events/s. Thus in order to obtain an image with not more than 2-3 particle tracks the exposure time of the readout system should be not more than about 6 ms/frame.…”

“…The optimal γ-ray source for the nitrogen GRA method is the de-excitation spectrum of the excited 14 N * 9.17 MeV level following 1.746 MeV proton capture via the reaction 13 C(p,γ) 14 N. Since the lifetime of the 9.17 MeV level (5.1x10 −18 s) is very short compared to ion stopping times (typically ∼1x10 −12 s) the emission of the γ-ray occurs in-flight following the recoil of the excited 14 N nucleus, resulting in Doppler-shifting of the γ-ray. At the resonant angle θ R = 80.66 o with respect to the proton beam, the nuclear recoil energy losses that occur during emission and absorption of the γ-ray by the 14 N nucleus are precisely compensated by the Doppler-shifted energy component.…”

“…Soreq NRC has hitherto circumvented this problem by developing nitrogen-rich resonantresponse detectors, that are selectively sensitive to photons in the 9.17 MeV region [2,11]. With such detectors the resonant flux is sampled, on an event-by-event basis, by counting 1.5 MeV photoprotons internally-produced via the resonant absorption reaction 14 N(γ,p), the inverse reaction to 13 C p-capture: γ(hν9.17 MeV) + 14 N g.s. −→ 13 C g.s.…”