12 C (p, p′)12 C reaction (Ep=19.5-30 MeV) for active interrogation of special nuclear material

Passive detection of special nuclear material (SNM) is challenging due to its inherently low rate of spontaneous emission of penetrating radiation, the relative ease of shielding, and the fluctuating and frequently overwhelming background. Active interrogation, the use of external radiation to increase the emission rate of characteristic radiation from SNM, has long been considered to be a promising method to overcome those challenges. Current active-interrogation systems that incorporate radiography tend to use bremsstrahlung beams, which can deliver high radiation doses. Low-energy ion-driven nuclear reactions that produce multiple monoenergetic photons may be used as an alternative. The 12C(p,p′)12C reaction is one such reaction that could produce large yields of highly penetrating 4.4- A nd 15.1-MeV gamma rays. This reaction does not directly produce neutrons below the approximately 19.7 MeV threshold, and the 15.1-MeV gamma-ray line is well matched to the photofission cross section of 235U and 238U. We report the measurements of thick-target gamma-ray yields at 4.4 and 15.1 MeV from the 12C(p,p′)12C reaction at proton energies of 19.5, 25, and 30 MeV. Measurements are made with two 3-in. EJ-309 cylindrical liquid scintillation detectors and thermoluminescent dosimeters placed at 0â and 90â, with an additional 1.5-in. NaI(Tl) cylindrical scintillation detector at 0â. We estimate the highest yields of the 4.4- A nd 15.1-MeV gamma rays of 1.65×1010 and 4.47×108sr-1 μC-1 at a proton energy of 30 MeV, respectively. The yields in all experimental configurations are greater than in a comparable deuteron-driven reaction that produces the same gamma-ray energies-11B(d,nγ)12C. However, a significant increase of the neutron radiation dose accompanies the proton energy increase from 19.5 to 30 MeV.