A new technique has been proposed for measurement of the boron concentration in primary cooling water in Pressurized Light Water Reactors (PLWRs). This designed system consists of two types of photon detectors and a compact external neutron source. The boron concentration is, in principle, determined by the count rule of 0.48 MeV prompt γ-rays emitted after ¹⁰B(n,α)⁷Li^* reactions induced by thermal neutron irradiation, diving by that of 2.2 MeV capture γ-rays from ¹H(n,γ) reactions. In the first design, a cadmium zinc telluride (CZT) semiconductor and a bismuth germante (BGO) scintillator were selected as promising detectors and their response functions were evaluated by Monte Carlo calculations. Pulse height distributions have been simulated considering the background photons emitted from a neutron source and radioactive impurities in cooling water. The error in evaluating the boron concentration was also estimated; for example, the concentration of 100 ppm with 1% error might be measured within 20 seconds in a thermal neutron field of 5×10⁴ cm^-2s^-1.