Thermal neutron fluence measurement using a cadmium differential method at the laser-driven neutron source

We produced high-energy neutrons by p + Be and d + Be reactions with protons and deuterons with continuous energies of up to 30 and 10 MeV/u, respectively, provided from laser-plasma interactions. Thermal neutrons were subsequently generated through deceleration by a small moderator with conjunction on the Be target. Here, we report the measurements on the thermal neutron fluence using the activation method with/without Cd filters. The unstable isotopes of Au-198, Mn-56, Co-60, and Hf-181 were produced through (n, gamma) reactions by a single laser shot. The nuclear reaction fractions were evaluated by measuring the gamma-rays emitted from these unstable isotopes. The thermal neutron fluences from the four nuclides are consistent within their experimental uncertainties. The fluence is evaluated to be (2.2 +/- 0.4) x 10(5) neutrons cm(-2) at the distance of approximately 9 mm at 90 degrees against the laser axis. The present result shows that the method to generate thermal neutrons from a compact neutron source with a diameter of 44 mm and length of 46 mm is expected to be a useful tool for various nuclear experiments and applications.