To verify the self-indication method, we have performed three kinds of experiments using a 46 MeV electron linear accelerator (linac) at the Kyoto University Research Reactor Institute. The experimental arrangement is shown in Fig. 3.1. Pulsed neutrons were produced from a water-cooled photo-neutron target assembly, 5 cm in diameter and 6 cm long, which was composed of 12 sheets of tantalum plates with total thickness of 29 mm [5]. This target was set at the center of an octagonal water tank, 30 cm long and 10 cm thick, as a neutron moderator. The linac was operated with a repetition rate of 50 Hz, a pulse width of 100 ns, a peak current of 5 A, and an electron energy of about 30 MeV. We used a flight path in the direction of 135 ° to the linac electron beam. To reduce the gamma flash generated

by the electron burst from the target, a lead block, 7 cm in diameter and 10 cm long, was set in front of the entrance of the flight tube.

First, a gold foil 10, 20, 30,40, or 50 qm thick was located as a sample at a flight length of 11.0 m, where the neutron beam was well collimated to a diameter of 24 mm. An indicator located at a flight length of 12.7 m was surrounded by a Bi4Ge3O12 (BGO) assembly, which consists of 12 scintillation bricks each 5 x 5 x 7.5 cm3 [6]. Prompt-capture у-rays from the indicator were detected with the BGO assembly in the TOF measurement. A 10B plug 8 mm thick or gold foil 50 qm thick was used as an indicator. Because the former thick indicator can absorb most neutrons with energies below the epi-thermal region, it was equivalent to the conventional NRTA. In the latter case, it was the self-indication measurement. The area densities of the samples with different thickness were estimated by area analysis for the 4.9 eV resonance of 197Au.

As the next step, a 50-qm-thick silver foil was added to a 10-qm-thick gold foil to form a sample and the area density of the gold foil was measured. It is worth noting that silver has a large resonance at 5.2 eV, close to the 4.9 eV resonance of 197Au. The 10B plug 8 mm thick or a gold foil 50 qm thick was also used as an indicator. Moreover, we demonstrated a nondestructive assay for nuclear materials using a mixture composed of a natural uranium plate and sealed minor actinide samples of ‘Np and Am. The natural uranium plate was 10 x 20 mm and weighed 5.8 g. The samples of Np and Am were oxide powder, which was pressed into a pellet 20 mm in diameter and encapsulated in an aluminum disk­shaped container 30 mm in diameter with 0.5-mm-thick walls. The activities of 7Np and Am were 26 and 868 MBq, respectively. In the third measurement, the

10B plug 8 mm thick or a natural uranium plate of 10 x 20 mm2 and weight 5.8 g was used as an indicator.