Calorimetric measurements of nuclear heating in small probes of plasma-facing materials

Direct measurements of nuclear heating in small probes of materials subjected to D-T neutrons from an accelerator based source were initiated during 1989 under USDOE/JAERI collaborative program. A calorimetric technique was utilized to make these measurements. The probes of plasma facing materials, among others, were kept very close, ∼3 to ∼7 cm, to the neutron source inside an evacuated vacuum chamber. A typical probe measured 20 mm in diameter by 20 mm in length. Typical source intensity was ∼2 x 1012 n/s. The temperature changes in the probe medium were detected by thermal sensors spatially distributed in the probe. The thermal sensors included bead-thermistors, and platinum RTD's. The change in resistance of a thermal sensor due to onset of nuclear heating was picked up by an automated data acquisition and control system that included a highly sensitive digital voltmeter that had a resolution of 100 nV in voltage range of 300 mV or less. Usually, an individual probe was subjected to spaced neutron pulses of time duration 3 m to 10 m. Two consecutive source neutron pulses were separated by a cooling interval of almost the same duration as that of a source pulse. This approach made it possible to clearly distinguish between the heating and drift phases of the probe medium, on one hand, and to ascertain and verify the reproducibility of measured heating rates from one neutron pulse to another, on the other hand