Comparison of Gamma Densitometer Detectors Used in Loss of Coolant Studies

Ionization chamber type gamma detectors are used in water-steam density measurements in loss of coolant studies at Oak Ridge National Laboratory. Ionization chambers have replaced current-mode scintillation detectors to obtain stability and freedom from magnetic field interference. However, this change results in some loss of fast transient response. Results of studies comparing the transient response of ionization chamber detectors, plastic scintillation detectors, and sodium iodide (NaI) detectors to rapid changes in gamma intensity demonstrate that plastic scintillation detectors have the fastest response and most closely reproduce the transient; ionization chambers have an initial fast response followed by a slower response, which may produce errors in fast transient measurements; and NaI scintillation detectors have a moderately fast initial response followed by an extremely slow response, which produces errors in even slow transient measurements. We are using ionization chamber detectors with data acquisition systems having response times of 1 and 16 ms. With the 16-ms system, the transient response is limited by the system rather than the detector. However, with the 1-ms system the response is detector limited. At present, the extreme stability of the ionization chamber detector system is more important than the loss in fast transient response.