Transient Nonlinear Response of Plastic Scintillators

The peak dose rate for which light output is linearly proportional to incident radiation intensity was measured by bombarding 80-mil-thick samples of NE 102 and MEL 150C scintillators with 12-MeV electrons from the EG&G/AEC linear accelerator. To obtain the equivalent gamma dose rate, the sensitivity of the scintillator-photodiode detector used in this work was measured with a calibrated Co60 source. Nonlinear behavior was observed for excitation pulse widths between six nanoseconds and 0.5 microseconds. Analysis of the detector signals indicates that nonlinearity is a function of total dose as well as dose rate. Thus for an excitation pulse width of 0.5 microseconds, scintillator output was found to be linear at the start of the burst, but showed marked nonlinearity by the end of the pulse. The data also indicate that if the dose rate drops below the threshold value after a scintillator is driven into nonlinear operation, the scintillator will continue to behave nonlinearily for some time. The decay time associated with recovery from nonlinearity is greater than 0.5 microsecond. Data are presented, showing the average dose rate required to produce 5% and 10% nonlinear behavior as a function of exposure time. As a typical case, 10% nonlinear behavior for NE 102 results from an average dose rate of 6.5 × 1011 R/sec for an exposure time of 10 nanoseconds.