Frequency domain multiplexing of pulse mode radiation detectors

The capability to multiplex scintillation detectors or other pulse mode radiation detectors is necessary in some applications where a large number of detectors is required. Frequency domain multiplexing has been previously implemented for applications in astronomy using amplitude modulation on radiation detectors such as transition-edge sensors. We propose an alternative method for multiplexing pulse mode radiation detectors in the frequency domain using convolution. We pass the detector signal to a resonator circuit that converts a detector pulse to a damped sinusoid of a specific frequency which gives a unique tag to the detector. We have developed a prototype frequency-domain multiplexed system for four EJ-309 organic scintillator detectors using four resonators of unique frequencies. The resonator outputs are combined using a fan-in circuit which is then connected to a single digitizer input. Using this system, we demonstrate that the charge collected under the original anode pulse can be estimated from the power spectrum of the damped sinusoid with a relative uncertainty of about 2%. The time-of-arrival of the anode pulse can be estimated using constant fraction discrimination applied to the leading edge of the damped sinusoid with an uncertainty of about 450 ps. We also used a CeBr3 detector to test the performance of our system for spectroscopic applications and found only small degradation in the resolution for a multiplexed detector.