Development of dual time-over-threshold method for estimation of scintillation decay time and energy

In the past decade, the time-over-threshold (ToT) method has been developed to measure pulse energy from radiation detectors with power consumption lower than that of ordinary analog-to-digital converters (ADCs). In addition, the ToT method is easily implemented in application-specific integrated circuits because it simply needs only a comparator and a time-to-digital converter. However, adapting the ToT method to scintillation-based radiation detectors worsens its energy resolution compared to that of ADCs owing to the pulse shape of the scintillator, because ToT is not proportional to energy. In this paper, a dual-ToT method that can estimate the energy without any non-linear terms is proposed. Although the proposed method needs two ToT circuits against one input signal, both circuits are simple ToT circuits. Using information from two ToT circuits enables calculation of not only the precise energy but also the decay time constant of the scintillator. As a proof of concept, an experiment was performed using a radiation detector composed of a small lutetium yttrium orthosilicate crystal and a multi pixel photon counter. Data were analyzed by using the dual-ToT method and both energy and decay time were successfully calculated with high accuracy. (author)