Abstract
PurposeThe time-over-threshold (TOT) technique is being used widely due to itsimplications in developing the multi-channel readouts, mainly when fast signal processing is required. Using the TOT technique, as a measure of energy loss instead of charge integration methods, significantly reduces the signal readout costs by combining the time and energy information. Therefore, this approach can potentially be utilized in J-PET tomograph which is built from plastic scintillators characterized by fast light signals. The drawback in adopting this technique lies in the non-linear correlation between input energy loss and TOT of the signal. The main motivation behind this work is to develop the relationship between TOT and energy loss and validate it by the J-PET tomograph setup.MethodsThe experiment was performed using a 22Na beta emitter source placed in the center of the J-PET tomograph. This isotope produces photons of two different energies: 511 keV photons from the positron annihilation (direct annihilation or through the formation of a para-positronium atom or pick-off process of ortho-positronium atoms) and a 1275 keV prompt photon. This allows the study of the correlation between TOT values and energy loss for energy ranges up to 1000 keV. Since the photon interacts predominantly via Compton scattering inside the plastic scintillator, there is no direct information of the energy deposition. However, using the J-PET geometry, one can measure the scattering angle of the interacting photon. Since the 22Na source emits photons of two different energies, it is necessary to know unambiguously the energy of incident photons and their corresponding scattering angles in order to estimate energy deposition. In summary, this work presents a dedicated algorithm developed to tag photons of different energies and studying their scattering angles to calculate the energy deposition by the interacting photons.ResultsA new method was elaborated to measure the energy loss by photons interacting with plastic scintillators used in the J-PET tomograph. We find the relationship between the energy loss and TOT is non-linear and can be described by the functions TOT = A0 + A1 * ln(E dep + A2) + A3 * (ln(E dep + A2))2 and TOT = A0 - A1 * A2^{E_{dep}}phantom {dot {i}!}. In addition, we also introduced a theoretical model to calculate the TOT as a function of energy loss in plastic scintillators.ConclusionsA relationship between TOT and energy loss by photons interacting inside the plastic scintillators used in J-PET scanner is established for a deposited energy range of 100–1000 keV.
Highlights
The time-over-threshold (TOT) technique was first time introduced by Nygren and Millaud [1] and proved to be an excellent solution for multi-channel readouts [2]
The application of the TOT method for the energy loss determination may be of particular advantage in the newly developed Jagiellonian-positron emission tomograph (J-PET) [3, 4] which is based on plastic scintillators characterized by fast light signals with rise and decay times of the order of ≈ 1 ns [5, 6] and being about two orders of magnitude shorter than signals from crystals used in the current PET devices [7,8,9]
The motivation of this work is the elaboration of a method to estimate the energy loss by incident photons interacting with plastic scintillators used in the J-PET tomography scanner
Summary
The time-over-threshold (TOT) technique was first time introduced by Nygren and Millaud [1] and proved to be an excellent solution for multi-channel readouts [2]. This method is less precise in comparison to the charge integration method It reduces the readout costs by using only the time to digital converter (TDC), combining both timing and energy information. The application of the TOT method for the energy loss determination may be of particular advantage in the newly developed Jagiellonian-positron emission tomograph (J-PET) [3, 4] which is based on plastic scintillators characterized by fast light signals with rise and decay times of the order of ≈ 1 ns [5, 6] and being about two orders of magnitude shorter than signals from crystals used in the current PET devices [7,8,9]. Despite many advantages like compactness of signals readout and low power consumption, the TOT technique confronts the challenge in terms of non-linear input energy to pulse width conversion [10,11,12]. It has been reported that using multiple fixed triggering thresholds [13, 14] or dynamic threshold levels [15, 16] for estimating the TOT values alleviates the problem of non-linearity to a significant extent
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