Speaker
Description
In this work, the concept of an area monitoring dosimeter and its operational regime for pulsed radiation dose rate measurements is presented.
A fast tissue equivalent plastic scintillator EJ-200 (Eljien Technology) is exploited as a detector material. In the energy range from 200 keV up to 4 MeV, the scintillator is tissue equivalent. This minimizes the influence of pile-up on absorbed energy measurements as simultaneous energy depositions in the detector material from multiple photons lead to the proportional value of the energy deposition as from summing up contributions from separately coming photons for a soft tissue. This means that for the dose rate measurements the higher weighting of pile-up events is achieved through their higher energy values. Fast response of the scintillator (in the range of ns) can provide time-resolved dosimetry if necessary.
The detector is connected to a fully digital signal processing board, which creates an active system with adjustable parameters. Signal losses due to pile-up, readout, and dead-time during the data acquisition can be estimated by analyzing generated listmode files and from the knowledge of the dead-time behavior of the system (completely non-paralyzable).
This system was used to measure the absorbed dose rate in conditions imitating radiation protection measurements outside the treatment room of a clinical linear accelerator for percutaneous cancer treatment. The detector was placed in a scattered photon field with a time structure mimicking a radiotherapeutic beam, but also in the presence of a constant radiation field. The LINAC at the γELBE (HZDR, Germany) operated in the macropulses mode with their duration of Δt of 5 µs and the period T of 5 ms. An additional source of continuous radiation ($_{}^{22}$Na) was used. For all measurements, the source was attached to the PMMA phantom opposite to the detector. The arrangement of the incident beam, PMMA phantom and the detector provides the scattered photon field with the maximum energy no more than 777 keV which is acceptable to conform the energy range where the scintillator is completely tissue equivalent.
The real-time distinction of pulsed and non-pulsed contributions is based on the time structure of a single interaction. Then, the pulsed radiation from the different macropulses of the accelerator has the time difference that belongs to intervals of
[kT - Δt; kT + Δt],
where k is used for positive integers. If events come from the same macropulse, the time difference between them belongs to the interval of
[long gate; Δt],
where the "long gate" is the integration gate of the data acquisition system. This algorithm can be easily implemented in the software of any active detector.
For the investigated dose rate range up to 8 μGy/h, the pulsed radiation dose rate estimated according to the presented algorithm shows a linear dependence on the accelerator current. Thus, the increase of the accelerator current by 1 μA leads to the increase of the pulsed radiation dose rate by (26.2±0.9) nGy/h.
If the accelerator power continues increasing, greater and greater part of pile-up pulses will be lost due to the limited integration gate. One of the possible decisions is to expand the long integration gate up to the macropulse duration when detector will integrate all events within the macropulse, and pile-up events will be accounted for completely. This approach is going to be tested during the next experiment.
The behavior of the non-pulsed radiation dose rate is not constant and approaches the reference value (measured with the $_{}^{22}$Na source only) while increasing the accelerator current. This can be explained by underestimating the pulsed radiation dose rate at low accelerator current values when mostly one event per macropulse is detected, and this event cannot be definitely related to the accelerator or the background and the $_{}^{22}$Na radiation. This means that the proposed algorithm will overestimate the pulsed radiation dose rate contribution in cases with very low accelerator current, while in high pulsed dose rate scenarios the accuracy of the approach should improve.
There are some challenges that apply to the improvement of the presented detector. The first one is to adjust it for measurements of operational dose quantities. The specific calibration that relates indications of the detector to the operational quantities can be performed for a number of standard calibration sources. But in the case of mixed radiation field, the deconvolution of spectrum will be required which is not a trivial task as no full absorption peaks are observed in low-Z organic scintillators.
The second challenge is the dosimetry of the low-penetrating radiation like laser-induced X-rays. Here, one deals with the energy range of about tens keV, which means that the housing of the detector will absorb a great part of radiation. Also, the behavior of the detector material in this energy region in comparison with a soft tissue is hardly correctable in the online regime without knowing the energy spectrum of the incident radiation.
