Anthony J. MacKay Student Paper Contest Winner / Le gagnant du concours de communications étudiantes Anthony J. Mackay

A novel method for resolving the energy spectrum of a radiation beam using doped plastic scintillators

By Humza Nusrat
Department of Physics, Ryerson University

Coauthors
Geordi Pang & Arman Sarfehnia
Odette Cancer Centre, Sunnybrook Health Sciences Centre
Toronto, ON

Introduction

Knowledge regarding the quality of radiation, the ability of radiation to cause damage to living tissue, in addition to absorbed dose can be very useful. The potential biological damage caused by low-energy X-rays or heavy charged particles can be many times more severe than the 1 MeV (or higher) photons typically used in cancer radiotherapy.

In cancer radiotherapy, absorbed dose is used to optimize the amount of radiation delivered to the patient, but does not account for variations in radiation quality (energy levels). Numerous studies have shown that the impact of low-energy scattered radiation on the radiation damage to the patient must be measured for optimal treatment. According to current literature, these low-energy particles may be causing more damage to tissue (by inducing cell death) than previously predicted.

In the radiation protection industry, a radiation-weighting factor (W_R) is used to account for variations in radiation quality. However, these factors are often gross estimations.

By measuring the energy spectrum, many characteristics of the radiation source/beam can be resolved. Using the National Institute of Standards and Technology (NIST) collisional stopping-power database, the linear energy transfer spectrum can be obtained if the energy spectrum is known. Similarly, the maximum relative biological effectiveness can be determined from the energy spectrum, as shown by Nusrat et al. (2018).

This paper presents a novel technique for resolving the energy spectrum using doped plastic scintillators.

Methods

Plastic scintillators are made of organic materials that emit visible light upon interacting with radiation. Their response is intrinsically dependent on the stopping power of incoming radiation, as shown through Birks’ Law (Birks, 1964).

Using organometallic chemistry, various amounts of high-Z dopants can be incorporated into the scintillator volume. By adding high-Z elements, the radiation interaction cross-sections of the scintillator can be tuned in order to change its sensitivity to certain energy ranges (e.g., adding high-Z dopants can induce greater sensitivity to low-energy X-rays).

Four different scintillators were used: 0.0%, 1.0%, 1.5%, and 5.0% Pb–doped plastic scintillators (Eljen Technology, Sweetwater, TX). In order to resolve the energy spectrum, two main challenges needed to be overcome:

• We needed to account for a number of considerations when designing and fabricating the prototype.
• The responses of each type of scintillator to known photon energies needed to be determined.

The detector apparatus was constructed to allow the scintillator light to be guided via an optical fibre to a Hamamatsu H17021 photosensor module (design shown in Figure 1). Measurements were conducted at Sunnybrook Health Sciences Centre for high-energy (6, 9, 12, and 15 MeV electron beam) and low-energy (100, 180, 250, and 300 kV X-rays) radiation beams.

Figure 1: Scintillator detector shown in a disassembled configuration. Scintillator housing was constructed in house using poly(aryl-ether-ether-ketone) (PEEK), allowing for coupling to the optical fibre.

Based on the response of each scintillator to various monoenergetic energy bins, the response matrix (R) was created using Geant4 10.3 Monte Carlo simulations. The signal matrix (S) was obtained through measurements. Using these two pieces of information, the energy spectrum (∅) can be resolved:

∅ = S × R⁻¹

Results

Each scintillator was modelled in Geant4 10.3, and the model was validated by comparing the light emitted by each scintillator in the simulated and measured cases. Through this, an accurate scintillator model was established.

The response matrix was created using energy bins of varying sizes; measurements were obtained using the detector apparatus. We discovered that doping with 5.0% Pb increased the sensitivity to low-energy radiation (100 kV X-rays) by 474% (± 0.78%) relative to the highest energy used (15 MeV electron beam). As the doping concentration was decreased, the low-energy sensitivity decreased significantly (141% ± 0.22% for the 1.0% Pb–doped scintillator). Using these measured and simulated results, the energy spectrum can be resolved.

Conclusion

In both radiotherapy and radiation protection, information regarding radiation quality is essential. By adding varying levels of high-Z elements to plastic scintillators, the energy spectrum and, subsequently, the linear energy transfer (LET) and maximum relative biological effectiveness (RBE), can be resolved using a simulated response matrix and the detector apparatus used in this study.

References

Nusrat, H., Pang, G., Ahmad, S.B., Sarfehnia, A. 2018. Evaluating the biological impact of increased scattered radiation in single and composite field radiation beams. Biomedical Physics & Engineering Express 4:035016. https://doi.org/10.1088/2057-1976/aab0db

Birks, J.B. 1964. The Theory and Practice of Scintillation Counting. Oxford: Pergamon Press.