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Characteristics of Prompt Gamma Compton Interactions in Detectors


D Mackin

D Mackin1*, J Polf2, S Avery3, E Lee4, D Dolney4, S Beddar5, (1) MD Anderson Cancer Center, Houston, TX, (2) Oklahoma State University, Stillwater, OK, (3) University of Pennsylvania, Sicklerville, NJ, (4) University of Pennsylvania, Philadelphia, PA, (5) MD Anderson Cancer Ctr., Houston, TX

SU-E-T-442 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Prompt gamma radiation produced from non-elastic scattering during proton therapy provides a signal that can be used to verify the beam range and possibly the dose in vivo. The purpose of this study is to highlight how the Compton interaction characteristics change based on the gamma energy and to determine how these characteristics will affect the detection system.

Methods:
The prompt gamma rays are produced at discrete energy levels ranging from 511 keV to 7.12 MeV. Over this energy range the interaction characteristics are known to vary widely. Thus, when designing a detection system, it may be advantageous to consider individual characteristic gamma energies (.511, 1.33, 2.2, 4.44 and 6.13 MeV) rather than all prompt gammas as a whole. We simulated 106 Compton scatter events for the 5 energies. For each event we randomly selected a scattering angle based on the Klien-Nishina scattering formula. We then calculated the corresponding Compton electron scattering angle, energy, and CSDA range.

Results:
The average Compton electron CSDA range in CdZnTe for the events from the lowest energy gammas in our study is 0.2 mm and for the highest energy gammas is as high as 7 mm. Over 80% of the Compton electrons from the highest energy events have a CSDA range greater than 3 mm. Thus, the stopping power of the detector material should be taken into consideration when designing a detector for the high energy gammas. The average scattering angle of the Compton photons ranged from 61° to 39° for low to high energies, respectively. Therefore, the high energy gammas, the stages of multistage detectors can be placed further apart with a lower impact on detection efficiency.

Conclusion:
The designers of gamma imaging systems for protons may benefit by considering each gamma energy independently rather than focusing on the entire spectrum.

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