1. Photoneutron Distributions around 18 MV X-ray Radiotherapy Accelerators using Nuclear Track Detectors Fazal-ur-Rehman, H. Al-Ghamdi, M. I. Al-Jarallah (Presenting Author) N. Maalej King Fahd University of Petroleum and Minerals

2. Outline Introduction Methods Results Conclusion

3. Introduction Neutrons contaminate the high energy Linear Accelerator (LINAC) photon beam in radiation therapy and contribution dose to the patient We studied the distribution of thermal and fast neutrons in the LINAC room during irradiation with an 18 MeV photon beam

4. Methods Neutrons are generated by high energy photon interaction with the high Z materials of the LINAC head Fast Neutron Measurement: –Bare NTDs are used –Tracks are generated by protons from the (n,p) recoil in the detector material (C 12 H 18 O 7 ) Thermal Neutron Measurement: –NTDs are covered with Li 2 B 4 O 7 –Tracks are generated by  -particles from 10 B(n,  ) 7 Li and 6 Li(n,  ) 3 H nuclear reactions

5. Methods Fig.1 Experimental arrangement showing the location of CR-39 NTDs around a radiotherapy linear accelerator.

6. Methods x 0 1 2 3 12 3 -2 -3 y Maze Isocente r 1 m 5 4 Insid e Maze Entrance Fig. 2 Experimental arrangement showing the location of CR-39 NTDs around a radiotherapy linear accelerator.

7. Methods NTDs advantages over active detectors: –There is no pulse pileup problem –No photon interference with neutron measurement –No electronics are required –Less prone to noise and interference

8. Results Fig. 4 Fast neutron relative intensity as a function of transversal distance from isocenter of linear accelerators located in a small and large treatment rooms.

9. Results Fig. 5 Fast neutron relative intensity as a function of longitudinal distance from the isocenter of linear accelerators located in a small and large treatment rooms.

10. Results Fig. 6 Fast neutron relative intensity as a function of longitudinal distance from the isocenter of linear accelerators located in a small and large treatment rooms.

11. Results Fig. 7 Thermal neutron relative intensity as a function of longitudinal distance from isocenter of linear accelerators located in a small and large treatment rooms.

12. Conclusion The fast neutron distribution along the transverse and longitudinal directions is symmetrical about the beam axis. The fast neutron relative intensities fall in an exponential-like fashion with distance from the isocenter of the accelerators. The thermal neutron distribution is uniform at all the locations along transverse and longitudinal directions. The larger size LINAC room had lower fast and thermal neutron relative intensities at all locations.

13. Acknowledgment Physics Department of King Fahd University of Petroleum and Minerals (KFUPM). The Department of Radiotherapy, King Fahd Specialist Hospital. Dhahran Health Center, Dhahran, for utilizing their accelerator facilities in this study.