Prog Med Phys.  2021 Dec;32(4):145-152. 10.14316/pmp.2021.32.4.145.

Measurements of Neutron Activation and Dose Rate Induced by High-Energy Medical Linear Accelerator

• Kwon NH
• Jang YJ
• Kim J
• Kim KB
• Yoo J
• Ahn SH
• Kim DW
• Choi SH

• KMID: 2524161
• DOI: http://doi.org/10.14316/pmp.2021.32.4.145

Abstract

Purpose
During the treatments of cancer patients with a linear accelerator (LINAC) using photon beams with energies ≥8 MV, the components inside the LINAC head get activated through the interaction of photonuclear reaction (γ, n) and neutron capture (n, γ). We used spectroscopy and measured the dose rate for the LINAC in operation after the treatment ended.
Methods
We performed spectroscopy and dose rate measurements for three units of LINACs with a portable high-purity Germanium (HPGe) detector and a survey meter. The spectra were obtained after the beams were turned off. Spectroscopy was conducted for 3,600 seconds, and the dose rate was measured three times. We identified the radionuclides for each LINAC.
Results
According to gamma spectroscopy results, most of the nuclides were short-lived radionuclides with half-lives of 100 days, except for ⁶⁰ Co, ⁶⁵ Zn, and ¹⁸¹ W nuclides. The dose rate for three LINACs obtained immediately in front of the crosshair was in the range of 0.113 to 0.129 µSv/h. The maximum and minimum dose rates measured on weekends were 0.097 µSv/h and 0.092 µSv/h, respectively. Compared with the differences in weekday data, there was no significant difference between the data measured on Saturday and Sunday.
Conclusions
Most of the detected radionuclides had half-lives <100 days, and the dose rate decreased rapidly. For equipment that primarily used energies ≤10 MV, when the equipment was transferred after at least 10 minutes after shutting it down, it is expected that there will be little effect on the workers’ exposure.

Keyword

Spectroscopy; Activation; Dose rate; Linear accelerator; Radionuclide

Figure

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  Fig. 1 Diagram showing the measurement procedure and evaluation of activation of medical linear accelerators (LINACs). HPGe, high-purity Germanium; CRM, certified reference material.

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  Fig. 2 Setup of the in-situ high-purity Germanium detector in front of the linear accelerator crosshair and relevant measurements.

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  Fig. 3 LINAC dose rate measurement outcomes. LINAC, linear accelerator.

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  Fig. 4 Dose rate changes after beam irradiation with the use of a 10 MV energy beam.

Reference

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