Fast neutrons from a 25-MeV betatron

Area of Science:

• Medical Physics
• Radiation Oncology
• Nuclear Physics

Background:

• Published fast-neutron dose per x-ray rad measurements from high-energy therapy installations vary significantly.
• Accurate neutron dose assessment is crucial for radiation therapy safety and efficacy.

Purpose of the Study:

• To accurately measure neutron production from a 25-MeV betatron.
• To quantify the fast-neutron dose per x-ray rad in a clinical setting.
• To compare neutron production in x-ray and electron beams.

Main Methods:

• Utilized fast-neutron activation of aluminum foils to measure neutron production.
• Eliminated photoneutron production in detectors, a confounding factor in previous studies.
• Obtained neutron spectra via interpolation and considered contributions from the platinum target, beam flattener, and background.

Main Results:

• Neutron fluences per rad of x rays were largely independent of field size and energy (18-23 MeV) without a phantom.
• A beam flattener contributed 15% of neutrons at 23 MeV; the target and background contributed equally.
• Estimated neutron dose within the treatment field was 2.2 x 10^-4 rad/x-ray rad, with 50% uncertainty.
• Neutron dose in the electron beam was ~50 times lower than in the x-ray beam.

Conclusions:

• The study provides crucial data on neutron dose from medical accelerators, addressing significant prior discrepancies.
• Findings highlight the importance of accounting for target material and beam modifiers in neutron production.
• The measured neutron dose is relatively low but warrants consideration in high-energy radiation therapy planning.