Answer to Question #5218 Submitted to "Ask the Experts"

The linatron is an industrial linear accelerator manufactured by Varian Medical Systems¹. In a linatron, the electron beam strikes a target, producing bremsstrahlung which is then collimated with internal collimators and sometimes with external collimators. The target and internal collimator are typically made of some high-Z material such as tungsten. The accelerator head is usually shielded with lead or tungsten. Neutron production will take place in any material struck by an electron or bremsstrahlung beam above a threshold energy (E_th). The minimum threshold for photoneutron production in tungsten and lead is 6.19 MeV and 6.11 MeV, respectively (NCRP 1984). Thus a linatron operating at 9 MV will produce neutrons. The photoneutron spectrum from the accelerator head resembles that of a fission spectrum. The spectrum changes after penetration through the head shielding. Since the linatron is usually operated in a concrete-shielded room, room-scattered neutrons will further soften the spectrum. Neutrons are classified as:

Thermal:  E_n = 0.025 eV at 20°C; typically E_n < 0.5 eV (cadmium resonance)
Intermediate:  0.5 eV <E_n <10 keV
Fast:  E_n > 10 keV
where E_n is the neutron energy.

The neutrons observed at the camera location will consist primarily of neutrons leaking directly from the accelerator head, room-scattered neutrons, neutrons produced in the object to be imaged by the primary bremsstrahlung beam, and neutrons scattered from the object to be imaged. Since the camera is not in the direct-beam path, photoneutrons can only be produced in the shielding by leakage photons or photons scattered from the object to be imaged. Since leakage photons are typically only 1 percent or 0.01 percent of the primary beam or even lower, neutron production in the camera shielding from leakage photons can be considered negligible, as can neutrons produced by photons scattered from the object. Therefore changing the shielding from tungsten to lead will not significantly reduce the neutrons at the camera location. The camera can be shielded from neutrons with several inches of a hydrogeneous material such as polyethylene. If thermal neutrons are also of concern, polyethylene doped with 5 percent boron (borated polyethylene) can be used. Boron has a much higher cross-section than polyethylene for thermal neutron capture. Boron interacts with thermal neutrons by undergoing an (n,α) reaction producing ⁷Li, which then undergoes an isomeric transition by emitting a 0.478 MeV photon or gamma ray. These photons are much lower in energy than the thermal neutron capture gammas (2.2 MeV) from hydrogen in the polyethylene. A sandwich construction of tungsten (or lead), polyethylene, and tungsten (or lead) will work best.

Nisy E. Ipe, PhD, CHP

 

National Council on Radiation Protection and Measurements. Neutron contamination from medical electron accelerators. Bethesda, MD: NCRP; Report No. 79; 1984.

Footnote:
¹ Varian Medical Systems, Industrial Products, Palo Alto, CA  94304-1000