Biological shielding calculations for a 10 MeV Rhodotron electron accelerator facility

This thesis focuses on biological shielding calculations for a 10 MeV Rhodotron electron

beam accelerator operating in dual mode to produce a 10 MeV primary electron beam

and 7.5 MeV secondary X-rays, generated by hitting an optimized tantalum-73 (73Ta)

target with the primary electron beam.

The first part of this thesis describes the Rhodotron electron accelerator and its oper ating principle. A model is presented for the transverse optics of a TT-100 Rhodotron

accelerator. Potential industrial applications of a Rhodotron accelerator for Botswana

are highlighted, including sterilization of food products, medical devices, pharmaceuti cals, wastewater and sludge treatment, cargo screening and postal mail decontamination.

Sterilization of food reduces harmful micro-organisms, thus prolonging shelf-life and min imizing health risks to humans. A particular emphasis is placed on the sterilization of

meat for local consumption and export as the meat industry plays an important role in

Botswana’s economy and represents the country’s third main income earner.

The second and main part of this thesis focuses on designing an optimal biological radi ation shielding structure for the Rhodotron and irradiation vaults to protect the general

public, radiation workers and the environment from unnecessary radiation exposure in

compliance with recommendations by the International Commission on Radiological Pro tection (ICRP).

viTwo methods of shielding calculations are employed, namely analytical estimates using

empirical formulas from the literature and Monte Carlo (MC) predictions based on the

FLUktuierende KAscade (FLUKA) simulation package. FLUKA has the added advantage

that the transport of particles (photons and electrons in this project) and their interactions

with matter are included for complex geometrical shielding structures. On the other hand,

FLUKA simulations are numerically intensive and require more computational time.

After the primary beam strikes the target, the main biological dose rate is attributed to

the secondary photons. The shield is designed to reduce the biological dose equivalent

rates to less than 5 µSvh−1 during normal operation. The analytic calculations show that

an ordinary concrete wall with a thickness of approximately 2.1 m is sufficient to shield

the Rhodotron accelerator vault. Using the latter value as the input for FLUKA, the

thickness is varied in steps of 0.01 m from a lower limit of 1.9 m until the biological dose

rate of 5 µSvh−1

is achieved. The FLUKA results show that a 2 m thick ordinary concrete

wall, roof and slab are sufficient to shield both the accelerator and irradiation vaults to

permissible levels for optimal and safe operation.