Optimisation of Radiation Shielding of Medical Cyclotrons

Effective radiation shielding is imperative to safe operation of modern Medical Cyclotrons producinglarge activities of short-lived radioisotopes on a commercial basis. Like the containment shielding of Medical Linear Accelerators , the optimal shielding design of Medical Cyclotrons demands a careful balance between the radiological, economical and often the sociopolitical factors. One is required to optimize the cost of radiation protection and the cost of radiological health detriment . The cost of radiation protection depends explicitly on the nature of the ionising radiation field produced by the cyclotron, its operational condition, the cost of
shielding material, the level of dose reduction, the projected net revenue from the from the sale of the radioisotopes, and the depreciation rate of the cyclotron facility. The mathematical methods of accelerator shielding optimisation  within the guideline of ALARA  have been reported by various investigators.

The important radioisotopes produced by Medical Cyclotrons for present day diagnostic nuclear
medicine include 201Tl (T1/2= 73.06 h) and67Ga (T1/2= 78.26 h). These radioisotopes are generated by bombarding the thick copper substrates electroplated with enriched parent target materials with 30 MeV protons at ~ 400A beam current. The target bombardments result in the production of intense fields of high-energy neutrons and gamma rays. Therefore, in order to avoid the radio-activation of the cyclotron, the ancillaries and radiation exposure to cyclotron workers and members of the public, the high performance target irradiation stations of modern negative ion Medical Cyclotrons  are housed in separate target vaults made of high density concrete. Thus, the efficient shielding of the target vaults plays the most important role in the radiological safety of the commercial Medical Cyclotron facilities.

The conventional optimisation method for the functions of multiple variables, such as the cost benefit-
analysis of Medical Cyclotron shielding design is too complex and prone to serious pitfalls. Hence, a new
method based on the Genetic Algorithm (GA) was used to solve this problem. The GA is a mathematical technique that emulates the Darwinian Evolution paradigm, also known as the “Survival of the Fittest” strategy.

It is ideally suited to search for a global optimum in a large multi-dimensional solution space, havingdemonstrated strength compared to the classical analytical methods. This paper highlights the application of an interactive spreadsheet macro program for the optimised shielding thickness calculation of the target vault of a Medical Cyclotron. The present optimisation method is based on a Genetic Algorithm search engine and runs on a Pentium 300 MHz Personal Computer in the Windows 98 platform.

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