My MSc Story

Mary PW Chin 钱碧慧博士
PhD (Wales), MSc (Surrey)

Neutron contamination in a radiotherapy maze

☞ more on bunker design & shielding
☞ more on photoneutron production

MSc Medical Physics, University of Surrey, 1998-1999


It has been a year of many blessed encounters. Dr Spyrou's encouragement and confidence nourished every moment of the course -- he is indeed the most supportive mentor I have ever met. Sue's help and friendship made such a difference to my stay in England. The special people in our class made the classroom experience wonderful.

I am most grateful to be granted the International Development Shared Scholarship Scheme by the Association of Commonwealth Universities, which made my attendance of the course possible.

I thank the people who contributed directly to this dissertation project. Once again, Dr Spyrou, whose supervision I enjoyed. Also Dr Robert Price, who answered via email some of my questions about MCNP. A few others helped briefly.

Yet another moment of grace, this is also a time to remember those who took part in my first altering steps. Dr Sivananthan and Dr Pathmanathan gave orthopaedic and paediatric care respectively. Sister Hermine was the physiotherapist, who remains an inspiration. All medical treatment was founded on papa's tremendous patience and commitment. Greater difficulty came from my primary school days which I miraculously survived, adding to mama's pain more than she could bear. This work brings the course to its finale. It is dedicated to papa and mama. May it be a delight.


Neutron contamination in a radiotherapy maze


This work attempts to investigate neutron contamination in a radiotherapy maze that houses a high-energy medical linear accelerator. While electrons and/or photons from the linac are destined for therapeutic effects by destroying cancerous cells, the production of neutrons which are hazardous is unintended but inevitable. This work attempts to quantify the degree of this contamination. The variation of neutron fluence in the treatment room, the variation of neutron dose along the passage of the maze, the changes in neutron energy distribution as the neutrons traverse the treatment head and the passage of the maze will be quantified using Monte Carlo simulation. Data will be compared with estimations from empirical formulae developed by McCall, for neutron fluence,and Kersey, for neutron dose equivalent. The presence of secondary photons will also be briefly demonstrated. To take advantage of the flexibility computer simulation offers, modified maze designs will be explored by comparing a double-bended maze with a single-bended one,and by comparing bunkers of varying heights.

Table of contents
  1. Chapter 1. The Hazard Defined
    • Neutron production
    • Neutron interactions
    • Residual activities
  2. Chapter 2. Shielding the Hazard
    • The material
    • The design
  3. Chapter 3. Quantifying the Hazard I: The Approach
    • Deterministic approach
    • A deterministic simulation
    • Monte Carlo approach
    • MCNP
  4. Chapter 4. Quantifying the Hazard II: Data & Analysis
    • Neutron dose
    • Neutron fluence
    • Neutron energy distribution
    • Secondary photons
  5. Chapter 5. Quantifying the Hazard III: Errors & Reliability
    • Variance reduction
    • The 10 statistical checks
    • The 2 dangers
    • Reproducibility
    • Neutron cross sections
    • Fluence-to-dose conversion
    • Limitations of the model
  6. Chapter 6. Conclusion
  7. References
  8. Appendices
Full dissertation
Download pdf (2.5 MB)

The idea of pursuing this course began as I was finishing my undergraduate study at Universiti Sains Malaysia. When the opportunity finally came, the joy was more than I could contain. Even after the fulfilment of the course, as I was preparing to return home, I still couldn't believe that I was in fact stepping on British grounds.