Example Animated Simulation

BNCT simulation using a silicon-on-insulator based  microdosimeter

Background

Boron neutron capture therapy (BNCT), is a relatively experimental form of cancer treatment. A compound loaded with B-10 is injected into the bloodstream. The compound is designed to have a specificity towards the tumour cells. The tumor region is then irradiated with low energy neutrons. The boron-10 nucleus absorbs these thermal (low energy) neutrons with a much higher probability then that of any other element in the surrounding tissue. However, upon absorption of the thermal neutron the boron-10 becomes unstable and fissions into two recoiling ionizing particles, an alpha particle (ie He nucleus) and an lithium ion. These products of the neutron capture reaction are very damaging to tissue but of a very short range (8 and 4um respectively) such that the ionizing energy is confined to the microscopic cellular region containing the boron. Providing sufficient boron is contained within a tumour cell and  sufficient thermal neutrons are available at that site, then there is a high probability of cell death in the boronated tumour region.

The microscopic distribution of energy deposited and the thermal neutron flux (n/cm2/s) within the irradiated region are important considerations in treatment planning. The silicon-on-insulator microdosimeter proposed by Anatoly  Rosenfeld and Peter Bradley of  the University of Wollongong Radiation Physics Group can simultaneously measure both these parameters of interest. The device essentially imitates a 2D array of tissue cells using an array of diodes with each diode of cellular dimensions (10 microns). The p+ (boron doped silicon) surrounding each diode is analogous to boron accumulating in the cytoplasm of the cell with the central junction of the diode representing the nucleus.

The diodes can measure the energy deposited by the alpha and lithium ion since the ionizing particles generate charge (electron-hole pairs) within the silicon (and energy  deposited is proportional to charge generated) . The central reverse biased n+-p diode junction has a depletion region with a strong electric field. Recombination of electron hole-pairs in regions of high electric field is small so that most of the charge generated in this region is collected.

Understanding the behavour of charge collection in a silicon device is a complex task. 2D and 3D semiconductor simulation packages are useful tools for characterising charge collection in a silicon based microdosimeter.
We have used a suite of tools produced by ISE (Zurich) to simulate the neutron capture reaction in the microdosimeter. The simulation research was undertaken in collaboration with Gernot Heiser (University of NSW)

Simulation Details

The simulation depicts a 1.75MeV alpha particle orginating from the p+ boronated contact at an angle of 80 degrees with the vertical. The alpha has a range of about 6.2um in silicon.
Details of the simulation include: The simulation shows the initial plasma track forming then  diffusing. Note the low minority carrier(electron)  density in the p+ region due to the short lifetime (doping dependent) in this region and hence greater recombination with holes. This reduces the charge collected by ion tracks which traverse the p+ region. The hole density plot shows high injection conditions (ie majority carrier density, holes, greater then substrate doping) continue until several nano-seconds. Of primary interest however is the flood of minority carrier electrons into the depletion region (n+ junction and surrounding 2um, right side of image). These carriers are collected at the n+ junction to form a current pulse at the output of the device. Connection to a charge amplifier integrates the current pulse to estimate total collected charge. The total collected charge is digitized and subsequently processed with a Mult-Channel Analyser. After many radiation interactions a spectrum of charge collected (proportional energy deposited) is produced.
Contact Peter Bradley for more information.