Monte Carlo simulation for coincidence detection of In-111 cascaded photons with innovative data processing

In In-111 decay, the emission directions of the two cascaded photons (171 and 245 keV) are angularly correlated. When the two photons are detected coincidentally, a counts distribution is obtained on one detector corresponding to a bin of the opposite detector. The peak of the distribution is used to determine the source position so that collimators are not needed. In this work, we carried out Monte Carlo simulations using a modified SimSET code to assess this new idea. The result shows that with a point source at 5 cm from the detector, the FWHM is about 10 cm. To improve the resolution, we developed a new method to calculate the separation of two point sources even if the projection image of the two sources appears as one peak. First we determine the direction of the line that connects the two sources by rotating the detectors several times. When the detector is parallel to the line, the peak value reaches the minimum value, while on the detector that is perpendicular to the line, the peak value achieves its maximum. Then the peak values and positions, the coincidence probability, and the detector geometric factor are used to calculate the separation of the two sources. We have applied the method to two cases. For two point sources located at (-6, -5) and (-5, -5) (1 cm apart and 5 cm from the detector), a separation of 0.99 cm and a standard deviation of 0.09 cm were obtained from the calculation. With two sources at (-3, -7) and (-2, -6.5) (1.1 cm apart and 26.57° tilted with respect to the x-axis), the calculation resulted in 1.1 cm separation with 0.2 cm standard deviation.