A performance study to the spatial and energy resolution of a hybrid pixel detector for alpha imaging applications

Abstract

Targeted Alpha Therapy (TAT) is an interesting technique in tumour treatments, especially in the treatment of local tumours or the treatments of solid tumours while minimizing unwanted radiation to surrounding tissue. The use of Ac-225 in TAT is especially interesting as in the decay of Ac-225 four alpha particles are emitted. The recoil effect will however disrupt the daughter nuclide from the targetting molecule, allowing the daughter nuclide to drift away from the tumour cells to healthy tissue. In order to study the redistribution of radionuclides due to the recoil effect imaging tools are required. An existing technique is the classic alpha camera, this technique is based on the scintillation of alpha particles. Emitted photons by the scintillating layer are registered by a charge coupled device. This technique allows for a spatial resolution of up to 35 μm but is limited by the uptake of other radiation forms (photon, beta).
Another alpha imaging technique is called the Timepix, this is a hybrid pixelated semiconducting sensor that consists of an array of 55 μm square pixels. The semiconducting material (silicon) converts ionizing radiation into charge carriers that are collected at pixel sites. In this research the limitations of the Timepix chip with regard to spatial & energy resolution in alpha particle imaging are investigated.
The spatial resolution was investigated by the use of fine edged alpha absorbing objects, a small pitched collimators to ensure a monochromatic beam of alpha particles, and an alpha emitting source (Am-241). The Timepix chip registered the incoming alpha particles. By binning the counted alpha particles in small inter- vals a step function was obtained, from which a Gaussian function was obtained. The Full Width at Halve Maximum (FWHM) is a measure of (spatial) resolution of the Gaussian function. Results showed a spatial resolution of 9.8 μm obtained by a collimator with pitch of 6 μm. The obtained spatial resolution shows that sub-pixel resolution in alpha particle imaging is possible.
The energy resolution of the Timepix chip was investigated by the use of a collimator to ensure a monochro- matic beam of alpha particles, several Mylar foils to slow down the alpha particles and an alpha emitting source (Am-241). The use of the time over threshold mode (TOT) indicates the amount of time a signal is above threshold, which is a direct indication of the particles energy. Output of the chip is Gaussian shaped, from which the energy resolution was obtained. The obtained energy resolution of the Timepix chip was 0.8 MeV, which allows for identification of most daughter nuclides from the decay chain of Ac-225. This energy resolution does not allow for the identification of the disintegration of Fr-221 (6.64 MeV) and Ac-225 (5.94 MeV).