Using Monolithic Active Pixel Sensors for fast monitoring of therapeutic hadron beams
Section snippets
Introduction and motivation
Nowadays, tumors are most frequently treated by irradiation with X-rays. Anyhow, in the last 60 years a new technology has been developed that is especially beneficial for deep seated tumors and which is in clinical use since 1990s (http://ptcog.web/psi.ch/): The treatment of cancer with hadron beams, most commonly with protons or carbon ions. From a biological point of view, heavy ions have one outstanding advantage over photons, namely the mechanism of energy loss along their path through the
Architecture of the sensor
The Mimotera (Minimum Ionizing MOnolithic active pixel sensor (MAPS) for the TERA foundation) is an MAPS of crystalline silicon. The epitaxial layer is only 14 μm thick and has an entrance window of about 100 nm (Bandano, 2005). This is possible by thinning the wafer down and back-illuminating it. The sensor consists of 112 × 112 = 12,544 pixels, each of the size of 153 × 153 μm2. This leads to an active area of 17 × 17 mm2, which is divided into four sub-arrays of 28 × 112 pixels that are read
Measurements at CERN
In June 2010 first tests have been carried out at ACE. The beam spill of 500 ns duration is contained in only one readout frame of 1.4 ms of the sensor. Therefore it is most convenient to analyze the data in a differential way, subtracting the frame with the spill from the previous one without beam. This has the advantage that one does not have to record pedestal files. All dead pixels and time-dependent background signals are eliminated automatically.
The crucial question for the first test run
Measurements at HIT
The Mimotera has not only been tested with antiprotons, but also with carbon ions at the Heidelberg Ion-Beam Therapy Center (HIT) in Heidelberg, Germany. The aim of these measurements was to investigate whether it could serve as a beam monitor for existing cancer treatment facilities. A scan through all available beam intensities from 5.0 × 106 s−1 to 8.0 × 107 s−1 has been performed, the results are presented in Fig. 3.
The detector response is very good, clear beam spots were obtained from the
Summary and outlook
It has been shown that the Mimotera is well suited for direct monitoring of very short pulses of antiprotons as well as of continuous beams of carbon ions. Further investigation will focus on monitoring proton beams. To be able to monitor even larger focus sizes, one could merge four of the sensors together. To reduce the beam straggling further, such that the sensor could stay in the beam during treatment, it is possible to remove that part of the printed circuit board support which lies
Acknowledgments
The Mimotera architecture is protected by the U.S. patent no. 7582875, it was developed by the SUCIMA collaboration, with a project supported by the European Commission under the contract no. G1RD-CT-2001-00561.
This work was supported by the DFG under contract WE3565-3 and the NSF under grant #CBET 0853157. MHH acknowledges support by the EU through a Marie Curie Fellowship under contract #PIIF-GA-2009-234814.
Special thanks go to M. Caccia at Università dell’ Insubria, Como, for providing the
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