Abstract : Intraoperative localization of tumor tissues labeled with positron radiotracers opens up new prospects to improve the efficiency of cancer surgery. Because Silicon Photomultipliers (SiPM) introduced a breakthrough for the development of miniaturized imaging devices, we are currently designing two intraoperative beta probes based on this technology: a light imaging device with a small field of view (~5cm2) to perform tumor localization and post-operative control of the surgical cavity, and a miniaturized counting probe to guide in real time the excision of the tumor lesion. We report here optimization of the detection head design of the two intraoperative probes using Monte Carlo simulations. Two different γ ray background rejection schemes were investigated for the imaging probe. The first one is based on the stack of two SiPM arrays each coupled to a continuous plastic scintillator. The second configuration implements two layers of plastic scintillator separated by a light guide and mounted onto a single SiPM array. Events originating from the top and the bottom layers can be discriminated from the analysis of the width of the light distribution on the photosensor. We showed that the first configuration offers millimetric spatial resolution, low distortion and γ ray rejection efficiency better than 98%. The second configuration allows to further reduce the size of the probe but optimizations are still necessary to improve the trade-off between the accuracy on the layer identification and the spatial performances. Detailed description of the simulation study as well as the performance characterization of the first prototypes of the counting and imaging intraoperative probes will be presented at the conference.