  The recent interest in beta- radionuclides for radio-guided surgery derives
from the feature of the beta radiation to release energy in few millimeters of
tissue. Such feature can be used to locate residual tumors with a probe located
in its immediate vicinity, determining the resection margins with an accuracy
of millimeters. The drawback of this technique is that it does not allow to
identify tumors hidden in more than few mm of tissue. Conversely, the
bremsstrahlung X-rays emitted by the interaction of the beta- radiation with
the nuclei of the tissue are relatively penetrating. To complement the beta-
probes, we have therefore developed a detector based on cadmium telluride, an
X-ray detector with a high quantum efficiency working at room temperature. We
measured the secondary emission of bremsstrahlung photons in a target of
Polymethylmethacrylate (PMMA) with a density similar to living tissue. The
results show that this device allows to detect a 1 ml residual or lymph-node
with an activity of 1 kBq hidden under a layer of 10 mm of PMMA with a 3:1
signal to noise, i.e. with a five sigma discrimination in less than 5 s.
