We have developed an irmaoperative probe for use in early detection of breast Cancer and aiding lumpectomy. The probe consists of a small plastic scintillator, 0.8mm diameter and 3mm length, coupled to a single clear opticalfiber strand, and solid state photomultiplier. Due to the small size of the probe, it can be placed inside of a small gauge biopsy needle. The scintillator is very efftcient in detecting betas and positrons while being very inefficient to energetic gammas due to its small

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... e and low density. High quantum efficiency, 80%, and high gain obtainable, solid state photomultiplier makes the probe very low noise device in detecting beta particles. Intrinsic resolution of the probe is expected to be around 1 millimeter. Preliminary results using beta sources and a rat bearing R320 adenocarcinoma tumor were very successful.