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At INFN-LNS we have developed DMNR, a prototype system for the online real-time monitoring of the radioactive waste repositories, consisting of an array of many radiation sensors (gamma rays and neutrons) to be deployed all around the radwaste drums. In order to be suitable for mass deployment these detectors are small, inexpensive, robust, easy-to-use, and reliable. The basic idea was to use well known materials and properties, avoiding too sophisticated technologies which would lead to expensive and delicate components.

From the monitoring standpoint a distinction is needed between two main categories, apart from their activity, depending on the origin of the waste. Radwaste originated directly from the nuclear fuel, either reconditioned or not, is expected to emit neutrons, whereas other radwaste basically emits only gamma rays (alpha and beta radiation is stopped inside the material itself, unless the waste is dispersed in the environment). This means that in order to monitor the radwaste one has to focus mainly on detectors for gamma rays and neutrons.

As for the gamma ray detection, there is a wide variety of devices and techniques on the market, and typically a detector is from moderately to highly expensive, therefore not well suited for a granular monitoring. By exploiting the recently born silicon photomultiplier (SiPM), along with plastic scintillating fibers, we have developed a new gamma ray counter capable of fulfilling this task. The scintillating optical fiber is coupled at each end to a SiPM whose single photon sensitivity, along with the left-right coincidence constraint, allows to achieve a reasonable sensitivity to gamma radiation even though using a thin 1 mm diameter fiber.

As for neutrons, the worldwide need to replace the 3He-based detectors has triggered research and development on new technologies and methods. Our choice fell on commercial solid state silicon detectors coupled with thin neutron converter layers containing 6Li. We proved the feasibility of this technique, also proving that its detection efficiency can be reasonably high and that the gamma/neutron discrimination capability is comparable to that of 3He tubes.

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