Il Rilevatore Gamma


The Radon sensor is divided in three sections: a gamma detector, an amplifier system and an analyser one.
The gamma detector is a plastic scintillator, NE110 (or NE102) with a total volume of 1030 (or 670) ± 5 cm3.
The characteristics of the commercial scintillator used in the equipment are a time of response of 2.4 ns, and light emission at 423 nm, which lies at the lower end of the violet.
The response in light is of 65%, compared with anthracene, organic crystal, which has the characteristic of having the highest response among all the organic scintillators.
The plastic scintillator is inserted in a PVC box and housed into a container made of a 7 cm-thick stable lead. The lead container is covered with a thin layer of Mylar (40 μm), used as a reflective coating with over 98% of reflectivity, in order to improve the device’s performance. The plastic scintillator is coupled with four or two photomultipliers (Photonis XP3462b), placed on opposite walls of the lead box, with a characteristic gain, given from suppliers, of ≈ 2 · 106.
The photomultipliers glass lens is an ultraviolet shield.
The photocathode is shielded with a μ-metal cylinder of Φ = 7,8 cm diameter. The received analogical signal in converted in a digital one by the discriminator and sent to an adapter board (NIM-TTL- NIM) that forwards it to the acquisition system, consisting of a scaler board interfaced with a computer, which acquires the output in real time. The sensor provides an indirect Radon measurement, based on detection of Rn daughters (214Pb and 214Bi, with their characteristic gamma emission at 351 and 609 KeV, respectively).
It automatically takes measurements at closely-spaced time intervals (10 minutes), and gives output signals as counts per hour (e.g. photons per hour).