55Fe measurements in radioactive waste with a triple GEM detector

Nowadays, accelerators have a large use both for experimental and medical physics. Throughout decommissioning and upgrade, there is a huge amount of radioactive waste because, depending on the type of accelerator, a considerable number of nuclides is produced by irradiation. Gamma emitters are easily identified by gamma spectrometer, but with this technique the efficiency to 6 keV photons of ⁵⁵Fe, is very low. Facing this situation is needed a detector with high efficiency to the x-rays of ⁵⁵Fe and high rejection of higher energy photons (such as those emitted by ⁶⁰Co). The present work discusses the development of a system based on a Triple Gas Electron Multiplier (GEM) detector. The GEM detector has the advantage of low gamma sensitivity and it can be built with a large active area. Therefore, it could be possible to characterize extended sample of waste. After preliminary measurements with a GEM with 3 mm drift, a second unit with 40 mm drift has been built. Detectors are filled with Ar-CO₂ 70%-30% gas mixture and from calculation; efficiency of 40 mm detector is up to seven times those of 3 mm detector. In order to optimize the detector FLUKA simulations are being performed with a new tool, to study the physical phenomena inside the detector event by event.