S. Puddu

The triple GEM detector as stray neutron monitor

A triple GEM detector for low energy neutrons and high rejection of gamma background was tested at the CERF facility at CERN as stray neutron monitor. The detector was exposed to a wide neutron spectrum generated by a 120 GeV/c positively charged hadron beam hitting a copper target. The beam particle rate on target ranged over three orders of magnitude. The neutron count rate measured with the GEM could be linearly correlated with the beam intensity and also compared well with Monte Carlo simulations. The detector performance suggests that it can be used as an independent low energy neutron monitor at high-energy particle accelerator facilities.

55 Fe 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.

The triple GEM detector as beam monitor for relativistic hadron beams

A triple GEM detector was tested at the CERF facility at CERN as an on-line beam imaging monitor and as a counting reference device. It was exposed to a 120 GeV/c positively charged hadron beam (approximately 2/3 pions and 1/3 protons), which hits a copper target generating a wide spectrum of different kinds of particles used for various experiments. The flux of beam particles ranged over three orders of magnitude, from 8·104 s-1 to 8·107 s-1. The profile of the beam acquired with the GEM was compared to the one measured with a MWPC and no saturation was observed. In addition, the count rate measured with the GEM was compared to the one measured with an Ionization Chamber, which is routinely used for monitoring the beam intensity. Another way of monitoring the intensity of the beam was also explored, which is based on the total current driven from the GEM foils. The digital readout allows making a 2D online image of the beam for the alignment with the copper target in the CERF facility. A low residual activation of the detector was observed shortly after irradiation.

Neutron beam profile measurements with a triple GEM for thermal neutrons at the CERN n_TOF facility

A triple GEM detector for thermal neutrons, with a side-on geometry, was tested at the CERN n_TOF facility as beam profile detector. The n_TOF, neutron time-of-flight facility, provides a neutron beam from thermal energy to 1 GeV with a flight path of 185 m. The triple GEM detector used in this measurement has a cathode specifically conceived for thermal neutron conversion, with a series of thin strips of 10B deposited on glass supports. Filled with an Ar-CO2 70-30% mixture, it was installed in the experimental area, using a step motor to change the position and to image the whole beam area. Furthermore, using the n_TOF trigger it was possible to synchronize the GEM data acquisition in order to select a given neutron energy window and measure the detector efficiency as a function of neutron energy between thermal and 2 eV. Working at low gain, it was possible to perform measurements with a low γ-background level. The neutron beam spot and the efficiency of the detector were measured.