## 3 Gamma-Ray Detectors
### Gamma-Ray Detectors-PGNAA  
The Prompt Gamma Neutron Activation Analysis is based on the detection of prompt emissions of nuclear electromagnetic radiation (nuclear de-excitation), following the radiative capture of a neutron by a nucleus. PGNAA enables both qualitative and quantitative analysis, the former consisting in the identification of the elements present in a sample, the latter aiming at determining the concentration (or, equivalently, the mass) of each of the elements traced. A very appealing feature of PGNAA is the possibility to carry out simultaneous non-destructive and non-invasive bulk analysis, without sample preparation. PGNAA enables potentially detecting all the elements of the periodic table and in particular the elements with low Z and those with a high neutron capture cross section, such as B, Cd, Sm and Gd. It has allowed the analysis of a wide range of materials, such as metals, rocks and minerals, ceramics and glass, stone objects (obsidian, flint, porphyry, metabasic rocks, etc.). The PGNAA facility installed at the LENA Laboratory of the University of Pavia, devoted to studies in CH, is a unique installation in the national scenario and was funded and developed within the CHNET_TANDEM project, a collaboration between INFN-CHNet and LENA. The horizontal channel B of the TRIGA Mark II nuclear research reactor was selected, due to higher thermal-to-fast neutron flux ratio with respect to other channels. For the PGNAA facility construction, a campaign of Monte Carlo simulations was performed with the MCNP code using a complete model of the TRIGA MARK II nuclear reactor of LENA. The model was implemented adding the geometry and materials of the PGNAA facility, shown in Figure 11.  
Two groups of calculations were performed: one to optimize the collimators and to characterize the neutron beam, and another to evaluate the shielding of the facility for radioprotection purposes. The main components of the facility are the collimators and the filters inserted in the beam hole, two shutters, the shielding labyrinth made of bar  
Figure 11: **Left-hand side**: The PGNAA facility at LENA. **Right-hand side**: The design of the PGNAA facility used for Monte Carlo simulations. detector (\(n\)-type) and the data acquisition system in a separate room. The thermal flux available at the irradiation position is approximately 5 \(\times\) 10\({}^{6}\) cm\({}^{-2}\) s\({}^{-1}\). The detector is a hyper-pure germanium (HPGe) \(n\)-type detector, i.e., with a reverse electrode in coaxial geometry. The main characteristic of this HPGe is that the positive electrode is on the outer surface of the Ge crystal and consists of a thin layer of Ge with implanted boron ions (thickness: 0.3 \(\upmu\)m). The negative electrode is on the surface of the internal cavity of the crystal and consists of a thin layer of Ge with diffused lithium ions (thickness: 700 \(\upmu\)m). The reverse configuration enables a much longer detector life in the presence of high neutron fluxes, indeed the external contact consisting of implanted boron ions, has not only the function of guaranteeing the low-energy (up to 3 keV) photons detection, but also of protecting the crystal from the effects of radiation damage, that causes hole trapping in Ge. Unlike the case of \(p\)-type coaxial detectors, holes are collected by the external electrode of the \(n\)-type detector and thus they have less distance to travel. With less distance to travel, they are less probable to be trapped in the detector material damaged by irradiation. The consequence is an extension of the life-time of the detector. Of course, the resistance to radiation damage depends also on other factors, but experimental evidence suggests that the \(n\)-type detectors are approxi[mately ten times more resistant to damage with respect to conventional Coaxial Ge detectors. Detector signals are fed to a Canberra 2002 CS preamplifier, whose gain has been reduced from 500 mV/MeV down to 100 mV/MeV.