## 5 Particle Detectors
### Mass Spectrometry
#### 5.2.5 Ams  
A great breakthrough in detection capabilities in MS has been the development of accelerator-based systems, which enabled overcoming the limits of conventional spec  
Figure 16: Schematic layout of the ToF analyzer with reflectron ion mirror at the LASR3 Surface Laboratory Roma Tre. trometers when detecting rare isotopes in ultra-low concentrations [293; 294]. With the expression of rare isotopes, we indicate those radioactive isotopes that are characterized by long, or medium-long, half-lives and by a very low abundance. Indeed, the combination of both the long half-life and the low concentration makes their measurement by traditional spectroscopic methods (such as beta counting) not as efficient and sensitive as needed. Accelerator mass spectrometry (AMS) enables obtaining very good sensitivity, keeping sample mass very small (normally, a few tens of milligrams, considering the mass of the collected sample) and, at the same time, keeping measurement time quite short (30 min per sample for the AMS measurement are enough). Among all the possible isotopes for which concentration can be measured by AMS, the most well known is \({}^{14}\)C, or radiocarbon, used in dating and environmental applications. \({}^{14}\)C measurements can be carried out coupling the magnetic and electrostatic elements, which are common also to mass spectrometry, to a small particle accelerator, most often a tandem electrostatic one, as the 3 MV tandem accelerator installed in Florence [295], where radiocarbon measurements are routinely performed in the framework of the INFN-CHNet network. A tandem accelerator is necessary to obtain the required high sensitivity (typically down to 10\({}^{-15}\)): the most important and abundant isobars are suppressed either in the source, through extraction of negative ions, or at the high-voltage terminal of the accelerator, thanks to the stripping process and its capability to contribute to the breakup of molecules. After acceleration, while the more abundant stable isotopes (e.g., \({}^{12}\)C and \({}^{13}\)C in the case of radiocarbon measurements) are collected through Faraday cups, the rare isotope ions (\({}^{14}\)C in radiocarbon) are counted via a particle detector. In our \({}^{14}\)C experimental setup in Florence, typical count rates for modern samples are of the order of few tens of Hz, with the required counts statistics (approximately 40,000 counts at least for each of the certified concentration standard samples) acquired in approximately 30 min. In some cases, when isobars cannot be fully suppressed along the beam line, for instance in the case of \({}^{10}\)Be, the final detector is also used as a further discrimination element [296]. Even though, in the early times, measurements were performed with terminal voltages of few tens of MV, research has moved to smaller and smaller accelerator machines, with terminal voltages even as low as 200 kV, now 0.5-3 MV, the typical voltage range. In such conditions, the most probable charge states after stripping are relatively low, with the consequence of an accelerated beam on the high-energy side consisting of very low MeV/AMU ions. The ideal detector is thus a device with a very thin entrance interface, so that energy loss and straggling of impinging ions are minimized, able to withstand the direct irradiation of the incoming rare-isotope beam. Typically, gas ionization detectors are used (see, e.g., [297]), possibly with a segmented anode to allow for the measurement of both dE/dx and the residual E, to discriminate the ions with respect to their atomic number. Typical filling gas is isobutane. An entrance window, a material which can withstand high pressure differences, even when arranged in very thin foils, is used: in higher terminal voltage setups, 1 um thick Mylar was normally chosen; in recent times, with the typical terminal voltage being lower, new silicon nitride windows of few tens of nm thickness have been introduced, minimizing energy straggling and thus improving energy resolution.