## 5 Particle Detectors
### Mass Spectrometry
#### 5.2.5 Ams
When the typical MeV/AMU of accelerated ions is not so low as in the most recent 200 kV tandem machines, considering that in AMS the expected counting rate is in the order of some tens of Hz, solid-state detectors can be used as an alternative to gas ionization chambers. Energy resolution and timing response can clearly suffer for radiation damage: using a solid-state detector in front of a stable ion beam would not be possible. The detector is thus installed on a retractable arm in order to be inserted on the beam axis only when the rare isotope beam is injected (the detector is also protected by a beam stopper when not in use). An example is represented by the setup installed at the aforementioned laboratory in Florence, where a simple and low-cost 10 \(\times\) 10 mm Hamamatsu nude photodiode is installed on a bellows to count the \({}^{14}\)C ions. When residual interferences are somehow expected in the final detector, a Time-of-Flight (ToF) system [298] can be used. The basic idea of ToF is relatively simple: different ions with the same energy but different masses have different speeds and thus travel the same distance in different amounts of time. Measuring these intervals can allow us to discriminate the type of the particles. A better and further criterium can be added while also measuring the residual ion energies. What is challenging in a ToF setup for AMS is the required timing resolution; in particular, the START signal has to be generated without affecting too much the energy loss and energy straggling of the beam (a deterioration of these characteristics would result in a broadening of the measured times of flight, with the consequence of worsening the timing resolution). As an example, we report the Florence setup, where the START signal is generated by secondary electrons produced by the ion beam impinging on a 50 nm thick carbon foil and amplified by a microchannel plate. The STOP signal is given by the same mechanism or, as an alternative, by the silicon diode already mentioned. The overall timing resolution achieved is 0.7 ns with a 10 MeV C beam, on a flight path of 1.75 m, and the system enables easy discrimination of the \({}^{14}\)C ions from the unwanted \({}^{12}\)C and \({}^{13}\)C contaminants present even after the very last filter, the electrostatic analyzer [299]. The AMS feature of working with very low-intensity rare-isotope beams can be the key to exploit particle detectors not only as the final detectors or counters, but also to monitor the transmission of the beam itself. Indeed, optimizing the beam transport is highly mandatory in AMS, since the count rate in the final detector is expected to be relatively low if compared to other nuclear setup. Nevertheless, some of the typical monitoring devices installed along a tandem beam line, e.g., the beam profile monitors based on the measurement on the induced current on two oscillating filaments, do not have enough sensitivity. In this context, position-sensitive particle detectors can be used to monitor the spread of the beam in the transversal plane and its relative position with respect to the beam reference trajectory. Of course, such detectors should be either as much transparent as possible or mounted on a retractable arm, just not to interfere with the detection of the rare isotope particles during the \"real\" acquisition time. The latter is for instance the path followed by the Florence group, where both gas and solid-state detectors have been used as \({}^{14}\)C beam profile monitors. In [300], the use of a multiwire gas proportional chamber allowed us to monitor both the dimension of the beam spot and its position. The device consisted of a central cathode made of a double-sided aluminized Mylar foil placed between 2 grids of anode wires, oriented perpendicular one to each other. In each plane, 40 gold-plated tungsten wires, with a pitch of 1 mm, were mounted. Entrance and exit windows were 40 \(\times\) 40 mm\({}^{2}\), 1.3 \(\upmu\)m thick Mylar foils.