T HE EXPERIMENT AT LNS

97 tion products are detected by a multi-detector array.

The experimental setup has been simulated with the Geant4 Monte Carlo toolkit and its predictions have been compared with the experimental data, as shown in the next sections. Another experiment has already been performed and data analysis is still in progress. Other experiments at higher energies (up to 400 AMeV) are planned for the next year in other laboratories, so that further benchmarks of the Geant4 models will be able in the future.

4.1 The experiment at LNS

Experimental data, reported in this work, have been collected at Laboratori Nazionali del Sud (LNS, Catania, Italy) in the framework of TPS project (see section 1.3.1). The aim of the experiment is to measure the angular distribution of the emitted fragments and absolute differential cross section.

Figure 4.1. Schematic map of beam lines in the experimental rooms of LNS.

Inside the scattering chamber a three positions target holder (empty frame

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and 113.5 µm ¹⁹⁷Au and 1 mm ¹²C) is set up. Empty frame was used for back-ground events measurements (due, for example, to particles interaction with frame edges) and ¹⁹⁷Au frame for the experiment. After fragmentation of the primary beam on the target, reaction products are detected by a complex system of telescopes and resolved with the ∆E-E technique. In Figure 4.2 a picture and a scheme of the scattering chamber have been shown.

Figure 4.2. Left: picture of the experimental set up. Right: schematic view of the ex-perimental set up in the scattering chamber.

4.1.1 The detection system

The interaction of ion beams at this energy is mainly dominated by projectile fragmentation. Reaction products have velocities similar to the primary beam and are emitted mainly in the forward direction. Hence, different isotopes have to be detected with atomic mass ranging from that of protons up to mass equal to the primary particle one. To investigate the beam fragmentation influence on dose, in hadrontherapy, the measurements of the yield of the different isotopes is necessary. Therefore, a detection system used for this aim has to be able to dis-tinguish particles in term of atomic Z and mass number A, with a good resolution in energy and high efficiency, covering a solid angle relatively small around the incident beam direction.

Hence, in our experiment two Si-CsI hodoscopes of two-fold and three-fold telescope detectors with different granularity have been used, called Hodo-small

4.1 The experiment at LNS

99 and Hodo-big, respectively placed at a distance of 0.8 m and 0.6 m from the tar-get [111].

Each element of the Hodo-small consists of 300 µm thick silicon ∆E (∆E) detector 1x1 cm² followed by a CsI(Tl) crystal E (E) detector with the same area and 10 cm long, with photodiode read-out. The detectors of the Hodo-small are arranged in a cube of 81 telescopes, covering a total angular range of θ_lab = ± 4.5° (Figure 4.3, right).

In the final version of the Hodo-big each element of the multi-detector array is a three-fold telescope consisting of a 50 µm thick Silicon ∆E (∆E1) detector followed by a 300 µm thick Silicon ∆E (∆E2) detector and a 6 cm long CsI(Tl) crystal E (E) detector with photodiode read-out. Each element has an active area of 3x3 cm². The implementation of the 50 µm ∆E1 detector in the Hodo-big al-lows a lower energy threshold of 3 AMeV. The 88 modules of the Hodo-big are arranged on a spherical surface of radius r = 60 cm centered on the target posi-tion. They cover a total solid angle of 0.23 sr with a geometrical efficiency of 90%. The Hodo-big covers the angular range between ± 6.09° ≤ θ_lab ≤ ± 20.47°

(Figure 4.3, left).

Figure 4.3. Left: the 88 modules of the Hodo-big arranged on a spherical surface of ra-dius r = 60 cm centred on the target position. Right: the detectors of the Hodo-small are arranged in a cube of 81 telescopes at 80 cm far from the target.

The two hodoscopes cover a solid angle of 0.34 sr with a geometrical

effi-100

ciency of 72% [112].

In Figure 4.4 one of the Hodo-big telescopes is shown. In order to maximize packing there is no housing around the scintillators, and the ∆E detectors are self-supporting silicon chips.

Figure 4.4. Schematic representation (left) and image (right) of a telescope of Hodo-big.

The CsI(Tl) scintillator has been chosen because of the advantages that it presents with respect to other possible candidates. It is only slightly hygroscopic so that hermetic sealing is not needed. Moreover it has good mechanical proper-ties so that it can hold mechanical shocks fairly well and can be easily machined.

Its high density (4.51g/cm³) makes the scintillator suitable to stop ions, in the energy range of interest, in few centimeters. Furthermore the light emission spectrum fits well with the response curve of silicon photodiodes: the quantum efficiency is around 70% at 550 nm. A disadvantage is represented by the de-pendence on the charge, mass and energy of the particle and the scintillation quenching, which requires specific calibration procedures for each ion. The CsI crystals of the Hodo-big have been accurately cut in a pyramid-trunk shape (31x31 mm² and 34x34 mm²) in order to be centered at the same distance (60 cm) on the target location, and to make possible each of them subtends an equal solid angle of 2.8 msr. The CsI scintillators of the Hodo-small do not require to be shaped accordingly since they are simply packed one over the other because the small angular aperture with respect to the distance of the target ensures that

4.1 The experiment at LNS

101 they are all placed at around the same distance within mechanical errors. Photo-diodes have been chosen because they require small space and have a good quantum efficiency for the CsI(Tl) light emission spectrum.

4.1.2 ∆E-E identification technique and data acquisition

Working principle of a generic radiation detector depends essentially on the interaction modalities between the incident radiations and the absorbing mate-rial. In case of charged particle beams at intermediate energies, the main mecha-nism for ion energy loss consists in the Coulomb interaction with orbital elec-trons of the traversed medium. In particular, traversing a thickness of material, charged particles collide with electrons close to their trajectory and delivery, in every collision, a small fraction of their initial kinetic energy producing excita-tion or ionizaexcita-tion of the medium atoms. The stopping power S, described by the Bethe-Bloch formula (see section A.1.1), is proportional to S∝ (mz²/v²) where m, z and v are respectively mass, charge and velocity of the incident particle. The

∆E-E technique is based on this relation. Indeed, telescopes generally consist of a thin detector ∆E, where the projectile loses only a small fraction of its total en-ergy, positioned before a second detector E, thicker than the first one, where the incident ion is completely stopped. In this way, considering only events in coin-cidence in the detectors, the energy loss (∆E signal) and the total energy (E_tot=

∆E+E) of the projectile are measured (the last as sum of ∆E and E signals).

Thus, it is possible to identify the particle which has traversed the telescope and to resolve also for the different isotopes [113]. Figure 4.5 shows a typical ∆E-E calibrated matrix relative to one of the Hodo-big telescopes used during the ex-periment, where it is possible to resolve the atomic and mass number of the iso-tope produced. It is also clearly visible the elastic peak of the incident ¹²C parti-cles.

Signals coming from the detectors are handled by means of standard elec-tronics chains for the pulse height analysis, based on charge preamplifier, shap-ing amplifier, stretcher and analog-to-digital converter. The signals are then

digi-102

tized by 96-channel Fastbus LeCroy QDC (charge-to-digital converter) with a common gate derived from the data acquisition trigger.

Figure 4.5. A ∆E2-ECsI matrix of one of the Hodo-big telescopes (θ = 8.62°). It is clear that the resolution in terms of mass A is good up to Z ≤ 4; i.e. for H, He, Li and Be iso-topes. Instead, Boron and Carbon isotopes are not well distinguishable by means of a simple graphic analysis.

4.2 Simulation of the experiment with