**2. The position-sensitive detector**

The PSD prototype has a sensitive area of 90 **×** 90 mm2 , which is made of two ribbons, layers of pre-aligned BCF-12 SCIFI, manufactured by Saint-Gobain Crystals, juxtaposed and orthogonally oriented, named the X and Y planes. The SciFi have 500 μm nominal square section. In detail, each single layer is composed of four ribbons of 40 fibers. The ribbons are optically isolated from each other by means of 220-μm-thick black adhesive tape to reduce cross talk between adjacent and overlapped ribbons. Each fiber is coated with white extra mural absorber (EMA) [6] to further reduce the cross talk between individual fibers. Particles intersecting the PSD's sensitive area deposit energy in the fibers which is partially converted in scintillation light. A fraction of this light is channeled in the core and propagated in the fiber toward the photo-sensor. When a particle loses suitable energy in all four SciFi layers, the coordinates of the intersection of its trajectory and the sensitive area can be measured. A picture of PSD detector is shown in **Figure 3**.

The PSD has 640 optical channels (four layers of 160 fibers each). The channel reduction system reduces the number of the readout channels without any data loss or degradation in the position measurement. The readout is performed in time coincidence, strongly reducing the effect of noise and chance coincidences, enhancing at the same time the performances of the system. The working principle of channel

**Figure 3.**

*(a) Picture of PSD detector during the assembly phase. The dashed red box highlights the sensitive area of the detector. (b) A sketch of the arrangement of the four layers is shown.*

reduction can be argued by considering a strip detector able to detect one particle at a time. Each strip can be read from both ends, and the signals are grouped following the scheme reported in **Figure 4**, where a two-dimension 16-strip detector is illustrated. All strips are read out, on one end, in *m* groups of *n* neighboring strips, named *NeigSet*, while at the other end, the first strips of each group are grouped in *StripSet1*, the second strips of each group in *StripSet*2, and so on to *m*. The two numbers are not necessarily the same. The minimum number of total channels, obtained by choosing *n* = *m*, is 4√N, where N is the total number of strips per layer, X or Y. This implies that the number of readout channels on the second side is equal to the number of fibers per channel on the first side. A particle crossing one

**85**

*Real-Time Particle Radiography by Means of Scintillating Fibers Tracker and Residual Range…*

strip generates a signal at both ends of the fiber. Then, we have a signal from the *i*

*Striphit* = (*i − 1*) *∙ n + j*. (1)

The developed readout scheme reduces the number of readout channels to 16, instead of 32 channels without the application of the compression. When N is large, the reduction factor becomes important, allowing a compact and low-cost real-time acquisition. Notice that to reconstruct the point where the particle crosses the detector (event), an energy release in both planes is needed. In the PSD, readout occurs in time coincidence between the two layers of fibers for each plane. The channel reduction system can read the signal from the whole detector with only 112 channels (less than a fifth respect to the 640 total channels). Each of the 56 optical channels per plane is optically coupled to one of the 64 channels of a SiPM array. For a detailed

As previously stated, the signals from the SiPM arrays are acquired by a DAQ chain divided in two main sections. The first section consists of the FE electronics which operate the analog-to-digital conversion. The digital-encoded data output from the FE is sent to the RO board which hosts a system on module (SoM) manufactured by the National Instrument (NI) for decoding and filtering. The SoM basically consists of a field-programmable gate array (FPGA) and a real-time processor, communicating by means of a direct memory access (DMA) data bus. The real-time processor has gigabit Ethernet connection for data transfer toward a PC, where the

Two FE boards, one for each direction, are required. Each FE board hosts a 64-channel SiPM array manufactured by Hamamatsu Photonics, mod. S13361-

boards amplify and filter the analog signals from each SiPM array channel and compare them, by means of an array of fast comparators, to an individual threshold, remotely settable by a DAC. An individual threshold per channel is useful to compensate the unavoidable mismatch of the SiPMs gain and of the optical coupling of SiPM to fibers. The output of each FE board is an asynchronous digital data bus of 56 bits which represents the status of the SiPM signals and is acquired by the NI SoM on the RO board. The FPGA samples the FE output at high frequency, up to 250 MHz. This data is transferred toward the SoM processor via DMA. The processor applies real-time filtering algorithms and, after discarding the spurious events, reconstructs the impact point of the particles. The SoM's FPGA can be programmed via a graphical approach by means of the LabVIEW platform. The LabVIEW platform also manages the entire acquisition chain and data processing in real time.

photosensitive area per channel. These custom designed

area. Any layer is a

description of the readout channel reduction system, see Refs. [7–10].

real-time visualization and storage of the results are accomplished.

The RRD prototype is a stack of 60 layers, 90 × 90 mm2

ribbon of 180 BCF-12 SciFi. A picture of the detector is shown in **Figure 5(a)**. The ribbons are oriented horizontally and optically coupled at both ends to 1 mm square section wavelength-shifting fibers (WLS), see **Figure 5(b)** and **(c)**. To avoid optical cross talk between adjacent RRD layers, which would degrade range resolution and therefore energy measurement, each layer is optically isolated from the others by

*th StripSet* that univocally identifies the *Striphit*

*th*

*DOI: http://dx.doi.org/10.5772/intechopen.81784*

*NeigSet* group and another from the *j*

hit, according to equation (1):

**2.1 The PSD DAQ chain**

3050AE-08, with 3 **×** 3 mm<sup>2</sup>

**3. The residual range detector**

*Real-Time Particle Radiography by Means of Scintillating Fibers Tracker and Residual Range… DOI: http://dx.doi.org/10.5772/intechopen.81784*

strip generates a signal at both ends of the fiber. Then, we have a signal from the *i th NeigSet* group and another from the *j th StripSet* that univocally identifies the *Striphit* hit, according to equation (1):

$$\text{Strrip}\_{hit} = \left\langle i - \mathcal{I} \right\rangle \cdot n + j. \tag{1}$$

The developed readout scheme reduces the number of readout channels to 16, instead of 32 channels without the application of the compression. When N is large, the reduction factor becomes important, allowing a compact and low-cost real-time acquisition. Notice that to reconstruct the point where the particle crosses the detector (event), an energy release in both planes is needed. In the PSD, readout occurs in time coincidence between the two layers of fibers for each plane. The channel reduction system can read the signal from the whole detector with only 112 channels (less than a fifth respect to the 640 total channels). Each of the 56 optical channels per plane is optically coupled to one of the 64 channels of a SiPM array. For a detailed description of the readout channel reduction system, see Refs. [7–10].

### **2.1 The PSD DAQ chain**

*Applications of Optical Fibers for Sensing*

**Figure 3.**

reduction can be argued by considering a strip detector able to detect one particle at a time. Each strip can be read from both ends, and the signals are grouped following the scheme reported in **Figure 4**, where a two-dimension 16-strip detector is illustrated. All strips are read out, on one end, in *m* groups of *n* neighboring strips, named *NeigSet*, while at the other end, the first strips of each group are grouped in *StripSet1*, the second strips of each group in *StripSet*2, and so on to *m*. The two numbers are not necessarily the same. The minimum number of total channels, obtained by choosing *n* = *m*, is 4√N, where N is the total number of strips per layer, X or Y. This implies that the number of readout channels on the second side is equal to the number of fibers per channel on the first side. A particle crossing one

*An example of the application of the channel reduction scheme to a two-dimension 16-strip detector.*

*(a) Picture of PSD detector during the assembly phase. The dashed red box highlights the sensitive area of the* 

*detector. (b) A sketch of the arrangement of the four layers is shown.*

**84**

**Figure 4.**

As previously stated, the signals from the SiPM arrays are acquired by a DAQ chain divided in two main sections. The first section consists of the FE electronics which operate the analog-to-digital conversion. The digital-encoded data output from the FE is sent to the RO board which hosts a system on module (SoM) manufactured by the National Instrument (NI) for decoding and filtering. The SoM basically consists of a field-programmable gate array (FPGA) and a real-time processor, communicating by means of a direct memory access (DMA) data bus. The real-time processor has gigabit Ethernet connection for data transfer toward a PC, where the real-time visualization and storage of the results are accomplished.

Two FE boards, one for each direction, are required. Each FE board hosts a 64-channel SiPM array manufactured by Hamamatsu Photonics, mod. S13361- 3050AE-08, with 3 **×** 3 mm<sup>2</sup> photosensitive area per channel. These custom designed boards amplify and filter the analog signals from each SiPM array channel and compare them, by means of an array of fast comparators, to an individual threshold, remotely settable by a DAC. An individual threshold per channel is useful to compensate the unavoidable mismatch of the SiPMs gain and of the optical coupling of SiPM to fibers. The output of each FE board is an asynchronous digital data bus of 56 bits which represents the status of the SiPM signals and is acquired by the NI SoM on the RO board. The FPGA samples the FE output at high frequency, up to 250 MHz. This data is transferred toward the SoM processor via DMA. The processor applies real-time filtering algorithms and, after discarding the spurious events, reconstructs the impact point of the particles. The SoM's FPGA can be programmed via a graphical approach by means of the LabVIEW platform. The LabVIEW platform also manages the entire acquisition chain and data processing in real time.

### **3. The residual range detector**

The RRD prototype is a stack of 60 layers, 90 × 90 mm2 area. Any layer is a ribbon of 180 BCF-12 SciFi. A picture of the detector is shown in **Figure 5(a)**. The ribbons are oriented horizontally and optically coupled at both ends to 1 mm square section wavelength-shifting fibers (WLS), see **Figure 5(b)** and **(c)**. To avoid optical cross talk between adjacent RRD layers, which would degrade range resolution and therefore energy measurement, each layer is optically isolated from the others by

### **Figure 5.**

*(a) View of the RRD prototype. (b) Detail of the optical coupling between SciFi and WLS fibers in a RRD layer. (c) View of the sensitive area of the RRD.*

means of 100 μm black adhesive film. The SciFi used in the RRD are not coated with EMA because cross talk between adjacent fibers in the same layer does not affect detector resolution. The scintillation light produced in each layer at the passage of the particle is partially absorbed, reemitted, and channeled by the WLS. The two WLS coupled to a layer transfer the collected light to a channel of a SiPM array. The FE output is, then, processed by the DAQ chain. The SiPM array and the DAQ chain are identical to those used in the PSD described in the previous section.

A charged particle crossing the RRD passes through a number of layers as a function of its initial energy, before stopping. The dose deposited in each layer increases with depth up to the Bragg peak, where the particles produce the maximum amount of scintillation light. This point corresponds approximately to the end of the particles' path in the detector, so, by detecting the layer in which the light signal is the more intense, it is possible to measure their range. The RRD's working principle is reported in **Figure 6**. A calibration of the detector allows to obtain a

### **Figure 6.**

*Working principle of the RRD. In this figure, the height of the yellow column is proportional to the energy released in each layer by a particle crossing the RRD.*

**87**

**Figure 7.**

*Real-Time Particle Radiography by Means of Scintillating Fibers Tracker and Residual Range…*

*0.* In

range-energy characteristic curve, which can be fitted by a function *R0 = αEp*

this way, it is possible to retrieve the initial energy of the particles from the measured range. For the actual RRD, the maximum measurable range was about 36 mm in polystyrene/PVC corresponding to the range of protons with 67 MeV initial energy, but this maximum range can be easily extended up to higher energies by placing a stack of calibrated water-equivalent range shifters between the beam exit

Measurements at CATANA were carried out to fully characterize the performance of the prototypes, with protons up to 58 MeV at the output in treatment room. During the last year, other measurements have taken place at TIFPA proton irradiation facility. The spatial resolution of the PSD was measured by means of a calibrated brass collimator applied at the beam pipe exit in the treatment room at CATANA. After data analysis, it is possible to estimate the holes' centers and compare them with the projection of the collimator holes on the detector plane. Then, the mean distances between the reconstructed centers and the collimator hole centers were calculated for each hole, and the mean distance was about 130 μm, comparable with the (a priori) spatial resolution of the PSD, given by 500 μm/√12. The maximum spatial resolution is an intrinsic characteristic of the

In order to calibrate the RRD, several measures of the range have been acquired changing the initial energy of protons. At CATANA facility, the proton beam energy can be passively modulated by placing a different calibrated PMMA range shifter at the beam pipe exit. The energy of the protons at the beam pipe exit was calculated by means of Monte Carlo simulation. The Bragg peak position is not exactly at the real end of the particles' path but just before. It is an experimentally consolidated practice to assume that the particle range measurement corresponds to the distance

*Measured values of the range vs. the corresponding proton energy at TIFPA and resulting data fit.*

*DOI: http://dx.doi.org/10.5772/intechopen.81784*

detector, independent of the readout strategy.

and the RRD entry window.

**4. Experimental results**

*Real-Time Particle Radiography by Means of Scintillating Fibers Tracker and Residual Range… DOI: http://dx.doi.org/10.5772/intechopen.81784*

range-energy characteristic curve, which can be fitted by a function *R0 = αEp 0.* In this way, it is possible to retrieve the initial energy of the particles from the measured range. For the actual RRD, the maximum measurable range was about 36 mm in polystyrene/PVC corresponding to the range of protons with 67 MeV initial energy, but this maximum range can be easily extended up to higher energies by placing a stack of calibrated water-equivalent range shifters between the beam exit and the RRD entry window.
