ITRM20120273A1 - "METHOD OF COMPRESSION AND REDUCTION OF READ-OUT CHANNELS AND ITS APPLICATION FOR THE IMPLEMENTATION OF AN INVESTIGATION SYSTEM FOR PARTICLES LOADED, IN REAL TIME, OF LARGE AREA AND HIGH SPATIAL RESOLUTION" - Google Patents

Description

METHOD OF COMPRESSION AND REDUCTION OF READ-OUT CHANNELS AND ITS APPLICATION FOR THE CREATION OF AN INVESTIGATION SYSTEM FOR CHARGED PARTICLES, IN REAL TIME, OF LARGE AREA AND HIGH SPATIAL RESOLUTION;

DESCRIPTION

1. Field in which the invention belongs

The present invention relates to a method of compressing and reducing the number of reading channels which is applied to linear segmentation particle detectors while keeping the information intact, as well as the means for implementing this method and the investigation devices thus obtained.

Said method is directly applicable to all linear segmentation detectors, based on scintillating fibers, while it is suitably adaptable to those of other technologies and allows to improve their performance in terms of detection speed, costs and acquisition dead time.

It is known that in a linear segmentation particle detector as the sensitive area of the detector increases, once a certain spatial resolution is fixed, the number of reading channels increases proportionally.

The number of reading channels influences the detector reading speed and therefore the dead time, ie the time during which the detector is unable to detect an event. As the number of channels necessary to read the information from the detector increases, among other things, the complexity of the electronics required increases. However, there is a maximum number of channels that can be supported for a given technology, especially in real-time applications. There are, in fact, small detectors with excellent resolution and large detectors with lower resolution.

Advantageously, the channel compression system, part of the present invention, is in itself an innovation applicable to various scientific and industrial fields and allows to maximize, at a given resolution, the sensitive area, the reading speed and minimize the costs of any strip segmented detector of any technology, as long as no multiple events are expected.

• A first innovative feature of the present invention is the compression of the number of reading channels of a position detector segmented by charged particles, by means of an appropriate grouping of the strips at the two reading extremes of the same, with a compression factor equal to ✓ N / 2. The joint use of two or more position detectors, as aligned and parallel planes, such as the one just described, allows to realize tracking detectors.

• Another innovative feature of the invention is the halving of the reading channels of a tracer detector, based on scintillating fibers, obtained through the use of the previously described channel compression system and the use of only two folded sparkling fiber ribbons U-shaped and arranged to create two tracking planes instead of four planes (ribbons) of glittering fibers, two for x and two for y.

• Another innovative feature of the present invention is the use of innovative schemes for coupling the scintillating fibers with wavelength - shifter fibers in the detectors that use the channel compression system in order to reduce the encumbrance of the detector and which are usable in medical imaging

• A further innovative feature of the invention is a position detector based on ribbons of scintillating fibers, which uses the channel reduction system, according to the invention, in an alternative manner, resorting to the approach along a direction of N elementary structures each of which it consists of a ribbon of sparkling fibers folded back on itself and with an alpha angle defined as a design parameter.

• Another characteristic of the invention is a device for radiography with protons (with charged particles) which uses the channel compression and reduction system and the various techniques for making the markers, according to the invention, in the realization of a range meter residue of the particles after passing through a volume to produce the image of the same volume by the use of sparkling fiber ribbons.

2. State of the art

The technique of segmentation of the sensitive area of a particle detector is used in various fields, scientific and industrial, with applications ranging from imaging to environmental monitoring, from large physics experiments to industrial control systems.

An extreme segmentation, for example, is the pixels of an optical sensor in a video camera. The system to be patented is applied, more precisely, to sensors with linear segmentation, in strips.

There are different technologies used for the realization of these segmented detectors, in which the extracted information is not only the successful crossing of the sensitive area of the detector by a particle but also the exact position of this crossing: gas, those integrated on silicon or detectors in which the strips consist of sparkling optical fibers.

In each of the previous cases the number of channels increases with the size at a fixed resolution or the reading electronics have few channels but are complex and / or slow.

Below is table 1 in which some devices for tracking according to the known technique are compared.

TABLE 1

On the other hand, Table 2 shows some devices for imaging with protons as a reference to the state of the art.

TABLE 2

3. Problems of the prior art

With reference to the above, the limitations of the state of the art are evident:

• High number of channels on large detection areas and high resolution;

• High complexity and high costs of reading electronics and lower performance in terms of dead time.

4. Purpose of the invention

The main application is medical imaging with protons (charged particles) in order to allow an upgrade of the performance of existing systems. The main benefits in this area are those of allowing proton radiography in a very short time at high resolution, with large area and at low cost, allowing precise positioning of the patient in real time and beam diagnostics.

More generally, the advantages such as the reduction of the reading channels for a strip segmented detector and the consequent achievement of high performance for the acquisition electronics in the direction of real time, play a key role in various scientific sectors.

5. Detailed description, with reference to the drawings, of an embodiment of the invention

The description of the invention will be better followed with reference to the attached figures which represent, by way of non-limiting example only, some preferred embodiments, in the tables:

Figures 1a and 1b show the sensitive area of a detector before and after traditional strip segmentation, in one direction;

Fig. 2 shows a sensitive area divided in one direction into 15 strips, which shows how according to the invention the reading always takes place from both ends but after having grouped the strips, at each end, into a Primary Group and a Secondary Group ;

fig. 3 is an operating diagram of a scintillating fiber showing the photogeneration mechanism in correspondence with the passage of a particle P through a scintillating core C;

figs. 4a and 4b are respectively a schematic view from above and a side view of the positioning on two layers of two ribbons of scintillating fibers;

Figures 4c and 4d show two photos taken under the microscope with different enlargements of a portion of the sensitive area of a detector according to the invention;

figs. 5a, and 5b, respectively show a sparkling fiber ribbon and a wavelength shifter (green) fiber optically coupled to the same ribbon;

fig. 6 shows the elementary structure of the sensitive area of a position detector which is constituted by a ribbon of scintillating fibers folded on itself, and where each of the two ends of the ribbon is read with the already described method of reducing the channels;

fig. 7 shows a detector realized by the approaching along a direction of N elementary structures such as the one illustrated in fig. 6, where the useful detection area is a right-angled trapezoid;

fig. 8 shows a prototype made with sparkling fibers where a single tape is rewound several times to create a sensitive area of certain dimensions;

Fig. 9 is the basic diagram of a device for the radiography of a volume with charged particles based on the principle of measuring the residual range of the particles after crossing the volume itself;

figs. 10a, 10b and 10c show in the component parts and as a whole, a tracer for protons (charged particles) which uses channel reduction systems and only two sparkling fiber ribbons;

figs. ila and llb respectively show the diagram of a residual range meter where the tracers are made with the techniques described above and a detail of the sparkling planes (tapes) for the measurement of the residual range, integrated between the tracing planes;

fig. 12 shows the diagram of a residual range detector made with layers of scintillator or ribbons of scintillating fibers, to which the channel compression system according to the invention is applied.

With reference to the figures, the invention consists of a channel compression and reduction system for an innovative type of linear segmentation detector.

The application is general and brings the benefits already listed in the previous points

Consider a detector whose sensitive area is segmented in a strip direction, as shown in Figure 1b, and compare it with a non-segmented detector as shown in Figure 1b. there. In the example shown, the strips are 6 in number. As mentioned, the channel hit by the particle provides the impact position and is uniquely identified by the intersection of the vertical and horizontal strips that produce a signal above a suitable threshold. It is therefore necessary to read the strips from both ends.

Increasing the sensitive surface or increasing the resolution involves, according to the known art, an increase in the number of channels, with the consequence that the dead time increases, i.e. the time during which the detector is unable to detect an event, and becomes more complicated. the supporting electronics.

In a completely innovative way, according to the invention, it is provided that the strips are always read from both ends but that the meaning of the two signals is interpreted differently following a different grouping, as shown by way of example in figure 2.

In this figure, the segmentation strips are 15 in number. Once the number of stripsN has been fixed, in the example N = 15 the system according to the present invention provides for the choice of two prime numbers between them whose product is as close as possible a N. It is clear that the choice that optimizes the total number of channels starts from the square root of N and from the choice of the two prime numbers that are close to each other and whose sum is close to 2-✓N ·

In the example, ✓N ~ 4 and, so we can choose the two prime numbers 3 and 5, primary and secondary respectively.

At this point we will use these two numbers to properly group the strips at the two ends. At one end the i-th strips of each group of 3 consecutive fibers are read together forming 3 primary channels (CP). The same goes for the other extreme, for which we will have 5 secondary channels (CS).

When a particle passes through a strip, two signals are generated at both ends of the strip. In our case there will be a signal from a Primary channel and a Secondary channel.

The combination of these two signals uniquely identifies the affected strip, as each strip is identified by one and only one pair of channels, one Primary and one Secondary that produce signals above a suitable threshold.

It is clear that by reapplying the system described to a strip layer orthogonal to the previous one, it is possible to realize xy position detectors.

Other advantages are:

1) reducing the size of the detector. In fact, the interconnections occupy a volume which constitutes encumbrance in the case, for example, of telescopic detectors.

2) the greater robustness of the system towards cross-talk; in fact, in the system described, the cross-talk introduces a slight indeterminacy in the reconstruction of the point of impact, tolerable in almost all applications.

3) The system provides a quadruple coincidence and is therefore immune from the noise, for example, of the photosensor used for the conversion into an electrical signal of the optical signal generated in the scintillating fibers when a particle passes.

4) The strong compression of the number of channels remains valid, for N sufficiently large, with a compression factor equal to ✓N / 2 and the performance of the maximum supportable event rate is enhanced.

In fact, in a detector that adopts the system described, all the positions on the detector are coded by 4 channels, two Primary and two Secondary, and this allows the hardware implementation of the fastest possible coding electronics, which uses RAM memories. dynamics.

In a first embodiment, the method of compression and reduction of the channels which it is intended to patent finds application in the realization of a detector tracing large area particles.

This tracer is a large area detector based on appropriately grouped scintillating optical fibers. It consists of two ribbons of glittering fibers, folded into a "U", oriented at 90 ° with respect to each other, called ribbon X and ribbon Y.

Note that for the reconstruction of the detector crossing point by a particle (event), it is necessary that the particle releases energy in both layers of fibers.

At this point it would be possible through a matrix of light sensors, a front-end and an appropriate acquisition system, to identify which fibers have produced a light pulse and to reconstruct the position of the crossing point with resolution where d represents the size / diameter of the sparkling fiber.

To do this, each sparkling optical fiber must be optically coupled to a light sensor and each light sensor must be read from a front-end electronics channel and acquired.

The coupling between sparkling fibers and the optical sensor is achieved by mechanically joining and fixing the fibers and dipping them in optical grease, or by gluing the fibers directly to the sensor using a special two-component optical glue. An alternative is the collection of the light produced in one or more scintillating fibers by means of a wavelength shifter (WLS) fiber, as shown in figure 5 and subsequently coupling said fiber to the photosensor. In any case, the yield in terms of optical coupling is equivalent. The channel reduction technique ("U" strips) combined with the latter technique of collecting the light produced by a group of scintillating fibers (GP and GS), allows a considerable reduction of the overall dimensions of the optical interconnections. Multianode photomultipliers or SiPM arrays can be used as light sensors instead of separate light sensors for each fiber or group of fibers. The use of SiPMs has lower costs for large quantities, a simpler architectural implementation and takes advantage of the channel compression system as the quadruple coincidence mechanism provided in the tracer would allow to overcome the problem of the high rate of spurious signals characteristic of SiPM.

An alternative scheme of application of the channel reduction system for a position detector of charged particles, associated with the aforementioned compression of the channels, is carried out with a different use of the sparkling fiber ribbons and is described below.

All other considerations remain valid also in this case.

The proposed detector is realized through the approach, along a direction, of "N" elementary structures (see figure 6).

The elementary unit of the detection area consists of a ribbon of scintillating fibers, placed side by side, in the example of figure 7, with a square section of the size of 500 μm, about 1 m long. Each of the two ends of the tape is read with the aforementioned channel compression method.

Each elementary structure is made up of a ribbon of scintillating fibers folded back on itself, respecting the minimum radius of curvature characteristic of the type of fiber that has been chosen to use, and with an angle (alpha) defined as the design parameter (see figure 6 ). It is evident that the choice of this angle, in addition to the size of the entire detection area to be reached, will determine the number of tapes to be used, how many fibers the tape is composed of, the resolution along one of the two directions.

From a preliminary study carried out on the admissible variation of the size of the detection area and of the alpha angle, the following values were determined:

• side of the detection area: ≤ 45 cm

• 6 ° <alpha <45 °

It should be emphasized that these limits are certainly achievable and that only a more detailed study will determine how much and which of these can be overcome. In fact, if one thinks that the side of the detection area is constrained by the length of attenuation of the light inside the scintillating fibers, one could think of eliminating the curvature of the ribbons, which introduce a considerable attenuation factor, in an area that is not active of the detector, with a system of mirrors that allows the light to pass from the fiber of one layer to the same fiber of the other layer or to effect the bends with clear fibers, whose attenuation length is negligible compared to the quantities in question.

A numerical example can give an idea of the advantages of the method just described.

Given L = 450 mm, detector side, d = 0.5 mm, section or diameter of the scintillating fiber, F = 90, number of scintillating fibers per tape and alpha = 6 °, we obtain n = 29 for the number n of primary channels , m = 35 for the number m of secondary channels and, therefore, with the ribbon of 90 fibers each and with a length of about 1 m it is possible to make a large area detector (45x45 cm <2>) by reading only n + m = 64 digital channels.

The two ends of the tapes are read with the method already described, grouping them into Primary Groups (n) and Secondary Groups (m) of fibers, read by a multi-anode photomultiplier or by a SiPM matrix, whose anode, analogue signals , each will be converted into a single digital bit through comparison with a reference threshold. A crossing by a charged particle at any point in the detection area will cause the creation of scintillation light in only two fibers.

The light produced within each fiber will propagate in both directions over two different distances and will be collected at the ends of the ribbon. Due to the attenuation of the light introduced by the scintillating fiber, the two signals at the two ends will have a different and distinguishable duration above the threshold. It will therefore be possible to distinguish the corresponding Primary and Secondary Groups for each of the two fibers, uniquely identifying the position in which the particle passes through the detector. In this way there is a further reduction of the channels with respect to the first case illustrated in fig. 2. In fact, the groupings in the two directions, x and y, are not repeated. Furthermore, the amount of optical couplings is reduced while maintaining the advantage of coincidence for rejecting spurious signals.

A peculiarity of this configuration is that the pixels that are formed by the superposition of the two layers of fibers have a parallelogram shape. This parallelogram is all the more elongated the smaller the alpha angle of wrapping of the tapes.

That is, we will have a different resolution in the two directions, x and y. The first is comparable to the size of the fibers divided by ✓12 while the second depends on the alpha angle (about 3 times in the case of 20 °).

Figure 7 shows a numerical example to estimate the design parameters and how they can be chosen to make a tracer detector of the desired size and the number of desired readout channels.

Figure 8 shows a prototype made with bcf-20 scintillating fibers for the characterization of the attenuation length and the bending properties of scintillating ribbons. In this detector, a single tape is rewound several times to create a sensitive area of 13x16 cm <2>.

The tests have provided excellent results and the measurements are still ongoing. The optical coupling between scintillating fibers and photomultipliers is direct.

We will now describe a proton radiography device based on the method of measuring the residual range of particles and to which the previously described innovative features are applied.

Figure 9 shows the basic scheme of a device for radiography with charged particles based on the consolidated technique of the residual range of particles after crossing a volume, for producing the image of the same. Such a device made in large dimensions must integrate tracers that allow to reconstruct the correct range of particles. In fact, the residual range in the classical scheme, i.e. in the absence of integrated tracers, is capable of measuring with good precision only particles that travel an almost straight trajectory inside the detector. In applications, this situation is the least likely. A system that allows the measurement of the residual range and at the same time provides its trajectory overcomes the previous limitation but in the absence of the solutions proposed below, it would have a prohibitive number of reading channels from the point of view of a real-time acquisition.

There are two fundamental elements:

a) a system of tracers;

b) a residual range detector.

The application of the method of compression and reduction of the channels proposed to this measurement system translates in practice into a detector whose acronym is P.R.E.D.A.T.E. (Particle Residuai Energy Detector And Tracker Enhanced).

In particular, in this application, there are:

a) tracers such as those illustrated in Fig. 7 or Fig. 10, for measuring the inlet, intermediate and outlet directions of the particles. A pair of trackers is, in fact, able to measure in very low intensity beam conditions (single particle or imaging conditions) the direction of the track in real time and correlated to the energy released in the trackers.

b) A set of layers of plastic scintillators with a thickness of one millimeter and an area equal to 40x40 cm <2> or ribbons of 400 scintillating fibers of 1 mm diameter fibers for the measurement of the residual range.

To guarantee the arrest of 250 MeV protons (typical energy from proton imaging) and considering the density of plastic scintillators, close to 1 (water equivalent), we have to foresee about 400 layers.

The light produced in the layer (or of the sparkling fiber web) is collected by two wavelength shifter (WLS) fibers optically coupled to two opposite sides of the layer or at the ends of the sparkling fiber webs, as in figure 5.

The compression system of fig. 2 is applied to the layers, therefore, the two WLS fibers are grouped in a different way: one in Primary Groups (GP) (from 19 layers in the example of figure 12), the other in Secondary Groups (GS) (from 21 layers in the example of figure 12).

In figure 5, the coupling scheme of the WLS fibers is shown.

The channels thus produced, 19 primary channels CP and 21 secondary channels (CS), are optically coupled to the channels of a photosensor.

The sensor outputs are compared to the same suitable threshold through a matrix of comparators, producing a logic output.

A threshold is chosen such as to trigger the comparators only in the final part of the trace in the detector, near the Bragg peak.

Figure 12 also shows an example of the logic output, where 19 main CP channels are read at a high threshold (Bragg peak) as well as the 21 Secondary CS channels are read at high threshold: it is understood how it is possible to easily extract the information always considering the one bit to the right.

This system allows a radiography with protons in real time and at the same time produces the beam profile through the tracers.

A further simplification is illustrated in fig. 11b where there is the diagram of a device for radiography with protons (charged particles) which uses the compression and reduction systems of the channels already mentioned for the realization of a residual range meter. In this device the two planes of the tracker are made with any of the innovative methods previously described.

Figure 10 shows the previously described tapes arranged so as to create a plotter. The scenario regarding the layers of scintillators for the measurement of the residual range remains unchanged compared to the previous point.

Figure 11b shows a detail of the design of the system with the addition of the first 133 sparkling layers (or ribbons of sparkling fibers).

The system for the reconstruction of the different impact points on the tracers is based on the attenuation of the scintillation light produced almost simultaneously on them.

The attenuation produced by the stretches of tape interposed between the different points of impact allows the processing of the signals on the basis of the time above the threshold.

The resulting number of reading channels allows the use of even a single high-performance digital acquisition card and the use of the latest generation of programmable logic could favor the real-time processing of all the information coming from the detector. Note that in the example of the detector in figure 11, with dimensions 40x40x40 cm <3> and with 500 micron scintillating fibers and 1 mm scintillating layers (or ribbons of scintillating fibers) and 2 tracers, the number of reading channels is of 256 channels.

From what has been described up to now, the characteristics of the invention and its advantages are evident:

The method of compression and reduction of channels, with various applications;

The detector, with main application in proton imaging but innovative and competitive also in other scenarios.

The channel reduction system optimized with the use of sparkling fiber ribbons.

The proton radiography system based on the measurement of the residual range and with the application of the channel compression and reduction system.

The radiography system with protons based on the measurement of the residual range and with the application of the system for reducing and optimizing the channels according to the invention and of the winding system for sparkling fiber tapes.

The detector has multiple advantages over similar state-of-the-art detectors under imaging conditions (single event):

• large area up to 40x40 cm <2>;

• water equivalent;

• resolution up to 80 microns with 250 μm sparkling fibers;

• acquisition and processing in real time;

• maximum supported event rate up to 100 MHz;

• low cost;

It is also worth highlighting the possibility of making the detector in a short time by purchasing only devices and materials on the market, guaranteeing low costs and production times.

In fact, there is no need for full-custom integrated electronics for the front-end and for the acquisition electronics.

The two configurations for a tracker, in the embodiment examples described and illustrated with reference to Figures 7 and 10, have important features and can be well adapted to different applications.

By comparing the references to the state of the art, it is possible to evaluate how the simple application of the present invention in its various embodiments can bring enormous benefits, in terms of performance, to different applications by using or replacing already consolidated techniques.

Claims (2)

CLAIMS 1) Method of compression and reduction of the number of reading channels applicable to linear segmentation particle detectors while keeping the information intact, characterized in that it provides for the compression of the number of reading channels of a segmented position detector for charged particles by means of a appropriate grouping of the strips at the two reading ends of the same, with a compression factor equal to ✓N / 2, where N is the number of strips and where the grouping methods of the strips at one reading end are different from those of the strips at the other extreme of reading. 2) Method as in claim 1 characterized by the fact that the grouping of the strips at the two reading extremes occurs on the basis of the choice of two prime numbers between them whose product is as close as possible to the square root of N and whose sum is close a 2✓N 3) Method as in claim 1 characterized by the fact that in the case of application to position detectors based on scintillating fibers, it further provides for the use of a configuration of scintillating fibers to realize the detection area, chosen from the following two: one first configuration that involves the juxtaposition, along a direction, of N elementary structures each consisting of a ribbon of sparkling fibers folded back on itself, respecting the minimum radius of curvature characteristic of the type of fibers chosen, and with an alpha angle defined as a project parameter; And a second configuration in which only two ribbons of U-folded glittering fibers are used and oriented at 90 ° with respect to each other arranged in such a way as to create two tracing planes instead of with four planes (tapes) of scintillating fibers, two for x and two for y, with the consequent halving of the reading channels, the choice of one or the other configuration depending on the type of survey device to which said position detector belongs. 4) Method as in the previous claim characterized by the fact that using the first configuration, the choice of the alpha angle, in addition to the size of the entire detection area to be reached, determines the number of tapes to be used, of how many fibers it is composed the tape and the resolution along one of the two directions. 5) Method as in the previous claim characterized by the fact that the admissible variation of the dimension of the detection area and of the alpha angle is determined by the following values: • side of the detection area: ≤ 45 cm • 6 ° <alpha <45 ° 6) Method as in claim 3 characterized by the fact that in the first configuration the bending of the strips can be eliminated in a non-active zone of the detector, replacing it with a system of mirrors which allows the light to pass from the fiber of a layer to the same fiber of the detector. other layer. 7) Method as in claim 3 characterized by the fact that in the first configuration the bends are made with clear fibers, the attenuation length of which is negligible. 8) Method as in claim 3 characterized by the fact that in the first configuration the pixels that are formed by the superposition of the two layers of fibers of each ribbon of sparkling fibers folded back on itself have a parallelogram shape, which is all the more elongated the smaller the alpha winding angle of the tapes. 9) Tracer detector realized with the joint use of two or more position detectors, such as aligned and parallel planes, obtained with the method of claims 1 to 3. 10) Tracer detector obtained with the channel compression method of claim 1, in combination with sparkling fiber ribbons characterized by the second configuration of claim 3. 11) Tracer detector usable in the field of medical imaging, which uses the channel compression system according to claim 1, and is obtained by coupling the scintillating fibers with wavelength -shifter fibers in order to reduce the bulk of the interconnections optics. 12) A device for radiography with protons (charged particles) based on the consolidated technique of the residual range of the particles after the crossing of a volume, for the production of the image of the same, including a tracer system and a residual range detector , characterized by the fact that: said tracers are made with ribbons of scintillating fibers arranged according to the first or second of the fiber configurations of claim 3, for measuring the inlet, intermediate and outlet directions of the particles; and the residual range detector consists of a set of layers of plastic scintillators to which the compression system according to Riv .1 is applied. 13) A device for radiography with protons (charged particles) as per the previous claim characterized by the fact that the light produced in the layer (or ribbon of scintillating fibers) is collected by two wavelength shifter (WLS) fibers optically coupled to two opposite sides of the layer or ends of the sparkling fiber ribbons; the compression system according to claim 1 is applied to the layers, and the two WLS fibers are grouped in a different way: one in Primary Groups (GP) and the other in Secondary Groups (GS); the channels thus produced, primary channels CP and secondary channels (CS), are optically coupled to the channels of a photosensor; the sensor outputs are compared to the same suitable threshold through a matrix of comparators, producing a logic output; said threshold being such as to trigger the comparators only in the final part of the trace in the detector, near the Bragg peak, thereby obtaining an X-ray with protons in real time and at the same time the beam profile through the tracers. 14) A device for radiography with protons as per claims 12 and 13 characterized by the fact that, thanks to the resulting reduced number of reading channels, it uses a single high-performance digital acquisition card and programmable logics to favor processing in real time of all information coming from the detector.