• radiation damage
• physics
• calibration
• beam monitor
• scintillating fibres

Plastic scintillating fibers have many uses for particles detectors in high energy physics. They can be used for tracking detectors, since they produce optical photons when a charged particle deposits energy in the scintillating material and they are able to transport the optical photons to a read-out system. They have many advantages: the spatial granularity is proportional to the fibre diameter, the scintillation decay time is typically on the order of a few nanoseconds and the deadtime is shorter than in other tracking systems (as ionization chambers or multi-wire proportional chambers). However, plastic scintillating fibres are rather vulnerable to radiation damage, that over time compromises their response in terms of transmission of light.
The main goal of this thesis is the study and the investigation of the radiation damage in plastic scintillating fibers for an ion beam profile monitor. When a plastic scintillator material is exposed to a certain dose of radiation, a damage due to the dose exposition in the material is caused. In the case of this prototype detector, the detection of the radiation damage appears in different ways: reduction of the light output and attenuation length reduction.
All the measurements were conducted in the Heidelberg Ion Therapy Center (HIT), a facility for radiation treatments located at the Heidelberg University Clinic. A scintillating fibre based ion beam monitor could be an optimal replacement for the current tracking system at HIT, made of ionization chambers and multi-wire proportional chambers, at the end of its lifetime. In the studied
beam profile monitor it is important to measure a precise beam position, width and intensity in order to detect deviations from the planned parameters. The effects of the radiation damage appears on the reconstructed beam profile as a bias in the position and in the spot size (width) of the beam. In this context, it is important to underline that one of the goals of this scintillating fibers detector is to reconstruct the beam profile within given values of resolution for the beam position and beam width: <0.2 mm for the first and <0.4 mm for the latter.
The detector planes are made of scintillating fibers wound as a thin multi-layer ribbon. In this prototype detector four mats with two layers of fibres are used. Each fibre has a diameter of 250 μm and the pitch is 275 μm between each fibre. The exposure of two different types of fibre mat to the radiation is tested: the SCSF-78 MJ (blue fibers mat) and the SCSF-3HF (green fibers mat).
The first emits light with a peak wavelength of 480 nm and the latter has an emission spectra peak at 530 nm. The blue fibers mat are the ones originally used for the LHCb experiment. A better resistance to transmission loss due to radiation damage is expected for green fibers due to its emission of light at higher wavelength. The scintillating light is collected and converted in a electronic signal through a read-out system made of photodiode arrays. Four custom front-end electronic board are used to digitize the signal from the four detector planes. In each board two 64 photodiode arrays are used, providing 128 channels readout. Each board is located at the end of one side of the fibre mat.
The total expected dose due to the patient treatment in the HIT accelerator in one year is about 7kGy over the surface of the fibre, but greater than 1MGy deposited as a small spot in the center due to other accelerator activities; so the light loss is expected much more in the center of the fibres. In our tests, the detector planes were irradiated with a dose corresponding to around six months of accelerator activities. In order to investigate and estimate the impact of radiation damage in the fibres, the central active area of 10 × 10 cm 2 in each mat was scanned with a proton beam of 78.17 MeV of kinetic energy, intensity 2×10 9 particles/s and with a read out frequency of 10 kHz.
Data in 25 different positions of the active area (dividing the active area in a grid of 5 × 5) of the detector are taken, moving the beam in steps of 2 cm. The beam profiles in each position is reconstructed.
The radiation damage appears as a region of suppressed signal amplitude, compared to the Gaussian shaped expected from the nominal beam profile. It is located, as expected, in the center position of the fibers mat, where the dose deposited is greater than in the rest of the surface.
The reduction of the light intensity observed is up to 60%-70% for the blue fibers mat and up to 20%-30% for the green fibers mat. The blue fibers mat are not a good solution, due to the very large reduction in signal.
In order to compute the magnitude of the bias in the beam position and beam width due to the radiation damage, a toy model simulation is constructed, in which various beam profiles corresponding to the multiple beam settings available at the HIT in each different position are simulated, with the presence and without the presence of the damage. From the results of the toy model simulation it is clearly evident that in the position affected by the radiation damage, the resolutions of the beam parameters (the beam position and beam width) are out of the specified requirements. The toy model is later compared to data collected with the ion beam.
In order to remove the bias, a possible solution could be the calibration of the detector. In this way the correction of the data becomes a method to face the problem of radiation damage.
This could be also a convenient solution, in terms of costs and time,the solution to replace the fibres after a certain period of received dose. The calibration process used in this thesis is based on a simulation method of the beam profiles in the different positions of the 5×5 grid in the active area. In particular the sum of the simulated beam profiles in the five different positions from the photodiode (same Y coordinate in the grid) is taken and compared with the sum from the real data of the beam profiles in the same positions. Dividing the sum of the expected beam profiles by the one of the real data, a calibration factor for each channel is obtained. Multiplying the amplitude in each channel for the corresponded calibration factor in each beam position, the data correction of the beam profile is obtained.
For the data set recorded for this thesis, one would expect the beam position to be reconstructed within the resolution of the tracking detector (30-40 μm when unirradiated for the beam setting used). The bias introduced by the radiation damage results in a position and width reconstruction up to 1-2 mm from the expected position. The beam spots that are unaffected by the radiation damage show a reconstruction position in the nominal location. After calibration, the beam spots in the affected region agree with the unaffected regions within 100 μm for the green and blue mats, and can be considered as an additional uncertainty in the reconstructed beam position. This uncertainty for the blue and green mats is within the requested specifications and hence quantify the goodness of the algorithm used.
The next steps for this work will be to study the impact and the recalibration using a smaller beam spot of carbon ions. Moreover, a mirror will be introduced to the detector in order to increase the light output.
The thesis presented details a study performed at the Physikalisches Institut (PI) in Heidelberg (Germany), during a traineeship program.