Proton induced radiation damage studies on plastic scintillators for the Tile calorimeter of the atlas detector

Figure 4-6: The final product of a polished sample, and a sample mounted into the brass 
holder. 
4.1.3.
 
Proton irradiation at iThemba LABS Gauteng
Plastic scintillator samples were irradiated using 6 MeV protons accelerated by 
the 6 MV EM Tandem accelerator of iThemba LABS. A schematic of the tandem 
accelerator is shown in Figure 4-7 below.
Figure 4-7: Schematic representation of the tandem accelerator at iThemba LABS.
In order to produce a beam of 6 MeV protons, the following procedure occurs. 
First, negative hydrogen ions are produced through Cesium sputtering by an 860A 
SNICS ion source and injected into the tandem accelerator. A voltage of 3 MV is 
applied to the central high voltage terminal whilst the entrance and exit ends of 
the tank are kept grounded.

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A Pelletron charging system creates a potential difference, resulting in the 
negative ions being accelerated toward the terminal where they reach an energy 
of ~3 MeV. At the terminal, a gas stripping system removes the electrons from 
the ions.
A second Pelletron chain then creates a potential difference between the terminal 
and exit end, resulting in the now positive ions (protons) being further accelerated 
to reach ~6 MeV before being extracted from the tank. Bending and steering 
magnets then direct the extracted beam towards the nuclear microprobe. A 
photograph of the line leading to the microprobe is shown in Figure 4-8.  
Figure 4-8: Photograph of proton beam line leading to the nuclear microprobe chamber.
For the irradiations, plastic scintillator samples were mounted on a hexagonal 
carousal sample holder and housed within the nuclear microprobe chamber under 
vacuum conditions. The proton beam was passed through an object slit and 
collimator slit and a set of magnetic quadrupole triplets were used to focus the
beam onto a sample with a spot size of ~20-30 μm.

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The beam was scanned in the x and y plane using a raster pattern to achieve a 
uniformly irradiated area of approximately 1.8 mm by 1.8 mm. This irradiation 
technique was required because using a large diameter beam whic h is not scanned 
over the sample resulted in an inhomogeneous intensity distribution of a Gaussian 
shape which was not conducive to the required damage analysis techniques.
The beam current was determined by measuring the current generated across a 
metal plate situated on the side opposite to the sample on the carousal . The beam 
current, integrated per second, was recorded for the duration of each irradiation. 
Two samples of each plastic scintillator type were irradiated per dose for targeted 
doses of 0.8 MGy, 8 MGy, 25 MGy and 80 MGy.