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

Figure 5-16: Possible configurations resulting from dehydrogenation of benzene.

---

56 
5.5.2.
 
Healing of structural damage 
Raman spectra for the 8.11 MGy irradiated EJ200 sample were measured 3 days, 
10 days and 4 weeks after irradiation. The background subtracted Raman spectra 
for these are shown in Figure 5-17. A plot of the ratios between each peak 
intensity to the intensity of peak 8 (C-C aromatic) is shown in Figure 5-18.  
Three days after irradiation, the structure contains a larger amount of specie 
related to the vinyl backbone (peaks 15-21) verses benzene ring type vibrations, 
thus indicating damage to the benzene ring. There is an increase in the higher 
vibrational energy aliphatic modes and small decreases in the lowe r energy 
modes.
After 10 days, some recovery of the original ratios are observed. After 4 weeks, 
the ratios are significantly recovered. A small increase in some aliphatic modes 
remain as well as a 4% increase in the ratio of C=C bonds. It should also be noted 
that the additional peak forming at 500-650cm
-1
, loses a small amount of intensity 
as the recovery occurs 
Figure 5-17: Background subtracted Raman spectra for EJ200 sample measured several 
days after radiation exposure to a dose of 8.11 MGy . 

---

57 
Figure 5-18: Plot of Raman peak intensity ratios relative to the C-C aromatic peak 
intensity for EJ200 sample irradiated to 8.11 MGy.

---

58 
6
 
Conclusion 
The Tile Calorimeter of the ATLAS detector will be replacing the plastic 
scintillators employed in its Gap region as part of the 2018 upgrade. These 
upgrades are being implemented in order to ensure an optimal performance of the 
detector during the next several data taking periods, where particles will be 
collided at higher energies and increased luminosity. Since these conditions will 
result in a harsher radiation environment, the scintillators used will need to 
exhibit a strong radiation damage tolerance.
As such, the radiation damage undergone by several polyvinyl toluene and 
polystyrene based plastic scintillators was investigated. Samples of 350 µm 
thickness were subjected to 6 MeV proton irradiation at doses of approximately 
0.8, 8, 25 and 80 MGy using the 6 MV tandem accelerator of iThemba LABS . 
Transmission spectroscopy, fluorescence spectroscopy and light yield studies 
were used to assess the optical changes undergone by the damaged scintillators. 
Raman spectroscopy was used to study the struct ural properties.
Overall, the extent of damage undergone was proportional to the exposure dose. 
For doses of ~0.8 MGy, scintillators maintained their transmission character over 
the visible wavelength range, however a loss in absorption correlating to the fluor 
absorption region occurred. This lead to a loss in light yield, which corresponded 
to a reduced fluorescence over the fluor emission region. The scintillators 
maintained their structural properties and damage could therefore be correlated 
to a bleaching related effect of the fluors. 
As the exposure progressed to higher doses (> 8 MGy), the following were 
observed: 

Visible discolouration progressing from yellow to brown with dose . 

Formation of an absorptive component shifting to higher wavelengths with 
dose. As a result, scintillators lose transparency to their own scintillation 
light. Linked to free radicals or colour centre formation.

---

59 

Increasing loss to light yield with increasing dose.

Loss to the 229 nm excitation driven fluorescence yield over both base and 
fluor emission regions.

Structural changes to the PVT or PS base of the scintillators
The scintillators showed recovery of transmission character as well as structural 
recombination within 4 weeks after irradiation. The most significant recovery of 
transmission occurred within 3 days after irradiation with exposure to air. It was 
noted that oxygen could be a driving factor since it may react with free radicals 
and result in bleaching of the competitive absorption that they cause.
The scintillators were prone to photo-bleaching upon excitation with a 229 nm 
excitation laser (also in the presence of air ), and dose exposure impacted on the 
rate of bleaching undergone by the scintillators. This would be an interesting 
effect to study in further detail since bleaching of the fluors in plastic scintillators 
is a driving factor behind radiation damage as well . The same fundamental 
mechanism is responsible for bleaching effects, photo -bleaching and natural 
aging of scintillators driven by sunlight exposure.
The Raman spectra for 8 MGy samples showed small changes to the structure of 
the polymer base, with less aromatic ring type vibrations and additional 
alicyclic/aliphatic chain vibrations, C=C bonds and potentially 
𝐶 ≅ 𝐶
bonds. 
Damage to the C-H bonds of the benzene ring may lead to hydrogen degassing 
and hence could account for the observed structural change s. The degassed 
hydrogen may be lost to free radicals.
The Raman technique using the 514 nm excitation laser was not sufficient 
enough to accurately study the structural damage undergone for the higher dose 
exposures due to the additional fluorescence background. The use of a longer 
excitation wavelength, such as 718 nm, could reduce this background effect and 
may be considered for future experiments. The downside of this would be a loss 
in the number of Raman active modes that could be studied.

---

60 
Overall, EJ260 and EJ208 exhibited the most tolerance against radiation damage 
effects to their optical properties. Both these scintillators have secondary fluors 
that shift the final scintillation light to higher wavelengths. As a result, less 
competition for re-absorption of light by free radicals occurs in these scintillators 
as compared to the other common blue emitting ones.
EJ260, however, is a green emitting scintillator and its emission range will not 
couple well to the current optical fibers used by the Tile Calorimeter. These fibers 
were tested to have a greater radiation and stress tolerance versus clear fibers. If 
EJ260 is to be considered as a candidate for the upgrade, further studies on the 
radiation damage of the coupled scintillator -fiber-PMT system would be needed.
EJ208 on the other hand, performs considerably well in comparison to EJ260. It’s 
wavelength of maximum emission at 435 nm couples well to the absorption 
maximum at 430 nm of the Y11 optical fibers. It also has a faster response time 
verses EJ260 according to the manufacturers. EJ208 is therefore recommended 
as a viable candidate to consider for replacing the c urrent scintillators in the Gap 
region of the Tile calorimeter.
Since these studies were conducted on 350 µm thin samples, the influence of 
radiation damage on their bulk properties have not been considered. Future 
studies will therefore be conducted on thick scintillators so that the effect of their 
transmission loss to the attenuation length may be better understood. Scintillators 
having geometries similar to those used by the TileCal will be investigated.
Alongside this study, several other investigations into radiation damage of plastic 
scintillators are being conducted. These include electron paramagnetic resonance 
(EPR) studies [29], neutron induced radiation damage studies and dose rate 
dependence studies [30].The collaboration wishes to extend the studies further 
by investigating the radiation damage in coupled scintillator -fiber-PMT systems. 
The radiation damage in inorganic YSO (Y
2
SiO
5
) crystal samples will also be 
studied in collaboration with the JINR. The feasibility of plastic scintillators 
versus these YSO scintillating materials will be investigated for future HEP 
applications.

---

61