Radiation Damage in plastic scintillators

Plastic scintillators have properties that are favourable for the use as a stopping target such as a low density, low nuclear charge Z, good homogeneity and a good reproducibility.

However for certain applications a dose-dependent degradation of the light output has been observed. Up to now there is no comprehensive framework that quantitatively describes radiation damage for arbitrary setups. Nevertheless a variety of literature is available, the major part dating back to the late 80's and early 90's in the context of detector develop-ments for the Superconducting Super Collider[82]. However radiation damage in plastic scintillators is still the subject of more recent research projects, e.g. in the context of radiation hard detector components for the Large Hadron Collider at CERN[83]. Reliable data on radiation damage in PVT is rare and the available data spans a large parameter space, showing dierent results depending on the environment, geometry, dose, dose rate, particle type¹ and the scintillator compounds. An attempt to provide a theoretical fra-mework for radiation damage in PS which quantitatively links the observed degradation to the formation of radicals and the associated chemical reaction channels can be found in [86]. Neglecting the inuence of oxygen as a potential annealing agent and dose rate dependence, the radiation damage in plastic scintillators is described by an exponential decay of the initial LY in [83]:

L(d) =L0·e^−Dd (3.1)

where L is the actual LY, L0 is the initial LY before irradiation, d is the absorbed dose and D the material decay constant which depends on the sample properties, the sample environment and the nature of the irradiation.

Although the majority of publications cover the radiation damage in PS, radiation damage in PVT is expected to be based on qualitatively the same eects [87]. The remainder of this section provides a brief overview of the phenomena commonly described in literature and how it relates to the use of a scintillation target for MEG II.

Light loss during irradiation is usually accompanied by a change of the appearance of the sample. The former clear and transparent sample becomes yellow-brown [88]. After irradiation a partial annealing of the scintillation properties can be observed, that is also reected in a bleaching of the sample. The reduction in LY can be mainly associated with a wavelength-dependend decrease in transmission in the solvent [89]. This is related to the creation of absorption centres formed by radicals in the base material [82, 90]. Absolute numbers for the loss of scintillation light in BC400 after irradiation with a Cs-137 source in air are presented in [91]. The relative LY was measured with a FEU-110 PMT and therefore shows the convolution of the wavelength-dependent quantum eciency of the PMT and the generated light. After exposure to3.4·10⁴ Gy the light output degraded to 44.4 %. Further irradiation up to10⁵ Gy resulted in a decreased output corresponding to 31.0 % of the unirradiated sample. Figure 3.4 illustrates the asymmetric degradation of the transmission spectrum of NE102a for dierent irradiation doses. The light transmission degrades strongest on the short wavelength side, which implies that a search for future radiation hard plastic scintillators should not only focus on the base material and additives that support its annealing but also on the wavelength shifting uors[93]. The underlying principle in the creation of absorption centres is commonly described as follows:

1Related to the linear energy transfer during energy loss[84, 85] in the medium.

Figure 3.4.: Shown are the transmission spectra of NE102a, which is equivalent to BC400B, taken after dierent radiation exposures. As can be seen most of the absorption/LY loss occurs on the short-wavelength side. The gure was taken from [92].

Energy that is deposited in the base material can break up the polymer chains and build up radicals. During this process the environment plays an important role. Due to its high electron anity the presence of oxygen has a considerable inuence in this context.

Literature provides contradictory information on this point. On the one hand oxygen is suspected to increase the radiation damage [88, 85] since it oers more reaction channels in the formation of radicals, while on the other hand a more recent source [86] shows an increased radiation hardness in the presence of oxygen. Measurements presented in the same publication indicate that oxygen dissolved in the sample (due to previous storage in air) reduces the initial radiation damage until the oxygen is "used up", when irradiated in an inert Argon atmosphere. The benet of oxygen in the annealing process after irradiation is commonly accepted. As a conclusion three dierent types of absorption centres can be identied [86]:

ˆ Short-lived damage, with life times on the order of minutes to hours, that anneals even quicker in an oxygen environment

ˆ Long-lived radiation damage that can anneal when exposed to oxygen

ˆ Permanent radiation damage, that can not anneal

More information concerning radiation damage in plastic scintillators is available. However the limited data and the variance of the radiation damage for dierent setups still necessi-ates dedicated tests before application. This is certainly the case for the MEG II stopping target. One of the particular advantages in the MEG-case, using a camera to sample the light from a thin slab is, that one is sensitive to light-paths of the order of the thickness of the scintillator, i.e. path lengths several orders of magnitude less than the attenuation length in PVT [77], rather than, in the usual case of an edge read-out of the scintillator slab, where the light propagates parallel to the surface of the slab.