High energy charged particles, in particular protons, are one of the main components of the cosmic radiation. Fast neutrons in the cosmic radiation result from proton reaction with interstellar or atmospheric gaseous. Similar processes but in reaction with material in space instruments generate substantial neutron doses. Cosmic gamma radiation, for example gamma-ray bursts origin from processes in astrophysical objects.
Fast neutron interaction with organic materials is mainly due to elastic scattering, resulting in recoiled protons penetrating the crystal. Gamma-ray interaction at medium energies mostly involves Compton scattered electrons. For organic materials the neutron cross section is much higher than that for gamma absorption. Therefore these materials are characterized by much higher sensitivity to neutron induced radiation damage. The scattered protons and electrons while penetrating the crystal damage its structure. This results in creation of color centers, which are leading to absorption bands in a wide spectral region. The damage is manifested by reduction of the scintillation light yield and optical transmission as well as change of the samples mechanical properties. There are two groups of organic scintillators, namely monocrystals and plastics. Examples of the former are anthracene, naphthalene, stilbene, etc and are known to be much more sensitive to radiation damage than plastics. Roughly one order of magnitude higher radiation dose is required to generate similar damage in plastics compared to monocrystals . For these reasons monocrystals are omitted in the present review. There is a large chemical variety of plastic scintillators, polystyrene -, polyvinyl (polivinyltoluene) -, polymethyl based and others. Their scintillation properties are highly different, while material properties fairly similar.
Neutron induced radiation damage
Results by B. Bodmann and U. Holm are to our knowledge the only study on fast and thermal neutron damage in plastics . Some other investigations were performed at mixed neutron and gamma fields, however the results are considered less ambiguous. The experiments in ref.  were performed for polystyrene (PS) scintillator and polymethyl (PMMA) based samples, exposed separately to neutrons and gammas. The radiation damage was monitored as changes in transmission spectra. The results for permanent damage induced by fast and thermal neutrons are slightly much similar, however up to factor 5 higher than that for gammas (60Co source) at doses 5.2 kGy n and 7.0 kGy γ, respectively. This is mostly valid for PS scintillators, since PMMA samples indicate the opposite at much similar doses. PMMA, contrary to PS scintillators, is often used as light guide or basic material for wavelength shifter.
Gamma ray induced radiation damage
Radiation damage in plastic scintillators due to gamma radiation, measured as a drop of optical transmission and scintillation light yield, has been a subject to extensive studies, reported in a large number of publications. Most of the studies were performed for polystyrene (PS) and polivinyltoluene (PVT) based materials at doses up to 100 kGy (but typically at 1-10 kGy), utilizing strong 60Co sources. The results can be summarized as follows:
- Radiation damage is strongly depended on the total delivered dose.
- Radiation damage is influenced by the dose rate to deliver the same total dose, i.e. lower dose rates irradiation shows stronger radiation damage [3, 4]. The explanation is that oxygen reacts with radical species formed under irradiation as it diffuses into the sample. These oxidation products contain chemical groups which are characterized by stronger light absorption at longer wavelength. The production of radicals is higher at lower dose rates, due to the much longer exposure time.
- Gamma radiation damage much depends on environment in which the sample is situated. The damage is much severe in oxygen atmosphere and less severe in air. In contrary, same samples placed in nitrogen atmosphere indicate substantial radiation hardness . It is most possibly do to the effect of oxidation, as mentioned above.
- Irradiated samples indicate change in its mechanical properties. It is observed as tensile change, slightly higher at lower dose rates . On the other hand, polarized light examination of some gamma irradiated samples revealed presence of stress zones in the crystal structure .
- For some PVT based plastics were found that radiation affects more the light transmission that the light yield .
The damage is not totally permanent. It is recovered in course of time, much dependent on the delivered dose. The color centers, which are created during irradiation, can be spontaneously relaxed after the end of irradiation via thermoactivation. This thermodynamic process can be accelerated by heating of the crystal, which is a standard procedure to anneal radiation damage. In this way the recovery time, which is usually of the order of few days, can be shorten. There is also another aspect of the recovery process. Studies of some PVT based crystals shown that samples after 40 kGy absorbed gamma dose irradiated in nitrogen environment recovered faster with respect to samples irradiated in air .
Despite a long history of organic scintillators radiation damage studies, the number of publications is rather limited. This is in particular valid for neutron induced damage. Thus further research should be mostly addressed to studies with fast neutrons. For organic scintillators, as well as other scintillators, being a part of large detector systems, to shorten the recovery time is often of large importance. Thermal annealing is not always an option, since it requires the disassembly of the detector system. Studies of PWO inorganic scintillators, used in PANDA detector system, revealed that the recovery process can be much enhanced by illumination with UV light. This is a very interesting option, even for practical reasons. Thus research should be addressed to find if this process is valid for organic scintillators as well.
- I.M. Rozman and K.G. Zimmer, Intern. J. of Appl. Radiation and Isotopes, 36 (1958) 3
- B. Bodmann and U. Holm, Nucl. Instr. Meth. B 185 (2001) 299
- M.M. Hamada et al, Nucl. Instr. Meth. A 422 (1999) 148
- A.D. Bross and A. Pla-Dalmau, IEEE Trans. Nucl. Sci. 39 (1992) 1199
- Y. Sirois and R. Wigmans, Nucl. Instr. Meth. A 240 (1985) 262
- S. Illie et al, Nucl. Phys. B 32 (1993) 384
- S. Baccarro et al, Radiat. Phys. Chem. 40 (1992) 565
Radiation dose absorbed in scintillator when exposed to Genie 16 neutron generator
Neutron irradiation of Polar plastic scintillators
Neutron dose estimation
Polar detector element, front surface 17.5 cm x 0.5 cm=8.75 cm2
Test plastic scintillator, front surface 17.5 cm x 5 cm=87.5 cm2
Assumed experiment, test plastic scintillator, 5 cm wide, mounted on 2” PM tube.
14 MeV neutrons from neutron generator, flux 2 x 108 n/s in 4π solid angle
Distance from target r=10 cm, irradiation time t=10 h=36000 s
This gives a total number of neutrons impinged on scintillator of 5×1011 n,
which means 5.7×109 n/cm2
1015 n/cm2 is roughly corresponding to a dose of 3 Mrad
Therefore 5.7×109 n/cm2 =2 rad