Although inorganic scintillators are efficient light producers, they are however somewhat
slower (typical response time: microseconds) than organic scintillators and are generally g
harder to handle. Sodium Iodide, the most commonly employed inorganic crystal for
gamma spectroscopy is hydroscopic, hence cannot be used conveniently. The organic
scintillators are quite fast and are relatively easy to handle. The crystalline organics, like > anathracene and stilbene are brittle and have anisotropic response to radiation which
complicates measurements. Liquid scintillators are inconvenient due to the need for a container and geometry. The Scintillation fibres are made either from organic or inorganic scintillators and do not offer different properties (White, 1988).
Among the scintillators, the organic plastic phosphors have the best potential, They are easy to handle, cheap, are in condensed phase and there are no radiation damage problems. They have densies very close to those of tissues; have low atomic weight constituents. They are widely used in the spectroscopy of heavy charged particles in nuclear physics. The main feature being their fast response (ns) and the relative ease of fabricating (in the laboratory) films of a few mg/cm^ , which is less than the range of weakly ionising particles. They are used as transmission detectors (TED) which respond only to the fraction of the energy lost by the particles as it passes through the film (Goulding and
Harvey, 1975). Wide experience has been acquired from such investigations.
Aspects of the thin film detector (TFD) that have been investigated are its potential as a device for identifying particles and its time response for possible use in time-of-flight experiments. These investigations (Goulding and Harvey, 1975 and Brooks, 1979) confirm the well known nonlinearity of the light output with energy loss suggesting the inappropriateness of the TFD as a transmission detector in the dE/dX and dE technique of identifying low energy nuclear species. Since the plastics produce sufficient light output, at least to the passage of densely ionising radiation, and they can be easily made in ultra thin layers, it may helpful to explore ways (and limits) in which the plastic scintillators could be improved for application to unified dosimetry.
4.2. REVIEW OF EXPERIMENTAL DATA.
Birks (1964) has made an extensive review of the use of scintillators for radiation detection. The more recent developments have been reviewed by Brooks et al (1979) for organic scintillators, by Heath et al (1979) for NaI(Tl) detectors and by Harvey and Hill (1979) for neutron detection. The empirical data summarised here is restricted to that from laboratories which have published sufficiently extensive data. By so doing, problems such as differences in the scintillator concentration, light collection etc can be minimised.
4.2.1. RESPONSE OF THICX ORGANIC SCINTILLATORS.
Co too O ' E 40 - 0.001 0.01 0.10 1.0 10.0 100.0 dE/dx (MeV/mq/cm^)
FIGURE 4.1: Specific fluorescence versus calculated specific energy loss in NE 102. Adopted from Becchetti et al (1976),
The response of plastic scintillators to a variety of ions (Z=l-35) fully stopped in the phosphors was reported by Becchetti et al (1976). They exposed plastic scintillators to the ions of E < 170MeV at near normal-incidence, they observed the following.
(a) The intrinsic light output of three scintillators (NE102, NEl 11, NE 110) is basically the
same for given conditions of PM, light collection etc. »
(b) At the same energy, lighter ions are more efficient light producers than heavy ion, |
figures 4.1 and 4.2. The nonlinearity of the light output with energy is similar for ions with K energy less than 15MeV/amu, and is given by L a E", where n is 1.6 independent of Z of
the ions.
(c) The response is generally nonlinear with energy, but becomes linear for E /A »
6MeV/amu.
(d) The differential efficiency of fluorescence (dL/dX) is initially linearly related to dE/dX, but with increase in stopping power it saturates. (In the case of Nal(Tl), this occurs at about 7-8MeV per nucleon for the different ions (Newman and Steigert, 1960) there after it decreases, see figure 4.4). For ions of the same dE/dx, dL/dX increases with Z but is relatively independent of mass.
(e) The light output is slightly less (about 5%) for odd Z ions compared to that for adjacent even-Z ions.
(f) dL/dE is generally not constant except at high energies ( > 40MeV); its pattern of variation with energy is similar for the different ions, but its magnitude depends on Z and it decreases with increasing Z. This implies that the number of photoelectrons detected is proportional to the light output or, conversely, the photo-conversion efficiency depends on the ion energy and not the dE/dx and suggests a complicated mechanism of photon production in the scintillator.
(g) The pulse height resolution is given by 0.8L"1/^.
(h) The light output is best described as a function of the ion range, R, in the scintillator,
with a strong dependence on Z but little dependence on isotopic mass; thus L a “ (R - 0.04Z3
The curve L against R shows curvature and non-zero intercept which indicates, according to Becchetti et al (1976) an effective light producing portion of the range Rl which is less than the total range. The dead layer may be due to surface oxidation of the films. Its
thickness increases with the increase in the ions charge, but is independent of its energy.
i
The response of organic scintillators to electrons is quite linear with particle energy above lOOkeV. There is however a strong dependence on the mode of irradiation: external or internal. This is due to the back scattering of electrons, surface effects and surface escape of photons (Brannen and Olde, 1962). Smith et al (1968) compared the response of different scintillators (anthracene, stilbene, NE 123, NE230 and Pilot B) to external electrons of energies in the i^ g e (0.16-0.99MeV) with that to protons (0.24-15.0MeV). They reported that the response of the different scintillators to ionising particles of 2>1 is nonlinear, but the response to electrons is linear. They also observed that, at the same energy, the light output of the electrons is greater than those of protons. The investigators also noted that although there were large differences (a factor of three) in light output of the different scintillators, a similar trend in the energy dependence of the ratio of the light output of protons (or deutrons) to the response of electrons is observed. The ratio is constant at the same energy, but varied with energy. The ratio is about 0.1 for particles with energy 0.20-1.0MeV, increased to between 1.0 and 4.0MEV but levelled off to 0.5 in the energy range 10-15MeV.
5 r~rr-n tt î ; i r-rTTrrr]---r~i • i 11114
I— L _ ij.iij.a I— L—1.1 1 ! 1|J IÜ.0
E(MeV) 100.0
J — L.J I 11 ill
1000.0
HGURE 4.2: Scintillation efficiency versus energy. The Kght output are in a scale that the