معدل القشط العام لكاشف الأثر CR-39 المعرض لأشعة كاما باستخدام طريقة D-Le

الملخص EN

The use of solid state track detectors (SSNTDs) has already become well-known, having been extensively used in nuclear physics, space physics, biophysics, geology, nuclear engineering, monitoring radon concentrations of radon gas by recording its emitted alpha particles, as well as in many other fields.

It is known that some plastic types of solid state nuclear detectors such as polycarbonate CR-39 and PM-355, cellulose nitrate CN-85 and LR-355 or cellulose acetate CA-80 are highly sensitive to ionizing radiations and charged particles.

Operation of the solid-state nuclear track detector is based on the fact that a heavy charged particle which is heavier than an electron will cause extensive ionization of the material when it passes through a medium.

Along the path of the alpha particle, a zone enriched with free chemical radicals and other chemical species is then created.

This damage zone is called a latent track.

Now, when a piece of material containing latent tracks is exposed to some chemically aggressive solution, chemical reaction would be more intensive along the latent track.

The overall effect is that the chemical solution etches the surface of the detector material, but with a fast rate in the damaged region.

In this way, a track of the particle is formed, which may be seen under an optical microscope or by other counting techniques.

The secondary electrons or -rays should also contribute to producing damage in plastic detectors.

Though low LET radiation such as - rays or electrons cannot form any etchable tracks, it is observed in plastic detectors that some changes in macroscopic properties of the detector, such as sensitivity, etching rates, brittleness or change in color are all induced when the irradiation dose becomes higher than about Mrad.

An increase of bulk etching rate has been reported as the most striking change among macroscopic properties.

Recently, researchers keep on investigating irradiation effects of low LET radiation, such as - rays, UV-light or electrons, by using and developing various methods and different approaches for measuring parameters of solid state nuclear track detectors, such as the track etch rate (VT), the bulk etch rate (VB), the sensitivity (V), the etched track length (Le) and how these parameters are varied with the detector depth analog with the variations in the track growing stages.

Accordingly, this study deals with investigating irradiation effects of low LET radiation of -rays on the bulk etch rate of (VB) of the plastic polycarbonate solid state nuclear track detector CR-39 using the track diameter-length (D-Le) measurement approach.

A group of five pieces of plastic detector CR–39 of 250 m thickness and dimensions (1.5x1.5) cm2 were used.

Four of them were exposed to -rays from 60CO (368.21Ci) with a dose rate of 0.306 kGy/h and doses of (2.754, 4.590, 6.426, 8.262) kGy corresponding to irradiation times of (9, 15, 21, 27) h respectively, while the last detector piece was left without -ray irradiation.

Both -ray irradiated and un-irradiated detectors were then directly irradiated with alpha particles of 3.5 MeV energy from 214Am (1Ci) source.

In order to reveal alpha particle tracks, the detectors were etched by the aqueous solution of KOH of 6.5 N molarity at a temperature of (70±1) ºC for (1.5-4) h with a recurrent increase each 0.5 h.

The profiles of the tracks and their diameters and lengths were measured using the digital camera MDCE-5A fixed on an optical microscope and connected to a computer.

Using the track diameterlength (D-Le) measurement approach, the bulk etch rate (VB) was measured as a function of -rays absorbed dose.

It was found that the bulk etch rate (VB) increased exponentially with the dose (D), and its value was between (1.703-3.838) m/hr corresponding to the examined -ray doses.

The -irradiation characteristic parameter (g) was also calculated and found to be equal to 94.5 MGy-1.

However, the D-Le measurement method of the bulk etch rate of the detector CR-39 exposed to -doses showed good agreement with other measurement methods and approaches used by other researchers.