Proton beam dosimetry by CR-39 track-etched detector

Background: High and intermediate energy protons are not able to form a track in a solid state nuclear track detector (SSNTD) directly. However, such tracks can be formed through secondary particles created during primary radiation nuclear reactions in a SSNTD. Materials and Methods: The protons with primary energies of 9.6 and 30 MeV available at the cyclotron accelerator with corresponding low LETs of 5.87 and 2.40 keV/µm were taken into consideration. The nuclear tracks etch rate ratio V in CR-39 were measured and transformed into LET spectra for the absorbed and equivalent dose measurements. Results: The optimum etching condition of 6 N NaOH solutions at 65 to 70 oC over a 6-hour period for the CR-39 were found initially. The corresponding bulk etching rate reached a steady rate of about 0.62 to 1.3 um/h after nine hours for an optimum etching condition. Although the LET was low, but the energy range seemed sufficient enough to create secondary particles with much higher LET through the nuclear reactions in CR-39. The relative absorbed dose contribution of the created secondary particles to the primary particles for the 9.6 and 30 MeV protons in CR-39 at 1 Gy entrance dose were 7.5 and 29.6%, respectively. Conclusion: The contribution of the secondary particle increased relatively with the proton energy decrease. This phenomenon could modify the characteristics of the energy transfer process due to secondary particles when such particles are used for radiobiological studies and/or for radiotherapy. Iran. J. Radiat. Res., 2008; 6 (3): 113­120