Study on ion irradiation response of Gafchromic films for the intensity distribution measurement of a large-area beam

It is required to precisely evaluate the large-area transverse intensity distribution of various ion beams applied to materials and biological research at an azimuthally-varying-field cyclotron in Japan Atomic Energy Agency. The response of radiochromic films, Gafchromic HD-810 and EBT2, was, therefore, investigated using proton and several heavy-ion beams for developing a measurement technique of the transverse beam intensity distribution. The optical density of the films increased linearly with the particle fluence of the beam in a low-fluence region. The linear-response range of the fluence was strongly dependent on ion species. By analyzing the film response to absorbed dose, it was found that the sensitivity of the film decreased for heavy-ion beams. The practical fluence range of the Gafchromic film HD-810 for an arbitrary ion beam was determined from the measurement result. The present technique with the use of the two types of Gafchromic films enables us to measure the intensity distribution of a large-area ion beam easily in a wide fluence range. Using the technique, we measured the relative intensity distribution of a large-area ion beam focused by multipole magnets and evaluated its size and uniformity.