Application of nuclear emulsions for the identification of multi-MeV protons in laser ion acceleration experiments

Nuclear emulsions are applied to identify multi-MeV protons accelerated by an intense PW-class laser. Although nuclear emulsions, which record the three-dimensional trajectories of charged particles, feature high sensitivity and a sub-micron spatial resolution, they have never been used for the detection of laser-accelerated ions. This is because intense laser-matter interactions generate not only ions but also copious electrons, including secondary electrons induced by X-rays, which are recorded as background noise in a nuclear emulsion. Therefore, it is not a trivial task to identify ion tracks in a nuclear emulsion exposed to such extreme mixed radiation fields. Here, for the first time, we have succeeded in identifying the signals from laser-accelerated protons in nuclear emulsions by suppressing the background noise signals through the use of desensitized development conditions, i.e., a developer solution with a pH of 7.3 and a development time of 25 min. The energy spectrum obtained from a stack of nuclear emulsion films shows good agreement with that obtained from a stack of films of the conventional solid-state nuclear track detector material CR-39, which is sensitive only to ions. An additional benchmark experiment with 100-MeV protons from a conventional accelerator apparatus confirms that nuclear emulsions have the potential to measure 100-MeV-class laser-accelerated protons.