Enhancement of the Inelastic Nuclear Interaction Rate in Crystals via Antichanneling

The interaction rate of a charged particle beam with the atomic nuclei of a target varies significantly if the target has a crystalline structure. In particular, under specific orientations of the target with respect to the incident beam, the probability of inelastic interaction with nuclei can be enhanced with respect to the unaligned case. This effect, which can be named antichanneling, can be advantageously used in the cases where the interaction between beam and target has to be maximized. Here we propose to use antichanneling to increase the radioisotope production yield via cyclotron. A dedicated set of experimental measurements was carried out at the INFN Legnaro Laboratories with the AN2000 and CN accelerators to prove the existence of the antichanneling effect. The variation of the interaction yield at hundreds of keV to MeV energies was observed by means of sapphire and indium phosphide crystals, achieving an enhancement of the interaction rate up to 73% and 25%, respectively. Such a result may pave the way to the development of a novel type of nozzle for the existing cyclotrons, which can exploit crystalline materials as targets for radioisotope production, especially to enhance the production rate for expensive prime materials with minor upgrades of the current instrumentation.

Figure 1

Geant4 simulation of the distribution of the particle trajectories for a 655 keV proton beam impinging on a (110) Si crystal under channeling condition (a) and antichanneling conditions (b). (c) The average nuclei encounter probability as a function of the penetration depth for the two cases.

Figure 2

Geant4 simulations of the distribution of the protons interacting with an ${\mathrm{Al}}_2{\mathrm{O}}_3$ crystal under channeling (a) and antichanneling conditions (b). The potential of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ crystal oriented along the $⟨0001⟩$ axis is shown.

Figure 3

Schematic representation of the experimental setup.

Figure 4

Experimental and simulated interaction rates for ${\mathrm{Al}}_2{\mathrm{O}}_3$ (a) and InP (b) crystals as a function of the particle incoming angle. (a) The ${}^{18}\mathrm{O}(p,\alpha ){}^{15}\mathrm{N}$ reaction and RBS spectra for the $⟨0001⟩$ axis to the $[1\overline{2}10]$ plane in an ${\mathrm{Al}}_2{\mathrm{O}}_3$ crystal with protons at 642.5 keV. (b) The ${}^{31}\mathrm{P}(\alpha ,p){}^{34}\mathrm{S}$ reaction for the $⟨001⟩$ axis to the [010] plane in an InP crystal with $\alpha$ at 5.057 MeV.

Figure 5

Geant4 simulations of interaction rates for Y and Ni crystals as a function of the particle incoming angle. The values are normalized to the average density ratio in the crystal. The transition from the $[1\overline{1}0]$ plane to random for 13 MeV protons impinging on a Ni crystal and the transition from the $[1\overline{1}00]$ plane to random for 17 MeV protons impinging on a Y crystal are shown.