Dynamic Radiation Effects Induced by Short-Pulsed GeV U-Ion Beams in Graphite and h-BN Targets

Philipp Bolz,Alexey Prosvetov,P Simon ,P Drechsel ,C Trautmann ,M Tomut

doi:10.1155/2021/8825142

Abstract

Targets of isotropic graphite and hexagonal boron nitride were exposed to short pulses of uranium ions with ∼1 GeV kinetic energy. The deposited power density of ∼3 MW/cm³ generates thermal stress in the samples leading to pressure waves. The velocity of the respective motion of the target surface was measured by laser Doppler vibrometry. The bending modes are identified as the dominant components in the velocity signal recorded as a function of time. With accumulated radiation damage, the bending mode frequency shifts towards higher values. Based on this shift, Young’s modulus of irradiated isotropic graphite is determined by comparison with ANSYS simulations. The increase of Young’s modulus up to 3 times the pristine value for the highest accumulated fluence of 3 × 1013 ions/cm2 is attributed to the beam-induced microstructural evolution into a disordered structure similar to glassy carbon. Young’s modulus values deduced from microindentation measurements are similar, confirming the validity of the method. Beam-induced stress waves remain in the elastic regime, and no large-scale damage can be observed in graphite. Hexagonal boron nitride shows lower radiation resistance. Circular cracks are generated already at low fluences, risking material failure when applied in high-dose environment.

Highlights

With a new generation of particle accelerators like the Facility for Antiproton and Ion Research (FAIR) or the High Luminosity LHC at CERN, unprecedented high beam intensities will be reached
Experiments measuring pulsed-beam-induced effects were previously conducted at the HiRadMat beamline at CERN [3,4,5,6,7], at Brookhaven National Laboratory [8], and at SIS18 at GSI [9]. is work presents results from irradiations with ∼1 GeV uranium ions at the linear accelerator UNILAC at GSI Helmholtz Center for Heavy Ion Research in Darmstadt (Germany). e ion mass is higher, and the energy is lower than beams used in earlier studies [3,4,5,6,7,8,9]. e energy close to the Bragg peak provides maximum energy loss and leads to fast dose accumulation [10,11,12]. e lower energy has the advantage of rather limited sample activation
Discs of isotropic graphite and hexagonal boron nitride were exposed to U-ion pulses with GeV kinetic energy. e dynamic response of the targets was monitored by recording the surface velocity signal at the rear side using laser Doppler vibrometry

Summary

Introduction

With a new generation of particle accelerators like the Facility for Antiproton and Ion Research (FAIR) or the High Luminosity LHC at CERN, unprecedented high beam intensities will be reached. In a previous proton irradiation experiment, X-ray diffraction analysis showed a smaller shift and less broadening of the (002) reflection indicating a higher radiation hardness of h-BN compared to graphite [19] In the past, both materials have been considered as candidates for beam dumps and production targets [1, 20] or as luminescence screens in accelerators. Beam-induced heating of the samples was controlled with a thermal camera and kept below 200°C. is secures that the properties of the target materials remain close to the values at room temperature given in the data sheets. To simulate the effects of beam-induced material degradation, simulations were conducted for isotropic graphite with increased Young’s moduli and reduced thermal conductivities [26] in the irradiated volume. Density ρ (g/cm ) Young’s modulus E (GPa) Poisson’s ratio ] Coefficient of thermal expansion α (10−6/K)

Results and Discussion

30 Electronic energy loss

Glassy carbon

Conclusions