Radiation damage from energetic particles at GRad-level of SiO[formula omitted] fibers of the Large Hadron Collider ATLAS Zero-Degree Calorimeter (ZDC)

N Simos,G Atoian,A Bolotnikov,D Sprouster,A Tricoli,D Medvedev,M Palmer,D Asner,N Charitonidis,N Mokhov,Z Kotsina

doi:10.1016/j.nima.2020.164444

Abstract

Core SiO2 quartz fibers of the Large Hadron Collider (LHC) ATLAS Zero-degree Calorimeter (ZDC) are expected to experience integrated doses of a few giga-Rad (Grad) at their closest position to the LHC beam. An array of fibers was irradiated with 200 MeV protons and spallation-generated mixed spectra (primarily fast neutrons) at the Brookhaven National Laboratory (BNL) Linac. Specifically, 1 mm- and 2 mm-diameter quartz (GE 124) rods of 50 mm length were exposed to direct 200 MeV protons leading to peak integrated dose of ∼28 Grad (∼0.28 GGy). Exposure of 1 mm-diameter SiO2 fibers to a neutron flux was also achieved in the spallation field generated by 128 MeV protons. In the post-irradiation analysis, the quartz fiber transmittance was evaluated as a function of the absorbed dose. Significant degradation of the transmittance and increased radiation damage of the material were observed. Microscopic evaluation of the fibers revealed extensive micro-structural damage and irradiation-induced defects. The measurements revealed that a threshold fluence (∼2.6 1016 p/cm2) or dose of ∼10 Grad (0.1 GGy) appears to exist beyond which light transmittance drops below 10%. Also observed is that fiber transmittance loss increased drastically with SiO2 fiber diameter (1 mm vs. 2 mm diameter). This is attributed, in part, to the earlier lateral leakage from the 1 mm fiber of knock-on electrons and primary protons implying that more damage-inducing protons travel within the bulk of the 50 mm long 2-mm fibers. While Monte Carlo simulations performed tend to support such assumption, future experiments and sensitivity studies are envisioned to address the fiber diameter influence on degradation.

Highlights

The power dissipated in the ATLAS Zero-degree Calorimeter (ZDC) calorimeter (Fig. 1) during the Large Hadron Collider (LHC) p–p runs is expected to reach ∼6 W/kg or ∼6 kRad/s (∼0.06 kGy/s) [1]
In this work we report the results of transmittance measurements of 1 mm and 2 mm diameter SiO2 core fibers – fiber refers in this study to the pure quartz core with no cladding or coating – after the irradiation with 200 MeV protons and 1 mm diameter fibers after irradiation with spallation, secondary particles dominated by fast neutrons
It is only the authors’ assessment, and until further experimental exploration backed by sensitivity analyses over several parameters including fluence uniformity over the two fiber cross sections, incidence angle, fiber cross-communication, etc., that the observed higher deterioration of transmittance in the 2 mm diameter fibers at least partly stems from the higher leakage of beam protons leaving the 1 mm fiber

Summary

Introduction

The power dissipated in the ATLAS ZDC calorimeter (Fig. 1) during the LHC p–p runs is expected to reach ∼6 W/kg or ∼6 kRad/s (∼0.06 kGy/s) [1]. In this work we report the results of transmittance measurements of 1 mm and 2 mm diameter SiO2 core fibers – fiber refers in this study to the pure quartz core with no cladding or coating – after the irradiation with 200 MeV protons and 1 mm diameter fibers after irradiation with spallation, secondary particles dominated by fast neutrons. In both cases the specimens accumulated the dose of 0.48 GGy. Further, close examination of irradiated fibers was conducted under optical microscope to assess the structural changes from the interaction with the high-energy particles.

SiO2 quartz fiber irradiation

Photon spectra

Size dependent performance in 1 mm and 2 mm diameter fibers

Microscopic assessment of damage

Findings

Summary