Proton irradiated graphite grades for a long baseline neutrino facility experiment

In search of a low-Z pion production target for the Long Baseline Neutrino Facility (LBNF) of the Deep Underground Neutrino Experiment (DUNE) four graphite grades were irradiated with protons in the energy range of 140–180 MeV, to peak fluence of $\sim 6.1\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$ and irradiation temperatures between $120–200°\mathrm{C}$. The test array included POCO ZXF-5Q, Toyo-Tanso IG 430, Carbone-Lorraine 2020 and SGL R7650 grades of graphite. Irradiation was performed at the Brookhaven Linear Isotope Producer. Postirradiation analyses were performed with the objective of (a) comparing their response under the postulated irradiation conditions to guide a graphite grade selection for use as a pion target and (b) understanding changes in physical and mechanical properties as well as microstructure that occurred as a result of the achieved fluence and in particular at this low-temperature regime where pion graphite targets are expected to operate. A further goal of the postirradiation evaluation was to establish a proton-neutron correlation damage on graphite that will allow for the use of a wealth of available neutron-based damage data in proton-based studies and applications. Macroscopic postirradiation analyses as well as energy dispersive x-ray diffraction of 200 KeV x rays at the NSLS synchrotron of Brookhaven National Laboratory were employed. The macroscopic analyses revealed differences in the physical and strength properties of the four grades with behavior however under proton irradiation that qualitatively agrees with that reported for graphite under neutrons for the same low temperature regime and in particular the increase of thermal expansion, strength and Young’s modulus. The proton fluence level of $\sim {10}^{20}{\mathrm{cm}}^{-2}$ where strength reaches a maximum before it begins to decrease at higher fluences has been identified and it agrees with neutron-induced changes. X-ray diffraction analyses of the proton irradiated graphite revealed for the first time the similarity in microstructural graphite behavior to that under neutron irradiation and the agreement between the fluence threshold of $\sim 5\times {10}^{20}{\mathrm{cm}}^{-2}$ where the graphite lattice undergoes a dramatic change. The confirmed similarity in behavior and agreement in threshold fluences for proton and neutron irradiation effects on graphite reported for the first time in this study will enable the safe utilization of the wealth of neutron irradiation data on graphite that extends to much higher fluences and different temperature regimes by the proton accelerator community searching for multi-MW graphite targets.

Figure 1

Microstructural characterization and comparison of the four graphite grades using scanning electron microscopy.

Figure 2

Geometries of irradiated graphite specimens. Tensile sample (left) and CTE sample (right).

Figure 3

Specimen layout within their respective argon-filled capsules of the LBNF graphite grades. (a) POCO, (b) IG 430, (c) SGL.

Figure 4

Configuration of the 181 MeV proton irradiation experiment of graphite grades.

Figure 5

(a) Imprint of proton beam on the stainless steel window of the vacuum capsule just upstream of the first graphite target capsule in the array, and (b) profile of activation measurements on the CTE-type samples contained in the target capsules conducted following irradiation.

Figure 6

SEM images of (a) SGL and (b) C-2020 graphite after annealing in vacuum at $200°\mathrm{C}$.

Figure 7

Thermal expansion coefficient of as-received unirradiated graphite grades in the temperature range of $40–280°\mathrm{C}$.

Figure 8

Dimensional changes in as-received, unirradiated graphite grades (a) and following irradiation to the fluence of $4.07\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$ at $170°\mathrm{C}$. Heating rate of $2°\mathrm{C}/\text{minute}$ was used.

Figure 9

(a) Thermal cycle annealing of irradiated POCO graphite over three postirradiation cycles to $310°\mathrm{C}$ to fluence of $6.1\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$ and irradiation temperature of $200°\mathrm{C}$ and (b) postirradiation annealing of IG 430 irradiated to two fluence levels and corresponding irradiation temperatures.

Figure 10

Irradiation-induced changes in the CTE. (a) LBNF graphite grades irradiated at $170°\mathrm{C}$ and (b) pile grade A graphite (after Marsden [11]).

Figure 11

Proton fluence effects on thermal expansion coefficient for (a) POCO, (b) IG 430, (c) SGL and (d) C-2020 graphite grades. Indicated are fluence followed by corresponding irradiation temperature.

Figure 12

Stress-strain behavior of proton irradiated (a) POCO, (b) IG 430, (c) SGL and (d) C-2020 graphite grades.

Figure 13

Mechanical behavior of (a) unirradiated and (b) irradiated CARBONE C-2020 graphite.

Figure 14

Measured fractional change in the Young’s modulus of three graphite grades, POCO, IG 430 and SGL, as a function of proton fluence.

Figure 15

Postirradiation annealing effects on strength and Young’s modulus of (a) POCO and (b) IG 430 graphite grades.

Figure 16

X-ray diffraction of as-received, unirradiated graphite grades POCO, IG 430 and SGL.

Figure 17

3D X-ray phase map of proton-irradiated (a) POCO, (b) IG 430 and (c) SGL graphite grades for proton fluence of $5.66\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$. (002) diffraction peak is truncated for clarity.

Figure 18

XRD map of irradiated POCO graphite around the (002) diffraction peak and the effect of $5.66\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$ fluence. (002) diffraction peak is truncated for clarity.

Figure 19

Dimensional changes in $c$ and $a$ axes of proton-irradiated graphite grades POCO, IG 430 and SGL to fluence up to $6.1\times {10}^{20}\mathrm{p}/{\mathrm{cm}}^2$. Asterisks indicate data for irradiation temperature $\sim 150°\mathrm{C}$.

Figure 20

Dimensional changes in $c$ and $a$ axes of pyrolytic graphite by energetic ($>100\mathrm{eV}$) neutrons (after Marsden [11]).

Figure 21

(a) Evolution of (002) diffraction peak of graphite with proton fluence produced by the current study and (b) evolution of the (002) peak under fast neutrons (after Bollmann [9]).