Physical Thickness Characterization of the FRIB Production Targets

TL;DR

The paper addresses the need to quantify areal thickness variations in FRIB production graphite targets to maintain high rare-isotope beam production while managing thermal loads. It presents a custom non-contact optical thickness metrology system that continuously maps disc thickness around the outer rim during rotation, achieving fine angular and radial resolution. The authors characterize six nominal thickness targets (0.4–5 mm) from two suppliers, quantify both absolute and relative thickness variations, and identify a radial gradient in at least one disc, highlighting machining-related effects and a practical 2% thickness-variation tolerance. These results establish a fabrication baseline and inform design choices for single- and multi-slice graphite targets at FRIB, with implications for production reliability and thermal performance.

Abstract

The FRIB heavy-ion accelerator, commissioned in 2022, is a leading facility for producing rare isotope beams (RIBs) and exploring nuclei beyond the limits of stability. These RIBs are produced via reactions between stable primary beams and a graphite target. Approximately 20-40 \% of the primary beam power is deposited in the target, requiring efficient thermal dissipation. Currently, FRIB operates with a primary beam power of up to 20 kW. To enhance thermal dissipation efficiency, a single-slice rotating graphite target with a diameter of approximately 30 cm is employed. The effective target region is a 1 cm-wide outer rim of the graphite disc. To achieve high RIB production rates, the areal thickness variation must be constrained within 2 \%. This paper presents physical thickness characterizations of FRIB production targets with various nominal thicknesses, measured using a custom-built non-contact thickness measurement apparatus.

Figures (5)

• Figure 1: a) Photo of the test apparatus used for thickness measurement. The setup consists of a rotational unit, disc module assembly and displacement sensors. b) Detailed view showing the thickness measurement during target rotation. A green laser emitted from two opposing displacement sensor with a spot diameter of 0.5 mm. The measured thickness is averaged over the illuminated area.
• Figure 2: Representation of 3.5mm graphite disc. a) Two-dimensional contour plot of the measured thickness variation across the disc's outer rim (9 mm width) projected onto the graphite target diagram. b) Magnified view showing detailed the measurement traces in the selected region of the contour plot. c) Histogram illustrating the distribution of physical thickness measurements, where the x-axis represents thickness and the y-axis indicates the count corresponding to each thickness value measured across the disc.
• Figure 3: Measured thickness distributions of graphite discs with nominal values of a) 0.4 mm, b-c) 0.6 mm, d) 2.1 mm, e) 1.2 mm, f) 3.5 mm and g) 5.0 mm. Each panel displays individual measurements across the disc surface, with the x-axis representing the physical thickness and the y-axis indicating the count of each thickness value. The red vertical line denotes the nominal thickness for each disc. Discs a) and b) were manufactured by Supplier A. All Others were manufactured by Supplier B.
• Figure 4: Graphite discs manufactured by supplier A and B are represented by red and blue circles, respectively. a) Absolute and b) relative deviations of the average measured thickness from the nominal value, expressed in mm and %. c) Standard deviation of the measured thickness for each disc, expressed in mm and d) in %.
• Figure 5: Thickness profile analysis of a 1.2 mm graphite disc indicating radial variations. a) Two-dimensional contour map of the measured thickness across the disc's outer rim (9 mm width) , projected onto the graphite target schematic. b) Magnified view showing detailed measurement traces on the contour plot. c) The measured thickness distributions across the disc. r0 to r8 represent discrete radial positions from the outer edge toward the center (1 mm step).