Observation of narrow-band $γ$ radiation from a boron-doped diamond superlattice with an 855 MeV electron beam

TL;DR

This work demonstrates the first observation of narrow-band gamma radiation from a crystalline micro-undulator made of a boron-doped diamond superlattice, using a four-period sinusoidally bent (110) structure to generate undulator-like radiation for an 855 MeV electron beam. Through Monte-Carlo optimization and careful fabrication (period $\lambda_U \approx 5.0\,\mu$m, amplitude $A_U \approx 0.098\,\text{nm}$), the team predicts and measures a peak near $1.28$–$1.30$ MeV, with the observed energy in good agreement with simulations but a broader line width and reduced intensity due to backing-layer effects and stress relaxation. The results validate the undulator concept and illustrate the trade-offs between substrate thickness, channeling background, and undulator strength, while outlining practical pathways to higher-energy, highly directional $\gamma$ beams such as a 14.5 MeV peak for a 3 GeV electron beam with predicted flux on the order of $10^{12}$ s$^{-1}$ on target. The findings suggest that, at facilities like MAMI and with higher-energy accelerators, compact, tunable, and directionally intense gamma sources could enable applications in nuclear research, medicine, and industry, albeit with careful management of the high-energy tail and substrate-related effects.

Abstract

We report the first observation of narrow band 1.3 MeV $γ$ radiation from a crystalline micro-undulator. A diamond superlattice was grown with a periodical varying boron doping profile. Four sinusoidally deformed (110) planes resulted with a period length of 5.0 $μ$m and an amplitude of 0.098 nm. A channeling experiment was performed with the 855 MeV electron beam of the Mainz Microtron MAMI A accelerator facility. A clear peak was detected with a large sodium iodide scintillation detector close to the expected photon energy of 1.28 MeV. Key characteristics of the peak, including photon energy, width and intensity, were reproduced fairly well by Monte-Carlo simulation calculations. Based on the latter, optimized boron doping profiles were designed for the 1.6 GeV beam of MAMI C and a hypothetical 3 GeV beam, enabling preparation of highly directional $γ$-ray beams with photon energies of 4.2 and 14.5 MeV. The predicted spectral bandwidths are, respectively, about 18\% and 13\%, however, with a high energy continuum tail. The on-target photon flux at a beam current of 100 $μ$A would be about $10^{12}$/s for the 14.5 MeV photon beam.

Figures (7)

• Figure 1: Transverse potential $U(x,z)$ for a single (110) channel of a boron doped diamond superlattice. Shown is one period with length of $\lambda$ = 5.0 $\mu$m. Coordinates $(x,z)$ of a full period (0,0) and (-7.50 Å, 5 $\mu$m) define the $z'$ undulator direction around the potential wiggles. It makes an angle $\delta$ = -0.150 mrad with $z$ axis, the nominal electron beam direction. Notice, the longitudinal $z$ coordinate is squeezed by four orders of magnitudes. For details see appendix \ref{['simulation']} and also KorS26. Barrier heights are modified as functions of $z$ by centrifugal forces. Insets show examples, $x_1$ = -0.898 Å and $x_2$ = -6.604 Å. Figure based on design parameters of section \ref{['diamondUndulator']}.
• Figure 2: (a) Measured boron doping profile. A 250 x 250 µm² crater allowed for the analysis of the material as a function of depth with ToF-SIMS. On the left scale the boron concentration is shown, on the right one the corresponding relative lattice expansion $\Delta$a/a, according to the "atomic volume interpolation" WojA08, also called "modified Vegard’s law". (b) Depth profile obtained with the Rocking Curve Imaging (RCI) technique BraE06CalK23. The signal is the integrated intensity of Bragg diffraction peaks at (111) planes with 17 keV photons.
• Figure 3: Experimental setup at MAMI, not to scale. Radiation can be observed at virtual collinear geometry of the electron beam direction and the sodium iodide NaI(Tl) radiation detector. The inset depicts the undulator crystal with the position of three ToF-SIMS craters.
• Figure 4: (a) Experimental spectra of the undulator chip per electron, including the 182 $\mu$m thick backing crystal. Spectra taken with the NaI(Tl) detector at a typical beam current of 3.95 pA. Spectrum in green colour for detuning the aperture from $z$ direction by $\theta_x$ = - 0.2 mrad, and in blue colour by + 0.2 mrad. Both are dominated by channeling radiation of the backing crystal. (b) Low energy part of difference spectrum. Background spectrum taken at $\theta_x$ = +0.2 mrad aperture position (blue) subtracted from spectrum taken at -0.2 mrad (green). (c) and (d) Monte-Carlo simulated spectra per electron without a backing crystal. Ideal sinusoidal boron doping profile and infinitesimal small aperture size assumed, beam divergence $\sigma_x = \sigma_y$ =0.015 mrad. Mean of 200 single simulations.
• Figure 5: Simulated photon spectrum per electron for a sinusoidal boron doping profile at a beam energy of 3.0 GeV. Period length $\lambda_U$ = 5 $\mu$m, minimum barrier hight $U_B$ = 9.0 eV, amplitude $A_U$ = 0.656 Å, undulator parameter $K$ = 0.48, relaxation length $L_A = \infty$, and number of periods $N_U$ = 12. An infinitesimal small aperture was assumed positioned at $\theta_x = \delta$ = -0.0824 mrad and $\theta_y = 0$. Beam divergence $\sigma_x = \sigma_y$ = 0.06 mrad. Mean of 200 single simulations.