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

At a Glance

Metadata	Details
Publication Date	2025-04-25
Journal	arXiv (Cornell University)
Authors	H. Backe, J. Baruchel, Simon Bénichou, Rébecca Dowek, David Eon
Institutions	Institut Néel, Fraunhofer Institute for Applied Solid State Physics
Analysis	Full AI Review Included

Executive Summary

This research successfully demonstrates the observation of narrow-band gamma (γ) radiation generated by channeling ultra-relativistic electrons through a custom-fabricated, boron-doped diamond superlattice.

Core Achievement: A clear quasi-monochromatic γ-ray peak was experimentally observed at 1.30 MeV using an 855 MeV electron beam, validating the crystalline undulator concept in diamond.
Material Innovation: A 4-period diamond superlattice was grown via Chemical Vapour Deposition (CVD) with a sinusoidal boron doping profile, resulting in sinusoidally deformed (110) planes.
Undulator Performance: The superlattice exhibited a period length of 5.0 µm and a lattice constant amplitude variation of 0.138 nm.
Methodology Validation: Monte-Carlo simulations, assuming an idealized sinusoidal profile, accurately reproduced the measured 1.30 MeV peak energy, confirming the underlying physics.
Future Prospects: Simulations predict the generation of an intense 14.3 MeV γ-ray beam using a 3 GeV electron beam and an optimized 8-period undulator (triangular doping profile), suitable for photonuclear applications.
Application Focus: The technology offers a route to producing high-intensity, narrow-band multi-MeV photon beams for medical isotope production (e.g., ^99mTc via ¹⁰⁰Mo(γ,n)⁹⁹Mo) and basic nuclear research.

Technical Specifications

Parameter	Value	Unit	Context
Electron Beam Energy	855	MeV	Experimental setup (Mainz Microtron MAMI)
Observed Photon Energy (Peak)	1.30	MeV	Experimental result (NaI(Tl) detector)
Simulated Photon Energy (Peak)	1.29	MeV	Monte-Carlo simulation (sinusoidal profile)
Predicted Photon Energy (Prospects)	14.3	MeV	Simulated result (3 GeV beam, triangular profile)
Superlattice Period Length ([100] direction)	3.54	µm	CVD growth direction
Deformed Plane Period Length (λ)	5.0	µm	(110) plane deformation
Deformation Amplitude (A_u)	0.138	nm	Lattice constant variation
Number of Periods (N_u)	4	-	Fabricated superlattice
Undulator Parameter (K)	0.291	-	Calculated for 855 MeV beam
Minimum Boron Concentration (C_b,min)	1.0 x 10²⁰	cm^-3	Target doping level
Maximum Boron Concentration (C_b,max)	10.0 x 10²⁰	cm^-3	Target doping level
Diamond Substrate Thickness	80 to 270	µm	HPHT type IIa, 5x5 mm²
Substrate Miscut Angle	2	degrees	-
Growth Rate	0.58	nm/s	CVD process
Target Thickness (Experimental)	194	µm	Total thickness in [100] direction (including backing)
Predicted γ-ray Flux (14.3 MeV)	1 x 10¹³	photons/s	At 100 µA beam current

Key Methodologies

The crystalline undulator was fabricated using Microwave-Assisted Chemical Vapour Deposition (CVD) and characterized using advanced X-ray and mass spectrometry techniques.

A. Diamond Superlattice Fabrication (CVD)

Substrate Preparation: High-crystalline-quality HPHT type IIa diamond substrates (5x5 mm²) were used, featuring a (100) main surface orientation and a 2° miscut angle.
Cleaning: Samples underwent a tri-acid cleaning process (HClO₄, HNO₃, H₂SO₄) for 1 hour under boiling conditions to remove residual graphitic surface layers.
Reactor Setup: Samples were loaded into a microwave-assisted CVD reactor.
Plasma Stabilization: The plasma was ignited under a hydrogen atmosphere at 44 mbar pressure and 250 W power for 1 hour to ensure uniform temperature distribution (average temperature 850 °C).
Sinusoidal Doping: A tailored dilution stage with precise mass flow controllers was used to inject a variable B₂H₆ concentration into the gas mixture, achieving the desired sinusoidal boron doping pattern.
- Gas Flows: H₂ (100 sccm), CH₄ (4 sccm).
- Growth Conditions: Pressure maintained at 44 mbar, power at 250 W.
Layer Growth: Each doping cycle lasted 6015 seconds, resulting in a growth rate of 0.58 nm/s.

B. Crystal Characterization

Doping Profile: Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was used to confirm the sinusoidal boron concentration profile as a function of depth.
Lattice Deformation: Rocking Curve Imaging (RCI) technique, performed at the European Synchrotron Radiation Facility (ESRF), confirmed the periodic variation of the lattice constant corresponding to the doping profile.

C. Channeling Experiment (MAMI)

Electron Source: 855 MeV electron beam from the Mainz Microtron (MAMI).
Target Alignment: The diamond crystal was mounted on a goniometer and aligned using the signal from an ionization chamber registering scattered electrons (maximizing scattering probability parallel to crystal planes).
Radiation Detection: The forward γ radiation was detected by a large 10-inch sodium iodide (NaI(Tl)) scintillation detector positioned 8.5 m from the target.
Observation Direction: A movable cylindrical tungsten aperture (2 mm inner diameter) defined the observation angle. The peak search was performed by varying the aperture angle (θ_x) around the nominal electron beam direction (z-axis) and the undulator direction (z’-axis).

Commercial Applications

This technology provides a pathway for developing compact, high-intensity, quasi-monochromatic γ-ray sources, offering significant advantages over traditional bremsstrahlung or large-scale Compton scattering facilities.

Industry/Field	Specific Application	Technical Advantage
Medical Isotope Production	Production of critical isotopes like Technetium-99m (^99mTc) via photonuclear reactions (e.g., ¹⁰⁰Mo(γ,n)⁹⁹Mo).	High intensity and narrow energy bandwidth (e.g., 14.3 MeV peak) maximize reaction yield and minimize unwanted side reactions.
Nuclear Research	Inducing nuclear (γ,n) reactions and studying photonuclear processes (e.g., Giant Dipole Resonance).	Provides a highly directional, linearly polarized, and tunable multi-MeV photon beam.
Materials Science & Security	Non-destructive testing, elemental analysis, and cargo scanning requiring high-energy, penetrating radiation.	Compact source design compared to conventional accelerators; high flux concentrated in a small angular cone (0.5 mm spot size at 10 m).
Advanced Accelerator Technology	Development of compact, high-brightness light sources based on crystalline undulators.	Demonstrates the feasibility of using CVD diamond to create highly strained, periodic structures necessary for short-period undulators (µm scale).

View Original Abstract

A diamond superlattice with a period length of 3.54 $μ$m was grown on a high quality straight (100) diamond plate with the method of Chemical Vapour Deposition (CVD). A sinusoidal varying boron doping profile resulted in a periodic variation of the lattice constant, and in turn four sinusoidally deformed (110) planes with a period length of 5.0 $μ$m and an amplitude of 0.138 nm. A channeling experiment was performed with the 855 MeV electron beam of the Mainz Microtron MAMI accelerator facility. Part of the impinging electrons perform sinusoidal oscillations resulting in the emission of quasi-monochromatic $γ$ radiation. A clear peak was observed with a large sodium iodide scintillation detector close to the expected photon energy of 1.33 MeV. Gross properties like photon energy, width and intensity of the peak can be reproduced fairly well by idealized Monte-Carlo simulation calculations. Based on the latter, prospects of applying such $γ$ radiation sources are addressed with the example of the photonuclear reaction $^{100}$Mo($γ$,n)$^{99}$Mo at 14.3 MeV to produce the medical important $^{99m}$Tc isotope.

Tech Support

Original Source

DOI: None