Proton-irradiated Graphite Research Articles

Development of an intense 10Be radioactive beam in ‘off-line mode’ at the Catania Tandem accelerator

A 10Be beam having intensities up to 3 enA has been accelerated at the Tandem of the Laboratori Nazionali del Sud in Catania. The used 10Be was chemically extracted from a proton irradiated graphite target used for muon production at the Paul Scherrer Institute. In order to maximise the extracted current using a minimum amount of isotope, several preparatory tests, needed to optimise the preparation of the Tandem sputter source cathodes, were performed. The acceleration tests and accelerated beam characteristics will be discussed and examples of beam use for nuclear structure investigations will be presented.

Raman spectra and modulus measurement on the cross section of proton-irradiated graphite

Abstract A method combining proton irradiation and nano-indentation was explored in this study to measure the modulus of irradiated graphite. A IG-110 graphite sample was irradiated with 2.5-MeV protons to a fluence of 1.2 × 10 18 ions/cm 2 at room temperature. Proton irradiation produced a damaged layer with a thickness of ∼75 μm. A scan of Raman spectrum was performed on cross section of irradiated sample. Analysis of the Raman spectra indicates that the graphite breaks into nano-crystalline graphite at very low dose (0.02–0.04 dpa) during room-temperature irradiation. Nano-indentation tests were performed on the cross section of irradiated sample so that the damage along the loading direction is uniform. Thus, it was able to identify a dpa value for the nano-indentation tests. Using this method, the modulus at 0.015 dpa was deduced to be 2.39 times that of the un-irradiated graphite. The hardness is also more than doubled after irradiation to 0.015 dpa.

Separation of weighable amounts of 10Be from proton irradiated graphite

Large amounts of 10Be are produced at the PSI muon production facility by high-energy proton spallation in polycrystalline graphite. For the extraction of 10Be out of large amounts of carbon, pyrolysis followed by chemical purification has been performed. Approx. 270g of graphite from Target E92, which had received a total proton charge of 29 Ah between 2002 and 2005, have been burned at 1000°C in a stream of oxygen. The volatile radioactive oxidation product 3H2O was trapped in 3 water bubblers connected in series. The remainder, a white hygroscopic solid material mainly consisting of 7Li2O, 9/10BeO and 10/11B2O3, was dissolved in HF and subsequently purified by ion exchange chromatography. Radioactive impurities such as 22Na, 44Ti, 54Mn, 60Co, 101Rh, 133Ba and 172Hf have been separated from the final product. The purified material represents a mixture of approx. 6.5mg 9Be and 3.5mg (3.3MBq) 10Be. It is ready to be used for scientific investigations requiring large amounts of this precious isotope.

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 $\ensuremath{\sim}6.1\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{p}/{\mathrm{cm}}^{2}$ and irradiation temperatures between $120--200\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. 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 $\ensuremath{\sim}{10}^{20}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ 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 $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ 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.

Shubnikov-de Haas Oscillations in Proton-irradiated Graphite

Our study of the Shubnikov-de Haas (SdH) effect in highly-oriented pyrolytic graphite (HOPG) systems shows that low-density electrons may occupy a 3D Fermi surface and high-density holes a 2D surface in proton-irradiated HOPG while both carriers occupy a 2D Fermi surface in virgin HOPG. No massless Dirac fermions were evidenced by the SdH oscillations both in virgin HOPG and in proton-irradiated HOPG. Following the examination of the recent controversy on massless Dirac fermions in HOPG, we find that even a small amount of acceptor-like defects may play an important role in the interpretation of SdH oscillations.

Electron spin resonance study of proton-irradiation-induced defects in graphite

Electron spin resonance measurements of proton-irradiated graphite have revealed detailed nature of proton-irradiation-induced defects. Our results indicate that proton-irradiation creates confined defect regions of a metallic island surrounded by an insulating magnetic region which “isolates” the metallic island inside from the metallic graphite background outside. We have thus come up with a picture of phase separation in proton-irradiated graphite comprising three regions of distinct electrical and magnetic properties.

Magnetic order in proton irradiated graphite: Curie temperatures and magnetoresistance effect

Defect induced magnetic order is a new phenomenon in material science that refers to the triggering and manipulation of magnetic order and magnetic moments in nominally non-magnetic materials by lattice defects and/or non-magnetic add atoms. A noticeable example of this effect is the magnetic order at room temperature produced by proton irradiation of graphite. In this work we have managed to increase the ferromagnetic signal by cooling the graphite samples down to 110 K during proton irradiation, diminishing in this way annealing effects. SQUID measurements of the magnetization show a fluence dependent Curie temperature. The longitudinal magnetoresistance shows an irreversible behavior similar to that found in ferromagnetic films indicating spin/domain reorientation effects. The observed magnetoresistance effects and Curie temperatures above room temperature are promising facts that may lead to useful carbon-based devices in the near future.

X-ray photoelectron and Raman spectroscopic studies of MeV proton irradiated graphite

Poly-crystalline graphite samples were irradiated using 2.25 MeV H + ions with a fluence of 2 × 10 17 ions/cm 2. Magnetic ordering in highly oriented pyrolytic graphite samples have been reported earlier under the similar irradiation conditions [Esquinazi et al., Phys. Rev. Lett. 91 (2003) 227201]. In that study, the authors attribute the observed irradiation induced magnetic ordering to the formation of a mixed sp 2–sp 3 hybridized carbon atoms. In the present study, we report the X-ray photoelectron and Raman spectroscopic studies on pristine and irradiated samples. Irradiated samples are found to show an increased number of sp 3 hybridized carbon atoms. However, the Raman spectrum, specially the second order data, do indicate that the nature of the graphene lattice structure has been preserved in the irradiated samples. The mechanisms for the irradiation induced enhancement in sp 3 hybridization are discussed.

Radiochemical separation and analytical determination of 10Be from proton-irradiated graphite targets

High-concentration samples of carrier-free 10Be (up to 1 μg 10Be per gram graphite) were separated from proton-irradiated graphite targets used at the meson production unit of the Paul Scherrer Institute (PSI). 10Be concentrations were determined both with accelerator mass spectrometry (AMS) and inductively coupled plasma mass spectrometry (ICP-MS). The analytical results from both methods are in good agreement. The highly enriched 10Be-samples will be used in scientific experiments relating to basic nuclear research and nuclear astrophysics in the framework of an international collaboration aimed to exploit accelerator waste material for scientific applications.

Electron Spin Resonance of Proton-Irradiated Graphite

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.

3D-Hydrogen analysis of ferromagnetic microstructures in proton irradiated graphite

Recently, magnetic order in highly oriented pyrolytic graphite (HOPG) induced by proton broad- and microbeam irradiation was discovered. Theoretical models propose that hydrogen could play a major role in the magnetism mechanism. We analysed the hydrogen distribution of pristine as well as irradiated HOPG samples, which were implanted to μm-sized spots as well as extended areas with various doses of 2.25MeV protons at the Leipzig microprobe LIPSION. For this we used the sensitive 3D hydrogen microscopy system at the Munich microprobe SNAKE. The background hydrogen level in pristine HOPG is determined to be less than 0.3 at-ppm. About 4.8×1015H-atoms/cm2 are observed in the near-surface region (4μm depth resolution). The depth profiles of the implants show hydrogen located within a confined peak at the end of range, in agreement with SRIM Monte Carlo simulations, and no evidence of diffusion broadening along the c-axis. At the sample with microspots, up to 40at.% of the implanted hydrogen is not detected, providing support for lateral hydrogen diffusion.

Weak ferromagnetism and spiral spin structures in honeycomb Hubbard planes

Within the Hartree Fock RPA analysis, we derive the spin wave spectrum forthe weak ferromagnetic phase of the Hubbard model on a honeycomb lattice.Assuming a uniform magnetization, the polar (optical) and acoustic branches of thespin wave excitations are determined. The bipartite lattice geometry produces aq-dependent phase difference between the spin wave amplitudes on the two sub-lattices. Wealso find an instability of the uniform weakly magnetized configuration towards a weakantiferromagnetic spiraling spin structure, in the lattice plane, with wavevectorQ along theΓ–K direction, forelectron densities n>0.6. We discuss the effect of diagonal disorder on both the creation of electron bound statesand the enhancement of the density of states, and the possible relevance of these effectsto disorder-induced ferromagnetism, as observed in proton-irradiated graphite.

Examples of room-temperature magnetic ordering in carbon-based structures

In this article we review recent experimental evidence that supports the existence of magnetic ordering at room temperature in carbon-based structures. The results obtained for fullerenes and proton-irradiated graphite indicate that this magnetic ordering is not related to magnetic impurities.