Van De Graaff Accelerator Research Articles

A compact biological cell irradiation system with a Van de Graaff accelerator

This report describes the development of a compact microcell irradiation system in the 3MV KN Van de Graaff accelerator in the Accelerator Laboratory of National Tsing Hua University (NTHU). The main feature of this system is backscattering, a technique whereby a 100nm gold scattering foil is placed in the center of a scattering chamber, instead of a 90° bending magnet. The incident particles produced by the accelerator bombard the scattering foil and scatter isotropically. A set of micro pinholes was installed above the scattering foil and directed the 90° scattering particles to irradiate the cell target. This innovation simplifies the system design and massively reduces the cost and space requirements for system construction.

Neutron Spectroscopy Without Time-of-Flight Measurement: A DSP-Based Deuterated Scintillator Array

Development and evaluation of a large-volume multi-element digital-signal-processing (DSP)-based deuterated (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) liquid scintillator array for the study of reactions involving neutrons without the need for measurement of neutron time-of-flight (ToF) is described. In-beam testing was conducted at the University of Notre Dame (UND) 10 MV FN tandem Van de Graaff accelerator using a deuteron beam at E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> =16.0 MeV. The energy response and detector efficiency of 100 mm dia. ×; 150 mm long cylindrical deuterated liquid scintillator (Eljen 315) detectors was determined in the range E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> =4.8 to 8.7 MeV and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> =6.8 to 21.2 MeV, respectively. In addition, we determined that optimized digital pulse-shape discrimination (DPSD) in liquid scintillator detectors using enhanced liquid (Eljen 315MOD) and other methods can yield improved recoil-particle tagging. As we demonstrate, this is particularly important in deuterated scintillators at neutron energies above 15 MeV. This is especially advantageous for study of nuclear reactions involving radioactive beams, since these often have large, positive reaction Q values.

Fast Neutron Spectroscopy Using${\rm Cs}_{2}{\rm LiYCl}_{6}{:}{\rm Ce}$ (CLYC) Scintillator

The response of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Ce (CLYC) scintillator material to fast neutrons has been measured using a Van de Graaff accelerator. Beams of monoenergetic neutrons in the energy range 0.359 MeV to 4.703 MeV were used to irradiate a 9 mm × 9.5 mm × 12 mm CLYC crystal. Following pulse-shape discrimination to separate neutron and gamma-ray events, peaks are observed in the neutron spectrum and assigned to fast-neutron events in the scintillator. One of the peaks is interpreted as being due to the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sup> Cl(n, p) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sup> S reaction, and it is shown that the proton energy released in this reaction varies linearly with the energy of the incoming neutron. The linearity of the response may enable CLYC to be used for fast-neutron spectroscopy with well defined spectral peaks. The response of CLYC to thermal neutrons and gamma rays is well known, and the material has potential for simultaneous thermal-neutron detection, fast-neutron spectroscopy, and gamma-ray spectroscopy.

Fast Neutron Detection With ${\rm Cs}_{2} {\rm LiYCl}_{6}$

This paper discusses our initial investigation of fast neutron detection with a Cs2LiYCl6 (CLYC) scintillator. CLYC has been already developed for dual mode detectors (thermal neutrons and gamma rays). Described are results collected under mono-energetic irradiation from a Van de Graaff generator and a continuous irradiation from a 252Cf source. There are two reactions in which fast neutrons are captured 35Cl(n,p)35S and 6Li(n,t)α; both have been observed. They produce a proton and a pair of α/t particles, respectively. The response to mono-energetic fast neutrons due to the 35Cl(n,p) reaction produces a peak and due to the 6Li(n,t) reaction a continuum in the energy spectra. The relation between the peak (continuum) position and the excitation energy is linear within evaluated energy range. This allows for fast neutron spectroscopy. The α/β ratios for both reactions were found to be different and somewhat dependent on the excitation energy. The decay times under the proton excitation slightly differ from these under the α/t excitation, while both are significantly different from the gamma ray excited curves. This is important for pulse shape discrimination. Using continuous excitation from 252Cf initial efficiency was estimated for the 35Cl(n,p) reaction. For a 1-inch right cylinder crystal the measured intrinsic efficiency is at least 0.06%, while calculated is 0.5%. The discrepancy is due to experimental inaccuracies. The relative detection efficiency scales linearly with the volume. A convolution of the theoretical 252Cf fast neutron distribution and the 35Cl(n,p) cross-section curve was found to match the experimental data.

Neutron light output response and resolution functions in EJ-309 liquid scintillation detectors

Neutron light output response functions and detector resolution functions were measured at Ohio University's tandem Van de Graaff generator for three cylindrical EJ-309 liquid scintillator cells, having dimensions 12.7(∅)-by-12.7, 7.6-by-7.6, and 7.6-by-5.1cm. A 7.44MeV deuteron beam was used on an 27Al target generating a continuous spectrum over the energy range from a few hundred keV to over 10MeV. The light output response functions are determined using an exponential fit. Detector resolution functions are obtained for the 12.7-by-12.7 and 7.6-by-7.6cm detectors. It is demonstrated that the dependence on detector size is important for the light output response functions, but not to the same extent for the resolution function, even when photomultiplier tubes, detector material, and other detector characteristics are carefully matched.

Materials modification using ions with energies below 1 MeV/u

Materials modifications using swift heavy ion beams provided by large scale accelerators have been used for many years in a wide variety of ways, e.g. to produce ion tracks or to modify the shape of nanoparticles. In all those applications the most relevant parameter for the materials modification is the electronic stopping power and not the ion kinetic energy. For many materials, ions with energies below 1MeV/u delivered from medium and small size accelerators have already sufficiently high electronic stopping power to modify materials in different ways. Also, in this energy range the nuclear stopping power can be large enough to provide additional opportunities for materials modifications. In the present paper, we review recent experimental activities of the Zagreb group where ion beams with energies below 1MeV/u, obtained from a 6MV EN Tandem Van de Graaff accelerator have been used. Additionally, we present several novel examples of materials modifications and their analysis with such ion beams.

A compact stilbene crystal neutron spectrometer for EAST D-D plasma neutron diagnostics

A new compact stilbene crystal neutron spectrometer has been investigated and applied in the neutron emission spectroscopy on the EAST tokamak. A new components analysis method is presented to study the anisotropic light output in the stilbene crystal detector. A Geant4 code was developed to simulate the neutron responses in the spectrometer. Based on both the optimal light output function and the fitted pulse height resolution function, a reliable neutron response matrix was obtained by Geant4 simulations and validated by 2.5 MeV and 14 MeV neutron measurements at a 4.5 MV Van de Graaff accelerator. The spectrometer was used to diagnose the ion temperature in plasma discharges with lower hybrid wave injection and ion cyclotron resonance heating on the EAST tokamak.

Ion Source Optimisation for Proton Beam Quality of the Van De Graaff Accelerator at iThemba LABS for Ion Beam Analysis

The operating principles of the two duoplasmatron ion sources and the PIG source available for the Van de Graaff accelerator have been determined qualitatively, mainly by studying the extensive literature on ion sources. The main characteristics of the three sources have therefore been determined experimentally on an ion source test bench. Important parameters of the sources such as the pressure and temperature in different regions of the sources not measured, because of space limitations. However, in spite of these limitations in quantitative understanding of the sources, sufficient information to continue with the study of the beam transport through the Van de Graaff accelerator and its beamlines could be obtained from these measurements. With regard to beam intensity, lifetime and gas consumption the measurements showed that the hotcathode duoplasmatron is at present by far the most suitable source for the van de Graaff accelerator. The emittance of the ion source was measured with two slits, 90 mm apart, and a Faraday cup in the ion source test bench at an arc voltage of 83 V, an arc current of 1 Amp, a filament voltage of 24 V and an extraction voltage of 5 kV. The measured emittance for 90% of the beam intensity is 48π mm mrad. This figure will be used to calculate the beam transmission through the Van de Graaf accelerator. Different computer programs have been used for these calculations from the ion source through the terminal section, the accelerator and beam line up to the Nuclear Microprobe (NMP) for beam quality.

Isotope Exchange between &lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; Gas and Mechanoactivated Oxide NdMnO&lt;sub&gt;3+δ&lt;/sub&gt;

sotope exchange of oxygen 18О2 with oxides NdMnO3+δ was investigated. The oxide was obtained from oxides Nd2O3 and Mn2O3 using a ceramic technology with annealing in air at 1400°C for 90 hours followed by cooling in a furnace. A planetary mill AGO-2 with a centrifugal factor of g = 60 was used for mechanical treatment of oxides. The study of isotope exchange was carried out by nuclear microanalysis. The concentration of the isotopes 18O and 16O in oxides was determined using a Van de Graaff accelerator and 18O(p, α)15N and 16O(d, p)17O* reactions at the energies of incident beams 762 and 900 keV. Isothermal annealing of powders was carried out in oxygen, enriched to 80% by the isotope 18O. It was established that the concentration of the isotope 18O in mechanically activated powders was several times higher than in the initial micropowder under the same conditions of annealing. The effect increased with increasing of mechanical activation time (30 - 300 s). The isotope exchange parameters connected with the processes at the boundaries of the particles of mechano-activated powder and within their volume were analyzed.

Directional detection of fast neutrons by the Timepix pixel detector coupled to plastic scintillator with silicon photomultiplier array

Fast neutrons are conventionally detected by scintillators of large volume, low spatial resolution and poor, if any, directional sensitivity. In this paper we present a detection technique based on the tracking of protons recoiled by fast neutrons. In this approach we use the silicon pixel detector Timepix attached in contact planar geometry to a fast plastic scintillator. The protons recoiled by neutrons in the scintillator are detected by the pixel detector while scintillation light is sensed by a 4 × 4 array of silicon photomultipliers (SiPM). Each photomultiplier is equipped with an independent amplifier and discriminator providing a fast trigger signal to the pixel detector. Variable threshold level allows adjustment of the trigger sensitivity. Single events in the pixel detector can be tagged and triggered by the scintillating detector. Position and energy sensitivity of the scintillator together with the position and the energy sensitivity of the pixel detector allow obtaining information about the position and the spectrum of the neutron source. The Timepix detector is operated with the FITPix readout interface and the Pixelman software package providing control, DAQ and online visualization. The assembled prototype has been tested with fast neutrons from a laboratory radioactive source (AmBe) and a Van de Graaff accelerator (D-T reaction). The detector architecture, comprising the Timepix device, the scintillator and the segmented SiPM, allows stacking several such units for increased detection efficiency and enhanced directional sensitivity.

The Origin of the Elements

There are about 35 nuclei found in nature, which are not susceptible to neutron capture and are explained by the p-process. The modeling for this process requires thousands of nuclear reactions involving both stable and unstable nuclei including (α,α), (α,p) and (α,γ) reactions. In a recent experiment, the cross section of the reaction 120Te(α,p)123I was measured in the energy range of astrophysical interest for the p-process. The α beam from the Notre Dame FN Tandem Van de Graaff accelerator bombarded highly enriched self-supporting 120Te targets and the γ-rays from the activated 123I was counted with a pair of Ge clover detectors in close geometry.Â

Development of a GVM-based ion beam energy stabilization system at the Bucharest Van de Graaff FN tandem accelerator

This paper presents a new, home-made, GVM (Generating Voltmeter) ion beam energy stabilization system, currently in operation at the Bucharest Tandem Van de Graaff accelerator. The new design combines the signal from the GVM preamplifier with the signal of an original digital reference, in order to obtain an error signal needed by the high-voltage stabilization loop. It is shown that the structure of the employed algorithm provides a high efficiency operation mode for the slow variation of accelerating voltage, with the exception of discharges in the high voltage system. The block structure design, construction features, and tests of this new stabilization system are reported in detail.

The Influence of Crystallinity Degree on the Glycine Decomposition Induced by 1 MeV Proton Bombardment in Space Analog Conditions

Glycine is the simplest proteinaceous amino acid and is present in all life-forms on Earth. In aqueous solutions, it appears mainly as zwitterion glycine (+NH3CH2COO-); however, in solid phase, it may be found in amorphous or crystalline (α, β, and γ) forms. The crystalline forms differ from each other by the packing of zwitterions in the unitary cells and by the number of intermolecular hydrogen bonds. This molecular species has been extensively detected in carbonaceous meteorites and was recently observed in the cometary samples returned to Earth by NASA's Stardust spacecraft. In space, glycine is exposed to several radiation fields at different temperatures. We present an experimental study on the destruction of zwitterionic glycine crystals at room temperature by 1 MeV protons, in which the dependence of the destruction rates of the α-glycine and β-glycine crystals on bombardment fluence is investigated. The samples were analyzed in situ by Fourier transform infrared spectrometry at different proton fluences. The experiments occurred under ultrahigh vacuum conditions at the Van de Graaff accelerator lab at the Pontifical Catholic University at Rio de Janeiro (PUC-Rio), Brazil. For low fluences, the dissociation cross section of α-glycine was observed to be 2.5×10(-14) cm2, a value roughly 5 times higher than the dissociation cross section found for β-glycine. The estimated half-lives of α-glycine and β-glycine zwitterionic forms extrapolated to the Earth orbit environment are 9×10(5) and 4×10(6) years, respectively. In the diffuse interstellar medium the estimated values are 1 order of magnitude lower. These results suggest that pristine interstellar β-glycine is the one most likely to survive the hostile environments of space radiation. A small feature around 1650-1700 cm(-1), tentatively attributed to an amide functional group, was observed in the IR spectra of irradiated samples, suggesting that cosmic rays may induce peptide bond synthesis in glycine crystals. Combining this finding with the fact that this form has the highest solubility among the other glycine polymorphs, we suggest that β-glycine is the one most likely to have produced the first peptides on primitive Earth.

Feasibility study of the proton yield from the reaction D(3He,p)4 He as a possible tool for radiotherapy treatment

Recent achievements in proton and carbon ions therapy have shown the importance of the hadron therapy methods. Aiming at radiotherapy applications such as dermatological and intra-operative procedures, where a short range treatment is needed, we have studied the use of nuclear reactions induced by low energy ions from small accelerators. A very suitable reaction is D(3He,p)4He, using 3He+ ions with energies of about 800 keV. The resulting protons have energies above 17 MeV and could deliver significant radiation dose depending on the accelerator 3He+ beam current and the irradiation time. The deuterium containing target was prepared by reactive magnetron sputtering of titanium in Ar and Ar + D2 radiofrequency plasma on a substrate of Silicon. The Ti-Dx stoichiometry and deuterium content was determined by Ion Beam Analysis. The accelerated 3He+ beam was provided by the 2.5MV Van de Graaff accelerator at the National Laboratories of Legnaro, INFN, Italy. Proton yield as a function of the beam current at different forward scattering angles has been studied for the energies of the incoming 3He+ in the 700keV – 800keV energy interval. The irradiated volume and the radiation dose in biological tissues as a function of the proton energy and proton yield has been estimated. Possible applications in small animal treatment studies as well as potential clinical radiotherapy applications are discussed.

Characterization of actinide targets for fission cross section measurements

In this work the characterization of thin actinide targets ( 237Np, 238U, 235U), man- ufactured for measurements of fission cross section, will be presented. The determi- nation of the mass of the actinide content of each target has been performed via alpha spectrometry, utilizing two SSB detectors, with 50mm2 and 3000mm2 active surface, in order to combine good resolution and good statistics respectively and obtain a smaller error in the final mass result. The impurities of the targets were quantitatively estimated with the same technique. The homogeneity of the samples was examined with two independent methods. Firstly, RBS measurements were per- formed using a proton beam of 2 MeV, provided by the 5.5 MV Tandem Van de Graaff accelerator of the N.C.S.R. ”Demokritos”. The thickness of each target was quantitatively estimated at various points. Secondly, CR-39 plastic track detectors were used to measure the alpha tracks from the surface of the targets and provide an image of the activity of the surface.

Response of BC-418 plastic scintillator to low-energy protons

The response of fast plastic scintillators to protons is unknown for energies below approximately 300keV. The response of BC-418 plastic scintillator to protons from 100keV to 1MeV was measured using elastic scattering of neutrons at the University of Kentucky. Protons of precise energies from a Van de Graaff accelerator impinged on a thin LiF target to produce neutrons in narrow energy bands. A coincidence was detected between the recoiling proton in the plastic scintillator and the elastically scattered neutron in a liquid scintillator. The energy of the recoil proton was determined by the elastic scattering kinematics, with the scattered neutron energy precisely determined by time of flight. The results are compared with previous measurements of the response of similar plastic scintillators in the energy region where they overlap.

Preliminary Study on Neutron Radiography with Several Hundred keV Fast Neutrons

Several hundred keV fast neutron radiography (HKFNR) can be a complementary technique to common thermal neutron radiography (TNR) and several MeV fast neutron radiography (MFNR). We tested HKFNR on a 4.5 MV Van de Graaff accelerator, and the experimental results show that the spatial resolution of this technique is better than MFNR and close to TNR. Several hundred keV fast neutrons can penetrate some thermal neutron absorbers such as Cd, and it is feasible to investigate its use on some materials which are transparent to cold/thermal neutrons, such as aluminum, using this technique.

Laboratory studies in search of the critical hydrogen concentration

Addition of H2 to primary coolant water is widely used in the nuclear reactor industry to suppress water radiolysis and lower the corrosion potential. The critical hydrogen concentration (CHC) – the minimum concentration of excess H2 that can completely suppress O2, H2O2, and H2 formation from water radiolysis – is an important quantity for the management of reactor water chemistry. For the design of future supercritical water cooled reactors, we have investigated whether water radiolysis can be suppressed with a reasonable overpressure of H2. Experiments were carried out using 2.5–2.8MeV electrons from a van de Graaff accelerator, which can easily produce dose rates on the order of one kilogray/second, typical of power reactors. Radiolytic H2 and O2 production was measured as a function of excess dissolved H2. The results indicate that net radiolysis of water can be suppressed in supercritical water. Anomalous high H2 concentrations were obtained using metal (hastelloy or titanium) irradiation tubing rather than sapphire or silica. We ascribe these results to a radiation-stimulated corrosion process at high temperature. Throughout the subcritical temperature regime, almost no oxygen is measured, even though kinetic modeling suggests there should be concentrations well above our detection threshold. To explain this result we recommend that unmeasured rate constants for •H+O2− and (e−)aq+O2− should be considered completely diffusion-limited. At 300°C, the (high dose rate) steady state H2 concentration in pure water is almost completely determined by the equilibrium H2+•OH⇔•H+H2O. The measured steady-state H2 is in good agreement with the recent equilibrium constant estimate of Bartels (Radiation Physics and Chemistry 78(3): 191–194, (2009)).

A shell model study of the high spin states of 88Y

Experiments were carried out at the Wright Nuclear Structure Laboratory at Yale University using the 21MV ESTU Tandem Van de Graaff accelerator with the purpose of studying 88Y. A beam of 18O impinged at laboratory energies of 60, 65 and 70 MeV on a 600 μg/cm2 74Ge target with a thick (10mg/cm2) 197Au backing. This experiment was performed with the specific aim of accessing medium spin states of the nucleus of interest. A second experiment was undertaken to populate the nucleus of interest in higher spin states by impinging the same 18O beam on a thin 62 μg/cm2 76Ge target with a 20 μg/cm2 carbon backing at a laboratory beam energy of 90 MeV. Gamma rays emitted following the decay of excited states in 88Y and other nuclei populated in the reactions were measured using the YRAST ball detector array, consisting of 10 Compton suppressed HPGe clover detectors. In conjunction with the experimental study presented here, nuclear shell model calculations using a truncated valence space have also been performed in an attempt to describe the single-particle make-up of the states observed. Preliminary results from these experiments and theoretical calculations are presented.

Impact ionization mass spectra of anorthite cosmic dust analogue particles

Anorthite, the Ca‐rich end‐member of plagioclase feldspar, is a dominant mineral component of the Lunar highlands. Plagioclase feldspar is also found in comets, meteorites and stony asteroids. It is therefore expected to contribute to the population of interplanetary (and circumplanetary) dust grains within the solar system. After coating micron‐ and submicron‐sized grains of Anorthite with a conductive layer of Platinum, the mineral was successfully accelerated to hypervelocity speeds in the Max Planck Institut für Kernphysik's Van de Graaff accelerator. We present impact ionization mass spectra generated following the impacts of anorthite grains with a prototype mass spectrometer (the Large Area Mass Analyser, LAMA) designed for use in space, and discuss the behavior of the spectra with increasing impact energy. Correlation analysis is used to identify the compositions and sources of cations present in the spectra, enabling the identification of several molecular cations (e.g., CaAlO2, CaSiO2, Ca2AlO3/CaAlSi2O2) which identify anorthite as the progenitor bulk grain material.