Tandem Accelerator Research Articles

CALIBRA: A national research infrastructure for accelerator-based research and applications

CALIBRA stands for Cluster of Accelerator Laboratories for Ion-Beam Research and Applications, i.e., the Research Infrastructure proposed in 2014 to the General Secretariat for Research and Technology (GSRT) in response to the call for the establishment of the Greek National Roadmap for Research Infrastructures (RI). CALIBRA is planned to be completed in phases with the first one being implemented through the homonym project that is funded by the currently running Partnership Agreement for the Development Framework (ΕΣΠΑ 2014-2020). The CALIBRA project aims at establishing and operating an accelerator-based research infrastructure open to the national and the European scientific community to conduct research at excellence level, develop innovative applications of increased socioeconomic impact, and provide highly-specialized services, unique at the country level, to the public and private sector. CALIBRA is implemented at the Tandem Accelerator Laboratory of NCSR “Demokritos”. The project had its kickoff meeting in November 2017.

Study of the 232Th(n,f) Cross section at NCSR ‘Demokritos’ using Micromegas Detectors

The accurate knowledge of neutron-induced fission cross sections in actinides, is of great importance when it comes to the design of fast nuclear reactors, as well as accelerator driven systems. Specifically for the 232Th(n,f) case, the existing experimental datasets are quite discrepant in both the low and high energy MeV regions, thus leading to poor evaluations, a fact that in turn implies the need for more accurate measurements.In the present work, the total cross section of the 232Th(n,f) reaction has been measured relative to the 235U(n,f) and 238U(n,f) ones, at incident energies of 7.2, 8.4, 9.9 MeV and 14.8, 16.5, 17.8 MeV utilizing the 2H(d,n) and 3H(d,n) reactions respectively, which generally yield quasi-monoenergetic neutron beams. The experiments were performed at the 5.5 MV Tandem accelerator laboratory of N.C.S.R. “Demokritos”, using a Micromegas detector assembly and an ultra thin ThO2 target, especially prepared for fission measurements at n_ToF, CERN during its first phase of operations, using the painting technique. The masses of all actinide samples were determined via α-spectroscopy. The produced fission yields along with the results obtained from activation foils were studied in parallel, using both the NeusDesc [1] and MCNP5 [2] codes, taking into consideration competing nuclear reactions (e.g. deuteron break up), along with neutron elastic and inelastic scattering with the beam line, detector housing and experimental hall materials. Since the 232Th(n,f) reaction has a relatively low energy threshold and can thus be affected by parasitic neutrons originating from a variety of sources, the thorough characterization of the neutron flux impinging on the targets is a prerequisite for accurate cross-section measurements, especially in the absence of time-of-flight capabilities. Additional Monte-Carlo simulations were also performed coupling both GEF [3] and FLUKA [4] codes for the determination of the detection efficiency.

NatMg(p,p0) differential cross-section measurement relevant to the EBS technique

In the present work the natMg(p,p0)natMg reaction was investigated in the energy range Ep,lab ~ 2700–4250 keV, at six backward detection angles (120o-170o) suitable for analytical purposes. The measurements were performed using the 5.5 MV TN11 HV Tandem Accelerator of N.C.S.R. ‘‘Demokritos”, Athens, Greece and a high-precision goniometer. The experimental data are compared to data from the literature, when available, and similarities and discrepancies are presented and analyzed.

Fusion of C12+Mg24 far below the barrier: Evidence for the hindrance effect

Background: The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behavior of heavy-ion fusion at extreme subbarrier energies, including reactions involving lighter systems, e.g., reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the $S$ factor vs energy. Whether there is an $S$-factor maximum at very low energies for systems with a positive fusion $Q$ value is an experimentally challenging question.Purpose: Our aim has been to search for evidence of fusion hindrance in $^{12}\mathrm{C}+^{24}\mathrm{Mg}$ which is a medium-light system with positive $Q$ value for fusion, besides the heavier cases where hindrance is recognized to be a general phenomenon. $^{12}\mathrm{C}+^{24}\mathrm{Mg}$ is very close to the $^{16}\mathrm{O}+^{16}\mathrm{O}$ and $^{12}\mathrm{C}+^{12}\mathrm{C}$ systems that are important for the late evolution of heavy stars.Methods: The experiment has been performed in inverse kinematics using the $^{24}\mathrm{Mg}$ beam from the XTU Tandem accelerator of LNL in the energy range 26--52 MeV with an intensity of 4--8 pnA. The targets were $^{12}\mathrm{C}$ evaporations $50\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{g}/{\mathrm{cm}}^{2}$ thick, isotopically enriched to $99.9%$. The fusion-evaporation residues were detected at small angles by a $E\ensuremath{-}\mathrm{\ensuremath{\Delta}}E$-ToF detector telescope following an electrostatic beam deflector.Results: Previous measurements of fusion cross section for $^{12}\mathrm{C}+^{24}\mathrm{Mg}$ were limited to above-barrier energies. In the present experiment the excitation function has been extended down to $\ensuremath{\simeq}15\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{b}$ and it appears that the $S$ factor develops a clear maximum vs energy, indicating the presence of hindrance. This is the first convincing evidence of an $S$ factor maximum in a medium-light system with a positive fusion $Q$ value. These results have been fitted following a recently suggested method and a detailed analysis within the coupled-channels model that has been performed using a Woods-Saxon potential and including the ground state rotational band of $^{24}\mathrm{Mg}$. The coupled-channels calculations give a good account of the data near and above the barrier but overpredict the cross sections at very low energies.Conclusions: The hindrance phenomenon is clearly observed in $^{12}\mathrm{C}+^{24}\mathrm{Mg}$, and its energy threshold is in reasonable agreement with the systematics observed for several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (${\ensuremath{\sigma}}_{s}=1.6\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$) is indeed found for this system. Therefore it may even be possible to extend the measurements further down in energy to better establish the position of the $S$-factor maximum.

Deuterium retention in tungsten and reduced activation steels after 3 MeV proton irradiation

Abstract Nuclear fusion plasma-facing materials (PFM) will suffer from irradiation, leading to significant changes in the material properties. This study investigates the impact of displacement damage on the deuterium retention near room temperature. ITER grade tungsten, Eurofer-97, and HiperFer 17Cr5 steel samples are irradiated with a tandem accelerator with ~3 MeV protons at currents of 100–600 nA on 250–550 µm spots at 320±10 K. In total 33 spots from 0 to 0.9 displacements per atom (DPA) at 0–4 µm depth are irradiated on 5 samples. After irradiation, the samples are exposed to D2 plasmas with a peak ion-flux of 2.1 × 1021 D/m²s for 4 h at The long-term D retention in both W and steel increases with DPA with a saturation starting around 0.2 DPA. Retention in W increased by a factor 12 with up to 3.2 at.% D, while in steel increases up to 180 times with up to 0.08 at.% D were observed. The results highlight the importance of using steels also in PFMs. Compatibility of the results with heavy ion irradiations boosts the confidence in inter-comparability between different ion types, but also between ions and neutrons.

Proton-induced single-event effects on 28 nm Kintex-7 FPGA

Proton experiments were performed on Kintex-7 XC7K70T field programmable gate array by using a self-developed test system at EN tandem accelerator in Peking University. The single-event upset and multiple-bit upsets induced by low-energy proton were presented, and a cross section peak under the low-energy protons was tested. Also, the experiments under high-energy protons were performed on the proton cyclotron accelerator in the China Institute of Atomic Energy, cross sections of some blocks in FPGA were obtained. Data of low-energy and high-energy experiments showed that the LET threshold of some resources were different. Results suggest that the low-energy proton-induced single event effect must be considered.

Novel Approaches for Intensifying Negative C60 Ion Beams Using Conventional Ion Sources Installed on a Tandem Accelerator

We developed novel methods for producing negative C60 ion beams at the accelerator facility Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) to increase the current intensity of swift C60 ion beams accelerated to the MeV energy region using a tandem accelerator. We produced negative C60 ion beams with an intensity of 1.3 µA, which is several tens of thousands of times greater than the intensity of beams produced using conventional methods based on the Cs sputtering process. These beams were obtained by temporarily adding an ionization function based on electron attachment to an existing ion source that is widely used in tandem accelerators. The high-intensity swift C60 ion beams can be made available relatively easily to institutes that have tandem accelerators and ion sources of the type used at TIARA because there is no need to change existing ion sources or install new ones.

Energy calibration of He-3, He-4 Neutron Proportional Counters

Two helium filled proportional counters (He-3, He-4) were irradiated with neu- trons in the energy range 0.025 eV - 10.7 MeV, in the Tandem accelerator, NCSR Demokritos, Athens. The full energy peak in He-3 proportional counter as well as the recoil peaks in He-3 and He-4 counters show linear dependence with neutron energy. A study of gamma ray contribution to the neutron spectra of He-4 counter was performed as a function of the pulse shaping time.

Measurement of the 241Am(n,2n) Reaction Cross Section, by the Activation Method

The measurement of the cross section of the reaction 241Am(n,2n)240Am, has been performed, for the first time at neutron energies of 8.8, 9.6, 10.6 and 11.4 MeV, by the activation method. The monoenergetic neutron beam was produced at the 5.5MV TANDEM accelerator of NCSR "Demokritos", using the H{d,n) He reaction. During the 5-day long irradiation, the neutron flux was monitored by a BF3, counter. The neutron yield as well as the beam current, were recorded in 100 sec intervals by two multiscaling units. The radioactive target consisted of a 37GBq 241Am source enclosed in a Pb container. A natural Au foil, 27Al foil and a 93Nb foil were used as reference materials for the neutron flux determination. The activity of the irradiated targets, was measured off-line by a 56% relative efficiency, HPGe detector. The first preliminary results are presented and compared with evaluated libraries and existing data.

Proton-capture Reaction Cross Sections on the Sr Isotopes Relevant to the p-process Nucleosynthesis

Proton-capture reaction cross sections on the 86,87,88Sr isotopes have been determined at energies from 1.4 to 5 MeV by measuring γ-angular distributions at the 4 MV single-ended Dynamitron accelerator of the University of Stuttgart as well as at the 5 MV VdG Tandem accelerator of NCSR "Demokritos", Athens. In the former case an array of 4 HPGe detectors with relative efficiency εr≈100%, each shielded with BGO crystals, were used. In the case of the measurements carried out at "Demokritos" we used only one HPGe detector (εr≈80%) with no BGO shield. Cross sections ranging from 0.5 μb to 5 mb as well as the relevant S factors were obtained. The data were compared with statistical model calculations using the code MOST. In the calculations, various combinations of microscopic and phe- nomenological models of the nucleon-Nucleus Optical Model Potentials (OMP) and Nuclear Level Densities (NLD) were used and a good agreement between the data and theoretical predictions was found.

High Spin States in 193Hg

The high-spin structure of 193Hg was investigated by in-beam γ-ray spectro­scopic techniques. The tandem accelerator at Daresbury Laboratory, U. K., was used to populate excited states of 193Hg through the reaction 150Nd(48Ca,5n)193Hg at a beam energy of 213 MeV and the EUROGAM detector array was used to de­ tect the γ-rays emitted by the deexciting nuclei. The normal level scheme has been further extended and a new band has been observed. In addition two new ΔI=1 structures of competing dipole and quadrupole transitions were found which will be discussed in detail.

Irradiation hardening behaviors of tungsten–potassium alloy studied by accelerated 3-MeVW2+ ions**Project supported by the National Natural Science Foundation of China (Grant Nos. 11975160, 11775149, and 11475118) and the International Thermonuclear Experimental Reactor (ITER) Program Special, China (Grant No. 2011GB108005).

Tungsten–potassium (WK) alloy with ultrafine/fine grains and nano-K bubbles is fabricated through spark plasma sintering (SPS) and rolling process. In this study, 3-MeV W2+ ion irradiation with a tandem accelerator is adopted to simulate the displacement damage caused by neutrons. As the depth of irradiation damage layer is limited to only 500 nm, the hardening behaviors of WK alloy and ITER (International Thermonuclear Experimental Reactor)-W under several damage levels are investigated through Bercovich tip nanoindentation test and other morphological characterizations. The indenter size effect (ISE), soft substrate effect (SSE), and damage gradient effect (DGE) are found to influence the measurement of nano-hardness. Few or no pop-ins in irradiated samples are observed while visible pop-in events take place in unirradiated metals. Extensive pile-up with different morphology features around the indentation exists in both WK and ITER-W. The WK shows a smaller hardness increment than ITER-W under the same condition of displacement damage. This study provides beneficial information for WK alloy serving as a promising plasma facing materials (PFMs) candidate.

Nucleon inelastic scattering cross sections on O16 and Si28

This paper reports cross-section measurements of the ($n,{n}^{\ensuremath{'}}$) and ($p,{p}^{\ensuremath{'}}$) reactions on $^{16}\mathrm{O}$ and $^{28}\mathrm{Si}$ at Geel Electron Linear Accelerator and at the 9-MV Tandem Accelerator of Horia Hulubei National Institute for Physics and Nuclear Engineering, respectively. The main purpose was to measure the neutron- and proton-induced inelastic $\ensuremath{\gamma}$-production cross sections for all observed transitions in $^{16}\mathrm{O}$ and $^{28}\mathrm{Si}$, followed by the calculation of the corresponding total inelastic cross section. The results are compared with theoretical calculations performed using the talys 1.9 code, evaluated nuclear data, and previously reported experimental data. The broader goal of this work is to study whether and to what extent the neutron-induced inelastic cross sections of these nuclei can be inferred from those obtained using suitable charged particle reactions. We show that, by making use of the formal similarities between the neutron- and the proton-target optical model potentials and isospin symmetry in mirror nuclei, one can develop a procedure that combines experimental proton-induced inelastic cross sections with theoretical calculations to infer neutron inelastic cross sections. For $^{16}\mathrm{O}$ and $^{28}\mathrm{Si}$, the precision associated with this procedure is around 10--20% for most of the incident energy range.

Measurement of 85Rb (n, 2n) 84Rb reaction cross-section from 12 MeV up to 19.8 MeV * * Supported by National Natural Science Foundation of China (11575163)

The cross-section data of the 85Rb(n, 2n)84Rb reaction have been measured with the neutron energies of 12 MeV to 19.8 MeV using the activation technique and the relative method. The 85Rb samples were irradiated on the surface of a two-ring orientation assembly with neutrons produced from the 3H(d, n)4He reaction at the 5SDH-2 1.7-MV Tandem accelerator in China. Theoretical model calculations were performed with the TALYS-1.9 code. The present data were compared with previously obtained experimental data and the available evaluated data.

Experimental study of the spectroscopic factors of Sn116–125

Enriched targets of $^{116,118,120,122,124}\mathrm{SnO}_{2}$ were bombarded with proton and deuteron beams, and the angular distributions of $(p,d)$ and $(d,p)$ on tin isotopes were accurately measured using the high-precision Q3D magnetic spectrograph at the Beijing HI-13 tandem accelerator of the China Institute of Atomic Energy. Distorted-wave Born approximation calculations were performed to extract the neutron spectroscopic factors (SFs) using two different sets of systematic optical potential parameters for these neutron transfer reactions. The SFs of $^{116--125}\mathrm{Sn}$ were obtained and compared to previous values. Our results are consistent with the average of the previous data within the error range. It is worth noting that the reaction products corresponding to $^{119}\mathrm{Sn}_{\text{G.S.}}$ and $^{119}\mathrm{Sn}_{\text{0.024}}^{*}$, to $^{123}\mathrm{Sn}_{\text{G.S.}}$ and $^{123}\mathrm{Sn}_{\text{0.025}}^{*}$, and to $^{125}\mathrm{Sn}_{\text{G.S.}}$ and $^{125}\mathrm{Sn}_{\text{0.028}}^{*}$ were first distinguished by the present experiment; therefore, our results of the low lying states of $^{119,123,125}\mathrm{Sn}$ are more reliable. However, the first excited state of $^{121}\mathrm{Sn}$ is only 0.006 MeV; we failed to identify the products that correspond to the ground state and the first excited state, and the extracted SFs of $^{120}\mathrm{Sn}_{\text{G.S.}}\ensuremath{\bigotimes}n\ensuremath{\rightarrow}^{121}\mathrm{Sn}_{\text{0.006.}}$ and $^{121}\mathrm{Sn}_{\text{0.006}}\ensuremath{\bigotimes}n\ensuremath{\rightarrow}^{122}\mathrm{Sn}_{\text{G.S}}$ are not reliable. A simple linear formula was used to analyze the relationship of SFs with neutron separation energy ${S}_{n}(N)$ and the even-$A$ Sn pairing gap $\mathrm{\ensuremath{\Delta}}(N)$, and SFs are found to be positively correlated to ${S}_{n}(N)$ and $\mathrm{\ensuremath{\Delta}}(N)$.

Search for proton emission of the isomeric mathbf {10^+} state in mathbf {^{54}}Ni

Several experiments were conducted at the 10 MV Van-de-Graaff tandem accelerator at the Institute of Nuclear Physics, Cologne, to detect proton emission from the isomeric 6457-keV 10^+ state in ^{54}hbox {Ni}. Excitation functions for two fusion–evaporation reactions were measured to maximise the population of the rare two-neutron evaporation channel from a ^{56}hbox {Ni} compound nucleus. The search for delayed proton emission was based on the ^{28}hbox {Si}(^{28}hbox {Si},2n)^{54}hbox {Ni} reaction at a beam energy of 70 MeV. For this reaction, a cross-section limit for the population of the 10^+ state in ^{54}hbox {Ni} and its proton-decay branch was determined to be sigma < 22 nb.

Toward short-lived and energy-dependent fission product yields from neutron-induced fission

Fission product yields (FPYs) are an important source of information that are used for basic and applied physics. They are essential observables to address questions relevant to nucleosynthesis in the cosmos that created the elements from iron to uranium, for example, in energy generating processes from fission recycling in binary neutron star mergers; resolving the reactor neutrino anomaly; decay heat release in nuclear reactors; and many national security applications. While new applications will require accurate energy-dependent FPY data over a broad set of incident neutron energies, the current evaluated FPY data files contain only three energy points: thermal, fast, and 14-MeV incident energies.Recent measurements using mono-energetic and pulsed neutron beams at the Triangle Universities Nuclear Laboratory (TUNL) tandem accelerator and employing a dual fission ionization chambers setup have produced self-consistent, high-precision data critical for testing fission models for the neutron-induced fission of the major actinide nuclei. This paper will present new campaign just beginning utilizing a RApid Belt-driven Irradiated Target Transfer System (RABITTS) to measure shorter-lived fission products and the time dependence of fission yields, expanding the measurements from cumulative towards independent fission yields.

Calibration of CR-39 solid state track detectors with monoenergetic protons from 0.3 MeV to 2.5 MeV

The2H(d,p)3H reaction is one of the most crucial reactions in the Big Bang nucleosynthesis (BBN). It is of particular interest to investigate this kind of reactions in plasma environments, generated by high intensity lasers, which are similar to real astrophysical conditions. We have experimentally investigated the2H(d,p)3H reaction using laser-driven counter-streaming collisionless plasmas at the Shenguang-II laser facility. CR-39 track detectors are widely employed as the main diagnostics in such experiments and laser-driven ion acceleration. In this work, we performed calibration of CR-39 track detectors with monoenergetic protons from the tandem accelerator, and then presented their track diameters for proton energies ranging from 300 keV to 2.5 MeV and for etching times between 4 and 28 hours. In addition, we recommended the optimal etching time at the typical etching conditions, which will be very useful for the following massive data analysis from the CR-39 detectors.

Fission Product Yield Measurements from Neutron-Induced Fission of 235,238 U and 239Pu

Fission product yields (FPY) are one of the most fundamental quantities that can be measured for a fissioning nucleus and are important for basic and applied nuclear physics. Recent measurements using mono-energetic and pulsed neutron beams generated using Triangle Universities Nuclear Laboratory’s tandem accelerator and employing a dual fission chamber setup have produced self-consistent, high-precision data critical for testing fission models for the neutron-induced fission of 235,238U and 239Pu between neutron energies of 0.5 to 15.0 MeV. These data have elucidated a low-energy dependence of FPY for several fission products using irradiations of varying lengths and neutron energies. This paper will discuss new measurements just beginning utilizing a RApid Belt-driven Irradiated Target Transfer System (RABITTS) to measure shorterlived fission products and the time dependence of fission yields, expanding the measurements from cumulative towards independent fission yields. The uniqueness of these FPY data and the impact on the development of fission theory will be discussed.

Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of 239Pu as a test of the Bohr hypothesis in nuclear fission

Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The 239 Pu(n,f) reaction at En = 4.6 MeV and the 240 Pu(γ,f) reaction at Eγ = 11.2 MeV were used to produce a 240 Pu∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HIγS) facility. The FPYs from these two reactions are compared quantitatively for the first time.