Experimental study and simulation of the residual activity induced by high-energy argon ions in copper

The paper presents new experimental results and FLUKA-simulations of residual activation induced by high-energy argon ions in copper. It follows the previous residual activation studies performed at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt with uranium ions as a preparatory work for constructing the FAIR facility. Copper samples were irradiated by 1 GeV/u and 500 MeV/u ⁴⁰Ar ions and investigated by gamma-ray spectroscopy. The samples were irradiated in the stacked-foil geometry. The isotopes with dominating contribution to the total residual activity were identified and their partial activities were quantified. Depth-profiling of the partial residual activities of all identified isotopes was performed by measurements of individual target foils. The experimental results were compared with simulations by the FLUKA-code. A satisfactory agreement between the experiment and the simulations was observed.     

First results of an experimental study of the residual activity induced by high-energy uranium ions in steel and copper

First results of an experimental study of the residual activity induced by high-energy uranium ions are presented. As a preparatory work for constructing the FAIR facility at GSI, samples of stainless steel and copper were irradiated by 500 MeV/u ²³⁸U ions and investigated by gamma-ray spectroscopy. The isotopes that contribute dominantly to the residual activity have been identified and their contributions have been quantified. Depth-profiles of residual activity of individual isotopes have shown that target activation due to projectile fragments and neutrons extends far beyond the range of primary particles and reaches the same level as in the region irradiated by primary particles.     

Activation of aluminum by argon: Experimental study and simulations

The paper presents the results of irradiation of aluminum targets by 430 and 500 MeV/u argon beams. Gamma-spectra were measured after the end of the irradiation in order to identify the induced nuclides as well as to determine their residual activity depth-profiles. The results of this experiment are compared with Monte Carlo simulations by FLUKA, GEANT4, MARS, PHITS and SHIELD-A codes. In case of a thin target, the agreement between the experiment and simulations is satisfactory, while in case of a thick target, some discrepancies are observed.     

Production of Fission Product Xenon

Radioactive Xe isotopes were produced by in-pile melting of irradiated uranyl nitrate hexali yd rate crystals. An aluminum pipe closed at the end was placed adjacent to the reactor core of the TTR-1 (swimming pool type reactor), and uranyl nitrate hexahydrate crystals in a stainless steel capsule were irradiated therein at a neutron flux of 2×10¹¹/cm²-sec. After in-pile cooling for 10–15hr, the capsule was heated to melt the crystals. The rare gas activity released from the crystals was led out of the reactor by helium sweep and collected in a charcoal trap cooled by liquid nitrogen. More than 99% of the rare gas activity was recovered. The composition of the activity at the time of separation, while depending on cooling time and target irradiation history, was in the present case ¹³⁵Xe(–80%), ¹³³Xe(–10%) and small amounts of Kr isotopes. The remaining uranyl nitrate, crystallizes again upon removal of heat, and can serve for succeeding irradiations. About 30 mCi of radioactive Xe isotopes were produced by 4 hr irradiation using 5 g of 13% enriched U. Radioactive Xe produced in this way is being used for medical purposes.     

Cross sections for the production of helium, neon and argon isotopes by proton-induced reactions on iron and nickel

We measured integral thin target cross sections for the proton-induced production of ³He, ⁴He, ²¹Ne, ²²Ne, ³⁶Ar and ³⁸Ar from Fe and Ni from the respective reaction thresholds up to 1.6 GeV. The production of noble gas isotopes, especially ⁴He, from Fe and Ni is of special importance for design studies of accelerator driven systems and/or energy amplifier, because Fe is the main structural material in almost every design study. Furthermore, the cross sections are needed to establish the first physical model calculations for the production of cosmogenic nuclides in iron meteorites. As a result of our new measurements there now exist for both target elements a complete and consistent database for the production of noble gas isotopes. The experimental data are compared to results from the theoretical nuclear model codes INCL4/ABLA and TALYS. This comparison clearly demonstrates again that experimental data are still needed because the predictive power of nuclear model codes, though permanently improving, does still not allow reliably predicting the cross sections needed for most applications and irradiation experiments remain indispensable.     

Cross sections for the proton-induced production of He and Ne isotopes from magnesium,aluminum, and silicon

We measured integral thin target cross sections for the proton-induced production of the rare gas isotopes ³He, ⁴He, ²⁰Ne, ²¹Ne and ²²Ne from Mg, Al, and Si from the respective reaction threshold up to 1.6 GeV. These target elements were chosen since they account for more than 50% and 95% of the cosmogenic He and Ne production in extraterrestrial matter, respectively. In order to minimize the influences of secondary particles on the production of residual nuclides a so-called “mini-stack”-approach was used instead of the well known “stacked-foil-technique” for all irradiation experiments with proton energies above 200 MeV. With this new data base a complete and consistent set of excitation functions for the proton-induced production of He and Ne isotopes is established for all target elements relevant for deciphering the cosmic ray record in extraterrestrial matter.     

Change in the Isotopic Composition of Transmutation 135Cs Targets during Radiochemical Recycling

¹³⁵Cs transmutation in the presence of other cesium isotopes is studied. The change in the concentration of the ancillary starting nuclides and the activity of the targets when they are recycled are analyzed. It is shown that decreasing the initial ¹³⁷Cs content in a target to 1% is an adequate level of isotopic separation of cesium. It is established that the ¹³³Cs and ¹³⁴Cs concentration is quasistationary, the increase in target activity is negligible and is determined by the accumulation of ¹³⁷Cs, and for a long time there is no need to separate the cesium isotopes from irradiated targets.     

Study of the excitation functions for K, Sc and Ti by proton irradiation on Sc up to 37 MeV

The excitation functions of proton induced reactions on Sc targets (100% ⁴⁵Sc), leading to the formation isotopes ⁴³K, ⁴³Sc, ^44mSc, ^44gSc and ⁴⁴Ti were studied by the stacked foil activation technique up to 37 MeV. High-resolution gamma-spectrometry measurements were performed on an HPGe detector in order to determine the activity of the irradiated Sc₂O₃ pellets and Ti monitor foils. The reaction cross-sections were measured from their respective thresholds up to E_p = 36.4 MeV and were compared with previous values reported in literature. Possible batch yields and optimal irradiation parameters for generator ⁴⁴Ti -^44gSc in high current accelerators are discussed.     

Protactinium-231: A new radionuclide for AMS

For the first time, an AMS system was used to determine concentrations of the actinide Protactinium-231. ²³¹Pa has widespread applications in the earth sciences. It can be used for U-series disequilibrium dating, and ²³¹Pa is an important tracer in Paleoceanography. The Pa isotopes were measured with the compact ETH/PSI-AMS facility TANDY operating at about 300 kV. The linearity of the AMS is documented with two standard-dilution experiments and first measurements of natural samples from very different climate archives are presented. Our results show that it is possible to determine ²³¹Pa amounts in the lower femtogram (1 fg = 10⁻¹⁵ g) range with the TANDY. In natural samples, ²³¹Pa concentrations between 1 and 4 pg/g (1 pg = 10⁻¹² g) were measured with an average total error of 3% (one sigma). The average over all reproducibility of these first measurements was better than 2% with clear potential for improvement.     

Alpha induced reactions on Cd and Cd: An experimental study of excitation functions

Excitation functions of alpha induced reactions on enriched ¹¹⁴Cd and ¹¹⁶Cd targets, leading to the formation of the ^117m,119mSn, ^{114m₁,115m,116m,117m,g}In, ^115gCd isotopes, were studied by the stacked foil activation technique. Reaction cross-sections were measured from their respective thresholds up to E_α = 38.9 MeV. Quantification of induced isotopes has been made by gamma and X-ray spectrometry. The experimental cross-sections are compared, where available, with values reported previously in literature. Thick target yield for the medically important radionuclide ^117mSn is calculated based on discrete values of measured cross-sections.     

Nucleogenic Cl, U and Pu in uranium ores

The nucleogenic isotopes ³⁶Cl, ²³⁶U and ²³⁹Pu are produced naturally in subsurface environments via neutron capture of thermal and epithermal neutrons. Concentrations are, however, very low and accelerator mass spectrometry (AMS) is required for quantitative measurements. A particular challenge is presented by the measurement of ²³⁶U/²³⁸U ratios down to the level of 10⁻¹³ that is expected from rocks with low uranium concentration. Here, we present the AMS methodology that has been developed at the ANU for measuring ²³⁶U/²³⁸U ratios at this level. The more established methodologies for ³⁶Cl and ²³⁹Pu measurements are also summarised. These capabilities are then used to characterize the ³⁶Cl, ²³⁶U and ²³⁹Pu concentrations in a range of uranium ores. A simple model of the neutron production and capture processes in subsurface environments has been developed and is presented. It is shown that nucleogenic ³⁶Cl, ²³⁶U and ²³⁹Pu can be used to determine both thermal and epithermal neutron fluxes in subsurface environments. Potential applications include uranium exploration and monitoring of the environmental impact of uranium mining.     

Comparison of the analytical sensitivities for non-1/v elements in different neutron beams

Irregular nuclides such as ¹¹³Cd, and some rare-earth isotopes show different analytical sensitivities in PGAA performed at different facilities, because the cross-sections of these nuclides have strong low-energy resonances which partly overlap the energy range of typical neutron beams used for activation. A series of systematic measurements has been performed in the spectrally different cold and thermal neutron beams of the Budapest Research Reactor, Hungary and at the research reactor of NIST, Gaithersburg, USA to quantitatively study the non-1/v behaviour of irregular nuclides. Samples were prepared that contained one of the irregular nuclides and also a regular one (¹⁰B, ³⁵Cl and ⁵⁶Fe) and their activation ratios were compared as measured in five different beams. Theoretical values of the activation ratios were calculated from estimates of the actual neutron spectra and cross-section data, and show a generally good correspondence to the experimental results, although some details are still not reproduced.     

Production of noble gas isotopes by proton-induced reactions on bismuth

We measured integral thin target cross sections for the proton-induced production of He-, Ne-, Ar-, Kr- and Xe-isotopes from bismuth (Bi) from the respective reaction thresholds up to 2.6 GeV. Here we present 275 cross sections for 23 nuclear reactions. The production of noble gas isotopes from Bi is of special importance for design studies of accelerator driven systems (EA/ADS) and nuclear spallation sources. For experiments with proton energies above 200 MeV the mini-stack approach was used instead of the stacked-foil technique in order to minimise the influences of secondary particles on the residual nuclide production. Comparing the cross sections for Bi to the data published recently for Pb indicates that for ⁴He the cross sections for Bi below 200 MeV are up to a factor of 2-3 higher than the Pb data, which can be explained by the production of α-decaying Po-isotopes from Bi but not from Pb. Some of the cross sections for the production of ²¹Ne from Bi are affected by recoil effects from neighboured Al-foils, which compromises a study of a possible lowering of the effective Coulomb-barrier. The differences in the excitation functions between Pb and Bi for Kr- and Xe-isotopes can be explained by energy-dependent higher fission cross sections for Bi compared to Pb. The experimental data are compared to results from the theoretical nuclear model codes INCL4/ABLA and TALYS. The INCL4/ABLA system describes the cross sections for the production of ⁴He-, Kr- and Xe-isotopes reasonably well, i.e. mostly within a factor of a few. In contrast, the model completely fails describing ²¹Ne, ²²Ne, ³⁶Ar and ³⁸Ar, which are produced via spallation and/or multifragmentation. The TALYS code is only able to accurately predict reaction thresholds. The absolute values are either significantly over- or underestimated. Consequently, the comparison of measured and modelled thin target cross sections clearly indicates that experimental data are still needed because the predictive power of nuclear model codes, though permanently improving, does still not allow to reliably predict the cross sections needed for most applications and irradiation experiments remain indispensable.     

Production of high-purity medical radio isotope 64Cu with accelerator-based neutrons generated with 9 and 12 MeV deuterons

We conducted a feasibility study for producing a high-purity medical radioisotope ⁶⁴Cu from natural zinc with accelerator-based neutrons. ⁶⁴Cu isotopes were produced via the ⁶⁴Zn(n,p) reaction. The accelerator-based neutrons were generated via the C(d,n) reaction using low-energy deuterons of 9 and 12 MeV on a 1-mm-thick carbon target. First, the production purity was estimated using the evaluated nuclear data library JENDL-4.0 and our previously measured thick target neutron yield. We found that even when natural zinc was used as the starting material, significantly high-purity ⁶⁴Cu could be obtained. Next, irradiation experiments for producing ⁶⁴Cu using natural zinc were conducted at Kyushu University Tandem Laboratory, with the amounts of ⁶⁴Cu isotopes and other gamma-emission nuclides measured by a high-purity germanium detector. As a result, high-purity ⁶⁴Cu isotopes of 1.11(49) × 10⁰ and 3.70 (17) × 10⁰ Bq/g/μC were produced with incident deuteron energies of 9 and 12 MeV, respectively.     

Measurement of neutron total cross-section and resonance parameters of xenon

We measured the neutron total cross-sections of natural xenon in the neutron energy region from 0.1 to 40 eV by using the time-of-flight method at the Pohang neutron facility, which consists of an electron linear accelerator, a water-cooled tantalum target with a water moderator, and a 12-m long time-of-flight path. A ⁶Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm was used as a neutron detector. Notch filters composed of Co, In, Cd were used to estimate the background level and to calculate the neutron flight path length. The present measurement was compared with the existing experimental and the evaluated data. The resonance parameters of Xe isotopes were obtained from the transmission ratio by using the SAMMY code and were compared with other previous results.     

Plutonium measurements on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA)

Plutonium isotopes have been recently added to the list of radionuclides that can be measured with the new generation of compact AMS facilities. In this paper we present first experimental results concerning the development of the plutonium AMS technique at 680 kV on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) in Sevilla, Spain. This is the first compact AMS machine designed and manufactured by High Voltage Engineering Europa. As we demonstrate, the obtained backgrounds for ^239,240Pu, of about 10⁶ atoms, and the ²³⁹Pu/²³⁸U mass suppression factor, in the range of 10⁻⁹, compare to the ones achieved on other AMS facilities. With the measurement of reference materials provided by the International Atomic Energy Agency (IAEA-375, IAEA-Soil-6, IAEA-381) and samples already studied on the 600 kV compact ETH/PSI AMS system at Zürich, we show that the CNA system can be perfectly used for the routine measurement of plutonium isotopes at environmental levels.     

Isotopic separation of silicon atoms by atomic mirror

The isotopic separation of silicon by an atomic mirror has been demonstrated numerically, by which each silicon isotope encounters a different dipole force depending on the frequency detuning due to the different frequencies resonant to the silicon isotopes. It was found that the atomic mirror, which comprised a wave guide structure designed for the enhancement of evanescent waves at 252 nm, allows us to discriminate ²⁹Si from the other isotopes.     

Obtaining molybdenum-99 in the IRT-T research reactor using resonance neutrons

The yield of ⁹⁹Mo from the ⁹⁸Mo(n,γ)⁹⁹Mo reaction significantly depends of the energy spectrum of the neutron flux. It is well known that the cross-section for this reaction is about 130 mb, whereas the resonance integral of the reaction is 6.9 b. The aim of this work was to investigate the conditions that let to increase ⁹⁹Mo yield from the targets with natural and enriched isotope composition under irradiation by resonance neutrons at the IRT-T research reactor.The calculations of integrated cross-sections of all Mo isotopes in the region of the ⁹⁸Mo resonances showed that screening in the target with natural isotope composition by other isotopes is relatively small. So the ⁹⁸Mo in the natural mixture can be activated by resonance neutrons approximately in the same manner as pure ⁹⁸Mo.Experimental measurements of the ⁹⁸Mo(n,γ) effective cross-section using the MoO₃ sample with natural and enriched composition in the reactor channels with the beryllium moderator with the thickness of 20 up to 90 mm showed that the effective cross-sections in these channels reach the value of 700 mb. The contribution of the epithermal neutrons into the ⁹⁸Mo activity was 68% for the enriched targets and 78% for natural molybdenum, respectively.At that channel it is possible to produce ⁹⁹Mo with specific activity up to 3.4 Cu/g with samples of natural isotope composition and up to 15 Cu/g with enriched samples on the base of reactors with neutron flux of (1.7 × 10¹⁴ n/(cm² s)). Such ⁹⁹Mo specific activity is enough not only to realize extraction technologies production of ^99mTc, but to manufacture sorption generators of ^99mTc without wastes.     

A Time of Flight-Energy spectrometer for stopping power measurements in Heavy Ion-ERD analysis at iThemba LABS

The quantitative analysis of thin layers using Heavy Ion-Elastic Recoil Detection (HI-ERD) can be reliably performed if the stopping powers of the probing ions and recoils in a given target matrix are known accurately. Unfortunately for many projectile/target combinations experimental data is limited and where available, deviations of up to 50% between experiment and theory have been reported. This presentation describes the assembly of a Time of Flight-Energy (ToF-E) detector system developed for HI-ERD analysis and adapted for stopping power measurements at iThemba LABS. First results from energy loss measurements of 0.1-0.5 MeV/nucleon ²⁸Si and ⁸⁴Kr ions in ZrO₂ are presented and compared with predictions of the widely used SRIM2003 (Stopping Range of Ions in Matter).     

Rutherford backscattering (RBS) with lithium ions

⁷Li ions at 4.5 MeV and ⁴He ions at 1.5 MeV were scattered at 170° from the same target under the same conditions. The distribution of elements as a function of distance from the surface is more clearly revealed with ⁷Li than ⁴He. We have rewritten our RBS analysis program to handle all ion beams and to take into account the normal isotopic abundance of the target elements. With 4.5 MeV ⁷Li ions the minority isotopes can not be ignored, even with silicon. At present, the accuracy for determining elemental depth profiles with ⁷Li is limited by the accuracy of Li ion stopping powers.