R. Brandt

Determination of energetic neutron spatial distribution using neutron induced nuclear recoil events

Abstract Neutron induced nuclear recoils were used to determine the spatial distribution of the weakly moderated spallation neutrons produced in the interaction of 1 GeV protons with lead and uranium–lead targets. CR39 plastic track detectors were used to record neutron-induced recoil tracks. The track density measurements were carried out using a fully automated optical microscope. The experimental results were compared with Monte Carlo simulations using MCNPX-2.1.5 code and an extension code that was written for this purpose. A good agreement was found between the experiment and calculations for normalised results. Applicability of the MCNPX-2.1.5 code for absolute recoil track density determination is discussed.

Transmutation experiments on 129I, 139La and 237Np using the Nuclotron accelerator

Abstract Transmutation experiments have been carried out using the NUCLOTRON, a new accelerator for relativistic particles. The experiments were carried out in the Veksler and Baldin Laboratory of High Energies of the Joint Institute for Nuclear Research in Dubna, Russian Federation. Earlier experiments using an 8 cm Ø Pb-target of 20 cm length and surrounded by 6 cm paraffin (GAMMA-2 target) have been continued to study the transmutation of 139La and the highly radiotoxic radwaste nuclides 129I and 237Np using spallation neutrons produced by relativistic protons with energies 0.5 GeV ≤ Ep ≤ 4.15 GeV. Results of previous experiments were complemented with additional results, thus yielding data systematics with small uncertainties over the probably commercially relevant energy range of 0.5 GeV ≤ Ep ≤ 1.0 GeV and also in the scientifically interesting range above 1 GeV. Moreover, neutron density distributions in irradiations with 0.65 GeV, 1 GeV and 3.7 GeV protons were determined on the surface of the paraffin moderator for two energy regimes of slow and intermediate/fast neutrons. A large set of high quality experimental data with small uncertainties has been generated that can serve as benchmark data for modelling purposes.

SSNTD and radiochemical studies on the transmutation of nuclei using relativistic ions

Extended targets were irradiated for transmutation studies with relativistic heavy ions. For this, a metal core was surrounded by a paraffin moderator. The metal is either copper or lead and it was irradiated with deuterium, alpha, or carbon beams of 1.5 or 3.7 GeV/u at the SYNCHROPHASOTRON, LHE, JINR, Dubna, Russia. During this irradiation copious amounts of secondary neutrons are produced and studied with SSNTD detectors and radiochemical sensors, for example: 139 La (n,γ) 140 La→ B- . The yield of reaction products allows an estimation of secondary neutron fluxes. The yields of all kinds of reactions produced with deuterium and alpha beams obey to some extent the law of limiting fragmentation, i.e. they show little influence on the energy and the kind of incoming particles. However, one observes with 44 GeV 12 C ions always enhanced nuclear cross-sections induced by secondary particles. This behavior could not be confirmed with theoretical estimations based on the Dubna Cascade Model in its Cascade Evaporation Model version (DCM-CEM). Finally, some results for transmutation studies on 127 I and Cu will be presented.

The etch-induction time (Tind) and other registration properties in CR-39 detectors for well-defined ions

Abstract The etch-induction time ( T ind ) in CR-39 for tracks of various ions, such as 4 He, 12 C, 86 Kr, 136 Xe, fission fragments, 208 Pb and 238 U has been measured. Three different kinds of CR-39 plastic detectors, made by Homalite, Pershore and Acrylic were used. The influence of the etching temperature on T ind has been measured. It is possible to differentiate fast light ions (84 MeV/amu 12 C), common alpha particles, and heavy ions (including fission fragments) by their T ind . Some applications to nuclear reaction studies are discussed.

Secondary neutron production from thick Pb target by light particle irradiation

Abstract Neutron multiplicities from spallation neutron sources were measured by Solid State Nuclear Track Detectors. Light particles as protons, deuterons and alphas in the GeV range were used on Pb targets. For neutron thermalization the targets were covered by 6 cm paraffin moderator. Neutron multiplicity distributions were studied inside and on the moderator surface. Comparison of SSNTDs results were made for thermal-epithermal neutrons with 139 La activation method as well as with Dubna DCM/CEM code. Discussion including previous 12 C results are given.

First experiments on transmutation studies of129I and237Np using relativistic protons of 3.7 GeV

First experiments on the transmutation of long-lived129I and237Np using relativistic protons of 3.7 GeV are described. Relativistic protons generate in extended Pb-targets substancial neutron fluences. These neutrons get moderated in paraffin and are used for transmutation as follows:129I(n,γ)130Iβ→ and237Np(n,γ)238Npβ→. The isotopes130I (T1/2-12.36 h) and238Np (T1/2=2.117 d) were identified radiochemically. One can estimate the transmutation cross-section (n,γ) in the given neutron field as σ(129I(n,γ))=(10±2)b and σ(237Np(n,γ))=(140±30)b The experiments were carried out in November 1996 at the Synchrophasotron, LHE, Dubna, Russia. The investigation has been performed at the Laboratory of High Energies, JINR, Dubna.

Nuclear track microfilters: gas separation ability and β-radiation stability

Nuclear track microfilters were produced by irradiating the polyimide foil UPILEX with highly energetic heavy ions and subsequent etching with sodium hypochlorite solution. Pore diameters and shape were investigated by scanning electron microscopy and a replica technique where the pores were electrochemically filled with metal. The results show that it is possible to form straight cylindrical pores with diameters in the range of 100 nm. The flow rate of air through the microfilters was determined. It turned out to be a linear function of the pore diameter at this pore size. With increasing pressure, the flow rate increases. At a certain level, the increase becomes non-linear, almost exponential. It can be assumed that this is due to stress-induced deformation of the polymer with an increase in the pore diameter. With the model system carbon monoxide and carbon dioxide the microfilters were tested for their ability for gas separation. Gas chromatographic measurements show that carbon monoxide penetrates faster through the microfilter, rendering gas separation possible. Possible applications of such microfilters for separating gases include radiation environment such as fusion reactors. For testing the β-radiation stability, the microfilters were stored in tritium gas. The microfilters appeared to be radiation resistant. After the storage time, no changes in the microfilter properties were found.

Transmutation of 239 Pu with spallation neutrons

Incineration studies of plutonium were carried out at the Synchrophasotron of the Joint Institute for Nuclear Research (JINR), Dubna, using proton beams with energies of 0.53 GeV and 1.0 GeV. Solid lead targets (8 cm in diameter and 20 cm long) were surrounded with 6 cm thick paraffin as neutron moderator and then irradiated. The transmutation of 239 Pu and the associated production of fission products 91 Sr, 92 Sr, 97 Zr, 99 Mo, 103 Ru, 105 Ru, 129 Sb, 132 Te, 133 I, 135 I and 143 Ce were studied in the present work. The plutonium samples (each 449 mg) were placed on the outer surface of moderator. For 1.0 GeV proton beam, the fission rate of 239 Pu is 0.0032 atoms per proton in one gram plutonium samples, for 0.53 GeV proton, this value is 0.0022. The experimental uncertainty is about 15%. The experiments are compared to two theoretical model calculations with moderate success, using the Dubna Cascade Model (CEM) and the LAHET code. The practical incineration rate of 239 Pu is very high. For example: if one uses 10 mA, 1 GeV proton beams under the same (fictive) experimental conditions, the incineration rate of 239 Pu via fission is 3 mg out of the 449 mg sample per day. For 0.53 GeV protons the corresponding rate is 2 mg per day.

Application of activation methods on the Dubna experimental transmutation set-ups.

High spallation neutron fluxes were produced by irradiating massive heavy targets with proton beams in the GeV range. The experiments were performed at the Dubna High Energy Laboratory using the nuclotron accelerator. Two different experimental set-ups were used to produce neutron spectra convenient for transmutation of radioactive waste by (n,x) reactions. By a theoretical analysis neutron spectra can be reproduced from activation measurements. Thermal-epithermal and fast-super-fast neutron fluxes were estimated using the 197Au, 238U (n,gamma) and (n,2n) reactions, respectively. Depleted uranium transmutation rates were also studied in both experiments.

Fission track autoradiography as a means to investigate plants for their contamination with natural and technogenic uranium

Fission track autoradiography has been applied to investigate plants for their uranium content. Uranium was either assimilated by the plant from uranium-bearing soil or deposited at the leaves by uranium-bearing dust particles.