J.-S. Wan

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.

Monitor reactions in Al-foils for high energy proton beams bombarding a thick target

Abstract During the bombardment of U and Pb targets (each 21 cm thick) with protons of energy 1.0 and 1.5 GeV, the reactions 27Al(p, 3pn)24Na and nat.Cu(p, X)24Na were studied as monitor reactions. The influence of 27Al(n, α)24Na on the monitor reaction 27Al(p, 3pn)24Na was investigated experimentally and theoretically. In order to avoid the influence of (n, α) reaction, placing of the Al-monitors 35 cm upstream the massive targets is recommended. This method may be used in Accelerator-Driven-Transmutation (ADT) studies of long-lived nuclear waste, where one always uses a massive heavy element target irradiated with protons at about 1 GeV. The experiments were performed at the Laboratory of High Energies, JINR, Dubna, Russian Federation.

Transmutation studies using ssntd and radiochemistry and the associated production of secondary neutrons

Abstract Experiments using 1.5 GeV, 3.7 GeV and 7.4 GeV protons from the Synchrophasotron, LHE, JINR, Dubna, Russia, on extended Pb- and U-targets were carried out using SSNTD and radiochemical sensors for the study of secondary neutron fluences. We also carried out first transmulation studies on the long-lived radwaste nuclei 129I and 237Np. In addition, we carried out computer code simulation studies on these systems using LAHET and DCM/CEM codes. We have difficulties to understand rather large transmutation rates observed experimentally when they are compared with computer simulations. There seems to be a rather fundamental problem understanding the large transmutation rates as observed experimentally in Dubna and CERN, as compared to those theoretical computer simulations mentioned above.

Studies on neutron production in the interaction of 7.4 GeV protons with extended lead target

Abstract A cylindrical lead target of diameter 8 cm and length 20 cm was irradiated with 7.4 GeV protons along the axis of the cylinder. The lead target was surrounded with a paraffin layer of thickness 6 cm to moderate the neutrons produced in p + Pb reactions. The spatial distribution of the slow and fast neutrons on different surfaces of the moderator were determined using LR 115 2B detectors (through 10 B(n,α) 7 Li reactions) and CR39 detectors (through proton recoils) respectively. Such results can be valuable in the studies and design of Accelerator Driven Subcritical Nuclear Reactors and Nuclear Waste Incinerators.

Precision half-life measurement of 140La with Ge-detector

Abstract Half-life is one of the fundamental properties of radioactive nuclei, and the precision required for its numerous applications in modern physics sometimes approaches the level of 10−4–10−5. Most part of the T1/2 measurements performed up to now was made with proportional chambers, and the results were sometimes hardly reproducible within the error limits. Using Ge-detectors for that purpose brought some significant advantages but electronic unit related effects and spectra analysis procedures still remain the sources of the errors influencing the accuracy of the T1/2 attained. In this work, 140 La samples were obtained in the 139 La(n,γ) 140 La reaction, employing a microtron as a neutron source and the half-life measurements were performed with a HPGe-detector. Influencing factors such as photopeak and background shape, electronic circuitry dead time and deadtime variations during the measurements, as well as pulse pileup are studied altogether. Values of the 140 La T1/2=1.6808(18) d, λ=0.47749(20)×10−5, agreeing within the uncertainities with the most accurate evaluated ones (T1/2=1.6781(3) d, λ=0.47807(9)×10−5) [2] were obtained in two series of measurements.

Fission of Pb nuclei induced by 0.5, 1.0, 1.5, 3.7 and 7.4 GeV protons in the volume of massive U/Pb and Pb targets

Abstract Experiments with relativistic protons accelerated at the Synchrophasotron LHE, Dubna, with energies of 0.5, 1.0, 1.5, 3.7 and 7.5 GeV hitting massive targets of (nat. U)/Pb and Pb were carried out using SSNTD during the years 1996–1999. The beam profiles and intensities of both primary particles and fast secondary neutrons were measured inside the massive cylinder blocks of Pb and U by counting fission fragment tracks due to the induced fission of Pb nuclei. The beam diameter typically increases by 20–30% at the depth 10 and 20 cm . With increasing the energy of protons the number of secondary neutrons increases with the depth of the target. Further studies on beam profile measurements inside the massive heavy metal targets are discussed.

Determination of slow neutron flux and spatial distribution using LR-115 2B detectors and radiochemical sensors and comparison of the results with Monte Carlo simulations

Abstract The fluence and spatial distribution of slow neutrons on the surface of a cylindrical paraffin moderator, surrounding a Pu/Be neutron source was measured using LR-115 2B detectors via 10B(n,α)7Li reaction and 139La radiochemical sensors. The obtained results were compared with Monte Carlo simulation using MCNP-4B2 code.

SSNTD studies of lead nuclei fission induced by relativistic p, d, He and 12C projectiles inside massive Pb and U targets

Abstract A series of experiments was carried out with relativistic protons, deuterons, helium and carbon-12 projectiles accelerated at SYNCHOPHASOTRON LHE, Dubna which hit massive Pb and U targets. The beam profiles and intensities of both primary particles and secondary fast neutrons were measured using plastic SSNTD inside the massive cylinder blocks of Cu, Pb and U by counting of fission fragment tracks due to the induced fission of Pb nuclei. The beam diameter increases typically by 20–30% at the depth 10 and 20 cm. With increasing the energy of projectiles the number of secondary neutrons rises with the depth for protons, deutrons and helium ions. Nevertheless, for 12 C ions beams with changing the energy from 18 GeV to 44 GeV we first observe the effect of significant increase both the yield of secondary fast neutrons and the half-width of the beam. The observed enhanced yield of secondary fast neutrons confirms unusual behavior of nuclear interaction cross section of 44 GeV 12 C ions observed earlier in our studies with massive blocks of Cu, Pb and U.

Measurements of neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets bombarded by 0.5 and protons

Abstract Measurements have been carried out on neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets and the surrounding paraffin moderators bombarded by 0.533 and 1.0 GeV protons. CR-39 detectors were deployed on the surfaces of targets and moderators to record the neutrons produced in the targets. The measurements show that: (1) Neutron yield of Hg target is less than U/Pb and Pb targets on bombardment by protons. Neutron yield ratio of U/Pb and Pb to Hg target are 2.01±0.10 and 1.76±0.33 , respectively. This means that Hg target is less preferable in order to get more secondary neutrons. (2) Neutron yields along 20 cm thick targets decrease approximately linearly with the thickness through which the incident protons have penetrated. The lower the incident energy, the steeper the decrease of neutron yield. (3) Secondary neutron spectra for different incident energies are close to each other, while some differences have been observed.

Studies of neutron emission from relativistic nuclear interactions

Abstract Studies were carried out on the yields and spatial distributions of secondary neutrons produced in the relativistic nuclear interactions of 1.5 GeV to 14.4 GeV projectiles p, d and α-particles with targets Pb and U/Pb. CR-39 track detectors were used to measure the neutrons. It shows that: (1) Secondary neutrons are produced in the whole length of Pb or U targets having a thickness of 20 cm. The neutron intensities produced by proton bombardments are reduced along the proton beam direction in the targets. The higher the energy of protons, the lower the reduction rate of the neutrons. The reduction rate of neutrons in U target is higher than in Pb target for the same energy of protons. (2) The radial intensities of neutrons decrease as the distance increases from the target central line. (3) The neutron yield in U target by proton bombardments is ∼55% higher than in Pb target. (4) The ratio of neutron yield by 14.4 GeV α to 7.3 GeV d bombardment in Pb target is 1.74 ± 0.20.