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 ¹³⁹La (n, γ) ¹⁴⁰La→β⁻. 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 ¹²C 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 ¹²⁷I and Cu will be presented.     

On the behavior of spallation neutrons from extended Pb targets plus moderator: A comparison between SSNTD measurements and calculation

Thick Pb targets of different lengths were irradiated by 1 GeV protons at the Nuclotron accelerator of the High Energy Laboratory, JINR, Dubna. To favor transmutation via (n,γ) reactions a paraffin moderator is used. Solid-state nuclear track detectors (SSNTDs) measured neutron distribution. A comparison of experimental results with calculation on the moderator surface is given.     

Neutron production in extended Cu-target irradiated with relativistic C-ions at Dubna, as studied with SSNTD and nuclear chemistry

An extended Cu-target was irradiated with 22 and 44 GeV carbon ions. The target was in contact with a (CH₂)_n-block for the moderation of secondary neutrons. Small holes in the moderator were filled with either lanthanium salts or uranium oxide. The reaction ¹³⁹La (n,γ) ¹⁴⁰La was studied via the decay of ¹⁴⁰La (40 h), and the reaction ²³⁸U (n, γ) ²³⁹U ²³⁹Np was studied via the decay of ²³⁹Np (2.3 d). In addition, a variety of solid state nuclear track detectors (SSNTD) were used. Results will be presented. The yields for the formation of (n, γ) products agree essentially with other experiments on extended targets carried out at the Synchrophasotron LHE, JINR (Dubna). To a first approximation, the breeding rate of (n, γ) products, as well as the specific track density, seen with several SSNTDs, doubles when the carbon energy is increased from 22 to 44 GeV. If, however, results at 44 GeV are compared in detail to those at 22 GeV, we observe an excess of (37 ± 9) % in the experimentally observed ²³⁹Np-breeding rate over theoretical estimations. Experiments using solid state nuclear track detectors are giving similar results. We also observed in the past such excess in the yield of other secondary particles in relativistic heavy ion interactions above a total energy of approximately 35–40 GeV.     

Experimental and Monte-Carlo studies of the spatial distribution of neutrons around extended Pb-spallation target

The spatial distribution of thermal and fast neutrons on the surface of a paraffin moderator surrounding a cylindrical lead target, irradiated with 1 GeV protons was studied. The lead target had 8 cm diameter and 20 cm length. The thickness of the paraffin around the target was 6 cm. The slow and fast neutron distributions were determined using LR 115 2B and CR-39 detectors via the ¹⁰B(n,) reaction and neutron induced nuclear recoils, respectively. The observed slow and fast neutron distributions on the surface of the paraffin were compared with Monte-Carlo simulations using the MCNPX-2.1.5 code.     

Neutron yields from massive lead and uranium targets irradiated with relativistic protons

Long-lived isotopes can be transmuted into stable or short-lived elements either by neutron captures or neutron induced fission. The need of a large excess of neutrons has led to the use of accelerator driven sources (ADS). A series of experiments were carried out at the Synchrophasotron/Nuclotron of the Joint Institute for Nuclear Research (JINR) Dubna, using protons of 1.0 GeV. Solid Lead and Uranium targets surrounded by paraffin moderator were irradiated. On the outer surface of the moderator a number of Solid State Track Detectors were placed to monitor neutron spatial distribution. The results showed that the maximum neutron production was reached within the range of one to two proton mean free paths in the target. Then decreasing neutron production follows the proton beam attenuation along the target. Moreover, the results showed both targets neutron production evolution along the target, to be the same. However, neutron flux per incident proton is depended on the target mass, which was found to be higher for the heavier target.     

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

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.     

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

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,)⁷Li reaction and ¹³⁹La radiochemical sensors. The obtained results were compared with Monte Carlo simulation using MCNP-4B2 code.     

Status of NICA complex at JINR

New scientific program is proposed at Joint Institute for Nuclear Research (JINR) in Dubna aimed a study of hot and dense baryonic matter in the wide energy region from 2 GeV/amu to √s _NN = 11 GeV, and investigation of nucleon spin structure with polarized protons and deuterons maximum energy in the c.m. 27 GeV (for protons). To realize this program the development of JINR accelerator facility in high energy physics has started. This facility is based on the existing superconducting synchrotron—Nuclotron. The program foresees both experiments at the beams extracted from the Nuclotron, and construction of ion collider—the Nuclotron-based Ion Collider fAcility (NICA).     

Intermediate energy proton scattering and nuclear transition densities in nickel isotopes

Using the high-resolution spectrometer SPES-I, the angular distribution of 1.047 GeV protons scattered from ⁵⁸Ni, ⁶⁰Ni, ⁶²Ni, ⁶⁴Ni have been measured in the angular range 4–18°, up to 4.8 MeV excitation energy. The data are analyzed within the framework of Glauber multiple scattering theory using a spin-dependent NN scattering amplitude. The parameters of the neutron density distributions have been determined by a χ² analysis of elastic scattering. The inelastic data corresponding to the first 2⁺ and 3⁻ states are analyzed with the collective model and an attempt has been made to extract information about the corresponding neutron transition densities.     

Measurements of neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets bombarded by 0.5 and 1.0 GeV protons

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:

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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

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.     

Spallation reactions in isotopically enriched targets

Yields of ⁴²K and ⁴³K have been measured in spallation reactions at 590 MeV and at 7 GeV in isotopically enriched targets from ⁵⁸Ni to ⁶⁸Zn. The results are correlated with other recent measurements in the development of a new empirical spallation-yield formula which takes into account the isotopic composition of the target. A practical example of the use of this formula is discussed.     

Polarization of protons from the Be(d, p)Be reaction

The polarization of protons from the ⁹Be(d, p)¹⁰Be reaction has been measured for deuteron energies between 1 and 6 MeV. Angular distributions of the polarization for the ground state proton group (Q=4.59 MeV) have been measured for laboratory angles 10° and 135° at 2.5, 4.0 and 5.5 MeV. The polarization at a laboratory angle of 30° was measured in 0.2 MeV steps between 1 and 6 MeV. In addition the polarization of protons leaving ¹⁰Be in its first excited state (Q=1.22 MeV) was measured between 10° and 90° for a deuteron bombarding energy of 5.5 MeV. Measurements were made by conventional double-scattering techniques using elastic scattering from helium at 45° as the analyser. The results show a strong energy dependence contrary to what one might expect on the basis of the lack of structure in the cross section. The maximum polarization observed at each energy is about 0.35.     

Detection, dosimetry and microdosimetry using high energy C beams

Samples of polyallyldiglycolcarbonate (PADC) track etch detectors (TED) were exposed to high energy ¹²C nuclei at the particle beam of the Dubna synchrophasotron. The energy of ¹²C nuclei varied between 0.1 and 1.5 GeV per amu.

At the low studied energies the linear energy transfer (LET) of these nuclei is higher than the detector threshold value. Then, the primary particle tracks are directly etched in the detector surface. The detection efficiency approaches to 100% at perpendicular incidence. Their LET has been established by means of standard authomatized procedure recently developed. The LET values found here are in good agreement with theoretical ones.

At 1.5 GeV per amu (LET 8.4 KeV μm⁻¹) the secondary particle tracks were evaluated in all the exposed detectors. The energy deposited by these particles was compared to the energy deposited through primary ionization losses. It was found out that its contribution to the total dose is relatively lower than for protons of comparable energies. In some of these samples even the tracks of the primary nuclei were observed. It follows that the detection threshold of the developed LET spectrometer should be below 10 keV μm⁻¹.     

The (He, t) reaction at intermediate energies : Spin-isospin multipole transitions

Data for the (³He, t) reaction at 900 MeV and 2 GeV on the targets ²⁶Mg, ⁴⁰Ca, ⁴⁸Ca, ⁵⁴Fe, ⁹⁰Zr and ²⁰⁸Pb are presented. A multipole decomposition for the data at 900 MeV has been made and the different distributions have been analysed. From the L = 0 cross section the Gamow-Teller strength distributions are extracted and compared with (p, n) data. The L = 1 and 2 distributions are analysed in a schematic model which describes the general systematics fairly well. The spectra at 2 GeV and Θ = 4° show for all targets a well-developed quasi-elastic peak. The A-dependence of the cross section is analysed in a simple model.     

Neutron Generation in an Extended Cu Target Irradiated with 22 and 44 GeV Carbon Ions (Preliminary Results)

An extended Cu target was centrally irradiated with 22 and 44 GeV carbon ions for about 11 and 15 hours, respectively. The upper side of the target was in contact with a paraffin block for the moderation of secondary neutrons. Small holes in the moderator were filled with lanthanum salts (LaCl₃7H₂O) where the neutrons induced the ¹³⁹La(n,γ) ¹⁴⁰La reaction. The β-decay of ¹⁴⁰La(40h) was followed using radiochemical methods. The yields for the formation of ¹⁴⁰La agree within large errors with other experiments on extended targets carried out at the LHE, JINR (Dubna). To a first approximation, the breeding rate of ¹⁴⁰La doubles when the ¹²C energy is increased from 22 to 44 GeV. If results at 44 GeV are compared to those at 22 GeV, there cannot be excluded an excess of (20 ± 7)% in the experimentally observed ¹⁴⁰La production over theoretical estimates. A definite conclusion in this respect must await further experimental and theoretical work.

These experiments are the beginning of a research program to develop a subcritical reactor with neutron fluxes sufficiently high to permit the TRANSMUTATION of long-lived radioactive waste into short-lived or stable nuclei. Such transmutation would considerably relieve the ecological problems related to the final deposition of radioactive waste.     

Some contributions from SSNTD towards nuclear science: from multifragmentation towards accelerator driven systems (ADS)

Three examples are chosen to show the importance of SSNTD as one of the essential tools in nuclear science:

(1) Multifragmentation into more than two heavy reaction products: Starting with the observation of three heavy reaction products in the interaction of relativistic protons or 414 MeV ⁴⁰Ar with actinides in the early 1960s, up to the observation of five heavy reaction products in the interaction of 2400 MeV ²³⁸U with uranium, SSNTD had a leading role in this research.

(2) In the search for superheavy elements (SHE: Z around 114): Many different techniques are used. However, SSNTD are exclusively decisive in the possible observation of SHE within the heavy element component of galactic cosmic rays.

(3) Accelerator driven systems: They are increasingly important in the discussion of energy producing nuclear power stations and in the corresponding ability to transmute long-lived poisonous radioactive materials (above all plutonium) into shorter lived fission fragments or stable nuclides. SSNTDs play an important role in the determination of the energy dependent neutron fluence in small volumes (≈cm³) or in the exact beam profile determinations of the primary proton beams.

This contribution ends with an outlook into possible future fields of physics research: With the advent of a new generation of relativistic heavy ion accelerators, such as the NUCLOTRON at the JINR in Dubna, Russia, and RHIC in Brookhaven in the United States, one can continue to study (and finally confirm or disprove) all phenomena mentioned already, as well as additional controversial phenomena, such as “enhanced nuclear cross-sections over short distances”, called colloquially “anomalons”. Again SSNDT can be used in at least a twofold manner as an important tool: (a) the enhanced neutron production with ¹²C ions or heavier ions in thick targets at energies above approximately 50 GeV and (b) the reduced “mean-free-path” of secondary fragments produced by the same heavy and energetic ions.     

Interactions of relativistic heavy ions in thick heavy element targets and some unresolved problems

Interactions of relativistic heavy ions with total energies above 30 GeV in thick Cu and Pb targets (≥ 2 cm) have been studied with various techniques. Radiochemical irradiation experiments using thick Cu targets, both in a compact form or as diluted “2π-Cu targets” have been carried out with several relativistic heavy ions, such as 44 GeV ¹²C (JINR, Dubna, Russia) and 72 GeV ⁴⁰Ar (LBL, Berkeley, USA). Neutron measuring experiments using thick targets irradiated with various relativistic heavy ions up to 44 GeV ¹²C have been performed at the JINR. In addition, the number of “black prongs” in nuclear interactions (due to protons with energies less than 30 MeV and emitted from the target-like interaction partner at rest) produced with 72 GeV ²²Ne ions in nuclear emulsion plates has been measured in the first nuclear interaction of the primary ²²Ne ion and in the following second nuclear interaction of the secondary heavy (Z > 1) ion. Some essential results have been obtained. (1) Spallation products produced by relativistic secondary fragments in interactions ([44 GeV ¹²C or 72 GeV ⁴⁰Ar] + Cu) within thick copper yield fewer products close to the target and many more products far away from the target as compared to primary beam interactions. This applies also to secondary particles emitted into large angles (Θ > 10°). (2) The neutron production of 44 GeV ¹²C within thick Cu and Pb targets is beyond the estimated yield as based on experiments with 12 GeV ¹²C. These rather independent experimental results cannot be understood within well-accepted nuclear reaction models. They appear to present unresolved problems. The text was submitted by the authors in English.     

The quasi-elastic charge exchange reaction pHe→ nFppp at intermediate energies

The basic characteristics of the quasi-elastic charge exchange reaction p³He→ n_Fppp are investigated using the 80 cm ITEP liquid-hydrogen bubble chamber, which is exposed to beams of ³He nuclei with momenta of 2.5 and 5 GeV/c (the kinetic energy of primary protons in the rest system of the nucleus are, respectively, T_p = 0.318 and 0.978 GeV). The experimental data are compared with the predictions of the Glauber-Sitenko multiple scattering theory and of the pole model, taking into account the interaction between spectator nucleons in the final state. In the mass spectrum of the 3p system at 3.05 GeV a clearly denned structure has been revealed, which is not described within the pole model. A possible resonance origin of the given structure is discussed.     

Projectile fragmentation of O and S at 3.65 GeV/n on several targets

We investigate the charge changing collisions for ¹⁶O and ³²S beams at 3.65 GeV/n on several targets, by using Solid State Nuclear Track Detectors (SSNTD). Track measurements were performed by an automatic measurement system. We determined the total charge changing cross sections and the elemental cross sections for the production of fragments of charge 9 ≤ Z ≤ 14. Comparison with theoretical models as well as with experimental data are given.