Cylindrical cumulation of fast ions in a ring focus of a high-power subpicosecond laser

A new method of cylindrical cumulation of fast ions undergoing ponderomotive acceleration at the focus of a high-power subpicosecond laser is proposed. When a laser beam is focused in a preionized gas at a ring focus, radial acceleration of ions by the ponderomotive force occurs. The ions accelerated from the inner side of the ring form a cylindrical shock wave converging toward the axis. As the shock wave cumulates, the ion density increases rapidly and the ion-ion collision probability increases along with it. A numerical simulation for a ~100 TW subpicosecond laser pulse predicts the generation of up to 200 keV ions and up to 100-fold volume compression of the plasma in a cylinder ~1 μm in diameter. The lifetime of the dense plasma filament over the length of the laser caustic is several picoseconds. It is suggested that laser cumulation of ions be used for the production of a bright and compact subpicosecond source of fast neutrons, media for x-and γ-ray lasers, and multiply-charged ions and for the initiation of nuclear reactions. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 20–25 (10 January 1999)     

Powerful thermonuclear neutron source based on laser excitation of hydrothermal dissipation in a volume-structured medium

An efficient method is proposed for generating thermonuclear neutrons by irradiating with a laser pulse a volume-structured material of subcritical density, consisting of a series of thin layers of condensed matter separated by interlayers of low-density matter (or a vacuum gap). The plasma ions are heated up to thermonuclear temperatures much higher than the electron temperature by hydrothermal dissipation of the energy of the laser radiation, as a wave of thermal explosions of the layers propagates along the laser beam axis, followed by collisions of plasma counterflows with conversion of the kinetic energy into thermal energy of ions. Different variants of the targets and experimental conditions are discussed in order to demonstrate the proposed method of neutron generation. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 521–526 (25 October 1997)     

Analysis of formation of primary radiation-induced point defects in graphite under action of secondary neutrons and π<Superscript>−</Superscript>-mesons generated in various channels of nuclear reactions upon graphite irradiation by 450-GeV protons

A theoretical model is constructed and the rates of formation are calculated for primary radiation-induced point defects in graphite by secondary neutrons and π⁻-mesons generated in various nuclear reaction channels upon irradiation of graphite by a 450-GeV proton beam. Calculations are performed using the ENDF-VII nuclear data base for energies of secondary particles lower than 150 MeV. It is shown that neutrons produce the highest generation rate of point radiation-induced defects in graphite (the rate of formation of defects under the action of neutrons is an order of magnitude higher than the corresponding value for π⁻-mesons).     

Initiation of the (<Emphasis Type="Italic">γ, n</Emphasis>) and (<Emphasis Type="Italic">p,n</Emphasis>) nuclear reactions in the picosecond laser plasma at a laser intensity of 2 × 10<Superscript>18</Superscript> W/cm<Superscript>2</Superscript>

The experimental results on the initiation of various (γ, n) and (p, n) nuclear reactions in the picosecond laser plasma are presented. It is demonstrated that the following (γ, n) and (p, n) nuclear reactions can be initiated at a laser intensity of 2 × 10¹⁸ W/cm² with the threshold energies of γ-quanta ranging from 1.67 to 7.56 MeV and the threshold energies of protons ranging from 1.88 to 5 MeV: ⁹Be(γ, n)2α, ¹⁸¹Ta(γ, n)¹⁸⁰Ta, ⁷Li(p, n)⁷Be, ⁶³Cu(p, n)⁶³Zn, ⁴⁸Ti(p, n)⁴⁸V. The method based on the detection of fast neutrons using the 3He counters is employed for the measurement of the number of the initiated (γ, n) and (p, n) nuclear reactions. The measured yield of the (γ, n) and (p, n) nuclear reactions ranges from 5 × 10¹ to 10⁵ reactions per laser pulse.     

Neutron cross-sections above 20 MeV for design and modeling of accelerator driven systems

One of the outstanding new developments in the field of partitioning and transmutation (P&T) concerns accelerator-driven systems (ADS) which consist of a combination of a high-power, high-energy accelerator, a spallation target for neutron production and a sub-critical reactor core. The development of the commercial critical reactors of today motivated a large effort on nuclear data up to about 20 MeV, and presently several million data points can be found in various data libraries. At higher energies, data are scarce or even non-existent. With the development of nuclear techniques based on neutrons at higher energies, nowadays there is a need also for higher-energy nuclear data. To provide alternative to this lack of data, a wide program on neutron-induced data related to ADS for P&T is running at the 20–180 MeV neutron beam facility at ‘The Svedberg Laboratory’ (TSL), Uppsala. The programme encompasses studies of elastic scattering, inelastic neutron production, i.e., (n, xn′) reactions, light-ion production, fission and production of heavy residues. Recent results are presented and future program of development is outlined.      

A primer for electroweak induced low-energy nuclear reactions

Under special circumstances, electromagnetic and weak interactions can induce low-energy nuclear reactions to occur with observable rates for a variety of processes. A common element in all these applications is that the electromagnetic energy stored in many relatively slow-moving electrons can — under appropriate circumstances — be collectively transferred into fewer, much faster electrons with energies sufficient for the latter to combine with protons (or deuterons, if present) to produce neutrons via weak interactions. The produced neutrons can then initiate low-energy nuclear reactions through further nuclear transmutations. The aim of this paper is to extend and enlarge upon various examples analysed previously, present order of magnitude estimates for each and to illuminate a common unifying theme amongst all of them.     

3.5-μm high-resolution gas sensing employing a LiNbO3 QPM-DFG waveguide module

Diode laser technology coupled with a wavelength-conversion unit to produce mid-infrared narrow bandwidth laser light applicable to trace-gas detection and with the potential for high-resolution spectroscopy is described. Quasi-phase-matched difference-frequency generation (QPM-DFG) in a compact and fibre-coupled periodically poled lithium niobate (PPLN) waveguide module mixing 1063 and 1525-nm radiations has been adopted for generating 34 μW of 3.5-μm wavelength laser light. Optical detection methods, including sensitive wavelength modulation spectroscopy and a rapid wavelength chirp technique, have been employed with a single-pass cell to investigate methane and formaldehyde absorption profiles around 2855 cm⁻¹, as proof of principle experiments for high sensitivity and resolution spectroscopy on atmospherically important molecules.     

Secondary doses delivered to an anthropomorphic male phantom under prostate irradiation with proton and carbon ion beams

Secondary radiation exposure of patients undergoing radiation therapy with light ions is of great concern due to possible tissue damage and risk of induction of secondary cancers.Secondary particles such as neutrons, protons and heavier ions are produced when the primary ions interact through nuclear inelastic reactions with the beam-line components, and with the tissues of the patient.Evaluations of secondary doses delivered to an anthropomorphic male phantom under prostate irradiation with ¹H and ¹²C ion beams with energies 172 MeV and 330 MeV/u, respectively, have been performed with the Monte Carlo code SHIELD-HIT.Fluences of secondary particles with atomic mass A = 1–7 and energies up to 200–600 MeV/u are observed in organs even at larger distances (40–50 cm) from the irradiated volume. The secondary absorbed doses in selected organs are discussed taking into account the dose contribution from secondary neutrons, and the contribution from charged fragments that are not the products of neutron interactions. For ¹²C ion irradiation, a substantial contribution to the absorbed organ dose is due to charged fragments. This contribution decreases from 81% in the organs close to the irradiated volume to 35–40% in the organs at larger distances.     

Limits on spurious contributions to integrated crosssections for photoexcitation and de-excitation of isomers

The large values of integrated cross sections for the excitation and de-excitation of nuclear isomers in (γ, γ′) reactions provide strong encouragement for the feasibility of an optically pumped gamma-ray laser. For this reason, sources of possible spurious contamination of the measurements were carefully considered. This paper reviews an analysis of possible sources as well as experimental limits on contamination of the isomeric yields. The question of spurious contributions from (n, γ) or (n, n′) reactions was examined by estimating the level of thermal, epithermal and fast neutron fluxes based on possible source material in the accelerator environments. Such possibilities were severely reduced by the range of photon energies employed in the studies of 1.5-6 MeV. The expected fluxes were below levels necessary to produce significant isomeric yields in this energy range. Next, experiments were conducted in accordance with standard neutron activation-foil techniques to directly measure any fluxes of neutrons in the accelerator environments. Measurements for fast neutron fluxes were completely negative under even the most likely conditions with a 6 MeV medical linac. Measurable fluxes of thermal and epithermal neutrons were obtained. However, in typical cases the amount of isomeric activation due to “slow” neutrons was 1% of the total activation and 6% in the worst case based on measured fluxes and known values of cross sections. This revised version was published online in August 2006 with corrections to the Cover Date.     

Theoretical investigation of the bunching of an electron beam in relativistic power amplifiers

The modern approach to designing an injector for the driver of a two-beam accelerator is based on the use of a bunched electron beam. The results of simulation and comparison of the processes leading to bunching of a relativistic electron beam in a free-electron laser and in a traveling-wave tube at low electron beam energies are discussed. The simulation and existing experimental results for bunching of an electron beam in a free-electron laser are compared. Zh. Tekh. Fiz. 69, 98–102 (February 1999)     

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable ³⁶S beam on deuteron and ⁹Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 10⁶ particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on ²H and ⁹Be targets at REX-ISOLDE energies of 2.2 MeV ^. A are confirmed. Received: 9 October 2000 / Accepted: 28 January 2001     

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.     

Effect of charge exchange on the energy distribution of fast multiply charged ions propagating through matter

A technique for calculating, in the diffusion approximation, energy distributions of multiply-charged ions with an arbitrary number of charge states propagating through matter has been suggested. Examples of numerical solutions of kinetic equations taking into account charge exchange between ions and matter are given. A compact solution for the special case of two charge states has been found. The calculations are compared to experimental data. Zh. éksp. Teor. Fiz. 111, 2226–2236 (June 1997)     

Bohr–Weisskopf effect: influence of the distributed nuclear magnetization on hfs

Nuclear magnetic moments provide a sensitive test of nuclear wave functions, in particular those of neutrons, which are not readily obtainable from other nuclear data. These are taking added importance by recent proposals to study parity non-conservation (PNC) effects in alkali atoms in isotopic series. By taking ratios of the PNC effects in pairs of isotopes, uncertainties in the atomic wave functions are largely cancelled out at the cost of knowledge of the change in the neutron wave function. The Bohr–Weisskopf effect (B–W) in the hyperfine structure interaction of atoms measures the influence of the spatial distribution of the nuclear magnetization, and thereby provides an additional constraint on the determination of the neutron wave function. The added great importance of B–W in the determination of QED effects from the hfs in hydrogen-like ions of heavy elements, as measured recently at GSI, is noted. The B–W experiments require precision measurements of the hfs interactions and, independently, of the nuclear magnetic moments. A novel atomic beam magnetic resonance (ABMR) method, combining rf and laser excitation, has been developed for a systematic study and initially applied to stable isotopes. Difficulties in adapting the experiment to the ISOLDE radioactive ion beam, which have now been surmounted, are discussed. A first radioactive beam measurement for this study, the precision hfs of ¹²⁶Cs, has been obtained recently. The result is 3629.515(∼0.001) MHz. The ability of ABMR to determine with high precision nuclear magnetic moments in free atoms is a desideratum for the extraction of QED effects from the hfs of the hydrogen-like ions. We also point out manifestations of B–W in condensed matter and atomic physics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of H− ions with foil targets in the charge exchange system of a beam transport channel

The possibility of experimentally modeling the interaction of high energy H⁻ ions with foil targets using beams with more accessible lower energies that have the same dimensionless interaction parameter and similar current characteristics is pointed out. Results are presented from the first stage of a study of the beam-foil stripping of 2 and 7 MeV H⁻ ions. An analysis of the charge composition of the beam after a carbon foil serves as a basis for determining the corresponding cross sections for stripping of the ions and ionization of the product hydrogen atoms. The data from these and other beam-foil experiments are in good agreement with theoretical cross sections on carbon at different energies, as well with calculated values based on the superposition of experimental cross sections for gaseous carbon-containing targets. Zh. Tekh. Fiz. 68, 102–105 (August 1998) Deceased.     

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO₂ and Ar clusters with high-intensity femtosecond laser pulses with q _las≃10¹⁸ W/cm² are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters. Zh. éksp. Teor. Fiz. 115, 2051–2066 (June 1999)     

Determination of Atomic Masses and Nuclear Binding Energies via Neutron Induced Reactions

Information on atomic masses and nuclear binding energies can be extracted from (n,p) and (n, α) reactions with thermal and resonance neutrons. This is illustrated by means of several selected examples. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of nuclear spectroscopy at Saha Institute of Nuclear Physics

Experimental studies of nuclear excitations have been an important subject from the earliest days when the institute was established. The construction of 4 MeV proton cyclotron was mainly aimed to achieve this goal. Early experiments in nuclear spectroscopy were done with radioactive nuclei with the help of beta and gamma ray spectrometers. Small NaI(Tl) detectors were used for gamma-gamma coincidence, angular correlation and life time measurements. The excited states nuclear magnetic moments were measured in perturbed gamma-gamma angular correlation experiments. A high transmission magnetic beta ray spectrometer was used to measure internal conversion coefficients and beta-gamma coincidence studies. A large number of significant contributions were made during 1950–59 using these facilities. Proton beam in the cyclotron was made available in the late 1950’s and together with 14 MeV neutrons obtained from a C-W generator a large number of short-lived nuclei were investigated during 1960’s and 1970’s. The introduction of high resolution Ge gamma detectors and the improved electronics helped to extend the spectroscopic work which include on-line (p ₇ p′γ) and (p ₇ nγ) reaction studies. Nuclear spectroscopic studies entered a new phase in the 1980’s with the availability of 40–80 MeV alpha beam from the variable energy cyclotron at VECC, Calcutta. A number of experimental groups were formed in the institute to study nuclear level schemes with (α ₇ xnγ) reactions. Initially only two unsuppressed Ge detectors were used for coincidence studies. Later in 1989 five Ge detectors with a large six segmented NaI(Tl) multiplicitysum detector system were successfully used to select various channels in (α ₇ xnγ) reactions. From 1990 to date a variety of medium energy heavy ions were made available from the BARC-TIFR Pelletron and the Nuclear Science Centre Pelletron. The state of the art gamma detector arrays in these centres enabled the Saha Institute groups to undertake more sophisticated experiments. Front line nuclear spectroscopy works are now being done and new informations are obtained for a large number of nuclei over a wide mass range. Currently Saha Institute is building a multi-element gamma heavy ion neutron array detector (MEGHNAD), which will have six high efficiency clover Ge detector together with charged particle ball and other accessories. The system is expected to be usable in 2002 and will be used in experiments using high energy heavy ions from VECC.     

Heating of ultracold neutrons on beryllium surfaces

The temperature dependence of the loss factor for ultracold neutrons owing to heating at thermal energies on the surface of a beryllium sample is studied. The probability of heating ultracold neutrons is anomalously high throughout the entire measured temperature interval, but especially at low temperatures. Zh. éksp. Teor. Fiz. 114, 786–797 (September 1998)     

Quasi-elastic neutrino-nucleus reactions

The quasi-elastic contribution of the nuclear inclusive electron scattering model developed in [A. Gil, J. Nieves, and E. Oset: Nucl. Phys. A 627 (1997) 543] is extended to the study of electroweak charged current induced nuclear reactions at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for long-range nuclear (RPA) correlations, final state interaction and Coulomb corrections. RPA correlations are shown to play a crucial role in the whole range of neutrino energies, up to 500 MeV, studied in this work. Predictions for inclusive muon capture for different nuclei, and for the reactions ¹²C(ν _μ , μ ₋)X and ¹²C(ν _e, e⁻)X near threshold are also given. Presented by M. Valverde at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.