Positron lines from subcritical heavy ion-atom collisions

Positron lines were observed in heavy ion-atom collisions at bombarding energies close to the Coulomb barrier in subcritical systems with the sum of the atomic numbers of the colliding nucleiZ _u =Z ₁+Z ₂ being smaller thanZ _u =172. Each collision system, studied,²⁰⁸Pb +²⁰⁸Pb(Z_u=164),²³⁸U+¹⁸¹Ta(Z_u=165), and²³⁸Au(Z_u=171), exhibits the emission of two positron lines withZ _u -independent c.m. energies of ～ 258 keV and ～ 340 keV, and with widths of about 30 keV, superimposed on continuous positron spectra from nuclear pair decay and pair emission induced by the time changing Coulomb field of the collision. The production cross section of thee ⁺-lines rises with a high power ofZ _u (ocZ _u ²²), which is comparable to theZ _u ²⁰-dependence for the collision induced positrons.     

Study of high energetic electrons emitted during heavy ion collisions

Double differential cross sectionsd ² σ/dΩdE were measured for high energetic electrons emitted in heavy ion collisions. Electrons were detected in the energy range of 60 keV-500 keV for various target projectile combinations 66≦Z _u =Z _t +Z _p ≦145 and projectile velocities between 7 % and 10 % of the speed of light. Clear evidence was found that these electrons stem from the united atom formed during the collision. Slope and height of the spectra are discussed with respect to the momentum distribution of strongly bound states (i.e. theL-shell) at momenta far above the mean value. In addition for the systems S, Ni, Br→Pb electrons were detected in coincidence withK x-rays of Pb. By this method the contribution of theK-shell of the combined system to the total spectrum could be separated. Binding energies of theK-shell were estimated by a slope comparison between the coincident and single spectrum. The resulting values are close to the united atom limit.     

Measurements ofK-shell ionization with132Xe and208Pb projectiles in the energy range of 1.4–5.9 MeV/u

Projectile and targetK-shell ionization cross sections induced by 3.6-, 4.7-, and 5.9 MeV/u¹³²Xe ions and 1.4-, 3.6-, 4.7-, and 5.9 MeV/u²⁰⁸Pb ions from the UNILAC in thin solid targets between C and U are measured. The cross sections are discussed in terms of the molecular model of innershell vacancy production in heavy ion-atom collisions. The sharing of 2p _1/2σ vacancies between theK shells of the two collision partners in these very heavy ion-atom collisions is found to deviate from the Meyerhof-Demkov formula forR≦10^?2. The measured ionization cross sections are compared with theoretical calculations for 1sσ and 2p _1/2σ excitation cross sections. AZ _UA=Z₁+Z ₂ dependence is found independent ofZ ₁/Z ₂. Outer-shell vacancy configurations measured in these close encounters are reported.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive⁴He and⁴H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u²³⁸U with²³⁸U and^?197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For²³⁸U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For²³⁸+²³⁸U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In²³⁸U+¹⁹⁷Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.     

Spectroscopy of excited states in212Po,210Pb,and213At employing18O induced few-nucleon transfer reactions

γ-ray spectroscopic techniques have been applied to measure properties of excited states (E ^*<2MeV) in²¹⁰Pb,²¹²Po, and²¹³At after populating these neutron rich (N=128) nuclei via¹⁸O induced few-nucleon transfer reactions on²⁰⁸Pb and²⁰⁹Bi targets. In²¹²Po an isomeric state is located atE ^*=1,477 keV with a halflife oft _1/2=14.7±0.3 ns. This state is interpreted to be the 8⁺ yrast level which decays to the ground state via the measured $$8^ + \left( {121.1 keV} \right)6^ + \left( {223.3 keV} \right)4^ + \left( {405.1 keV} \right)2^ + \left( {727.8 keV} \right)0^ + $$ γ-cascade.α-decay (E _α≈10.2 MeV) of the isomer is also observed. In²¹⁰Pb the time spectra for members of the 4⁺ (297.8keV) 2⁺ (799.4keV)0⁺ γ-cascade show a delayed component with a halflife oft _1/2=152±13 ns which is attributed to the known 8⁺ yrast state atE ^*=1.27 MeV. For²¹³At first results on theγ-decay of excited states are presented.     

The new nuclide230Pu

In the heavy-ion complete fusion reaction²⁰⁸Pb+²⁶Mg at a beam energy of 135 MeV the new nuclide²³⁰Pu was produced. The measuredα-decay energy was found to be E_α=7050±15 keV. The new isotope was identified after in-flight separation with the kinematic separator VASSILISSA, followed by its implantation into a silicon surface-barrier detector and the observation of the genetic relationships of subsequentα-decays. The half-life of²²⁶U was measured more accurately.     

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( E _γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed. Received: 27 April 2000 / Accepted: 14 September 2000     

K-vacancy production by 3.6-, 4.75-, and 5.9-MeV/amu238U Ions

Projectile (Z ₁) and target (Z ₂)K-vacancy production cross sections induced by 3.6-, 4.75-, and 5.9-MeV/amu²³⁸U ions in selected targets between C and U are reported. The cross sections are discussed within the context of the molecular model of inner-shell vacancy production in heavy-ion-atom collisions. The measured 1sσ and 2pσ excitation cross sections are compared with semiempirical formulas, and with the perturbed-stationary-states calculations of Betz et al. The dependence of these cross sections onZ _u =Z₁+Z ₂ agrees well with the Betz calculations. However, the measured cross sections are factors of 2 to 4 (1sσ) and 4 to 5 (2pσ) larger than the theoretical values.     

The new isotopes223, 224U

In the heavy-ion complete fusion reaction²⁰⁸Pb+²⁰Ne the new isotopes^223,224U were produced. These nuclei were identified after in-flight separation with the kinematic separator VASSILISSA, followed by their implantation into a passivated ion implanted silicon detector and the observation of the genetic relationships of subsequent α-decays.²²³U was found to decay withE _α=(8780±40) keV and T_1/2=18 _?5 ⁺¹⁰ μs. For²²⁴U the α-line atE _α=(8470±15) keV and T_1/2=0.7 _?0.2 ^+0.5 ms was observed.     

Resonant processes causing photon multiplication in CaSO4:Tb3+

The emission spectra of single and polyterbium centers have been measured at the excitation of CaSO₄:Tb³⁺ phosphors with different charge compensators (Na⁺, calcium vacancies, etc.) by 3.8–35 eV photons or 5 and 300 keV electrons at 6–300 K. The possible mechanisms providing quantum yield above unity for green (⁵D₄ → ⁷F_J) and blue emission (⁵D₃ → ⁷F_J) of Tb³⁺ at the direct intracenter excitation, excitation of oxyanions or creation of hot (nonrelaxed) electrons and holes have been discussed. On the basis of thermally stimulated luminescence at 6–600 K, the peculiarities of the hopping diffusion of relaxed electrons and holes and their tentative low-temperature self-trapping have been considered.     

Scintillation response of Y3Al5O12:Pr3+ single crystal scintillators

In this paper we present the photoluminescence (PL) and scintillation response of Pr³⁺-doped Y₃Al₅O₁₂ (YAG:Pr) single crystals grown by the Czochralski method with different Pr³⁺ concentrations of 0.16, 0.33, and 0.65 mol%. PL spectra and decay curves were measured for both the fast 5d → 4f and slow 4f → 4f emissions. The PL decay times were evaluated which evidence concentration quenching especially in ¹D₂ → ³H₄ emission for the highest Pr³⁺ concentration. Light yield (LY) of 15,600 photons per MeV and energy resolution of 6.4% at 662 keV γ-rays were obtained with the YAG:Pr (0.33%) crystal. The LY non-proportionality and energy resolution versus γ-ray energy were measured and the intrinsic resolution was calculated. A good proportionality of the LY was found within 7% over the energy range from 1274.5 keV down to 32 keV. The estimated photofraction in the pulse height spectra of 320 and 662 keV γ-rays was also determined and compared with the theoretical one calculated using WinXCom program.     

Effect of hyperfine interaction on the luminiscence of Pb2+ and Bi3+ centers in alkaline-earth chalcogenides

Emission spectra of MgS, CaO, CaS, CaSe and SrS phosphors activated with a Pb²⁺ or a Bi³⁺ ion are measured at 4.2 K. By the analysis of the phonon structure of the ³A_1u → ¹A₁g emission band, it is found that the hyperfine interaction provokes the zero-phonon line on the emission band for the alkaline-earth chalcogenide phosphors activated with a Pb²⁺ or a Bi³⁺ ion.     

Nuclear bremsstrahlung and electron-positron pair creation in heavy-ion collisions

Nuclear bremsstrahlung is investigated as a background effect in the X-ray-spectroscopy of superheavy electronic molecules. The intensity ratio between electric dipole and quadrupole radiation as function of frequency is given. The total cross section for electron-positron pair creation due to conversion of bremsstrahlungs-photons was found to beσ=3.8·10^?8b for the ₅₄ ¹³² Xe- ₉₂ ²³⁸ U system andσ=5.8·10^?8b for the ₉₂ ²³⁸ U? ₉₂ ²³⁸ U system at the Coulomb barrier. This demonstrates that bremsstrahlung pair creation can be neglected compared with the induced positron production in overcritical quasimolecules.     

Half-life and α-ray energy of146Sm

Up-to-date α-decay data of the double-even,N=84 isotones are presented, including the presently remeasured values for¹⁴⁶Sm. For this nuclide a half-life,T _1/2 =1.031 (45) ×10⁸ y, and α-ray energy, E_α=2455(4) keV, were measured. The significance of these data for the subshell closure atZ=64 observed in α-decay is discussed.     

Decay of194Os andL 1 subshell yields atZ=77

The relative intensities of x rays and gammas emitted in the beta decay of¹⁹⁴Os are measured. No evidence for the feeding of a level at 83 keV in¹⁹⁴Ir is found. An upper limit of 1.7×10^?4 was set for theK shell internal ionization probability in the beta decay to the ground state of¹⁹⁴Ir. TheL-conversion coefficient of 43 keV transition is found to be 12.1 and the transition is mainly ofM1 type with an admixture of 1.3%E2 type. TheL ₁ subshell yields atZ=77 are determined to beω ₁=0.16±0.04,f ₁₂=0.11±0.04 andf ₁₃=0.37±0.03.     

On the possible existence of superheavy elements in monazite

Proton-induced x-ray studies of monazite grains from different occurrences show no evidence for primordial superheavy elements. It is demonstrated that the recently reported evidence for superheavy elements in monazite inclusions of biotite mica showing giant halo formation is not significant: the observed differences in the x-ray spectra of normal and giant halo inclusions are due to different backgrounds in the respective spectra. Moreover, it is shown that the 27.2 keV line tentatively interpreted as theL _α1x-ray of elementZ=126 can be entirely attributed to the reaction¹⁴⁰Ce(p,n γ)¹⁴⁰Pr.     

The electron capture decay of125I and145Pm

The productP _K ω _K of theK-capture probability and theK-fluorescence yieldω _K was measured in the EC decay of 17.7 y¹⁴⁵Pm and 60.14d¹²⁵I by means ofK x-ray-γ-ray coincidences with semiconductor detectors. Accepting a best value ofω _K =0.920 forZ=60 from the literature, values ofP _K =0.606±0.025 and 0.554±0.025 for decay of¹⁴⁵Pm to the 67.2 and 72.4 keV levels in¹⁴⁵Nd, respectively, were determined, from which the application of EC theory givesQ _EC=153±4 keV to the ground state. TheP _K ω _K ratio to the 67.2 and 72.4 keV levels also was determined independently to be 1.11±0.01, which serves as a good check on theγ-ray-gatedP _K values. Theγ-ray energies were determined more accurately to be 72.4±0.1 and 67.2±0.1 keV, together with a ratio of their relative intensities,I _72.4/I _67.2=3.35±0.09. In the decay of¹²⁵I, a value ofP _K ω _K =0.699±0.030 was measured, and by using an independent determination ofQ _EC=178±2 keV to the ground-state from the literature, a value ofP _K=0.801 is calculated from allowed EC theory, from whichω _K =0.873±0.017 atZ=52 is obtained. This value is in good agreement with theoretical values and with a value of 0.875±0.028 predicted by the best fit curve of Bambyneket al.     

Measurement of the astrophysical S factor for dd interaction at ultralow deuteron-collision energies using the inverse Z pinch

This paper is devoted to measurement of the astrophysical S factor and cross sections of the d + d → ³He + n reaction at ultralow deuteron-collision energies. Formation of the flow of the accelerated deuterons incident on the CD₂ solid-state target was made within the scheme of the inverse Z pinch. The liner in the initial state was a hollow supersonic deuterium jet of radius of 15 mm and length of 20 mm. The experiment was carried out at the pulsed high-current accelerator (I=950 kA, τ=80 ns) of the Institute of High-Current Electronics (Tomsk, Russia). Measurement of the deuteron energy distribution was performed through an analysis of the time distributions of the intensity of the liner radiation (H_α and H_β lines) generated during the liner radial movement from the axis. Recording of this radiation was carried out by optical detectors placed along the direction of the liner moving from its axis. The measured value of the astrophysical S factor for the dd reaction at the average deuteron collision energy E _coll=3.69 keV was equal to S(E _coll=3.69 keV)=58.2±18.1 keV b. The dd-reaction cross section calculated using the found value of the S factor and known representation of the reaction cross section as the product of the barrier factor and the astrophysical S factor was σ _dd ⁿ (E _coll=3.69 keV)=(1.33±0.41)×10^?30 cm².     

Molecular-orbital X-rays from 1 GeV 208Pb + 208Pb and 209Bi + 209Bi collisions

We have measured the spectra of continuum X-rays above the characteristic K lines for 4.5 to 4.8 MeV/N ²⁰⁸₈₂Pb → ²⁰⁸₈₂Pb, Pb → Bi, Bi → Pb and Bi → Bi collisions. Above ≈400 keV X-ray energy the spectral shape and intensity agree roughly with calculations of Kirsch et al. for the 1sσ molecular-orbital (MO) X-ray spectrum from Pb-Pb. Deviations from the theory below ≈400 keV suggest transitions to other MO's.     

Representations and inequalities for Ising model Ursell functions

We describe and investigate representations for the Ursell functionu _n of a family ofn random variables {σ _i}. The representations involve independent but identically distributed copies of the family. We apply one of these representations in the case that the random variables are spins of a finite ferromagnetic Ising model with quadratic Hamiltonian to show that (?1) ^n/2+1 u _n(σ ₁, ...,σ _n) ≧ 0 forn=2, 4, and 6 by proving the stronger statement \(( - 1 )^{\frac{n}{2} + 1} \frac{{\partial ^m }}{{\partial J_{i1j1} \cdots \partial J_{imjm} }}Z^{\frac{n}{2}} u_n \left| {_{J = 0} } \right. \geqq {}^\backprime 0\) forn=2, 4, and 6, theJ _ij being coupling constants in the Hamiltonian andZ the partition function. For generaln we combine this result with various reductions to show that sufficiently simple derivatives of (?1) ^n/2+1 Z ^n/2u_n, evaluated at zero coupling, are nonnegative. In particular, we conclude that (?1) ^n/2+1 u _n ≧ 0 if all couplings are nonzero and the inverse temperature β is sufficiently small or sufficiently large, though this result is not uniform in the ordern or the system size. In an appendix we give a simple proof of recent inequalities which boundn-spin expectations by sums of products of simpler expectations.