Investigation of alpha particle induced reactions on thulium

Alpha particle induced reactions on the target element thulium were investigated up to 75 MeV, using foil-stack activation technique and Ge(Li) gamma ray spectroscopy method. Excitation functions for eight reactions of the type¹⁶⁹Tm(α, xn),x=1 − 4;¹⁶⁹Tm(α, pxn),x=3; and¹⁶⁹Tm(α, αxn),x=1, 2, 4 were investigated. Of these, four reactions¹⁶⁹Tm(α, p3n),¹⁶⁹Tm(α, αn),¹⁶⁹Tm(α, α2n)¹⁶⁹Tm(α, α4n), were studied for the first time and in the remaining four reactions, some 19 new energy-point cross-sections were measured for the first time. The experimental cross-sections were compared with the predictions of pre-equilibrium hybrid model, as well as the more recent index model, using the initial excition number,n ₀=4 (4p0h). Both the models show better agreement in respect of (α, xnyp) type of reactions. However they are equally bad for (α, αxn) type of reactions which involve theα-particle in the exit channels, and for which some direct reaction contributions are indicated.     

Analysis of the excitation functions of (α, xnyp) reactions on natural copper

Excitation functions of the reactions⁶³Cu[(α, n), (α, 2n)+(α, pn)] and⁶⁵Cu[(α, n), (α, 2n), (α, 3n), (α, 4n)+(α, p3n)] were investigated up to 50 MeV using stacked foil activation technique and Ge(Li) gamma ray spectroscopy method. Since natural copper used as the target has two odd-mass stable isotopes of abundance,⁶³Cu(69.17%) and⁶⁵Cu(30.83%), their activation in some cases, gives the same residual nucleus through different reaction channels but with very different threshold energies. In such cases, the individual reaction cross sections are separated with the ratio of theoretical cross sections. The experimental results were compared with the predictions of preequilibrium hybrid model of Blann. A general agreement was found in all reactions using initial exciton numbern ₀=4(4p0h) and also preequilibrium fraction depends on the incident particle energy.     

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.     

Measurement and analysis of alpha-induced reactions on Ta,Ag and Co

Excitation functions for the reaction¹⁸¹Ta (α,xn)^185−x Re,^107,109Ag (α, ypxn) and⁵⁹Co (α, ypxn) were obtained from measurements of residual activity of stacked foils from threshold to 60 MeV. The excitation functions for the production of¹⁸¹Re,¹⁸²Re,¹⁸³Re,¹⁸⁴Re,¹⁰⁵Ag,¹¹¹In,⁵⁴Mn,⁵⁶Co,⁵⁸Co, and⁶⁰Co, are being presented. The experimental data are compared with calculations considering equilibrium as well as pre-equilibrium reactions according to the hybrid model of Blann. High energy part of the excitation functions is dominated by the pre-equilibrium reaction mechanism. Calculations were done using a priori calculational method of Overlaid Alice Code of Blann. Most of the excitation functions in the energy range mentioned above could be very well fitted with the hybrid model calculation for exciton numbern=4 withn _p=2 andn _n=2. The overall agreement with theory is good. Certain discrepancies, however, indicate the necessity to revise the hybrid model with respect to emission of complex particles.     

α-Particle induced reactions on rhodium

Excitation functions for¹⁰³Rh (α,xn);x = 1–4 and¹⁰³Rh (α,αxn);x = 1–3 reactions were measured up to 50 MeV bombarding energy using stacked foil activation technique and high purity germanium (HPGe) γ-ray spectroscopy method. The experimental results were compared with calculations considering equilibrium as well as pre-equilibrium hybrid model of Blann (ALICE/90). It is found that the initial exciton configurationn ₀ = 4(4p0h) gives fairly good agreements for (α,xn) reactions. There seems to be indication of direct inelastic scattering effects in (α,αxn)-type of reactions.     

Measurement and analysis of the excitation functions for alpha particle induced reactions on niobium

Stacked foil activation technique and Ge(Li) gamma ray spectroscopy have been used for determining the excitation functions up to 45 MeV, of six reactions⁹³Nb[(α, n), (α, 2n) ^m , (α, 2n) ^g , (α, 3n), (α,p3n) and (α, αn)]. Excitation functions were also calculated theoretically by means of the hybrid model with and without the inclusion of pre-equilibrium emission of particles. A general agreement was found in (α, xn) type of reactions.     

Measurement of neutron-induced activation cross-sections using spallation source at JINR and neutronic validation of the Dubna code

A beam of 1 GeV proton coming from Dubna Nuclotron colliding with a lead target surrounded by 6 cm paraffin produces spallation neutrons. A Th-foil was kept on lead target (neutron spallation source) in a direct stream of neutrons for activation and other samples of ¹⁹⁷Au, ²⁰⁹Bi, ⁵⁹Co, ¹¹⁵In and ¹⁸¹Ta were irradiated by moderated beam of neutrons passing through 6 cm paraffin moderator. The gamma spectra of irradiated samples were analyzed using gamma spectrometry and DEIMOS software to measure the neutron cross-section. For this purpose neutron fluence at the positions of samples is also estimated using PREPRO software. The results of cross-sections for reactions ²³²Th(n, γ), ²³²Th(n, 2n), ¹⁹⁷Au(n, γ), ¹⁹⁷Au(n, α), ¹⁹⁷Au(n, xn), ⁵⁹Co(n, α), ⁵⁹Co(n, xn), ¹⁸¹Ta(n, γ) and ¹⁸¹Ta(n, xn) are given in this paper. Neutronics validation of the Dubna Cascade Code is also done using cross-section data by other experiments.      

Measurement and analysis of alpha particle induced reactions on gold

Stacked foil activation technique and Ge(Li) gamma ray spectroscopy have been used for the measurement of excitation functions of¹⁹⁷Au(α,xn) (x=1−3),¹⁹⁷Au(α,2pn) and¹⁹⁷Au(α,αn) reactions up to 50 MeV. The experimental cross-sections were compared with the predictions of pre-equilibrium hybrid model, as well as with the more recent index model. A general agreement was found in all reactions using initial exciton numbern ₀=4(4p0h) except for¹⁹⁷Au(α,n) reaction, where index model gives fairly good agreement withn ₀=5(5p0h).     

Hybrid model analysis of the excitation function for alpha induced reaction on121Sb and123Sb

Excitation functions for the reactions¹²¹Sb(α, xn)^125−x I,¹²³Sb(α, xn)^127−x I and¹²¹Sb(α, p3n)¹²¹Te were obtained from the measurements of the residual activity of stacked foils of antimony trioxide evaporated on Al backings from threshold to 60 MeV. The excitation functions for the production of¹²¹I,¹²³I,¹²⁴I,¹²⁶I and¹²¹Te are presented. The experimental data are compared with calculations considering equilibrium as well as pre-equilibrium reaction mechanism according to the hybrid model of Blann. The high energy part of the excitation functions are dominated by the pre-equilibrium reaction mechanism. Calculations were done using a priori calculational method of Overlaid Alice Code of Blann. Most of the excitation functions in the energy range mentioned above could very well be fitted with the hybrid model calculation for exciton numbern=4 withn _n=2 andn _p=2. The overall agreement with the theory is good. Certain discrepancies for example¹²¹Sb(α, p3n)¹²¹Te excitation function, indicate that the production mechanism is different from the one presumed for the calculation.     

A study of pre-equilibrium emission of neutrons in <Superscript>93</Superscript>Nb(α, xn) reactions

With a view to study the pre-equilibrium emission mechanism in α-induced reactions the excitation functions for ⁹³Nb(α, n)^96m Tc, ⁹³Nb(α, n)⁹⁶Tc, ⁹³Nb(α, 2n)^95m Tc, ⁹³Nb(α, 2n)^95g Tc and ⁹³Nb(α, 3n)⁹⁴Tc reactions have been measured in the energy range threshold to ≈ 10MeV/nucleon using the activation technique. The measured excitation functions have also been compared with theoretical predictions based on the semi-classical code, which takes into account compound nucleus as well as pre-equilibrium emission. The analysis of the data indicates significant contribution from pre-equilibrium emission at these energies particularly in the high-energy tail portion of EFs. The effect of the variation of the parameters used in the code has been studied. The isomeric cross-section ratios have also been measured. It has been observed that the pre-equilibrium fraction increases rapidly with the increase in α-particle bombarding energy.     

Measurement and analysis of excitation functions for alpha induced reactions on iodine and cesium

The excitation functions for the reactions¹²⁷I(α, 2n)¹²⁹Cs,¹²⁷I(α, 4n)¹²⁷Cs,¹³³Cs(α, 2n)¹³⁵La and¹³³Cs(α, 4n)¹³³La have been measured up to ≈50 MeVα-particle energy using the stacked foil activation technique. Measured excitation functions are compared with pre-equilibrium geometry dependent hybrid model calculations. It has been found that theoretical calculations using an initial exciton numbern ₀=4 (2p+2n+0h) give good agreement with experimental excitation functions.     

Determination of <Superscript>68</Superscript>Ga production parameters by different reactions using ALICE and TALYS codes

Gallium-68 (T _1/2 = 68 min, I _β+ = 89%) is an important positron-emitting radionuclide for positron emission tomography and used in nuclear medicine for diagnosing tumours. This study gives a suitable reaction to produce ⁶⁸Ga. Gallium-68 excitation function via ⁶⁸Zn(p, n)⁶⁸Ga, ⁶⁸Zn(d, 2n)⁶⁸Ga, ⁷⁰Zn(p, 3n)⁶⁸Ga and ⁶⁵Cu(α, n)⁶⁸Ga reactions were calculated by ALICE-91 and TALYS-1.0 codes. The calculated excitation function of ⁶⁸Zn(p, n)⁶⁸Ga reaction was compared with the reported measurement and evaluations. Requisite thickness of the targets was obtained by SRIM code for each reaction. The ⁶⁸Ga production yield was evaluated using excitation function and stopping power.      

Four quasi-particle level at 2256 keV in182Re

In-beam nuclear spectroscopic studies of¹⁸²Re, following the reaction¹⁸¹Ta(α, 3n)¹⁸²Re have been made using gamma-ray and internal conversion electron techniques.K-conversion coefficients for several transitions have been measured and the multi-polarities of the various transitions assigned. In particular, the spin and parity of the four-quasi-particle isomeric level at 2256 keV were determined to be 16⁻. Theg-factor of this level has been measured to beg = 0·32 ± 0·05. On the basis of theg-factor and the decay pattern of this level, a configuration {v9/2⁺ [624↑]v7/2⁻ [514↓]v7/2⁻ [503↑]π9/2⁻ [514↑]} _k ^x = 16⁻ has been assigned to this level. The nature of the retardation of the gamma transitions deexciting this level is discussed. It is argued that the measured retardation factors can be explained if the nucleus has a triaxial shape.     

Lattice Dislocations in a 1-Dimensional Model

The spectral properties of the Schr?dinger operator T(t)=−d ²/dx ²+q(x,t) in L ²(ℝ) are studied, where the potential q is defined by q=p(x+t), x>0, and q=p(x), x<0; p is a 1-periodic potential and t∈ℝ is the dislocation parameter. For each t the absolutely continuous spectrum σ _ac (T(t))=σ _ac (T(0)) consists of intervals, which are separated by the gaps γ _n (T(t))=γ _n (T(0))=(α _n ⁻,α _n ⁺), n≥1. We prove: in each gap γ _n ≠?, n≥ 1 there exist two unique “states” (an eigenvalue and a resonance) λ _n ^±(t) of the dislocation operator, such that λ _n ^±(0)=α _n ^± and the point λ _n ^±(t) runs clockwise around the gap γ _n changing the energy sheet whenever it hits α _n ^±, making n/2 complete revolutions in unit time. On the first sheet λ _n ^±(t) is an eigenvalue and on the second sheet λ _n ^±(t) is a resonance. In general, these motions are not monotonic. There exists a unique state λ₀(t) in the basic gap γ₀(T(t))=γ₀(T(0))=(−∞ ,α₀ ⁺). The asymptotics of λ _n ^±(t) as n→∞ is determined. Received: 5 April 1999 / Accepted: 3 March 2000     

Microwave and far-infra-red dielectric absorption inn-alkanes

Summary The power absorption coefficient (α) forn-hexane (C₆),n-heptane (C₇),n-dodecane (C₁₂) andn-tetradecane (C₁₄), benzene and cyclohexane has been studied at 20 °C, in the wave number range 20 to 300 cm⁻¹ within an accuracy of 2%. The power absorption measurements onn-alkanes (C₅−C₁₄ except for C₈, C₁₁ and C₁₃) are discussed in conjunction with the dielectric loss in the frequency range (9⋎140) GHz. The results indicate that, unlike other nonpolar liquids,n-alkanes show a broad dielectric dispersion in the microwave frequency range followed by a second dispersion in the far infra-red. The results are interpreted in terms of a dipole moment of 0.09 D in alkanes. A possible explanation for the origin of the dipole moment is discussed. A reference is also made to measurements of the dielectric loss and the Stark effect on the lower-series alkanes in the gas phase.

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Riassunto Si è studiato il coefficiente di assorbimento di potenza (α) pern-esano (C₆),n-eptano (C₇),n-dodecano (C₁₂) en-tetradecano (C₁₄), nel benzene e nel cicloensano a 20 °C nell’intervallo di numeri d’onda da 20 a 300 cm⁻¹ con un’accuratezza del 2%. Le misurazioni dell’assorbimento di potenza neglin-alcani (C₅−C₁₄, eccetto che per C₈, C₁₁ e C₁₃) sono discusse insieme alla perdita dielettrica nell’intervallo di frequenza (9⋎140) GHz. I risultati indicano che, diversamente da altri liquidi non polari, glin-alcani mostrano un’ampia dispersione dielettrica nell’intervallo di frequenza delle microonde, seguíta da una seconda dispersione nel lontano infrarosso. I risultati sono interpretati in termini di un momento dipolare di 0.09 D negli alcani. Si discute una possibile spiegazione dell’origine del momento dipolare. Si fa anche un accenno alle misurazioni della perdita dielettrica e dell’effetto di Stark sugli alcani della serie inferiore in fase gassosa.

     

The Variational Principle for a Class of Asymptotically Abelian C*-Algebras

Let (A,α) be a C^*-dynamical system. We introduce the notion of pressure P _α(H) of the automorphism α at a self-adjoint operator H∈A. Then we consider the class of AF-systems satisfying the following condition: there exists a dense α-invariant *-subalgebra ? of A such that for all pairs a,b∈? the C^*-algebra they generate is finite dimensional, and there is p=p(a,b)∈ℕ such that [α ^j (a),b]= 0 for |j|≥p. For systems in this class we prove the variational principle, i.e. show that P _α(H) is the supremum of the quantities h _φ(α) −φ(H), where h _φ(α) is the Connes–Narnhofer–Thirring dynamical entropy of α with respect to the α-invariant state φ. If H∈A, and P _α(H) is finite, we show that any state on which the supremum is attained is a KMS-state with respect to a one-parameter automorphism group naturally associated with H. In particular, Voiculescu's topological entropy is equal to the supremum of h _φ(α), and any state of finite maximal entropy is a trace. Received: 19 April 2000 / Accepted: 14 June 2000     

Near-barrier fusion reactions induced by α on197Au,193Ir,191Ir,185Re,181Ta,121Sb and69Ga nuclei

Fusion-evaporation cross-sections for the α-induced reactions upon¹⁹⁷Au,¹⁹³Ir,¹⁹¹Ir,¹⁸⁵Re,¹⁸¹Ta,¹²¹Sb and⁶⁹Ga nuclei at bombarding energies near the Coulomb barrier have been measured by off-line observation of the γ-rays emitted in the radioactive decay of the residual nuclei using stacked foil technique. The total fusion cross-section for the systems have been compared with simple statistical model calculations using the code ALICE/91 as well as with the coupled channel calculations that include the β₂ and ν₄ slatic deformations and dynamic couplings of the vibrational/rotational states of the target and the projet tile using the code CCDEF.     

Possibility of implementing a nondirect experiment aimed at determining the cross section for nucleon interaction with an excited nucleus

With the aid of experimental data on the level densities in the even-odd tungsten isotopes ^183,185,187W as determined in the respective (n, 2ρ) reactions, the ratio of the cross section for the interaction of an excited nucleus with a neutron to its counterpart calculated on the basis of the optical model of the nucleus was obtained from the spectra of evaporated neutrons in the reaction ¹⁸¹Ta(p, n)¹⁸¹W. A significant local increase in this ratio is qualitatively interpreted as that which is due to the possible increase in the penetrability of the nuclear surface for nucleon reaction products because of the effect of phonon-type excitations. Sources of possible nonstandard systematic errors in experimentally determining the level density in an arbitrary nucleus at energies below the nucleon binding energy B _n are analyzed. The extraction of information from the distribution of the intensities of cascades belonging to the nucleon and photon type is simulated. The resulting information may be highly reliable if use is made of the entire body of experimental data on the nucleus under analysis.