Search Results (19 total)

A sample containing 6.3×10¹⁴ nuclei of the 16⁺ isomer of ¹⁷⁸Hf having a half life of 31 years and excitation energy of 2.446 MeV was irradiated with x-ray pulses derived from a device operated at 15 mA to produce bremsstrahlung radiation with an end point energy set to be 63 keV. Gamma spectra of the isomeric target were taken with two independent Ge detectors. Intensities of the 213.4 keV (4⁺→2⁺) and 325.5 keV (6⁺→4⁺) transitions in the ground state band of ¹⁷⁸Hf were found to increase when irradiated. The largest enhancement was 1.6±0.3% measured in the 213.4 keV transition. Such an accelerated decay of the ¹⁷⁸Hf isomer is consistent with an integrated cross section exceeding 2.2×10^-22 cm² keV if the resonant absorption takes place below 20 keV as indicated by the use of selective absorption filters in the irradiating beam.

A Reply to the Comment by Silviu Olariu and Agata Olariu. Also D. P. McNabb, et al. also P. von Neumann-Cosel and A. Richter.

A sample of 6.3×10¹⁴ nuclei of the 4-quasiparticle isomer of ¹⁷⁸Hf having a half-life of 31 yr and excitation energy of 2.446 MeV was irradiated with x-ray pulses from a device typically used in dental medicine. It was operated at 15 mA to produce bremsstrahlung radiation with an end point energy set to be 70 or 90 keV. Spectra of the isomeric target were taken with a high purity Ge detector. Intensities of selected transitions in the normal decay cascade of the ¹⁷⁸Hf isomer were found to increase by about 4%. Such an accelerated decay is consistent with an integrated cross section of 1×10^-21 cm ² keV for the resonant absorption of x rays to induce gamma decay.

It is shown that our method of ‘‘successive perturbations’’ is closely related to the method of analytic continuation of perturbation series via reexpansion about a shifted origin [Silverman, Sudhindra, and Olbrich, Phys. Rev. A 30, 1554 (1984)], in that the former procedure arises from successive incremental applications of the latter. The theory is illustrated with a second-order application to the anharmonic oscillator.

We consider the concept of gauge-independent parity I_Π with respect to a certain plane Π of a square-integrable state Ψ, I_Π=FΨ^*(Q)Ψ(Q₊)exp[(iq /ħc) F_Q+^Q A⋅ds]d³Q, where the points Q and Q₊ are symmetric relative to Π. We show that the parity of state I_Π can be expressed as an average of the nonintegrable phase factor R_PQQ+TP=exp/me punds his head into the wall[(iq/ħc)F_PQQ+TP (cφ dt-A⋅dr)] over all loops PQQ₊TP connecting the points P,T in the field-free incidence region to the points Q,Q₊ in the field-free region where I_Π is being observed. In the case of the scattering by an infinite magnetic string carrying the flux F, we find that the parity of a state which was symmetric in the incidence region becomes cos(qF/ħc) in the observing region behind the string. The measurement of the parity of a free state is discussed.

This paper is a review of the problem of the observable action of enclosed electromagnetic fluxes on the quantum-mechanical state of charged particles, known as the Aharonov-Bohm effect. The authors first describe the quantum effects of the fluxes in the quasiclassical approximation, and discuss their relation with basic quantum-mechanical principles. Then they examine the influence of modeling assumptions on the predicted effects of enclosed fluxes. They analyze the experiments demonstrating the reality of the quantum effects of electromagnetic and gravitational fluxes, and finally discuss the physical significance of these quantum effects, comparing the current interpretation, based on the concept of a nonintegrable phase factor, with alternative approaches.

In this paper we consider the scattering of charged particles by magnetic strings for several configurations of the incident state and of the enclosed flux. We analyze the scattering of a plane wave by an infinite magnetic string and show that the scattering contribution arises from flux-dependent phase shifts followed by quantum diffusion. Then we report that the amplitude for the scattering of a wave packet of finite extension by a magnetic string becomes exponentially small as the width of the packet goes to zero. At the same time the phase at the center of the incident wave packet is progressively shifted by an amount proportional to the enclosed flux, as predicted on the basis of the quasiclassical approximation. We consider further the scattering by a pair of parallel strings carrying opposite fluxes and show that the probability pattern is affected by the magnetic fluxes mainly in the vicinity of a finite surface spanning the two strings. We finally discuss the relevance for the Aharonov-Bohm effect of the multivalued gauge transformations which eliminate the potentials from the field-free region. The results reported in this paper confirm the reality of the quantum effects of remote fluxes, and prove that the state of the electromagnetic field is not completely determined by the local knowledge of the field strengths.

Recent calculations have shown that multiphoton transitions in a nucleus that involve a γ-ray photon could be induced by intense fields of optical radiation. The effect of such transitions was predicted to be a shift of the γ-ray energy by the addition or subtraction of the energy of the optical photon. A literature search has shown that such optical sidebands have been observed in certain instances on the ⁵⁷Fe Mössbauer line at 14 keV and not satisfactorily explained by conventional models. This work reports the formulation and evaluation of the multiphoton model for the values of experimental variables appropriate to those experiments. Using no fitted parameters, remarkable agreement was obtained between the predictions of the multiphoton theory and the previously reported results.

NUCLEAR REACTIONS Calculated B for multiphoton absorption; γ plus radio-frequency photons.

This paper reports the examination of the general problem of multiphoton processes which might occur at the nuclear level when one of the photons belongs to an intense radiation field of optical frequencies. Transition probabilities are reported which relate the rates for γ-ray transitions induced by the optical radiation to the Breit-Wigner cross sections for the absorption or emission of single γ photons. In emission the induced processes of both anti-Stokes Raman scattering and two-photon emission are considered and both are found to lead to the generation of tunable γ radiation. Transition probabilities are calculated to be large enough to support the production of usable spectral sources that could be tuned over a range of γ-ray energies approximately 10³ times greater than can be realized by conventional Mössbauer techniques. Analogous possibilities for the detection of nuclear reactions induced by intense optical fields also have been evaluated in this work.

NUCLEAR REACTIONS Calculated B for multiphoton absorption; γ+ optical photon from laser.

A two-photon technique is reported for the measurement of relative cross sections for the photolysis of simple molecules into particular product channels. In a demonstration of this method, two independently tunable dye lasers were used to dissociate molecules of Cs₂ sequentially for wavelengths in the range 445 to 505 nm, and then to excite the resulting products to determine the relative cross sections for the photolysis of the Cs₂ molecules into each of the energetically possible product states.

Recent concerns about the gauge dependence of transition amplitudes describing the changes of state of quantized systems interacting with electromagnetic radiations have led to the development of invariant schemes for the expression of the time-dependent quantities involved. It is shown in this work that such schemes are not uniquely specified by the requirement for gauge invariance. Moreover, they simply exchange a dependence upon the gauge of the electromagnetic potentials for a previously unrecognized dependence upon the representation of basis states describing initial and final states of the process that is equivalent to a dependence upon gauge. However, general expressions are developed in this work which show that resonant transitions which correspond to physically observable processes involving the pure or mixed absorption and emission of N photons are gauge invariant regardless of the multipolarity of the transition moments and independent of the representation of the basis states of the system. A preferred representation is introduced which is shown to accommodate the physical effects generally encountered.

This paper reports an analytic estimate developed from a multiphoton model of the cross section for the absorption of a γ-ray photon when the nuclear recoil is compensated by the simultaneous absorption of an optical photon from the radiation field of a high power laser. Tabulated data show 22 experimentally accessible nuclear transitions which might be expected to demonstrate this effect.

NUCLEAR REACTIONS Calculated B(E1) for multiphoton absorption; γ+optical photon from laser.

This paper reports an analytic estimate of the cross section for the absorption of a γ-ray photon when the nuclear recoil is compensated by the simultaneous absorption of an optical photon from the radiation field of a high-power laser. A multiphonon transition model is developed and cross sections of the order of 1 fm² are obtained when nearly resonant intermediate states of nuclear excitation are assumed to lie within a few tens of eV of the transition energy.

The multiphoton excitation of rubidium has been investigated over the 4600-6500-Å wavelength region with a tunable dye-laser source having a linewidth better than 0.1 Å and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the 5²S ground state of atomic rubidium to higher-lying n²D levels for values of n ranging from 9 to 34 and to n ²S levels for values of n from 11 through 20. The fine-structure intervals of the n ²D levels for n=9 toc 13 were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of Rb₂. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate pπ(³Σ_u1) and 0_g⁺ terms dissociating to give a 5P_{3 / 2} atom and resulting in the strong development of features corresponding to the 5²P_{3 / 2}→n²D_{3 / 2,5 / 2} part of the diffuse series, for n≤50 in absorption and the 5²P_{3 / 2}→n²S_{1 / 2} part of the sharp series. Components to 32²D and 30²S were recorded to a precision of 0.3 cm^-1 and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a 5²P_{1 / 2} atom were not observed in the wavelength interval available in this experiment.

Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 Å to measure the relative absorption probabilities for transitions of the cesium-principal-series (6²S-n²P_{1 / 2,3 / 2}) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of Cs₂*, were examined over a range of cesium-atom concentration varying from 1.2×10¹⁴ to 1.5×10¹⁵ cm^-3. From the corrected doublet line-strength ratios for 11≤n≤16, the segment of the curve of Fano X parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near 19²P.

The multiphoton excitation of cesium through potentially resonant continuum molecular states is investigated over the 6200-5000-Å wavelength region with a tunable dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. Conditions are established under which the lifetime of the resonant intermediate state against photoexcitation exceeds the lifetime against dissociation. Single- and double-photon resonances occur for the same wavelength and give absorption maxima corresponding to the line spectrum from the intermediate state following dissociation. These lines are modulated in amplitude as a function of wavelength by the more slowly varying absorption resonance from the initial to the intermediate continuum state. As a consequence, the resulting dispersion curve for two-photon absorption in cesium shows what appear to be resonant intermediate dπ(³Π_g) terms dissociating to give a 5²D atom and resulting in the strong development of features corresponding to the fundamental series (5²D→n²F) of atomic cesium in absorption. Components to n=50 were observed and recorded to a precision sufficient to determine the average quantum defect for F states to be 〈n-n^*〉=0.033 in the limit of large n. More complex structure is attributed to pπ(³Π_g) and pσ(¹Σ_u) terms dissociating to give a 6 ²P_{3 / 2} atom and resulting in the development of the n=8 to 32 components of the 6 ²P_{3 / 2}→n ²D_{3 / 2, 5 / 2} part of the diffuse series in absorption. Terms dissociating into 6 ²P_{1 / 2} were found only at higher photon energies (∼2.27 eV) corresponding to the n≥10 members of the 6 ²P_{1 / 2}→n ²D_{3 / 2} series.

The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-Å wavelength region with a tunable-dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 6²S_{1 / 2} level to resonant n²D and n²S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9 to 13 have been recorded for the 6²S_{1 / 2}-n²D_{5 / 2,3 / 2} series and from n=11 to 14 for the 6²S_{1 / 2}-n²S_{1 / 2} series.

This paper reports the observation of the two-photon ionization of molecular cesium as a continuous function of laser wavelength in the 6200-6600-Å region with a tunable dye laser having a 0.18-Cm^-1 linewidth. To the author's knowledge, it serves to illustrate the first extension to neutral molecular species of the important techniques of resonant multiphoton photodetachment spectroscopy. The resulting two-photon photoionization spectrum is intepreted in terms of the vibrational structure of the resonant intermediate molecular state.

Space-charge amplification of the direct and indirect ionization resulting from photoabsorption in alkali-metal vapor is shown to provide a useful complement to the conventional techniques of absorption spectroscopy. Because of its inherently greater sensitivity, this technique is particularly useful in spectroscopic applications involving weak transitions such as those to highly excited states or absorptive transitions from excited states of low population. This paper reports the use of this technique to extend prior measurements of the oscillator strengths of the (6²S→n ²P) series in Cs, the n ²D doublet separations, and the photoionization spectrum of Cs₂.