Equivalent Photon Research Articles

Probing the parton densities of polarized photons at a linear e+e− collider

The present theoretical status of spin-dependent parton densities Δf γ ( x, Q 2) of circularly polarized photons is briefly reviewed. It is then demonstrated that measurements of the deep-inelastic spin asymmetry A 1 γ ≅ g 1 γ / F 1 γ and of di-jet rapidity distributions at a future linear e + e − collider appear to be particularly suited for a determination the spin-dependent photonic quark and gluon densities, respectively. Special emphasis is devoted to a comparison of the different sources of polarized photons at a linear collider: the equivalent photon approximation and backscattered laser (Compton) photons. It is shown that backscattered laser photons are highly favorable, even indispensable, for decent measurements of the Δf γ ( x, Q 2).

Production of low mass electron pairs due to the photon-photon mechanism in central collisions

We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the gamma-gamma mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at RHIC are also given.

Determination of equivalent photon fields through integrated 1D convolution kernels

The equivalent fields concept has been studied using a variation of the convolution dose calculation method. This allows a better understanding of this classic, widely-used concept and its relation to modern dose calculation techniques. Total scatter energy contribution as a function of field size was computed by radially integrating three-dimensional point interaction scatter kernels representing cobalt-60, 6 MV and 24 MV photon beam spectra and utilizing reciprocity. For arbitrarily shaped treatment fields, equivalent square or circular fields are chosen based on a conservation of energy approach. Associated depth dose curves are generated via convolution of primary fluence with pre-integrated scatter kernels. For the energy spectra used, the resulting equivalent circle to square relationships agree with the empirical equation recommended in the British Journal of Radiology Supplement 25 (BJR 25) (to within 2%). Tables converting rectangular fields to equivalent squares and circles have been generated and most (~63%) of the equivalent square (and circular) field sizes are within 0.5 cm of the BJR 25 tables. This agreement provides an explanation for the successful use of a single relationship for all therapeutic energies. Fields of different shape will be approximately equivalent only when the scatter kernels are similar in shape and total energy content. The equivalent field concept inevitably results in an optimal match of depth dose curves at a single depth only. These ideas are demonstrated for a highly elongated (2 × 30 cm2) field and its equivalent square field. The invalidity of the equivalent fields concept off-axis is demonstrated by examining predicted dose deposition for several points in and around a 15 × 15 cm2 cobalt-60 beam. Modern convolution techniques are shown to offer a unique approach to the classic equivalent fields method. The appropriate use of the BJR 25 equivalent fields tables is recommended for all photon energies.

Absolute oscillator strengths for hydrogen sulphide: II. Ionic photofragmentation and photoionization in the valence shell continuum regions (10–60 eV)

Absolute oscillator strengths (cross sections) of ionic photofragmentation and photoionization for hydrogen sulphide in the valence shell continuum regions have been determined using dipole (e,e+ion) coincidence spectroscopy (∼1 eV fwhm) at equivalent photon energies from the first ionization threshold to 60.2 eV. These have been determined from the recently published absolute photoabsorption oscillator strengths [Feng et al., Chem. Phys. 244 (1999) 127] together with the photoionization efficiency and ionic photofragmentation branching ratios obtained from time-of-flight mass spectra reported in the present work. Consideration of the presently reported data together with the photoelectron branching ratios for H 2S and ionization potentials obtained from previously reported photoelectron and dipole (e,2e) spectroscopies yields quantitative information on the breakdown pathways of H 2S following absorption of radiation in the VUV and soft X-ray regions. A new photofragmentation product (H 2 +) and the doubly charged molecular ion (H 2S 2+) from hydrogen sulphide have been found in the present work. Partial photoionization oscillator strengths for production of the four valence shell electronic states of H 2S + have also been derived from the molecular and dissociative partial photoionization oscillator strengths over the entire energy range up to 60.2 eV.

Bremsstrahlung from electrons and positrons in peripheral relativistic heavy-ion collisions

We study the spectrum of the bremsstrahlung photons coming from the electrons and positrons, which are produced in the strong electromagnetic fields present in peripheral relativistic heavy ion collisions. We compare different approaches, making use of the exact pair production cross section in heavy ion collisions as well as the double equivalent photon approximation.

Bethe-Heitler cross-section for very high photon energies and large muon scattering angles

The cross-section for the process $\gamma + A \to \mu^+ + \mu^- + X$ is studied where the photon energy is of the order of several hundreds of GeV and where one of the leptons is scattered to large angles. This is of practical importance for muon shielding calculations at future linear electron colliders. In addition to the photon pole contribution which was previously considered especially by Y.S.Tsai, we identify another component due to the muon pole (equivalent photon and equivalent muon approximation). This is discussed following the usual Bethe-Heitler formula. Then we give practically useful formulae for inclusive lepton (muon) production along with some numerical examples.

Single- and double-strand breaks in solid pBR322 DNA induced by ultrasoft X-rays at photon energies of 388, 435 and 573 eV.

We measured strand breaks of pBR322 plasmid DNA irradiated with ultrasoft X-rays using monochromatic synchrotron radiation as a light source. Three photon energies, 388, 435 and 573 eV, a value below and above the nitrogen K-edge and above the oxygen K-edge, respectively, were chosen for the irradiation experiments as they have an equivalent photon transmittance of the sample. Irradiated DNA was analyzed by agarose gel electrophoresis and the numbers of single- and double-strand breaks (ssb and dsb) were determined by measuring the band intensity on the gel after ethidium bromide staining. The action cross-sections for the ssb and dsb slightly increased with the photon energy. The ratio between 388 and 573 eV was about 1.5 for both forms of strand breaks. The absorbed energy required for a strand break was about 60 eV for ssb and 1 keV for dsb, less than one fifth of the values obtained previously in the 2 keV region. On the other hand, the absorbed energies per strand break, as well as the ratio of the action cross-section for the ssb to that for the dsb, were constant regardless of the photon energy used. The K-shell photoabsorption on carbon, nitrogen and oxygen atoms in the DNA molecule, followed by an Auger cascade, induced DNA strand breaks with a constant efficiency in terms of the absorbed energy. These results indicate that the strand breaks of the DNA molecule in the solid state are mainly caused by the photo- and Auger-electrons and the efficiency of the strand breaks little depends on the atoms ejecting these secondary electrons.

Production of boundμ+μ−systems in relativistic heavy ion collisions

Dimuonium (the bound system of two muons, $\mu^+\mu^-$-atom) has not been observed yet. In this paper we discuss the electromagnetic production of dimuonium at RHIC and LHC in relativistic heavy ion collisions. The production of parastates is analyzed in the equivalent photon approximation. For the treatment of orthostates, we develop a three photon formalism. We determine the production rates at RHIC and LHC with an accuracy of a few percent and discuss problems related to the observation of dimuonium.

Relativistic antihydrogen production

Antihydrogen has recently been produced in collisions of antiprotons with ions. While passing through the Coulomb field of a nucleus an antiproton will create an electron-positron pair. In rare cases the positron is bound by the antiproton and an antihydrogen atom produced. We calculate the production of relativistic antihydrogen atoms by bound-free pair production. The cross section is calculated in the semiclassical approximation (SCA), or equivalently in the plane wave Born approximation (PWBA) using exact Dirac-Coulomb wave functions. We compare our calculations to the equivalent photon approximation (EPA).

Single pseudoscalar meson production in diffractive $ep$ scattering

Exclusive pseudoscalar meson production in \(ep\) scattering at high energies is a direct probe for a possible “odderon” exchange in soft hadronic processes. Using a simple phenomenological ansatz for the odderon, we demonstrate how it can be separated from the contribution due to photon-photon fusion, and the relevant parameters be measured. Total cross sections and differential distributions are presented for \(\pi^0,\eta,\eta'\), and \(\eta_c\) production. Results are given from both a full calculation and one using the equivalent photon approximation. The accuracy of the latter is discussed.

Antihydrogen production and accuracy of the equivalent photon approximation

The production of antihydrogen in flight in antiproton-nucleus collisions is calculated theoretically in the Plane Wave Born Approximation (which is equivalent to the straight line semiclassical approximation). Antihydrogen has been produced in this way at LEAR/CERN and is presently studied at FERMILAB at various antiproton energies. Dirac wave functions for the leptons are used, taking first order (Z.alpha) corrections into account. Analytical results are obtained for differential cross-sections. Total cross sections are obtained by numerical integration. The dependence on the transverse momentum transfer is studied and the accuracy of the equivalent photon approximation and a recent variant by Munger, Brodsky,and Schmidt is discussed as a function of beam energy.

Quantitative studies of the photoabsorption (4.5–488 eV) and photoionization (9–59.5 eV) of methyl iodide using dipole electron impact techniques

Absolute oscillator strengths (cross sections) for the valence shell, I 4d, 4p, 4s and C 1s inner shell photoabsorption of CH 3I have been measured using low resolution (1 eV fwhm) dipole (e,e) spectroscopy in the equivalent photon energy range from 4.5 to 488 eV. In addition, high resolution (0.05–0.1 eV fwhm) dipole (e,e) spectroscopy has been used to study the absolute valence shell and I 4d inner shell photoabsorption spectra of CH 3I from 4 to 65 eV in more detail. The photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined for CH 3I by dipole (e,e+ion) spectroscopy from the first ionization threshold to 59.5 eV. The major pathways of the ionic dipole induced breakdown scheme have been predicted from these results. Estimates of the absolute electronic state partial oscillator strengths of CH 3I + from photoionization of CH 3I were derived from the dipole induced breakdown scheme and the molecular and dissociative photoionization partial oscillator strengths.

The Quantum Radiation Formulae of a New Radiation Mechanism in Curved Magnetic Fields

A new radiation mechanism for relativistic charged particles moving in curved magnetic fields has been put forward recently by Zhang & Cheng. This radiation mechanism generalizes all the classical results of ordinary synchrotron and curvature radiation and reveals the inherent linkage and unification between them. Since the magnitude of the pulsar magnetic field is generally ~1011-1013 G, the quantum effects must be taken into account. Applying the generalized method of equivalent photon scattering developed recently by Lieu & Axford, we present the quantum-limited synchrocurvature radiation formulae in this paper, which elaborates the essence of this new radiation mechanism in a deeper sense. The formulae we present could be extensively applied to various kinds of practical curved magnetic fields, including those of pulsars.

Single pseudoscalar meson production in diffractive

Exclusive pseudoscalar meson production in ep scattering at high energies is a direct probe for a possible "odderon" exchange in soft hadronic processes. Using a simple phenomenological ansatz for the odderon, we demonstrate how it can be separated from the contribution due to photon-photon fusion, and the relevant parameters be measured. Total cross sections and differential distributions are presented for pi0, eta, eta', and eta_c production. Results are given from both a full calculation and one using the equivalent photon approximation. The accuracy of the latter is discussed.

Relativistic antihydrogen production

We calculate the production of relativistic antihydrogen atoms by bound-free pair production. The cross section is calculated in the semiclassical approximation (SCA), or equivalently in the plane wave Born approximation (PWBA) using exact Dirac-Coulomb wave functions. We compare our calculations to the equivalent photon approximation (EPA).

Ionization of Helium in the Attosecond Equivalent Light Pulse of 1 GeV/NucleonU92+Projectiles

Single and double ionization of helium by 1GeV/nucleon U{sup 92+} impact was explored in a kinematically complete experiment. The relativistic ion generates a subattosecond (10{sup {minus}18} s ) superintense (I{gt}10{sup 19} W/cm {sup 2} ) electromagnetic pulse, which is interpreted as a field of equivalent photons (Weizs{umlt a}cker-Williams method). Cross sections, the emission characteristics of ions and electrons as well as momentum balances, are quantitatively discussed in terms of photoionization of the atom in this broadband, ultrashort virtual photon field. {copyright} {ital 1997} {ital The American Physical Society}

Measurement of X-ray intensity in the medical diagnostic range by a ferroelectric detector

A pyroelectric detector is described for measuring the energy fluence rate (intensity) of X-ray pulses with duration from 0.8 to 6 seconds in the diagnostic energy range from 50 to 140 kVp corresponding to an equivalent photon energy of 29 to 45 keV. Detector response to a radiation pulse conforms to theoretical prediction based upon thermal and electrical analysis and is linear with energy fluence rate of the radiation over the range studied. The detector is small, inexpensive, rugged, and simple to construct.

Chlorine and sodium perfusion and electrolyte balance in human tissue and tumours before and during neutron and photon radiotherapy

Radiotherapy with nuclear reactor fission neutrons was applied in 49 cases of pre-treated patients with superficial metastases or relapses from primary carcinoma. Measurements of the decay rates of the radiation-induced radioactivity of , and in the irradiated tissue resulted in values for the simultaneous local kinetics of chlorine and sodium, and in approximate data on the electrolyte masses. The electrolytes were present in non-exchangeable and exchangeable compartments of soft tissue. Exchange times of the intravascular to extravascular turnover and the frequencies of the exchange fractions were determined for a series of irradiations. The results have been interpreted in terms of the mean electrolyte exchange rates, of a standardized functional blood flow, and of the supply capacity of the vascular system. In the average of all cases, the regional perfusion was reduced by about 30% by irradiation up to 14 Gy (equivalent photon dose ) connected with an increase in the non-exchangeable fractions. After fractionated doses higher than 14 Gy, functional blood flow and supply capacity increased to 120%, and fixed electrolytes were removed from the irradiated tissue. Data on electrolyte kinetics and vascularity are compared with the literature.

Comment on “Quantum-mechanical equivalent-photon spectrum for heavy ion physics”

We critically discuss the recently developed quantum-mechanical equivalent photon spectrum by Benesh {ital et al.} [Phys. Rev. C {bold 54}, 1404 (1996)]. We point out that the key point, the strong absorption in heavy ion collisions, is not treated adequately. Conclusions drawn from such a spectrum are invalid. Equivalent-photon spectra appropriate for heavy ions have been given before in quantal, as well as semiclassical versions, and were found to be very satisfactory. {copyright} {ital 1997} {ital The American Physical Society}

Absolute photoabsorption and photoionization studies of methyl bromide using dipole electron impact and synchrotron radiation PES techniques

Absolute oscillator strengths (cross sections) have been measured for the photoabsorption of CH 3Br at low resolution (1 eV fwhm) in the equivalent photon energy range from 6 to 450 eV and at high resolution (0.05 eV fwhm) from 5 to 100 eV using dipole (e,e) spectroscopy. In the valence shell and Br 3d inner shell regions of CH 3Br, from the first ionization threshold to 79 eV, the photoionization efficiency, ionic photofragmentation branching ratios and absolute partial oscillator strengths for molecular and dissociative photoionization have been determined by dipole (e,e+ion) spectroscopy. The major ionic pathways of the dipole induced breakdown scheme of CH 3Br have been predicted from these results and the partial photoionization oscillator strengths for production of the valence shell electronic states of CH 3Br + have been estimated. Photoelectron branching ratios for CH 3Br have been measured using monochromated synchrotron radiation at photon energies from 21 to 68 eV. These have been combined with the dipole (e,e) photoabsorption data to obtain partial photoionization oscillator strengths for production of the electronic states of CH 3Br +, which have been compared with the partial oscillator strength estimates derived from the (e,e+ion) data.