Total Photoabsorption Cross Section Research Articles

Photonuclear reactions at intermediate energies investigated via the Monte Carlo multicollisional intranuclear cascade model

The Monte Carlo multicollisional (MCMC) intranuclear cascade model is used to study photonuclear reactions at intermediate energies $(20\ensuremath{\leqslant}{E}_{\ensuremath{\gamma}}\ensuremath{\leqslant}140\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})$. This version of the code differs from previous versions in the following aspects: (i) the quasideuteron model of photoabsorption is consistently included by taking into account relative momentum correlations of the neutron-proton pair in a relativistic kinematics; (ii) a realistic treatment of the Pauli-blocking mechanism at the initial photoabsorption and at each binary nucleon-nucleon scattering during the cascade process is incorporated throughout the calculations; (iii) a criterion based on energy considerations is required by the end of the cascade. Differently from other transport models used so far, which are based on a randomly generated nuclear ground state with a stochastic treatment of the Pauli blocking, the present model incorporates a shell constrained momentum space of the nucleons which is preserved as the cascade evolves along time. The transition between the pre-equilibrium and evaporation phases is energetically determined, allowing the description of the cascade process without any free parameter, such as some ambiguous stopping time parameters adopted in similar time-structured cascade models. The occupation number distribution after the cascade corresponds to a typical Fermi distribution at a finite nuclear temperature, and the long-standing spurious depletion of the Fermi sphere, usually present in other cascade models, no longer appears. The Pauli-blocking factors are calculated and compared with previous approaches based on Fermi gas level density calculations. The evaporation-fission process of the compound nucleus is described in the framework of a Monte Carlo algorithm. Experimental data of the total photoabsorption cross section and the neutron multiplicities for $\mathrm{Sn}$, $\mathrm{Ce}$, $\mathrm{Ta}$, and $\mathrm{Pb}$ in the $20--140\text{\ensuremath{-}}\mathrm{MeV}$ range are described fairly well by the present calculations.

Total photoabsorption cross-sections of CF 3SF 5 in the C, F and S K-shell regions

Total photoabsorption cross-sections of trifluoromethyl sulfur pentafluoride, CF 3SF 5, were measured in the C, F and S K-shell regions by using a double ionization chamber and synchrotron radiation at the SPring-8 facility. The maximum cross sections were found to be 6.3, 5.0 and 1.1 Mb for the C, F and S K-edges, respectively. The spectral features in the F K-shell region were characterized by the photoabsorption spectra of CF 4 and SF 6 and those in the S K-shell region resemble the peaks of SF 6. The observed peaks for CF 3SF 5 were tentatively assigned on the basis of a comparison with those for CF 4 and SF 6 measured simultaneously for references.

Effect ofP-wave interaction inHe6andLi6photoabsorption

The total photoabsorption cross sections of six-body nuclei are calculated including complete final state interaction via the Lorentz Integral Transform method. The effect of nucleon-nucleon central P-wave forces is investigated. Comparing to results with central potentials containg S-wave forces only one finds considerably more strength in the low-energy cross sections and a rather strong improvement in comparison with experimental data, in particular for 6Li.

Photodisintegration of Light Nuclei

The first microscopic calculation of inelastic reactions on nuclei with mass number A = 6 is presented. In particular, we consider the total photoabsorption cross section of 6He and 6Li. It is shown that the halo nucleus 6He exhibits two types of dipole resonances, the normal giant dipole resonance and a low-energy soft dipole resonance associated with the excitation of the outer neutrons. Comparing our results with available experimental data we note the role of P-wave interaction. Calculations are performed using the Lorentz integral transform (LIT) method and the effective interaction in the hyperspherical harmonics formalism (EIHH).

Double photoionization of C(60) and C(70) in the valence region.

Photoion yields from gaseous fullerenes, C(60) and C(70), for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hnu=25-150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C(60) which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C(60) (2+) ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C(60) and C(70) are larger than unity at hnu>50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization.

The total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson production

Based on the two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low x ~= Q^2/W^2 <<1, we stress the intimate connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply-virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA.

Efficient Method for Lorentz Integral Transforms of Reaction Cross Sections

The Lorentz Integral Transform (LIT) method is reformulated via the Lanczos algorithm both for inclusive and exclusive reactions. The new technique is tested for the total photoabsorption cross section of 3H and 4He. Due to the rapid convergence of the algorithm one has a decrease in cpu time by two orders of magnitude, but at the same time an excellent agreement with the results of a conventional LIT calculation. The present work opens up the possibility of ab initio calculations for inclusive and exclusive processes for A greater equal 6 with inclusion of complete final state interactions.

First Measurement of the Gerasimov-Drell-Hearn Sum Rule for 1H from 0.7 to 1.8 GeV at ELSA.

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] microb.

Helicity dependence of the $\gamma N$ interaction and the GDH sum rule

First measurements of pion photoproduction using circularly polarized photons on longitudinally polarized protons were carried out at MAMI (Mainz) in the energy range E γ = 200–800 MeV. Results of the helicity dependence of the total inclusive photoabsorption cross-section and of the pion photoproduction channels will be presented. These data provide new input for multipole analyses and determine the main contribution to the Gerasimov-Drell-Hearn integral and the forward spin polarizability γ0.

High-energy photon beam production with laser-Compton backscattering

We have produced a beam of high-energy gamma-rays by Compton backscattering of 1064 nm laser photons from 1 GeV electrons circulating in a storage ring. Measuring the energy spectra of the backscattering photons with an HPGe detector, we have found that the maximum energy of 17.6 MeV and the measured energy spectra show agreement with the simulation calculations, while the detected photon yields were measured at about 4×10 3, 2×10 3 and 3×10 2 photons s −1 mA −1 W −1 for the 20, 10 and 2 mmφ collimators, respectively. The photon energy widths from the collimators correspond to 6.6– 17.6 MeV , 12.4– 17.6 MeV and 17.3– 17.6 MeV , respectively. By using these photons, we measured the total nuclear photoabsorption cross-sections in the E1 giant resonance energy region for 197 Au using the attenuation method and we have demonstrated that the photon attenuation method will be a useful tool for studying photonuclear reactions.

Statistical and direct aspects of64Zn(γ,n)and(γ,np)decay channels in the giant dipole resonance and quasideuteron energy regions

The investigation of statistical and direct aspects related to the $(\ensuremath{\gamma},n)$ and $(\ensuremath{\gamma},np)$ decay channels of ${}^{64}\mathrm{Zn}$ in the giant dipole resonance (GDR) and quasideuteron (QD) energy regions was performed by a trial function fitting to the respective $(e,n)$ and $(e,np)$ electrodisintegration yields measured by residual activity. The trial function incorporated the GDR and QD models to describe the initial photoabsorption mechanism and the geometry dependent hybrid exciton model used in the ALICE/LIVERMORE-82 code to calculate the relevant branching ratios, with the $E1$ virtual photon spectra being calculated in the distorted wave Born approximation. We compared our results for the $(\ensuremath{\gamma},n)$ cross section with other existing experimental measurements, and the long-standing normalization issue among different laboratories was revisited and addressed. We obtained for the first time the absolute $(\ensuremath{\gamma},np)$ cross section from threshold to 60 MeV. We succeeded in separating statistical and direct contributions of the $(\ensuremath{\gamma},np)$ process, the latter being remarkably well described by the QD model in the interval 40--60 MeV. A possible direct contribution for the $(\ensuremath{\gamma},n)$ decay in the GDR is also addressed. Finally, the total photoabsorption cross section of ${}^{64}\mathrm{Zn}$ was reevaluated up to 21 MeV, and the results were compared with previous estimates performed by other groups.

The [formula omitted] total cross section and elastic diffraction

The empirical scaling law, wherein the total photoabsorption cross section depends on the single variable η = ( Q 2 + m 0 2/ Λ 2( W 2), provides empirical evidence for saturation in the sense of σ gg ∗ p(W 2, Q 2)/σ γp(W 2) → 1 for W 2 → ∞ at fixed Q 2. The total photoabsorption cross section is related to elastic diffraction in terms of a sum rule. The excess of diffractive production over the elastic component is due to inelastic diffraction that contains the production of hadronic states of higher spins. Motivated by the diffractive mass spectrum, the generalized vector dominance/color dipole picture (GVD/CDP) is extended to successfully describe the DIS data in the full region of ξ ≤ 0.1, all Q 2 ≥ 0, where the diffractive two-gluon-exchange mechanism dominates.

Final-state interaction in spin asymmetry and GDH sum rule for incoherent pion production on the deuteron

The contribution of incoherent single pion photoproduction to the spin response of the deuteron, i.e., the asymmetry of the total photoabsorption cross section with respect to parallel and antiparallel spins of photon and deuteron, is calculated over the region of the $\Delta$-resonance with inclusion of final state $NN$- and $\pi N$-rescattering. Sizeable effects, mainly from $NN$-rescattering, are found leading to an appreciable reduction of the spin asymmetry. Furthermore, the contribution to the Gerasimov-Drell-Hearn integral is explicitly evaluated by integration up to a photon energy of 550 MeV. Final state interaction reduces the value of the integral to about half of the value obtained for the pure impulse approximation.

Absolute photoabsorption cross-sections of Ne and Xe in the sub-keV X-ray region

Total photoabsorption cross-sections of Ne in the energy region of 50–1300 eV and of Xe in 85–1100 eV have been measured in an absolute scale using a multi-electrode ion chamber combined with monochromatized synchrotron radiation. In order to improve the spectral purity of monochromatized synchrotron radiation, an electron storage ring was operated at an electron beam energy lower than the normal mode, and a thin foil was inserted into the photon beam. The photoabsorption cross-sections obtained for Ne are close to the data previously reported in most photon energies. However, for Xe, there are some discrepancies between the present results and the literature data at some photon energies, in particular near 300, 700, and 950 eV.

Diffractive production and the total cross section in deep inelastic scattering

We explore the consequences for diffractive production, gamma* p --> X p, in deep inelastic scattering at low values of x\sim Q^2/W^2 <<1 that follow from our recent representation of the total photoabsorption cross section, sigma_{gamma* p}, in the generalized vector dominance/ color dipole picture(GVD/CDP) that is based on the generic structure of the two-gluon-exchange from QCD. Sum rules are derived that relate the transverse and the longitudinal (virtual) photoabsorption cross section to diffractive forward production of q q-bar states that carry photon quantum numbers ("elastic diffraction"). Agreement with experiment in the W^2 and Q^2 dependence is found for M_X^2/Q^2<<1, where M_X is the mass of the produced system X. An additional component ("inelastic diffraction"), not actively contributing to the forward Compton amplitude, is needed for diffractive production at high values of M_X. Our previous theoretical representation of the total photoabsorption cross section sigma_{gamma* p}=sigma_{gamma* p}(eta), in terms of the scaling variable eta=(Q^2+m_0^2)/Lambda^2(W^2) is extended to include the entire kinematic domain, x=<0.1 and all Q^2 with Q^2>=0, where scaling in eta holds experimentally.

Fragmentation competing with energy relaxation in core-excited CF 3CN

Total photoabsorption cross-sections and peak assignments were presented for C(1s) excited CF 3CN. It was found that fragmentation of the core-excited CF 3CN molecule competes with intramolecular energy redistribution on the basis of two measurements: One is the analysis of the kinetic energies of the CF 3 +, CF 2 +, CF +, F + and CN + fragment ions in angle-resolved mass spectra observed by using a linear time-of-flight mass spectrometer. This is the first observation that the kinetic energy distribution of fragment ions depends on the site of excitation. The other is site-selective photofragmentation by use of a reflectron type mass spectrometer. These experiments strongly suggest that the CF 3 group acts as an effective internal energy reservoir.

Experimental verification of the GDH sum rule at E lsa and M ami

In order to verify the GDH sum rule for the first time the GDH-Collaboration measures the total photoabsorption cross section of circularly polarized real photons on longitudinally polarized nucleons in the photon energy range 0.14 – 3.1 GeV. These measurements are being pursued at the two electron accelerators M ami in Mainz and E lsa in Bonn. The experimental setup of the ongoing data taking campaign is described and results for the proton up to 1.9 GeV are presented.

The GDH-Detector

For the GDH-Experiment at E LSA, the helicity dependent total photoabsorption cross-section is to be determined. These measurements will be performed with the newly developed GDH-Detector which is presented here. The concept of the GDH-Detector is to detect at least one reaction product from all possible hadronic processes with almost complete acceptance concerning solid angle and efficiency. This is realized by an arrangement of scintillators and lead. The overall acceptance for hadronic processes is better than 99%. The electromagnetic background is suppressed by about five orders of magnitude by means of a threshold Cherenkov detector. In dedicated tests, it has been demonstrated that all individual components of the GDH-Detector fulfill the design goals. Measurements of unpolarized total photoabsorption cross-sections were performed to ensure that the complete GDH-Detector is operational.

In-medium broadening of nucleon resonances

We analyze the effects of an in-medium broadening of nucleon resonances on the exclusive photoproduction of mesons on nuclei as well as on the total photoabsorption cross sections in a transport calculation. We show that the resonance widths observed in semi-inclusive photoproduction on nuclei are insensitive to an in-medium broadening of nucleon resonances. This is due to a simple effect: the sizeable width of the nuclear surface and Fermi motion.

First Measurement of the Gerasimov-Drell-Hearn Integral forH1from 200 to 800 MeV

A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200<Eγ<800MeV. The experiment used a 4π detection system, a circularly polarized tagged photon beam, and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability γ0 determined from the data are 226±5(stat)±12(syst)μb and −187±8(stat)±10(syst)×10−6fm4, respectively, for 200<Eγ<800MeV.Received 13 March 2001DOI:https://doi.org/10.1103/PhysRevLett.87.022003©2001 American Physical Society