Search

The γ -radiation by electrons with the energy of ∼ 200 MeV in a 100 μ m-thick diamond crystal has been measured at the MAX-lab experimental facility, when the electrons are incident on the crystal along the 〈 100 〉 axis. In this case, the intensity of the -radiation with the energy ∼ 1–2 MeV is approximately 16 times higher than that in amorphous matter of the same thickness. Theoretical calculations based on the quasi-classical QED approximation are in good agreement with the experimental data. The analysis of the radiation mechanisms has shown that the main contribution to the radiation intensity resulted from electrons, moving in the above-barrier regime and at small angles to the crystal axis.

The cross section asymmetry of C12(γ,p01)B11 and C12(γ,p2−6)B11 reactions has been studied at the energy range 40–55 MeV, using linearly polarized tagged photons of the MAX-lab facility. The asymmetry of the C12(γ,p01)B11 processes, which assume the one-body mechanism of the reaction, is Σ≈0.82±0.05 for photon energies 45–50 MeV. The asymmetry for the C12(γ,p2−6)B11 reactions, which produce a maximum at excitation energy ∼6 MeV, is Σ≈0.53±0.13 for a photon energy 49 MeV. It is close to the asymmetry of reaction of the free deuteron photodisintegration, and can be resulted from the two-body mechanism of the photon absorption.

The first experimental investigation of the near-threshold cross section for incoherent $\pi^-$ photoproduction on the deuteron $\gamma$ d -> $\pi^-$ pp is presented. The experimental technique involved detection of the ~131 MeV gamma ray resulting from the radiative capture of photoproduced $\pi^-$ in the target. The total cross section has been measured using an unpolarized tagged-photon beam, a liquid-deuterium target, and three very large NaI(Tl) spectrometers. The data are compared to theoretical models that give insight into the elementary reaction $\gamma$ n -> $\pi^-$ p and pion-nucleon and nucleon-nucleon final-state interactions., Comment: 8 pages, 6 figures, to be submitted to Physical Review C

In this work properties of a vacuum phototriode (VPT) and preamplifier unit designed for the electromagnetic calorimeter of the PANDA experiment being built at FAIR are investigated. With the use of lead tungstate and lanthanium bromide scintillators the VPT properties are studied at low photon energies, from tens of key in the lanthanium bromide measurements and between 10 MeV and 60 MeV in the lead tungstate measurements. At these energies the noise of the VPT unit can be expected to influence its performance significantly. It is shown that the noise contribution to the measured energy resolution, under optimal conditions, is consistent with a fluctuation of (one standard deviation) approximately 200 electrons at the VPT anode. For a lead tungstate crystal this is equivalent to a noise of 1.2 MeV. For lanthanium bromide this makes it possible to use VPTs for gamma ray spectroscopy above a few hundreds of keV without noticeable effects on the energy resolution compared to measurements with a standard photomultiplier. (C) 2014 Elsevier B.V. All rights reserved.

A linearly polarized photon beam has been produced at MAX-lab using the coherent bremsstrahlung of electrons with an energy of 192.6 MeV in a 0.1 mm thick diamond crystal. The intensity and shape of the coherent maxima and their dependence on the crystal orientation are similar to the features observed at higher electron energies (~ 1 GeV) and are well described by coherent bremsstrahlung theory. The linear polarization of the uncollimated beam at the coherent peak energy ≈50–60 MeV is about 20% and can be increased to 40–45% if collimation of half the characteristic angle is used. At present the degree of polarization is high enough to allow the study of polarization observables in photo-nuclear reactions at MAX-lab in the energy range from Giant Dipole Resonance up to ≈80 MeV.

A description is given of the upgraded photon tagging facility at the MAX IV Laboratory. Two magnetic spectrometers are used to momentum analyze post-bremsstrahlung electrons. The tagged photon range extends from 10 to 180 MeV with an energy resolution of about 300 keV. The system has been operated at rates up to 4×106 photons s-1 MeV −1. Different diagnostic tools are described as well as the experimental program.

The electric and magnetic polarizabilities of the proton are now well known, owing to Compton scattering measurements on hydrogen targets; however, the neutron polarizabilities are still relatively uncertain, compared to the proton values. To address this problem, a multiinstitutional collaboration has conducted a program of elastic Compton scattering experiments on deuterium at the MAX IV Laboratory in Lund, Sweden using tagged photons in the energy range Eγ= 65-115 MeV. We assembled at one laboratory, for the first time, three of the world's largest NaI detectors, each having better than ΔE/E ∼ 2% photon energy resolution. We have measured elastic Compton scattering cross sections at lab angles of θγ= 60°, 120° and 150° over this energy range in 8 MeV steps. This effectively doubles the world's set of elastic Compton scattering data from deuterium and provides valuable input for chiral effective field theory (χEFT) calculations. These new deuterium data overlap previous measurements and extend them by 20 MeV to higher energies. Based on χEFT fits to the expanded world data set, new values for the neutron electric and magnetic polarizabilities have been obtained with greater accuracy than previously achieved, decreasing the statistical error by more than 30%. (Less)

The response of a matrix of 25 lead tungstate (PWO) scintillator detectors, operated at -25 degrees C, to photons in the range 13 MeV-64 MeV has been measured at the tagged-photon facility at MAX-l ...

The upgrade of the MAX-lab Injector and the construction of MAX III, provided the opportunity for upgrading the tagged-photon facility and thus lead to the possibility of more extensive program in nuclear physics research This upgrade increased the injected electron energy to all eventual maximum of 250 MeV and allows for the extraction of electrons from the MAX I ring operated in the stretcher mode The first stretched bean was delivered in September 2005. The tagged-photon facility was commissioned in parallel with the commissioning of new experimental equipment. The PAC approved experimental program is current in progress, including measurement of pion photoproduction below the Delta(1232) The efforts at the tagged photon-facility are pursed within all international collaboration with around fifty members

Real or virtual pions are important ingredients of nuclear reactions at intermediate energies, and their couplings to major nuclear de-excitation modes can determine the final reaction products starting from several beam species. One of these modes is the important fission decay channel for heavy nuclei, induced with real pion beams over a wide range of energies and nuclei, largely using SSNTD. The data have been compared with the predictions of several standard INC reaction models. This review of the data and calculations will provide a status report for this class of reactions. Comparison to fission induced by protons and photons is included to set a context for that induced by real pion beams.

In photonuclear experiments precise location of the photon beam relative to the experimental sample is critical. Previously used techniques such as using photographic film to identify the position, intensity and centroid of the beam is time-consuming and a faster method is required. PILATUS is a single-photon-counting pixel detector developed at the Paul Scherrer Institute (PSI), Switzerland. It is a silicon-based, two-dimensional detector with a large dynamic range and zero readout noise. Designed as an X-ray detector, its optimal quantum efficiency is between 3 and 30 keV. This paper reports measurements carried out at the MAX-lab tagged photon facility in Lund, Sweden. The beam endpoint energy of approximately 200 MeV is far above the designed optimal energy detection range of PILATUS, and provides a critical test of the use of PILATUS under high energy conditions. The detector was placed in the photon beam and images were taken both downstream of other experiments, and in close range of a 19 mm collimator. The successful measurements demonstrate the versatility and robustness of the detector and provide an effective way of quickly and accurately monitoring beam position and profile in real time. (C) 2009 Elsevier B.V. All rights reserved. (Less)

A new program of Compton scattering on deuterium is under way at the tagged-photon facility at MAX-Lab in Lund, Sweden. We will measure differential cross sections between 60A degrees and 150A degrees over the photon energy range 60-115 MeV in 5 MeV steps, with the ultimate goal of obtaining new precision information on the electric and magnetic polarizabilities of the neutron.

The very first data from MAX-lab in Lund, Sweden on pion photoproduction at threshold energies are presented. The decrease of the total π+ yield in γ+12C, 27Al reactions below 200 MeV as well as the dσ/dΩ cross-section data essentially follow the predictions of an intranuclear-cascade model with an attractive potential for the pion–nucleus interaction. However, View the MathML source, cross-section data at 176 MeV show deviations which call for refinements of the model and possibly also for the inclusion of coherent pion-production mechanisms.

A multi-cell He gas-scintillator active target, designed for the measurement of photoreaction cross sections, is described. The target has four main chambers, giving an overall thickness of 0.103 $\mathrm{g/cm^{2}}$ at an operating pressure of 2 MPa. Scintillations are read out by photomultiplier tubes and the addition of small amounts of $\mathrm{N}_{2}$ to the He, to shift the scintillation emission from UV to visible, is discussed. First results of measurements at the MAX IV Laboratory tagged-photon facility show that the target has good timing resolution and can cope well with a high-flux photon beam. The determination of reaction cross sections from target yields relies on a Monte Carlo simulation, which considers scintillation light transport, photodisintegration processes in $^{4}\mathrm{He}$, background photon interactions in target windows and interactions of the reaction-product particles in the gas and target container. The predictions of this simulation are compared to the measured target response., Comment: 24 pages, 15 figures

Baryon-to-meson Transition Distribution Amplitudes (TDAs) encoding valuable new information on hadron structure appear as building blocks in the collinear factorized description for several types of hard exclusive reactions. In this paper, we address the possibility of accessing nucleon-to-pion ($��N$) TDAs from $\bar{p}p \to e^+e^- ��^0$ reaction with the future PANDA detector at the FAIR facility. At high center of mass energy and high invariant mass squared of the lepton pair $q^2$, the amplitude of the signal channel $\bar{p}p \to e^+e^- ��^0$ admits a QCD factorized description in terms of $��N$ TDAs and nucleon Distribution Amplitudes (DAs) in the forward and backward kinematic regimes. Assuming the validity of this factorized description, we perform feasibility studies for measuring $\bar{p}p \to e^+e^- ��^0$ with the PANDA detector. Detailed simulations on signal reconstruction efficiency as well as on rejection of the most severe background channel, i.e. $\bar{p}p \to ��^+��^- ��^0$ were performed for the center of mass energy squared $s = 5$ GeV$^2$ and $s = 10$ GeV$^2$, in the kinematic regions $3.0 < q^2 < 4.3$ GeV$^2$ and $5 < q^2 < 9$ GeV$^2$, respectively, with a neutral pion scattered in the forward or backward cone $| \cos��_{��^0}| > 0.5 $ in the proton-antiproton center of mass frame. Results of the simulation show that the particle identification capabilities of the PANDA detector will allow to achieve a background rejection factor of $5\cdot 10^7$ ($1\cdot 10^7$) at low (high) $q^2$ for $s=5$ GeV$^2$, and of $1\cdot 10^8$ ($6\cdot 10^6$) at low (high) $q^2$ for $s=10$ GeV$^2$, while keeping the signal reconstruction efficiency at around $40\%$. At both energies, a clean lepton signal can be reconstructed with the expected statistics corresponding to $2$ fb$^{-1}$ of integrated luminosity. (.../...), 19 pages, 7 figures (some multiple), 2 tables (each double), preprint of an article for epj - v2

Differential cross sections for elastic scattering of photons from the deuteron have recently been measured at the Tagged-Photon Facility at the MAX IV Laboratory in Lund, Sweden. These first new measurements in more than a decade further constrain the isoscalar electromagnetic polarizabilities of the nucleon and provide the first-ever results above 100 MeV, where the sensitivity to the polarizabilities is increased. We add 23 points between 70 and 112 MeV, at angles 60deg, 120deg and 150deg. Analysis of these data using a Chiral Effective Field Theory indicates that the cross sections are both self-consistent and consistent with previous measurements. Extracted values of \alpha_s = [12.1 +/- 0.8(stat) +/- 0.2(BSR) +/- 0.8(th)] X 10^{-4} fm^3 and \beta_s = [2.4 +/- 0.8(stat) +/- 0.2(BSR) +/- 0.8(th)] X 10^{-4} fm^3 are obtained from a fit to these 23 new data points. This paper presents in detail the experimental conditions and the data analysis used to extract the cross sections., Comment: 13 pages, 8 figures, to be submitted to Physical Review C

The response of the liquid scintillator array Nordball to neutrons in the energy range 1.5 < T_n < 10 MeV has been measured by time of flight using a 252Cf fission source. Fission fragments were detected by means of a thin-film plastic scintillator. The measured differential and integral neutron detection efficiencies agree well with predictions of a Monte Carlo simulation of the detector which models geometry accurately and incorporates the measured, non-linear proton light output as a function of energy. The ability of the model to provide systematic corrections to photoneutron cross sections, measured by Nordball at low energy, is tested in a measurement of the two-body deuteron photodisintegration cross section in the range E_gamma=14-18 MeV. After correction the present 2H(gamma,n)p measurements agree well with a published evaluation of the large body of 2H(gamma,p)n data., 20 pages 10 figures, submitted Nucl. Instr. Meth. A

The electromagnetic polarizabilities of the nucleon are fundamental properties that describe its response to external electric and magnetic fields. They can be extracted from Compton-scattering data --- and have been, with good accuracy, in the case of the proton. In contradistinction, information for the neutron requires the use of Compton scattering from nuclear targets. Here we report a new measurement of elastic photon scattering from deuterium using quasimonoenergetic tagged photons at the MAX IV Laboratory in Lund, Sweden. These first new data in more than a decade effectively double the world dataset. Their energy range overlaps with previous experiments and extends it by 20 MeV to higher energies. An analysis using Chiral Effective Field Theory with dynamical \Delta(1232) degrees of freedom shows the data are consistent with and within the world dataset. After demonstrating that the fit is consistent with the Baldin sum rule, extracting values for the isoscalar nucleon polarizabilities and combining them with a recent result for the proton, we obtain the neutron polarizabilities as \alpha_n = [11.55 +/- 1.25(stat) +/- 0.2(BSR) +/- 0.8(th)] X 10^{-4} fm^3 and \beta_n = [3.65 -/+ 1.25(stat) +/- 0.2(BSR) -/+ 0.8(th)] X 10^{-4} fm3, with \chi^2 = 45.2 for 44 degrees of freedom., Comment: 6 pages, 3 figures, comments from Physical Review Letters Referees addressed

A near-threshold He-4(gamma, n) cross-section measurement has been performed at MAX-lab. Tagged photons from 23 < Ey < 42 MeV were directed toward a liquid He-4 target, and neutrons were detected by time-of-flight in two liquid-scintillator arrays. Seven-point angular distributions were measured for eight photon energies. The results are compared to experimental data measured at comparable energies and Recoil-Corrected Continuum Shell Model, Resonating Group Method, and recent Hyperspherical-Harmonic Expansion calculations. The angle-integrated cross-section data are peaked at a photon energy of about 28 MeV, in disagreement with the value recommended by Calarco, Berman, and Donnelly in 1983.

Elastic scattering of photons from C-12 has been investigated using quasimonoenergetic tagged photons with energies in the range 65-115 MeV at laboratory angles of 60 degrees, 120 degrees, and 150 degrees. at the Tagged-Photon Facility at the MAX IV Laboratory in Lund, Sweden. A phenomenological model was employed to provide an estimate of the sensitivity of the C-12(gamma,gamma)C-12 cross section to the bound- nucleon polarizabilities.

The upgraded tagged photon facility at the MAX IV Laboratory is presented. The pulsed electron beam from a linac system is stretched in the MAX I ring and the electron beam hits a thin aluminium foil in which bremsstrahlung is produced. One of two available magnetic spectrometers is used to determine the energy of the post-bremsstrahlung electrons. The tagged photon ranges from 10 to 180 MeV with an energy resolution of about 300 keV. Rates as high as 4x10(6) photons s(-1) MeV-1 have been used. The experimental area and facilities are described as well as examples of ongoing experiments.

J. R. M. Annand, G. I. Crawford, V. Holliday, J. C. McGeorge, and G. J. Miller** Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland (Received 27 May 1999) The 16O g,pn reaction was measured at Eg 72 MeV with resolution sufficient to distinguish the low-lying states of the residual 14N nucleus. Cross sections averaged over the acceptance of the detector system were determined for each of the resolved states. The relative population of residual states indicates that proton-neutron pairs coupled to J ,T 0, 1 play a minor role in the photon absorption process compared to 1, 0 pairs and that both L 0 and L 2 pairs participate in the reaction.

A combined Bragg-PPAC ionization detector has been developed for registration and identification of light charged particles from photonuclear reactions at tagged photon beams. The detector is described and also its performance, which was obtained in a test experiment at the tagged photon beam.

Differential cross sections (σ(Eγ,θn)) for the 4 He(γ,n) reaction have been measured at Eγ=50–71 MeV and θn=30–120°. These data are compared with theoretical predictions where a microscopic calculation of the 3 He and 4 He wavefunctions has been made within the Alt-Grassberger-Sandhas, integral-equation formalism.

Relative population of states in 11B following the 12C(γ,p) reaction has been measured with high resolution using the deexcitation γ-ray technique. The states near 7 MeV in 11B are clearly resolved and the measured population clarifies earlier conflicting data. Comparison of the results with new calculations indicates the importance of both one-nucleon and multinucleon processes.

The reaction ${}^{10}\mathrm{B}(\ensuremath{\gamma},p)$ has been studied using tagged photons of mean energies ${E}_{\ensuremath{\gamma}}=57.6$ and 72.9 MeV. Angular distributions and derived single-particle momentum distributions for protons leading to the ground state of ${}^{9}\mathrm{Be}$ and higher excited states are compared to various calculations made using model parameters constrained by ${}^{10}\mathrm{B}{(e,e}^{\ensuremath{'}}{p)}^{9}\mathrm{Be}$ and ${}^{9}\mathrm{Be}{(p,p}^{\ensuremath{'}}{)}^{9}\mathrm{Be}$ measurements. The effects of varying final-state interactions (including channel couplings) and meson exchange currents are considered. A sizable discrepancy between direct-knockout calculations and the experimental results is observed. If meson exchange currents are included in an approximate fashion, a good description of the ${}^{10}\mathrm{B}(\ensuremath{\gamma}{,p}_{0}{)}^{9}\mathrm{Be}$ data is found.

The forward-to-backward asymmetry of neutrons emitted in the 40Ca(γ,n0) reaction was measured at photon energies in the range of 27–50 MeV. An energy-dependent asymmetry was observed that is interpreted as evidence of interference between the isovector quadrupole resonance and the giant dipole resonance. Data analysis in terms of semiclassical and direct-semidirect models estimate the isovector quadrupole resonance to be at an excitation energy of 31.0±1.5 MeV, with a width of 16.0±1.5 MeV, and exhausting most of the energy-weighted sum rule for the isovector quadrupole resonance.

A broad range tagging spectrometer together with a new beam transport system for photonuclear experiments at the MAX-laboratory in Lund is described. The spectrometer consists of a quadrupole followed by an Elbek-type dipole and has a large momentum acceptance. It can produce both polarized and unpolarized tagged photons in the energy range 10–80 MeV with an energy resolution of about 300 keV.

Cross sections have been measured for the reactions 208Pb(γ,p) and 12C(γ,p) leading to the low-lying and continuum states in 207Tl and 11B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the 12C(γ,p) data by typically a factor five depending on angle and missing energy. For 208Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for 208Pb as compared to 12C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on 208Pb despite the difference in the electromagnetic couplings involved.

Rate-dependent effects in the electronics used to instrument the tagger focal plane at the MAX IV Laboratory have been investigated using the novel approach of Monte Carlo simulation. Results are compared to analytical calculations as well as experimental data for both specialized testing and production running to demonstrate a thorough understanding of the behavior of the detector system., Comment: 31 pages, 17 figures, modified after submission to Nuclear Instruments and Methods in Physics Research Section A

Absolute differential cross sections for the $^{12}\mathrm{C}$(\ensuremath{\gamma},p${)}^{11}$B reaction have been measured for low-lying regions of residual excitation energy in $^{11}\mathrm{B}$. Cross sections with low systematic uncertainties are presented for proton detection angles ranging from 30\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{}, measured with tagged photons of 25--75 MeV energy. The experimental results resolve the discrepancies between the older data sets. In addition, a reinterpretation of the low-lying states excited in $^{11}\mathrm{B}$ is presented. It is concluded that HF-RPA calculations with essential contributions of meson exchange currents (MEC) provide a qualitative description of the angular distributions obtained for the negative parity states in the (\ensuremath{\gamma},p) reaction. \textcopyright{} 1996 The American Physical Society.

A systematic study of the mass dependence of meson-exchange currents in the reaction (γ,p) has been carried out. This study became possible with the availability of new high resolution (γ,p) data from 12C to 208Pb. In order to determine the relative importance of meson-exchange currents the direct knockout part of the (γ,p) cross section has been constrained by results obtained with the (e,e′p) reaction in similar momentum ranges. It is found that the enhancement of the (γ,p) cross section due to meson-exchange currents reduces by a factor three in going from 12C to 208Pb. This effect is tentatively attributed to the increased importance of higher multipolarities in the description of the coupling to meson exchange currents in heavy nuclei.

Differential cross sections for elastic scattering of photons by C-12 and O-16 are measured at 58 and 75 MeV photon energies and 45 degrees to 150 degrees scattering angles using the tagged photon ...

Differential cross sections for Compton scattering by 4He have been measured at photon energies ranging from 83.7 to 93.8 MeV with a weighted average of 87 MeV using the bremsstrahlung beam of the MAX-LAB (Lund) nuclear-physics facility. The experimental differential cross sections show a pronounced increase at backward angles, strongly differing from predictions based on zero-angle dispersion relations and on the standard method used to calculate the differential cross section at nonzero angles. It is shown that the attempt to explain this effect in terms of quadrupole strength leads to unreasonable properties of the photoabsorption cross section. A parametrization in terms of a shift in the multipolarity ratio of the electromagnetic polarizabilities of the bound nucleon leads to Δβ = − Δα = 7 × 10−4 fm3.

The values of the angle-integrated cross section for the decay to various low-lying states in39K following the40Ca(γ,π) reaction at a mean photon energy of 60 MeV are reported on, together with the values of the reduced Legendre coefficientsa i . The40Ca(γ,gp) and40Ca(γ,n) reactions yield the same angular cross-section values and these agree with the corresponding RPA-calculations.

Measurements of the reaction {sup 16}O({gamma},{ital n}), made with tagged photons at {ital E}{sub {gamma}}{similar_to}58 MeV, are presented. Neutron energy determination was by time of flight, giving a resolution of {similar_to}2 MeV which enabled the ground-state excitation in {sup 15}O to be resolved from states at 5.2 and 6.2 MeV. Differential cross sections were measured in the angular range 30{degree}--110{degree} and are compared with the predictions of a microscopic, self-consistent Hartree-Fock, random phase approximation model of ({gamma},{ital N}) reactions. The implications for the theoretical description are discussed, notably the dependence of the ({gamma},{ital n}) cross section on meson exchange currents and final-state rescattering.

The 6 Li(γ,p) reaction was measured at average tagged photon energies of 〈 E γ 〉 = 59 and 75 MeV. Protons were detected at 5 different angles between 30° and 150°. Most of the observed strength is apparently due to the three-body breakup channels. In particular the semi-inclusive (γ,p(n)) and (γ,p(t)) channels are discussed.

Angular distributions within the range 25°–125° are presented for the 16O(γ, p) reaction populating low-lying states in 15N at Eγ = 60 and 72 MeV. A comparison of the data with self-consistent Hartree-Fock RPA calculations shows the importance of meson-exchange currents in the photon-absorption process.

The tagged photon technique has been used to measure the excitation energy spectra resulting from the (γ, p) reaction in 6Li and 12C at 90 degrees for incident photon energies around 61 and 77 MeV. Strong evidence is found for the quasideuteron absorption mechanism, not only in the high excitation energy regions, but also in bound states. Furthermore, for the 12C(γ, p) reaction the photon seems to be absorbed on pn-pairs in both T = 0 and T = 1 states.

The results of a study of the 6 Li (γ, cc) (c, c : charged particle ) reaction with the tagged-photon technique are presented. It is shown that the (γ, pt) reaction can be understood within the framework of a semi-phenomenological quasi-alpha reaction model. The total (γ, pt) cross section is found to be larger than the 6Li(γ, pn)α cross section at the same photon energy. No evidence is found for a three-body absorption mechanism, but the possible role of final-state interactions in the (γ, pd) reaction is outlined.

High resolution measurements of the reaction [sup 12]C([gamma],[ital n]) at [ital E][sub [gamma]][similar to]58 MeV are presented. The distribution of strength to the resolved bound final states in [sup 11]C is compared with that of [sup 11]B obtained in previous analogous ([gamma],[ital p]) measurements and the implications for the theoretical description of ([gamma],[ital N]) reactions are discussed. These new results confirm the importance of two-nucleon effects in intermediate energy photon absorption and highlight inadequacies in state-of-the-art microscopic calculations of ([gamma],[ital N]) reactions.

A study of the (γ, p) reaction on 12C, 14N, 27Al and 51V targets, at photon energies Eγ ∼60 MeV, is reported. Angular distributions in the range tp = 60°-120° are presented for transitions leading to the ground state and several low-lying states in each residual nucleus. The relative population of residual nucleus slates is similar to those found in (e, e'p) and (d, 3He) experiments, apart from the well-known exception of the peak at 6.8 MeV in the 12C(γ, p) reaction. Direct-knockout calculations fail to account for the magnitude of the (γ, p) results, which indicates the need for calculations that include effects such as meson exchange or short-range correlations. Strength seen at higher excitation energies is attributed to inclusive (γ, p) events. The excitation-energy spectra have been compared with quasi-deuteron model calculations which assume a (γ, np) reaction where the neutron goes undetected. The shape of the 51V(γ, p) excitation spectrum is compared to a pre-equilibrium model calculation. The results appear to indicate that two-nucleon absorption of the photon is important.

The linearly polarized photon beam for photonuclear researches has been produced at MAX-lab facility on the base of coherent bremsstrahlung process of electrons in a diamond crystal. Test experiments have been performed at electron energies 143.9 and 192.7 MeV with a diamond crystal 0.1 mm thick. The measured coherent bremsstrahlung spectra demonstrate typical features to be observed at higher electron energies. The polarization ~35% was obtained at coherent peak energy of ~60 MeV. The experiments have shown that produced polarized photon beam can be used for photonuclear investigations at energy range up to 60 MeV. To extend the energy range it is necessary to increase maximal energy of the electron beam up to 250 MeV.

Using 58 and 73 MeV tagged photons and scattering angles from 60° to 150°, it is shown that it is possible to observe Compton scattering by “mesons in nuclei” through an incomplete cancellation of the mesonic (exchange-current) seagull amplitude by parts of the nuclear resonance amplitude related to the giant-dipole resonance of 208 Pb. This phenomenon is a property of an extended nucleus and, therefore, cannot be studied on the deuteron. Predictions of the exchange from factor which determines the angular distribution of the exchange seagull amplitude are compared with experimental data.

A new, improved method to define the random particle energy spectrum in tagged photon experiments is described. This procedure yields the random spectrum with very good statistical accuracy, and can be applied when no hard coincidence with the focal plane is required in order to accept events.

The polarizabilities of nucleons imbedded in 12C and 16O nuclei are measured via Compton scattering of 61 and 77 MeV photons, leading to α N = (11.5±0.8±2.1) × 10 −4 fm 3 and β N = (2.5∓0.8∓2.1) × 10 −4 fm 3 in close agreement with the free-nucleon values.

The two-body photodisintegration of He-3 has been investigated using tagged photons with energies from 14-31 MeV at MAX-lab in Lund, Sweden. The two-body breakup channel was unambiguously identified by the (nonsimultaneous) detection of both protons and deuterons. This approach was made feasible by the overdetermined kinematic situation afforded by the tagged-photon technique. Proton-and deuteron-energy spectra were measured using four silicon surface-barrier detector telescopes located at a laboratory angle of 90 degrees with respect to the incident photon-beam direction. Average statistical and systematic uncertainties of 5.7% and 6.6% in the differential cross section were obtained for 11 photon-energy bins with an average width of 1.2 MeV. The results are compared to previous experimental data measured at comparable photon energies as well as to the results of two recent Faddeev calculations which employ realistic potential models and take into account three-nucleon forces and final-state interactions. Both the accuracy and precision of the present data are improved over those obtained in the previous measurements. The data are in good agreement with most of the previous results, and favor the inclusion of three-nucleon forces in the calculations. (Less)

Excitation-energy spectra resulting from the {sup 12}C, {sup 16}O, and {sup 40}Ca({gamma},{ital p}) reactions at 90{degree} have been measured with tagged photons of about 61 and 77 MeV. Decay to several discrete states is discussed. Evidence is shown for an absorption process where the photon interacts with a {ital T}=1 proton-neutron pair, instead of the {ital T}=0 {ital p}-{ital n} pair implicit in the quasideuteron model.

A description is given of the photon tagging facility at the MAX accelerator system in Lund for use in photonuclear experiments in the energy region between the giant delta resonances. The system enables experiments with high-energy resolution, 300 keV at 60 MeV photons, and a tagged rate of 5 × 10 6 /sMeV.

Spectra of protons and deuterons emitted in the photodisintegration of $^{27}\mathrm{Al}$ and $^{\mathrm{nat}}\mathrm{Ca}$ have been measured with the tagged photon technique. The experimental data are compared to the results of calculations using the hybrid model for pre-equilibrium reactions. For the first time it is possible to determine the relevant initial doorway configuration, i.e., a 2p-1h doorway, appropriate for quasideuteron absorption.