J.-O. Adler

Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems.

Bremsstrahlung photon beams produced by linac accelerators are currently the most commonly used method of radiotherapy for tumour treatments. When the photon energy exceeds 10 MeV the patient receives an undesired dose due to photoneutron production in the accelerator head. In the last few decades, new sophisticated techniques such as multileaf collimators have been used for a better definition of the target volume. In this case it is crucial to evaluate the photoneutron dose produced after giant dipole resonance (GDR) excitation of the high Z materials (mainly tungsten and lead) constituting the collimator leaves in view of the optimization of the radiotherapy treatment. A Monte Carlo approach has been used to calculate the photoneutron dose arising from the GDR reaction during radiotherapy with energetic photon beams. The simulation has been performed using the code MCNP4B-GN which is based on MCNP4B, but includes a new routine GAMMAN to model photoneutron production. Results for the facility at IRCC (Istituto per la Ricerca e la Cura del Cancro) Candiolo (Turin), which is based on 18 MV x-rays from a Varian Clinac 2300 C/D, are presented for a variety of different collimator configurations.

Compton Scattering from the Deuteron and Extracted Neutron Polarizabilities

Differential cross sections for Compton scattering from the deuteron were measured at MAX-Lab for incident photon energies of 55 and 66 MeV at nominal laboratory angles of 45 degrees, 125 degrees, and 135 degrees. Tagged photons were scattered from liquid deuterium and detected in three NaI spectrometers. By comparing the data with theoretical calculations in the framework of a one-boson-exchange potential model, the sum and the difference of the isospin-averaged nucleon polarizabilities, alpha(N)+beta(N)=17.4+/-3.7 and alpha(N)-beta(N)=6.4+/-2.4 (in units of 10(-4) fm(3)), have been determined. By combining the latter with the global-averaged value for alpha(p)-beta(p) and using the predictions of the Baldin sum rule for the sum of the nucleon polarizabilities, we have obtained values for the neutron electric and magnetic polarizabilities of alpha(n)=8.8+/-2.4(total)+/-3.0(model) and beta(n)=6.5-/+2.4(total)-/+3.0(model), respectively.

Photoneutron yields from tungsten in the energy range of the giant dipole resonance

Photoneutron production on the nuclei of high-Z components of medical accelerator heads can lead to a significant secondary dose during a course of bremsstrahlung radiotherapy. However, a quantitative evaluation of secondary neutron dose requires improved data on the photoreaction yields. These have been measured as a function of photon energy, neutron energy and neutron angle for natW, using tagged photons at the MAX-Lab photonuclear facility in Sweden. This work presents neutron yields for natW(gamma, n) and compares these with the predictions of the Monte Carlo code MCNP-GN, developed specifically to simulate photoneutron production at medical accelerators.

A broad range tagging spectrometer for the MAX-laboratory

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.

Near-threshold measurement of the He-4(gamma,n) reaction

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.

The 16O(γ, p) reaction at Eγ = 60 and 72 MeV

Abstract 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.

Measurement of the He-4(gamma,n) reaction from 23 gamma <70 MeV

A comprehensive set of {sup 4}He({gamma},n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E{sub {gamma}}<70 MeV were directed toward a liquid {sup 4}He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983.

Electromagnetic polarizabilities of nucleons bound in 40Ca, 16O and 4He

Differential cross sections for elastic scattering of photons have been measured for 40Ca at energies of 58 and 74 MeV and for 16O and 4He at 61 MeV, in the angular range from 45° to 150°. Evidence is obtained that there are no significant in-medium modifications of the electromagnetic polarizabilities except for those originating from meson exchange currents.

Photon-induced proton knockout from 208Pb and 12C

Abstract 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.

The 4He(γ,n) reaction: a potential testing ground for the alpha-particle wavefunction

Differential cross sections (σ(Eγ,θn)) for the 4He(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 3He and 4He wavefunctions has been made within the Alt-Grassberger-Sandhas, integral-equation formalism.