H. Condé

A facility for studies of neutron-induced reactions in the 50–200 MeV range

A facility for studies of neutron-induced reactions has been built at the upgraded Gustaf Werner cyclotron of the The Svedberg Laboratory, Uppsala, Sweden. Well-collimated, monoenergetic neutron beams are produced with a fairly long distance between the neutron source and the reaction target, to reduce background radiation. A magnetic spectrometer with large angular and momentum acceptance has been constructed to allow measurements of energy and angular distributions of light ions produced in neutron-induced reactions. Initially, studies will be undertaken of isovector monopole and Gamow-Teller resonances, induced by the (n,p) reaction in various nuclei. The performance is illustrated in a measurement of the doubly differential cross section of the 12C(n,p)12B reaction at 100 MeV.

The 54,56Fe(n, p)54,56Mn reactions at En = 97 MeV

Abstract Double-differential cross sections of the 54,56Fe(n, p) reactions have been measured at 97 MeV in the angular range 0–30° for excitation energies up to 40 MeV. The spectra have been decomposed into different multipolarities by a technique based on the use of sample angular distributions calculated within the distorted-wave Born approximation. From the identified Gamow-Teller strength, Sβ+ values were obtained for 54Fe and 56Fe. Comparisons with available shell-model calculations of the GT strength were made. The results are important for models of supernova explosions since electron-capture rates, which are proportional to Sβ+, in 1f2p-shell nuclei affect the dynamics of the star. At higher excitation energies, the spectra are dominated by L = 1 strength in broad distributions with a maximum at about 12 MeV. Microscopic calculations based on the random-phase approximation were performed and compared with the experimental data.

THE C-12(N,P)B-12 REACTION AT E(N)=98 MEV

Double-differential cross sections of the C-12(n, p) reaction have been measured at 98 MeV in the angular range 0-degrees-33-degrees for excitation energies up to 35 MeV. The spectra have been analysed in terms of multipole components, using peak fitting and decomposition techniques. The data have been compared with cross sections obtained in the distorted-wave Born approximation, using matrix elements from shell-model calculations. The unit cross section and the related strength of the spin-isospin part of the nucleon-nucleon interaction were determined from the extracted Gamow-Teller strength. Cross sections and fractions of strength exhausted by dipole and higher-multipole transitions were extracted and compared with data available in the literature.

The Be-9(n,p)Li-9 reaction and the Gamow-Teller unit cross section

Double differential cross sections of the Be-9(n,p)Li-9 reaction have been measured at 96 MeV in the angular range 0 degrees-27 degrees up to about 20 MeV excitation energy. In addition, the C-12(n,p)B-12 reaction has been measured in the same angular and

Measurement of neutron-induced fission cross-sections for natPb, 208Pb, 197Au, natW, and 181Ta in the intermediate energy region

Neutron-induced fission cross-section ratios natPb∕209Bi, 208Pb∕209Bi, 197Au∕209Bi, natW∕209Bi, 181Ta∕209Bi, and 209Bi∕238U have been measured in the 30–180 MeV energy range using the neutron beam facility at The Svedberg Laboratory in Uppsala. The 7Li(p,n) reaction was employed as a neutron source. The fission fragments were detected by thin-film breakdown counters. Cross sections at specific energies were determined using unfolding techniques with respect to the excitation function and the neutron spectra, the latter obtained from recent measurements and an evaluation. The absolute fission cross sections were obtained using the standard 238U(n,f) cross section. The natW(n,f) and 181Ta(n,f) cross sections have been measured for the first time. The results for 209Bi(n,f), natPb(n,f), 208Pb(n,f), and 197Au(n,f) cross sections have been compared with available literature data. A universal easy-to-use parametrization has been suggested for all measured cross sections. The common features of subactinide neutron-induced fission cross sections are found to be similar to those of the proton-induced fission data.

The Possible Use of a Spallation Neutron Source for Neutron Capture Therapy with Epithermal Neutrons

Spallation is induced in a heavy material by 72-MeV protons. The resulting neutrons can be characterized by an evaporation spectrum with a peak energy of less than 2 MeV. The neutrons are moderated in two steps: first in iron and then in carbon. Results from neutron fluence measurements in a perspex phantom placed close to the moderator are presented. Monte Carlo calculations of neutron fluence in a water phantom are also presented under some chosen configurations of spallation source and moderator. The calculations and measurements are in good agreement and show that, for proton currents of less than 0.5 mA, useful thermal-neutron fluences are attainable in the depth of the brain. However, the dose contribution from the unavoidable gamma background component has not been included in the present investigation.

A facility for studies of giant multipole resonances by fast heavy-ion scattering

Abstract A technique to study giant resonances by heavy-ion scattering in the energy range 100 A –470 A MeV using one quadrant of the CELSIUS storage ring as a spectrometer is presented. A test experiment using 250 A MeV 16 O on a 40 Ar target has been performed. The inelastically scattered particles could be separated from the background originating from the beam halo using a focal-plane telescope in coincidence with neutron detectors around the target. The resolution in the excitation-energy spectrum was 3.5±0.5 MeV, mainly determined by the beam size at the target.

The Pb-208(n,p)Tl-208 reaction at E-n=97 MeV

Abstract Double-differential cross sections of the 208 Pb(n,p) reaction have been measured at 97 MeV in the angular range 0°–30° for excitation energies up to 40 MeV. The experimental proton spectra have been compared with calculated spectra obtained with a statistical multistep direct reaction theory, in which charge exchange and inelastic response functions are described microscopically in the quasiparticle random phase approximation. The direct parts of the spectra have also been distributed on different multipole components by using a decomposition technique, based on sample angular distributions calculated within the distorted-wave Born approximation.

Status Report on the Development of a Spallation Neutron Source for Neutron Capture Therapy (NCT)

The eventual aim of the present study is to construct an accelerator-based intermediate-energy neutron source that would permit irradiation of neoplasms in the central nervous system by an intermediate-energy fluence rate of at least 109 n·cm−2.s−1 in fields of 4–8 cm in diameter. The accelerator should be of a moderate size to permit accommodation in a hospital environment. In the first instance, a prototype source has to be designed that could be conveniently tested in a laboratory dedicated to radiation research, such as the Paul Scherrer Institute (PSI), where the measurements here reported were performed.

Neutrons for Capture Therapy Produced by 72 MeV Protons

Neutrons (and protons) move in nuclear matter with an energy which is roughly the same in all nuclei, and which can be thought of as a temperature (of about 1011K). Neutrons from the fission process therefore emerge with an energy corresponding to this temperature; so do most but not all neutrons from the ‘spallation’ process, in which fast protons strike nuclei and liberate neutrons. The use of a proton beam or a reactor for the production of neutrons for capture therapy (NCT) thus present basically similar problems: to reduce the energy of the neutrons from the MeV level at which they emerge to the few keV required for NCT. There are however significant differences: the total intensity of a spallation source is much lower – some four orders of magnitude between the layout planned at the Paul Scherrer Institute (PSI) and the Petten reactor, for example. On the other hand, the source is much smaller: of the order of a cubic centimetre as against the cubic metre of a typical reactor. This means that it can be made mobile, as in the PSI layout, and in practice one can work closer to it. Again, less γ radiation is produced by a spallation source.