H.H. Thies

Photonuclear experiments using combined energy and intensity sweeping

Abstract Accuracy and resolution of photonuclear bremsstrahlung experiments can be improved by using combined “energy and intensity sweeping”. This means that accelerator energy and intensity are changed with each accelerator pulse according to a repetitive pattern, and has the beneficial effect of greatly reducing systematic cross-section errors. The method and its limits are demonstrated by a universal model experiment.

Effective decorrelation of errors of photon difference data

Abstract The propagation of random errors of photonuclear yield data in a photon difference type transformation generates cross-section errors, which are correlated. Methods are discussed here which minimise the effects of this correlation and hence help to make the cross section interpretation straightforward and unambiguous.

Interference effects in cross sections calculated by photon difference methods

Abstract The photon difference type calculation transforms the photo-nuclear yield into the corresponding cross section. For a cross section containing very narrow resonances approximations in the transformation calculation can simulate interference like effects in the derived cross section, which can be caused by truncation errors and by the use of an incorrect bremsstrahlung spectrum.

The photoneutron cross section of 7Li

Abstract The 7Li photoneutron cross section was measured from threshold to 19.5 MeV using bremsstrahlung from an electron synchrotron. The accelerator was operated using yield scanning techniques. Neutrons were detected by an array of high-efficiency BF3 counters. The cross section shows one major maximum at 17 MeV, and some less prominent structure at 13.75 MeV and 14.65 MeV. A small resonance was observed at 7.46 MeV which can be attributed to an M1 transition to the known 7.456 MeV, 5 2 − state, in 7Li.

Measurement of photonuclear cross-sections with quasi-monochromatic photons using bremsstrahlung

Abstract A simple novel experimental method is described which makes it possible to measure individual photonuclear cross-section ordinates with bremsstrahlung in complete equivalence to an idealised experiment using a near-monochromatic gamma beam. Advantages and limitations of this method are discussed.

The photoneutron cross section in 13C

Abstract The photoneutron cross section in 13 C was measured using bremsstrahlung from an electron synchrotron operated in energy sweeping mode. Neutrons were detected by a multi-BF 3 -counter detection system. Decorrelated cross-section ordinates were calculated directly using a modified Leiss-Penfold analysis. The cross section from threshold to 25 MeV shows detailed structure, some nine maxima, in agreement with recent continuum shell-model calculations.

Systematics of nuclear level density parameters of nuclei deficient in one neutron

Abstract Photoneutron mean energies of 38 elements were measured as a function of peak bremsstrahlung energy for elements with 23 ≦ Z ≦ 83. Results are compared with neutron mean energies calculated from statistical theory, using for nuclear level densities modified Fermi gas formulae with and without pairing corrections and a constant temperature formula. Except near closed shells the Fermi gas formula with pairing corrections gives reasonable to good correlation between experimental and theoretical data. Derived values of the nuclear level density parameter a -except near Z = 82-are in quantitative agreement with those from recent neutron resonance data.

The photoneutron production cross section in 9Be

Abstract The photoneutron production cross section in 9 Be was measured with a synchrotron using energy sweeping. Neutrons were detected with a multi-BF 3 -counter flat-response detection system. Under high resolution the cross section shows 23 maxima between 6 and 27 MeV which are caused predominantly by E1 transitions.

Calculation of flat-response fast-neutron detection systems employing finite size BF3-counters

Abstract Making the assumption that φ the rate at which neutrons are emitted from a source, is proportional to the volume integral of the slow neutron density throughout the moderator, one can calculate a weighting function ⨍( r ) such that ∫ moderator ⨍( r )I( r ) d V is proportional to φ where I( r ) is the count-rate of a finite size BF3-counter at position coordinate r . The weighting function ⨍( r ) is independent of the energy spectrum of the source neutrons. The calculation is demonstrated by a sample calculation of a flat-response fast neutron detection system.

The photoneutron cross section of 6Li

Abstract The photoneutron cross section of 6 Li has been measured from threshold to 17 MeV using bremsstrahlung from an electron synchrotron. Neutrons were detected by a high-efficiency multi-BF3-counter detection system. The cross section shows broad peaks at 12 MeV and 16 MeV with no other structure apparent.