The study of prompt and delayed muon induced fission

Mass yield and total kinetic energy release (TKE) distributions of fragments from prompt and delayed muon induced fission, separately, have been measured for the isotopes²³⁵U,²³⁸U,²³⁷Np and²⁴²Pu. The distributions from prompt muon induced fission are compared with the corresponding distributions from hadronic reactions and from spontaneous fission (s.f.). The distributions from the delayed muonic fission processes are compared to the distributions for neutron and proton-induced fission. No mass distributions measured in the prompt muonic fission process show any signature, which can be attributed to the presence of the muon. Differences observed between the TKE distributions of prompt muon induced and hadron induced fission can be explained by the screening effect of the negative charge of the muon bound in the orbit of one of the fission fragments. The observed yield of symmetric muon induced fission was found to be defined merely by the value of the excitation energy.     

Paritätsreinheit der Kernzustände in Rb85

Nuclear γ-radiation must be circularly polarized if parity conservation does not hold strictly in nuclear states. From a polarisation measurement of the 513 keV γ-line in Rb⁸⁵ an upper limit for the relative amplitude of the irregular wave function F≦5,5·10^?6 was obtained.

     

Elastic scattering of35Cl from27Al,58, 62Ni,116, 120, 124Sn and141Pr

Elastic scattering angular distributions for³⁵Cl from²⁷Al,^{58, 62}Ni,^{116, 120, 124}Sn and¹⁴¹Pr have been measured at energies between 100 and 170 MeV. Optical model analyses have been performed to determine reaction cross sections, strong absorption radii and grazing angular momenta. The results are compared with the corresponding quantities extracted by means of the Fresnel scattering model. Near the interaction barrier the quarter point method yields reaction cross sections systematically smaller than the optical model results. The interaction radii, however, do not show significant differences or energy dependencies. These radii are compared with trend formulae for interaction radii, and comparisons with fusion radii are made.     

Polarisations-Korrelations-Messungen am Na24

An arrangement for the measurement of angular correlations betweenβ-particles and circular polarizedγ-radiation is described and a result on Na²⁴ is given. A correlation coefficientA=(+ 0,12± 0,03) was obtained, which indicates a ratio of Gamow-Teller to Fermi matrix elementM _G /M _F > 10.     

Investigation of α-induced reactions on niobium and tantalum

Excitation functions of (α,xn yp) reactions on⁹³Nb withy≦6 andx≦10 have been measured in four different irradiations from 16 to 171 MeV bombarding energy by Ge(Li) spectroscopy of the residual activity of stacked foils. Similarly, previous measurements of¹⁸¹Ta(α,x n y p) reactions from 15 to 103 MeV were extended up to 170 MeVα-energy for residual nuclei withy≦4 andx≦12. Including isomeric states a total number of 37 residual activities could be identified in⁹³Nb+α. From the rather complexγ-decay spectra altogether 28 different excitation functions of sizable length could be established. For181Ta+α 10 different residual activities were analyzed so that together with the former data a set of 15 excitation functions is available. The whole set of excitation functions provides a large data basis for probing the validity of combined equilibrium and preequilibrium reaction models in a wide energy range.     

The study of prompt and delayed muon induced fission

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture ratesΛ _c are 1.392(4)·10⁷/s for²³⁷Np, 1.290(7)·10⁷/s for²⁴²Pu and 1.240(7)·10⁷/s for²⁴⁴Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement ofΛ _c for²³⁹Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.     

Evaporation residue excitation functions for 35Cl induced reactions of 112, 120Sn and 141Pr

Evaporation residue excitation functions have been measured for ³⁵Cl ions up to 170 MeV (lab) on targets of ^{112, 120}Sn and ¹⁴¹Pr. These values are combined with previously reported fission excitation functions to give fusion excitation functions. Classical analysis of these results yield fusion barriers and fusion radii. Earlier results are confirmed: as the target atomic number increases the required density penetration at the fusion barrier increases. The percentage of the reaction cross section undergoing fusion monotonically decreases as the targets go from ²⁷Al to ¹⁴¹Pr. The nuclear contribution to the fusion barrier deduced from these measurements is compared with several model predictions and is found to be in satisfactory agreement with the models due to Bass and Krappe and Nix.     

Muon capture rates in233U,234U,235U,236U,238U,and237Np

The time spectra for muon induced fission of²³³U,²³⁴U,²³⁵U,²³⁶U,²³⁸U, and²³⁷Np were measured simultaneously, detecting both fragments in coincidence in a fast multi parallel plate avalanche detector. The observed mean lifetimes τ are 68.9 ± 0.3 ns, 70.6 ± 0.2 ns, 72.2 ±0.2 ns, 74.3 ± 0.3 ns, 77.0 ±0.4 ns, and 69.8 ±0.2 ns for the above isotopes, respectively. No second time component, tentatively ascribed to a possible fission isomer, was observed. The deduced total capture rates are compared with theoretical predictions.     

An upper limit for the (α, π −) reaction on181Ta

In order to search for traces of the¹⁸¹Ta(α, π ^?)¹⁸⁵Os reaction a foil stack of very pure tantalum was bombarded with anα-beam of 173 MeV. A radiochemical separation method was applied to four parts of the stack. So two sources corresponding to energies above and below the (α, π ^?) threshold were obtained and searched for contents of¹⁸⁵Os. Using a well shielded Ge(Li) detector weak 646 keV transitions belonging to¹⁸⁵Os were observed for all samples. Since the origin of this activity is uncertain for the samples below threshold only an upper limit of 100 nb can be set for the (α, π ^?) reaction on¹⁸¹Ta above threshold.