Speaker
Description
The detection of long-lived radionuclides through ultra-sensitive isotope ratio measurements via accelerator mass spectrometry (AMS) offers opportunities for precise measurements of neutron capture cross sections, e.g. for nuclear astrophysics. For specific reactions AMS provides a unique tool to pin down uncertainties which is important to address existing discrepancies. This approach directly counts the number of reaction products present in the sample after the neutron activation rather than measuring the associated γ-radiation or the emitted particles during the irradiation. A series of samples was irradiated at Karlsruhe Institute of Technology (KIT) with neutrons simulating a Maxwell-Boltzmann distribution of 25 keV, and also with quasi-monoenergetic neutrons of energies up to 500 keV and at thermal energies. In this way precise neutron-capture cross section data were obtained for a series of isotopes, among them $^{9}$Be, $^{13}$C, $^{35}$Cl, $^{40}$Ca, $^{54}$Fe and for $^{14}$N(n,p) by AMS.
Using AMS, the respective spectrum-averaged cross section is simply the product of two quantities, the neutron fluence and the isotope ratio of reaction product and educt; the latter quantity the direct result obtained from AMS. The reaction $^{197}$Au(n,γ) was used as fluence monitor for all activations. From these data, Maxwellian average cross sections (MACS) are derived by normalizing the known cross-section to energy dependence. The reaction products were measured as isotope ratios relative to their respective individual AMS standards that are completely independent for different nuclides. Thus, this technique represents a truly complementary approach, completely independent of any other experimental method (such as time-of-flight method or decay counting); in addition the AMS results are also fully uncorrelated for different reactions. When compared with existing data from complementary techniques, cross sections obtained through AMS suggest a small but consistent systematic difference. Potential reasons for this discrepancy and potential unknown systematic uncertainties will be discussed
