Monitoring of neutron flux changes in short-lived neutron activation analysis
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Monitoring of neutron flux changes in short-lived neutron activation analysis S. Landsberger • K. Dayman
Received: 31 July 2012 / Published online: 21 August 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract It is well known variations in neutron fluxes can adversely affect the final result in neutron activation analysis. The monitoring of neutron flux changes are usually described for medium and long-lived NAA using foils of cobalt, gold, zirconium, etc. However, for short-lived neutron activation analysis there appears to be no systematic study of the variations of the neutron flux. With our new automatic pneumatic system, where irradiation timing, decay and counting and position are very reproducible, we have performed a series of experiments using thermal and epithermal neutrons using aluminum wire as a monitor to monitor the neutron fluxes. Our experiments confirm that neutron flux fluctuations in the worst case can be up to ±12 % with a SD of 2–3 %. This effect can be seen regardless of the irradiation time and must be taken into consideration to achieve the best result. Keywords Neutron activation analysis Neuron flux variations Flux monitoring
Introduction Regardless of the methodology of determining the elemental concentrations in NAA the monitoring of neutron fluence samples receive is of utmost importance in achieving accurate results. Over the last two decades many groups have looked at the errors which are involved in NAA when using the comparator or k0 methods. Some of these findings include the overall measurement uncertainty of k0 [1], S. Landsberger (&) K. Dayman Nuclear Engineering Teaching Lab, University of Texas at Austin, R-9000, Austin, TX 78712, USA e-mail: [email protected]
uncertainty budget for k0 NAA [2], variation of neutron flux in an irradiation container [3] the 1993 ISO guide to the expression of uncertainty in measurement applied to NAA [4], and a very recent excellent review article on NAA as a primary method of measurement by Greenberg et al. [5] and all the references therein. What is surprising is that while many of the articles have sophisticated treatment of the uncertainty budget in NAA, almost all the focus is on changes in neutron fluence received by the samples pertains to medium- and long-lived NAA. Various flux monitors such as Al–Au, Co, Zr, etc. are very effectively used in the comparator and k0 methods. For many years in our own laboratory we have been using sulfur powder as a flux monitor in short-lived thermal NAA using the 36S(n,c)37S reaction with its 5.05 min half-life and 3.102 keV gamma [6–8] It has a poor thermal cross-sections making it ideal to handle due to its low induced radioactivity. However, by using about 1 g of material allows adequate counting statistics. On the other hand due to the epithermal flux being about 10 % of the thermal flux, the 36S(n,c)37S reaction is not adequate enough to yield enough statistics for quality control for epithermal NAA. To overcome this deficiency aluminum wire was chosen since it had both g
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