Measurement of the167Er content in absorbing reactor materials based on the detection of the prompt gamma radiation

In the last few years the use of burning-up absorbers in reactors has increased significantly. This is a result of their important functions. At the beginning of the operating period they compensate a large fraction of the reserve of reactivity for burnup and slagging of nuclear fuel, they make it possible to reduce the number of mobile control rods and thereby simplify the construction of the reactor, they aid in equalizing the energy-release field over the active zone of the reactor, and they make it easier to meet the nuclear-safety requirements. There are a number of works concerning the theory of the calculation and practical application of burning-up absorbers [1-3]. The isotopes of rare-earth elements, in particular, erbium, are promising burning-up absorbers for thermal and intermediate reactors. There are six isotopes of erbium, which, for all practical purposes, are not activated. The absorption cross section for thermal neutrons is maximum for the isotope 1~TEr, which determines the absorbing properties of the natural mixture. The work of the burning-up absorber in a reactor is based on the process of radiative capture, when the isotope that strongly absorbs neutron becomes after capture a nonabsorbing isotope. The purpose of this work is to study the possibility of recording acts of radiative capture of neutrons based on the prompt gamma radiation in the reaction 1~TEr (n, 7)168Er and to measure the content of the isotope 167Er with the help of this radiation. It is obvious from the scheme of gamma transitions of the excited 16SEr nucleus [4] that the probability for the emission of the 185 keV line is high (~82%); this line can be used to solve the problem posed. In this work we employed a Po-Be source of neutrons with a yield of ~10 s sec-1; it was placed at the center of a moderating parafin sphere 18 cm in diameter. In working out the method, to make it easier to separate the required gamma line reliably the measurements were performed with samples of the separated isotope 167Er with an enrichment of 95.6%. Er203 powder was poured into thin-walled polyethylene caps (24 mm in diameter and 3 mm high; the wall was 0.1mmthick) with a cover. The spectrum of the prompt gamma radiation from the sample was recorded with DGDK-32A semiconductor Ge-Li detector with a volume of 32 cm 3. The detector was surrounded with lead shielding 40 mm thick. The spectrometric channel included, aside from the detector, a low-noise preamplifier and a Langur amplifier with a model LP4900 Nokiya analyzer with different output devices. Figure 1 shows the spectrum of the prompt captured gamma radiation from a 0.4 g sample of the isotope 167Er, measured over a period of 1.5 h. The 185 keV line is clearly distinguished in the graph. The area under the total absorption peak was determined by the method described in [5]. The number of counts in the photo-peak is of a statistical nature and depends on the exposure time. To employ the prompt 185 keV gamma line of the isotope 168Er for quantitative determination of the 167Er content in the burning-up absorber material, it is first necessary to construct a calibration curve for standards of this material containing different known quantities of 167Er. Figure 2 shows a curve of the area of the 185 keV photopeak versus the mass of the isotope 1~TEr, contained in the burning-up absorber material. Erbium is usually employed in the rods of burning-up absorbers together with a fill whose absorbing and scattering properties differ from those of erbium. For this reason the curve obtained for the pure isotope 167Er cannot be employed as a calibration curve for the burning-up absorber material. The graph shown in Fig. 2 was employed as a calibration curve to determine the quantity of the isotope 167Er in a sample containing an unknown quantity of this isotope. The following conditions were satisfied in doing so: the sample studied and the standard has the same shape and size; the geometrical conditions of the experiment were identical both during the measurement process and during calibration; in both cases one and same neutron source was employed. For the