Thetis Reactor Research Articles

Recalibration of the irradiation facilities in the Thetis reactor, with an examination of the a versus E behavior in the keV neutron energy range

The reloading of research reactor Thetis (INW, Gent), in April 1996, involved the re-calibration of the irradiation facilities used for k0-NAA. In some channels, the values of the re-measured parameters f and a significantly deviated from the ones before reloading, and the effect of this on the concentration results is demonstrated. Some additional experimental work was done in two irradiation facilities, one well-thermalized and another with a hard neutron spectrum, in order to examine the behavior of a with neutron energies up to about 60 keV. This study was performed in the context of a lasting controversy regarding the accuracy of the k0 and Q0 values for the 95,97Zr-isotopes, especially in view of the high 94Zr(n,g) effective resonance energy. In the irradiation channels under consideration, no evidence was found of a versus energy dependence

Determination and evaluation of fission k0-factors for correctionof the 235U(n,f) interference in k0-NAA

In order to provide a tailored solution for the correction of the 235U interference in k0-NAA, fission k0-factors for the analytically relevant radionuclides/gamma-lines (versus gold as the comparator), are experimentally determined in four channels of the Gent THETIS reactor. A comparison is made with former data from the literature. The new evaluated results are to be implemented in the forthcoming upgrade of the DSMKayzero software package for k0-NAA.

The k0 and Q0 Values for the Zr-Isotopes: A Re-Investigation

The k0 and Q0 values for 94Zr(n,γ)95Zr(Eγ = 724.2+756.7 keV) and 96Zr(n,γ)97Zr(β−) 97mNb (Eγ = 743.4 keV) were re-investigated. The aim was to further improve the reliability of the neutron spectrum characterization (f and α monitoring) in k0-NAA, based on "bare monitor" methods with the use of these Zr radionuclides. So as to achieve this goal, experimental determinations were performed in three reactor centers: KFKI AEKI, Budapest (WWR-M reactor); INW, Gent (THETIS reactor); SCK·CEN, Mol (BR1 reactor). The results were: Q0(94Zr) = 5.306; Q0(96Zr) = 251.6; k0(95Zr, 724.2+756.7 keV) = 2.000E-4; k0(97Zr/97mNb, 743.4 keV) = 1.237E-5. This means that the newly evaluated k0-values are about 4.7% lower than the formerly reported ones. It is also emphasized that the 97Zr half-life is 16.74 hours, contrary to the ∼1% higher values usually reported.

Thirty years of naa developments and applications at the THETIS reactor of the Institute for Nuclear Sciences, Gent

A survey is given of the past and present fundamental developments and practical applications of neutron activation analysis performed at the reactor THETIS of the Institute for Nuclear Sciences, Gent, which has already now for more than 30 years played an internationally appreciated pioneering role in this domain. Whereas the applications were mainly dealing with the analysis of semiconductor and high-purity materials, biomedical matrices, geological materials, environmental samples and artefacts, the developments were directed towards the elaboration of irradiation, separation and counting schemes, the determination of nuclear activation and decay constants, and especially towards the introduction of a modern (so-called k0-) standardization method which—especially combined with a dedicated software package such as KAYZERO/SOLCOI© issued by DSM Research—proved to make NAA to a manageable and competitive analytical tool, nowadays applied in more than 50 NAA-labs worldwide.

Applicability of the 1/E1+α epithermal spectrum representation and the effective resonance energy $$\bar E_r$$ in NAA

The practical applicability of the 1/E1+α epithermal spectrum representation and of the effective resonance energy\(\bar E_r\) in NAA is investigated. Attention is paid to fundamental considerations such as definitions and approximations, error propagation functions, uncertainty statements, reliability of nuclear data and formal dimensioning problems. As to the latter, it is concluded that the applicability of the\(\bar E_r\)-concept is not dependent on the choice of a reference energy. The usefulness of the parameters α and\(\bar E_r\) in NAA is demonstrated by new experimental evidence obtained from α and Q0 determinations in three different reactors: the WWR-M reactor (Budapest, Hungary), the THETIS reactor (Gent, Belgium) and the DR-3 reactor (Riso, Denmark).

The accuracy and precision of the experimental α-determination in the 1/E1+α epithermal reactor-neutron spectrum

Some methods for the experimental α-determination in the 1/E1+α epithermal reactorneutron spetrum are critically compared with respect to their accuracy and precision. The analysis is based on the error propagation theory. Besides the general formulae numerical examples are elaborated for specific conditions in the Thetis reactor (Gent) and the WWR-M reactor (Budapest).

Instantaneous α-determination without Cd-cover in the 1/E1+α epithermal neutron spectrum

A new method is developed for instantaneous α-determination in the 1/E1+α epithermal neutron spectrum. It is based on coirradiation of three bare resonance detectors, followed by gamma-counting on a calibrated Ge(Li)-detector. The technique is applied in two channels of the Thetis reactor (Gent) and the results are critically compared with those obtained by other methods.

Modification and generalization of some methods to improve the accuracy of α-determination in the 1/E1+α epithermal neutron spectrum

Some methods described in the literature for the determination of α in the 1/E1+α epithermal neutron spectrum are critically reviewed with respect to their accuracy. The multi resonance—detector method with Cd-covered irradiations, as used by SCHUMANN and ALBERT, is generalized by subtracting the epithermal 1/v-tail and by introducing the effective resonance energy, as defined by RYVES. The two-detector method of RYVES is modified by using Cd-ratio measurements, thus eliminating the introduction of systematic errors due to the inaccuracy of absolute nuclear data. The adapted methods are applied in channel 15 of the Thetis reactor (Gent).

Average cross-sections for ( n, p) and ( n, α) reactions on 19F and 23Na in a fission neutron spectrum

The average cross-sections for the ( n, p) and ( n, α) reactions on 19F and 23Na in a 235U thermal fission neutron spectrum were determined by radioactivation. Calcium fluoride and sodium carbonate samples and an aluminum fast neutron flux monitor were irradiated in the Thetis reactor (Institute for Nuclear Sciences, Rijksuniversiteit Gent). The activities induced by the reactions of interest were measured with a detector of calibrated detection efficiency. Using σ F = 0.705 mb for the 27Al( n, α) 24Na monitor reaction, the following cross-sections were obtained: 19F( n, p) 19O: 0.83 ± 0.02 mb; 19F( n, α) 16N: 15.1 ± 0.2 mb; 23Na( n, p) 23Ne: 1.43 ± 0.02 mb and 23Na( n, α) 20F: 0.53 ± 0.02 mb. The results are compared with other experimental and evaluated data found in the literature.

Average cross-sections for (n, p) reactions on calcium in a fission-type reactor spectrum

The average cross-section in a fission-type reactor spectrum\(\bar \sigma _F\) was experimentally determined for the reactions42Ca(n, p)42K,43Ca(n, p)43K and44Ca(n, p)44K. Calcium carbonate samples and fast neutron flux monitors were irradiated with and without cadmium shielding in the Thetis reactor (Institute for Nuclear Sciences, Rijksuniversiteit, Gent). The potassium activities induced in the calcium carbonate samples were separated and purified by tetraphenylborate precipitation, after which they were measured with a Ge(Li)-detector of calibrated detection efficiency. On the basis of\(\bar \sigma _F = 0.64\) mb for the reaction27Al(n, α)24Na, the average cross-sections were as follows:42Ca(n, p)42K: 2.82±0.07mb;43Ca(n, p)43K: 1.89±0.05 mb;44Ca(n, p)44Ca(n, p)44K: 0.065±0.003 mb.