L. Lakosi

Half-lives of 111mCd, 113mIn and 115mIn

Abstract Bremsstrahlung from 4.5 MeV electrons on a platinum target was used to produce nuclear isomers in three nuclides through (γ,γ′) excitation of higher lying states and their half-lives were measured. The obtained values are 48.54±0.05 min for 111m Cd, 1.660±0.005 h for 113m In and 4.485±0.004 h for 115m In. The intensity ratio of 497 keV/336 keV gamma transitions in the decay of 115m In was determined as 1.04±0.05 × 10 −3 . The methods of analysis and the results are discussed.

A new detection method of99Tc by nuclear excitation

A new nuclear excitation process,99Tc (γ, γ′)99mTc reaction, was applied for the first time to radioactivation analysis of technetium. Bremsstrahlung irradiation of99Tc samples gave the reaction product99mTc which emits γ-ray measurable with ease by a semiconductor detector. The production rate of99mTc per μg99Tc was linearly correlated with the flux of bremsstrahlung. The detection limit of99Tc was estimated to be nanogram order (0.63 Bq99Tc) under the optimum irradiation condition. Possible interference by100Ru(γ, p)99mTc reaction was also studied, which could be discriminated from the (γ, γ′) reaction by simultaneously occurring98Ru (γ, p)97Ru reaction.

Integral cross section of the 99Tc(γ, γ′)99mTc reaction in the 15–50 MeV energy region

Photoexcitation of radioactive 99Tc was studied by irradiation with 15–50 MeV bremsstrahlung. The integral cross sections of the 99Tc(γ, γ′)99mTc reaction and the reference 115In(γ, γ′)115mIn reaction were found to be nearly constant as (5.8 ± 1.0) × 10−27 cm2 MeV and (9.3 ± 0.4) × 10−27 cm2 MeV, respectivel in the given energy range. The large second increase of the cross section described in earlier reports was observed neither for 115In nor for 99Tc.

Investigation of fast neutron emission of spent fuel assemblies with CR-39 track etch detector

A method based on fast neutron detection with CR-39 track etch detector has been developed for monitoring burnup of spent reactor fuel assemblies.

Neutron interrogation of shielded/unshielded uranium by a 4 MeV linac.

A non-destructive active assay method was developed for revealing illicit trafficking of uranium. Photoneutrons produced in beryllium or heavy water by bremsstrahlung from a linac induced fission in the samples. Delayed fission neutrons were detected by a neutron collar built up of (3)He counters embedded in polyethylene moderator. High-enriched uranium samples shielded and unshielded by lead up to 14 mm thickness were detected, with a performance practically unaltered. 25 mg (235)U can be revealed in a 1 min interrogation time.

Half-life of 77mSe, 167mEr and 197mAu

Abstract Isomer states of three nuclides were excited by bremsstrahlung from a 4.5 MeV linear electron accelerator, and their half-lives were measured by counting their γ radiation with a well-type NaI(Tl) detector. A pneumatic rabbit system was used for fast sample transport. The obtained half-life values in seconds are 77 m Se : 17.31 ± 0.08, 167 m Er : 2.269 ± 0.006 and 197mAu: 7.73 ± 0.06. The results are compared with literature data. New half-life measurements were performed on three short lived odd-A nuclear isomers, since the uncertainty of the values published earlier is very striking. The isomeric states were excited by (γ, γ′) reaction. A relatively low exciting energy (below the energy required for particle emission) was chosen to obviate excitation of other nuclides.

Determination of the production rate of low intensity isomeric transitions

Abstract Flat 2 π and cylindrical 4 π multiwire proportional counters were built for counting low energy internal conversion electrons from the nuclear isomers 83m Kr, 103m Rh and 189m Os, induced by irradiation with high intensity 60 Co and 4 MeV bremsstrahlung sources. The β-decay of 176m Lu was recorded by a plastic scinillator. In this way higher sensitivities were attained than by detecting γ-rays or characteristic X-rays associated with the isomeric transitions, and the excitation of 189m Os by low energy 137 Cs and 300 kV X-ray sources also became detectable. Comparatively large isomeric activities produced by linac irradiation were standardized by a Ge spectrometer for calibrating proportional and scintillation counting.

Disturbing effects in spent fuel assay using CR-39 detectors

Abstract Neutron emission of spent reactor fuel assemblies was studied with CR-39 solid state track detectors in a nuclear plant. To reduce the effect of disturbing events, a shape selection method was used. For typical exposures the γ-dose fell in the interval of 0.5–3 kGy. The sensitivity decrease was found to be 23±5%kGy γ-dose The overall effect of neighbouring assemblies was within the statistical uncertainty (5%), i.e negligible.

Anticoincidence counting of low intensity isomeric transitions

A new detector system was developed to count the highly converted low-energy isomeric transitions of long-lived 189mOs and 193mIr produced by (γ, γ′) reactions upon irradiating stable elemental Os and Ir samples with gamma photons from a 200 kV X-ray generator and high-activity 137Cs and 60Co sources. This system consists of multiwire flat 2π proportional chambers for internal counting, with sensitive areas 90, 20 and 6 cm2, surrounded by two large 2π counters connected in anticoincidence to reduce background. Specific activities in the order of Bq g−1, corresponding to counting rates lower than 0.1 cpm, were obtained. Photoproduction of 193mIr was detected for the first time below an excitation energy of 2.75 MeV. Excitation of 189mOs by 200 keV endpoint energy X-rays due to an interaction via nuclear excitation by electronic transition (NEET) was also observed.

Photoneutron production by bremsstrahlung in the target and the converter

Abstract A simple and fast method—based on a statistical cascade neutron evaporation model—is presented to estimate the photoneutron spectrum produced in the target upon A (γ, γ′) A m type photoexcitation experiments in the 15–30 MeV beam energy region. It is shown that in early measurements—under unfavourable conditions—the (n, n′) m contribution may have resulted in significant distortion in the measured yield curves. By the same approach the photoneutron spectrum accompanying bremsstrahlung is estimated in the 15–50 MeV beam energy region. For a 1 mm thick platinum converter the average energy of the resulting spectra is in the range of 0.9–1.2 MeV and the spectrum shape is characteristic of neutron evaporation process.