Á. Veres

The new prompt gamma-activation analysis facility at Budapest

Prompt gamma-activation analysis (PGAA) is an important complementary technique to conventional instrumental activation analysis that can be successfully used in a number of cases when INAA is not applicable. Therefore, a PGAA facility has been constructed at the recently refurbished and upgraded Budapest Research Reactor. It occupies the end position of a new curved themal guide of 30 m length and 2.5×10 cm2 cross section which provides a clean beam of low energy neutrons. The sophisticated HPGe-BGO γ-ray spectrometer system can be operated in Compton-suppression and pair-spectrometer modes simultaneously. The octal splitting of the main BGO improves efficient pair mode operation when coincidences between pairs of opposite segments and the HPGe detector are required separately. Gamma-gamma coincidence measurements will also be possible when the new multiparameter data acquisition system is completed. One of the main tasks at the new facility will be the accumulation of new spectroscopic data for detector calibration and standardisation, as well as for the construction of a more accurate prompt γ-ray library for the chemical elements. Various applications are planned, such as the determination of hydrogen in fullerenes and of toxic trace elements in environmental samples.

Comparison of experimental and theoretical high multipole-order internal conversion coefficients

Sixty-four accurately measured (1σ ⩽ 5%) internal conversion coefficients (ICCs) of high multipole-order (E3, M3, E4, M4) gamma-transitions have been compared with the theoretical values of Rosel et al. and Hager and Seltzer. Individual measurements were carefully examined and modified, if necessary. The discrepancies among the experimental values and the Rosel calculations tend to fall within a narrow band and do not reveal any dependence on atomic number, shell or transition energy. However, discrepancies between the experimental data and the calculations of Hager and Seltzer depend on transition energy, viz., increases up to 10% occur when the transition energy approaches the K binding energy. The calculations of Rosel et al. are preferred, and it is proposed that their third and fourth order ICCs are multiplied by factors of 0.975±0.010 and 0.975±0.005 respectively, to give better agreement with the experimental data. The accuracy of the corrected theoretical values is higher than most of the experimental data. This comparison also implies that the recommended source of ICCs and total transition probabilities needs to be reconsidered by the International Nuclear Structure and Decay Data Network.

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.

Cold neutron facility for prompt gamma-neutron activation analysis

The restart of the recently upgraded research reactor in Budapest is foreseen at the end of this year. A number of fast, thermal and cold neutron beams will serve for research, industrial and educational activities. One of the cold neutron guide end positions will be utilized for neutron capture prompt gamma-ray activation analysis (PGAA). Further development of the PGAA method as well as new applications in environmental research, biology and medicine are planned.

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.

Photoexcitation by gamma-ray scattering near threshold and giant dipole resonance

Photoexcitation of 4.5 h half-life 115mIn and 56 min half-life 103mRh isomers by inelastic gamma-ray scattering near threshold and in the giant dipole resonance region has been reviewed. In disagreement with earlier experimental results available in the literature, but in good agreement with our experiments published recently, present calculations indicate that above the photoneutron emission threshold the isomer excitation drops abruptly and remains orders of magnitude smaller than at the threshold, even around resonance maximum.

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.

Integral cross section of the99Tc(γ,γ′)99mTc reaction at 4 MeV

Abstract99mTc production was studied with the aid of photoexcitation by a 4 MeV endpoint energy bremsstrahlung from the LINAC of the Institute of Isotopes, Budapest. The intensity of the γ-flux was monitored by disc-shaped natural indium plates, placed in front of and behind the small cylindrical aluminium holders containing TcO2 samples in a powder form. Isomeric activities were measured through the 140 keV γ-line by a Ge spectrometer. The integral cross section at 4 MeV was found to be 63.3±7.1 μbMeV, which can be considered reasonable compared to the corresponding value of 55.3 μbMeV established for the115In (γ,γ′)115mIn reaction. We also attempted the photoexcitation of99mTc by irradiation with γ-rays from a 1.5×1015 Bq60Co source, but no isomeric activity could be observed. This places the first activation level between 1.33 and 4 MeV.