Standardization of radionuclide by beta(LS)-gamma coincidence counting using the geometry-efficiency variation method.

A liquid scintillation counting method consisting of three photomultiplier tubes for beta counters and a NaI(Tl) gamma counter has been developed for the standardization of radionuclides with the beta-gamma coincidence technique. The beta detection efficiency functions are obtained by means of a geometry-variation method developed in the present work; an array of beta detectors is moved uniformly at the same time from a centrally located counting vial to 50 mm. The method has been applied in the standardization of 60Co and 134Cs. Unquenched liquid scintillation samples with nominal count rates from 1000 to 6000 s-1 were prepared. The observed beta detection efficiencies with this method are from 90 to 45% in the case of 60Co, and from 84 to 50% for 134Cs. The output of each beta channel is summed together and compared with gamma data by the coincidence analyzer. The dead time of each counting channel is adjusted to be 20 micros, sufficiently long to suppress the afterpulses in the beta counting channel. The activity of each sample is determined by using the Cox and Isham formula. The obtained results are in good agreement with KRISS certified values.

Development of 3-PM liquid scintillation counting system with geometrical efficiency variation.

An improved triple-to-double coincidence ratio system has been developed to obtain efficiency variation by means of geometrical displacement, that is, three photomultiplier (PM) tubes move back and forth direction from a vial source. The detection unit of the system is located in a cylindrical chamber for light shielding, and a stepping motor connected to the three PM tubes controls the source-to-detector distance up to 50 mm with 1 mm resolution. In activity measurements of 14C and 204Tl using this measurement technique, the efficiencies varied from 0.60 to 0.90 for 14C and from 0.87 to 0.96 for 204Tl. In this study, the characteristics of the system are described as well as data acquisition and analysis methods.

An anticoincidence-shielded gamma-ray spectrometer for analysis of low level environmental radionuclides.

We developed an ultralow-level background gamma-ray spectrometer, using active and passive shield devices at the same time. Cosmic-ray-induced background is suppressed by means of active shield devices consisting of plastic scintillating plates of 50mm thick and anti-coincidence electronic system. The observed background rate was 0.34 s(-1) (=0.12s(-1) per 100 cm(3) Ge volume) for energy regions between 50 and 3000 ke V. The detection efficiency curve for 10(3)ml Marinelli beaker samples is obtained over all the energy regions. The advantages of the method are demonstrated by measuring the activity of 137Cs in powdered milk sample prepared without taking any chemical procedure. The MDA for 137Cs is estimated to be (17+/-1.7)mBq at a confidence level of 95% and it is about a factor of 10 lower than the MDA obtained from the previous cryostat assembly with 10-cm thick lead shielding.

Standardization of 152Eu and 88Y.

The activities of 152Eu and 88Y have been measured by the digital coincidence counting (DCC) technique and compared with those of the conventional coincidence counting technique. In the DCC, the pulse amplitudes from both beta and gamma channels are digitized in commercial ADCs and the pulse input times are simultaneously determined by 10 MHz oscillation clock installed in the coincidence module. The activity of the measured source is determined from the information of pulse height and input time for each pulse by off-line analysis. The activity of 152Eu was determined from extrapolations of efficiency functions obtained at 3gamma gates and the result showed a good agreement with that of the conventional method with 0.2% of uncertainty. The activity of 88Y was also determined by the DCC technique, and the reliability of the result was proved through Asia Pacific Metrology Program (APMP) inter-comparison of 88Y.