Half-lives of 221Fr, 217At, 213Bi, 213Po and 209Pb from the 225Ac decay series

The half-lives of (221)Fr, (217)At, (213)Bi, (213)Po, and (209)Pb were measured by means of an ion-implanted planar Si detector for alpha and beta particles emitted from weak (225)Ac sources or from recoil sources, which were placed in a quasi-2π counting geometry. Recoil sources were prepared by collecting atoms from an open (225)Ac source onto a glass substrate. The (221)Fr and (213)Bi half-lives were determined by following the alpha particle emission rate of recoil sources as a function of time. Similarly, the (209)Pb half-life was determined from the beta particle count rate. The shorter half-lives of (217)At and (213)Po were deduced from delayed coincidence measurements on weak (225)Ac sources using digital data acquisition in list mode. The resulting values: T1/2((221)Fr)=4.806 (6) min, T1/2((217)At)=32.8 (3)ms, T1/2((213)Bi)=45.62 (6)min, T1/2((213)Po)=3.708 (8) μs, and T1/2((209)Pb)=3.232 (5)h were in agreement only with the best literature data.

Decay data measurements on 213Bi using recoil atoms

In this work, (213)Bi has been separated from an open (225)Ac source by collecting recoil atoms onto a glass plate in vacuum. The activity of such recoil sources has been measured as a function of time, using an ion-implanted planar Si detector in quasi-2π geometry. From these measurements, a new half-life value of T1/2((213)Bi)=45.62 (6)min was derived. Additionally, high-resolution alpha-spectrometry measurements were performed at a solid angle of 0.4% of 4πsr, to verify the energies and emission probabilities of the α-emissions from (213)Bi. Using (225)Ac, (221)Fr, (217)At and (213)Po peaks as reference peaks, the measured (213)Bi α-peak energies at Eα,0=5878 (4)keV and Eα,1=5560 (4)keV were about 10keV higher than validated data. The relative α-particle emission probabilities of (213)Bi, Pα,0=0.9155 (11) and Pα,1=0.0845 (11), and the (213)Bi alpha branching factor, Pα=1-Pβ=2.140 (10)%, are compatible with recommended values, but have a higher accuracy.

Measurement of the 225Ac half-life

The (225)Ac half-life was determined by measuring the activity of (225)Ac sources as a function of time, using various detection techniques: α-particle counting with a planar silicon detector at a defined small solid angle and in a nearly-2π geometry, 4πα+β counting with a windowless CsI sandwich spectrometer and with a pressurised proportional counter, gamma-ray spectrometry with a HPGe detector and with a NaI(Tl) well detector. Depending on the technique, the decay was followed for 59-141 d, which is about 6-14 times the (225)Ac half-life. The six measurement results were in good mutual agreement and their mean value is T(1/2)((225)Ac)=9.920 (3)d. This half-life value is more precise and better documented than the currently recommended value of 10.0 d, based on two old measurements lacking uncertainty evaluations.

The use of solid angle for alpha detector efficiency in 226Ra analyses of soil samples

In the frame of proficiency tests organized by IAEA and IRMM, the specific activity concentration of radium-226 in soil has been measured. The BaSO(4) co-precipitation technique has been used. Normally, in this method, the detector efficiency of the alpha spectrometer is determined using a (226)Ra source with known activity. As an alternative to using a (226)Ra standard, we calculated the detector efficiency from the relative solid angle subtended by the detector on the soil samples. The accuracy of this method depends on the uncertainty of geometrical properties and the distribution of activity within the source. An uncertainty budget is provided. The method was applied successfully in the intercomparisons.

Measurement of the 109Cd half-life

The half-life of (109)Cd was measured by following the decay of sources from a radiochemically pure solution with two different measuring systems: an ionisation chamber and a high-purity germanium (HPGe) detector. The measurements were performed over a period of 3.6 years, i.e. about 2.8 half-lives of (109)Cd. The resulting half-life values and detailed uncertainty budgets (k=1) are presented for both systems. The result obtained with the ionisation chamber, 462.36 (33) days, and the one obtained with the HPGe detector, 461.92 (76) days, are mutually consistent. The weighted mean of our measured values, T(1/2)((109)Cd)=462.29 (30) days, is consistent with the currently recommended values of 461.4 (12) days (Schönfeld and Dersch, 2004; IAEA, 2007) and 462.0 (3) days (Xiaolong et al., 2010). From a set of selected experimental values published after 1970, a "partially weighted mean" (Pommé and Spasova, 2008) of T(1/2)((109)Cd)=462.36 (39) days was calculated. More measurements are needed to resolve the discrepancies among literature data and to reduce the final uncertainty on the (109)Cd half-life.

Measurement of the (54)Mn half-life

The half-life of (54)Mn was measured by following the decay of sources from a radiochemically pure solution using three different measuring systems: an ionisation chamber, a high-purity germanium (HPGe) detector and two 7.5 cm (diameter) × 7.5 cm (height) NaI(Tl) scintillation detectors in opposite position. The measurements were performed over a period of 3 years, i.e. about 3.5 half-lives of (54)Mn. The resulting half-life values and detailed uncertainty budgets are presented for the three measuring systems. The half-life obtained with the ionisation chamber, 312.32 (9) days, is consistent with but more precise than the ones obtained with the HPGe detector, 311.9 (5) days and the NaI(Tl) detectors, 311.9 (6) days, respectively. Our final half-life value of 312.32 (9) days is rather consistent with the currently recommended values of 312.29 (26) (IAEA, 2007) and 312.13 (3) days (Helmer and Schönfeld, 2004), even though the uncertainty of the latter may be underestimated. From a partially weighted mean (Pommé and Spasova, 2008) of selected experimental values published after 1970, a new best estimate of T(1/2)((54)Mn)=312.20 (8) days was calculated.

Half-life measurement of 124Sb

The half-life of (124)Sb was determined experimentally by following the decay of a source from a radiopure solution with a Centronic IG12 ionisation chamber. Thousands of measurements were performed over a period of 358 days, i.e. about six half-life periods. However, the data analysis was restricted to the first 221 days, in order to limit the dominant uncertainty component associated with the hypothetical possibility of a systematic error on background subtraction. The resulting value for the (124)Sb half-life, 60.212 (11) days, is found to be in very good agreement with published values, but carries a lower uncertainty. Major uncertainty contributions pertain to possible systematic errors in background correction, long-term changes in source-detector geometry and medium- and long-term instability of the instrument. Additional measurements were performed with a high-purity germanium detector to confirm the above value.

alpha-Particle and gamma-ray spectrometry of a plutonium solution for impurity determination

A highly enriched (240)Pu solution was measured by alpha-particle and gamma-ray spectrometry to determine other radionuclides present in the material as impurities. Low activities of (238)Pu, (241)Am, (243)Cm and (244)Cm were determined by measuring thin sources, made from the original solution, in a high-resolution alpha-particle spectrometer. The sources were prepared by evaporating the plutonium solution on quartz plates in a vacuum chamber. From the ingrowth of (241)Am in the original solution, the amount of (241)Pu could be calculated. After radiochemical separation of (241)Am, the plutonium was measured by high-efficiency alpha-particle spectrometry to determine the amount of (238)Pu. The enriched (240)Pu material was also measured by high-resolution gamma-ray spectrometry, using two different HPGe detectors to determine the impurities of (239)Pu and (241)Am. The preparation of the sources and the measurement methods are described and discussed. The measured impurities, given in % of the (240)Pu activity, are compared with the values on the certificate.

Cascades of pile-up and dead time

Count loss through a cascade of pile-up and dead time is studied. Time interval density-distribution functions and throughput factors are presented for counters with a series arrangement of pile-up and extending or non-extending dead time. A counter is considered, where an artificial dead time is imposed on every counted event, in order to control the length and type of dead time. For such a system, it is relatively easy to determine an average count-loss correction factor via a live-time clock gated by the imposed dead-time signal ('live-time mode'), or otherwise to apply a correction factor based on the inversion of the throughput function ('real-time mode'). However, these techniques do not account for additional loss through pulse pile-up. In this work, counting errors associated with neglecting cascade effects are calculated for measurements in live-time and real-time mode.

The solid angle subtended by a circular detector for a linear source

An exact solution is presented for the solid angle subtended by a circular detector for a linear source. It is based on Conway's equation for a non-coaxial point source, involving an integral of Bessel functions [Conway, J.T., 2006. Generalizations of Ruby's formula for the geometric efficiency of a parallel-disk source and detector system. Nucl. Instrum. Methods A 562, 146-153]. Some numerical examples are calculated and compared with the results obtained with a recently published closed formula (Galiano, E., Pagnutti, C., 2006. An analytical solution for the solid angle subtended by a circular detector for a symmetrically positioned linear source. Appl. Radiat. Isot. 64, 603-607), which is incorrect but can be improved by applying a correction factor.