Annika Tovedal

Isotope amount ratio measurements by ICP-MS: aspects of software induced measurement bias and non-linearity

The effect of a post-acquisition software induced bias on the measurement of the 235U/238U isotope amount ratio in the reference material IRMM-184 was studied. The total uranium concentration ranged from 7 to 2500 pg g−1 [80 (235U) to 3500000 (238U) counts per second], thus covering the entire pulse counting range of the ICP-MS instrument. The bias for the measured ratio increases with decreasing count rate, and it is mostly governed by the count rate of the 235U isotope. Blocking the data into large subgroups is one way of reducing the effects of the bias; however, a decreased number of blocks is accompanied by an increased scattering of the standard uncertainty. Using a more laborious, manual data evaluation procedure of a less processed raw measurement signal not only solves the problem, it also increases the linear measurement range by at least ten times. At the lowest concentration, 7 pg g−1 total uranium concentration, the combined uncertainty of the observed ratio was less than 5% (k = 2).

Time optimization of (90)Sr measurements: Sequential measurement of multiple samples during ingrowth of (90)Y.

The aim of this paper is to contribute to a more rapid determination of a series of samples containing (90)Sr by making the Cherenkov measurement of the daughter nuclide (90)Y more time efficient. There are many instances when an optimization of the measurement method might be favorable, such as; situations requiring rapid results in order to make urgent decisions or, on the other hand, to maximize the throughput of samples in a limited available time span. In order to minimize the total analysis time, a mathematical model was developed which calculates the time of ingrowth as well as individual measurement times for n samples in a series. This work is focused on the measurement of (90)Y during ingrowth, after an initial chemical separation of strontium, in which it is assumed that no other radioactive strontium isotopes are present. By using a fixed minimum detectable activity (MDA) and iterating the measurement time for each consecutive sample the total analysis time will be less, compared to using the same measurement time for all samples. It was found that by optimization, the total analysis time for 10 samples can be decreased greatly, from 21h to 6.5h, when assuming a MDA of 1Bq/L and at a background count rate of approximately 0.8cpm.

Time optimization of 90Sr determinations: sequential measurement of multiple samples during decay of 90Y

This work presents an optimized method for the determination of multiple samples containing 90Sr when its daughter 90Y is measured after chemical separation and in sequence, i.e. during its decay. Consequently the measurement times will increase for each subsequent sample, since there has been a longer time for decay before measurement. Compared to a previously published approach, when 90Y is measured during its ingrowth, the gain in total analysis time (time for ingrowth+ summation of measurement times) is not that large, particularly not for low background instruments. However, results for a large part of the samples can be delivered earlier.

Overall approaches and experiences of first-time participants in the Nuclear Forensics International Technical Working Group’s Fourth Collaborative Material Exercise (CMX-4)

The Fourth Collaborative Material Exercise (CMX-4) of the Nuclear Forensics International Technical Working Group (ITWG) registered the largest participation for this exercise in nuclear forensics, with seven of the 17 laboratories participating for the first time. Each of the laboratories had their strategic role to play in its respective country, analyzing real-world samples using their in-house resources. The scenario was fictitious but was thoughtfully crafted to engage participants in nuclear forensic investigations. In this paper, participants from five of the first-time laboratories shared their individual experience in this exercise, from preparation to analysis of samples.