Speaker
Description
Authors (affiliation): 1. Robert Shearman (NPL, UK), 2. Séan M. Collins (NPL, UK / University of Surrey, UK), 3. John D. Keightley (NPL, UK).
Cadmium-109 is one of the most challenging radionuclides to standardise; the pure electron capture decay from the ground state (I pi = 5/2+; T1/2 = 461.9 a) populates one delayed (I pi = 7/2+; T1/2 = 39.7 s) state only, followed by an 88.0 keV gamma ray (T = 26.3). The temporal decoupling between the atomic rearrangements and the gamma transition makes conventional coincidence counting measurements problematic, often requiring some analogous tracer radionuclide. One common tracer is 125I, the ground state of which decays similarly (I pi = 5/2+, T1/2 = 59.388 d), but to a less-delayed state (I pi = 3/2+; T1/2 = 1.48 ns) followed by a 35.5 keV gamma ray (T = 14.08). Although 125I is simpler to standardise, and has been via numerous techniques [1], due to their differences in chemistry it is by no means an ideal tracer in liquid form for 109Cd.
Previously, the National Nuclear Array (NaNA) at the National Physical Laboratory (NPL) has been used to perform a primary standardisation of 60Co by the gamma-gamma coincidence technique[2]. NaNA has recently undergone an upgrade, now composed of up to twenty 1" by 2" CeBr3 scintillation crystals coupled with Hamamatsu photo-multiplier tubes. The current configuration has a maximum angular efficiency of 22.5 % and 33 angles between detector pairs. This upgrade reduces the internal radioactive background and allows for less uncertain peak areas in the low- and mid- energy regime, compared to the previously used LaBr3 detectors.
In this work, sources of 125I were dropped deposited onto thin films before adding drops of AgNO3, to stop the iodine escaping and left to dry before being mounted in source holders. Gamma-X coincidence measurements were made of numerous sources at several distances, and the calculation of the single and coincidence in-gate count rates in all detector pairs allowed for the determination of the efficiency of detection. These values were then used to extrapolate and derive the source activity following formulae as described in previous literature [1,3]. Following this, "sandwich" sources were made containing 125I and 109Cd, in which the activity of the 109Cd was calculated via the tracer method [3].
[1] Pommé S, Altzitzoglou T, Van Ammel R, Sibbens G. Standardisation of 125I using seven techniques for radioactivity measurement. Nucl. Instrum. Methods Phys. Res. A: Accel. Spectrom. Detect. Assoc. Equip. 2005;544(3):584-592.
[2] Collins SM, Shearman R, Keightley JD, Regan PH. Investigation of γ-γ coincidence counting using the National Nuclear Array (NANA) as a primary standard. Appl. Radiat. Isotopes. 2018;134:290-296.
[3] Schrader H. Photon–photon coincidences for activity determination: I-125 and other radionuclides. Appl. Radiat. Isotopes. 2006;64(10):1179-1185.
