57. Validity of ²⁴¹Am as a Tracer of Plutonium Dioxides in Lungs in External Chest Counting

Nobuhito Ishigure, Hiroko Enomoto, Takashi Nakano and Jiro Inaba

Keywords: Am-241, tracer, plutonium, external chest counting

The most difficult radionuclides to measure by external chest counting are the isotopes of plutonium. They are detected through weak emission of low energy L X-rays. The plutonium treated in nuclear fuel cycle is usually accompanied by ²⁴¹Am produced from ²⁴¹Pu by disintegration, which emits -rays of 60 keV with the emission rate of 0.36, being more penetrable than the L X-rays. The ²⁴¹Am could improve the detection limit of chest counting of plutonium by using it as a tracer of plutonium in the lungs. The ratio of lung burdens of ²⁴¹Am and ²³⁹Pu was followed for 15 months by in vivo counting using rats inhaled with plutonium dioxide that contained 4 % of ²⁴¹Am in -activity.

The suspension of Pu(OH)₄ was nebulized using a compressed-air operated nebulizer. The resultant droplets were passed successively through a heated tube to dry the droplets and on to a high temperature furnace heated to 1150°C to oxidize the dried particles. The size of the PuO₂ particles was 0.47 m (AMAD) with the geometric standard deviation of 2.1. The inhalation was conducted using a multi-port nose-only exposure chamber, in which the aerosols flow vertically downwards. Young adult female Wistar rats, 12 weeks old and weighing 230 g at the time of exposure, were used. The lung contents of ²⁴¹Am and ²³⁹Pu were measured with three thin NaI(Tl) scintillation detectors, 51 mm in diameter and 1mm in thickness which were housed within a shielded chamber with a 50 mm thick lead wall and a lining of 5 mm of copper and acrylic resin. The usual counting time was 1000 s.

The follow-up of lung content of ²³⁹Pu indicated that a proportion of the initial alveolar deposition after exposure was well approximated with a two-component exponential function; about 77% was cleared at the half time of 53 days and 23% is at 794 days.

The ratio of ²⁴¹Am/²³⁹Pu in the lungs is shown in Fig.19. The ratio was almost constant throughout the follow-up period, that is, the ²⁴¹Am was cleared from lungs at the same rate as ²³⁹Pu.

The retention function of ²³⁹Pu indicated that the ²³⁹Pu inhaled in this study was cleared from lungs very slowly as compounds of Type S. Though the ICRP has assigned americium to Type M for all chemical forms, the ²⁴¹Am, assumed to be in a matrix of plutonium dioxide, behaves as if it were a material of Type S.

In addition this result suggests that the plutonium dioxide particles are retained in lungs in a particulate state for a long time after exposure and are hard to dissociate.

The concomitance of ²⁴¹Am with ²³⁹Pu obtained in this study supports the validity of the proposal that in some cases ²⁴¹Am can be used as a substitute for isotopes of plutonium to assess them by external chest counting.

Fig.19. Ratio of lung burdens of ²⁴¹Am and ²³⁹Pu.

Publications:
Ishigure, N., Enomoto, H., Nakano, T. and Inaba, J.: Radiat. Prot. Dosim., 79, 133-136, 1998.