A SEARCH FOR LONG LIVED 242mAm ISOTOPE IN FOREST LITTER SAMPLES FROM POLAND

J.W. Mietelski and E. Tomankiewicz

The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences; Radzikowskiego str. 152, 31-342 Krakow, Poland.

1 INTRODUCTION

The 242mAm is a long lived (T1/2=143 years) transuranic isotope which is produced in nuclear reactors mainly as a result of a neutron capture of 241Am. It can be also produced in this way by high neutron fluxes during nuclear explosion. For many particular environmental sources, as for example global fallout, one can expect the 242mAm activity on the level of about 1% of that of 241Am. 242mAm decays by internal transition into 242Am (T1/2=16 h). The ground state 242Am decays by beta minus (83%) or electron capture (16%) into an alpha emitter, 242Cm (T1/2=163 days), which then decays into another alpha emitter, 238Pu (T1/2=88 years). As discussed elswhere the direct detection of 242mAm at environmental levels is not technically possible and only indirect methods are suitable. In our approach, the ingrowth of 238Pu was determined on an old alpha source of americium. The original forest litter samples were collected in Poland in 1991 or 1987. Samples were subjected to typical radiochemical methods during 1991-1996. Americium was separated and alpha spectrometric sources were prepared (using NdF3 method), measured and results were published. For the same set of samples, data on Pu activity (including 241Pu) are also available. In the present paper original codes are kept to enable any further comparisons. Some of the litter samples showed clear dominant Chernobyl influence (for example traces of 244Cm, strongly enhanced activity ratios 238Pu to 239+240Pu and 241Pu to 239+240Pu), although for the majority global fallout was concluded to be the main origin of transuranic isotopes. In our laboratory 25 sources from this project were preserved and still available. After about 12 years (on average: 4065 days) which elapsed since the preparation of the Am alpha sources (plastic foil filters with several micrograms of NdF3 crystals on it glued on to a metal plate) were taken for radiochemical separations in order to determine the ingrowth of plutonium alpha activity within them.

2 METHOD AND RESULTS

Prior to the radiochemical procedures all samples were re-measured by alpha spectrometry to ensure the self consistency between our past and present measurement techniques. The average ratio between past and present 241Am results (decay corrected) was 0.95 with standard deviation of 0.12. Thus it was considered satisfactory. Radiochemical work started with the dismounting of alpha sources. Filters were carefully removed from the metal discs and then dissolved in hot aqua regia with some boric acid added. A spike of 242Pu was added (13.3 mBq) to each sample. After evaporation a few drops of perchloric acid was added to destroy any remains of filter material. Then the Pu oxidation state was adjusted to +4 using hydrazine and NaNO2 followed later by a standard procedure used to separate Pu from 8 M HNO3 on Dowex 1x8 anion-exchange resin. The NdF3 sources were then prepared and measured by means of alpha spectrometry.

In the original study we used a relatively high activity of 243Am tracer (usually 226 mBq per sample) and in some samples traces of 244+243Cm were observed. These isotopes all result in the production of 239Pu or 240Pu, and the decay of americium tracer is the dominant source. Spectra therefore revealed traces of 239+240Pu as well as 238Pu. The results of Pu measurements in Am sources are presented in Table 1. The last column of this table shows the 242mAm activity calculated from Beteman equation. In about half of our samples an excess of 239+240Pu activity above that expected from 243Am decay is noticed, which suggests possible contamination of original Am fraction with tiny traces of Pu. This means that the observed 238Pu activities are the upper limits of that ingrown from 242mAm decay. The calculated 242mAm activities do not correlate with measured 241Am activity (see Fig. 1), which is of mixed Chernobyl and global fallout origin.

The mean value for 242mAm/241Am activity ratio is 0.023 with standard deviation of 0.033 so, in general, the 242mAm to 241Am ratio is on the level of a single percent. The high standard deviation reflects the mix of two sources (global fallout and Chernobyl) which differ in 242mAm to 241Am activity...