DIRECT ALPHA COUNTING OF SOIL SAMPLES
DOE/ERSP PROCEDURE NO. 9
APPROVED:
1
DATE DRAFTED: 30 January 1978
8 February 1978 by Don R. Martin (ERSP Manager)
General
The purpose of this procedure is to provide a method of determining the plutonium activity in soil by
counting the alpha activity present. This procedure provides for only an estimate of the plutonium activity.
Several variables {nteract which could cause significant error in the inter
etpretation «of direct alpha counting,
such as the
238py to 239,240py ratio and the 239,240py to
lam ratio. When an accurate
determination of the concentration is desired alpha spectrometry should be used.
IL.
Sample Preparation
After logging in the sample, an aliquot of soil is transferred to an AC-3 probe face plastic cover. The
volume of the aliquot should fill the bottom of the cover to a depth of approximately 0.5em. Remove
organic debris and rocks with diameters larger than 0.25¢em. Spread the sample evenly over the bottom of
the cover and break larger chunks of soil into granules to homogenize the sample.
Dry sample for several hours to remove all moisture. Water entrapped in the sample will shield the alphas
emitted from the soil and cause as much as a 50% reduction in the gross alpha counts detected. To dry the
sample at higher temperatures an aluminum foil drying pan may be substituted for the plastic face cover.
TIL
Sampling Counting
Prior to counting, place an open AC-3 probe face, with webbing removed, on the sample to prevent the
probe from resting directly on the soil. The spacer thickness should be kept to a minimum, thick enough
only to prevent contamination of the probe face. Care should be taken when placing the AC-3 probe on the
spacer so that the mylar window of the probe is not punctured; then count the sample for 10 minutes with
the AC-3 probe on the spacer above the sample. The concentration of plutonium in soil is calculated by
dividing the net counts (gross 10-minute count minus the 10-minute background count) by 1.07. This is an
empirically derived conversion factor obtained by Dr. Bramlitt, of DNA, while he was at Enewetak.
A-9-1