C. Efficiency Values for Wipe Testing
All isotopes for which wipes are to be counted shall either be prepared from a known liquid
concentration or if possible, a standard source can be purchased in a wand form from various
manufacturers. Generally, the isotopes Cs-137, Co-57, Co-60, Ba-133, and Am-241 are available
in wand form with a stated activity and uncertainty. The activity should be kept in the range of
0.01 to 0.1 µCi, and should not exceed 0.2 µCi. This will generally keep the overall counting
rate below 10,000 cps and will eliminate any spectrum distortion or non-linearity from playing a
role in the efficiency calculation.
A suggested procedure for source preparation is outlined on the next page. The liquid source
will be deposited on an absorbent material in the bottom of a plastic vial which fits inside the
well. It is recommended that no more than 10 ul of liquid be dispensed in order to reduce
spilling radioactive material.
NOTE: If a probe is being tested for efficiency, a method for holding the source or vial in
proximity of the probe, approximately one (1) cm, must be arranged and then the following
procedure followed with modification as needed. If a greater distance is used then you must
increase activity according to the inverse square law.
Source Preparation:
1. Prepare an activity of 0.1 to 0.01 mCi of the isotope in 10.0 ml of saline. This will result in a
concentration of approximately 0.01 to 0.001 mCi/ml. The volume should be either
controlled as close as possible to 10.0 ml or precisely measured.
2. Assay the activity in the vial in a dose calibrator which has a resolution of 0.01 µCi.
Compensate for background before making the measurement. Record the time of
measurement.
3. Calculate concentration by dividing the measured activity by the source volume and record
with the time of measurement.
4. Prepare the vial with absorbent material (such as a portion of a wipe pad or the end of a
Q-tip) at the bottom of the vial.
5. Draw off 10 ul of source material and carefully place the end of the syringe tip at the bottom
of the vial. Deposit all of the source material onto the absorbent material.
6. Withdraw the syringe and immediately rinse it in saline at least ten (10) times. This will
reduce the chance of contamination of the next source which will be a different isotope.
7. Record the activity in the test vial as Ax
x
-123 `at the time T is recorded in step 2. (The Xx-
123 subscript is meant to designate the isotope symbol which should be recorded, i.e.,
Am-241.)
NOTE: A tuberculin syringe is not suitable in this procedure. Hamilton microliter syringe, model
701-N, has a capacity of 10 ul with 0.2 ul resolution and will provide the necessary precision.
Hamilton Co., P.O. Box 10030, Reno, Nevada, 89520. Phone: 800-648-5950.
After the source preparation, the efficiency can be measured by counting the source in the
Administration Mode “Isotope Efficiency” page of the Atomlab 960.
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