Acceptable values meet these system specifications:  

• 3H: ≥ 59% in wide open window

• 14C: ≥ 95%

The specifications mentioned above are relevant for Beckman calibrated standards that are traceable to the National Bureau of Standards. To ensure accurate results, it is imperative to calibrate the LS counter properly.

To ensure accurate and consistent results, it is essential to intimately mix any sample with the cocktail according to Beckman. Low efficiency and unpredictable results can occur if this step is not followed. Additionally, dirty detector or reflector housing, as well as aged PMTs, can also cause low efficiency reports. One can easily identify dirty reflector housing by visually inspecting it; a clean and reflective internal paint maximizes counter efficiencies, whereas a dark and dirty coating indicates contamination. Replacing the reflector housing is a simple and cost-effective solution to resolve efficiency issues in Beckman LS6500 counters.

To detect possible radioactive contamination and check overall performance, it is recommended to periodically conduct a background count and efficiency check following the procedure below.

Set up a rack with the supplied 3H, 14C, and unquenched background standards, followed by three or four of your own samples that are giving questionable results.

Set up a User Program as follows:

Count Time: 5 minutes

Isotope 1: 3H

Isotope 2: Wide

Data Calculation: CPM

Count Sample: 3 times

H# (if installed): ON

Lum-Ex: ON

3H and 14C precision: 1 %

3H Counting Efficiency:

For 3H it will be necessary to correct for the radioactive decay since the samples were prepared. This can be done by using the half-life table. The 3H standard will have the date of calibration and the dpm on that date printed on the vial. Calculate t (current date minus date of calibration. in years].

Divide t by the half-life T (12.35 years for 3H). Find this value in the t/T column of the half-life table. Use the corresponding fraction remaining value to multiply the standard DPM. The result is the DPM remaining on the selected date.

Use the CPM for Isotope I for the 3H standard, and the CPM for Isotope I for the background sample. The background for this calculation is insignificant compared to the CPM of the standard but is given for completeness. Calculate the 3H efficiency from the equation:

14C Counting Efficiency:

For the 14C efficiency. use the 14C CPM for Isotope 2, Wide. Calculate the efficiency as described for 3H.

Background:

As per Beckman, the background count in the Wide window should be less than 60 CPM. The background in an LS counter is influenced by the noise within the system (phototubes. electronics). Naturally occurring radioactivity, and cosmic rays. The background also varies with the size and type of vial (plastic has lower background than glass), and the altitude (about a 7% increase in background for each 1000 feet above sea level).

If any results fail to meet these values, check the following:  

1. Inspect the sample vials to ensure they are securely sealed.

2. Ensure the instrument is calibrated and set up properly.

3. Verify that you have accurately corrected for decay using the Half-life Tables, and that your computations are correct.

4. Replace LS6500 Elevator Cap.

5. Inspect the detector reflector housing. If it looks dirty, replace the reflector housing.