How to Leak Test the Cs-137 Source in Your Liquid Scintillation Counter
Service AI
A practical guide to wipe testing and interpreting results for tabletop LS counters.
Why This Test Matters
Beckman LS counters use an internal ~30 µCi Cs-137 sealed source as an external standard for quench monitoring — including H-number calculation. On instruments from the LS 5000 and LS 6000/6500 series manufactured before January 1994, this source used an early pellet/matrix encapsulation design. Over extended operational lifespans, these suffered mechanical degradation and seal deterioration, causing microscopic leaks that showed up as rising background counts (exceeding 60 CPM) and calibration failures. Beckman Coulter responded with customer advisories and free replacement programs.
The fix was a transition to doubly-encapsulated stainless steel welded sources, largely supplied by Eckert & Ziegler / Isotope Products Laboratories. These comply with ISO 9978 and ANSI sealed-source integrity standards and effectively eliminated the leakage problems of the older design.
For instruments with the modern encapsulated source, a genuine leak is unlikely — but periodic contamination checks remain good practice, and most radiation safety programs require them regardless. The wipe test takes under an hour using the LS counter itself.
Safety Requirements
Before starting, ensure the following:
- Wear rubber or plastic gloves and your radiation dosimeter.
- Follow ALARA principles — time, distance, and shielding — throughout.
- Treat all gloves and swabs as potentially contaminated until the instrument is proven clean.
- Conduct all work in compliance with your institution's radiation safety program. Notify your Facility Radiation Safety Officer (RSO) as required.
Step 1: Check Instrument Background
Set the instrument to count for 5 minutes with a wide-open window and count the sealed Background Standard. Counts should be less than 80 CPM. Higher readings suggest existing contamination in or near the counting chamber — resolve this before proceeding.
Step 2: Access the Source
- Remove the upper panel and the two centre pieces of lead shielding.
- Remove the two screws from the bracket holding the upper source tube.
- Manually slide the source spring upward until the source just protrudes from the end of the tube.
Step 3: Prepare Reference and Source Swabs
- Reference swab: Place a clean swab or Q-tip (tip only) into a clean vial half-filled with scintillation cocktail. Label it "Reference."
- Source swab: Wipe the end of the source and around the outside of the spring with a fresh swab. Place the tip into a second vial half-filled with cocktail. Label it "Source."
Step 4: Program and Count
Set up the LS counter as follows:
| Parameter | Setting |
|---|---|
| Counting Time | 10.0 minutes |
| Isotope 1 Window | 0 – 300 |
| Isotope 2 Window | 300 – 1000 |
| Isotope 3 Window | Wide open |
| Data Calc | CPM |
| H# | OFF |
| All other parameters | NO or OFF |
Count in order: (1) sealed Background Standard, (2) Reference swab, (3) Source swab.
Step 5: Interpret Results
The source swab should read close to the reference. Swab material itself contributes 40–80 CPM, so a similar reading from the source swab is normal. A reading more than 100 CPM above background warrants action.
Pay attention to which window the counts fall in:
- 300–1000 window: Cs-137 signature — elevated counts here indicate a genuine source leak.
- 0–300 window only: Likely grease fluorescence, which can read as high as 300–400 CPM without any radioactive contamination. Not a cause for alarm on its own.
The LS 6500's Hot Graph spectral display is particularly useful here — it gives an immediate visual of where counts are falling across the energy spectrum, making it straightforward to distinguish a real leak from fluorescence interference.
If a Leak Is Detected
Stop operating the instrument immediately.
- Carefully remove the source spring from the instrument.
- Cut the end of the spring off and place the spring and source into the vial from the ship kit.
- Contact your RSO immediately. Dispose of the source and all contaminated materials through your institution's radiation safety program.
- Do not return the instrument to service until it has been inspected and cleared by a qualified service professional.
How Often to Test
Most radiation safety programs recommend leak testing:
- At least every 6 months for instruments in routine use
- After any service or access to the source assembly
- After any incident — unusual counts, a drop, or suspected contamination
- Before returning an idle instrument to service
Real-World Example: What a Leaking Source Looks Like
The printout below is from an actual wipe test on an LS 6500 with a leaking source. Five samples were counted — the results tell the story clearly.
Samples 1 and 2 are clean — the sealed Background Standard reads 33.00 CPM in Window 2 (300–1000), and the reference swab reads 33.50 CPM. Both are normal baselines. Samples 3, 4 and 5 are swabs from the leaking source: Window 2 readings of 883.00, 445.40 and 513.60 CPM respectively — 13 to 26 times the reference level. The signal is unambiguous and entirely in the Cs-137 window (300–1000), confirming genuine radioactive contamination rather than grease fluorescence.
The Hot Graph below shows the sample spectrum for the leaking source swab. Note how all counts cluster above the 0.30K dashed line — squarely in the Cs-137 energy range. This spectral signature is what a leaking source looks like on the LS 6500's Hot Graph display, and why it is such a useful diagnostic tool for distinguishing a real leak from lower-window interference.
Summary Checklist
- Background under 80 CPM before starting
- Gloves and dosimeter on
- Source spring kept in place — do not remove source
- Reference swab prepared before wiping source
- 10-minute count across all three windows
- Flag source swab readings more than 100 CPM above background
- Distinguish Cs-137 counts (300–1000) from grease fluorescence (0–300)
- Contact RSO immediately if leak detected
- Document and retain all results
