The Genesys gamma counter family ships with a feature called Self-Efficiency Check — a calibration routine that measures detector efficiency for I-125 without a purchased calibrated standard. Most labs already have what they need on the shelf: I-125 tracer that comes free with every standard RIA / IRMA assay kit.
This article is the practical how-to. If you want the physics behind why this works, see the companion article: How Self-Calibration actually works (the two-peak method).
Why this works (the short version)
I-125 emits two distinct peaks: a main peak around 28 keV and a secondary summation (or coincidence) peak. The Genesys Self-Efficiency routine uses the ratio of those two peaks to derive absolute DPM — a calculation that doesn’t depend on how efficient your detector is or how the sample is shaped. As long as your sample geometry is reasonable and your activity is high enough to give the routine clean statistics, the system can compute its own efficiency.
What you need
- An I-125 tracer source. Any standard assay kit’s tracer vial works. You’re going to pipette a small amount — you don’t need a "calibrator," you need a few microliters of tracer.
- A pipette capable of measuring around 250 µL. A standard adjustable lab pipette is fine; pipette accuracy is more than adequate for this purpose.
- A 75 mm test tube (the standard size that fits the Genesys well). One tube if you’re calibrating a Gamma 1; one tube per well if you want to calibrate all wells of a Genii simultaneously.
- Your Genesys gamma counter — either a Gamma 1 (single well) or a Genii (2–10 wells).
Step 1 — Decide your target activity
Aim for roughly 30,000 CPM (about 0.01 µCi) per tube. That’s LTI’s recommended target — it gives the Self-Efficiency routine plenty of counts to work with on the summation peak, which is the smaller of the two peaks and the one that drives statistical precision. The minimum the calculation will run on is about 5,000 CPM (~0.002 µCi), but you’ll get noisier results.
Step 2 — Match the volume to your assay tube geometry
This is the part that catches people. The Self-Efficiency calculation is geometry-independent mathematically, but the measured efficiency it reports depends on how much of the summation peak the crystal actually captures — which depends on where the radioactive material physically sits inside the tube.
LTI recommends a sample volume of around 250 µL. That spreads the activity across a roughly 10 mm diameter pool at the bottom of a 75 mm tube — the same general geometry as a typical coated-tube RIA assay. Your day-to-day patient samples will read at efficiencies close to what the Self-Efficiency check reports, which is what you want.
If your assay protocol uses a different sample form (smaller volume, larger volume, sample on a column, etc.), match the form. You can use whatever sample form works for your workflow — the Self-Efficiency check is a performance measurement of your gamma counter, so what matters is that the form is consistent from day to day and across all the tubes you use for the check.
Step 3 — Make your set of tubes
For a single-well Gamma 1, prepare one tube. For a multi-well Genii, prepare one tube per well — you can run the Self-Efficiency check on all wells in parallel, which is faster and exercises the whole detector array in one routine.
- Pipette your tracer aliquot into each tube.
- Top up to your target volume (~250 µL) with diluent.
- Cap and gently mix.
- Cross-check tube-to-tube uniformity isn’t required to be perfect — standard pipette accuracy is more than sufficient.
Step 4 — Run the Self-Efficiency Check
From the Genesys main menu, enter the Calibration Menu (4 then #), then select Option 2: Efficiency Check. Load your tracer tube(s), run the routine, and the Genesys will report the measured efficiency for each well, along with the normalizing factor (NF) and the variance from the previously stored value.
If this is the first Self-Efficiency check after migrating from purchased calibrators, expect to see some difference between the calculated efficiency and the historical "measured" efficiency from your old Multi-Calibrator log. That’s normal and expected — it’s the geometry difference between a point-source-like calibrator (LTI’s old Multi-Calibrator design suspends the I-125/Resin core in a 5×5 mm inverted cone, ~5 mm off the tube bottom) and your pipetted sample (spread across a 10 mm pool). It does not indicate a problem with your counter. See the companion article on the physics for the full explanation.
Step 5 — Lock in your routine
Whatever sample form you choose for the Self-Efficiency check, be consistent about it. Pick a tracer source, pick a target volume, document them, and use the same approach every check. Day-to-day reproducibility is what makes the trend data meaningful.
If you ever switch tracer source, dilution buffer, or sample volume, run a fresh efficiency check immediately and update your stored reference. The numbers will shift — not because anything is wrong, but because the geometry changed. Document the change and move on.
The math: what you save
A calibrated I-125 source costs hundreds of dollars and decays at about 1% per day — meaning if you don’t use it constantly, you’re paying for activity that’s gone before you got value out of it. The tracer that comes with your assay kits costs nothing extra; you’re going to use it for assays anyway. The Self-Efficiency check turns a few microliters of that tracer into a working efficiency reference.
For a lab running monthly efficiency checks across an active Genii, the recurring savings is real and goes on for the life of the instrument.
Further reading
- How Self-Calibration actually works (the two-peak method) — the physics behind why a tracer sample can substitute for a calibrated standard.
- Calibration sources & Reflex Industries — where to buy actual calibrated sources for the Gain Adjust routine.
- Genesys Genii product page — multi-well configurations, full assay library, Self-Efficiency Check standard.
- Genesys Gamma 1 product page — single-well bench-top configuration.