If you’ve owned a Genesys gamma counter for a while, you already know about the Self-Efficiency Check — it’s the routine that lets you measure I-125 detector efficiency without buying a calibrated standard. What’s less obvious is how the system pulls that off. It’s a clever piece of physics that depends on a specific property of I-125 and a specific feature of well-type NaI(Tl) detectors.

This article is the explainer. If you want the practical step-by-step on building your own tracer sample for the routine, see the companion piece: Build your own I-125 calibrator using tracer from any assay kit.

Step 1 — I-125 has two peaks, not one

Most isotopes you count emit a single characteristic gamma at a known energy (or a small handful of distinct lines). I-125 is different. It emits a main peak around 28 keV — the one every NaI(Tl) detector reads — and it also produces a summation peak (also called the coincidence peak) at higher energy.

The summation peak comes from two of I-125’s emission products being captured by the crystal at essentially the same instant. The detector electronics see them as a single event with the combined energy of both photons. That summed event is what shows up as the second peak in the spectrum.

Step 2 — Why the summation peak only shows up in deep wells

Here’s the geometry constraint: the summation peak only appears when the crystal can capture both photons from the same decay. That requires the sample to be surrounded by crystal on more than one side. A flat detector or a shallow through-hole crystal will catch one photon and miss the other most of the time, so the summation peak is small or absent in those configurations.

A deep well-type NaI(Tl) crystal — the kind every Genesys gamma counter uses — surrounds the sample on five sides. Both photons from a given decay have a high probability of being captured. The summation peak appears clearly in the spectrum, alongside the main 28 keV peak. This is the geometry that makes Self-Efficiency Check possible; it doesn’t work on flat or shallow crystal designs.

Why LTI deep-well crystals matter here The Genesys deep-well design (with extra crystal mass below the well) was engineered for high I-125 capture efficiency in routine RIA work. The same geometry is what makes the summation peak strong enough to use as the reference signal in the Self-Efficiency calculation. The two are the same engineering choice, used twice.

Step 3 — The ratio is the trick

This is the key insight. The Self-Efficiency routine measures the ratio of counts in the summation peak to counts in the main peak.

Both peaks come from the same decay events. Whatever the detector’s absolute efficiency is, it scales both peaks together. Whatever the sample geometry is, it affects both peaks together. The ratio cancels those terms out. What’s left is a quantity that depends only on a known nuclear-physics property of I-125 itself — and from that the routine can derive the absolute DPM of the sample, with no reference standard required.

Once the routine knows the absolute DPM of the sample in your well, it can compare that to the measured CPM and report your detector’s actual percent efficiency. That’s the number you see on the printout.

Step 4 — Why the calculated efficiency might differ from your old calibrators

Switch from a purchased calibrator to a Self-Efficiency check and you may see a small difference between the new "calculated" efficiency and the historical "measured" efficiency from your Multi-Calibrator log. That’s expected. The two numbers come from samples with different geometry:

  • Purchased Multi-Calibrators are designed as near-point sources. The I-125/Resin core sits in a 5 mm × 5 mm inverted cone suspended ~5 mm off the bottom of the tube — a tight, point-source-like geometry that maximizes gamma capture and minimizes counts lost through the open top of the well.
  • A pipetted tracer sample spreads out as a pool on the bottom of the tube — ~10 mm diameter for a 75 mm tube. That changes the geometric coupling to the crystal and reduces the fraction of summation events the detector sees.

The result: a pipetted sample reads at a lower efficiency than a Multi-Calibrator on the same instrument. This is not a defect. The instrument is exactly as efficient as it was; you’re just measuring it through a different geometry. Once you commit to a consistent sample form, the day-to-day reproducibility is excellent — which is what efficiency tracking actually needs.

Step 5 — Why even purchased calibrators don’t agree perfectly

You can prove the Self-Efficiency math by running it on your old Multi-Calibrator set and comparing the calculated DPM to the decayed Logbook value. Two important details:

  • Calculated DPM is the absolute DPM of the specific sample in front of the detector. Logbook DPM is an average across the entire calibrator lot, calculated for 12:00 AM on the date listed.
  • I-125 decays at about 1% per day. The combination of the lot-average framing and the time-of-day reference means any individual calibrator set can sit up to ~4% off from its Logbook number on any given day.

If you compare a Self-Efficiency calculation against a Logbook DPM and see a few percent of difference, that’s entirely consistent with the inherent uncertainty in the Logbook number itself. The math is fine.

Where Self-Efficiency Check fits in the Calibration Menu

Self-Efficiency is one of five calibration routines on the Genesys. Knowing where each fits in the system helps you understand when to reach for which:

  • Background Count. Required at least every 24 hours. The system will prompt you if you haven’t done it in time. Sets the baseline that’s subtracted from every subsequent measurement.
  • Efficiency Check (Self-Efficiency). The routine this article is about. Tracks detector efficiency drift over time using either a Self-Efficiency tracer sample or a purchased calibrated standard.
  • Gain Adjust. The deeper detector calibration. Sets overall voltage, LLD, zero offset, and balances gain across detectors. Run with Co-57 (single or matched) or Cs-137. This is where you do need a calibrated source.
  • Gain, FWHM, Chi-Square. Adds two diagnostic checks alongside the gain adjust: detector resolution (FWHM — should typically be 20% or less) and a goodness-of-fit Poisson check (Chi-Square — reports PASSED or FAILED).
  • View Spectrum. Live multi-channel spectrum display, 4096 channels across 0–999 keV (0–2 MeV on the HE series). Useful for inspecting unfamiliar isotopes, confirming peak positions, or troubleshooting.

Self-Efficiency Check is the routine you’ll run most often after the initial setup. It’s also the only one that can run without recurring source-purchase cost, which is why it tends to become the calibration backbone of a Genesys-equipped lab.

The bottom line

Self-Efficiency Check works because I-125 emits two correlated peaks, the Genesys deep-well crystal sees both, and the ratio of those peaks is a function of the isotope’s nuclear properties — not the detector or the sample geometry. That’s a clever exploitation of physics that removes a recurring cost from your calibration program forever, as long as the sample is high enough activity (~30,000 CPM target, ~5,000 CPM minimum) and the geometry is consistent from day to day.

Further reading