Short answer for the technician already in the menu. After Auto Gain Adjust, before you run Efficiency Check, you need to run Measure Efficiency for each isotope you’ll be counting. It lives in the edit screen of the Isotope Library — not under Calibration. Enter the source’s reference activity and reference date, then select Measure Efficiency. Now Efficiency Check will work.

What you’re seeing

You ran Auto Gain Adjust after a service event — an annual recalibration, a detector swap, an HV adjustment, or just routine drift correction. The procedure ran clean. Gain landed in range, the printout looked good, every well passed. Confidence high.

Then you ran Efficiency Check on your Cs-137 (or Co-57) source as the standard post-calibration QC step.

And the instrument printed DPM Invalid.

It feels like the calibration broke something. It didn’t. The instrument is doing exactly what it was designed to do — it’s refusing to compute a fictional number from data it knows is a placeholder. There’s one extra step between Auto Gain Adjust and Efficiency Check that the menu order doesn’t make obvious, and once you do it, the error goes away.

Why it happens

Every time Auto Gain Adjust runs, it changes the detector’s electrical response. Energy bins shift, the spectrum re-aligns, the photopeak channel moves. Any previously-stored efficiency value for any isotope is no longer accurate — the detector that produced those efficiency numbers is, in a measurement sense, a different detector after re-calibration.

Rather than leave stale efficiencies in the library to be silently used by the next count, the firmware does something protective: it resets every stored measured efficiency value to 100% as a placeholder. The 100% isn’t a measurement — it’s a flag that says “this isotope hasn’t been measured since the last calibration; the value here is not valid.”

When Efficiency Check then tries to compute a DPM by dividing measured CPM by the stored efficiency, it sees the 100% placeholder and recognizes it for what it is. Rather than print a misleading number, it prints DPM Invalid. The error is the firmware refusing to lie to you.

What "Measure Efficiency" vs "Efficiency Check" actually do. They’re different tools for different jobs, and the names are confusingly similar.
  • Measure Efficiency is the setup-once-per-isotope-per-calibration tool. It uses a known calibrated source to compute the actual detector efficiency for that isotope and store it. This is what we need to run after Auto Gain Adjust.
  • Efficiency Check is the daily QC tool. It uses the value Measure Efficiency stored, compares today’s reading against it, and reports drift. You run this every day for routine QC; you cannot run it until Measure Efficiency has been run at least once for that isotope since the last calibration.

The fix — three steps

Step 1 — You’ve already done this: Auto Gain Adjust

You’ve already done the calibration. If you haven’t, do it now — click through the on-screen warning that says “running this will reset all stored efficiency values” (that’s the placeholder behavior we just described — expected and intended). When the routine completes, the detector is calibrated and the efficiency library is reset to 100% placeholders.

Step 2 — Run Measure Efficiency from the Isotope Library (THIS is the missing step)

This is the step the menu structure makes hard to find. Measure Efficiency does not live under the Calibration menu. It lives inside the edit screen of the Isotope Library, and only appears after you’ve entered the calibrator information for that isotope.

  1. Open the Isotope Library from the main menu.
  2. Navigate to the page containing your isotope (Cs-137, Co-57, I-125, etc.).
  3. At the bottom of the screen you’ll see an option to edit the isotope entry — usually 7 1 followed by #, but the exact prompt is shown on the screen. The system will ask which isotope ID to edit; enter the number shown next to the entry.
  4. You’re now on the isotope edit screen. This is where the source data goes.
  5. Confirm the calibrator setup is single (not multiset) — almost all Cs-137 and Co-57 sources sold for this purpose are singles. If it’s set to multiset, change it.
  6. Enter the source’s reference activity (from the source certificate — e.g., 0.559 µCi or whatever the cert shows), the reference date (the date the activity was certified — the firmware will decay-correct forward from this date automatically), and the lot number if you have it.
  7. Once the calibrator info is saved, the Measure Efficiency option appears on the edit screen. It wasn’t there before — the system hides it until the source data is on file. Select it.
  8. With the source in the appropriate well, the system runs the measurement. It computes the actual measured efficiency from observed CPM versus expected DPM, and stores that value as the reference for this isotope going forward.

Keep the printout from Measure Efficiency — it documents the source data and the date the efficiency was measured. Inspectors often want to see this in the QC binder.

Step 3 — Now run Efficiency Check

With the stored efficiency populated, Efficiency Check works the way it was always supposed to. It computes DPM cleanly, reports the variance against the stored value, and prints the result. This is the tool you’ll use every day for routine QC going forward — you do not need to repeat Measure Efficiency unless the unit is recalibrated.

One source, many isotopes — what to repeat

If the only isotope your facility measures is Cs-137 (or only Co-57, or only I-125), you’re done after Step 2. The library entries for isotopes you don’t use can stay at 100% placeholder forever — the firmware will only block Efficiency Check on the ones you actually call.

If your facility runs multiple isotopes (a common nuclear medicine workflow: Tc-99m for daily wipe surveys, Co-57 for tracer calibration, I-131 for therapy doses), you need a calibrated source of each isotope you’ll be measuring and you repeat Step 2 once per isotope. Same edit-screen procedure each time — reference activity, reference date, Measure Efficiency.

A word on source aging

If your last calibration source is more than a few half-lives old, the source may be too weak for a clean Measure Efficiency — not broken, just low-statistics. Quick decay math:

  • Cs-137 — 30.1 year half-life. A source that was 0.5 µCi ten years ago is still about 0.40 µCi today (~79% strength). Still a usable check source. Twenty years in, it’s about 0.31 µCi (~63%); still fine for most check work, but starting to get noisier.
  • Co-57 — 271.8 day half-life. After two years, a 0.5 µCi source is down to about 0.08 µCi (16%). After three years, ~6%. Co-57 sources should be replaced approximately every 18–24 months for routine use, and they have a real shelf life on the cert.
  • I-125 — 59.4 day half-life. A 100 µCi patient-dose aliquot is down to ~0.5 µCi after one year. Don’t use old I-125 as a check source; that’s what the patient-dose self-calibration procedure exists for.

If Measure Efficiency reports unusually low or noisy results, the source may be the issue, not the instrument. A source with healthy activity should produce a stable measured efficiency that matches what the cert and the well geometry predict.

Same fix on every LTI counter

The calibration architecture is shared across the Genesys platform. The procedure above applies identically to the Multi-Wiper, Wiper Gold, Genesys Genii, Genii HE, and Gamma 1 — same firmware family, same Isotope Library edit flow, same Measure Efficiency dependency, same DPM Invalid behavior when it’s missed. If you’ve done this once on one counter, you know how to do it on every counter we’ve shipped since the platform was introduced.

The exact key prompts (which number to press to enter the edit screen, etc.) are documented in the operator manual for your specific unit, but the order — Auto Gain Adjust → Measure Efficiency → Efficiency Check — is universal.

Why is this the #1 service call? The menu structure separates Measure Efficiency (under Isotope Library) from Efficiency Check (under Calibration), which makes intuitive sense if you already know the difference between the two — and is confusing if you don’t. The next firmware revision after this article publishes may move the two closer together; until then, this article is the workaround.

Related articles

If the steps above didn’t resolve it

If you’ve completed all three steps with a known-good source and Efficiency Check still reports DPM Invalid or a wildly off-target measurement, the situation is no longer procedural — it’s a real instrument-side issue worth a phone call. Most likely causes at that point: a failed PMT in one well, a cracked NaI(Tl) crystal, a degraded preamp, or in rare cases a contaminated detector well. We’ve seen them all.

Open a service ticket or call 800.542.1123 and we’ll work the diagnostic from there.