Sealed Source Inventory & Accountability (35.67)

Leak testing proves a sealed source is intact; inventory and accountability prove you still have it, know where it is, and have not exceeded your license. These are two different jobs, governed by the same rule — 10 CFR 35.67 — and inspectors check both. A facility can have perfect leak-test records and still take a violation for a missing semi-annual inventory, an unlocated source, or a possession total that drifted over the license limit. ^{1, 2}

This guide covers the sealed-source inventory and accountability program the Radiation Safety Officer (RSO) owns: the semi-annual physical inventory, the records that must accompany it, decay-corrected activity reconciliation, possession limits, and how source tracking and security requirements fit on top. DRPS provides this support as part of its radiation safety officer and radioactive material license support services across Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware.

Introduction

Nuclear medicine departments hold more sealed sources than people often realize: cobalt-57 flood sheets for uniformity QC, cesium-137 and barium-133 rod and point sources for dose-calibrator checks, germanium-68 references, and sometimes therapy or calibration sources. Each one is byproduct material the facility is licensed to possess, and each one must be accounted for continuously, not just leak-tested periodically. ^{1, 7}

Accountability is a quiet discipline. A source does not announce that it has been loaned to another room, dropped behind a cabinet, sent out with decayed waste, or simply never logged. The semi-annual physical inventory is the scheduled moment when the facility reconciles what the records say it has against what is physically present, radionuclide by radionuclide, location by location. When those two lists disagree, the program has found a problem before an inspector — or a lost source — does. ^{1, 2, 7}

This is also where radiation safety and security meet. The same inventory that satisfies 10 CFR 35.67 is the foundation for demonstrating that a facility knows the whereabouts of its licensed material, which is the premise of the NRC's source-tracking and physical-protection framework. ^{5, 6}

Topic Explanation

Leak testing versus inventory

These are distinct program elements that answer different questions. 10 CFR 35.67 contains both, and conflating them is a common weak point in an RSO program.

Leak testing checks source integrity. The licensee tests each applicable sealed source for leakage at intervals not to exceed six months, measuring the wipe sample so the test can detect the presence of 185 becquerels (0.005 microcurie) of removable radioactive material. If a test reveals 185 becquerels (0.005 microcurie) or more of removable contamination, the source is withdrawn from use and a report is filed with the appropriate NRC regional office within five days. Sources in storage and not in use do not require routine leak testing, but each must be tested before use or transfer unless it was leak-tested within the preceding six months. ¹
Physical inventory checks accountability. Under 10 CFR 35.67(g), a licensee in possession of sealed sources or brachytherapy sources — except gamma stereotactic radiosurgery sources — must conduct a semi-annual physical inventory of all such sources. The inventory confirms each source exists, is where the records say it is, and is correctly identified. ¹

A facility that treats "we leak-tested everything" as equivalent to "we accounted for everything" has a gap. Leak testing can be, and often is, performed on the sources in routine use; the inventory must reach every source, including those in storage or decaying in a shielded safe.

The accountability chain

Accountability does not begin at the semi-annual inventory; it runs from receipt to disposal. Under 10 CFR 30.51 the licensee keeps records of the receipt, transfer, and disposal of byproduct material. A defensible program links four moments for every source: ³

Receipt — the source is logged in with radionuclide, model and serial number, calibration activity and date, and location, and the package is surveyed and wipe-tested on arrival.
Use and movement — any relocation is recorded, so the "location" field in the inventory is always current.
Physical inventory — twice a year, the records are reconciled against the physical sources.
Disposal or transfer — when a source is decayed in storage, returned to the vendor, or transferred, it is logged out and the possession total is updated.

This chain is the same record system that supports license renewal and preparing for an NRC inspection, and it is reviewed during both.

Key Technical Principles

Decay-corrected activity

A sealed source is not a fixed quantity of activity; it decays continuously. Accountability therefore tracks a source by its identity (model, serial number, radionuclide) rather than by a frozen activity value, and it reports a realistic present activity using decay correction. The activity $A$ remaining after a time $t$ from a calibration activity $A_{0}$ is:

A = A_{0} e^{- λ t}, λ = \frac{ln 2}{T _{1/2}}

Consider a cobalt-57 flood source, the workhorse of gamma-camera uniformity QC, with a half-life of 271.74 days and a calibration activity of 740 MBq (20 mCi). ^{8, 10} Its decay constant is:

λ = \frac{ln 2}{271.74 d} = 2.55 \times 1 0^{- 3} d^{- 1}

After one half-life (271.74 days) the activity is 370 MBq by definition. After one full year (365 days):

A = 740 MBq \times e^{- (2.55 \times 1 0^{- 3}) (365)} = 740 \times 0.394 \approx 292 MBq

So a source logged at 740 MBq is realistically about 292 MBq (7.9 mCi) a year later. This matters for three reasons: the inventory and QC logs should reflect the present activity so a flood acquisition collects adequate counts; possession totals are evaluated against current activity; and a source that has decayed to near-background may be a candidate for disposal, updating the possession total. The same decay arithmetic underpins decay-in-storage waste management. ^{1, 3}

Possession limits and reconciliation

A radioactive material license authorizes possession of each radionuclide up to a possession limit. The accountability program's quantitative job is to confirm that, for each radionuclide, the sum of all possessed activity — sources in use, in storage, and decaying in storage — stays at or below the licensed limit:

i \sum A_{i} (radionuclide) \leq A_{licensed limit}

Reconciliation is the act of comparing the record-based list against the physical inventory and resolving every discrepancy. A source present but not on the list, or on the list but not found, is an accountability failure that must be investigated and corrected — and, if a source cannot be located, escalated under the facility's procedures and the applicable reporting rules. ^{1, 7}

The table summarizes how common nuclear medicine sealed sources fit the program.

Source	Typical half-life	Primary use	Leak test (35.67(b))	Semi-annual inventory (35.67(g))
Cobalt-57 flood sheet	271.74 days	Gamma-camera uniformity QC	Yes, when in use	Yes
Cesium-137 rod/point	~30 years	Dose-calibrator constancy, survey-meter check	Yes, when in use	Yes
Barium-133 point/mixed	~10.5 years	Dose-calibrator and camera check	Yes, when in use	Yes
Germanium-68 reference	~271 days	Dose-calibrator reference	Yes, when in use	Yes
Decayed source in storage	—	Awaiting disposal/return	Before any reuse/transfer	Yes (still accountable)

Note that a source decaying in storage is still a possessed, accountable source: it leaves the possession total only when it is disposed of or transferred and logged out. ^{1, 3}

Clinical Impact

Accountability is not paperwork for its own sake — it protects patients, staff, and the public, and it keeps the department operating.

QC integrity. A cobalt-57 flood source used without decay correction collects too few counts, producing a noisy uniformity image that can mask or mimic a camera defect. Accurate activity tracking keeps gamma-camera uniformity QC meaningful.
Regulatory standing. A missed semi-annual inventory, incomplete records, or an unlocated source are among the findings that surface during inspection and can lead to enforcement. A clean accountability program is a direct contributor to avoiding common radiation safety violations.
Security. Knowing where every source is at all times is the premise of source security. Even when a facility's sources are below the high-security thresholds, the inventory is the evidence that the material is under control.
Continuity of care. A source that cannot be located triggers investigation, potential reporting, and loss of confidence — all of which disrupt a department that depends on reliable QC sources to keep cameras and dose calibrators in service.

Practical Optimization Tips

1. Keep one master source list

Maintain a single, current list of every sealed source with model and serial number, radionuclide, calibration activity and date, current decay-corrected activity, and location. Every other record (leak tests, inventories, possession totals) flows from it.

2. Put the inventory and leak test on a calendar

Schedule the semi-annual inventory and the six-month-maximum leak tests as recurring tasks with owners, not as things remembered when convenient. Missing the interval is itself the violation.

3. Record exactly what the rule requires

For each inventory, capture the model and serial number, radionuclide and nominal activity, location, and the name of the person who performed it, and retain the record for three years as 10 CFR 35.2067 requires. Leak-test records carry the same retention and a parallel content list. ²

4. Reconcile against the possession limit every time

At each inventory, sum the current decay-corrected activity by radionuclide and compare to the license limit. Document the comparison so compliance is demonstrable, not assumed.

5. Check against the tracking and security thresholds

Confirm whether any source — alone or aggregated — reaches a Category 1 or Category 2 quantity that would invoke the National Source Tracking System or 10 CFR Part 37. For most imaging QC sources it will not, but the determination should be documented. ^{5, 6, 9}

Common pitfalls to avoid

Treating leak testing as the whole program. Integrity and accountability are separate; both are required.
Forgetting stored and decaying sources. They remain possessed and accountable until logged out at disposal or transfer.
Letting the "location" field go stale. An inventory is only as good as the movement records feeding it.
Ignoring decay. Logging the calibration activity forever overstates possession and undermines QC.
Assuming the thresholds do not apply without checking. Document the Category determination rather than assuming.

Regulatory Considerations

The sealed-source inventory and accountability program is built on NRC rules for medical use of byproduct material, supported by recordkeeping and source-security requirements, and interpreted through NRC and IAEA guidance. The RSO documents the program so it is defensible during inspection.

10 CFR 35.67 — Requirements for possession of sealed sources and brachytherapy sources: the six-month-maximum leak test with a 185 becquerel (0.005 microcurie) detection capability and five-day leak report, and the semi-annual physical inventory of all applicable sealed sources. ¹
10 CFR 35.2067 — Records of leak tests and physical inventories: required content and three-year retention. ²
10 CFR 30.51 — Records of receipt, transfer, and disposal of byproduct material, which anchor the accountability chain. ³
10 CFR Part 20 — Standards for Protection Against Radiation, including the recordkeeping and radiation protection program framework the inventory supports. ⁴
10 CFR Part 37 and 10 CFR 20.2207 — physical protection of Category 1 and Category 2 quantities and reporting to the National Source Tracking System, which apply when source activities reach those thresholds. ^{5, 6}
NUREG-1556, Volume 9, Revision 3 (2019) — NRC program-specific guidance for medical-use licenses, including model procedures for source inventory, leak testing, and accountability. ⁷
IAEA Safety Guide RS-G-1.9 (2005) — the international basis for categorizing radioactive sources by activity relative to dangerous quantities, which informs the security tier a source falls into. ⁹

Byproduct material is regulated by the NRC or by an Agreement State, not by the FDA. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that administer their own equivalent medical-use rules, while Washington DC and Delaware are regulated directly by the NRC. A facility must confirm which authority issues its license and keep its inventory, leak-test, and accountability records to that authority's requirements. This work is typically coordinated with radioactive material license support and the broader radiation safety officer program. ^{1, 7}

Frequently Asked Questions (FAQs)

What is the difference between a leak test and a physical inventory?

A leak test checks the integrity of a sealed source — whether radioactive material is escaping — by wipe-testing the source or its holder and measuring for removable contamination. A physical inventory checks accountability — whether the licensee still has each source, knows where it is, and has not exceeded its possession limit. Both are required under 10 CFR 35.67, and they answer different questions.

How often must a nuclear medicine facility inventory its sealed sources?

Under 10 CFR 35.67(g) a licensee in possession of sealed sources or brachytherapy sources must conduct a semi-annual physical inventory of all such sources, except gamma stereotactic radiosurgery sources. Leak testing under 10 CFR 35.67(b) is on a separate schedule not to exceed six months.

What must a physical inventory record contain?

Under 10 CFR 35.2067 the inventory record must include the model number of each source and its serial number if assigned, the identity of each source by radionuclide and its nominal activity, the location of each source, and the name of the individual who performed the inventory. These records must be retained for three years.

What is decay-corrected activity and why does it matter for inventory?

Radioactive sources decay continuously, so a source's current activity is lower than its activity at calibration. Decay-corrected activity is the calibration activity reduced by the exponential decay factor for the elapsed time. It matters because the inventory should reflect the realistic present activity, because possession limits are checked against current activity, and because quality-control sources such as flood sheets must be decay-corrected when used.

What is a possession limit?

A possession limit is the maximum activity of each radionuclide a facility is authorized to possess under its radioactive material license. The sealed-source inventory supports compliance by confirming that the sum of possessed activity for each radionuclide, including sources in use, in storage, and decaying in storage, stays at or below the licensed limit.

Are nuclear medicine quality-control sources nationally tracked or high-security?

Usually not. The National Source Tracking System under 10 CFR 20.2207 and the enhanced security of 10 CFR Part 37 apply to Category 1 and Category 2 quantities of specific radionuclides. Typical imaging quality-control sources — cobalt-57 flood sheets, cesium-137 and barium-133 check sources, germanium-68 dose-calibrator references — are far below those thresholds. A facility must still confirm its own source activities against the thresholds rather than assuming.

Who is responsible for the sealed-source inventory?

The Radiation Safety Officer is responsible for ensuring the inventory and leak-test program is conducted, documented, and reconciled against the license. The physical work may be delegated to trained staff, but the RSO owns the program, the records, and the response when a source cannot be accounted for.

Key Takeaways

Integrity and accountability are separate jobs. Leak testing proves a source is intact; the semi-annual physical inventory proves you still have it and know where it is. 10 CFR 35.67 requires both.
The inventory reaches every source. Sources in storage and decaying in storage remain possessed and accountable until logged out at disposal or transfer.
Track identity, report decay-corrected activity. Account for sources by model, serial number, and radionuclide, and reconcile current activity against the license possession limit.
Records are prescriptive. 10 CFR 35.2067 sets the required content and a three-year retention for both inventory and leak-test records.
Check the security tier. Confirm and document whether any source reaches a Category 1 or 2 quantity invoking the National Source Tracking System or 10 CFR Part 37 — usually imaging QC sources do not.

Conclusion

A sealed-source program lives or dies on two habits: testing that the sources are intact, and proving that they are all accounted for. The second habit is the one facilities underinvest in, because a source that is quietly misplaced does not set off an alarm until an inventory — or an inspector — asks for it.

The semi-annual physical inventory, backed by a current master source list, decay-corrected activity, clean receipt-to-disposal records, and a documented comparison against the possession limit, turns accountability from a scramble into a routine. That routine is what lets an RSO say, with evidence, that the department knows exactly what licensed material it holds and where every piece of it is.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine facilities build and maintain defensible sealed-source programs as part of its radiation safety officer and radioactive material license support services. DRPS support includes designing the master source list and inventory procedure, aligning leak-test and inventory intervals and records with 10 CFR 35.67 and 35.2067, reconciling possession totals against license limits, and confirming source-tracking and security applicability — all documented for inspection readiness.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR 35.67: Requirements for possession of sealed sources and brachytherapy sources. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 35.2067: Records of leak tests and inventory of sealed sources and brachytherapy sources. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 30.51: Records of receipt, transfer, and disposal of byproduct material. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 37: Physical Protection of Category 1 and Category 2 Quantities of Radioactive Material. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.2207: Reports to the National Source Tracking System. ecfr.gov
U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Consolidated Guidance About Materials Licenses — Program-Specific Guidance About Medical Use Licenses. 2019. nrc.gov
National Nuclear Data Center, Brookhaven National Laboratory. Nuclear data for Co-57 (half-life 271.74 days). nndc.bnl.gov
International Atomic Energy Agency. IAEA Safety Standards Series No. RS-G-1.9: Categorization of Radioactive Sources. Vienna: IAEA; 2005. iaea.org
International Atomic Energy Agency. Nuclear Medicine Physics: A Handbook for Teachers and Students. Vienna: IAEA; 2014. iaea.org