What is Activity? Becquerels, Curies, and How DOT Uses Them

Q: Why does DOT use TBq instead of Ci?

International harmonization. DOT aligned with IAEA Transport Regulations, which use SI units. Curies appear in parentheses for information only.

Q: Does activity tell me the dose rate?

Not directly. Activity measures decay rate; dose rate measures radiation exposure at a distance. They depend on radiation type, energy, shielding, and distance. For labeling, you measure dose rate directly with a survey instrument.

Q: Does decay affect my classification?

Absolutely. Activity at time of shipment is what matters. For short-lived isotopes like Tc-99m and I-131, the difference can be significant over even a few hours.

Quick Answer

Activity is the rate at which radioactive atoms decay — how many disintegrations occur per second. It's measured in Becquerels (Bq) in SI units or Curies (Ci) in traditional units. The key conversion: 1 Curie = 37 GBq. Activity is the single most important number in RAM shipping because it determines whether your material is regulated, what package type you need, and what label goes on it.

Becquerel (Bq): 1 disintegration per second — the SI unit used in DOT regulations
Curie (Ci): 37 billion disintegrations per second — the traditional unit still widely used in the US
DOT uses TBq as the primary unit in 49 CFR 173.435, with Curies in parentheses

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Why Understanding Activity Matters

Activity is the foundation of every classification decision in radioactive material shipping. It determines whether your material is even regulated as Class 7, which UN number applies, what package type is required, and how many packages you can load on a vehicle.

I have seen shippers make costly mistakes because they confused units, miscalculated conversions, or used the wrong activity value. A medical facility I worked with was preparing a shipment of I-131 therapy doses and had the activity listed as 3.7 GBq on their source certificate. They entered 3.7 into RAMcalc but accidentally selected “Ci” instead of “GBq.” That one unit error turned a 3.7 GBq shipment (which is 0.1 Ci) into what looked like a 3.7 Ci shipment (136.9 GBq) — a 37-fold overestimate that changed the package type entirely.

Who Needs to Know This

Every RAM shipper needs to understand activity:

Anyone who classifies radioactive material for transport
Anyone who reads or creates shipping papers listing activity values
Anyone who uses A1/A2 values to determine package type
Radiation safety officers who verify shipment documentation
Anyone who converts between Becquerels and Curies

Important: DOT regulations use SI units (Bq, TBq) as the official primary units, with Curies shown in parentheses “for information only.” However, many US facilities and source certificates still use Curies. You must be comfortable working in both systems.

What Activity Actually Means

Activity measures how fast a radioactive material is decaying — the number of atomic disintegrations per second. Each disintegration releases radiation, which is why higher activity means more radiation output.

The Becquerel (Bq)

Named after Henri Becquerel, who discovered radioactivity in 1896:

1 Becquerel = 1 disintegration per second

SI activity units based on the Becquerel
Unit	Symbol	Value	Common Use
Kilobecquerel	kBq	10³ Bq	Leakage limits, small check sources
Megabecquerel	MBq	10⁶ Bq	Medical doses, calibration sources
Gigabecquerel	GBq	10⁹ Bq	Medical therapy, small industrial sources
Terabecquerel	TBq	10¹² Bq	A1/A2 values, large industrial sources

The Curie (Ci)

Named after Marie and Pierre Curie. Originally defined as the activity of 1 gram of Ra-226:

1 Curie = 3.7 × 10¹⁰ disintegrations per second = 37 GBq

Traditional activity units based on the Curie and their Becquerel equivalents
Unit	Symbol	Equivalent in Bq	Common Use
Microcurie	µCi	37 kBq	Small check sources, smoke detectors
Millicurie	mCi	37 MBq	Medical diagnostic doses
Curie	Ci	37 GBq	Industrial sources, therapy sources

Converting Between Becquerels and Curies

The conversion is exact:

1 Ci = 37 GBq = 3.7 × 10¹⁰ Bq
1 mCi = 37 MBq
1 µCi = 37 kBq

The number 37 is your anchor. Curies to Becquerels: multiply by 37, then adjust the prefix.

Tip: Memorize: 1 Ci ≈ 37 GBq and 1 mCi ≈ 37 MBq. For precise work, use RAMcalc — it handles all unit conversions automatically.

The biggest mistake shippers make with conversions is getting the prefix wrong. The jump between kBq, MBq, GBq, and TBq is a factor of 1,000 each step. I have seen a shipping paper list activity as 3.7 MBq when the actual value was 3.7 GBq — a factor of 1,000 off. There really are no shortcuts for getting this right.

How DOT Uses Activity in Shipping Regulations

Step 1: Is It Regulated?

Per 49 CFR 173.436, material is only regulated as Class 7 if both the activity concentration (Bq/g) and the total activity (Bq) exceed the exempt values. See our What is Radioactive Material per DOT? guide.

Step 2: What Package Type?

Activity determines the package tier through A1 and A2 values:

Excepted package: Activity is a small fraction of A1 or A2
Type A package: Activity up to A1 (special form) or A2 (normal form)
Type B package: Activity exceeds A1 or A2 (requires NRC-approved packaging)

Critical: DOT regulations express A1 and A2 values in TBq as the primary unit with Curies shown “for information only” per 49 CFR 173.435. If your source certificate uses Curies, you must convert to SI units for regulatory compliance.

Specific Activity vs Total Activity

Total Activity

The absolute amount of radioactivity in your shipment (Bq or Ci). This is what you use for shipping classification.

Specific Activity

Activity per unit mass (Bq/g or Ci/g), defined in 49 CFR 173.403. An intrinsic property of each radionuclide — shorter half-life means higher specific activity.

Specific activity examples showing the relationship between half-life and activity per gram — Source: 49 CFR 173.403
Isotope	Half-Life	Specific Activity	Context
Co-60	5.27 years	~41.8 TBq/g	High — industrial sources are physically small
Cs-137	30.17 years	~3.2 TBq/g	Moderate — common in gauges
Ra-226	1,600 years	~37 GBq/g	Low — defined the original Curie
U-238	4.5 billion years	~12.4 kBq/g	Very low — large mass, little activity

In practice, total activity is what drives your package type selection. Specific activity matters for LSA classifications and for determining whether natural or depleted uranium requires regulation.

Activity Changes Over Time: Radioactive Decay

Activity decreases over time as radioactive atoms decay. This matters for shipping because:

Activity must be determined at time of shipment — not at time of calibration
Short-lived isotopes (like Tc-99m with a 6-hour half-life) may decay significantly during transit
Source certificates often list activity at a reference date — you must decay-correct to the ship date

I have worked with facilities shipping I-131 where the source was calibrated several days before shipment. For I-131 (8-day half-life), waiting 3 days reduces activity by about 23%. That being said, always use the higher value if you're unsure — overstating activity means more conservative packaging, which is always compliant. Understating activity is the dangerous error.

Common Activity Ranges in Shipping

Typical activity ranges encountered in common radioactive material shipments
Shipment Type	Typical Activity	Common Isotopes	Typical Package
Smoke detectors	~33 kBq (~1 µCi)	Am-241	Excepted
Medical diagnostic	100 MBq – 40 GBq	Tc-99m, F-18	Type A
Industrial gauges	GBq to TBq	Cs-137, Am-241	Type A or B
Industrial radiography	0.37 – 9 TBq	Ir-192, Co-60	Type B

How RadShip.com Helps

RadShip.com takes the guesswork out of activity calculations:

RAMcalc — Enter activity in any unit (Bq, kBq, MBq, GBq, TBq, Ci, mCi, µCi) and it converts automatically
RAMcalc — Compares your activity against A1/A2 values to determine package type instantly
LabelCalc — Uses activity and dose rate to determine the correct label and Transport Index

Here's the reality: the most error-prone step in RAM shipping is the conversion between units. Having the calculator accept any unit and do the conversion for you eliminates that entire class of errors.

Try it free for 7 days.

Common Questions

Why does DOT use TBq instead of Ci?

International harmonization. DOT aligned with IAEA Transport Regulations, which use SI units. Curies appear in parentheses “for information only.”

Does activity tell me the dose rate?

Not directly. Activity measures decay rate; dose rate measures radiation exposure at a distance. They depend on radiation type, energy, shielding, and distance. For labeling, you measure dose rate directly with a survey instrument.

Does decay affect my classification?

Absolutely. Activity at time of shipment is what matters. For long-lived isotopes the change is negligible over weeks, but for short-lived isotopes (Tc-99m, I-131) the difference can be significant.

In my experience, most classification errors trace back to activity — either the wrong units, a missed conversion, or using the calibration date instead of the ship date. Getting comfortable with both Becquerels and Curies is the single most practical skill a RAM shipper can develop.

Summary: Your Activity Checklist

☐ Activity value obtained from source certificate, assay, or measurement
☐ Units clearly identified (Bq, MBq, GBq, TBq, or Ci, mCi, µCi)
☐ Converted to SI units (Bq/TBq) for comparison against DOT thresholds
☐ Decay-corrected to the date of shipment (especially for short-lived isotopes)
☐ Compared against exempt values in 49 CFR 173.436 (is it regulated?)
☐ Compared against A1/A2 values in 49 CFR 173.435 (what package type?)
☐ Activity listed on shipping papers and labels in proper units

Regulatory References

DOT Requirements:

49 CFR 173.403 — Definitions (activity, specific activity, A1, A2)
49 CFR 173.435 — Table of A1 and A2 values
49 CFR 173.436 — Activity values for exempt materials
49 CFR 172.403 — Class 7 label requirements (activity in SI units)

NRC Reference:

10 CFR 20.1005 — Units of radioactivity (Becquerel and Curie definitions)

About the Author

Scott Brown is the Subject Matter Expert and co-creator of RadShip.com. He has been a trained hazmat shipper for over 15 years and specializes in DOT Class 7 radioactive material shipping.

This guide is based on the requirements of 49 CFR (DOT), 10 CFR (NRC), and the IATA Dangerous Goods Regulations as of the publication date. As regulations are amended, RadShip.com is committed to keeping its guides current with the latest requirements.