Plan around decay
Ba-133 has a half-life of 10.55 years and loses about 0.1 percent of its activity each week. Ask for the activity you need on the day you start using it.
- 1 year93.6%
- 5 years72%
- 10 years51.9%
- 20 years26.9%
Sealed gamma reference sources
Feasibility review
Barium-133 for research
Gamma lines from 81 to 384 keV and cesium X-rays near 31 keV: a standard for low- and mid-energy calibration.
- Half-life
- 10.55 years
- Main gamma
- 356.0 keV
- Decay
- Electron capture to Cs-133
- Exempt quantity
- 10 µCi
Ba-133 at a glanceIAEA evaluated data
Research applications
Where Ba-133 does the work.
Figures are simulated examples of each measurement.
Radiation detection
Efficiency calibration from 31 to 384 keV1, 2
Five gamma lines and the cesium K X-rays calibrate gamma-ray spectrometers across the low and middle energy range.
More: Efficiency calibration from 31 to 384 keV
The IAEA lists Ba-133 among its recommended calibration nuclides, with evaluated emission probabilities for the 81, 276, 303, 356 and 384 keV lines and the cesium K X-rays.
Radiation detection
Low-energy lines and a close pair2, 3
Below 100 keV, Ba-133 gives cesium K X-rays and two gamma lines only 1.4 keV apart, at 79.6 and 81.0 keV.
More: Low-energy lines and a close pair
The IAEA evaluation notes that resolving the 79 and 81 keV lines is a problem, and the GERDA experiment's dead-layer analysis sums the two peaks.
Detector physics
Germanium dead-layer scans3, 4
The ratio of the 81 keV to the 356 keV peak measures the inactive surface layer of a germanium detector.
More: Germanium dead-layer scans
Low-energy gamma rays are absorbed in the dead layer at the crystal surface, while 356 keV gamma rays mostly pass through it. The thickness is where the measured peak ratio meets the ratio simulated for that thickness.
The GERDA experiment measured the full charge collection depth of its germanium detectors this way, with Ba-133 sources of several tens of kBq.
Radiation shielding
Attenuation at several energies4, 5
One Ba-133 source measures attenuation coefficients at lines on both sides of the 88 keV K edge of lead.
More: Attenuation at several energies
A sample between the source and the detector gives the attenuation coefficient at each line, which is compared with calculated reference values.
A study of lead and bismuth salts, for example, measured mass attenuation coefficients from 81 to 1333 keV with Ba-133 and other point sources and a germanium detector, in good agreement with WinXCOM calculations.
Forms
Ba-133 forms researchers ask for
Every source is confirmed with its manufacturer before you order.
Point or disk source
Sealed, encapsulated Ba-133
Energy and efficiency calibration, dead-layer scans
Feasibility reviewThin-window source
Ba-133 behind a thin window
X-ray and low-energy calibration
Feasibility review
Activity, certificate and lead time are set with the manufacturer at quote.
Before you order
Plan for decay and licensing.
Licensing
Exempt quantityNeeds a license10 µCi
A single Ba-133 source at or below 10 µCi is an exempt quantity under 10 CFR 30.71, Schedule B. Above that, your institution needs a license that covers it.
- Combining exempt sources to exceed the limit is not covered.
- Agreement States apply equivalent rules. Your RSO reviews the exact source before you order.
Start a request
Request a feasibility review for Ba-133.
No license file needed to ask. A scientist reads every request and replies with the next step.
- Name the measurementCalibration range, geometry and detector set the source form.
- Give a reference dateState the activity you need on the day you start using it.
- Plan the next sourceActivity halves about every 10.6 years.
Decay data
Ba-133 decay data
Decay schemeBa-133 to Cs-133
- Half-life
- 10.551 years ± 0.011
- Decay mode
- Electron capture (100%)
- Daughter
- Cs-133
- Decay energy (Q)
- 517.4 keV
| Radiation | Energy (keV) | Per 100 decays |
|---|---|---|
| Gamma | 53.16 | 2.14 |
| Gamma | 79.61 | 2.65 |
| Gamma | 81 | 32.9 |
| Gamma | 160.61 | 0.638 |
| Gamma | 223.24 | 0.453 |
| Gamma | 276.4 | 7.16 |
| Gamma | 302.85 | 18.3 |
| Gamma | 356.01 | 62 |
| Gamma | 383.85 | 8.94 |
| X-ray | 4.75 | 15.3 |
| X-ray | 30.63 | 32.6 |
| X-ray | 30.97 | 60.2 |
| X-ray | 35.09 | 17.6 |
| X-ray | 35.41 | 21.9 |
Source: IAEA LiveChart of Nuclides, ENSDF evaluation by YU. KHAZOV AND A. RODIONOV and F.G. KONDEV (literature cutoff 31 October 2010), retrieved 25 September 2026 [1]. Reference values; a manufacturer's certificate states the activity of a specific source.
Questions, answered
Ba-133 questions
How long does a Ba-133 source last?
Its activity halves every 10.551 years, so about 94 percent remains after one year. How long a source stays useful depends on the lowest activity your measurement can use.
Can we buy Ba-133 without a radioactive materials license?
For small sources, possibly. A single Ba-133 source at or below 10 microcuries is an exempt quantity under 10 CFR 30.71, and a distributor licensed under 10 CFR 32.18 can supply it to users without a license. Larger sources need a license that covers them. Your RSO decides how your institution handles each case.
Why does a Ba-133 spectrum show cesium X-rays?
Ba-133 decays by electron capture to cesium-133. As the cesium atom fills the vacancy in its inner shell it emits K X-rays near 31 and 35 keV, which also serve as calibration lines.
What activity should we ask for?
State the activity you need at a reference date close to first use, and the instrument or measurement it serves. We confirm a matching source and certificate with the manufacturer before you order.
Does the certificate state the activity on delivery?
It states the activity at the manufacturer's reference date. We convert it to your delivery or first-use date on the quote, and the chart at the top of this page shows how the activity falls.
Can a decayed source be returned?
Some manufacturers take back their own sealed sources. A return is a separate shipment with its own RSO and shipping review, so ask us before the source reaches the end of its use.
More isotopes
Related isotopes
Sources
References
- IAEA Nuclear Data Section. LiveChart of Nuclides, Ba-133 decay data (ENSDF evaluation). Retrieved 25 September 2026. www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
- International Atomic Energy Agency. X-ray and Gamma-ray Standards for Detector Calibration. IAEA-TECDOC-619. Vienna: IAEA; 1991. www-pub.iaea.org/MTCD/Publications/PDF/te_619_web.pdf
- GERDA Collaboration, Agostini M, Bakalyarov AM, et al. Characterization of 30 76Ge enriched Broad Energy Ge detectors for GERDA Phase II. Eur Phys J C. 2019;79(11):978. doi.org/10.1140/epjc/s10052-019-7353-8
- Hubbell JH, Seltzer SM. Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients (version 1.4). NIST Standard Reference Database 126. Gaithersburg, MD: National Institute of Standards and Technology; 2004. physics.nist.gov/PhysRefData/XrayMassCoef/
- Sayyed MI, Akman F, Kaçal MR, Kumar A. Radiation protective qualities of some selected lead and bismuth salts in the wide gamma energy region. Nucl Eng Technol. 2019;51(3):860-866. doi.org/10.1016/j.net.2018.12.018
- U.S. Nuclear Regulatory Commission. 10 CFR 30.71, Schedule B (exempt quantities). eCFR, current as of 23 September 2026. www.ecfr.gov/current/title-10/section-30.71
- U.S. Nuclear Regulatory Commission. 10 CFR 30.18, Exempt quantities. eCFR, current as of 23 September 2026. www.ecfr.gov/current/title-10/section-30.18
Technical review by ConductScience, 25 September 2026. Next review 25 September 2027.
Need Ba-133 for a specific instrument or measurement?
Start with what you know. Your RSO and supplier documents come after review. See all radioactive isotopes.
