Plan around decay
Cs-137 has a half-life of 30.08 years. Ask for the activity you need on the day you start using it.
- 1 year97.7%
- 5 years89.1%
- 10 years79.4%
- 20 years63.1%
Sealed gamma reference sources
Feasibility review
Cesium-137 for research
One gamma line at 662 keV and a 30-year half-life: a long-lived reference for detector and calibrator checks.
- Half-life
- 30.08 years
- Main gamma
- 661.7 keV
- Decay
- Beta minus to Ba-137
- Exempt quantity
- 10 µCi
Cs-137 at a glanceIAEA evaluated data
Research applications
Where Cs-137 does the work.
Figures are simulated examples of each measurement.
Radiation detection
Energy calibration at 662 keV1, 2
Its single 662 keV line is an IAEA recommended standard for calibrating gamma-ray spectrometers.
More: Energy calibration at 662 keV
The IAEA evaluates its emission probability at 85.1 per 100 decays. The line comes from barium-137m, the 2.55-minute excited state reached in 94.7 percent of decays.
Scintillator research
Scintillator energy resolution3, 4, 5
New scintillators are compared by their energy resolution at the 662 keV line of Cs-137.
More: Scintillator energy resolution
A commercial sodium iodide crystal resolves the line to about 7 percent. Van Loef and colleagues measured 2.85 percent for cerium-doped lanthanum bromide.
A 2018 review puts co-doped lanthanum bromide at around 2 percent, the best scintillator resolution it reports at 662 keV.
Detector physics
Compton-edge calibration6, 7
In organic scintillators gamma rays mostly Compton scatter, so their energy scale is set at the 477 keV Compton edge of the Cs-137 line.
More: Compton-edge calibration
Two classic rules locate the edge on the measured spectrum: the point at 89 percent of the full height marks the edge, or the half-height point sits at 104 percent of it.
Laplace and colleagues compared three organic scintillators this way with a 4.2 µCi Cs-137 source; the liquid EJ-309 resolved the edge to about 11 percent.
Radiation shielding
Attenuation at 662 keV8, 9
A narrow Cs-137 beam measures how lead, iron and other materials attenuate 662 keV gamma rays.
More: Attenuation at 662 keV
From NIST attenuation coefficients, the half-value layer at 662 keV is about 0.55 cm of lead, 1.2 cm of iron and 3.4 cm of aluminium.
A study of lead and bismuth salts used Cs-137 among its point sources, with a germanium detector, and its results matched WinXCOM calculations.
Environmental science
Time markers in sediment cores10
Fallout Cs-137 from atmospheric nuclear tests peaked in 1963, a date marker in sediment cores.
More: Time markers in sediment cores
Cs-137 first appears in layers from the mid-1950s and peaks in layers from 1963, and the depth of that peak gives the average sedimentation rate since.
A review of 1351 dated cores found the test fallout in about half of them and notes that the 1963 peak is described as the most reliable time marker worldwide.
Radionuclide metrology
Calibrator constancy checks11
A Cs-137 check source measured each day shows whether a radioactivity calibrator still reads the same.
More: Calibrator constancy checks
The IAEA recommends measuring a long-lived source in a reproducible geometry each day before use. It names caesium-137 a good option for its long half-life and radionuclidic purity, with check sources of about 5 MBq.
A Cs-137 check source loses about 2.3 percent of its activity each year, so readings are compared with the decay-corrected value.
Forms
Cs-137 forms researchers ask for
Every source is confirmed with its manufacturer before you order.
Point or disk source
Sealed, encapsulated Cs-137
Detector checks, energy calibration and resolution tests
Feasibility reviewCalibrator reference source
Sealed vial geometry
Calibrator constancy checks
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 Cs-137 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 Cs-137.
No license file needed to ask. A scientist reads every request and replies with the next step.
- Name the measurementDetector checks, calibration or shielding work sets the source form and activity.
- Give a reference dateState the activity you need on the day you start using it.
- Plan the next sourceActivity halves about every 30 years.
Decay data
Cs-137 decay data
Decay schemeCs-137 to Ba-137
- Half-life
- 30.08 years ± 0.09
- Decay mode
- Beta minus (100%)
- Daughter
- Ba-137
- Decay energy (Q)
- 1175.63 keV
| Radiation | Energy (keV) | Per 100 decays |
|---|---|---|
| Beta | 514 max, 174.3 mean | 94.7 |
| Beta | 1176 max, 416.3 mean | 5.3 |
| Gamma | 661.66 | 85.1 |
The 661.66 keV gamma ray comes from Ba-137m (half-life 2.552 minutes), which Cs-137 sources hold in equilibrium.
Source: IAEA LiveChart of Nuclides, ENSDF evaluation by E. BROWNE and J. K. TULI (literature cutoff 1 October 2006), retrieved 25 September 2026 [1]. Reference values; a manufacturer's certificate states the activity of a specific source.
Questions, answered
Cs-137 questions
How long does a Cs-137 source last?
Its activity halves every 30.08 years, so about 98 percent remains after one year. How long a source stays useful depends on the lowest activity your measurement can use.
Can we buy Cs-137 without a radioactive materials license?
For small sources, possibly. A single Cs-137 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.
Where does the 662 keV gamma ray come from?
In 94.7 percent of decays, Cs-137 beta decays to barium-137m, an excited state with a half-life of 2.55 minutes that emits the 662 keV gamma ray. Inside a sealed source the two stay in balance, so the line follows the Cs-137 activity.
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.
Can you supply high-activity Cs-137 sources?
No. We handle research and check-source quantities only, not irradiator or other high-activity sources.
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, Cs-137 and Ba-137 decay and level 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
- Hawrami R, Ariesanti E, Farsoni A, Szydel D, Sabet H. Growth and evaluation of improved CsI:Tl and NaI:Tl scintillators. Crystals. 2022;12(11):1517. doi.org/10.3390/cryst12111517
- van Loef EVD, Dorenbos P, van Eijk CWE, Krämer K, Güdel HU. High-energy-resolution scintillator: Ce3+ activated LaBr3. Appl Phys Lett. 2001;79(10):1573-1575. doi.org/10.1063/1.1385342
- Yanagida T. Inorganic scintillating materials and scintillation detectors. Proc Jpn Acad Ser B Phys Biol Sci. 2018;94(2):75-97. doi.org/10.2183/pjab.94.007
- Mauritzson N, Fissum KG, Perrey H, et al. GEANT4-based calibration of an organic liquid scintillator. Nucl Instrum Methods Phys Res A. 2022;1023:165962. doi.org/10.1016/j.nima.2021.165962
- Laplace TA, Goldblum BL, Bevins JE, et al. Comparative scintillation performance of EJ-309, EJ-276, and a novel organic glass. J Instrum. 2020;15(11):P11020. doi.org/10.1088/1748-0221/15/11/P11020
- 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
- Foucher A, Chaboche PA, Sabatier P, Evrard O. A worldwide meta-analysis (1977-2020) of sediment core dating using fallout radionuclides including 137Cs and 210Pbxs. Earth Syst Sci Data. 2021;13(10):4951-4966. doi.org/10.5194/essd-13-4951-2021
- International Atomic Energy Agency. Quality Assurance for Radioactivity Measurement in Nuclear Medicine. Technical Reports Series No. 454. Vienna: IAEA; 2006. www-pub.iaea.org/MTCD/Publications/PDF/TRS454_web.pdf
- 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 Cs-137 for a specific instrument or measurement?
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