Example: a Cs-137 field at 2.5 m — air kerma rate, lead HVL and Hp(10)
Field Pro · updated 28 October 2026
A caesium-137 irradiator is the workhorse of a radiation-protection calibration room: one line at 661.657 keV, a half-life long enough to plan around and short enough to matter. This example takes a source certified ten years ago, brings its activity to today, and gives the air kerma rate, the dose-equivalent rates and the lead half-value layer at a 2.5 m test point.
Inputs
Open the workbench with Cs-137 selected and enter the values below. The activity and dates are illustrative; use your own certificate.
| Panel | Field | Value |
|---|---|---|
| 01 Source | Source | Cs-137 |
| 01 Source | Activity | 10 GBq |
| 01 Source | Reference date (certificate) | 2016-01-01 |
| 01 Source | Evaluation date | 2026-10-28 |
| 01 Source | Distance | 2.5 m |
| 02 Geometry | α / HVL material | 0° / Auto (lead) |
Results
| Quantity | Value |
|---|---|
| Elapsed time | 3953 d (10.82 a) |
| Activity on the evaluation date | 7.793 GBq (decay factor 0.7793) |
| Air kerma rate at 2.5 m | 94.53 µGy/h |
| Hp(10, 0°)/Ka, slab | 1.216 Sv/Gy → 115.0 µSv/h |
| H*(10)/Ka | 1.203 Sv/Gy → 113.7 µSv/h |
| H′(0.07, 0°)/Ka | 1.200 Sv/Gy → 113.4 µSv/h |
| HVL1 in lead (narrow beam) | 5.600 mm Pb |
| Homogeneity coefficient | 1 (single line) |
| Radionuclide check against S-Cs | 661.657 keV vs 661.6 keV nominal |
Panel C does not search the X-ray table for a nuclide; it compares the field with the S-Cs quality of ISO 4037 instead.
What the numbers say
Dates are part of the calibration. Over 10.82 years the source has lost 22% of its activity, and it keeps losing 2.3% a year. A rate table written at commissioning and never re-dated is 2.3% wrong after one year and 11% wrong after five. Entering the evaluation date each time costs nothing.
The rate scales with the inverse square of distance, and only that. At 1 m the same source gives 590.8 µGy/h, 6.25 times the 2.5 m value. The point-source formula ignores the air in between; with the engine's own attenuation data, 2.5 m of air would remove 2.3% of the primary kerma, while scatter from the air, the collimator and the room adds some back. Those effects are why a laboratory fixes its rates by measurement with a calibrated chamber and uses a calculation like this one to plan and to check.
The calculated rate constant sits slightly low. Dividing the rate by activity and multiplying by d² gives 0.0758 µGy·m²·MBq⁻¹·h⁻¹, 1.7% below the literature value of 0.0771 used in the test suite. The engine models only the gamma line; the barium K X-rays near 32 keV are absent, and with them a small low-energy share of the kerma.
Lead attenuators. In the narrow-beam model each 5.6 mm of lead halves the air kerma rate, and 22.4 mm (four half-value layers) leaves 6.25%, about 5.9 µGy/h at 2.5 m. Real attenuators in a collimated beam add forward scatter, so the measured reduction is smaller; the narrow-beam value is the right one for describing beam quality and the wrong one for shielding a room.
Choose the quantity before the coefficient. At this energy Hp(10) on the slab exceeds H*(10) by only 1.1%, so mixing them up hides easily; it is still a different quantity with a different phantom, and a personal dosemeter must be compared with Hp(10).
The X-ray counterpart of this example is the N-80 field; how the decay and kerma formulas are built is on the Methods page.
Sources
- IAEA Nuclear Data Section, Live Chart of Nuclides: Cs-137 half-life 30.08 a, 661.657 keV line at 85.1% per decay, Ba K X-rays.
- ISO 4037-3:2019, recommended conversion coefficients for S-Cs: Hp(10) 1.21, H*(10) 1.20, H′(0.07) 1.21 Sv/Gy.
- J. H. Hubbell, S. M. Seltzer, X-Ray Mass Attenuation Coefficients, NIST Standard Reference Database 126 (lead and air).
- Literature rate constant as used in the engine's decay tests.