Chlorine-36

Cl-36 · Chlorine, Z = 17, A = 36

A gigabecquerel of Cl-36 is 819 mg of material, which follows from a specific activity of 1.22e+9 Bq/g (0.0330 Ci/g). Chlorine-36 decays by electron capture with beta-plus with a half-life of 3.013 × 10⁵ years; forty years leaves 99.99% of today's activity and ten thousand years leaves 97.73%.

The air kerma rate constant is small — 0.0000195 mGy·m²/(GBq·h), 4.1e+3× less than Cs-137 and 1.6e+4× less than Co-60, ranking 87 of 96 by Γ — near the bottom of the photon emitters, but above the cutoff, which 51 nuclides in this dataset are not. 1 GBq at 1 m reads 0.0000195 mGy/h, and 1 Ci at the same distance 0.000720 mGy/h. It takes 1.03 TBq at a metre to reach 20 µSv/h from the photons alone.

One line at 511.0 keV carries all of that.

Halving the air kerma rate calls for 3.90 mm of lead, or 10.6 mm of steel where lead is unwelcome, and a factor of ten calls for 13.0 mm of lead — sheet thicknesses that a glovebox or a transport container can carry. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.

The beta endpoint is 0.710 MeV, mean 0.251 MeV over 2 branches. That endpoint stops in 2.24 mm of acrylic or 1.06 mm of glass. Of the beta energy, 0.15% turns into X-rays in acrylic and 2.04% in lead.

Half-life, specific activity, dose rate and beta energies

Half-life	3.013 × 10⁵ years (9.508e+12 s)
Decay mode	electron capture with beta-plus
Specific activity	1.22e+9 Bq/g (0.0330 Ci/g)
Air kerma rate constant Γ (δ = 20 keV)	0.0000195 mGy·m²/(GBq·h)
Dose rate, 1 GBq at 1 m	0.0000195 mGy/h
Dose rate, 1 Ci at 1 m	0.000720 mGy/h
Kerma-weighted mean photon energy	511.0 keV
Beta endpoint / mean	0.710 MeV / 0.251 MeV

A single photon line at 511 keV

3 further lines below the 20 keV cutoff, the highest at 2.47 keV and 0.137% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.

Energy (keV)	Emission probability (%)	Share of dose rate (%)
511.00	0.028	100.00

3.90 mm of lead halves this spectrum

Solved numerically across all 1 line, narrow beam. Why not one representative energy.

Material	HVL (mm)	TVL (mm)	TVL / HVL
lead	3.90	13.0	3.32
tungsten	2.68	8.90	3.32
iron	10.6	35.1	3.32
copper	9.35	31.1	3.32
concrete	34.1	113	3.32
water	72.2	240	3.32
aluminum	30.7	102	3.32

A single energy would give 3.32. What a spread of energies does instead.

2.24 mm of acrylic stops the 0.710 MeV endpoint

Katz–Penfold fit to the 0.710 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.

Absorber	Range for the endpoint (mm)
acrylic	2.24
aluminum	0.978
water	2.64
glass	1.06

Shield material	Fraction of beta energy converted to X-rays
acrylic (Z ≈ 6)	0.15%
lead (Z = 82)	2.04%

Activity over geological time

Ten half-lives is 3.01 million years. On any timescale a facility can be planned over the activity is constant — 99.99% is left after forty years — and the mean life 1/λ is 434,675 years.

Elapsed	Fraction remaining
1 half-life	50.0 %
2 half-lives	25.0 %
5 half-lives	3.13 %
10 half-lives	0.0977 %
Time to fall to 10 % of today's activity	1.00 million years
Time to fall to 1 %	2.00 million years
Time to fall to 0.1 %	3.00 million years

Limits of these dose rates

0.0000195 mGy/h at a metre — bare point source, no capsule, no self-absorption.
3.90 mm of lead halves this spectrum, narrow beam, scatter not added back.
Γ excludes 3 lines under 20 keV, carrying 0.137% of all emissions.
2.24 mm of acrylic for the 0.710 MeV endpoint: a ±10% fit, and not a skin dose.
What every sheet leaves out, internal dose included.

Gamma, beta and decay calculators for Cl-36

Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.