| Isotope | Z | A | Stable? | Half‑life | Decay mode(s) | Daughter | Radiation | Origin | Uses | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| ³He | 2 | 3 | Stable | stable | — | — | — | Natural (trace) & Synthetic | Neutron detection; dilution refrigerators; hyperpolarized gas MRI | Fermion; rare on Earth, enriched from tritium decay. |
| ⁴He | 2 | 4 | Stable | stable | — | — | — | Natural | Cryogenics (He‑I/He‑II); carrier gas; leak detection | Produced primordially and by α‑decay in crust. |
| ⁵He | 2 | 5 | Unstable (unbound) | ≈ 7.6×10⁻²² s (from Γ) | n emission | ⁴He + n | neutrons (no α/β/γ) | Synthetic (accelerators) | Research (n–α resonance) | Lifetime inferred from resonance width; unbound to n+α. |
| ⁶He | 2 | 6 | Unstable | 806.7 ms | β⁻ | ⁶Li | β⁻ (no significant γ) | Synthetic | Precision β‑decay tests; few‑body/halo physics | Classic 2‑neutron halo nucleus. |
| ⁷He | 2 | 7 | Unstable (unbound) | ≈ 3×10⁻²¹ s (from Γ) | n emission | ⁶He + n | neutrons (no α/β/γ) | Synthetic (accelerators) | Research (unbound N=5 system) | Narrower resonance than ⁵He; decays to ⁶He+n. |
| ⁸He | 2 | 8 | Unstable | 119.1 ms | β⁻; β⁻, n (β‑delayed) | ⁸Li; ⁷Li + n | β⁻; β‑delayed n | Synthetic | Halo‑nucleus benchmark; β‑delayed‑n studies | Dominant β⁻ to ⁸Li; minor β‑delayed n branch to ⁷Li. |
| ⁹He | 2 | 9 | Unstable (unbound) | ≈ 7×10⁻²¹ s (from Γ) | n emission | ⁸He + n | neutrons (no α/β/γ) | Synthetic (accelerators) | Research (neutron‑unbound isotone) | Low‑lying unbound states observed; width‑derived lifetime. |
| ¹⁰He | 2 | 10 | Unstable (unbound) | ≈ 2.7×10⁻²¹ s (from Γ) | 2n emission (sequential/simult.) | ⁸He + 2n (via ⁹He) | neutrons (no α/β/γ) | Synthetic (accelerators) | Research (extreme dripline, 2n emission) | Evidence for direct/through‑⁹He 2n paths. |
Totals — Helium
- Stable: 2
- Unstable: 6
- Total: 8
Running cumulative totals (up to Z = 2)
- Cumulative total isotopes: 15
- Cumulative stable: 4
- Cumulative unstable: 11
Sources
- Data authority for half‑lives/decays: IAEA LiveChart of Nuclides (ENSDF backbone) and NNDC NuDat 3 interfaces. (IAEA Nuclear Data Services, National Nuclear Data Center)
- ⁶He half‑life and β⁻ to ⁶Li: KAERI isotope page for He‑6 (806.7 ms; β⁻). (KAERI Atom)
- ⁸He half‑life and β‑delayed neutron branch: TUNL compilation for ⁸He (β‑delayed n studies); concise isotope table with 119.1 ms and β‑delayed‑n branch. (TUNL Nuclear Data Project, ChemLin)
- Unbound resonances (⁵He, ⁷He): experimental/analysis papers on n–α scattering and ⁷He spectrum (widths ⇒ zepto‑/yoctosecond lifetimes). (Physical Review Links)
- Round‑up helium isotope list/values (for quick cross‑check): ChemLin helium isotope table (aggregates ENSDF/NuDat values). (ChemLin)
Next: Lithium (Z = 3)
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