Polonium-222

Properties and data of the isotope ²²²Po.

Contents

Polonium-222 is a radioisotope of the chemical element polonium, which has 138 neutrons in its atomic nucleus in addition to the element-specific 84 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 222. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²²²Po is exclusively for academic purposes.

In 2010, the previously unknown isotope polonium-222 was reported for the first time. In the experiment described, a 670-MeV/u uranium-238 beam impinged on a beryllium target, producing projectile fragments that were subsequently selected with the FRS fragment separator. The identification of ²²²Po was achieved by time-resolved Schottky mass spectrometry in the ESR storage ring [1].

See also: List of individual Polonium isotopes (and general data sources).

General data

Name of the isotope:Polonium-222; Po-222Symbol:²²²Po or 22284PoMass number A:222 (= number of nucleons)Atomic number Z:84 (= number of protons)Neutrons N:138Nucleon pairing (Z - N):even - evenNuclear ratio (N/Z ratio):1.6428571428571 (= neutron-proton ratio)Neutron excess (N-Z):54Isotopic mass:222.02414(4) u (atomic weight of Polonium-222)Nuclide mass:221.9780647 u (calculated nuclear mass without electrons)Mass excess:22.48627 MeVMass defect:1.828921096 u (per nucleus)Nuclear binding energy:1703.6290274 MeV (per nucleus)
7.67400463 MeV (average binding energy per nucleon)Separation energy:S_N = 5.359(45) MeV (first neutron)
S_P = 9.003(303) MeV (first proton)Half-life:68.7(21) sDecay constant λ:0.010089478610771 s^-1Specific activity α:2.736651086186 × 10⁺¹⁹ Bq g^-1
739635428.69916 Ci g^-1Spin and parity:
(nuclear angular momentum)0+Year of discovery:2010

Radioactive Decay

Half-life T_½ = 68.7(21) s respectively 6.87 × 10¹ seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
β^-	²²²At	100 %	1.53(4) MeV

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 138) and isobaric (same nucleon number A = 222) with Polonium-222. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 138	Isobar A = 222
82	²²⁰Pb
83	²²¹Bi	²²²Bi
84	²²²Po	²²²Po
85	²²³At	²²²At
86	²²⁴Rn	²²²Rn
87	²²⁵Fr	²²²Fr
88	²²⁶Ra	²²²Ra
89	²²⁷Ac	²²²Ac
90	²²⁸Th	²²²Th
91	²²⁹Pa	²²²Pa
92	²³⁰U	²²²U
93	²³¹Np	²²²Np
94	²³²Pu
95	²³³Am
96	²³⁴Cm
97	²³⁵Bk

Data changes (history)

Half-life: A new, shorter value for the half-life of polonium-222 - t_1/2 = 68.7 ± 2.1 s - was published in a 2025 conference report [2]. Other, older sources cite significantly less precise and widely divergent values ranging from approximately 2 (or 2.42) to 9.1 minutes (NuBase2020).

External data and identifiers

Adopted Levels, Gammas:NuDat 222Po

Literature and References

[1] - L. Chen, W. R. Plaß, H. Geissel et al.:
Discovery and investigation of heavy neutron-rich isotopes with time-resolved Schottky spectrometry in the element range from thallium to actinium.
In: Physics Letters B, 691, 5, (2010), DOI 10.1016/j.physletb.2010.05.078.

[2] - Giovanna Benzoni, Marta Polettini:
New half-lives in the polonium isotopic chain.
In: EPJ Web of Conferences, 324, 00006, (2025), DOI 10.1051/epjconf/202532400006.