Radium-221

Properties and data of the isotope ²²¹Ra.

Contents

General Data

Radioactive Decay

Parent Nuclides

Isotones und Isobares

Literature

Radium-221 is a radioisotope of the chemical element radium, which has 133 neutrons in its atomic nucleus in addition to the element-specific 88 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 221. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²²¹Ra is exclusively for academic purposes and experimental research.

In 1949 a report appeared describing the first evidence for the isotope radium-221. In the experiments discussed there, thorium, was irradiated with 100–120 MeV helium-4 from the 184-inch cyclotron, giving access to complex alpha-decay chains originating from the uranium isotopes produced in these reactions. After chemical separation, these chains could be followed spectroscopically, leading to a clear identification of radium-221 [1].

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

General data

Name of the isotope:Radium-221; Ra-221Symbol:²²¹Ra or 22188RaMass number A:221 (= number of nucleons)Atomic number Z:88 (= number of protons)Neutrons N:133Nucleon pairing (Z - N):even - oddNuclear ratio (N/Z ratio):1.5113636363636 (= neutron-proton ratio)Neutron excess (N-Z):45Isotopic mass:221.013917(5) u (atomic weight of Radium-221)Nuclide mass:220.965648 u (calculated nuclear mass without electrons)Mass excess:12.9636 MeVMass defect:1.827119644 u (per nucleus)Nuclear binding energy:1701.95098567 MeV (per nucleus)
7.70113568 MeV (average binding energy per nucleon)Separation energy:S_N = 5.380(9) MeV (first neutron)
S_P = 5.807(6) MeV (first proton)Half-life:26.2(1) sDecay constant λ:0.026455999258013 s^-1Specific activity α:7.209129145798 × 10⁺¹⁹ Bq g^-1
1948413282.6481 Ci g^-1Spin and parity:
(nuclear angular momentum)5/2+Magnetic dipole moment:μ(μ_N) = - 0.1799(17)Quadrupole moment Q:1.98(11) barn (100 fm²)Nuclear g-factor:g_l = -0.07196Charge radius:5.6795(228) femtometer fmYear of discovery:1949

Radioactive Decay

Half-life T_½ = 26.2(1) s respectively 2.62 × 10¹ seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
α	²¹⁷Rn	100 %	6.8804(20) MeV
CD ²⁰⁷Pb	¹⁴C	1.2(9) · 10^-10 %	-

Parent Nuclides

Direct parent isotopes are: ²²⁵Th, ²²¹Fr.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 133) and isobaric (same nucleon number A = 221) with Radium-221. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 133	Isobar A = 221
80	²¹³Hg
81	²¹⁴Tl
82	²¹⁵Pb
83	²¹⁶Bi	²²¹Bi
84	²¹⁷Po	²²¹Po
85	²¹⁸At	²²¹At
86	²¹⁹Rn	²²¹Rn
87	²²⁰Fr	²²¹Fr
88	²²¹Ra	²²¹Ra
89	²²²Ac	²²¹Ac
90	²²³Th	²²¹Th
91	²²⁴Pa	²²¹Pa
92	²²⁵U	²²¹U
93	²²⁶Np	²²¹Np

Data changes (history)

Half-life: At this point, a value for the half-life of radium, published in 2024 and determined during three different experimental campaigns, was adopted: 26.2(1) s [2]. In the relevant literature, the value for t_1/2 lies between 25(4) s (NUBASE2020) and 28(2) s.

External data and identifiers

Adopted Levels, Gammas:NuDat 221Ra

Literature and References

[1] - W. W. Meinke, A. Ghiorso, G. T. Seaborg:
Three Additional Collateral Alpha-Decay Chains.
In: Physical Review, 75, 314, (1949), DOI 10.1103/PhysRev.75.314.

[2] - S. Bara et al.:
Half-life determination of ²¹⁵At and ²²¹Ra with high-purity radioactive ion beams.
In: Applied Radiation and Isotopes, 208, 111289, (2024), DOI 10.1016/j.apradiso.2024.111289.