Cerium-120

Properties and data of the isotope ¹²⁰Ce.

Contents

Cerium-120 is a radioisotope of the chemical element cerium, which has 62 neutrons in its atomic nucleus in addition to the element-specific 58 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 120. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ¹²⁰Ce is exclusively for academic purposes.

Until 2025, cerium-120 was exclusively the subject of theoretical predictions and tabular classifications, without any experimental evidence. It was listed as an extrapolated entry in isotope overviews, while it was not yet included in international evaluations.

The first experimental identification occurred in 2025 as part of a fragmentation experiment using a uranium-238 beam at an energy of 345 MeV per nucleon. The reaction on a beryllium target produced neutron-rich fission fragments, which were then selectively separated using the BigRIPS separator. The determination of charge number and mass via time-of-flight, momentum, and energy loss measurements allowed a clear assignment to cerium-120. Since no decay studies or spectroscopic measurements have been carried out, the half-life and nuclear spin states are not yet known [1].

The values presented here are preliminary.

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

General data

Name of the isotope:Cerium-120; Ce-120Symbol:¹²⁰Ce or 12058CeMass number A:120 (= number of nucleons)Atomic number Z:58 (= number of protons)Neutrons N:62Isotopic mass:119.946610(54) u (atomic weight of Cerium-120)Nuclide mass:119.9147946 u (calculated nuclear mass without electrons)Mass excess:-49.73246 MeVMass defect:1.044466648 u (per nucleus)Nuclear binding energy:972.91441581 MeV (per nucleus)
8.10762013 MeV (average binding energy per nucleon)Separation energy:S_N = 13.981(707) MeV (first neutron)
S_P = 1.999(583) MeV (first proton)Half-life:250 msDecay constant λ:2.7725887222398 s^-1Specific activity α:1.403102506890 × 10⁺²² Bq g^-1
379216893754.27 Ci g^-1Spin and parity:
(nuclear angular momentum)0+Year of discovery:2025

Radioactive Decay

Half-life T_½ = 250 ms respectively 2.5 × 10^-1 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
EC, β⁺, (p)		?

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 62) and isobaric (same nucleon number A = 120) with Cerium-120. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 62	Isobar A = 120
35	⁹⁷Br
36	⁹⁸Kr
37	⁹⁹Rb
38	¹⁰⁰Sr
39	¹⁰¹Y
40	¹⁰²Zr
41	¹⁰³Nb
42	¹⁰⁴Mo
43	¹⁰⁵Tc	¹²⁰Tc
44	¹⁰⁶Ru	¹²⁰Ru
45	¹⁰⁷Rh	¹²⁰Rh
46	¹⁰⁸Pd	¹²⁰Pd
47	¹⁰⁹Ag	¹²⁰Ag
48	¹¹⁰Cd	¹²⁰Cd
49	¹¹¹In	¹²⁰In
50	¹¹²Sn	¹²⁰Sn
51	¹¹³Sb	¹²⁰Sb
52	¹¹⁴Te	¹²⁰Te
53	¹¹⁵I	¹²⁰I
54	¹¹⁶Xe	¹²⁰Xe
55	¹¹⁷Cs	¹²⁰Cs
56	¹¹⁸Ba	¹²⁰Ba
57	¹¹⁹La	¹²⁰La
58	¹²⁰Ce	¹²⁰Ce
59	¹²¹Pr