Data for table Atomic Mass, Half Life, and Decay


The data in the table are partially from the data sets of nubase2020 and mass2020 at the Atomic Mass Data Center, the International Atomic Energy Agency. The explanation for the data table can be found in articles "The Ame 2020 atomic mass evaluation (II)" and  "The NUBASE2020 evaluation of nuclear properties". 




Atomic Mass (from mass2020) :


Atomic mass Atomic mass M and its uncertainty in u for data in PubChem element pages, µu in original data file mass16.txt. For more information, see the publication: The Ame2020 atomic mass evaluation (II).


Half-life and Decay information (from nubase2020): 

Data are presented in groups ordered according to increasing mass number A.



N     Number of neutrons.
Z     Number of protons.
A     Mass number A = N + Z


Nuclidic name: mass number A = N + Z and element symbol (for Z > 109 see Section 2). Elements with upper suffix ‘m’, ‘n’, ‘p’, ‘q’, ‘r’ or ‘x’ indicate assignments to excited isomeric states (defined as higher states with half-lives greater than 100 ns). Suffixes ‘p’ and ‘q’ also indicate non-isomeric levels, but used in the AME. Suffix ‘r’ also indicates a state from a proton resonance occurring in (p,γ) reactions (e.g. 28Sir ). Suffix ‘x’ also applies to mixtures of levels (with relative ratio R, given in the ‘Half-life’column), e.g. occurring in spallation reactions (indicated ‘spmix’ in the ‘Jπ’ column) or fission (‘fsmix’).


# instead of a decimal point: value and uncertainty are not derived only from experimental data, but at least partly with estimates from TMS (see the reference publication).



s = seconds; m = minutes; h = hours; d = days; y = years; 1 y = 31 556 926 s or 365.2422 d


adopted values for NUBASE


STABLE = stable nuclide, or nuclide for which no finite half-life value has been found.

# value estimated from trends in neighboring nuclides with the same Z and N parities. subunits:


  ms: 10-3 s millisecond     ky : 103 y kiloyear

  µs: 10-6 s microsecond     My: 106 y megayear

  ns: 10-9 s nanosecond      Gy: 109 y gigayear 

  ps: 10-12 s picosecond     Ty: 1012 y terayear

  fs: 10-15 s femtosecond    Py: 1015 y petayear

  as: 10-18 s attosecond     Ey: 1018 y exayear

  zs: 10-21 s zeptosecond    Zy: 1021 y zettayear

  ys: 10-24 s yoctosecond    Yy: 1024 y yottayear


R: For isomeric mixtures only, it is the production ratio of the excited isomer state to the ground state isomer.


Year of discovery

Discovery year for ground states and for excited isomers. 


Decay modes and intencities

Decay modes followed by their intensities (in %), and their one standard deviation uncertainties. The special notation 1.8e-12 stands for 1.8×1012.


The uncertainties are given - only in this field - in the ENSDF-style: α=25.9 23 stands for α=25.9 ± 2.3 %


The ordering is according to decreasing intensities.


α ?     α decay is energetically allowed.

α=?     α decay has been observed but not yet quantified.


α       α emission

p 2p    proton emission   2-proton emission

n 2n    neutron emission  2-neutron emission

ε       electron capture

e+      positron emission

β+      β+ decay (β+ = ε + e+)

β-      β- decay

-     double β- decay

+     double β+ decay

β-n     β- delayed neutron emission

β-2n    β- delayed 2-neutron emission

β+p     β+ delayed proton emission

β+2p    β+ delayed 2-proton emission

β-α     β- delayed α emission

β+α     β+ delayed α emission

β-d     β- delayed deuteron emission

IT      internal transition

SF      spontaneous fission

β+SF    β+ delayed fission

β-SF    β- delayed fission

24Ne    heavy cluster emission

...     list is continued in a remark, at the end of the A-group


For long-lived nuclides:


IS      Isotopic abundance 


* A remark on the corresponding nuclide is given below the block of data corresponding to the same A.


Remarks. For nuclides marked with an asterisk at the end of the line, extra comments have been added. They are collected in groups at the end of each block of data corresponding to the same A. They start with a letter code, similar the ones following the reference key-number, as given above, indicating to which quantity the remark applies. They give:


  i)    Continuation for the list of decays. In this case, the remark starts with three dots.

  ii)   Information explaining how a value has been derived.

  iii)  Reasons for changing a value or its uncertainty as given by the authors, or for rejecting it.

  iv)  Complementary references to updated data.

  v)   Separate values used in the adopted average.


TNN : Trends from neighboring nuclides.

τ : meanlife (or lifetime) T = τ ×ln2


Was this information helpful?

The page cannot be found

The page you are looking for might have been removed, had its name changed, or is temporarily unavailable. Please make sure you spelled the page name correctly or use the search box.