Nuclide Naming Conventions

One of the most frustrating aspects of nuclear data software is the large number of different ways that people choose to nuclide names. Functionally, there are three pieces of information that should be included in a radionuclide’s name

  1. Z Number: The number of protons.

  2. A Number: The number of nucleons (neutrons + protons).

  3. Metastable Level: The metastable state of the nucleus as defined by ENSDF.

Some common naming conventions exist. The following are the ones currently supported by PyNE. Functions to convert between forms may be seen in Naming Convention Casting Functions.

  1. id (zas): This type places the charge of the nucleus out front, then has three digits for the atomic mass number, and ends with four state digits (0 = ground, 1 = first excited state, 2 = second excited state, etc). Uranium-235 here would be expressed as ‘922350000’. This is th canonical form for nuclides.

  2. name: This is the more common, human readable notation. The chemical symbol (one or two characters long) is first, followed by the atomic weight. Lastly if the nuclide is metastable, the letter M is concatenated to the end. For example, ‘H-1’ and ‘Am242M’ are both valid. Note that nucname will always return name form with the dash removed and all letters uppercase.

  3. zzaaam: This type places the charge of the nucleus out front, then has three digits for the atomic mass number, and ends with a metastable flag (0 = ground, 1 = first excited state, 2 = second excited state, etc). Uranium-235 here would be expressed as ‘922350’.

  4. zzzaaa: This type places the charge of the nucleus out front, then has three digits for the atomic mass. It contains no information about the excited state.

  5. zzllaaam: The ZZLLAAAM naming convention is similar to name form. However, it is preceded by the nuclides two AA numbers, followed by the two LL characters. Of the two LL characters, only the first letter in the chemical symbol is uppercase, the dash is always present, and the the meta-stable flag is lowercase. For instance, ‘95-Am-242m’ is the valid serpent notation for this nuclide.

  6. SZA: This type places three state digits out front, the charge of the nucleus in the middle, and then has three digits for the atomic mass number. Uranium-235M here would be expressed as ‘1092235’.

  7. MCNP: The MCNP format for entering nuclides is unfortunately non-standard. In most ways it is similar to zzaaam form, except that it lacks the metastable flag. For information on how metastable isotopes are named, please consult the MCNPX documentation for more information.

  8. Serpent: The serpent naming convention is similar to name form. However, only the first letter in the chemical symbol is uppercase, the dash is always present, and the the meta-stable flag is lowercase. For instance, ‘Am-242m’ is the valid serpent notation for this nuclide.

  9. NIST: The NIST naming convention is also similar to the Serpent form. However, this convention contains no metastable information. Moreover, the A-number comes before the element symbol. For example, ‘242Am’ is the valid NIST notation.

  10. CINDER: The CINDER format is similar to zzaaam form except that the placement of the Z- and A-numbers are swapped. Therefore, this format is effectively aaazzzm. For example, ‘2420951’ is the valid cinder notation for ‘AM242M’.

  11. ALARA: In ALARA format, elements are denoted by the lower case atomic symbol. Nuclides are specified by appending a semicolon and A-number. For example, “fe” and “fe:56” represent elemental iron and iron-56 respectively. No metastable flag exists.

  12. Groundstate: In Groundstate format, the nuclide is stored in a form similar to the standard id form, but the last four digits are zero to eliminate the information about the nuclide’s state.

  13. state_id: The state id naming convention uses the form zzzaaassss. It is different from the canonical zzzaaassss form in that ssss refers to a list of states by ordered by energy. This is derived from the levels listed in the ENSDF files for a given nuclide. Using this form is dangerous as it may change with new releases of ENSDF data.

If there are more conventions that you would like to see supported, please contact the PyNE Development Team.

Canonical Form

The zzzaaammmm integer form of nuclide names is the fundamental form of nuclide naming because it accurately captures all of the needed information in the smallest amount of space. Given that the Z-number may be up to three digits, A-numbers are always three digits, and the excitation level is one digit, all possible nuclides are represented on the range 0 <= zzzaaammmm < 10000000000. This falls well within 32 bit integers (but sadly outside of the smaller 16 bit ints).

On the other hand, name string representations may be anywhere from two characters (16 bits) up to six characters (48 bits). So in general, zzzaaammmm is smaller by 50%. Other forms do not necessarily contain all of the required information (MCNP) or require additional storage space (Serpent). It may seem pedantic to quibble over the number of bits per nuclide name, but these identifiers are used everywhere throughout nuclear code, so it behooves us to be as small and fast as possible.

The other distinct advantage that integer forms have is that you can natively perform arithmetic on them. For example:

# Am-242m
nuc = 942420001

# Z-number
zz = nuc/10000

# A-number
aaa = (nuc/10)%1000

# Meta-stable state
m = nuc%10

Code internal to PyNE use zzzaaammmm, and except for human readability, you should too! Natural elements are specified in this form by having zero A-number and excitation flags (zzzaaammmm('U') == 920000).

Well-Defined vs Ambiguous Situations

In situations where the input naming convention is well-defined, it is highly recommended that you use the direct <form>_to_id() functions (e.g. mcnp_to_id()) to convert from a nuclide in the given form to the id form representation. When a high level of quality assurance is required, it is advisable to require an specific input format to leverage the exactness of the direct-to-id functions.

However, in situations where arbitrary nuclide naming conventions are allowed, you must use the id() function. An example of such a situation is when accepting human input. This function attempts to resolve the underlying nuclide identifier. For most nuclides and most normal spellings, this resolution is straightforward. However, some nulcides are ambiguous between the various supported naming conventions. For more information please refer to the nucname theory manual.

Examples of Use

In [1]: from pyne import nucname

In [2]: nucname.zzaaam('U235')
Out[2]: 922350

In [3]: nucname.zzaaam(10010)
Out[3]: 10010

In [4]: nucname.zzllaaam(942390)
Out[4]: '95-Am-242m'

In [5]: nucname.name(10010)
Out[5]: 'H1'

In [6]: nucname.serpent('AM242M')
Out[6]: 'Am-242m'

In [7]: nucname.name_zz['SR']
Out[7]: 38

In [8]: nucname.zz_name[57]
Out[8]: 'LA'

In [9]: nucname.alara('FE56')
Out[9]: 'fe:56'

Further information on the naming module may be seen in the library reference Nuclide Naming Module – pyne.nucname.