XPath Selection

Since Webnucleo network codes can read data from and store data to XML, you can use XPath to select data for a reaction network. This cookbook helps you get familiar with how to use XPath with Webnucleo codes.

Begin by retrieving a network XML file. You can do this directly by pulling one down from OSF. To retrieve a network XML file based on the V2.2 JINA Reaclib database, type:

$ curl -L -J -o net.xml https://osf.io/kyhbs/download

Now open python and read the file into a network instance:

>>> import wnnet as wn
>>> net = wn.net.Net('net.xml')

Next, get and print out the nuclides in the network along with their atomic and mass numbers:

>>> nucs = net.get_nuclides()
>>> for nuc in nucs:
...     print(nuc, nucs[nuc]['z'], nucs[nuc]['a'])
...

Get all the data about a particular nuclide by typing, for example:

>>> print(nucs['o16'])

Now retrieve a subset of the nuclides. Select only nuclides with neutron number 50:

>>> nucs_n_50 = net.get_nuclides(nuc_xpath = "[a - z = 50]")

and print them out:

>>> for nuc in nucs_n_50:
...     print(nuc, nucs_n_50[nuc]['z'], nucs_n_50[nuc]['n'], nucs_n_50[nuc]['a'])
...

You must select on a - z because the network XML does not store n separately. Try other XPath expressions to get other nuclide subsets.

You can also use XPath to select reactions. Begin by selecting all reactions and printing them out:

>>> reacs = net.get_reactions()
>>> for reac in reacs:
...     print(reac)
...

Now try selecting only \((n,\gamma)\) reactions:

>>> reacs_ng = net.get_reactions(reac_xpath="[reactant = 'n' and product = 'gamma']")
>>> for reac in reacs_ng:
...     print(reac)
...

Now print out only reactions with \(^{90}{\rm Zr}\) as a product:

>>> for reac in net.get_reactions(reac_xpath="[product = 'zr90']"):
...     print(reac)
...

XPath has many functions. Some may be useful for Webnucleo XML. For example, you can find the reactions with exactly three products:

>>> for reac in net.get_reactions(reac_xpath="[count(product) = 3]"):
...     print(reac)
...

One can further refine that with:

>>> for reac in net.get_reactions(reac_xpath="[count(product) = 3 and not(contains(., 'neutrino'))]"):
...     print(reac)
...

The . in this XPath expression refers to the current XML node that is being evaluated by the parser.

Valid reactions in a Webnucleo reaction network are ones that are between species actually included in the network (and that conserve baryon number, charge, and lepton number). Select valid neutron-capture reactions for zinc isotopes:

>>> reacs_zn_ng = net.get_valid_reactions(nuc_xpath = "[z = 30]", reac_xpath = "[reactant = 'n']")
>>> for reac in reacs_zn_ng:
...     print(reac)
...

In fact, that prints out no reactions. The reason is that you have not included the neutrons in the network. Fix that by typing:

>>> reacs_zn_ng = net.get_valid_reactions(nuc_xpath = "[(a - z = 1) or (z = 30)]", reac_xpath = "[reactant = 'n']")

This emphasizes that all species involved in a reaction must be present for the reaction to be valid.

This cookbook has used wnnet to illustrate XPath relevant to Webnucleo. The C++ network codes similarly use XPath, with nuclide data selected by the nuc_xpath option and reaction data selected by reac_xpath. The default in those codes is to use all nuclides and all reactions in the network data. Those defaults are equivalent to using the expressions nuc_xpath = “” or nuc_xpath = “[.]” and *reac_xpath = “” and reac_xpath = “[.]”.